Assessment of Regional Earthquake Hazards and 

Risk Along the Wasatch Front, Utah 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-A-J 

Contributions from Utah Geological and Mineral Survey, 

University of Utah, and Utah State University 
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Assessment of Regional Earthquake Hazards and 

Risk Along tht~ Wasatch Front, Utah 

PAULA L. GORI and 'VALTER W. HAYS, Editors 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-A-J 

Contributions from 

Utah Geological and Mineral Survey, 

University of Utah, and 

Utah State University 

Chapters A-] are issued as a single volume 

and are not available sepaTately. 

Chapter titles are listed in the volume 

table of contents. 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1992 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by the 

U.S. Government 

Library of Congress Cataloging in Publication Data 

Assessing regional earthquake hazards and risk along the Wasatch Front, Utah I Paula L. Gori and Walter W. Hays, editors. 

p. em. - (U.S. Geological Survey professional paper; 1500) 

Supt. of Docs. no.: I 19.16:1500A-J 

1. Earthquake hazard analysis-Wasatch Range (Utah and Idaho) 2. Seismology-Wasatch Range (Utah and Idaho) 

3. Natural disasters-Utah. 4. Faults (Geology)-Wasatch Range (Utah and Idaho) 5. Geology, Stratigraphic. 

6. Geology-Wasatch Range (Utah and Idaho) I. Gori, Paula. II. Hays, Walter W. Ill. Series. 

QE535.2. U6A84 1991 

551.2'2'097922-dc20 90-26385 

For sale by Book and Open-File Report Sales, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225 

CIP 
CONTENTS 

[Letters designate the chapters] 

(A) Paleoseismology of the Wasatch Fault Zone: A Summary of Recent Investigations, Interpretations, and Conclusions, by Michael N. Machette, Stephen F. Personius, and Alan 

R. Nelson 

(B) Persistent and Nonpersistent Segmentation of the Wasatch Fault Zone, Utah: Statistical 

Analysis for Evaluation of Seismic Hazard, by Russell L. Wheeler and Katherine B. 

Krystinik 

(C) Subsurface Geology Along the Wasatch Front, by Don R. Mabey 

(D) Observational Seismology and the Evaluation of Earthquake Hazards and Risk in the 

Wasatch Front Area, Utah, by W.J. Arabasz, J.C. Pechmann, and E.D. Brown 

(E) Superimposed Late Cenozoic, Mesozoic, and Possible Proterozoic Deformation Along the 

Wasatch Fault Zone in Central Utah, by Mary Lou Zoback 

(F) Neotectonic Framework of the Central Sevier Valley Area, Utah, and Its Relationship to 

Seismicity, by R. Ernest Anderson and Theodore P. Barnhard 

(G) Neotectonics of the Hansel Valley-Pocatello Valley Corridor, Northern Utah and Southern 

Idaho, by James McCalpin, Robert M. Robison, and John D. Garr 

(H) Structure of the Salt Lake Segment, Wasatch Normal Fault Zone: Implications for Rupture Propagation During Normal Faulting, by R.L. Bruhn, P.R. Gibler, W. Houghton, 

and W.T. Parry 

(I) Late Quaternary Displacement on the Morgan Fault, a Back Valley Fault in the Wasatch 

Range of Northeastern Utah, by J. Timothy Sullivan and Alan R. Nelson 

(J) Late Quaternary History of the James Peak Fault, Southernmost Cache Valley, NorthCentral Utah, by Alan R. Nelson and J. Timothy Sullivan 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

INTRODUCTION 

By PAULA L. GoRI and WALTER W. HAYS 

THE EARTHQUAKE THREAT 

There are many geologists who are very wise, but even they do 

not understand the forces which produce mountains. And yet it 

must be admitted, not only that mountains have been made, 

but that some mountains are still rising. The mysterious forces 

appear to act in different ways in different places, and it is 

possible that their nature is not universally the same. Suffice it 

to say that in the Great Basin the movements they cause are 

vertical. It is as though something beneath each mountain was 

slowly, steadily, and irresistibly rising, carrying the mountain 

with it. 

So began an article in 1883 by Grove Karl Gilbert, the 

first Chief Geologist of the U.S. Geological Survey 

(USGS), warning the citizens of the Great Basin about 

the potential for damaging earthquakes in their region. 

Describing the damage and loss of life in a small town in 

California that he had just visited after an earthquake, 

Gilbert cautioned that a similar earthquake could occur in 

Salt Lake City. "It is useless to ask when this disaster 

will occur. Our occupation of the country has been too 

brief for us to learn how fast the Wasatch grows; and 

indeed, it is only by such disasters that we can learn. By 

the time experience has taught us this, Salt Lake City 

will have been shaken down .... " (Gilbert, 1883, p. 4). 

After more than a century of research, geologists and 

other scientists have increased their understanding of 

the tectonic processes at work in the Wasatch Front and 

are now beginning to answer the questions about the 

seismicity and tectonics of the Wasatch Front that 

Gilbert and others first posed. Scientists, engineers, 

architects, urban planners, and emergency managers are 

not waiting for a major earthquake disaster to learn that 

measures must be implemented to mitigate an ~arthquake's effects. They are taking actions now to prepare 

for and to mitigate the physical effects of such an 

earthquake. 

The majority of Utah's population lives adjacent to the 

Wasatch fault zone (fig. 1), an active, north-trending 

zone of normal faulting that extends approximately 370 

km' at the western foot of the Wasatch Range. Earthquakes have been reported since the arrival of the 

Mormon pioneers in 1847. During the last two centuries, 

eight earthquakes ofmagnitude1 greater than or equal to 

6 have occurred in Utah. The two largest were the 

magnitude (ML) 6.6 Hansel Valley earthquake of 1934 

(fig. 2) and the ML 6.5 Richfield earthquake of 1901. 

The historical record of seismicity in Utah has been 

broadened by incorporating geologic evidence and monitoring small earthquakes. Although no large earthquakes (M>7.0) have occurred since Utah was settled, 

clear geologic and geomorphic evidence (Hamblin, 1976) 

demonstrates that large earthquakes have occurred 

repeatedly throughout the late Pleistocene (2 million 

years before present) and Holocene (last 10,000 years) on 

segments within the Wasatch fault zone. For this reason 

and because of the continuing low level of seismicity, 

scientists believe that some parts of segments of the 

Wasatch fault zone are overdue for a damaging earthquake (Schwartz and Coppersmith, 1984). Machette and 

others (this report) have identified 10 segments of the 

Wasatch fault that have produced large earthquakes on 

the average of every several hundred to 1,000 years. 

1 Magnitude is a "number that characterizes the size of an earthquake, based on 

measurement of the maximum motions recorded by a seismograph for earthquake 

waves of a particular frequency, Scales most commonly used in the Western 

United States are (1) local magnitude (ML) (commonly referred to as "Richter 

magnitude"), (2) surface-wave magnitude (M8), and (3) body-wave magnitude 

(mb). None of these scales satisfactorily measures the largest possible earthquakes because each relates to only certain frequencies of seismic waves and 

because the spectrum of radiated seismic energy changes with earthquake size. 

The recently devised moment magnitude (M) scale, based on the concept of 

seismic moment, is uniformly applicable to all sizes of earthquakes" (Ziony, 1985, 

p. 18). 

1 
2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

C3 

~ IJJI 

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113' IDAHO 

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~---------(~~------L_ ______ _ I ' ( 

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FIGURE 1. -Location map of the Wasatch Front study area (shaded), northern Utah. Generalized fault locations are also 

shown. 
INTRODUCTION 3 

FIGURE 2. -Fault scarp formed in Utah's Hansel Valley north of Great 

Salt Lake in the 1934 Hansel Valley earthquake. 

Utah's largest urban centers, where more than 80 

percent of the State's 2 million people live, are located 

along the Wasatch Front. A large earthquake centered 

near Salt Lake City has the potential to cause extensive 

damage to buildings, lifelines, and public facilities. The 

expected level of peak ground acceleration would be in 

the range of 0.2 to 0.4 g or greater. Surface-fault rupture 

and tectonic deformation (fig. 3) as well as landslides and 

liquefaction would be expected to occur in many areas 

(fig. 4). Death and injury rates could be high, depending 

on the time of day that the earthquake struck and the 

nature and extent of pre-event mitigation actions. 

The October 28, 1983, Borah Peak, Idaho, magnitude 

(M8 ) 7.3, earthquake reminded scientists and policymakers that large earthquakes will recur in the Intermountain Seismic Belt (fig. 5). Scientists believe that the 

Borah Peak earthquake is a model of what might happen 

on the Wasatch Front (Smith and Richins, 1984 ). A 

magnitude 7.0 to 7.5 earthquake is now being considered 

as a scenario earthquake for emergency response planning in Utah. 

PURPOSE AND SCOPE OF THIS VOLUME 

In 1983, the USGS proposed the Wasatch Front of 

Utah as the target of a 5-year program of focused 

research on earthquake hazards and hazards reduction. 

The goals were to (1) accelerate the development of the 

knowledge base on seismic sources, size, frequency of 

occurrence, and physical effects of earthquakes in a 

10-county area along the Wasatch Front, including Salt 

Lake, Davis, Juab, Weber, Wasatch, Summit, Morgan, 

Cache, Utah, and Box Elder Counties and (2) foster 

implementation of earthquake hazards mitigation measures. 

The USGS earthquake hazards program in the 

Wasatch Front area was conducted under the auspices of 

the "Regional Earthquake Hazards Assessment" element, one of five elements in the USGS' Earthquake 

Hazards Reduction Program. The element was established to provide concentrated and coordinated attention 

to geographic regions containing large urban areas at 

risk from earthquakes by utilizing past research and 

fostering partnerships with universities, the private 

sector, local government, and State and Federal agencies. The effort provides a technical basis for devising 

and implementing mitigation and loss reduction measures. 

The five interrelated goals in the Utah comprehensive 

research and implementation program were: 

Information systems.-To produce quality data along 

with a comprehensive information system, available to 

both USGS and non-USGS users, for use in earthquake 

hazards evaluations, risk assessment, and implementation of loss reduction measures. 

Synthesis of geological and geophysical data. -To 

prepare synthesis reports describing the nature, extent, 

frequency of occurrence, and physical effects of the 

earthquake hazards of ground shaking, surface faulting, 

earthquake-induced ground failure, and tectonic deformation and recommend future research to increase the 

knowledge base required for the creation and implementation of mitigation and loss-reduction measures. 

Ground-motion modeling.-To produce deterministic 

and probabilistic ground-motion models and maps of the 

ground-shaking hazard and commentaries on their use in 

building codes and land use regulations. 

Loss estimation models.-To devise economical methods for acquiring inventories of structures and lifeline 

systems in urban areas, to create a standard model for 

loss estimation, to produce loss and casualty estimates 

for urban areas, and to prepare commentaries giving 

guidelines for use by agencies of State and local governments. 

Implementation.-To foster the creation and implementation of measures to mitigate the earthquake hazards of ground shaking, surface-fault rupture, 

earthquake-induced ground failure, and tectonic deformation in urban areas and to provide high-quality scientific information that can be used by local government 

decisionmakers as a basis for implementing and enforcing loss-reduction measures. 

This report provides a comprehensive treatment of the 

knowledge gained by the research and implementation 

program in the Wasatch Front area. The two volumes 

are divided into three sections dealing with individual 

components of the program that collectively define the 

nature of the earthquake hazards in the Wasatch Front 

area. The first volume deals with tectonic framework and 
4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FIGURE 3. -Interested bystanders studying fault rupture formed in the 1934 Hansel Valley, Utah, earthquake. From the Thomas Adams 

Collection; courtesy of the Department of Geology and Geophysics, University of Utah, Provo. 

earthquake potential. The second volume covers (1) 

ground-shaking hazards and aspects of loss estimation 

and (2) use of earthquake hazards information for urban 

and regional planning and development. The research 

results came from studies conducted by scientists and 

engineers of the USGS, the Utah Geological and Mineral 

Survey, universities, and private consulting firms. 

Funding was provided primarily by the USGS in the 

form of grants to non-USGS scientists and engineers. 

TECTONIC FRAMEWORK AND EARTHQUAKE POTENTIAL 

OF THE WASATCH FRONT AREA 

The first volume of Professional Paper 1500 contains 

chapters devoted to geologic and geophysical studies 

aimed at improving the fundamental understanding of 

the potential for the occurrence of large, damaging 

earthquakes in the Wasatch Front area. In some cases, 

researchers have drawn on studies of other parts of Utah 

and the Intermountain Seismic Belt to gain an understanding of the large-magnitude earthquakes that may 

occur in the Wasatch Front. The geologic and seismological/geophysical studies comprising this section are 

described in Chapters A through J: 

Chapter A: Paleoseimology of the Wasatch Fault Zone: A 

Summary of Recent Investigations, Interpretations, 

and Conclusions, by Michael N. Machette, Stephen F. 

Personius, and Alan R. Nelson. Ten discrete segments have been identified on the Wasatch fault zone. 

The fact that eight of these segments have demonstrable Holocene movement increases the possible number 

of separate localities where earthquakes may occur. 

Chapter B: Persistent and Nonpersistent Segmentation 

of the Wasatch Fault Zone, Utah: Statistical Analysis 

for Evaluation of Seismic Hazard, by Russell L. 

Wheeler and Katherine B. Krystinik. The Wasatch 

fault zone has been segmented at four salients-Pleasant View, Salt Lake, Traverse Mountains, and Payson-throughout much or all of its 10-m.y. history and 

will likely continue to be segmented there throughout 

the next several millenia, which is the time span of 

interest for hazard evaluation. 

Chapter C: Subsurface Geology Along the Wasatch 

Front, by Don R. Mabey. Magnetic data suggest 

segment boundaries of the Wasatch fault zone that are 

generally consistent with segment boundaries inferred 

from surface mapping of the fault zone. 

Chapter D: Observational Seismology and the Evaluation of Earthquake Hazards and Risk in the Wasatch 

Front Area, Utah, by W.J. Arabasz, J.C. Pechmann, 

and E.D. Brown. Background seismicity predominates 

on second-order faults in the Wasatch Front area. 
INTRODUCTION 5 

FIGURE 4. -Sand boils caused by liquefaction associated with the 1934 

Hansel Valley, Utah, earthquake. 

Small to moderate earthquakes are the largest contributor to the probabilistic ground-shaking hazard for 

exposure periods of 50 years or less. The earthquake 

data imply an average return period of24±10 years for 

potentially damaging earthquakes of magnitude 5.5 or 

greater along the Wasatch Front. 

Chapter E: Superimposed Late Cenozoic, Mesozoic, and 

Possible Proterozoic Deformation Along the Wasatch 

Fault Zone in Central Utah, by Mary Lou Zoback. 

Thrust ramping and late Cenozoic normal faulting may 

be localized by a major west-dipping normal fault zone 

formed during the early phases of late Precambrian 

rifting of the western Cordillera. 

Chapter F: Neotectonic Framework of the Central 

Sevier Valley Area, Utah, and Its Relationship to 

Seismicity, by R. Ernest Anderson and Theodore P. 

Barnhard. Normal faults in the Wasatch fault zone 

such as the Sevier fault probably cut one or more levels 

of potential structural detachment and penetrate to 

the base of the seismogenic part of the crust. Such 

faults are more likely to be the source of infrequent 

large earthquakes than are faults in the complex 

structural junctures where late Quarternary deformation is concentrated. 

Chapter G: Neotectonics of the Hansel Valley-Pocatello 

Valley Corridor, Northern Utah and Southern Idaho, 

by James McCalpin, Robert M. Robison, and John D. 

Garr. The 1934 ML 6.6 earthquake may be a typical 

interpluvial maximum event (that is, long recurrence 

time and small displacement in comparison with the 

larger, more frequent surface-faulting events that are 

triggered by pluvial lake water loading). 

Chapter H: Structure of the Salt Lake Segment, 

Wasatch Normal Fault Zone: Implications for Rupture 

Propagation During Normal Faulting, by R.L. Bruhn, 

P.R. Gibler, W. Houghton, and W.T. Parry. There are 

two potential sites of rupture initiation for large earthquakes at the central and southern ends of the Salt 

Lake fault segment of the Wasatch normal fault zone. 

The central site may have been the most common 

position for repetitive rupture initiation during the last 

17 million years. 

Chapter I: Quaternary Displacement on the Morgan 

Fault, a Back Valley Fault in the Wasatch Range of 

Northeastern Utah, by J. Timothy Sullivan and Alan 

R. Nelson. Paleoearthquakes having magnitudes in 

the range of 6.5 to 7 have occurred on the Morgan 

fault. 

Chapter J: Late Quaternary History of the James Peak 

Fault, Southernmost Cache Valley, North-Central 

Utah, by Alan R. Nelson and J. Timothy Sullivan. The 

James Peak fault may be a westerly splay of the East 

Cache fault rather than a separate valley-bounding 

fault. 

THE GROUND-SHAKING HAZARD AND ASPECTS OF LOSS 

ESTIMATION IN THE WASATCH FRONT AREA 

The second volume of Professional Paper 1500, which 

will be published under separate cover, is concerned with 

predicting the effects of local site conditions on ground 

shaking in the Wasatch Front area and developing loss 

(risk) estimation procedures. Ground-motion and loss 

estimation studies comprising this section are described 

in Chapters K through T: 

Chapter K: Site Amplification in the Salt Lake CityOgden-Provo, Utah, Corridor and the Implications for 

Earthquake-Resistant Design, by Walter W. Hays. 

Evaluation of site amplification effects in Utah indicates that soil deposits in the Salt Lake City-OgdenProvo urban corridor should amplify earthquake 

ground motions, especially when the level of shear 

strain induced in the soil column is less than about 0.5 

percent. 
6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FIGURE 5. -Graben formed in the fault rupture of the 1983 Borah Peak, Idaho, earthquake. 

Chapter L: Predicting Strong Ground Motion in Utah, by 

Kenneth W. Campbell. Near-source attenuation relationships for predicting peak horizontal acceleration 

and velocity in terms of earthquake magnitude, 

source-to-site distance, and several sources and site 

parameters are used to estimate strong ground motion 

in north-central Utah. 

Chapter M: Probabilistic Analy'>is of Earthquake 

Ground-Shaking Hazards Along the Wasatch Front, 

Utah, by R.R. Youngs, F.H. Swan, M.S. Power, D.P. 

Schwartz, and R.K. Green. The ground-shaking hazard along the Wasatch Front urban corridor has been 

assessed by using the geologic and seismologic data 

base that has been developed for the region. 

Chapter N: Relative Ground Response in Salt Lake City 

and Areas of Springville-Spanish Fork, Utah, by Kenneth W. King, Robert A. Williams, and David L. 

Carver. To predict the ground-shaking intensity 
INTRODUCTION 7 

potential for the Wasatch Front urban corridor, a 

better understanding of how shallow underlying geology affects ground shaking was developed. 

Chapter 0: In Situ Poisson's Ratio Measurements Near 

Provo, Utah, by Richard D. Miller, Don W. Steeples, 

Kenneth W. King, and Ralph W. Knapp. Classification 

of sites according to their response to earthquake 

energy enables engineers to accurately determine 

structural requirements for individual sites in high- to 

moderate-risk earthquake zones. 

Chapter P: Earthquake Losses in Central Utah, by T.S. 

Algermissen, Edward P. Arnold, Karl V. Steinbrugge, Margaret G. Hopper, and Philip S. Powers. 

Estimates of losses in central Utah have been made for 

a series of simulated earthquakes. Expected maximum 

losses in the Salt Lake City urban corridor in 50 years 

at a 10-percent chance of exceedance were also found. 

The losses range from $830 million for a M L 5. 5 shock 

on the Provo segment to $5.5 billion for a M8 7.5 quake 

on the Salt Lake segment. 

Chapter Q: Isoseismals of Some Historical Earthquakes 

Affecting the Wasatch Front Area, Utah, by Margaret 

G. Hopper. Isoseismals were drawn for 13 historical 

earthquakes that caused damage-level intensities 

(MMI> VI)2 in the four Wasatch Front counties of 

Weber, Davis, Salt Lake, and Utah. The highest 

historical intensity within any of the four counties was 

MMI VII. 

Chapter R: Effects of Six Damaging Earthquakes in Salt 

Lake City, Utah, by Sherry D. Oaks. Modified Mercalli intensities of historical earthquakes indicate that 

intensity increases across Salt Lake City from the 

Wasatch fault zone to the Jordan River Valley by 2 to 

3 intensity units and from east of the Utah State 

Capitol building south to 17th Street by 1Vz intensity 

units (increase in ground-shaking from east to west 

and from north to south). 

Chapter S: Seismic Risk Methods and Estimates for 

Utility Systems and State-Owned Buildings Along the 

Wasatch Front, by Craig E. Taylor, Delbert B. Ward, 

and Jerold M. Haber. Results of seismic risk projects 

on Utah facilities reaffirm the iterative nature of these 

projects. 

Chapter T: Earthquake Hazards to Domestic Water 

Distribution Systems in Salt Lake County, Utah, by 

Lynn M. Highland. In a magnitude 7.5 earthquake 

occurring along the Wasatch Front in Utah, significant 

damage may occur to Salt Lake County's domestic 

water system. 

2

Intensity is "a subjective numerical index describing the severity of an 

earthquake in terms of its effect on the Earth's surface and on humans and their 

structures" (Ziony, 1985, p. 18). 

USE OF EARTHQUAKE HAZARDS INFORMATION FOR 

URBAN AND REGIONAL PLANNING AND DEVELOPMENT 

IN THE WASATCH FRONT AREA 

Chapters U through Z describe the use of earthquake 

hazards information to reduce potential loss of life and 

damage to property after a damaging earthquake. 

Chapter U: Making the Implementation Process of the 

National Earthquake Hazards Reduction Program 

Work in Utah, by Walter W. Hays. Utah has the 

necessary elements of a program to implement loss 

reduction measures designed to mitigate earthquake 

hazards. 

Chapter V: Reducing Earthquake Risk in Utah: Past 

Trends and Future Opportunities, by Genevieve 

Atwood and Don R. Mabey. Utah's political and social 

structure produces factors that both enhance and 

impede programs to reduce earthquake risk. 

Chapter W: Emergency Management in Utah for Earthquakes, by James L. Tingey and Ralph F. Findlay. 

The major challenge facing the Utah State Division of 

Comprehensive Emergency Management has been to 

provide leadership for the implementation of preparedness and other mitigation strategies within the 

cities, counties, and State agencies of Utah. 

Chapter X: Utah Geological and Mineral Survey Excavation and Inspection Program: A Tool for Earthquake 

Hazard Recognition in Utah, by Harold E. Gill. 

Results of the Utah Geologic and Mineral Survey 

excavation inspection program have shown the zone of 

deformation along the Wasatch fault to be highly 

variable and, in some locations, up to several hundred 

meters wide. 

Chapter Y: Fall Hazard Susceptibility Due to Earthquakes, Central Wasatch Front, Utah, by William F. 

Case. Loss of life and property along a widespread 

area of the Wasatch Front could result from rock falls 

triggered by nearby earthquakes. 

Chapter Z: Legal Issues Related to Hazard Mitigation 

Policies, by James E. Slosson. Professionals involved 

in the design, construction, and maintenance of structures and facilities may be held to the high standards of 

practice established outside of Utah. Also, planners 

and public officials may become targets in future 

litigation and share the burden of court-imposed penalties. 

SUMMARY 

Efforts to assess regional earthquake hazards and risk 

along the Wasatch Front in Utah are dynamic and 

multidisciplinary. This report reflects the nature of that 

program and the research conducted before 1988. 

Not only are the topics varied-geology, seismology, 
8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

engineering geology, urban planning, law, economics, 

and so on-but the level of complexity is also varied. 

Seismologists and geologists can draw upon many years 

of research in their disciplines in Utah, whereas 

researchers in engineering, land use planning, and implementation have had to be pioneers in their fields. 

The timeliness and importance of the research contained herein have made it desirable to publish this 

report before the completion of all the research conducted during the 5 years of the program. The results 

from ongoing research in Utah will be included in a 

second professional paper (Professional Paper 1519) published as a sequel to this one. Eventually, the tectonic 

processes and their physical and social effects in the 

Wasatch Front will be truly understood and documented. 

REFERENCES CITED 

Gilbert, G.K., 1883, Earthquakes: Salt Lake Daily Tribune, Sept. 16, 

1883, p. 4. 

Hamblin, W.K., 1976, Patterns of displacement along the Wasatch 

fault: Geology, v. 4, p. 619--622. 

Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquakes: Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

Smith, R.B., and Richins, W.D., 1984, Seismicity and earthquake 

Hazards of Utah and the Wasatch Front: Paradigm and paradox, 

in Hays, W.W., and Gori, P.L., eds., Proceedings of Conference 

XXVI, a workshop on Evaluation of regional and urban earthquake hazards and risk in Utah: U.S. Geological Survey Open-File 

Report 84-763, p. 73-112. 

Ziony, J.I., ed., 1985, Evaluating earthquake hazards in the Los 

Angeles region-An earth-science perspective: U.S. Geological 

Survey Professional Paper 1360, 505 p. 
Paleoseismology of the Wasatch Fault Zone: 

A Summary of Recent Investigations, 

Interpretations, and Conclusions 

By MICHAEL N. MACHETTE, STEPHEN F. PERSONIUS, and 

ALAN R. NELSON 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-A 

CONTENTS 

Abstract ......................................................................... . 

Introduction .................................................................... . 

Trenching Along the Wasatch Fault Zone ...................... . 

Age Determinations ................................................... . 

Acknowledgments ...................................................... . 

Regional Quaternary Geologic Framework ........................... . 

Segmentation of the Wasatch Fault Zone ............................. . 

Segments and Segment Boundaries .............................. . 

Types of Segment Boundaries ...................................... . 

Segmentation Models for the Wasatch Fault Zone ........... . 

Segment Lengths ...................................................... . 

Historical Earthquakes and Surface Ruptures ................ . 

Northern Wasatch Fault Zone .......................................... .. 

Segments Along the Malad Range ................................ . 

Collinston Segment .................................................... . 

Central Wasatch Fault Zone .............................................. . 

Brigham City Segment ............................................... . 

Weber Segment. ........................................................ . 

Salt Lake City Segment .............................................. . 

Provo Segment .......................................................... . 

American Fork Part ............................................. . 

Central Part ....................................................... . 

Spanish Fork Part ............................................... . 

Page 

Al 

1 

2 

6 

7 

7 

10 

10 

11 

11 

13 

14 

17 

17 

19 

20 

20 

23 

29 

34 

34 

39 

40 

Page 

Central Wasatch Fault Zone-Continued 

Nephi Segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A42 

Southern Wasatch Fault Zone............................................. 45 

Levan Segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

Fayette Segment........................................................ 47 

Summary of Segmentation of the Wasatch Fault Zone . . . . . . . . . . . . 48 

Chronology of Recent Movement on the Central Wasatch 

Fault Zone: A Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 

Recurrence of Large Earthquakes on the Wasatch Fault 

Zone......................................................................... 50 

Discussion of Slip-Rate Data for the Wasatch Fault Zone........ 51 

Possible Causal Relation Between Deep Cycles of Lake 

Bonneville and High Rates of Slip on the Wasatch 

Fault Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 

Conclusions...................................................................... 54 

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 

Dating Techniques and Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

Radiocarbon Calibration............................................... 63 

Apparent Mean Residence 14C Ages of Soils.................... 68 

Dating the Formation of an A Horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 

ILLUSTRATIONS 

Page 

FIGURE 1. Map showing location of sites along segments of the Wasatch fault zone.................................................................. A3 

2. Schematic diagram showing probable levels of Lake Bonneville during the past 150,000 years.................................... 9 

3. Diagrams showing four types of structural features at some suggested segment boundaries of the Wasatch 

fault zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

4, 5. Nomograms showing: 

4. Lengths of the surface traces of proposed segments on some major faults in the northern Basin and Range 

province . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

5. Proposed segment lengths for prehistoric faults less than 100 km long and more than 100 km long in the 

northeastern part of the Basin and Range province . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

6, 7. Maps showing: 

6. Northern part of the Wasatch fault zone between Malad City, Idaho, and Honeyville, Utah.......................... 18 

7. Brigham City segment of the Wasatch fault zone between Honeyville and North Ogden, Utah....................... 21 

8. Part of the log of trench BC-1 near Brigham City, Utah...................................................................................... 22 

9, 10. Maps showing: 

9. Weber segment of the Wasatch fault zone between North Ogden and Bountiful, Utah.................................. 24 

10. Vertical exposure across a fault scarp at the mouth of Garner Canyon near Ogden, Utah.............................. 25 

11, 12. Diagrams showing: 

11. Amounts of displacement and times of fault events recorded in five trenches at the East Ogden site............... 26 

12. Fault displacements and times of fault events derived from trenching and dating studies at three sites along 

the Weber segment of the Wasatch fault zone.................................................................................. 28 

III 
IV CONTENTS 

Page 

FIGURE 13. Map showing the traces of the Salt Lake City segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A30 

14. Aerial photograph showing the surficial geology at the South Fork of Dry Creek site, Salt Lake City segment of the 

Wasatch fault zone................................................................................................................................... 31 

15. Map showing the main trace of the Provo segment of the Wasatch fault zone between the Traverse Mountains and 

Dry Mountain.......................................................................................................................................... 35 

16. Part of the log of trench AF-1 at the American Fork Canyon site, Provo segment of the Wasatch fault zone................ 37 

17. Schematic diagram showing times of faulting deciphered from three trenches at the American Fork Canyon site, 

Wasatch fault zone................................................................................................................................... 38 

18, 19. Maps showing: 

18. Nephi segment of the Wasatch fault zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 

19. Levan and Fayette segments of the Wasatch fault zone........................................................................... 46 

20. Histogram showing estimated timing of major surface ruptures on segments of the Wasatch fault zone during the past 

6,000 years . ... ... . .. . .. . .. ... ... . .. . .. ... ... . .. . .. . . . . . . .. . ... ... . .. . . . . . . . . . . . . . . . .. . . . . .. ... .. . . . . ... . . . .. . . . . . . . . . .. . .. . . . . . . . .. . .. . ... .. . .. . ... ... . 49 

21. Schematic diagram showing some slip rates on the Wasatch fault zone for different time intervals as determined 

from deposits of Holocene to middle Quaternary age...................................................................................... 52 

TABLES 

Page 

TABLE 1. Trench and natural exposures along the Wasatch fault zone, Utah................................................................................ A4 

2. Lengths of segments and position of boundaries proposed for the Wasatch fault zone, Utah.............................................. 13 

3. Lengths of surface rupturing from recent M8 6.5 or larger earthquakes in the northern Basin and Range province............... 17 

4. Timing and number of surface-faulting earthquakes and recurrence intervals for Holocene movement on the Wasatch 

fault zone............................................................................................................................................... 33 

Al. Data for radiocarbon and thermoluminescence analyses cited in the text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE: 

A SUMMARY OF RECENT INVESTIGATIONS, 

INTERPRETATIONS, AND CONCLUSIONS 

By MICHAEL N. MACHETTE, STEPHEN F. PERSONIUS, and ALAN R. NELSON 

ABSTRACT 

The Wasatch fault zone forms the core of the structural transition 

zone between the Basin and Range province to the west and the 

Colorado Plateaus and Middle Rocky Mountains provinces to the east. 

The fault zone extends from Malad City, Idaho, on the north, to 

Fayette, Utah, on the south, a distance of about 343 km (383 km along 

its irregular trace). Previous studies of the fault zone suggested that it 

is composed of six discrete segments, each of which tends to rupture 

independently during major earthquakes (M>7). Since 1983, we have 

been mapping the surficial geology along the Wasatch Front at a 

publication scale of 1:50,000. The results of this mapping and synthesis 

of recent topical studies have led us to modify the positions of some of 

the previously defined boundaries between segments, suggest new 

boundaries, and subdivide several of the segments originally defined 

along the Wasatch fault zone. 

We now recognize 10 segments along the Wasatch fault zone. 

Repeated Holocene movement has been documented along five adjacent segments (the central two-thirds) of the fault zone, and a sixth 

segment moved once in the Holocene. The heavily urbanized part of the 

Wasatch Front-between Ogden and Provo, Utah-coincides with the 

part of the fault zone that shows the highest slip rates, the shortest 

recurrence intervals for faulting events, and the most recent fault 

activity. 

The 10 segments that we propose have lengths (along surface trace) 

that are as short as 11 and 17 km for the northernmost and southernmost segments, respectively, to as long as 51 and 70 km for the 

centrally located Weber and Provo segments, respectively; the average 

trace length of all the segments is about 37 km. The segments 

exhibiting Holocene movement, which comprise the central part of the 

fault zone, average 48 km in length. In comparison, the proposed 

segment lengths of late Quaternary normal faults in the northeastern 

part of the Basin and Range province typically average 20 to 30 km. 

There have been three large historical earthquakes in nearby parts of 

the northeastern Basin and Range province and the Intermountain 

Seismic Belt. These earthquakes had magnitudes (M8 ) of6.6 to 7.5 and 

produced surface ruptures 11.5 to 36 km in length. Although the 

correlation between magnitude and the surface rupture lengths of 

earthquakes on historical normal faults is clearly nonlinear, we infer 

Manuscript approved for publication November 20, 1990. 

that major surface-rupturing earthquakes that broke entire segments 

of the Wasatch fault zone would be similar in magnitude to the major 

historical earthquakes in the northern Basin and Range province. One 

implication of our new segmentation scheme is that the expected length 

of surface rupturing during a major earthquake on the Wasatch fault 

zone could be as much as 60 to 70 km. Because length of surface 

rupturing is generally dependent on earthquake magnitude, the largest 

earthquake on a single segment could be M 7.5 or greater. 

Recent trenching of fault scarps and detailed mapping of Quaternary 

geology along the Wasatch Front show that the Wasatch fault zone has 

been active repeatedly through the Holocene (past 10,000 years). In 

addition, the recognition of 10 discrete segments of the Wasatch fault 

zone, 6 of which have demonstrable Holocene movement, increases the 

possible number of separate localities where earthquakes and their 

associated surface ruptures may occur along the Wasatch Front. 

Although recurrence intervals for individual segments are still being 

analyzed, we now think that major earthquakes have struck the 

central, densely urbanized part of the Wasatch fault zone on an average 

of once every 395 years (this value is termed the composite recurrence 

interval of faulting). 

This estimate is similar to the preferred composite recurrence 

interval of 444 years calculated by Schwartz and Coppersmith (1984). 

Dating of paleoseismic events along the Wasatch fault zone indicates 

that large earthquakes (those that cause significant surface faulting and 

major ground-shaking damage) have occurred more frequently in the 

late Holocene than the average value would suggest; thus, we consider 

that some process of temporal clustering of earthquakes may be active. 

Recent surface faulting involved at least six faulting events between 

about 400 and 1,500 yr B.P., or about one event every 220 years. This 

average is about twice as frequent as that calculated from the paleoseismic record of the past 6,000 years. A lack of movement on the 

Brigham City segment during the past 3,600 years suggests that it may 

be the next segment to undergo movement on the Wasatch fault zone. 

INTRODUCTION 

In Utah, the Wasatch fault zone is the most obvious 

and continuous structural element of a prominent transition zone between the Basin and Range province and 

Al 
A2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

the Colorado Plateaus and Middle Rocky Mountains 

provinces. The late Quaternary trace of the Wasatch 

fault zone extends from Malad City in southern Idaho to 

Fayette in central Utah, a straight-line distance of about 

343 km (fig. 1). The Wasatch fault zone is nearly 383 km 

long along its irregular surface trace and, as such, is the 

longest continuous and active normal fault zone in the 

Western United States. Other fault zones, such as the 

one associated with the Central Nevada Seismic Belt, 

approach this length but have discrete gaps in surface 

rupturing or seismicity. 

Although the Wasatch fault zone has been the subject 

of much scientific interest since G.K. Gilbert's pioneering 

studies (Gilbert, 1890, 1928), the first comprehensive 

study of the Wasatch fault zone was not completed until 

the 1970's. Cluff and others (1970, 1973, 1974) used 

low-sun-angle photographs to map ruptures along the 

entire length of the Wasatch fault zone from Malad City 

to Gunnison, which is about 15 km south of Fayette, 

Utah. Although their 1:24,000-scale maps show the most 

detailed trace of the Wasatch fault zone, they sometimes 

confused other faultlike features (such as landslide scars 

and shorelines) with bona fide fault scarps. As a logical 

extension of this reconnaissance work, other geologists 

at Woodward-Clyde Consultants made detailed investigations of the surficial geology at four trench sites along 

the Wasatch fault zone during .the period 1978 to 1982. 

This work resulted in many important preliminary 

reports and culminated in two major reports that focused 

on paleoseismological aspects of the Wasatch fault zone. 

Recent U.S. Geological Survey (USGS) mapping along 

the Wasatch Front in central and northern Utah has 

focused on differentiating middle to upper Quaternary 

deposits (those younger than 750,000 years) that record 

long- to short-term slip rates on the Wasatch fault zone. 

Studies of historical earthquakes on major normal faults 

in the Basin and Range province and studies of paleoseismicity on prehistoric faults in the Western United 

States have shown that surface rupturing tends to occur 

on discrete sections of long faults or fault zones. These 

discrete elements, called fault segments, are generally 

defined as portions of the fault that tend to move 

independently of one another during large earthquakes. 

Our scenario of segmentation of the Wasatch fault zone is 

based on apparent recency of fault movement as determined from geomorphic, stratigraphic, and dating evidence, on amounts of displacement of faulted deposits, on 

geometric patterns of surface ruptures and their relations to bedrock geology of the fault zone, and on the 

overall history of movement of the Wasatch fault zone 

during the Holocene and latest Pleistocene. 

Our mapping along the Wasatch Front has allowed us 

to further divide the six segments of the Wasatch fault 

zone (fig. 1) proposed by Swan and others (1980) and 

Schwartz and Coppersmith (1984). In addition to our 

detailed mapping studies, the USGS and the Utah Geological and Mineral Survey (UGMS) are engaged in a 

cooperative study of specific sites along the Wasatch 

fault zone, where trenching will reveal the time of the 

most recent ruptures and permit further characterization of major faulting events, such as determinations of 

recurrence intervals and displacements per event. 

Although this trenching work continues, summaries of 

published reports and preliminary results from our 

trench studies are discussed in this report. Readers 

should be aware that many of the values reported here 

for recurrence intervals, timing of fault movement, and 

slip rates are provisional and may change as further 

results become available from the cooperative program 

of trenching and isotopic dating. In addition, recent 

reports on some of the new major trenching sites (for 

example, Forman and others, 1991; Personius, 199la, b) 

along the Wasatch fault zone include more detailed 

discussions of age determinations, amounts of fault offset, slip rates, and interpretations of faulting history. 

Much of the work reported here has been summarized in 

a recent paper by Machette and others (1991). 

TRENCHING ALONG THE WASATCH FAULT ZONE 

The Wasatch fault zone has been the focus of extensive 

trenching efforts since the initial investigations of Cluff 

and others (1970, 1973, 1974). The results of early 

(1978-83) trenching have been summarized by Swan and 

others (1980) and Schwartz and Coppersmith (1984). As 

of 1987, 47 trenches and 4 natural exposures have been 

logged and described from 20 sites on 6 segments of the 

Wasatch fault zone and the associated West Valley fault 

zone (table 1). Most of the trenches have provided some 

control on the time of most recent faulting, but only a few 

have set limits on recurrence intervals and slip rates for 

faulting events. Our new dating control helps to refine 

recurrence intervals and times of most recent faulting at 

the trench sites as well as to highlight changes in slip 

rates through time. 

As an extension of our geologic mapping, the USGS 

and the UGMS undertook a study of several trench sites 

along previously unstudied parts of the Wasatch fault 

zone (fig. 1). In September 1986, 13 trenches were 

excavated at 3 sites on the Wasatch fault zone near 

Brigham City, East Ogden, and American Fork Canyon 

(sites 1, 4, and 9, table 1). The history of faulting that 

is being developed from these and additional trenches 

has helped to test our earlier suggestion (Machette and 

others, 1986) that the Wasatch fault zone has at least 

10 to as many as 12 discrete segments, each of which 

may act independently during major earthquakes (see 

"Fault Segmentation"). From 1985 to 1987, various 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE 

11 0"00' 

U ~Malad City 1 

__ l~AHO _ _ __ _ _ __ _ _ ~Clarkston - -i 

UTAH Mountain 1 

~~ I 

Collinston : Collinston I 

Brigham City 

'---- WYQ~ING ---- _ 

41"00' 

Provo 

40"00' 

Nepni 

Levan 

0 

I 

0 

Weber 

Salt Lake City 

American Fork 

section -

Utah La Eft ~~~ section Provo 

11~ 

12 Spanis~ Fork --:t :hi section 

...... ~ 

~ Levan 

18 .. 

.... ~ Fayette 

~ 

I 

20 

20 

I 

I 

40 

I 

40 MILES 

I 

60 KILOMETERS 

UTAH 

FIGURE 1.-Location of sites along segments of the Wasatch 

fault zone. Segments proposed by Schwartz and Coppersmith 

(1984) are shown to the left (west) of the fault zone, and those 

proposed in this study are to the right (east) of the fault zone. 

The main trace of the Wasatch fault zone is shown by a 

heavy line having a ball and bar on the downthrown side. 

Solid arrows indicate segment boundaries. Major towns are 

shown by hachures. Study sites, shown by numbered triangles, are listed in table 1. 

A3 
TABLE 1.-Trench and natural exposures along the Wasatch fault zone, Utah > 

[Trench sites and natural exposures are listed by fault segment from north to south on the Wasatch fault zone and along the West Valley fault zone to the west] ~ 

Number of datable samples Parameters determined2 

Site Trench/natural Most Recurrence 

Principal Dates of exposures Age of Organic Charcoal Thermolumi- recent interval Slip rate3 

No. Name investigator1 investigation (number logged) faulted deposits carbon sample nescence event of faulting (interval, in 1,000 years) 

Brigham City segment 

1 . . . Brigham City S.F. Personius September 1986 5/0 (1) Early to middle Holocene 5 0 2 Yes Yes Yes (3.6-4.7, 0-14) 

(USGS, Denver). ~ 

t_:l:j 

2 . . . Pole Patch S. F. Personius September 1985 1/0 (1) Latest Pleistocene 2 0 0 Yes AV Yes (0-15) Q 

(USGS, Denver). ~ 

0 

z 

Weber segment > 

~ 

t_:l:j 

3 . . . Garner Canyon A.R. Nelson July 1985 011 (1) Middle to late Holocene 3 0 0 Yes Yes No > 

(USGS, Denver). ~ 

1-3 

4 . . . East Ogden A.R. Nelson September 1986 5/0 (5) Middle to late Holocene 16 3 11 Yes Yes Yes (0.5-4.2, 0-15) .0 = 

(USGS, Denver). c:: 

5 . . . Kaysville Woodward-Clyde Summer 1978 7/0 (7) Latest Pleistocene 0 1 0 MN AV Yes (0-8) > 

Associates. ~ 

= Salt Lake City segment > N 

Yes (0-15) > 6 . . . Dresden Place John Garr (Delta November 1986 2/0 (2) Latest Pleistocene 0 0 6 No No ~ 

Engineering). t::l 

00 

7 . . . Little Cottonwood Woodward-Clyde Summer 1979 3/0 (3) Latest Pleistocene 0 3 0 No AV Yes (0-19) > 

Canyon. Associates. z 

t::l 

8 ... Dry Creek D.P. Schwartz Summer 1985 5/0 (5) Middle to late Holocene 5 0 0 Yes Yes Yes (0-5) ~ 

(USGS, Menlo Park), ~ 

00 

W.R. Lund (UGMS). ~ 

> 

Provo segment ~ 

0 

z 

9 . . . American Fork M. N. Machette September 1986 3/0 (3) Middle to late Holocene 3 3 10 Yes Yes Yes (0.5--5.3, 0-15) Q 

Canyon. (USGS, Denver). 1-3 

= 10 ... Rock Creek M. N. Machette July 1985 0/1 (1) Late Holocene 2 0 0 Yes No No t_:l:j 

(USGS, Denver). 

W.E. Lund (UGMS), May 1988 1/2 (3) Late Holocene 7 3 3 Yes, DP No No 00 

~ D.P. Schwartz > 

(USGS, Menlo Park). 1-3 

0 

11 . . . Hobble Creek Woodward-Clyde Summer 1978 3/0 (3) Latest Pleistocene, middle 0 0 0 MN AV Yes (0-14, 0-17) = 

(Deadmans Hollow) Associates. to late Holocene. l'%j 

~ 

12 . . . Mapleton W.E. Lund (UGMS) Summer 1978 5/0 (3) Latest Pleistocene to late 8 1 1 Yes Yes Yes (0-3) 0 

z Holocene. 1-3 

13 . . . Water Canyon D.A. Ostenaa Spring 1987 3/0 (3) Holocene (plus snails and 7 18 5 Yes AV Yes (0-8) 

(USBR, Denver). artifacts). 

14 ... Woodland Hills M. N. Machette November 1986 2/0 (1) Late Pleistocene to 1 0 6 MN AV Yes (0-150) 

(USGS, Denver). Holocene(?). 
Number of datable samples Parameters determined2 

Site Trench/natural Most Recurrence 

Principal Dates of exposures Age of Organic Charcoal Thermolumi- recent interval Slip rate3 

No. Name investigator1 investigation (number logged) faulted deposits carbon sample nescence event of faulting (interval, in 1,000 years) 

Nephi segment 

15 ... North Creek Woodvvard-Clyde September 1980 3/0 (2) Middle to late Holocene 2 2 0 MN Yes Yes (0-5) 

Associates. 

16 ... Red Canyon M.E. Jackson June 1987 1/0 (1) Latest(?) Pleistocene 4 0 12 MX Yes Yes (0-5) 

(University of and Holocene. 

Colorado). 

Levan segment 

17 ... Deep and Pigeon Woodvvard-Clyde Summer 1980 0/2 (0) Holocene 0 1 0 MX MN No 

Creeks. Associates. 

Deep Creek M.E. Jackson June 1987 0/1 (1) Holocene 0 0 4 Yes MN No ~ (University of t-t 

t:z:j 

Colorado). 0 

18 ... Skinner Peaks M.E. Jackson June 1987 110 (1) Holocene 5 7 10 MX MN Yes 00 

t:z:j 

(University of -

00 

Colorado). ts: 

0 

t-t 

West Valley fault zone 0 

~ 19 ... Granger fault J.R. Keaton 1985-86, 1987-88 2/0 (2) Late Pleistocene to 0 0 4 MN AV Yes (0-12, 0 

(Dames and Moore, Holocene. 0-58, 0-140). 1-:!:j 

Salt Lake City). ~ 

~ 

20 ... Taylorsville fault J.R. Keaton 1985-86, 1987-88 2/0 (2) Latest Pleistocene and 0 0 0 MN AV Yes (0-12) t:z:j 

(Dames and Moore, Holocene. 

Salt Lake City). 00 

~ 1

USGS, U.S. Geological Survey; UGMS, Utah Geological and Mineral Survey; USBR, U.S. Bureau of Reclamation. > 

~ 

2AV, average value; MN, minimum value; MX, maximum value; DP, data pending. 0 

~ 3

Calculated over intervals before present. 1-:!:j 

> 

c::::! 

t-t 

~ 

N 

0 

z 

t:z:j 

~ 
A6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

governmental agencies and private contractors excavated several additional trenches (Machette, 1987). 

Three sites were excavated along the Salt Lake City 

segment-at Dresden Place (site 6, table 1) along the 

East Bench fault by Delta Engineering; at the South 

Fork of Dry Creek (site 8, table 1) along the main 

Wasatch fault by the USGS and UGMS; and at the 

Forestdale Golf Course site (not logged) along the East 

Bench fault by the UGMS. To the south on the Provo 

segment, a trench site on the Woodland Hills fault (site 

14, table 1) was excavated by the U.S. Bureau of 

Reclamation (USBR). In addition, two sites off the 

Wasatch fault zone that supplied information pertinent to 

this report have been excavated-the West Valley fault 

zone (sites 19 and 20, table 1) studies by Keaton and 

others (1987) and Keaton and Currey (1989) and the East 

Cache fault zone (no fault number) study by McCalpin 

(1987) and McCalpin and Forman (1991). 

Starting in the spring of 1987, new trenches along the 

Wasatch fault zone were opened by the UGMS and 

USGS at Rock Creek, east of Provo, and at a site 

southeast of Mapleton on the Provo segment (sites 10 and 

12, table 1); by the USBR at Water Canyon near the 

southern end of the Provo segment (site 13, table 1); and 

by Michael Jackson of the University of Colorado at sites 

west of Red Canyon on the Nephi segment (site 16, table 

1) and north of Skinner Peaks on the Levan segment (site 

18, table 1). As a result of the recent flurry of trenching 

operations (36 trenches or new exposures in 2 years), we 

now have data from at least one major trenching site on 

each of the heavily populated segments and from as many 

as three major sites on the Provo and Weber segments. 

Because the areas of the northernmost and southernmost 

fault segments are sparsely populated and show little or 

no evidence of Holocene movement, no trenching operations have been conducted there. 

Displacement values and slip rates cited throughout 

this report refer only to the vertical component of 

tectonic displacement or slip (NVTD of Schwartz and 

Coppersmith, 1984), which includes compensation for 

backtilting, antithetic faults, and warping. However, in 

many cases, a lack of good exposures or poor preservation of the faulted surfaces allowed us to determine 

displacement only across a narrow zone or exposure 

rather than across a wider zone that may reflect the 

far-field effects of tectonism. In such cases, cited values 

of displacement are crude approximations of the common 

maximum vertical displacement. 

It is our practice to use metric units, both in the field 

and in this report, for measurements of length and 

height. However, because most of the geologic and 

geomorphic features decribed herein have been mapped 

on USGS 1:24,000-scale topographic-quadrangle maps, 

we report the altitudes of important map-oriented features, such as shorelines, in both metric (meters) and 

English (feet) units. 

AGE DETERMINATIONS 

We cite 36 new 14C ages that we have obtained for this 

study. The procedures that we have developed for using 

these age determinations to estimate the ages of faulted 

soil materials and the times of faulting are discussed in 

detail in the appendix, along with the limitations 

involved in their interpretations. However, we use a 

number of terms (in acronym form) in the text to 

describe different procedures or elements of dating techniques. The following list describes these acronyms. 

AMRT 

AMS 

cal yr B.P. 

ka 

Ma 

LHC 

MRC 

CAS 

UHC 

yrB.P. 

Apparent mean residence time (age) 

of soil organic carbon 

Accelerator mass spectrometry (a 

measurement technique) 

Calendar years before present (1950 

A.D.) 

Carbon 14 or, more commonly, radiocarbon 

Thousands of years (ago) 

Millions of years (ago) 

Lower horizon contact (usually the 

base of a deposit, such as faultscarp colluvium) 

Mean residence correction (AMRT at 

time of burial of sampled unit); 

UMRC refers to upper contact, 

LMRC refers to lower contact 

Carbon age span (the total time interval represented by different ages of 

carbon in the soil sample) 

Upper horizon contact (usually the 

surface of a buried soil and its associated deposit) 

Years before present (1950 A.D.) 

The 14C ages are of two main types: conventional 

gas-proportional and AMS ages on charcoal samples and 

AMRT ages on organic concentrates from the A horizons 

of surface and buried soils. Although both types of ages 

are measured in 14C years e4

C yr B.P.) and converted to 

calendric dates (cal yr B.P.), the geologic interpretation 

of AMRT ages is complex, and the associated errors 

(both analytical and geologic) commonly are much larger 

than those for charcoal samples (see discussion in appendix). 

In addition to the 14C ages, we obtained 18 thermoluminescence (TL) age estimates for materials associated 

with faulting (see appendix). The TL technique directly 

dates the last exposure of a mineral to sunlight and, as 

such, reflects the time of its burial. 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A7 

ACKNOWLEDGMENTS 

This study could not have been completed without the 

able assistance of many others. We thank the staff of the 

UGMS for funding the trenching program and some of 

the 14C ages; we especially acknowledge the assistance of 

William Lund, who coordinated much of the trenching 

operations. Genevieve Atwood (former Director of the 

UGMS) authorized the excavation of 13 trenches and 

allowed us to employ her technical staff during exploration of the trenches. Genevieve was an ardent supporter 

throughout the course of this study and is particulary 

memorable for her "Science is Fun" admonition. Many 

thanks are extended to UGMS staff members William 

Mulvey, Suzanne Hecker, William Case, Bob Klauk, and 

Hal Gill, each of whom spent considerable time in the 

trenches. 

As part of the National Earthquake Hazards Reduction Program, the State of Utah sponsors a County 

Geologist Program. In this capacity, Robert Robison 

(Utah County), Craig Nelson (Salt Lake County), and 

Michael Lowe (Weber and Davis Counties) each contributed to the investigation of trenches and provided information about land ownership at potential trenching sites. 

The U.S. Bureau of Reclamation (Provo office) provided 

hydraulic shoring and the manpower to install it. John 

Garr (Delta Engineering, Salt Lake City, Utah) helped 

with trench logging at East Ogden, and Donald Currey 

(University of Utah) and John Garr provided unpublished information for the Dresden Place site. 

We appreciate the cooperation of the following landowners who permitted us to excavate trenches during 

the fall of 1986: at Brigham City, Elbert Beecher and 

George Anderson; at East Odgen, the Fife Equipment 

and Investment Company of Brigham City; and, at 

American Fork Canyon, the Hayes brothers (John K., 

Robert D., and David R.) of Provo, Utah. Pat Cashman 

(then at Weber State College) provided storage facilities 

for research equipment during our excursions to Utah. 

We appreciate the forthright and stimulating discussions of Basin and Range faulting with our colleague 

David Schwartz (formerly of Woodward-Clyde Associates) and are particularly grateful to him for sharing his 

knowledge of paleoseismology. David continues to be 

involved with all aspects of research along the fault zone. 

Accurate chronologie dating has been the crux of many 

former studies and a critical element of this study. We 

are indebted to the following persons and laboratories for 

14C analyses: Stephen Robinson and Deborah Trimble of 

the USGS at Menlo Park, Calif.; Robert Stuckenrath of 

the University of Pittsburgh; Irene Stehle of Dicarb 

Radioisotope Co., Norman, Okla.; and A.J.T. Jull and 

colleagues at the University of Arizona-National Science 

Foundation Accelerator Facility for Radioisotope Analysis, Tuscon. Thermoluminescence analyses were provided under a USGS-sponsored research grant to Steve 

Forman, Michael Jackson, and Paula Mott of the Institute for Arctic and Alpine Research (INSTAAR) of the 

University of Colorado and James McCalpin of Utah 

State University. Rolf Kihl, also of INSTAAR, and 

Kathleen Haller of the USGS prepared most of the 

samples of soils and organic-rich sediment for 14C analysis. 

We appeciate the helpful comments of participants on 

Wasatch fault zone field trips and at earthquake hazard 

conferences that we have attended. This manuscript was 

improved by the critical reviews and comments of Russell Wheeler, Donald R. Nichols, and R. Ernest Anderson, all of the USGS. 

REGIONAL QUATERNARY GEOLOGIC 

FRAMEWORK 

Much of our mapping along the Wasatch Front utilizes 

stratigraphy based on relatively new interpretations of 

the late Quaternary (past 130,000 years) history of Lake 

Bonneville. The results of research from the 1970's and 

the 1980's have yielded a simplified stratigraphic framework (see Scott and others, 1983; Currey and Oviatt, 

1985; Currey and Burr, 1988) compared with that developed in the 1950's and 1960's (see Morrison's (1965) 

discussion of pre-1970 stratigraphic concepts). The new 

stratigraphic interpretations of the lake's history hinge 

on strategic ages coupled with careful discrimination of 

transgressive and regressive lacustrine sediments 

deposited before, during, and after isostatic adjustments 

in the lake basin. '· 

Deposition of Quaternary deposits along the Wasatch 

Front appears to be closely associated with climatic 

cycles-cool periods of greater effective moisture (glacial 

or pluvial intervals) or warm periods of restricted moisture (interglacial or interpluvial intervals). We suspect 

that depositional or erosional cycles along the Wasatch 

Front are stimulated by rapid changes in climate and 

vegetation; that is, these changes may cause dynamic 

thresholds in surface processes to be exceeded and 

thereby trigger periods of erosion or deposition. 

Stratigraphic and paleontologic evidence suggests that 

the climate in the eastern Basin and Range province was 

conducive to forming large and relatively deep lakes 

(Lake Bonneville, for example) at least three times since 

200 ka (200,000 years ago) (Scott and others, 1983). 

In addition, scant evidence has been found for middle 

(130-750 ka) and early (750 ka-2.0 Ma) Pleistocene 

predecessors of Lake Bonneville. Deposits of well-dated 

airfall volcanic ash interbedded with lacustrine sediments in the Bonneville Basin show that several 
AS REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

ancestral Lake Bonnevilles existed intermittently in the 

past 2 Ma. Near Beaver, Utah (about 280 km south of 

Salt Lake City), basin-fill sediments record a major 

freshwater lake (Lake Beaver) during late Pliocene and 

early Pleistocene time (Machette, 1985). Lake Bonneville 

and Lake Beaver are but two examples of the many lakes 

that must have been present in the eastern Basin and 

Range province during Quaternary time (past 2 Ma). 

Unfortunately, lake deposits older than late Pleistocene are only sparsely exposed in the Basin and Range 

province because much of the area, including the Bonneville drainage basin, is still relatively undissected. 

Young lacustrine and alluvial sediments related to Lake 

Bonneville largely obscure the record of all but the most 

recent lacustrine and fluvial cycles. Nevertheless, there 

are some exposures of fluvial and lacustrine deposits 

along the Wasatch Front that may be middle Pleistocene 

(750-130 ka) in age. For example, extensive alluvial-fan 

deposits in the Loafer Mountain area of the southeastern 

part of Utah Valley have thick, red argillic B (clay-rich) 

horizons and thick Stage III to IV calcic (calciumcarbonate-rich) horizons that suggest ages of at least 

several100 ka (see Birkeland's (1984) review of soils and 

Quaternary geology). The highest and oldest of these 

fans contain two volcanic ash beds, which have been 

identified by Izett and others (1970) as the Lava Creek B 

(0.62 Ma) (lzett, 1981) and the Bishop (0. 74 Ma) (lzett, 

1982). Also, in the Salt Lake Valley, there is a small 

remnant of fan alluvium or glacial outwash of middle 

Pleistocene age exposed south of Bells Canyon (Scott and 

Shroba, 1985; Personius and Scott, 1990, in press) that 

has a uranium trend age of 250±90 ka (J.N. Rosholt, 

written commun. to W.E. Scott, 1984). 

The sparsely preserved surficial deposits that predate 

the last deep cycle of Lake Bonneville (Bonneville lake 

cycle, 32-10 ka) (see fig. 2) include lacustrine deposits 

associated with the Little Valley lake cycle (latest middle 

Pleistocene) (Scott and others, 1983) and till of the 

largely contemporaneous Dry Creek glacial advance. 

Deposits of the intervening Cutler Dam lake cycle (middle late Pleistocene) (Oviatt and others, 1987), also 

known as the Hansel Valley lake cycle of McCalpin and 

others (this volume), are also present along the Wasatch 

Front. These two pre-Bonneville glacial-pluvial cycles 

most likely correlate with marine isotope stages 6 and 4, 

respectively (Martinson and others, 1987, fig. 18; Richmond and Fullerton, 1987, fig. 2). Till of Madsen and 

Currey's (1979) penultimate Dry Creek advance is 

exposed locally at the mouths of large canyons that cut 

deeply into the Wasatch Range. Scott and Shroba (1985) 

suggested that this till may be about 150 ka on the basis 

of its weathering characteristics. The outwash and 

alluvial-fan deposits of this same general age, which are 

more widespread than the till, are preserved as remnants of formerly widespread geomorphic surfaces at 

many places along the Wasatch Front. Although Scott 

and others (1983) considered the Little Valley lake cycle 

to have culminated about 150 ka, the lake cycle (and 

associated glaciation) probably persisted for several tens 

of thousands of years and ended about 130 ka (McCoy, 

1987), as determined from stratigraphic, soil morphologic, and amino-acid-ratio evidence. 

As previously mentioned, Oviatt and others (1985), 

McCalpin (1986), and Keaton and others (1987) found 

evidence of a major intermediate-level lake cycle that 

predates the Bonneville lake cycle and postdates the 

Little Valley lake cycle. Oviatt and others (1987) 

reported 14C dates and amino-acid analyses from sediments exposed along the Bear River north of Brigham 

City that suggest an age of more than 40 ka for deposits 

of the Cutler Dam lake cycle; however, these lake 

deposits must be significantly younger than those of the 

Little Valley lake cycle, as the development of soils on 

relict surfaces of the deposits demonstrates. McCalpin 

(1986) reported TL age estimates of 76 and 82 ka for 

deposits of his Hansel Valley lake cycle in the northwestern part of the Bonneville Basin. In addition, Keaton and 

others (1987, table 2) reported amino-acid-racemization 

age estimates of 45 to 80 ka for sediments of an intermediate lake cycle (that is, the Cutler Dam lake cycle) in the 

Salt Lake Valley and preferred an age of 58 ka for the 

end of the cycle. If we assume that the intermediate-age 

lake sediments from these three distant areas of the 

Bonneville Basin all were deposited during a pluvial cycle 

associated with marine isotope stage 4, then 60 to 75 ka 

seems to be a reasonable time for most sediment of the 

Cutler Dam lake cycle of Lake Bonneville. 

Most of the units that we have mapped along the 

Wasatch Front were deposited during the transgression 

(rise) and regression (fall) of the latest cycle of Lake 

Bonneville (the Bonneville lake cycle) and the largely 

contemporaneous glaciation in the Wasatch Range 

between 32 and 10 ka and as alluvial fans during the 

Holocene. The following summary of the geologic history 

during the latest Pleistocene and Holocene is abstracted 

largely from reports by Madsen and Currey (1979), 

Currey and others (1983), Scott and others (1983), and 

Currey and Oviatt (1985) and from evidence uncovered 

during our geologic mapping and dating investigations. 

In the latest Pleistocene (35-10 ka), valley glaciers 

were extensive in the Wasatch Range and high-elevation 

mountains of northeastern and central Utah. Along the 

Wasatch Front, valley glaciers advanced beyond the 

precipitous front of the Wasatch Range and into the 

eastern parts of the Salt Lake Valley and Utah Valley 

before 22 ka, while Lake Bonneville stood at a low to 

intermediate level (fig. 2). Till from these glaciers was 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A9 

~Q) 

=u >> Q) C.) 

c Q) 

...J c~ 

w Oas 

> Illw 

...J 

~Q) <( 

w =u CO> en 5000 >C.J w Q)Q) 

-~ > =as 0 :_:jIll 

<( 

~ 

w 

w 

LL 

~ EQ) _j 

w ~g. > .... C.) 

w 

...J ~~ w 4500 ::I as 

~ u-

<( 

...J 

75 60 30 0 

TIME BEFORE PRESENT, IN THOUSANDS OF YEARS 

FIGURE 2.-Probable levels of Lake Bonneville during the past 150,000 years. Modified from Currey and Oviatt (1985); extended past 30,000 years 

on the basis of recent stratigraphic studies of pre-Bonneville lake cycle deposits by Scott and others (1983), McCoy (1987), Oviatt and others 

(1987), and McCalpin and others (this volume). 

deposited as large end moraines that extended as much 

as 1 km beyond the mountain front at Little Cottonwood 

Canyon and Bells Canyon southeast of Salt Lake City 

and at Dry Creek east of Alpine in the northern part of 

Utah Valley. Meltwater streams from these glaciers and 

others higher in the Wasatch Range deposited gravelly 

outwash in large alluvial fans along the Wasatch Front 

and, farther basinward, as fan-delta complexes that were 

graded to a transgressing Lake Bonneville. The lake's 

expansion, which started about 32 ka, was interrupted 

by several pauses and minor regressions (such as the one 

that formed the Stansbury shoreline) (fig. 2) and culminated about 15 ka at the highest of the Bonneville 

shorelines (post-rebound altitudes of 1,573-1,584 m or 

5,160-5,200 ft above sea level along most of the Wasatch 

Front). The outwash and alluvial fans along the mountain 

front were inundated by the lake and, except for small 

areas near the canyon mouths that stood above the level 

of the lake, were covered by a veneer of relatively fine 

grained lake sediment. 

Although most workers agree that the Bonneville lake 

cycle culminated in a major high stand that eventually 

overtopped its natural sill at Red Rock Pass in southern 

Idaho, Currey and Oviatt (1985) found evidence of a drop 

in lake level and a readvance to the overflow level (the 

Keg Mountain oscillation) at about 16 to 15 ka. Some 

workers remain skeptical about this oscillation, but 

Machette's (1988, 1989) mapping near the American 

Fork Canyon and Scott's reconnaissance of the northern 

Oquirrh Mountains west of Salt Lake City (W.E. Scott, 

oral commun., 1987) tend to support an oscillation near 

the end of the final transgressive phase of the Bonneville 

lake cycle. Gravel pits in the fan-delta complex at the 

mouth of the American Fork Canyon record two closely 

timed series of transgression and regression at the 

Bonneville level (Machette, 1988), which Machette 

inferred to be the physical manifestation of the Keg 

Mountain oscillation. 

After Lake Bonneville attained an altitude of about 

1,552 m (5,092 ft), it overtopped its natural sill near Red 

Rock Pass and rapidly downcut about 108m (355ft) to 

an altitude of about 1,444 m (4,737 ft), where the outlet 

was stabilized and the lake formed the highest of the 

Provo shorelines. This fall, which occurred about 14.5 ka 
A10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

(Scott and others, 1983; Currey and Burr, 1988) marked 

the end of the transgressive phase of the Bonneville lake 

cycle and initiated rapid stream erosion of unconsolidated 

lake sediment along former shoreline areas and in mountain canyons. Much of the eroded sediment was deposited 

in deltas, bars, and spits graded to the Provo shoreline of 

the Bonneville lake cycle. Modeling of isostatic rebound 

by Currey and Burr (1988) suggests that Lake Bonneville may have occupied the main Provo level for only 

about 300 years (14.5-14.2 ka). As the lake began to 

retreat below the Provo shoreline (about 14 ka), streams 

cut into the higher delta and shore complexes to form 

new terraces and deltas graded to successively lower 

shorelines of the regressing Lake Bonneville. The lake's 

retreat from the Provo shoreline level was caused by 

additional (but minor) downcutting of the outlet at Red 

Rock Pass, by continued rapid isostatic uplift of the 

deeper parts of the Lake Bonneville basin, and by lake 

evaporation caused by a progressively warmer climate 

that marked the end of the Pleistocene Epoch. 

By 11 ka, the lake had fallen to a level at or below that 

of the present Great Salt Lake (altitude 1,283 m or about 

4,210 ft in 1987). After reaching a low, extremely dessicated level, the lake rose briefly at 10.3 to 10.5 ka 

(Currey and others, 1983, fig. 13) to form the Gilbert 

shoreline at about 1,294 m altitude (4,246 ft); we consider 

this final rise to be the culmination of the Bonneville lake 

cycle. Since the beginning of the Holocene (10 ka), Great 

Salt Lake (a saline remnant of Lake Bonneville) has 

remained within about 10 m of its present level (Scott 

and others, 1983; Currey and Oviatt, 1985). 

In contrast to the late Pleistocene, the Holocene is 

marked by climatic conditions similar to those of the 

present and by deposition of local alluvial fans close to 

the mountain front. Most of the deposition occurs near 

the mouths of steep canyons, which are sites of intermittent debris flows and floods. Away from the mountain 

front, reduced flow in streams prevents transport of 

gravels more than a few kilometers, whereas, farther 

downstream, the flood plains are depositional centers for 

sandy to silty alluvium. 

One of the significant stratigraphic results from our 

14C dating and TL analyses is recognition of a major 

episode of debris-flow deposition and fan formation that 

began after about 8 ka and ended about 5 ka (details of 

dating are discussed later in this report). Because similar 

sequences of landscape destabilization and stripping of 

upland hillslopes during the early Holocene have been 

recognized over wide regions of the Basin and Range 

province (Dohrenwend, 1987, p. 307), there is probably a 

regional, perhaps climatic cause for the activity. This 

time interval generally correlates with the Altithermal 

(warm) climatic period of Antevs (1948, 1955), which 

originally was defined from the pollen record in Europe. 

SEGMENTATION OF THE WASATCH FAULT 

ZONE 

Swan and others (1980) were the first investigators to 

suggest that movement on the Wasatch fault zone was 

naturally divided among segments-portions of the fault 

that tend to move independently of one another during 

large-magnitude earthquakes. In their report, they speculated on the number of possible segments for the 

Wasatch fault zone. They suggested at least 6 segments 

on the basis of historical microseismicity on the Wasatch 

fault zone to as many as 10 segments on the basis of 

geometric variations along the fault zone and a 30- to 

40-km rupture length associated with large-magnitude 

earthquakes on normal faults worldwide. In the second 

report, Schwartz and Coppersmith (1984) used geomorphic, geophysical, paleoseismic, and geodetic data to 

propose that the Wasatch fault zone is composed of six 

major segments (fig. 1). 

These two papers introduced the concept that individual slip events from large-magnitude earthquakes associaed with surface ruptures on major normal-slip fault 

zone, such as the Wasatch, are largely confined to 

discrete parts. These parts-or segments, as they were 

called-represent only a fraction of the fault's total 

length; the location of individual segments may be controlled by local geometric and structural properties of the 

fault zone. However, the term "segment" was not 

defined explicitly and has since been used in contexts 

that range from "a portion" (that is, a geometric part) to 

"a structural entity" (that is, a structural segment). 

dePolo and others (1989, 1991) have discussed this problem of nomenclature and suggested that the term "earthquake segment" be used for those parts of a fault or 

faults that "rupture as a unit during an earthquake." We 

use the term "segment" with the same meaning, 

although our determination of segments of the Wasatch 

fault zone is based on paleoseismological data rather than 

seismological data. Nevertheless, the segments defined 

herein indicate the extent of surface rupturing that we 

would expect during large-magnitude earthquakes that 

nucleate on the Wasatch fault zone. 

SEGMENTS AND SEGMENT BOUNDARIES 

We now recognize 10 segments along the Wasatch 

fault zone (fig. 1). Boundaries between these segments 

represent persistent (long-term) structural features. 

However, some apparent boundaries may be geologically 

transient (short-term or nonpersistent) features (see 

Wheeler and Krystinik (this volume) for a discussion of 

nonpersistent and persistent segment boundaries). Some 

nonpersistent boundaries occur along long segments, and 

these boundaries may be the initiation or termination 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE All 

points for some surface ruptures, depending on stress 

conditions and the size of the causative earthquake. For 

example, a magnitude 5.5 to 6.5 earthquake could rupture only a portion of a long segment. 

If segment boundaries are persistent and repeated 

barriers to lateral propagation of surface faulting, they 

should cause structural anomalies along fault zones. The 

anomalies may present themselves as either abrupt 

changes in structural relief between segments (owing to 

different long-term slip rates) or more likely as areas 

where structural relief is less than it is in adjacent parts 

of the range (see Wheeler, 1989; Wheeler and Krystinik, 

this volume). Changes in structural relief may be 

expressed as (1) subsurface ridges between deep structural basins (that is, gravity saddles), (2) depressed 

structural levels in the range, (3) bedrock blocks (salients) stranded at intermediate structural levels between 

parallel strands of the fault zone, or (4) any combination 

of these three. 

TYPES OF SEGMENT BOUNDARIES 

As a result of our detailed mapping along the central 

and southern parts of the Wasatch fault zone (Honeyville 

to Fayette, Utah), we recognize four features that mark 

persistent and nonpersistent segment boundaries 

(Machette and others, 1987) (fig. 3): 

1. Major bedrock blocks (spurs) or salients that extend 

into the basin at intermediate structural levels. These 

features are commonly bound by Quaternary faults 

that are less active than the range-bounding faults. 

The bedrock spurs north of Salt Lake City and Ogden 

are salients, as are the Transverse Mountains. Salients can persist as barriers for millions of years. 

2. Major lateral (echelon) steps in the fault zone that 

cross bedrock-cored ranges, such as at Dry Mountain 

south of Payson (a variety of a salient) and between 

the Levan and Fayette segments, or are within 

bedrock, such as at Provo Canyon. Lateral steps can 

be either persistent or nonpersistent barriers; dePolo 

and others (1989, 1991) found many examples of 

historical faulting where echelon steps in a fault failed 

to stop propagating ruptures. 

3. T -junction fault boundaries that involve the oblique 

intersection of two fault traces. One of the traces 

extends either basinward as a branching fault 

(Springville fault in the Utah Valley) or as a cross 

fault into bedrock (the northern part of the Brigham 

City segment). These features have been classified as 

geometric segment boundaries by dePolo and others 

(1989, 1991); they are not usually associated with 

reduced structural relief in either the ranges or the 

basin and thus are largely nonpersistent in the geologic record. 

4. Long (10-15 km) gaps in the fault zone. These features in the fault zone probably reflect the reduction 

of structural relief across boundaries. These features 

must occur on a time scale of hundreds of thousands of 

years (that is, long cycles of slip versus no slip). 

Through time, gaps may move in response to changes 

in stress accumulation or orientation. The NephiLevan segment boundary is marked by an obvious gap 

of 15 km in Holocene and latest Pleistocene faulting, 

but ancient scarps on middle(?) Pleistocene deposits in 

the gap attest to the presence of a throughgoing, 

albeit inactive, fault. 

SEGMENTATION MODELS FOR THE WASATCH FAULT 

ZONE 

The six segments proposed by Schwartz and Coppersmith (1984) are shown on the left-hand side of figure 1. 

Although they ended their northernmost segment (Collinston) at the Bear River, they followed Cluff and 

others' (1970) belief that movement on the Wasatch fault 

zone continued much farther to the north. The southern 

end of their Collinston segment has been moved 5 km 

north on the basis of mapping north of Brigham City by 

Personius (1988a, 1990), and its northern end has been 

extended about 11 km past the Bear River (Machette and 

others, 1987). In addition, we recognize another 41 km of 

the Wasatch fault zone that extends north along the 

Malad Range to Malad City, Idaho. This extension of the 

Wasatch fault zone has been tentatively divided into the 

Clarkston Mountain and Malad City segments (fig. 1), 

which are separated by the 6-km-wide Woodruff spur 

(Machette and others, 1987). The original Ogden segment, which extended from north of Brigham City to 

North Salt Lake City, is now divided into the 40-km-long 

Brigham City segment and the 61-km-long Weber segment. 

The Provo segment, which borders the eastern margin 

of Utah Valley, appears to be a single segment, as 

evidenced by similarities in the timing of the most recent 

and the penultimate events determined from major 

trenching investigations at the American Fork Canyon 

and Mapleton sites (table 1). Schwartz and Coppersmith 

(1984) originally proposed a single Provo segment, but 

Machette and others (1986, 1987) tentatively subdivided 

the Wasatch fault zone into three shorter segments 

named American Fork, Provo (restricted sense), and 

Spanish Fork. 

In our 10-segment model for the Wasatch fault zone, 

the Malad City-Clarkston Mountain segments and the 

Levan-Fayette segments are differentiated somewhat 

arbitrarily, mainly because we lack dating control to 

prove that these segments have different histories. As 

the following discussion of segments will show, there has 
A12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

NORTH 

T junction 

N 

. .. ·. 

···-~· 

• 

~~ ~ •. 

:o< 

''''" ;•?./'}; 

5 

SF 

2 

Salient 

~~ ~.,,. ,,. 

_ ( .. -

~~~~ l 

:'·:' {\ }, 

:;.,~.······ 

En echelon step 

FIGURE 3.-Four types of structural features at some suggested 

segment boundaries of Wasatch fault zone. Examples 1 through 7 

are from north to south; segment names are abbreviated as 

follows: C, Collinston; BC, Brigham City; W, Weber; SLC, Salt 

Lake City; SF, Spanish Fork; N, Nephi; L, Levan; F, Fayette. 

6 

Gap 

3 

Salient 

7 

En echelon step 

SOUTH 

4 

Salient 

The main trace of the Wasatch fault zone is shown by a heavy line 

having a ball and bar on the downthrown side. Solid arrows mark 

the ends of rupture segments or the centers of overlaps or gaps. 

Bedrock areas are indicated by shading. 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A13 

TABLE 2.-Lengths of segments and position of boundaries proposed for the Wasatch fault zone, Utah 

[Length rounded to 0.51an] 

Fault segment or part1 

Malad City segment .................... . 

Clarkston Mountain segment ............ . 

Collinston segment ..................... . 

Brigham City segment ................. . 

Weber segment ........................ . 

Salt Lake City segment ................ . 

Provo segment ........................ . 

American Fork part ................ . 

Central part ....................... . 

Spanish Fork part ................. . 

Nephi segment ........................ . 

Levan segment ........................ . 

Fayette segment ....................... . 

Total length of segments ............... . 

Average length of segments ............ . 

Total length of Holocene segments ....... . 

Average length of Holocene segments .... . 

Total length of Wasatch fault zone ....... . 

Length of segment (km} 

SUrlace trace Straight line 

17 

19 

30 

40 

61 

46 

69.5 

(22.5) 

(18.5) 

(31.5) 

42.5 

30 

11 

366 

36.6 

289 

48.2 

383 

16.5 

17 

29.5 

35.5 

56 

39 

59 

(21) 

(17) 

(24) 

37.5 

25.5 

10.5 

326 

32.6 

252.5 

42.1 

343 

1

Schwartz and Coppersmith (1984) proposed six segments. Their Collinston 

segment is divided here into the Malad City, Clarkston Mountain, and Collinston 

(restricted sense) segments. Their Ogden segment is divided into the Brigham 

City and Weber segments (the Brigham City includes about 51an of the original 

Collinston segment). We divided their Provo segment into three partsbeen repeated Holocene movement along five segments 

(the central two-thirds of the Wasatch fault zone), and a 

sixth (the Levan) shows evidence of a single movement in 

the late Holocene. The heavily urbanized part of the 

Wasatch Front-between Ogden and Provo-is the section of the Wasatch fault zone that shows the highest slip 

rates, the shortest recurrence intervals, and the most 

recent fault activity. 

Comments 

No latest Quaternary movement. Length does not include 6-km-wide 

Woodruff spur at southern end. 

No latest Quaternary movement. Extends from Woodruff spur south to 

Malad River. Has 7-km left step to east and 2-km north-south 

overlap with Collinston segment to south. 

No movement later than 15 ka. Northern end at Short Divide; fault's 

position in Bear River area uncertain. 

Repeated Holocene movement. Has 1-km left step and 1.5-km northsouth overlap at southern end; includes most of Pleasant View 

salient. 

Repeated Holocene movement. Length to where fault loses evidence of 

Holocene movement, east of Bountiful and 2 km north of Salt Lake 

salient (11 km wide). 

Repeated Holocene movement. East Bench and main Wasatch fault 

south to Corner Canyon, at Traverse Range. Length excludes 

7.5-km left step across Traverse Range salient and 4- to 6-km-long 

Warm Springs fault west of Salt Lake salient. 

Repeated Holocene movement extends from Chipman Creek to Payson 

Canyon. Includes the American Fork, Central, and Spanish Fork 

parts. 

Repeated Holocene movement. Extends from Chipman Creek near 

Alpine south to Provo River. Has 2-km-wide left step into bedrock at 

southern end. No north-south overlap at southern end. 

Repeated Holocene movement. Extends from Provo River Canyon 

to Springville. Includes Springville fault. Overlaps 3 km with Spanish Fork part. 

Repeated Holocene movement. Pronounced concave-to-west trace. 

Has 4-km right step and more than 6 km of north-south overlap with 

Nephi segment. 

Repeated Holocene movement. Extends from Payson to Nephi. Has 

major right step 8.5 km in bedrock between eastern (Santaquin 

Canyon) and western (Juab Valley) strands. Separated from Levan 

segment by a 15-km gap in faulting to south. 

Single(?) Holocene movement. Length includes two major gaps (6 km 

net) in faulting within segment. Steps 3.5 km left and 5 km south to 

Fayette segment. 

No Holocene movement. Has a western strand (4 km long) and major 

range-bounding eastern strand (9 km long). Ends just west of 

Fayette Cemetery. 

Total length of segments; excludes gaps, overlaps, and salients. 

Total length divided by 10 segments. 

Total length of Holocene segments divided by six segments. 

Including gaps and salients, subtracting overlaps. 

American Fork, Central, and Spanish Fork (Machette and others, 1986). However, further study reveals that this division is not warranted. The length values 

for these parts are shown in parentheses. Their Levan segment is divided into the 

Levan (restricted sense) and Fayette segments. 

SEGMENT LENGTHS 

The segments that we propose have lengths (along 

their surface traces) that are as short as 11 and 17 km for 

the segments at the ends of the fault zone to as long as 61 

and 70 km for the centrally located Weber and Provo 

segments, respectively; the average length is about 37 

km (table 2). 
A14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

The average length along the surface trace for all of 

the segments of the Wasatch fault zone is 37 km (fig. 4A). 

The central five segments, which show repeated Holocene movement, have an average length of about 48 km 

along the surface trace (see table 2 for the calculated 

range in segment lengths). The three northernmost 

segments and the two southernmost segments have an 

average length of about 20 km, whereas the inboard two 

segments (the Collinston and Levan) are each 30 km 

long. 

Segmentation of Basin and Range-style faults is a 

subject that is receiving broad attention, partly as a 

result of the 1983 Borah Peak earthquake, which ruptured only a small fraction of the total length of the Lost 

River fault zone in Idaho, and partly as a result of 

increased awareness of earthquake hazards and risks. 

We have found that the lengths of many of our proposed 

segments of the Wasatch fault zone are generally substantially longer than segments proposed for other major 

late Quaternary normal faults in the northern Basin and 

Range province (fig. 4). For example, mapping of the 

135-km-long Lost River fault zone (fig. 4B) by Crone and 

others (1987) and reconnaissance by Scott and others 

(1985) suggest a division into six segments that range in 

length from 18 to 29 km. Crone and Haller (1989, 1991) 

have completed reconnaissance studies of the 150-kmlong Lemhi and Beaverhead fault zones in east-central 

Idaho and the 27-km-long Red Rock fault zone in southwestern Montana. Their studies suggest segment 

lengths of 12 to 43 km (Lemhi) (fig. 4C), 20 to 42 km 

(Beaverhead) (fig. 4D), and 11 to 16 km (Red Rock) (fig. 

4E) for these range-bounding fault zones. A.J. Crone and 

M. N. Machette (unpub. data, 1988) have measured scarp 

profiles along the Bear Lake fault, which borders the 

eastern side of Bear Lake in Utah and Idaho. Analysis of 

the morphology of scarps along the fault suggests two 

segments (26 and 32 km long) of distinctly different age 

(fig. 4E). Closer to the Wasatch fault zone, McCalpin 

(1987, 1989) and McCalpin and Forman (1991) have 

divided the East Cache fault zone into three segments 

(greater than 14 to as much as 26 km long) (fig. 4E). By 

comparison, the Wasatch fault zone segments range from 

11 to 70 km in length and average 37 km (fig. 4A). 

The regional studies of paleoseismicity cited above 

suggest a commonality of segment lengths for rangebounding faults believed to be associated with largemagnitude (Ms 7 +) earthquakes in the northern part of 

the Basin and Range province and the Intermountain 

Seismic Belt of Smith and Sbar (1974). Figure 5 summarizes the lengths of segments found in two classes of 

faults: (1) those along major range fronts (Wasatch, Lost 

River, Beaverhead, Lemhi, and Teton faults) that are 

typically longer than 100 km and (2) those along lesser 

range fronts (Red Rock, Star Valley, East Cache, and 

East Bear Lake faults) that are typically less than 100 

km long. The proposed segments of the longer faults 

commonly range from 20 to 40 km in length and average 

25 km (and more for the Wasatch), whereas segments of 

the shorter faults range from 15 to 30 km and average 

about 20 km. 

HISTORICAL EARTHQUAKES AND SURFACE RUPTURES 

Well-documented surface ruptures caused by historical large-magnitude earthquakes have occurred in the 

Central Nevada Seismic Belt (Wallace and Whitney, 

1984) and the Intermountain Seismic Belt (Smith and 

Sbar, 1974), which form parts of the margins of the Basin 

and Range province. These data provide a comparative 

base for surface ruptures formed during prehistoric 

earthquakes along the Wasatch fault zone. Table 3 shows 

data for nine Ms 6. 5 or larger earthquakes that have 

struck the northern Basin and Range province since 

1915; these events include the 1915 Pleasant Valley, 1932 

Cedar Mountain, and 1954 Rainbow Mountain-Dixie 

Valley-Fairview Peak sequence of earthquakes in westcentral Nevada; the 1934 Hansel Valley earthquake in 

northern Utah; the 1959 Hebgen Lake earthquake in 

western Montana; and the 1983 Borah Peak earthquake 

in central Idaho. 

The Central Nevada Seismic Belt defines a northnortheast- to south-southwest-trending band of historically active faults. Major earthquakes have produced 

surface ruptures once every 15 to 20 years (on average) 

during the past 115 years (Wallace and Whitney, 1984; 

Pearthree and Demsey, 1987). The Pleasant Valley 

earthquake of 1915 (Wallace, 1984a) had an estimated 

magnitude of Ms 7.6 to M 7. 75 (table 3) and produced 

about 62 km of surface rupturing. It is the largestmagnitude earthquake recorded in the Central Nevada 

Seismic Belt. Seventeen years later, in 1932, a major 

earthquake (M 7.2) (table 3) struck the Cedar Mountain 

area and produced about 61 km of distributed surface 

faulting (Slemmons, 1977). 

In the summer of 1954, the Central Nevada Seismic 

Belt again became the locus of seismic activity as two 

major earthquakes in the Rainbow Mountain area of 

western Nevada caused surface rupturing (table 3). On 

December 16, the Fairview Peak-Dixie Valley earthquakes occurred at two discrete hypocenters separated 

in time by only 4 minutes (Doser, 1986). Doser's bodywave analysis indicates Ms 7.1 for the Fairview Peak 

earthquake and Ms 6.8 for the Dixie Valley earthquake. 

The Fairview Peak-Dixie Valley earthquakes reportedly 

produced 47- to 62-km-long surface ruptures, although 

the reported lengths vary by 10 to 14 km, depending 

on the source (see Slemmons, 1957, 1977; Bonilla and 

others, 1984; dePolo and others, 1989, 1991). dePolo 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A15 

C/) 

a: 

70 

60 

~ 50 w 

~ 

g 40 

i2 

~ 30 

::i 

1-

~ 20 w 

...J 

10 

0 

0 

e 

0.. 

WASATCH FAULT ZONE 

SEGMENTS (37 KM) 

C/) 

a: 

w 

50 

~ 40 

~ 

g 30 

i2 

z 20 

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It!) 10 

z 

w 

...J 

0 

D 

A 

C/) 30 a: 

'~ ::Z:w 20 ~--~ (.!)0 

z...J 

~:;;2 10 

~ 0 

LOST RIVER FAULT ZONE 

SEGMENTS (23 KM) 

8 

BEAVERHEAD FAULT ZONE 

SEGMENTS (25 KM) 

FIGURE 4.-Lengths of the surface traces of proposed segments on 

some major faults in the northern Basin and Range province. The 

number in parentheses below each fault name is the average length 

of all segments in that fault. Sources of data: Wasatch fault zone, this 

report, table 2; Lost River fault zone, Scott and others (1985), Crone 

40 

0 

50 en 

a: 

w 

1- 40 w 

~ 

0 30 ...J 

i2 

~ 20 

::z:' 

1- (.!) 10 z 

w 

...J 

0 

c 

RED ROCK 

FAULT ZONE 

SEGMENTS 

(13 KM) 

E 

LEHMI FAULT ZONE 

SEGMENTS (25 KM) 

EAST BEAR LAKE 

FAULT ZONE 

SEGMENTS 

(26KM) 

EAST CACHE 

FAULT ZONE 

SEGMENTS 

(18 KM) 

and others (1987); Lemhi, Beaverhead, and Red Rock fault zones, 

Crone and Haller (1989, table 1; 1991); East Bear Lake fault zone, 

A.J. Crone and M.N. Machette (unpub. data, 1988); East Cache fault 

zone, McCalpin (1989) and McCalpin and Forman (1991). 
A16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

12 

10 

u. 

0 

c:: 

~ 4 

:2 

::::> 

z 

2 

PRE-HOLOCENE 

SEGMENTS OF WFZ 

(19 KM) 

t 

1959 HL 

(29 KM) 

t 

ALL SEGMENTS 

OF WFZ 

(37 KM) 

t 

1983 BP 

(36 KM) 

f 

HOLOCENE SEGMENTS 

OF WFZ 

(48 KM) 

1954 DV 

(45±2 KM) 

f 

t 

1954 FP 

(67 KM) 

" 

0--5 5-10 10--15 15-20 20--25 25-30 30--35 35-40 40--45 45-50 50-55 

LENGTHS OF SEGMENTS, IN 5-KM CLASSES 

FIGURE 5. -Proposed segment lengths for prehistoric faults less than 

100 km long (light shading) and more than 100 km long (dark shading) 

in the northeastern part of the Basin and Range province. The 

lengths of proposed prehistoric segments along the Wasatch fault 

and others' (1989, 1991) recent compilation of historical 

faulting in the Basin and Range province listed rupture 

lengths of 67 km for the Fairview Peak earthquake and 

45±2 km for the Dixie Valley earthquake. We prefer to 

use these latter rupture lengths because they are based 

on surface mapping and a systematic treatment of all the 

historical earthquake data. Motion on all of these historical faults has been either normal dip slip or obliquenormal slip. 

Table 3 also shows the magnitudes and lengths of 

surface rupturing associated with M~6.5+ earthquakes 

in the northeastern part of the Basin and Range province. In 1934, aM 6.6 earthquake struck Hansel Valley 

about 45 to 50 km west of the northern end of the 

Wasatch fault zone (Shenon, 1936; Doser, 1989; McCalpin 

and others, this volume). This earthquake produced 

about 11.5 km of ruptures (Slemmons, 1977) and is the 

only earthquake to have ruptured the ground surface in 

Utah in historical time (see Arabasz and others, this 

volume). In 1959, a M8 7.5 earthquake struck Hebgen 

Lake in Montana (Doser, 1985) and caused extensive 

ground deformation (surface rupturing and differential 

subsidence). Movement occurred along along two major 

fault systems (Witkind, 1964): the Red Canyon and 

Kirkwood faults, which lie mainly in bedrock, and the 

range-bounding Hebgen Lake fault. Surface rupture on 

the two overlapping fault systems (24 and 13 km, respeczone (WFZ) and the lengths of historical surface ruptures produced 

during four M8>6.5 earthquakes are shown by arrows. HL, Hebgen 

Lake, Mont.; BP, Borah Peak, Idaho; DV, Dixie Valley, Nev.; FP, 

Fairview Peak, Nev. 

tively) produced 29.1 km of surface rupturing as measured from end to end (Slemmons, 1977; as measured from 

Witkind, 1964, pl. 2). 

Some small-displacement surface ruptures may not be 

included in the measurements discussed above. For 

example, Crone and others (1987) recorded 36±3 km of 

surface rupturing as a result of the 1983 M8 7.3 Borah 

Peak earthquake (Doser and Smith, 1985; Stover, 1987), 

although only 26 km of the 1983 ruptures on the Lost 

River fault zone had more than 25 em of offset. Scarps 

less than 25 em high on the Lost River fault zone may not 

have been identified if the earthquake had occurred 

several decades ago, before geologists began using lowaltitude low-sun-angle photography to help map surface 

rupture. This point is important, because most investigations of prehistoric faulting (paleoseismology) have not 

found evidence for small-displacement (less than 25 em) 

surface-rupturing events. 

In summary, historical large-magnitude earthquakes 

in the northern Basin and Range province have been 

associated with surface ruptures that range in length 

from as little as 11 km (Hansel Valley) to as much as 62 

km (Fairview Peak); generally, as magnitude increases, 

so do length of rupture and amount of displacement. 

Although this statement seems intuitively correct, we 

cannot be more specific, because the empirical relation 

between magnitude and length for normal and normal-
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A17 

TABLE 3.-Lengths of surface rupturing from recent Ms 6.5 or larger earthquakes in the northern Basin and Range province 

[-, not determined] 

A 

Slemmons 

(1977, table 16) 

B 

Bonilla and others 

(1984, table 2) 

Length of 

c Doser (1985, 1986), 

Crone and others (1987) 

Length of 

Earthquake location 

(Reference from column C) 

Fault 

Length of 

surface faulting surface faulting surface faulting 

Date type1 M2 (km)2 Msa (km)4,5 Mss (km)4 

Pleasant Valley, Nev. . ........................... . 10/03/15 NS 7.75 62 7.6 61±1 

Cedar Mountain, Nev ............................. . 12/20/32 OS 7.2 61 

Hansel Valley, Utah .............................. . 03/12/34 OS 6.6 11.5 

Rainbow Mountain, Nev. (Doser, 1986) ............. . 07/06/54 NS 6.6 17.7 6.3 20±2 6.6 (6.2) 16 

08/24154 NS 6.8 30.6 7.0 26±5 6.8 (6.5) 25 

Fairview Peak, Nev. (Doser, 1986) ................. . 12/16/54 OS 

Dixie Valley, Nev. (Doser, 1986) ................... . 12/16/54 NS 

Hegben Lake, Mont. (Doser, 1985) ................. . 08/17/59 NS 

Borah Peak, Idaho (Crone and others, 1987) ........ . 

1 NS, normal dip slip; OS, oblique dip slip. 2

Unspecified Richter magnitude. 3

Mean of surface-wave magnitude determinations. 

10/28/83 

4

Single value for length of surface faulting generally conforms with length 

along surface trace. 5

Range of values described by error limits indicates maximum and minimum 

lengths owing to uncertainties as described by Bonilla and others (1984, p. 

2380). 

oblique faults has so much scatter that Bonilla and others 

(1984, p. 2391) recommended against using this relation 

for estimating earthquake magnitude. In addition, Doser's studies of the historical earthquakes of the Central 

Nevada Seismic Belt and the Intermountain Seismic Belt 

indicate that most large earthquakes are complex 

events, often composed of discrete but closely spaced 

earthquake subevents. Thus, some large-magnitude 

earthquakes and surface ruptures may be multiplesegment-rupturing events. 

One implication of our segmentation model is that the 

anticipated length of surface rupturing during a major 

earthquake along some parts of the Wasatch fault zone 

could be less-by as much as a factor of 2-than previously reported. If we assume that length of surface 

rupturing and amount of slip are dependent on the 

energy released during an earthquake (that is, moment 

magnitude), then the largest earthquake that could be 

expected along an entire single segment would be somewhat less for our revised segments. Another implication 

is that our finer subdivision (10 segments versus 6) 

increases the number of possible segments on which 

discrete surface rupturing could occur along the Wasatch 

fault zone. 

For convenience of discussion we have divided the 

Wasatch fault zone into northern, central, and southern 

sections on the basis of their fault activity during the 

Holocene. The northern section shows no evidence of 

Holocene faulting, the central section shows evidence of 

OS 

7.1 58.0 7.2 48±5 7.1 (7.2) 62 

6.8 61.2 47±4 6.8 (6.7) 36 

7.25 29.1 7.6 26±2 7.5 (7.3) 729 

8

30 

7.3 (7.0) 9

36 

6

Values in parentheses indicate maximum moment magnitudes calculated 

from seismic moment (Stein and Bucknam, 1985). 7

Length of fault having more than 10 em (0.3 ft) of offset. Total length of 

surface faulting may be several kilometers longer. 8

Length estimated from geodetic modeling (Savage and Hastie, 1966). 9

Includes gaps and long sections exhibiting less than 25 em of offset. Length 

of faulting displaying more than 25 em of offset is about 26 km (Crone and others, 

1987, fig. 4). 

repeated Holocene faulting, and the southern section 

shows evidence of one or no Holocene faulting events. 

NORTHERN WASATCH FAULT ZONE 

Segmentation of the northern section of the Wasatch 

fault zone is difficult to recognize because there is little 

evidence of faulting in deposits younger than the Bonneville lake cycle. Our primary criteria for delineating 

segments and segment boundaries on the more active 

parts of the Wasatch fault zone are changes in latest 

Pleistocene and Holocene fault behavior, but, because 

the northern section of the Wasatch fault zone lacks fault 

scarps of this age, we have instead used geomorphic and 

structural relations to identify probable fault segments 

and boundaries. Although we are less confident of these 

segments, the structural and geomorphic patterns suggest that the 70-km-long northern section of the Wasatch 

fault zone is composed of a 17-km-long Malad City 

segment, a 19-km-long Clarkston Mountain segment, 

and a 30-km-long Collinston segment (figs. 1, 6). 

SEGMENTS ALONG THE MALAD RANGE 

The northern end of the Wasatch fault zone, which 

commonly has been mapped along the western flank of 

the Malad Range, dies out 4 km northeast of Malad City, 

Idaho (Cluff and others, 1974). On the basis of our aerial 

photographic analysis and brief field reconnaissance of 
A18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

EXPLANATION ·71111/11//J.; 

':il::f::p:: 

~ Segment boundary ;.:-:·-. 

0 

I 

0 

I 

5 

FIGURE 6.-Northern part of the Wasatch fault zone 

between Malad City, Idaho, and Honeyville, Utah, showing the Malad City (MCS), Clarkston Mountain (CMS), 

and Collinston (CS) segments. Other geographic and 

tectonic features mentioned in the text are the Woodruff 

5 

I 

I 

10 

I 

10 MILES 

I 

15 KILOMETERS 

spur (WSP), Short Divide (SD), Junction Hills (JH), 

Cutler Dam (CD), the Madsen spur (MSP), Jim May 

Canyon (JMC), and part of the West Cache fault zone 

(WCFZ). The dotted contact shows the near-surface 

limit of bedrock on the Madsen spur. 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A19 

this region, we have divided the Wasatch fault zone along 

the western flank of the Malad Range into the Malad City 

and Clarkston Mountain segments. 

The Malad City segment is expressed as a steep, 

relatively continuous escarpment along the western base 

of the Malad Range, from 4 km northeast of Malad City 

south to the vicinity of West Cherry Creek, Idaho (fig. 

6). Photographic reconnaissance by us and by Cluff and 

others (1974) has shown that younger lacustrine deposits 

of the Bonneville lake cycle lie undisturbed across most 

of this section of the fault zone. The presence of steep 

escarpments and steep, linear gravity gradients parallel 

to the fault zone (Peterson, 1974; Zoback, 1983) and the 

possible presence of fault scarps on "older'' alluvium 

(Cluff and others, 1974) suggest that the Malad City 

segment has been active during the late Pleistocene. 

The boundary between the Malad City and the Clarkston Mountain segments is expressed as a bedrock spur 

that extends 6 km along the front of the Malad Range, 

northeast ofWoodruff, Idaho. Just south of West Cherry 

Creek, the front of the Malad Range changes from the 

steep, linear escarpment that characterizes the Malad 

City segment to an irregular zone (the Woodruff spur) 

that extends southward to Muddy Creek. The Woodruff 

spur is coincident with a prominent saddle in the gravity 

data (Peterson, 1974; Zoback, 1983); this relation indicates that net slip across the Wasatch fault zone 

decreases at this point. Photographic reconnaissance by 

Cluff and others (1974) suggests that the spur is fractured by numerous faults rather than by a single welldefined fault or narrow fault zone. Fractured bedrock in 

the spur may have dispersed and absorbed energy as 

ruptures approached from the north or south, thereby 

decoupling the two adjacent fault segments (King and 

Nabelek, 1985; Wheeler and Krystinik, this volume). The 

Woodruff spur is shown as a major transverse structural 

feature on Zoback's (1983, figs. 3, 5) gravity map. 

The Clarkston Mountain segment is expressed as a 

steep, linear escarpment that extends from the vicinity 

of Muddy Creek at the southern end of the Woodruff 

spur southward along the western flank of the southern 

Malad Range (Clarkston Mountain) and dies out near the 

Malad River, 4 km west of Plymouth, Utah. The southern 4 km of the Clarkston Mountain segment are 

expressed as a linear escarpment covered by sediment of 

the Bonneville lake cycle; however, regressional (Provo) 

shorelines clearly wrap sharply around this escarpment 

and suggest that it is a fault scarp that slightly predates 

the Bonneville lake cycle. Thus, the latest movement on 

the Clarkston Mountain segment probably occurred in 

late Pleistocene time, during or before the end of the 

Bonneville lake cycle. 

Near the southern end of Clarkston Mountain at Short 

Divide, a prominent down-to-the-south east-striking normal fault places Tertiary lacustrine sediment (Salt Lake 

Group of Williams (1958)) against lower Paleozoic sedimentary rock. This fault crosses the Cenozoic structural 

grain in the region and appears to have been active in 

late Tertiary and Quaternary time. It marks the boundary between the Clarkston Mountain segment and the 

northern end of the Collinston segment (fig. 6). The two 

segments are separated by a 7-km left step and gap in 

late Pleistocene faulting. This pattern of faulting differs 

from that of Cluff and others (1974), who mapped the 

Wasatch fault zone as a continuous feature around the 

southern end of Clarkston Mountain by connecting the 

fault on the western side of the Malad Range (our 

Clarkston Mountain segment) with the fault along the 

western side of the Junction Hills (our Collinston segment). From our photographic and map interpretations, 

we believe their presumed connecting fault to be an 

escarpment formed by the Bonneville shoreline. The 

Clarkston Mountain-Collinston segment boundary also 

appears as a major transverse structural feature on 

Zoback's (1983, figs. 3, 5) gravity map. 

COLLINSTON SEGMENT 

The surface trace and geologic history of the Collinston 

segment are poorly defined because our surface mapping 

and photographic reconnaissance have revealed no evidence of surface faulting in deposits younger than the 

Bonneville lake cycle along most of its length. The 

northern end of the segment is probably near Short 

Divide, where the Wasatch fault zone appears to intersect the previously discussed east-striking fault near the 

southern end of Clarkston Mountain. Farther south, the 

Bear River flows southwest from Cache Valley through 

a major topographic saddle between the southern end of 

Clarkston Mountain and the northern end of the Wellsville Mountains. This saddle is marked by a gradual 

decrease in height of the mountains to the north and 

south, and the southern margin of the saddle coincides 

with the northerly plunge of bedrock at the northern end 

of the Wellsville Mountains (Oviatt, 1986a, b). The 

topography of this region suggests that the total throw 

on the Collinston segment decreases from south to north. 

However, the steep escarpment along the western side 

of Junction Hills (north of the Bear River) and a northtrending gravity gradient (Zoback, 1983) suggest that 

normal faulting probably extends continuously along the 

western flank of Junction Hills, across the Bear River, 

and south to the western flank of the Wells ville Mountain~. 

One possible explanation for the decrease in throw on 

the central and northern parts of the Collinston segment 

is the presence of the West Cache fault zone. This 

down-to-the-east normal fault forms the western margin 
A20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

of Cache Valley and lies at the eastern base of the 

Wellsville Mountains and the Malad Range (fig. 6). Cluff 

and others (1974) and Oviatt (1986a) have mapped several fault scarps on the West Cache fault zone that cut 

Bonneville lake cycle deposits in the topographic saddle 

between Junction Hills and the eastern side of the 

Wells ville Mountains. The presence of these scarps and 

the steeper topography of the eastern side of the northern Wellsville Mountains and Malad Range suggest that 

the northern part of the West Cache fault zone has been 

the most active part of this fault zone. Perhaps the 

apparent eastward transfer of extension from the 

Wasatch fault zone to the West Cache fault zone in both 

the long (Quaternary) and the short (Holocene) term may 

help explain the geomorphic patterns on the Collinston 

segment. 

Near the southern boundary of the Collinston segment, Personius (1988a, 1990) mapped a 2-km-long fault 

scarp on gravel of the Bonneville lake cycle and on lower 

Holocene fan alluvium. This short Holocene scarp (shown 

on fig. 6 as a solid line at the southern end of the 

segment) is considered part of the Collinston segment, 

although it probably formed in response to surface 

faulting originating on the more active, adjacent 

Brigham City segment. This relation is similar to the 

pattern of surface faulting that accompanied the 1983 

Borah Peak earthquake in Idaho. Near the northern end 

of the Lost River fault, the 1983 surface faulting jumped 

over the segment boundary at the northern end of the 

Thousand Springs Valley and ruptured a small part of 

the Warm Springs segment (Crone and others, 1987). 

Sympathetic or subsidiary rupturing is a reasonable 

explanation for the short Holocene scarp at the southern 

end of the Collinston segment, because no evidence of 

Holocene faulting is found on the remainder of the 

segment to the north. Although the most recent faulting 

on most of the Collinston segment occurred before the 

Bonneville lake cycle, the steep escarpments along the 

Wells ville Mountains and the Junction Hills suggest that 

the segment has probably been active in late Pleistocene 

time. 

The boundary between the Collinston and Brigham 

City segments is placed at a reentrant on the western 

flank of the Wells ville Mountains near the mouth of Jim 

May Canyon, 2 km northeast of Honeyville, Utah. The 

boundary is marked by a change in the trend of the fault, 

differences in the amount of displacement of similar aged 

pre-Bonneville lake cycle deposits, and a general lack of 

geomorphic evidence for Holocene movement on most of 

the Collinston segment (Personius, 1986, 1988a, b, 1990). 

Directly west of the reentrant at Jim May Canyon, a 

topographic bench known as the Madsen spur (Gilbert, 

1928) has been mapped by Oviatt (1986b) and Personius 

(1988a, 1990). Oviatt (1986b) interpreted the entire spur 

as a huge bedrock landslide, but we believe that exposed 

landslide deposits are restricted to small bedrock erratics 

at the surface near the mountain front (Personius, 1988a, 

1990). The large bedrock outcrop mapped by Oviatt 

(1986b) and Personius (1988a, 1990) near the western 

edge of the spur (fig. 6) dips to the northeast, into the 

southern end of the Collinston segment. We believe that 

the size and geometry of this deposit suggest that it is an 

outcrop of intact bedrock, suspended at an intermediate 

structural level between the active trace of the Wasatch 

fault zone and a buried fault (Davis, 1985) that forms the 

western margin of the Madsen spur (Personius, 1988a, b, 

1990). Just north of the bedrock exposure (fig. 6), two 

short faults offset Bonneville lake cycle gravels (Personius, 1988a, b, 1990); the presence of these faults may 

indicate that the structural complexities within the spur 

have absorbed and dispersed fault ruptures on the 

Brigham City and (or) Collinston segments of the 

Wasatch fault zone. 

CENTRAL WASATCH FAULT ZONE 

The central section of the Wasatch fault zone is composed of recently active fault segments, all of which have 

experienced repeated movement in the Holocene. From 

north to south, the five segments are Brigham City, 

Weber, Salt Lake City, Provo, and Nephi (fig. 1). The 

following discussion highlights evidence from recent 

investigations along each segment. Final determination 

of revised slip rates and recurrence intervals along this 

section of the Wasatch fault zone awaits analysis of dated 

samples from recently excavated trenches. 

BRIGHAM CITY SEGMENT 

The Brigham City segment (fig. 7) is the northernmost 

segment of the Wasatch fault zone that exhibits clear 

evidence of recurrent Holocene faulting along its entire 

length (fig. 7). Slip rates on the northern part of the 

segment (north of Brigham City to Honeyville) appear to 

have been lower than those on the southern part of the 

segment. However, near the segment's northern boundary, evidence from Personius (1988a, b, 1990) clearly 

indicates that two or more faulting events occurred after 

Lake Bonneville fell below the Provo shoreline (14 ka). 

Trench studies conducted in September and October 

1986 near Brigham City (fig. 7) (Personius and Gill, 1987; 

Personius, 1991a) revealed evidence for three surfacefaulting events in lower or middle Holocene alluvial-fan 

deposits. The timing of the two most recent events 

evident in the Brigham City trench (BC-1) are constrained by 14C analysis of buried A horizons on faultscarp colluvium (fig. 8). The results of AMRT ages 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A21 

112°15' 41~7'30" 

112"00' 111°52'30" 

30' 

..... 

, . .... 

•• I. 

LITTLE 

~MOUNTAIN 

:&& , ...... ,. 

... 

.. ... 'I!..Jio 

:·· :::::·· .::::·.·:.:::: , . .. ... . . ...... , . 

, ............ ... . .. . . ...... ..... 

,. ......... 

;, .. . ...... 

. ....... .. .... . .. ,. .... . .. 

... .... ,..,. .. .. .. .. . ... , ... ... 

. . . ... . ... ...... 

. .,) . ....... 

• .... .... ,1, 

............. ..... ... .. 

G~eat Salt L~k~. · · 

EXPLANATION 

~ Segment boundary 

A Trench or field site 

~ 

~«' 

~ ~(«';. 

Willard Reservoir~ 

North~~~~~ ~··:~~~ttftfttt 41°22'30" L.....J.. ________ ..c;:;:::::.~, _______________ ..A., ______ n m ,&;,;,;~.W ~=:::::::::::::::::::::::::::::::::: ....................... 

0 

I 

0 

I 

2 

FIGURE 7.-Brigham City segment (BCS) of the Wasatch fault 

zone between Honeyville and North Ogden, Utah; also shown 

are the southern part of the Collinston segment (CS) and the 

northern part of the Weber segment (WS). Other geographic 

and tectonic features mentioned in the text are the Madsen spur 

(MSP), Jim May Canyon (JMC), part of the West Cache fault 

2 

I 

I I 

3 4 

3 

I 

I 

4 MILES 

I 

5 KILOMETERS 

zone (WCFZ), Box Elder Canyon (BEC), and the Pleasant View 

salient (PVS). Sites are labeled as follows: BC-1, Brigham City 

trench 1; PP-1, Pole Patch trench 1. The dotted contacts show 

the near-surface limit of bedrock on the Madsen spur and the 

Pleasant View salient. 
A22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

WEST EAST 

en 

a: 

w 

Iw 

~ 

~ 

ui 

u 

z 

~ en 

c 

...J 

<( 

u 

i= 

a: 

w 

> 

3 

2 

3 4 5 6 7 8 9 10 11 12 

HORIZONTAL DISTANCE, IN METERS 

FIGURE 8.-Part of the log of trench BC-1 near Brigham City, Utah. 

The figure shows the relations of faults (heavy lines), scarp colluvial 

deposits (units 1-5), and Holocene alluvial-fan deposits (units 6-10) in 

the northern wall of the trench. The solid rectangles show the 

on organic matter concentrated from soils buried by the 

two youngest colluvial wedges (table A1) suggest that 

these events occurred about 4. 7±0.5 and 3.6±0.5 ka. The 

resulting recurrence interval between the two youngest 

events is 1,100±1,000 years (Personius, 1991a). A colluvial wedge from an earlier event that occurred about 5 to 

7 ka lacks suitable material for 14C analysis and therefore 

has not been dated. Although the size of individual 

faulting events at this site is difficult to determine, the 

total vertical offset of the fan deposits (6 m) and the 

number of events (three) exposed in trench BC-1 indilocations of organic-rich sediment sampled for 14C dating; ages are 

labeled with laboratory sample numbers (see table A1). Wavy 

vertical lines indicate A horizons. Horizontal and vertical distances 

on axes are arbitrary. 

cate an average of 2 m of displacement per surfacefaulting event and a post-middle Holocene slip rate of 

0. 75±0.3 mm/yr on the central part of the Brigham City 

segment. 

Just south of the trench site, there are large scarps 

(15-20 m high) on the Provo-level delta at the mouth of 

Box Elder Canyon and elsewhere along the southern 

part of the segment (Personius, 1988a, 1990). These 

scarps and the 2 m of average displacement recorded in 

trench BC-1 indicate that as many as 6 to 10 surfacefaulting events may have occurred on the central part of 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A23 

the Brigham City segment since 14 ka. The distribution 

of fault scarps on latest Pleistocene and Holocene deposits indicates that the entire Brigham City segment has 

remained active throughout most of the early and middle 

Holocene. However, only a few small, short, discontinuous scarps near the southern segment boundary have 

been mapped on what could be upper Holocene deposits 

(Personius, 1988a, 1990). The general lack of fault scarps 

on young deposits and evidence in the Brigham City 

trench for only one faulting event in the past 3,000 to 

4,000 years are in sharp contrast to the abundance of 

fault scarps on upper Holocene deposits along the Weber 

segment and evidence for multiple faulting events in the 

same general time period (Nelson and others, 1987; 

Nelson and Personius, 1990, in press) (see following 

discussion of the Weber segment). 

The boundary between the Brigham City and Weber 

segments is near the northeastern corner of the Pleasant 

View salient, 3 km north of North Ogden, Utah (fig. 7) 

(Personius, 1986, 1988a, b, 1990; Nelson and Personius, 

1987, 1990, in press). This salient, which was first 

recognized by Gilbert (1928), seems to be bounded on 

most sides by normal faults (Davis, 1985). The Pleasant 

View salient is a foundered block of Proterozoic and 

lower Paleozoic rock (Crittenden and Sorenson, 1985) 

that has been stranded at an intermediate structural 

level. The salient is broken by a network of short normal 

faults that are subsidiary to the main trace of the 

Wasatch fault zone. At least five of these faults offset 

deposits of the Bonneville lake cycle. Results from a 

trench excavated across one of these faults (PP-1, fig. 7) 

show about 5 m of vertical offset and evidence for three 

faulting events in the past 10 to 15 ka. Radiocarbon 

analysis of soil organics deposited in a tectonic crack in 

the penultimate colluvial wedge indicates that the most 

recent event occurred 4.6±0.5 ka (table A1) (Personius, 

1991b). Analysis of 14C ages suggests that the most 

recent event exposed in trench PP-1 is probably the 

same as the penultimate event recorded in trench BC-1 

discussed above. 

WEBER SEGMENT 

The 61-km-long Weber segment of the Wasatch fault 

zone extends from Barrett Canyon, 3 km north of North 

Ogden, to the Salt Lake salient, a major footwall spur of 

bedrock that extends westward between Bountiful and 

northern Salt Lake City. The Weber segment comprises 

the southern two-thirds of Schwartz and Coppersmith's 

(1984) 70-km-long Ogden segment; we place the northern 

boundary of the Weber segment at the Pleasant View 

salient, well south of their boundary for the Ogden 

segment (fig. 9). 

Rates of faulting differ along the Brigham City and 

Weber segments of the main fault on either side of the 

salient. Surface offset recorded by scarps and trench 

data from the Brigham City segment suggest that high 

rates of surface faulting occurred in latest Pleistocene to 

middle Holocene time but that tectonic activity must 

have slowed or ceased along the Brigham City segment 

in late Holocene time (Personius, 1988b). In contrast, 

fault scarps having 2 to 5 m of surface offset are common 

on deposits of late Holocene age on the northern part of 

the Weber segment. These differences in fault activity 

confirm that the Pleasant View salient is a Holocene 

boundary between two fault segments (Nelson and Personius, 1987). 

Recent mapping (Nelson and Personius, 1990, in 

press) shows that the Weber segment of the Wasatch 

fault zone is expressed at the surface by almost continuous scarps on Holocene deposits from its northern end 

to as far south as North Canyon, south of Bountiful (fig. 

9). Most of these scarps are formed on Holocene alluvialfan and debris-flow deposits. 

Upper Holocene alluvial fans composed of stream and 

debris-flow sediment are of at least two different ages 

along the Weber segment, but the degree of soil development and fan surface morphology cannot be used 

consistently to differentiate two or more ages of fans. 

Radiocarbon ages from the East Ogden site (discussed 

below and listed in table A1) indicate that even the older 

(Holocene) fan remnants exposed along the fault are less 

than 6 ka. Fault scarps of differing heights on the upper 

Holocene fans provide the only practical way of distinguishing fans of two ages, and such evidence is available 

at only a few stream drainages. Distinguishing fans by 

differing scarp heights assumes a uniform slip rate along 

the segment throughout the late Holocene. 

Surface offsets measured along the fault indicate slip 

rates of 0. 9 to 1. 9 mm/yr over the past 15 ka for the 

central three-quarters of the Weber segment. Slip rates 

decrease as one approaches both ends of the segment. 

This relation was demonstrated by making topographic 

profiles of 77 scarps in the field and by using the USGS's 

computer-assisted PG-2 photogrammetric plotter to 

make an additional298 profiles from 1:10,000-scale photographs (Nelson and Personius, 1990, in press). Comparison of field and plotter profiles of the same scarps (97 

profiles) shows that the computer-assisted profiles generally yield surface offset values within ± 10 percent of 

values measured from field profiles. This measurement 

error is similar to errors in estimating offsets from 

field-measured profiles, especially for scarps more than 5 

min height. In some areas, pre-1960 aerial photography 

has been used to measure profiles across scarps that 

were destroyed as much as 30 years ago by urbanization. 
A24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

41°00' 

EXPLANATION 

J( Segment boundary 

& Trench or field site 

0 

I 

0 

I 

5 

5 

I 

-~ ::11. ;:. 

-,< ..w: 

\\ ,-t+t+t 

I 

10 

n 

I 

17 

10 MILES 

I 

;:: 

15 KILOMETERS 

I~ 

FIGURE 9.-Weber segment of the Wasatch fault zone between North Ogden and Bountiful, Utah, 

showing the main trace of the fault. Trench exposure sites are abbreviated as follows: GC, Garner 

Canyon; EO, East Ogden; KV, Kaysville. 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A25 

en 

a: 

w 

1-

WEST 

6 

5 

EAST 

w 1300±80 yr B. P. ~ 4 (DIC-3279) 

~ 

u) 

x 0 

<( 

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<( 

u 

i= 

a: 3 ~, w @) > -. 

(!) 

0 

z - ...1 

<( 

w 

u 2 

z 

~ en 

C5 

7 6 5 4 3 2 0 

DISTANCE ALONG HORIZONTAL AXIS, IN METERS 

FIGURE 10.-Vertical exposure across a fault scarp at the mouth of 

Garner Canyon (Weber segment) near Ogden, Utah, showing the 

fault zone and colluvium produced by faulting events. The scarp (6 m 

high, 4.4-m vertical displacement) was produced by three or four 

earthquakes during the middle to late Holocene. Solid patches show 

boulders and large cobbles. Buried A horizons (units 3, 5) overlie 

cobbly debris wedges (units 4, 6) derived from a free face formed 

The exposures at the East Ogden and Garner Canyon 

sites, near the northern end of the Weber segment (fig. 

9), reveal a detailed middle and late Holocene fault 

history (Nelson and others, 1987; Nelson, 1988). Colluvial wedges, which were found in six exposures at these 

two sites, were deposited following the last three or four 

events on the fault. The vertical displacements measured 

in all exposures are similar to values of surface offset 

calculated from topographic profiles across the scarps. 

At the Garner Canyon site on the northern edge of 

Ogden, the last two events, each having about 1.0-m 

displacement, occurred about 1.5 to 2.0 ka and 0.8 to 1.2 

in stream and debris-flow material (units 7-12). Unit 5 has been 

displaced by a small antithetic fault. Radiocarbon dates (and laboratory numbers) (table A1) for A horizons are shown. Interpretation of 

these AMRT ages and correlations with estimated times of events at 

the East Ogden site suggest that the three most recent events 

occurred at more than 2.2 ka, 1.5 to 2.0 ka, and 0.8 to 1.2 ka. 

ka. These ages are based on correlations with the East 

Ogden site and on 14C AMRT ages on organic matter 

concentrated from soil A horizons (calibrated AMRT's) 

(fig. 10; ARN85-03 and ARN85-05, table A1). At the 

East Ogden site, two scarps on middle Holocene fan 

deposits (dated at about 5-6 ka) (ARN86-21, table A1) 

record about 5.3 and 8.1 m of total displacement. Where 

these scarps cross two upper Holocene fan deposits, the 

displacements are only about 1.2 and 1.8 m. Trenches 

across the 5- and 8-m scarps exposed deltaic deposits 

beneath fan deposits in the faults' hanging-wall block. 

The deltaic deposits probably formed at the Provo level 
A26 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

10~---------------------------------------------------------, 

8 

en 

a: 

w 

Iw 

~ 

~ 

.,.: 

z 6 w 

~ 

w 

u 

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a.. 

en 

i5 

..J 4 <( 

u 

t= 

a: 

w 

> 

Iw 

z 

2 

MIDDLE HOLOCENE FANS 

52 

E0-1 EQ-2 E0-3 

EXPLANATION 

Fault events 

~ d (young) 

me 

Fib 

[] a (old) 

UPPER HOLOCENE FANS 

E0-4 E0-5 

FIGURE 11. -Amounts of displacement and times of fault events recorded in five trenches at the East 

Ogden site (see fig. 9 for location of site). Correlation of faulting events is shown by patterned boxes. 

Event c is recorded in five trenches that cross both the 8-m scarp (S2) and the 5-m scarp (S1). Scarp 

S2a (trench E0-4) is antithetic to scarp S2 (trench E0-2), so most of the displacement for event c in 

trench E0-4 should be subtracted from the displacement for this event on the adjacent main scarp S2 

(trench E0-2) to obtain the net displacement for this event near the 8-m (S2) scarp. Trench E0-3 

contains evidence of a small young event (event d), for which no evidence was recognized in trenches 

E0-2 and E0-5 on the 8-m (S2) scarp. 

of the Bonneville lake cycle about 14 ka. The stratigraphy and lithology of colluvium exposed in a trench across 

the 5-m scarp suggest two events, each of about 2.2-m 

displacement during the middle Holocene, followed by a 

0.9-m event in the late Holocene (fig. 11) (ARN86-19, 

table A1). The colluvial sediments exposed in an adjacent 

8-m scarp reveal two middle or late Holocene events of 

about 2.5- and 3.5-m displacement, followed by a 2.2-m 

event during the late Holocene (ARN86-06 and 

ARN86-13, table A1). 

Thermoluminescence analyses of fine-grained distal 

colluvium in trenches E0-1 and E0-2 (Forman and 

others, 1991) help to constrain the age of the East Odgen 

fault events, particularly the ea:tlier events that are 

poorly dated by 14C analyses. A TL age estimate of 

2, 700±300 years (ITL-24, table A1) is probably a minimum for the second event in trench E0-1. Five age 

estimates from samples above ITL-24 (ITL-72, ITL-75, 

ITL-112, ITL-47, and ITL-113, table A1) suggest a 

nonuniform sedimentation rate for the distal colluvium 

adjacent to the 5-m scarp. Distal sediment 'Yas deposited 

in pulses in response to the two most recent faulting 

events. A TL age estimate of 3,200±300 years (ITL-74, 

table A1) also constrains the minimum time for the first 

event in trench E0-2 and the maximum time for the 

second event. This age estimate agrees well with a 

calibrated AMRT age of about 3.5 ka on a sample having 

a very low organic content from an equivalent stratigraphic position in trench E0-2 (ARN86-06, table A1). 

Thus, the TL age estimates suggest that the second 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A27 

rupture event in both trenches (E0-1 and E0-2) is 

probably the same event-a conclusion that we could not 

draw solely from our 14C ages. 

The results of 14C analyses of 3 charcoal samples and 

16 samples of concentrated A-horizon sediment from the 

five trenches at the East Ogden site (see table Al) 

highlight many of the problems with using AMRT ages to 

estimate the time of faulting events on normal faults. Of 

the 10 samples collected within 1 m of the present 

surface, two pairs of samples have inverted ages (lower 

sample younger than a stratigraphically higher sample) 

(ARN86-23, ARN86-24 and ARN86-03, ARN86-04, 

table Al), and 3 samples yielded more than 112 percent 

modern carbon (ARN86-08, ARN86-14, and ARN86-15, 

table Al). These results are probably due to the incorporation of modern bomb (thermonuclear-blast-induced) 

carbon into modern A horizons by mixing of surface 

sediment into lower parts of the A horizon through 

unrecognized animal burrowing and, possibly, translocation of fine organic matter within coarse-grained horizons. Several of the AMRT ages may also reflect a 

reworking of old A-horizon sediment exposed in fault 

scarps into newly developing soils on colluvial debris 

wedges adjacent to the scarps (ARN86-24 and possibly 

ARN86-19, table Al). Comparison of all TL age estimates and 14C ages shows that 14C ages for deeper 

(buried) horizons are more consistent than those for 

shallower samples. Thus, fault events recorded by nearsurface units (less than 1-m depth) are difficult to date 

accurately by means of AMRT ages, at least at sites like 

East Ogden, where most units are pervious sand or 

gravel. 

Stratigraphic relations indicate that at least three, and 

probably four, faulting events have been recorded in 

trenches at the East Ogden site since the debris-flow 

units were deposited 5 to 6 ka (fig. 11). Both of the main 

faults probably ruptured to the ground surface during 

the three largest events (a, b, and c, fig. 11). Displacement at East Ogden during event c seems to have been 

only about half that during events a and b. On the basis 

of our interpretation of all the 14C and TL analyses 

currently available, we estimate the timing of event a at 

3.5 to 4.0 ka, that of event bat 2.5 to 3.0 ka, and that of 

event c at 0.8 to 1.2 ka. Mixing of near-surface stratigraphic units by burrowing, possible pedogenic translocation of carbon, and the interpretive problems with the 

AMRT ages on these units make it difficult to determine 

if the 8-m scarp at the site has been offset by a smalldisplacement event within the past 500±200 years 

(ARN86-04 and ARN86-43, table Al). If so, then the 

displacement was probably less than 0.8 m. These conclusions raise questions about the minimum size of displacement events that can be recognized in trenches, 

about whether large later events destroy evidence of 

small earlier events, and about the limits of AMRT ages 

in resolving faulting events spaced only 500 to 1,000 

years apart. 

Our studies show that the general history of faulting at 

the Garner Canyon and East Ogden sites is similar to the 

history determined from the Kaysville site (23 km to the 

south) (figs. 9, 12) by Swan and others (1980). They 

demonstrated at least three surface-faulting events during the past 8 ka (±2 ka), although the timing of each 

event was poorly constrained. Discrete colluvial wedges 

and correlative faulted and back-tilted graben deposits in 

the trenches were the basis for recognizing three faulting 

events. The two latest events occurred within the past 

1. 6 ka, and stratigraphic and geomorphic evidence suggests that the most recent event occurred within the past 

500 years but before settlement of the area in 1847. Swan 

and others' (1980) best estimate for the elapsed time 

between faulting events is 500 to 1,000 years, the longer 

interval being preferred. The long-term record at 

Kaysville is recorded by a lower Holocene alluvial fan 

displaced 10 to 11m. Because displacements produced by 

the two most recent events were 1. 7 and 1.8 m, the prior 

6.5 to 7.5 m of displacement may have occurred during 

the older third event. However, it seems more likely that 

the remaining fan displacement represents the cumulative displacement from two or more events that were not 

recognized in the trench. 

Our limited age control at the trench sites prevents us 

from drawing more detailed conclusions about the history of faulting along the northern two-thirds of the 

Weber segment (fig. 12). Except for the last event, 

displacements at Garner Canyon are less than half the 

size of displacements at East Ogden for the same events, 

as would be expected for a site near (6 km) the end of a 

major fault segment. Even less dating control is available 

at the Kaysville site near the center of the Weber 

segment. A 14C date of 1,580±150 14C yr B.P., which 

limits the maximum time of the two youngest events at 

Kaysville, suggests that event c and possibly event d 

(East Ogden site) are recorded here. If so, displacements 

during event c were comparable at the two sites, but 

displacement during event d was twice as large at 

Kaysville as it was at East Ogden. Possibly, event dis 

the result of an earthquake that ruptured only the 

central part of the Weber segment within the past 

500±200 years. Other alternatives could be that evidence for event d at East Ogden has been misinterpreted 

and no event occurred or that a small youngest event 

ruptured the surface at the East Ogden site but did not 

reach or was not recorded at the Garner Canyon and 

Kaysville sites. 

The southern end of the Weber segment is bounded by 

one of the most prominent salients along the Wasatch 

Front (Personius and Scott, 1990, in press; Nelson and 
A28 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

12~------------------------------------------------------------, 

(/J 

a: 

w 

1-

w 

~ 

10 

~ 8 

r: 

z 

w 

~ 

w 

u s 6 a.. 

(/J 

0 

...J 

<( 

u 

~ 4 w 

> 

1-

w 

z 

2 

EXPLANATION 

Fault events 

~ d (young) 

mJc .b 

(2] a (old) 

D ee (older) 

>>2.2 

12 km 

34km 

Several events 

<8±2 

0~~------------~------~~~~~~~L-----~------------~~ Garner East Kaysville 

Canyon Ogden (1984) 

FIGURE 12.-Fault displacements and times of fault events (in thousands of years) derived from trenching 

and dating studies at three sites along the Weber segment of the Wasatch fault zone (fig. 9). Timing at the 

East Odgen site is summarized from data in table A1 and figure 11. Letter designations for events and 

shading for the East Ogden (EO) site match those shown in figure 11. Age estimates based on 14C and TL 

analyses for faulting events are shown within patterned boxes. Shading patterns indicate only one of 

several reasonable event correlations from site to site. Because of limited age control, we cannot correlate 

events from site to site with certainty. If a small-displacement event d occurred at the East Ogden site, 

it may correlate with the last event at the Kaysville site, as suggested here. However, the last event at 

Kaysville (d) may also correlate with the last large event at East Ogden (event c). The last event at Garner 

Canyon is almost certainly the same event as the last large event at East Ogden (c). TL analyses from 

distal colluvium in trench E0-1 suggest that event bon that scarp is older than the penultimate event at 

Garner Canyon. Thus, the penultimate event at Garner Canyon does not seem to be recorded at East 

Ogden. Event a at East Ogden probably correlates with the first event (a) at Garner Canyon. In summary, 

at least three and probably four major fault events have occurred at each site in the past 5 ka, but 

uncertainties in correlation prevent us from determining whether the fault ruptured each site during each 

event. 

Personius, 1990, in press). However, fault scarps in this 

area are easily confused with nontectonic features such 

as shorelines, scarps related to large lateral spreads, and 

small landslide headscarps. For this reason, the distribution of Holocene and latest Pleistocene scarps is less 

certain here than it is elsewhere along the segment. The 

main trace of the fault extends south of North Canyon 

(fig. 9). Other prominent north-south fault scarps also 

cross the Bonneville and lower shorelines along the 

middle portion of the northern edge of the salient. These 

faults extend into the Tertiary bedrock of the salient and 

have moved since the fall of the lake from the Bonneville 

shoreline. Other smaller scarps south of Stone Creek 

that parallel the shorelines on the salient's northern edge 

indicate that northeast-southwest-oriented latest Pleistocene or Holocene faults also bound the northern edge of 

the salient. Although all fault scarps may not have been 

identified in this area, we conclude (1) that the northern 

part of the Salt Lake salient contains many small faults 

with both north-south and northeast-southwest orientations; (2) that some of these faults have had significant 

displacements in the past 15 ka but that many have not 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A29 

moved during the late Holocene; and (3) that, on the 

basis of scarp heights, latest Pleistocene and Holocene 

slip rates on faults near the salient are much lower than 

rates for the main trace of the fault along the remainder 

of the segment (Nelson and Personius, 1990, in press). 

As with the Pleasant View salient, which bounds the 

northern end of the Weber segment, these characteristics are typical of segment boundaries. 

SALT LAKE CITY SEGMENT 

The Salt Lake City segment of the Wasatch fault zone 

traverses the most populous part of Utah's Wasatch 

Front. Detailed mapping along the segment by Scott and 

Shroba (1985) and by Personius and Scott (1990, in press) 

indicated that most of the recent faulting is concentrated 

on the Warm Springs fault (west-bounding fault of the 

Salt Lake salient), on the East Bench fault (intrabasin 

fault and possible east-bounding fault of the Salt Lake 

salient), and on the main strand of the Wasatch fault zone 

(Cottonwood section of Personius and Scott, 1990, in 

press) that bounds the range front from Olympus Cove 

south to Corner Canyon on the northern flank of the 

Traverse Mountains (fig. 13). 

Scott and Shroba (1985), Scott (1988), and Personius 

and Scott (1990, in press) summarized evidence for 

Holocene movement along the Warm Springs fault. The 

prominent north-trending scarps of the Warm Springs 

fault crossed young fans along the western edge of the 

Salt Lake salient near Beck's Hot Springs before gravel 

mining destroyed the surface evidence of this part of the 

fault. Gilbert (1928) noted that these scarps were 10 to 14 

m high on two adjacent Holocene (post-Bonneville lake 

cycle) alluvial fans just south of Beck's Hot Springs (see 

Hunt, 1981, p. 27-29; Scott and Shroba, 1985, p. 6). The 

north-south trend of the scarps, their continuity (as 

shown on 1952 aerial photographs and old geologic 

maps), and their suspected Holocene age suggest that 

the Warm Springs fault is the main trace of the Wasatch 

fault zone in this area. Discontinuous scarps on the 

northwestern end of the salient appear to step eastward 

around the salient, filling a 2-km-wide gap between the 

northern end of the Warm Springs fault and the southern 

end of the Weber segment (Personius and Scott, 1990, in 

press; Nelson and Personius, 1990, in press). Between 

the southern end of the Warm Springs fault and the East 

Bench fault to the southeast (fig. 13), there is a 4-km gap 

in Holocene faulting, which coincidently is the location of 

downtown Salt Lake City. 

The East Bench fault is the northeastward extension 

of the range-bounding Wasatch fault zone, which departs 

from the range front just south of Olympus Cove and 

extends basinward into the southern part of Salt Lake 

City. Farther north, the East Bench fault turns sharply 

to the northeast near the Dresden Place site (DP, fig. 13) 

and trends toward the mountain front, where it loses 

definition in surficial deposits. The northern end of the 

East Bench fault may coincide with bedrock faults that 

bound the eastern side of the Salt Lake salient (fig. 13) as 

mapped by Davis (1983a). From Olympus Cove north to 

downtown Salt Lake City, there is no evidence of active 

range-front faulting, whereas Holocene movement is 

concentrated on the East Bench fault. 

In the fall of 1986, two deep exploratory trenches were 

excavated at the Dresden Place site in eastern Salt Lake 

City (DP, fig. 13; site 6, table 1). Although these 

trenches were open for inspection for only about 2 weeks, 

they have greatly increased our knowledge of this poorly 

studied part of the Wasatch fault zone. Inspection of the 

trenches by D.R. Currey (University of Utah) and M.N. 

Machette and mapping of the excavations by John Garr 

(written commun., 1987) revealed at least 7 m of deformation in sediment deposited during the transgressive 

phase of the Bonneville lake cycle (about 26-15 ka at this 

site). Of the 7 m of deformation, 3 m is expressed as 

monoclinal warping (plastic deformation) of deep-water 

sediment (laminated silt and clay). Currey suggested 

that, because plastic deformation probably requires saturation of the sediment, this deformation must have 

occurred before Lake Bonneville fell far enough below 

the altitude of the site (about 1,326 m or 4,350 ft) to cause 

dewatering of the sediment. The falling lake reached this 

altitude about 12.5 ka, as estimated from the hydrograph 

of Lake Bonneville that was constructed by Currey and 

Oviatt (1985, fig. 2). The rapid drawdown in Lake 

Bonneville and its successor's (Great Salt Lake) low level 

have caused the ground-water table at the Dresden Place 

site to be low throughout the Holocene. Therefore, the 

plastic deformation of the Lake Bonneville sediment 

must have occurred in latest Pleistocene time (15-10 ka, 

regressive phase of the Lake Bonneville cycle). At this 

site, the absence of colluvium within the plastically 

deformed lake beds suggests that the deformation 

occurred in a single tectonic event near or after deposition of the lake sediment but before dewatering 

occurred. 

Plastic deformation was followed by at least 4 m of 

brittle deformation, as indicated by planar fault ruptures 

extending to the top of the undisturbed sediment in the 

trenches. Although the brittle deformation must be 

younger than 12.5 ka, it seems likely that it corresponds 

with one or more episodes of Holocene movement on the 

East Bench fault, as indicated by mapping of faulted 

Holocene sediment in areas immediately south of Dresden Place (Scott and Shroba, 1985; Personius and Scott, 

1990, in press). 

South of Salt Lake City, results of detailed mapping, 

exploratory trenching, and 14C dating by Swan and 
A30 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

40°45' ~ 

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PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A31 

others (1980) at the mouth of Little Cottonwood Canyon 

(LCC, fig. 13; site 7, table 1) indicated several surfacefaulting events during the past 19 ka (assumed age of 

faulted till) and at least one event within the past 8 ka. 

Their study suggested an average slip rate of 0. 76 mm/yr 

( +0.6 mm/yr, -0.2 mm/yr), an average displacement of 2 

m per event and an average recurrence interval of 2,400 

to 3,000 years since the culmination of the transgressive 

phase of the Bonneville lake cycle (15 ka). However, the 

complex pattern of deformation in this part of the 

Wasatch fault zone prevented an accurate determination 

of the timing of the most recent event at the Little 

Cottonwood Canyon site. 

In 1985, at the South Fork of Dry Creek (SFDC, fig. 

13; site 8, table 1), the UGMS and USGS began a 

program of aerial photograph interpretation, scarp profiling, exploratory trenching, and 14C dating to obtain 

additional paleoseismic information on the Salt Lake City 

segment. (This site is referred to as the South Fork of 

Dry Creek because there is a second, larger Dry Creek 

northeast of Alpine, Utah, along the Provo segment of 

the Wasatch fault zone.) Interpretations of the stratigraphic and structural relations of the site (South Fork of 

Dry Creek) have been finalized by Lund and Schwartz 

(1987), but they have not yet published their final 

analysis of recurrence intervals and slip rates or data for 

the timing of the faulting events. 

The site at the South Fork of Dry Creek is about 2.5 

km south of Little Cottonwood Canyon, between the 

steep front of the Wasatch Mountains and the highest of 

the Bonneville shorelines. Here, the Wasatch fault zone 

has as many as six west-facing scarps that form a 

complex en echelon zone of deformation as much as 305 m 

wide (fig. 14). Unconsolidated surficial deposits between 

the mountain front and the Bonneville shoreline include 

(from youngest to oldest) Holocene debris-flow deposits, 

late Pleistocene or early Holocene alluvial-fan and 

debris-flow deposits, and small remnants of middle Pleistocene alluvial-fan deposits (Scott and Shroba, 1985; 

Lund and Schwartz, 1987; Personius and Scott, 1990, in 

press). Geomorphic surfaces associated with these 

deposits are dropped progressively down to the west 

across the fault zone. The cumulative displacement of 

these surfaces and their deposits becomes greater with 

<IIIII FIGURE 13.- Traces of the Salt Lake City segment, including the 

main Wasatch fault zone (WFZ), the East Bench fault (EBF), 

and the Warm Springs fault (WSF). Traces of the Taylorsville 

and Granger faults of the West Valley fault zone are also shown. 

Faults are shown by heavy lines having a bar and ball on the 

downdropped side. Also shown are important geographic features and towns. Trench and field sites are abbreviated as 

follows: DP, Dresden Place; BCC, Big Cottonwood Canyon; 

LCC, Little Cottonwood Canyon; SFDC, South Fork of Dry 

Creek; CC, Corner Canyon; DC, Dry Creek. 

0 

I 

0 

500 

I 

1000 

I 

I 

250 

1500 2000 FEET 

I I 

I 

500 METERS 

EXPLANATION 

chs Holocene hillslope colluvium derived from bedrock 

cd1 Upper Holocene debris flow deposits having narrow 

levees along South Fork of Dry Creek 

cd2 Middle to lower Holocene coalesced debris-flow 

deposits 

af2 Middle-to-lower Holocene alluvial-fan deposits 

lbs Uppermost Pleistocene Bonneville lake cycle 

lacustrine sands and gravelly sands 

af4 Middle Pleistocene alluvial-fan deposits 

rx Bedrock-Quartzite and quartz monzonite 

__t____ Fault scarp-Ball and bar on downthrown side 

Geologic contact 

1,...._.... Locations of trenches excavated by W.R. Lund and 

D.P. Schwartz 

FIGURE 14. -Surficial geology at the South Fork of Dry Creek site, 

Salt Lake City segment of the Wasatch fault zone (Lund and 

Schwartz, 1987). Geology modified from Scott and Shroba (1985) by 

W.R. Lund and D.P Schwartz (written commun. , 1988). Figure 13 

shows the location of this site (SFDC). (Photograph courtesy ofW.R. 

Lund, Utah Geological and Mineral Survey.) 

increasing age and thus indicates a history of recurrent 

faulting in the late Pleistocene and Holocene. 

The youngest deposits at the trench site are two upper 

Holocene debris flows; the northern one flowed several 

hundred meters downslope (west) from the mouth of the 

South Fork of Dry Creek, and a second one deposited a 

lobe at the mouth of Dry Gulch, the next canyon to the 

south (fig. 14). The debris flow along the South Fork of 

Dry Creek formed distinct levees, which were later 
A32 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

displaced by movement on five of the six faults that form 

laterally continuous scarps in the study area (fig. 14). 

The sixth, most basinward fault does not displace the 

levees but is buried by them, an indication that at least 

two discrete episodes of surface faulting occurred at the 

site. This relation is consistent with the results of Lund 

and Schwartz's exploratory trenching, which shows that 

both single- and multiple-event scarps are present along 

the faults at this site but that no more than two surfacerupture events were recognized in any trench (W.R. 

Lund and D.P. Schwartz, oral commun. , 1987). By 

comparing the amount of displacement in the levees with 

that in the trenches, Lund and Schwartz have shown 

that, for multiple-event scarps, only the most recent 

event displaced the levee; hence, the earlier faulting 

event must have occurred before deposition of the debris 

flow. In addition, the displacement observed on singleevent faults in the trenches corresponds closely with that 

measured from scarps on the levees. 

In 1985, W.R. Lund and D.P. Schwartz collected 

organic matter from buried A horizons exposed in the 

trenches. So far, they have obtained three AMRT ages 

from a buried soil that is formed on the older of two 

colluvial wedges exposed in the trenches. The older 

wedge was downdropped during the most recent faulting 

event and subsequently buried by a younger colluvial 

wedge. The first age obtained from the buried soil was 

1,890 14C yr B.P. (Lund and Schwartz, 1987). A second 

sample from the same soil (age) yielded an age that is 

significantly younger than the first determination 

(Schwartz and others, 1988) but in accord with a minimum age estimate of 900 years (Lund and Schwartz, 

1987) as determined from diffusion modeling of the 

morphology of the single-event fault scarp. Calendar (but 

not AMRT) calibration of these two 14C ages constrains 

the most recent event to between 1,130 and 1,830 cal yr 

B.P. (Schwartz and others, 1988). The third sample from 

the buried soil has recently yielded an AMRT date of 

about 1,500 14C yr B.P. (D.P Schwartz and W.R. Lund, 

written commun., 1989), which is near the midpoint of 

the earlier range of dates. Schwartz and Lund currently 

interpret the most recent faulting at the South Fork of 

Dry Creek to have occurred about 1.5±0.3 ka. 

We have found that 14C dating of organic matter from 

modern (surface) A horizons along the Wasatch fault 

zone commonly yields AMRT ages of several hundred 

years (see discussion in appendix); these ages reflect the 

accumulation and cycling of carbon in A horizons over 

time. Thus, AMRT ages from buried A horizons should 

predate the time of A horizon burial. 

The maximum time for the older event at the site is 

constrained by calendar-corrected 14C ages from soil 

organics of 5,455 and 5,975 cal yr B.P. (Schwartz and 

others, 1988) and a TL age estimate of 5,525±390 cal yr 

from the soil on the older, twice-faulted alluvial-fan 

deposits. These 14C dates (which do not consider the age 

of the soils at the time of burial) show that the faulted 

deposits are middle Holocene rather than latest Pleistocene and thus are similar in age to many other remnants 

of alluvial fans along the Wasatch Front. 

The site at the South Fork of Dry Creek records two 

large displacements (each about 4.5 m) during the past 

5.5 ka and a Holocene slip rate that exceeds 1 mm/yr 

(4.5-m displacement in as little as 3, 700-4,400 years). At 

present, the recurrence interval on this segment of the 

Wasatch fault zone appears to be rather long (that is, 

3,500-4,000 years) if one assumes that the younger event 

is 1.5±0.3 ka and the earlier event is middle Holocene 

(5.0-5.5 ka, the preferred range). An important conclusion of Schwartz and Lund's trenching is that the fault 

scarps are the product of either one or two large displacements rather than many small displacements closely 

spaced in time. In addition, the observation that similarsized displacements were repeated at this site on the 

Wasatch fault zone fits Schwartz and Coppersmith's 

(1984) model of characteristic fault behavior. 

Keaton and others (1987) and Keaton and Currey 

(1989) have completed a study of the West Valley fault 

zone (fig. 13), part of which lies about 5 km west of Salt 

Lake City. Although this fault zone is 8 to 10 km west of 

the East Bench fault of the Wasatch fault zone, discussion of the West Valley fault zone is included here 

because it is a major (down-to-the-east) fault zone that 

may be related to the adjacent Wasatch fault zone. 

The West Valley fault zone strikes north-northwest of 

the Jordan River in the north-central part of Salt Lake 

County (fig. 13) (Personius and Scott, 1990, in press). 

Two principal surface traces of the fault zone-the 

Granger and Taylorsville faults-were identified in the 

early 1960's by Marsell and Threet (1964). Keaton and 

others (1987) found that the two principal faults each 

have evidence of Holocene movement. Numerous previously unmapped but suspected fault scarps and lineaments were found as far as 9 km north of the limits of the 

fault zone as recognized by Marsell and Threet (1964) 

(see Keaton and others, 1987, figs. 2, 18). These suspect 

features extend the West Valley fault zone north to the 

vicinity of Salt Lake City International Airport (fig. 13); 

as such, the fault zone forms an area of deformation 

about 7 km in width and 16 km in length. 

The Granger fault (site 19, table 1) forms the westernmost of the two scarps of the West Valley fault zone and 

shows evidence of 4.5 to 12.2 m of normal slip (down to 

the east) within the past 12 ka; these data yield a 

minimum slip rate of 0.4 to 1.0 mm/yr (Keaton and 

others, 1987). Faulted sediment of a previous lake cycle 

(probably the Cutler Dam lake cycle of Oviatt and others 

(1987)) was found by Keaton and others (1987) on the 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A33 

TABLE 4.-Timing and number of surface-faulting earthquakes and recurrence intervals for Holocene movement on the Wasatch fault zone 

[All values for age and time are rounded to the nearest 50 years] 

A B c D 

Number of faulting events 

Segment (no.) Site 

Age of oldest faulting 

event or datum 

(cal yr B.P.) 

Estimated time of 

most recent faulting 

(cal yr B.P.) 

Time between and intervals 

A and B 

Cyd Events Intervals 

Brigham City (1)..................... Trench BC-1 24, 700±500 3,600±500 1,100±1,000 2 1 

Weber (2)........................... East Ogden 2

3,750±250 500±300 3,250±550 4 3 

Salt Lake City (3).. . . . . . . . . . . . . . . . . . . Dry Creek 25,250±250 1,500±300 3,750±550 2 1 

Provo (American Fork part) (4) . . . . . . . AF-1, AF-2 2

5,300±300 500±200 4,800±500 3 2 

Nephi (5) . . . . . . . . . . . . . . . . . . . . . . . . . . . North Creek 3

5,300±200 4400 4,900±200 3 2 

Levan (6) . . . . . . . . . . . . . . . . . . . . . . . . . . . Deep Creek 3

7,300 1,000 5 1 0 

Total (based on segments 1-5) ................................................... . 17,800±2,800 15 9 

Calculated recurrence interval6 (yr) 

Average recurrence 7 on: 

Single segment of the Wasatch fault zone................ 1,980±310 

All segments of Wasatch fault zone having 

repeated Holocene movement (CRI)8 •••••••••••••••••• 395±60 

1

Time intervals for some sites include time between the oldest event at a site 

and the age of the datum; thus, some values in this column are maximums. 2 Age of oldest recorded faulting event. 3 Age of faulted datum estimated from dating and stratigraphic and tectonic 

considerations. 4

N o error limits assigned to this estimate. 5

Not applicable. Only one faulting event was recorded, so no recurrence 

interval can be calculated. 

footwall of an exploratory trench and in drill holes on the 

hanging wall. The calculated displacement of the sediment ranges from 12.2 to 19.8 m; Keaton and others 

(1987) assigned an age of 58 ka to the lacustrine beds on 

the basis of correlation with the end of marine isotope 

stage 4, which should have been represented by a pluvial 

episode in the Bonneville Basin. The resulting average 

slip rates for the Granger fault since the end of the Cutler 

Dam lake cycle are 0.21 to 0.34 mm/yr. In addition, 

Keaton and others (1987) suggested that slip rates on the 

Granger fault may have been about 0.1 mm/yr since the 

culmination of the last major pre-Bonneville lake cycle 

(about 130 ka). These estimates are derived from the 

projected offset of lacustrine beds penetrated by drill 

holes on both sides of the fault. 

The markedly different average slip rates recorded by 

pre-Bonneville lake cycle and post-Bonneville lake cycle 

(less than 12 ka) suggested to Keaton and others (1987, p. 

25) that the deep water of Lake Bonneville may have 

suppressed strain release along the West Valley fault 

zone, whereas strain release may have been accelerated 

after Lake Bonneville started to drain at about 14.5 ka. 

Similar evidence and interpretations of large changes in 

average slip rates before and after Lake Bonneville's 

culmination have been suggested for a number of points 

along the Wasatch fault zone (Machette, 1984; Machette 

and others, 1986) by comparing the net displacement in 

late Pleistocene and Holocene deposits of different ages 

6

Three significant figures are used to compute average values of recurrence 

from the mean of the sum of column C. Reported values are rounded to the 

nearest 5 years. 7 Average recurrence interval is determined by dividing the sum of time in 

column C by the sum of intervals between faulting events in column D. 8

Composite recurrence interval (the recurrence interval for movement on any 

of segments 1-5). 

(see "Discussion of Slip-Rate Data for the Wasatch Fault 

Zone" and the accompanying fig. 21). 

The Taylorsville fault (site 20, table 1), the easternmost of the two topographic escarpments of the West 

Valley fault zone, was formed by 1.5 m of monoclinal 

flexure (down to the east) and laterally discontinuous 

faulting in the past 5 to 7 ka (Keaton and others, 1987; 

Keaton and Currey, 1989). The resulting uplift (deformation) rate across the Taylorsville fault has been 0.21 to 

0.30 mm/yr, which is comparable to the longer term (58 

ka) values determined from the adjacent Granger fault. 

Keaton and others (1987) speculated that the Taylorsville 

fault may be the surface expression of a fault that has 

earthquakes just slightly above the threshold magnitude 

(taken as M8 6.5) that would create a continuous surface 

rupture along a discrete fault plane. Their seismological 

analysis (table 4) suggests that earthquakes as large as 

M8 7.0 could be reasonable for a surface rupture having 

a displacement of as much as 1. 5 m along the West Valley 

fault zone. Such faulting would have had an average 

recurrence interval of 1,000 to 4,000 years on the 

Granger fault and 5,000 to 7,000 years on the Taylorsville 

fault during the past 15,000 years. 

The west-trending Traverse Mountains form a major 

bedrock salient at the southern end of the Salt Lake 

Valley and are the boundary between the Salt Lake City 

and Provo segments of the Wasatch fault zone. The 

Traverse Mountains are one of four such salients that 
A34 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

form major structural barriers along the Wasatch fault 

zone (Wheeler and Krystinik, this volume). 

Scott and Shroba (1985) and Personius and Scott (1990, 

in press) traced surface ruptures of the Salt Lake City 

segment along the Wasatch Front as far south as Corner 

Canyon on the northern side of the Traverse Mountains, 

where the fault scarps turn from a south to an eastsoutheast strike. In this area, the Wasatch fault zone 

forms a broad gouge zone into which Corner Canyon has 

been excavated. From Corner Canyon (CC, fig. 14), the 

Wasatch fault zone turns eastward and dips shallowly to 

the south (Bruhn and others, 1987), where it separates 

intrusive rock of the Little Cottonwood stock (middle 

Tertiary) from late Tertiary fanglomerate (Salt Lake 

Formation), extrusive volcanic rock (middle Tertiary), 

and the underlying platform of the upper Paleozoic 

Oquirrh Formation (Personius and Scott, 1990, in press). 

This geology differs from the mapping done by Davis 

(1983a), who mapped the fanglomerates as Oquirrh Formation), and indicates more structural relief than previous work had suggested. Machette (1989, in press) has 

traced the Wasatch fault zone continuously across the 

salient from Corner Canyon to the mouth of Dry Creek 

(DC, fig. 15), where uppermost Pleistocene glacial outwash is displaced 3 to 5 m (this Dry Creek is about 3 km 

northeast of Alpine, Utah). Upper Quaternary alluvium 

at the head of the Traverse Mountains (salient) has been 

offset by the Wasatch fault zone, an indication that the 

salient is being actively deformed, albeit at an appreciably slower rate than the segments to the north and south. 

The major bend in the Wasatch fault zone in this area, 

named the Fort Canyon fault by Bruhn and others 

(1987), corresponds to the westward projection of the 

south-dipping Deer Creek fault (DCF, fig. 15), which is 

the northern limb of an east-vergent Laramide thrust 

fault. These transverse structures allow the Wasatch 

fault zone to step 8.5 km to the east (left) across the 

Traverse Mountains. 

PROVO SEGMENT 

Schwartz and Coppersmith (1984) suggested that the 

section of the Wasatch fault zone from the TraverseMountains south to Payson Canyon is a single segment 

and named it the Provo segment. This segment, reported 

as 55 km long, was one of the longest of the fault zone. In 

1986, as a result of his preliminary mapping and scarp 

morphology studies in the Utah Valley, Machette tentatively subdivided the original Provo segment (which he 

measured at 70 km long) into the American Fork, Provo 

(restricted sense), and Spanish Fork segments 

(Machette and others, 1986). Since then, detailed mapping and trenching at three new sites have indicated that 

Schwartz and Coppersmith's (1984) suggestion of one 

segment is probably correct. Thus, in the following 

discussions, the American Fork, Provo (restricted 

sense), and Spanish Fork segments of the fault are 

considered to be parts of the Provo segment for the 

purpose of discussion. In this sense, they are much like 

the geometrically defined segments of de Polo and others 

(1989, 1991). 

All of the data from exploratory trenching along the 

Provo segment are compatible with earthquakes that 

rupture the whole 70-km-long segment. Although this 

length of rupture seems long from historical and paleoseismic perspectives, we cannot disprove complete rupturing during M>7 earthquakes. 

AMERICAN FORK PART 

The American Fork part of the Provo segment forms 

the northeastern border of the Utah Valley and is short 

in comparison with most of the proposed segments along 

the central, active part of the Wasatch fault zone (about 

22.5 km as opposed to an average of 48 km) (table 2). The 

Wasatch fault zone has an en echelon 2-km left step at the 

Provo River, which Machette considered to be the probable southern boundary of the proposed American Fork 

segment. This proposed segment boundary was partly 

supported by Zoback's (1983) compilation of gravity for 

the region, which shows a prominent gravity saddle (a 

local high) within the Utah Valley adjacent to Provo 

River Canyon, between strong lows to the north and 

south that probably represent subbasins of the Utah 

Valley. 

Most of the paleoseismic data for this part of the Provo 

segment comes from a trenching site near American 

Fork Canyon, where the Quaternary geology had been 

mapped at a scale of 1:10,000 by Machette (1989, in 

press). The American Fork Canyon site (AFC, fig. 15) is 

characterized by a large, well-preserved fan-delta complex that was constructed during the transgressive phase 

of the Bonneville lake cycle. 

A widespread but thin mantle of calcareous loess was 

deposited across the landscape as newly exposed sediment of the Bonneville lake cycle was eroded by the 

FIGURE 15.-Main trace of the Provo segment of the Wasatch ...... 

fault zone (WFZ) between the Traverse Mountains and Dry 

Mountain, showing the American Fork part (to the north), the 

central part, and the Spanish Fork part (to the south). Faults 

are shown by heavy lines having a bar and ball on the downdropped side (dotted where uncertain). Also shown are important geographic features and towns. Trench and field sites are 

abbreviated as follows: CC, Corner Canyon; DC, Dry Creek; 

AFC, American Fork Canyon; RC, Rock Creek; HC, Hobble 

Creek; MN, Mapleton North; MS, Mapleton South; WC, Water 

Canyon. Faults are abbreviated as follows: WH, Woodland Hills 

splay; SF, Springville (near the middle of the Provo segment); 

BF, Benjamin (northern end of Nephi segment). 
40°15' 

PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE 

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15 KILOMETERS 

A35 
A36 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

wind. This process probably continued from about 13 ka 

well into the early Holocene. Overlying the loess are local 

alluvial fans that consist mainly of coalesced debris-flow 

deposits and alluvium of early(?) to middle Holocene age. 

Textural analysis of the matrix of the debris flows (M. N. 

Machette, unpub. data, 1987) shows that they were 

derived from the loess mantle, which now has been 

completely eroded from the mountainous slopes. An age 

of 7,290±100 14C yr B.P. (AA-2268, table A1) was 

obtained from charcoal in the loess in trench AF -3 (see 

following trench discussion), whereas an age of 4, 740±90 

14C yr B.P. (AA-2266, table A1) was obtained from 

charcoal in debris flows that overlie the loess. These two 

age determinations correspond to calendar-calibrated 

dates of 8,061 and 5,518 cal yr B.P. (the respective 

one-sigma error limits are generally ±200 years) (table 

A1). The younger date comes from charcoal in a burn 

layer between flows about 1 m below the surface of the 

debris-flow package and may predate the surface of the 

alluvial fan by 100 years or less (there are no intervening 

soils). Therefore, the 14C dates constrain the timing of a 

major phase of debris-flow deposition and associated fan 

building at the American Fork site from about 8. 0 to 5.4 

ka-that is, from early to middle Holocene. 

The American Fork part of the Provo segment is 

marked by a prominent set of scarps at the foot of the 

Wasatch Range, both north and south of American Fork 

Canyon (Machette, 1989, in press). Repeated movement 

across the fault zone has produced progressively more 

net displacement in middle to late Pleistocene deposits 

than it has in latest Pleistocene (Bonneville lake cycle) 

and Holocene deposits. The fan-delta complex at American Fork Canyon was abandoned about 14.5 ka during 

the fall to the Provo shoreline level. South of the canyon, 

gravel of the highest Bonneville shoreline is displaced 15 

to 20 m across a 50-m-wide graben. North of the canyon, 

gravel at the same level is displaced 26 m across a large 

scarp lacking a graben. Maximum and minimum values of 

net displacement of the gravel yield maximum and 

minimum slip rates of 1. 7 and 1.0 mm/yr for the past 14.5 

ka. In contrast, the long-term slip rates recorded by 130-

to 250-ka alluvium along this part of the Provo segment 

and other segments of the central Wasatch fault zone are 

only 0.1 to 0.3 mm/yr (Machette, 1984). These long-term 

slip rates are almost an order of magnitude less than slip 

rates recorded since Lake Bonneville fell from its highest 

level14,500 years ago (see "Discussion of Slip-Rate Data 

for the Wasatch Fault Zone"). 

In 1986, Machette and Lund excavated three 25- to 

50-m-long trenches across the Wasatch fault zone at the 

American Fork Canyon site. All three trenches were 

located on faulted alluvial fans that consist primarily of 

debris-flow deposits, the surface of which we consider to 

have become stabilized by about 5.4 ka on the basis of the 

previously mentioned 14C ages (table A1). The trenches 

showed conclusively that the middle Holocene deposits 

commonly are displaced 7 to 8 m by three faulting events 

where the fault zone has a simple geometry of single or 

multiple parallel normal faults (Machette and Lund, 

1987). 

Trench AF -1, the deepest and longest of the trenches, 

was placed across the 8-m-high main fault scarp and 

penetrated a relatively simple stack of three colluvial 

wedges, each of which was derived from erosion of 

material newly exposed in the scarp during faulting (fig. 

16). In turn, each wedge is separated by thick, welldeveloped A horizons and weakly developed calcic A 

horizons (Ak) or Ck horizons. The uppermost buried soil 

in trench AF-1 (sample F86-U8, fig. 16) yielded a TL 

age estimate of 400± 100 cal yr (ITL-23, table A1). 

Organic carbon from the upper part of the A horizon of 

the same soil (sample AF-1C, fig. 16) yielded a 14C age of 

980±70 14C yr B.P. (USGS-2532, table A1). The calibrated AMRT age for the A horizon sample is 713 cal yr 

B.P. (the range is 620-830 yr B.P.). This date and the TL 

age estimate place a maximum limit on the time of most 

recent faulting. However, we estimate an age of about 

0.5 ka for the burial of the upper horizon contact (UHC) 

of the soil (see table A1) by inferring a mean-residence 

correction (MRC) of 200 years at the time of burial. In 

addition, charcoal within the youngest fault-scarp colluvium (sample AF-1A, fig. 16) was dated at 140±120 14C 

yr B.P. (AA-2267, table A1); it has a permissible calendric age range of modern (post-1950) to 480 cal yr B.P. 

owing to three possible dendrochronologic dates. We 

consider 207 cal yr B.P. (the mean value of the three 

dates) as an absolute minimum time limit for the most 

recent event, owing to the position of the charcoal within 

the wedge and to the possibility of contamination by 

modern carbon (see prior discussion of ''Weber Segment"). 

Dating of samples from trench AF -2 also yielded a 

combination of TL and 14C ages that places additional 

constraints on the time of most recent faulting (fig. 17). 

Trench AF -2 crossed an east-facing scarp that is antithetic to the main fault scarp, which here is nearly 10 m 

high. These two scarps form a graben that is about 25 m 

wide and 50 m long. Faulted sag-pond silt that is buried 

by the youngest fault-scarp colluvium has a TL age 

estimate of 500±200 cal yr (ITL-3, table A1), whereas 

the upper 5 em of a thick organic-rich buried A horizon 

immediately below the silt has an age of 620± 150 14C yr 

B.P. (USGS-2533, table A1). 

The lack of coarse clasts in the silt layer in the sag pond 

indicates that it is an eolian accumulation; therefore, the 

sediment must have been fully exposed to sunlight and 

should have had no TL signal at the time of burial by the 

youngest colluvial wedge. The age determined for the silt 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A37 

EAST 

8 

7 

en 

a: 6 w 

1-

w 

::2!: 

~ 5 

~' 

:::::> 

1-

< 4 0 

0 

a: 

w 

N 

w 3 

> 0 

m 

< 

w 2 

u 

z --- j:! 

en 

i5 1 

0 

35 

0 

-- 0 

40 

WEST 

EXPLANATION 

mm A horizon 

1-3 

4 

7-8 

10 

AF-1A 

F86-U7)!p 

Colluvial wedges 

Holocene debris-flow deposits 

Old slope and scarp colluvium 

Transgressive Bonneville gravels 

Organic matter and charcoal 

samples 

Thermoluminescence samples 

45 

DISTANCE FROM TRENCH HEADWALL, IN METERS 

FIGURE 16. -Part of the log of trench AF-1 at the American Fork Canyon site, Provo segment of the Wasatch fault zone. Diagram shows the 

relation of three colluvial wedges and soils (units 1-3) to units on the upthrown and downdropped fault blocks on the southern wall of the 

trench. Field sample numbers refer to dating control shown in table Al. 

should approximate the time at which the sag pond was 

buried and, thus, date the time of the most recent 

faulting event. The buried A horizon has a thick accretionary profile that must have required a long time to 

form (that is, probably many hundreds to perhaps a 

thousand years). However, we estimate that the carbon 

in the upper 5 em of the accretionary A horizon has an 

estimated age span (CAS) of only about 150 years (table 

A1). The AMRT age of the A horizon sample is 620±150 

14C yr B.P. (USGS-2533, table A1), which yields a 

calibrated date of 612 cal yr B.P. (the age range is 

360-830 cal yr B.P.). Because we sampled only the 

uppermost 5 em of the buried A horizon, we subtract 100 

years for the MRC of the upper soil contact (table A1). 

Using the calibrated AMRT date and MRC, we estimate 

an age of about 0.5 ka for the UHC of the buried A 

horizon. 

The combination of two 14C ages and two TL age 

estimates from trenches AF -1 and AF -2 provides four 

constraints on the time of the most recent faulting at 

the American Fork Canyon site. The calibrated AMRT 

ages of 713 and 612 cal yr B.P. provide maximum limits. 

The TL age estimates, which should also provide close 

maximum dates for the time of faulting, are 400±100 and 

500±200 cal yr (fig. 17). Thus, our age estimates for the 

buried soil and silt suggest a probable age of 0.5 ka for 

the UHC of the soil formed on colluvial wedges that were 

downfaulted and buried during the most recent faulting 

event. 

Interpretation of the error limits for the AMRT ages is 

problematic, owing to corrections for the MRC of the 

upper soil contact. The analytical errors associated with 

the determinations carry over to the UHC age estimates, 

but all of the ages appear to fall within overlapping error 

limits. However, the accuracy ofTL age estimates of less 

than 1,000 years is suspect (S.L. Forman, oral commun., 

1988) because of low light levels measured in the laboratory and potential errors in determining equivalent doses 

(see discussion in appendix). Therefore, in this young age 

range, we place less importance on the TL age estimates 

(400 and 500 yr) and more importance on the two UHC 

age estimates of 500 cal yr B.P. On this basis, we 
A38 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

t C/) 0 

LO ..J (¥) 

<( +I 

> 0 a: LO w .... 1- N 

z 

w t u 

z t w 0 a: LO a: (¥) 

:::> +I u 0 w LO a: <0 

N 

t 

Colluvial wedge 

Colluvial wedge 

Event 2 

2650±150 ~ 

years ago 

Colluvial wedge 

Event 1 

5300±200 ~ 

years ago 

Debris flow 

Eolian silt 

TRENCH AF-1 

(2.6 ka) 

Weak soil 

(ca. 5.3 ka) 

TRENCH AF-2 

TRENCH AF-3 

~- ----------- r:-7290±100 (14C) (8.1 ka)-_-_-:.. 

... 

~===================~ ------7700±300 (TL).:-_-_-:. 

,.:---_-------_-_-_------:, 

FIGURE 17.-Times of faulting deciphered from three trenches at the American Fork Canyon site, Wasatch fault zone. 

Cited age estimates (in thousands of years) are derived from thermoluminescence (TL) and radiocarbon e4

C) ages listed 

in table Al. AMRT, apparent mean residence time of soil organic carbon. 

suggest using 500±200 years as the probable time of the 

most recent faulting at American Fork Canyon (fig. 17). 

Evidence for and dating of the penultimate faulting 

event is clearest in trench AF-1 (fig. 16). Samples for TL 

and 14C analysis were collected from the upper part of 

the buried soil that formed on the third (and oldest 

exposed) colluvial wedge in the trench. These samples 

provided maximum limiting dates on the time of burial of 

the soil. Sample F86-U7 yielded a TL age estimate of 

2, 700±200 cal yr (ITL-16, table A1), whereas the 14C 

AMRT age from the same soil (sample AF-1B) is 

2,620±70 14C yr B.P. (USGS-2531, table A1). This 

AMRT determination results in a calibrated date of 2, 777 

cal yr B.P. (the range is 2,549-2,879 cal yr B.P.). We 

estimate an age of about 2.6±0.2 ka for the 14C-dated 

UHC by subtracting 200 years (MRC at the time of 

burial) (see table Al). Because these two age estimates 

(2. 6 and 2. 7 ka) are so close and fall within concordant 

error limits (about 200 years each), we use the midpoint 

of the two age estimates to place the second faulting 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A39 

event at 2,650 yr B.P. (see fig. 17). We use an error limit 

of 250 years (2,400-2,900 yr B.P.), which includes errors 

associated with both the TL and the 14C ages. The 

recurrence interval for the most recent faulting event 

(500±200 yr B.P.) and the second faulting event 

(2,650±250 yr B.P.) is 2,150±450 years. 

The timing of the first (oldest) faulting event recorded 

in trench AF -1 is constrained by a 14C accelerator date 

on charcoal from within debris-flow deposits, which are 

buried by the third and stratigraphically lowest colluvial 

wedge. Calibration of the charcoal's age determination of 

4, 740±90 yr B.P. (AA-2266, table A1) resulted in three 

possible calendar dates: 5,467, 5,534, and 5,553 cal yr 

B.P. The mean of these dates is 5,518 cal yr B.P., but the 

associated errors create a possible range of from 5,289 to 

5,721 cal yr B.P. Assuming an age of 5.5±0.2 ka for the 

charcoal, we infer that the surface of the overlying debris 

flow was buried by a wedge of fault-scarp colluvium 

about 5.3±0.3 ka. This age estimate includes 200 years 

for the burial of the charcoal (a surface burn layer), 

deposition of the overlying 1.2-m-thick debris-flow surface, and formation of a very weak (incipient) A horizon 

on its surface (the oldest colluvial wedge buried nearly 

fresh debris-flow material). If our estimate of timing is 

reasonable for the first (oldest) event recorded in AF-1, 

then the recurrence interval between the first (5.3±0.3 

ka) and the second (2.65±0.25 ka) events is about 2,650 

years; as in the younger interval, 450 years seems to be 

a reasonable (cumulative) error limit. 

Trenching of the fault scarps on Holocene deposits 

shows that three surface-faulting events contributed to 

the 7 to 8 m of net displacement recorded since 5.3 ka. 

Reconstructing the volume of each colluvial wedge indicates that these events had similar (2.2-2. 7 m) amounts 

of displacement. The three Holocene events each produced discrete colluvial wedges, although only trench 

AF -1 showed evidence of all three events. Trench AF -2 

records. the most recent (0.5 ka) and probably the second 

(2.65 ka) faulting events, whereas trench AF-3 records 

the second and an earlier (greater than 5 .. 3 ka, less than 

0.8 ka) event not seen in trenches AF-1 or AF-2. The 

amount of displacement and distribution of this faulting 

event (in the other trenches) is unknown. 

The preliminary analysis of paleoseismicity at the 

American Fork Canyon site relies on both conventional 

and accelerator-method 14C dating of charcoal and soilorganic carbon (AMRT dates), TL analysis, and detailed 

mapping of lithologic and stratigraphic units. The results 

form the basis for an interpretation of three Holocene 

fault events at about 0.5, 2.65, and 5.3 ka. The strength 

of development of the soils on each colluvial wedge in the 

trenches (fig. 16) supports the early suggestion by 

Machette and Lund (1987) that faulting must have 

occurred over long intervals (that is, thousands rather 

than hundreds of years). If the most recent surfacefaulting event is correctly dated at 0.5 ka and if 5 to 6 m 

of displacement occurred since 5.3±0.3 ka (estimated 

time of burial of unit 4) (fig. 16), then two major faulting 

events occurred during an interval of 4,800±500 years. 

The resulting slip rate since middle Holocene time is 

between 1.0 and 1.4 mm/yr (5-6 m/4,800±500 yr), which 

is consistent with the range of slip rates (1.0-1. 7 mm/yr) 

determined for the past 15 ka from the adjacent faulted 

Bonneville lake cycle deposits. These values confirm that 

the rate of slip on the Wasatch fault zone in the northern 

part of the Utah Valley has been relatively high since the 

culmination of the Bonneville lake cycle and especially 

during the later part of the Holocene. 

CENTRAL PART 

The central part of the Provo segment is the middle of 

three segments that Machette and others (1986, 1987) 

subdivided from the original Provo segment of Schwartz 

and Coppersmith (1984). The central part extends from 

Provo Canyon (where its most recent scarps are in 

bedrock) south to and including the Springville fault (SF, 

fig. 15) at Springville, Utah, a distance of 18.5 km. South 

of Little Rock Creek (3 km north of Rock Creek) (RC, 

fig. 15), the active trace of the Wasatch fault zone is 

primarily in surficial deposits along the western base of 

the Wasatch Range. 

Between the canyons of the Provo River and Little 

Rock Creek, the western front of the Wasatch Range is 

characterized by numerous large but poorly recognized 

Quaternary landslides. We believe that Davis' (1983b) 

compilation of the geology of this area incorrectly shows 

large blocks of upper Paleozoic rock (Oquirrh Formation) 

thrust eastward and uplifted in the Wasatch mountain 

block. In contrast, Machette (1989, in press) indicated 

that exposures in much of the "Oquirrh-like" material are 

comprised of fanglomerate facies of the Salt Lake Formation (upper Tertiary) or of large landslide masses of 

Oquirrh Formation that have slid on upper Paleozoic 

Manning Canyon Shale along steep parts of the range 

front. Extensive upper Tertiary(?) and Quaternary landslide deposits that predate the Bonneville lake cycle also 

are present in this area; these deposits have been derived 

largely from the catastrophic landslide failure of the 

Manning Canyon Shale. These landslide deposits and the 

Salt Lake Formation are locally preserved in an overlapping left step of the Wasatch fault zone at an intermediate structural level; usually, these deposits are deeply 

buried in the basins and eroded from the ranges. 

Only one exposure along the central part of the Provo 

segment of the Wasatch fault zone has been studied in 

detail. In 1986, Machette and William Mulvey of the 
A40 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

UGMS mapped a natural exposure of a single fault on the 

southern bank of the modern drainage from Rock Canyon, which is at about the midpoint of the central part, 

and measured about 2 m of net displacement (1.5 m of 

tectonic displacement, 0.5 m of drag) in middle(?) to 

upper Holocene debris flows and alluvium. The resulting 

scarp has been partly buried by later debris flows and 

alluvium. Radiocarbon dating of soil-organic matter in 

the fault-scarp colluvium yielded an AMRT age of 

1,110±50 14C yr B.P (DIC-3236, table A1). Two factors 

need to be considered before interpreting this date. 

First, soil organics in the colluvium that was dated may 

span several hundred years, so this amount of time 

should be considered in estimating the time of faulting. 

Second, some of the organic matter was derived from the 

soil on the upthrown fault block and thus may have added 

a significant amount of old carbon to the sample; this 

reworked carbon may have increased the 14C age by 

several hundred years. Thus, the calibrated AMRT age 

of 1,005 cal yr B.P. (range is 950-1,150 cal yr B.P.) (see 

table A1) could reflect the approximate time of faulting. 

However, this date probably has a much larger uncertainty owing to reworking or accumulation of soil organics. 

Control on the minimum time of faulting at the natural 

exposure was provided by a 14C age from a soil that had 

formed on the fault-scarp colluvium before burial by 

upper Holocene fluvial and debris-flow deposits. This 

organic-rich soil postdates the faulting and predates the 

burial of the fault scarp. Organic carbon from this A 

horizon yielded an AMRT date of 455±35 14C yr B.P. 

(PITT-0091, table A1), which corresponds to a calibrated 

AMRT age of 512 cal yr B.P. (the range in age is 457-552 

cal yr B.P.). We estimate a MRC of 100 years for the soil 

at the time of burial (see table A1) and suggest an age, 

although poorly constrained, of about 0.4 ka for burial of 

the UHC of the soil and the adjacent fault scarp. These 

preliminary investigations suggested that the most 

recent faulting at Rock Canyon occurred since about 1.0 

ka and that the resultant scarp was buried by alluvium at 

about 0.4 ka. 

The poor dating control described above and the 

possibility of nonsynchronous movement on three short 

parts of the Provo segment prompted further studies at 

the Rock Canyon site. In the spring of 1988, W.R. Lund 

(UGMS) and D.P. Schwartz (USGS) trenched a 4.5-

m-high scarp on alluvium just north of Rock Canyon 

Road, about 100 m south of the stream exposure. In 

addition, they cleaned off the stream embankment with a 

backhoe and exposed a previously unmapped splay of the 

fault. These two fault splays merge to the south to form 

the scarp that was trenched by Lund and Schwartz. 

Preliminary mapping of the trench (W.R. Lund and 

D.P. Schwartz, oral commun., 1989) revealed a single 

colluvial wedge and, thus, evidence for only one faulting 

event. Although the size of this scarp ( 4. 5 m) had 

suggested two or three faulting events (Machette and 

others, 1987), Lund and Schwartz found that backtilting 

and graben formation account for the 2-m difference 

between stratigraphic offset (about 2.5 m) and scarp 

height. Although no charcoal was found in the trench, 

they sampled several organic soils for bulk (AMRT) 14C 

dating, all of which should predate the most recent 

faulting. At the new exposure along the stream channel, 

14C dating of both charcoal from above and soil organics 

from below the colluvial wedge should allow them to 

place tighter limits on the time of most recent faulting. 

The central part of the Provo segment is characterized 

by multiple parallel to anastomosing strands of the fault 

zone, and rarely is there a single fault strand on which 

most of the movement has taken place. The pattern of 

ground rupture is complicated south of Slate Canyon 

(due east of Provo) (fig. 15), where the Wasatch fault 

zone offsets transgressive gravel of the Bonneville lake 

cycle and regressive gravel at and below the Provo level. 

The gravel is draped across steeply sloping alluvial-fan 

deposits that predate the Bonneville lake cycle. The 

history of faulting in this area is difficult to decipher for 

three reasons: (1) existing fault scarps (pre-Bonneville 

lake cycle) were modified by shore processes during the 

rise and fall of Lake Bonneville, (2) most of these faults 

were reactivated by Holocene movement, and (3) much 

of the gravel cover has been disturbed by quarrying or 

by regrading of abandoned gravel pits or has been 

covered by homes. 

The southern end of the central part is considered to be 

the Springville fault (SF, fig. 15), a basin ward splay of 

the Wasatch fault zone that forms a 3-km-long fault scarp 

on Holocene alluvium of Hobble Creek. The fault extends 

from just east of the State Fish Hatchery (1 km north of 

the northern edge of Springville) southward to Utah 

Highway 77. Scarps along this fault are less than 2 m 

high and, within Springville, have been extensively 

modified. Bissell's (1963) map shows another fault scarp 

about 1 km south of Springville. However, we agree with 

Miller's (1982) decision not to map the feature as a fault; 

it appears to be both a part of a young Provo shoreline 

and the head of a massive lateral spread (see Machette 

1989, in press). 

SPANISH FORK PART 

The Spanish Fork part of the Provo segment is 

restricted to the Wasatch fault zone between Springville 

and Payson Canyon. It is the southernmost of three 

segments that Machette and others (1986, 1987) subdivided from Schwartz and Coppersmith's (1984) original 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A41 

Provo segment. The Spanish Fork part forms a major 

concave-to-the-west bend in the Wasatch fault zone, the 

only such prominent bend along the entire fault zone. 

The Spanish Fork part is bounded on the north and west 

by the central part of the Provo segment, which extends 

into the basin as the Springville fault (SF, fig. 15). The 

lack of a salient or a spur at the intersection of the Provo 

and Spanish Fork parts indicates that there is no prominent deficit or change in structural throw along the fault 

zone. Thus, if this intersection is a structural boundary, 

it must be a nonpersistent feature- one that may only 

intermittently or occasionally control the starting or 

stopping points of surface rupturing. On the south, the 

Spanish Fork part of the Provo segment is bounded by 

the Payson salient (Dry Mountain) and the northward 

extension of the Nephi segment (the Benjamin fault of 

Hintze (1973)) (BF, fig. 15). The Payson salient represents a persistent structural barrier. The Benjamin and 

Springville faults both have down-to-the-west displacement, which suggests that they could connect in the 

subsurface of the southern Utah Valley. However, 

Zoback's (1983) gravity data suggest that the deep part 

of the basin extends, uninterrupted, from east of Spanish 

Fork northwest into the area of Utah Lake across the 

alignment of the two faults. Thus, it appears that the 

Springville and Benjamin faults must die out as they 

extend into Utah Valley. 

The Spanish Fork part has been the focus of detailed 

mapping and trenching studies, especially during 1987. 

In the past 10 years, four major trenching sites have 

been investigated on this part of the Provo segment: 

three on the main Wasatch fault zone and a fourth on a 

splay of the Wasatch fault zone (fig. 15; sites 11-14, table 

1). Swan and others' (1980) study of tectonic, stratigraphic, and geomorphic relations at Hobble Creek (HC, 

fig. 15), coupled with trenching at Deadmans Hollow 

about 1 km to the west, provided the first evidence of 

repeated Holocene movement on the Provo segment of 

the Wasatch fault zone. They found evidence for six or 

seven surface-faulting events that resulted in 11.5 to 13.5 

m of net vertical tectonic displacement since formation of 

the Provo fan-delta complex at the mouth of Hobble 

Canyon. The three youngest events were defined on the 

basis of colluvial deposits observed in trenches across a 

sequence of faulted alluvial fans at Deadmans Hollow. 

Three or four earlier events were inferred from tectonic 

strath terraces preserved on the upthrown fault block 

along Hobble Creek. The number of post-Provo surfacefaulting events is probably reasonable, but the absence of 

organic material in exposures of the terrace alluvium or 

in the trenches prevented determination of individual 

recurrence intervals. The average interval between 

faulting events for the past 14 ka is 1, 700 to 2,600 years 

(as revised by Schwartz and Coppersmith, 1984). The 

age of the most recent event was estimated to be at least 

1 ka on the basis of relatively degraded fault scarps, 

although more recent trenching shows that the most 

recent faulting is younger than this estimate. Individual 

amounts of displacement for post-Provo events were not 

well constrained, but the average value ranges from 1. 6 

to 2.3 m for each of the six or seven inferred events. 

Scarps on sediment of the Bonneville lake cycle south 

of Hobble Creek are both steep and large. However, the 

true net displacement in transgressive sediment of the 

Bonneville lake cycle is difficult to determine because of 

extensive backtilting, graben formation, and colluviation. Swan and others (1980, table 1) preferred a displacement value of 30±0.5 m for the trangressive sediment, whereas we estimate 40 to 45 m on the basis of the 

geometry of backtilting that is proportional to the adjacent well-preserved Provo alluvial surface. If one 

assumes that the faulted lake sediment is 17±2 ka 

(permissible age range for sediment at this altitude), 

then the resulting average slip rate at Hobble Creek has 

been between 1. 8 mm/yr (Swan and others, 1980, table 1) 

and 2.5 mrnlyr. The latter value is based on the midpoints 

of our displacement and age values. Because the net 

displacement recorded by offset of transgressive deposits (30-45 m) is two to three times greater than that 

recorded by regressive deposits (11.5-13.5 m) of the 

Bonneville lake cycle and the potential difference in age 

(1-4 ka) is only slight, we suspect that faulting at this site 

occurred at very high rates (perhaps as much as 10 

mm/yr) during and after the catastrophic fall of Lake 

Bonneville 15,000 years ago (see "Possible Causal Relation Between Deep Cycles of Lake Bonneville and High 

Rates of Slip on the Wasatch Fault Zone"). 

In June 1987, W.R. Lund and D.P. Schwartz dug six 

exploratory trenches at two sites east and south of 

Mapleton, Utah, to determine recurrence intervals and 

to further refine the time of most recent faulting along 

this part of the Provo segment of the Wasatch fault zone. 

The trenching sites (fig. 15, MN, Mapleton North; MS, 

Mapleton South) are 4 and 5.5 km south of the Hobble 

Creek site, respectively. Lund and Schwartz's investigations (Schwartz and others, 1988; Lund and others, 1991) 

indicated two surface-faulting events in the past 3 ka, the 

most recent being bracketed by 14C ages on charcoal of 

445±75 and 770±100 14C yr B.P., which correspond to 

calendar dates of 512 and 689 cal yr B.P., respectively. 

They believe that the most recent faulting event at 

Mapleton probably occurred about 0.6±0.1 ka and, thus, 

could well be the same event recorded at the American 

Fork Canyon site (0.5±0.2 ka). In addition, Lund and 

Schwartz obtained a preliminary TL age estimate of 

3.3±0.3 ka and a 14C date of 2,810±95 yr B.P. (which 

yields possible calendar dates of 2,890, 2,900, and 2,930 

cal yr B.P.) from a soil buried by the older of two 
A42 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

colluvial wedges (Schwartz and others, 1988). These TL 

and 14C dates suggest that the second event occurred at 

3.0±0.3 ka (Lund and others, 199I) or about 2,400 years 

before the most recent faulting event. Given the problems involved in dating, the timing of the second event is 

essentially the same as the 2.65±0.25 ka estimate for the 

penultimate event at the American Fork Canyon site. 

The coincidence in dating from these two sites demands 

that they are on a master (Provo) segment whose last 

two faulting events have ruptured its entire length. 

Dean Ostenaa of the USBR (Denver) is investigating 

recent movement on the Spanish Fork part of the Provo 

segment at the Water Canyon site (WC, fig. I5), which is 

about 14 km southwest of Hobble Creek. This investigation is part of a study for the USER's Central Utah 

Project. In 1986, before trenching began at the Water 

Canyon site, the USBR excavated two trenches across a 

1.5- to 3-m-high fault scarp parallel to and above the 

prominent high shoreline of Lake Bonneville (altitude 

5,095 ft, or I,553 m) about 1.5 km southwest of Water 

Canyon at the southeastern end of Utah Valley. This 

fault scarp is herein named the Woodland Hills splay 

(WH, fig. I5; site 14, table 1). The larger, western trench 

was mapped by Machette, Robert Robison (Utah County 

Geologist), Craig Nelson (Salt Lake County Geologist), 

Rodney Weisser (USBR, Provo), and James McCalpin 

(Utah State University, Logan). Their interpretations 

suggest three or four major surface-faulting events in the 

past I50 ka and a total of about 3 m of net vertical 

displacement. Radiocarbon AMRT dates of 1,190±50 14C 

yr B.P. (USGS-2500, table A1) and I,380±60 14C yr B.P. 

(BETA-I9594, table AI) were obtained from two splits 

of organic matter in a buried A horizon. These dates 

correspond to calibrated AMRT ages of I,102 and 1,306 

cal yr B.P., respectively. The error limits on the AMRT 

dates are generally about 80 to I30 years (table A1), and 

the two calibrated AMRT ages are concordant at about 

1,200±200 cal yr B.P. From this estimate, we subtracted 

200 years to compensate for MRC at the time of burial 

(see table AI). Thus, we suspect that the most recent 

faulting event occurred about 1.0±0.2 ka. The timing of 

the most recent movement on the Woodland Hills splay 

of the Wasatch fault zone and of that on the main 

segment dated at Mapleton would be compatible only if 

one argues for subtracting a much larger MRC (about 

600 years) from the age of the buried A horizon at 

Woodland Hills. 

Inasmuch as movement on the Woodland Hills splay 

produced small amounts of displacement and has only a 

short rupture length, faulting along this splay probably 

was triggered by movement on the main Wasatch fault 

zone. However, based on recurrence intervals, it 

appears that the Woodland Hills splay is an active 

participant in Wasatch fault zone rupturing only a fraction of the time, perhaps once for every 5 to IO events 

that occur on the Provo segment. 

The USER's study was expanded in I987 with the 

excavation of three trenches at Water Canyon (WC, fig. 

I5; site 13, table 1). Two trenches were excavated in 

Holocene fan-head terraces at the mouth of the canyon, 

and a third trench was excavated in slope colluvium 

several hundred meters south of Water Canyon. 

Although final analysis of the trenching investigations 

awaits accelerator-method 14C dating of charcoal samples, Dean Ostenaa (oral and written communs., 1987) 

suspects three or possibly four faulting events in the past 

6 to 8 ka. If his estimate of the number of events and the 

time range holds, there would be a conflict between the 

fault chronology determined here and that determined at 

Mapleton, 14 km to the north-northeast. For example, 

Dean Ostenaa (written commun., 1987) interpreted two 

events in the past 0.8 ka at Water Canyon, whereas 

W.R. Lund and D.P. Schwartz have evidence for one 

event about 0.6 ka. 

Between the Mapleton and Water Canyon sites, the 

trace of the Spanish Fork part makes a spectacular 100° 

clockwise bend at the mouth of Spanish Fork Canyon. 

This geometry is suggestive of a potential segment 

boundary. However, detailed mapping of this area by 

Machette (1989, in press) reveals continuous Holocene 

faults around the bend and several prominent antithetic 

faults that crosscut the bend. The apparent displacement 

recorded in deposits of the Bonneville lake cycle 

decreases at the bend in comparison with sites to both 

the north and the west. This decrease in displacement 

probably results from drag in the bend (that is, the 

confining geometry created by perpendicular basinwarddipping fault planes). The continuity of faulting and the 

evidence of repeated Holocene faulting all along this part 

of the segment suggest that it is continuous from just 

north of Springville to Payson Canyon, where the fault 

turns southward and is concealed by massive Quaternary 

landslide deposits. We suspect that recent faulting continues several kilometers up Payson Canyon, but evidence for faulting is hidden by landsliding and tree cover. 

NEPHI SEGMENT 

The Nephi segment, which extends from Payson to 

Nephi, is comprised of a main western strand and an 

eastern strand. The western strand bounds the Wasatch 

Range in northern Juab Valley, and the eastern strand 

bounds the western side of Dry Mountain, east of Santaquin Creek (fig. I8). Holocene ruptures on the western 

strand continue as far north as Mendenhall Creek; from 

there, they turn northeast and strike into the bedrock of 

the range. Northward-striking scarps persist north of 

Mendenhall Creek, but they are discontinuous and more 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE 

Segment boundary 

.& Trench or field site 

0 

I 

0 

I 

5 

5 

I 

I 

10 

I 

UTAH VALLEY 

10 MILES 

I 

15 KILOMETERS 

FIGURE 18.-Nephi segment of the Wasatch fault zone (WFZ), which extends from just north of Payson to Nephi, 

Utah. Faults are shown by heavy lines having a bar and ball on the downdropped side. The area south of the 

segment boundary lacks evidence of Holocene faulting. Also shown are important geographic features and 

towns. Trench and field sites are abbreviated as follows: S, Santaquin; NC, North Creek; GC, Gardner Creek; 

RC, Red Canyon. 

A43 
A44 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

degraded (older) than those to the south. Although 

evidence of recent faulting between the two strands in 

the Wasatch Range is obscured by vegetation and extensive landslides, we suspect that the two strands of the 

Wasatch fault zone are connected by a major downto-the-north bedrock fault that strikes northeast toward 

the canyon of Santaquin Creek (Davis, 1983b). The 

Manning Canyon Shale, an incompetent unit susceptible 

to landsliding and intraformational faulting, is nearly 

vertical and strikes in a northeasterly direction between 

Mendenhall Creek and the end of the eastern northstriking strand at Santaquin Creek. We suspect that the 

Manning Canyon Shale is a convenient pathway for 

transferring near-surface movement between the two en 

echelon strands of the Nephi segment. 

At the northwestern end of Dry Mountain (about 2 km 

east of Santaquin), the eastern strand of the Nephi 

segment turns northeastward and becomes discontinuous in post-Bonneville lake cycle sediment. Young fault 

scarps (originally mapped by Cluff and others (1973)) 

continue across Payson Canyon, turn northward, and 

bound a narrow bedrock-cored horst block. This horst is 

the northern extension of the Payson salient and is 

mantled by beach gravel (Bonneville lake cycle) north of 

Tithing Mountain. At Payson, the Nephi segment loses 

definition and is largely coincident with fluvial terrace 

scarps graded to regressional (Provo) levels of Lake 

Bonneville (Machette, 1989, in press). 

The Wasatch fault zone continues another 3.5 km north 

of Payson to the vicinity of the Benjamin Cemetery, 

which is on a low north-south elongated hill underlain by 

sediment of the upper Tertiary Salt Lake Formation 

(Davis, 1983b). This hill is bounded on the west by the 

Benjamin fault (BF, fig. 15) of Hintze (1973), which 

forms a 1- to 2-m-high fault scarp on Bonneville lake cycle 

silt and clay of the regressive (Provo) phase. The scarp 

extends about 2 km south of the hill to within 1.5 km of 

Payson. Hintze (1973) reported that the fault forms a 

series of aligned hot springs that extends almost 4.5 km 

north of the Benjamin Cemetery to near Lake Shore, 

Utah, 8 km north of Payson (fig. 15). We consider the 

Benjamin fault to be a northward, less active part of the 

Nephi segment of the Wasatch fault zone. 

The Nephi segment is one of two of Schwartz and 

Coppersmith's (1984) segments that we have not modified except for the postulated cross-range fault that 

extends from Mendenhall Creek northeast to the canyon 

of Santaquin Creek. The surface trace of the Nephi 

segment is 42.5 km long (Payson to Nephi); if the 

Benjamin fault and its subsurface projection to the north 

are included, the total length is about 50 km. Detailed 

maps of Cluff and others (1973) show a number of linear 

features within the city limits of Nephi, but our reconnaissance (M.N. Machette, unpub. mapping, 1987) could 

not confirm them as faults. They show up as sharp 

changes in tone on aerial photographs but are not associated with topographic escarpments. They may, however, have exhibited topographic relief before settlement 

of the town. The southern limit of continuous faulting on 

the Nephi segment is at Utah Highway 41, about 2 km 

north of the center of Nephi. There are discontinuous 

fault scarps on alluvial fans of pre-Holocene(?) age as far 

as 2 km south of Salt Creek (fig. 18), but there is no 

evidence that recent faulting has displaced the intervening upper Holocene alluvial fan of Salt Creek. 

Schwartz and others (1983) excavated three trenches 

at North Creek, the fourth major site excavated by 

Woodward-Clyde geologists along the Wasatch fault 

zone. At North Creek, the Wasatch fault zone offsets an 

alluvial fan that contained charcoal dated at 4,580±250 

14C yr B.P. (USGS W-4057) (Bucknam, 1978), which 

corresponds to a calibrated date of 5,300 cal yr B.P. 

(range is 4,869-5,589 cal yr B.P.). Topographic profiles 

across the fan show that the surface has been offset 

7.0±0.5 m. Schwartz and others (1983) reported that this 

displacement is the result of three surface-faulting 

events, the most recent and penultimate events being 

represented by scarp-derived colluvium. An older, third 

event is inferred from a tectonic strath terrace, which is 

incised in an uplifted part of the North Creek fan. 

The timing of the most recent event at North Creek is 

constrained by two 14C dates and from age estimates 

based on scarp morphology. Age determinations of 

1,110±60 and 1,350±70 14C yr B.P. from charcoal and 

organic matter in the trenches provide a maximum age 

for the most recent event. These two 14C age determinations correspond to calibrated dates of 975 or 1, 048 and 

1,289 cal yr B.P., respectively, although the latter date 

has not been corrected for the AMRT effect. Thus, the 

youngest event at North Creek could have occurred as 

much as 1,000 cal yr B.P. However, the steep scarp 

angles (40°-42°) along this segment of the fault, the lack 

of upstream migration of knickpoints in many stream 

channels above the fault scarp, and a generally continuous 15-km-long fault scarp that lacks vegetation suggested very recent movement (400±100 years ago) to 

Schwartz and others (1983). They estimated 2.0 to 2.2 m 

of displacement per event on the basis of the thickness of 

scarp-derived colluvial wedges, topographic profiles of 

the faulted surface, and the height of the tectonic strath 

terrace. 

Constraint on the minimum time since the penultimate 

event at North Creek was provided by a 14C age of 

3,640±75 14C yr B.P. from a buried soil on the second 

colluvial wedge. The 14C age corresponds to calendar 

dates between 3,930 and 3,980 cal yr B.P. (the one-sigma 

error describes a range of 3,841-4,088 cal yr B.P.) or 

about 4.0±0.2 ka. This maximum estimate, however, 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A45 

does not include an MRC for the soil, which could be 

several hundred years. Hanson and Schwartz (1984) 

reported several other conventional and accelerator 14C 

dates from the North Creek site that are considerably 

younger but argued that the above mentioned 14C date 

provided the most reliable time constraint. The displacement during the second event was 2.0 to 2.5 m. 

The first (oldest) event occurred between 4.0±0.2 and 

5.3±0.3 ka (see previous discussion of charcoal in the 

alluvial fan) and produced 2.6 m of displacement, as 

indicated by the depth of inset of the strath terrace just 

above the fault. Although the ages listed above provide 

general limits on the times of faulting at North Creek, 

none of the determinations come from samples that are in 

stratigraphically definitive positions (for example, charcoal at the buried surface of a proximal colluvial wedge). 

In June 1987, Michael Jackson (University of Colorado) excavated a trench on the distal portion of the 

alluvial fan of Red Canyon at the southern end of the 

Nephi segment. His site (RC, fig. 18) is just east of 

Interstate Highway 15, about 3.5 km north of Nephi. 

However, because the trench is close to the end of the 

Nephi segment, not all of the faulting events at North 

Creek (near the center of the segment) may have ruptured as far south as the Red Canyon site. Jackson's 

trench crossed a 5-m-high fault scarp and exposed interbedded, distal alluvial-fan sediment (pebbly sand) and 

thin, silty, debris-flow deposits. Three discrete but thin 

colluvial wedges were recognized along the main fault, 

and an additional wedge was recognized along a subsidiary fault. 

A TL age estimate of 14.6±1.2 ka from what appears 

to be nonbleached material in the oldest colluvial wedge 

suggests that the faulted alluvial-fan sequence at Red 

Canyon is latest Pleistocene in age (Jackson, 1988, in 

press). Relations between the stratigraphic and tectonic 

units exposed in the Red Canyon trench and Jackson's 

(1988) 14C dates suggest three or four major surfacefaulting events during the middle and late Holocene. 

Jackson (1988, fig. 5-4), inferring synchroneity with 

faulting at the North Creek site, suggested that the 

"most probable" times for faulting events at Red Canyon 

are 1,200, 3,000 to 3,500, and 4,000 to 4,500 cal yr B.P. 

The southern end of the Nephi segment and the 

northern end of the Levan segment are separated by a 

15-km-long gap in recent faulting. M.N. Machette 

(unpub. data, 1987) found no evidence of latest Pleistocene or Holocene faulting in this gap, although older fault 

scarps are preserved on middle Pleistocene alluvial fans 

that emanate from the northeastern side of the San Pitch 

Mountains (see lower part of fig. 18). Zoback's (1983) 

gravity map shows a deep, continuous gravity trough in 

the valley adjacent to the gap in recent faulting; 

together, these data suggest that the Wasatch fault zone 

is a continuous structural feature beneath the gap. 

The middle Pleistocene alluvial fans in the gap are part 

of a broad, extensive alluvial cone (known informally as 

Levan Ridge) that forms a drainage divide within the 

Juab Valley. Inspection of topographic maps of the 

northern San Pitch Mountains and the adjacent Juab 

Valley suggests that the streams supplying sediment to 

Levan Ridge are grossly underfit. There is a prominent 

north-south valley within the San Pitch Mountains that 

projects north to the Salt Creek drainage (several kilometers east of Nephi) and south to the head of Levan 

Ridge. Machette suspects that the ancient course of Salt 

Creek formed the aforementioned valley in the San Pitch 

Mountains and that Salt Creek deposited the bulk of 

alluvium that underlies Levan Ridge. If so, then headward incision of the lower reach of Salt Creek, perhaps as 

recently as the late Pleistocene, resulted in capture of 

the upper Salt Creek drainage and subsequent entrenchment of Salt Creek Canyon. This stream capture would 

explain the deposition of a young fan complex at the 

mouth of Salt Creek Canyon. 

SOUTHERN WASATCH FAULT ZONE 

The southern section of the Wasatch fault zone comprises segments of the fault zone that do not exhibit 

repeated Holocene movement, in contrast to the more 

active central section of the Wasatch fault zone. The 

southern section of the fault zone consists of the Levan 

segment, which Schwartz and Coppersmith (1984) 

defined as extending from near Pigeon Creek (PC, fig. 

19) east of Levan, south to near Gunnison. Machette and 

others (1986, 1987) suggested that the Levan segment 

should be subdivided into the Levan segment (restricted 

sense) and the Fayette segment (fig. 19). The intervening segment boundary is located at the left step that the 

fault trace takes from the Juab Valley to Flat Canyon 

(the south-trending canyon headed by Maple Creek, 

north of Fayette). We make this subdivision on the basis 

of apparent recency of movement (late Holocene movement on the Levan segment versus no evidence of 

Holocene movement on the Fayette segment) and on the 

basis of the major left step and gap in faulting (Machette 

and others, 1986, 1987). 

LEVAN SEGMENT 

Our aerial photographic reconnaissance, geologic mapping, and analysis of scarp morphology indicate that the 

Levan segment has Holocene ruptures from Hartley's 

Creek, 3.8 km northeast of Levan (fig. 19) to as far south 

as Botham Road, 18 km south of Levan. Near Botham 
A46 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

0 5 10 MILES 

0 5 10 15 KILOMETERS 

FIGURE 19.-Levan (northern) and Fayette (southern) segments of the Wasatch fault zone. Faults are 

shown by heavy lines having a bar and ball on the downdropped side (dotted where uncertain). Also 

shown are important geographic features and towns. Trench and field sites are abbreviated as follows: 

PC, Pigeon Creek; DC, Deep Creek; SP, Skinner Peaks. 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A47 

Road, the Levan segment steps left about 0.5 km and 

enters bedrock. However, there is clear evidence of 

Quaternary faulting for another 3 km south of Botham 

Road, but we cannot tell if the most recent movement is 

Holocene. The southern end of the Levan segment 

(including the part of uncertain age) is about 0.5 km east 

of Utah State Highway 28 at the southern jog in the 

Juab-Sanpete County line. From this point south, the 

trace of the late Quaternary Wasatch fault zone makes a 

3.5-km step to the left (to the east) and 5 km to the south 

to the vicinity of Flat Canyon. We suspect that the 

western front of the San Pitch Mountains between Maple 

Creek and Hells Kitchen Creek also is fault controlled, 

but there is no evidence of young fault scarps on alluvial 

fans in this area. The morphology of fault scarps along 

the Fayette segment, which extends from Hells Kitchen 

Creek south to Fayette, shows no evidence of Holocene 

movement (M. N. Machette, unpub. data, 1986). 

Most of the fault scarps on Holocene deposits along the 

Levan segment are less than 3 m high and probably are 

the result of a single late Holocene faulting event. Fault 

scarps on upper to middle Pleistocene alluvium are 

commonly 5 to 10 m high or higher; this height suggests 

recurrent late Quaternary faulting. 

At Pigeon Creek (PC, fig. 19), 2 km north and due east 

of Levan, two samples of charcoal from faulted alluvium 

in a natural exposure were dated by the accelerator 

method at 1,750±350 and 2,100±300 14C yr B.P. by 

geologists from Woodward-Clyde and Associates (see 

Crone, 1983, p. 15; Schwartz and Coppersmith, 1984). 

These ages placed a maximum limit on the time of most 

recent faulting. Extensive flooding in 1984 and the 

subsequent construction of flood-control levees have 

prevented us from restudying the Pigeon Creek site. 

However, a second natural exposure at Deep Creek, 

about 5 km south of Levan, has provided datable material. Here the fault forms a 2.5-m-high scarp and is 

exposed in the north wall of the creek. Schwartz and 

Coppersmith (1984) and M. N. Machette (unpub. mapping, 1984-86) studied this exposure and concluded that 

the scarp was produced by a single faulting event. 

Machette measured a net vertical displacment of about 

1. 75 m, which is comprised of 2. 3 m of displacement on 

the main fault and 0.55 m of offsetting displacement on a 

graben-bounding antithetic fault about 15 m to the west. 

Charcoal from an unspecified position in the faultedalluvium at this site has been dated at 7,300±1,000 14C yr 

B.P. (Schwartz and Coppersmith, 1984). 

Michael Jackson also mapped the Deep Creek exposure and measured 2.3 to 2.4 m of displacement on the 

main fault. Jackson also determined a TL age estimate of 

about 1,000±100 cal yr for the soil on alluvium that is 

buried by fault-scarp colluvium (Jackson, 1988, in press). 

This TL age suggests that the most recent faulting event 

is about 1.0±0.1 ka. 

Jackson (1988, in press) also excavated a 5-m-deep 

trench across a 3.3-m-high fault scarp on middle Holocene alluvial-fan sediment near the southern end of the 

Levan segment at the Skinner Peak site (SP, fig. 19; site 

18, table 1). The site is about 1.5 km northwest of 

Skinner Peaks and about 200 m east of Utah State 

Highway 28. Jackson (1988, in press) found only one 

colluvial wedge, but there was at least twice as much 

alluvium (about 5 m) on the downdropped block as on the 

upthrown block. These relations suggest that the alluvium must have been deposited across or, more likely, 

against a preexisting escarpment. Thus, there appears to 

have been one faulting event after formation of the 

alluvial fan and one event during or before deposition of 

the fan. A combination of 14C dates and TL age estimates 

from this trench provides a maximum time for the most 

recent event of less than 3.1 to 3. 9 ka, which supports 

Schwartz and Coppersmith's (1984) earlier interpretation of a young (less than 1, 750 yr B.P.) faulting event 

and is consistent with results from the Deep Creek site. 

The stratigraphic and structural relations exposed in 

Jackson's trench suggest to us that the penultimate 

faulting event on the Levan segment could have occurred 

in latest Pleistocene or early Holocene time. If this 

speculation is correct, then distal segments of the 

Wasatch fault zone (such as the Levan) may have recurrence intervals on the order of 10,000 years, or about 

one-fifth that of the more active central segments. 

FAYETTE SEGMENT 

The Fayette segment is the southernmost segment of 

the 383-km-long Wasatch fault zone (fig. 1). It is similar 

in many ways to the segments of the northern Wasatch 

fault zone, except that Lake Bonneville was shallower 

here and remained below (west of) the main trace of the 

fault zone. Fault scarps on latest Pleistocene and early(?) 

Holocene alluvial fans are present from Hells Kitchen 

Creek south to the Fayette Cemetery, about 1 km east of 

Fayette (fig. 19). The Fayette segment has two strands: 

a 6-km-long western strand that strikes north-south and 

a concave-to-the-west, 11-km-long eastern strand that 

strikes southeast and south. These two strands join near 

the northern end of the segment, about 1 km south of 

Hells Kitchen Canyon. The fault scarps along this segment are commonly eroded at the mouths of canyons but 

preserved on terraces that are elevated 2 to 5 m above 

stream level. Profiles were measured across a number of 

scarps on faulted stream terraces along both strands to 

determine the relative ages of the scarps. The morphometric data (scarp height and maximum slope angle) 
A48 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

from these profiles plot between the regression lines for 

data from the Drum Mountains fault scarps in westcentral Utah (early Holocene) (Crone, 1983; Pierce and 

Colman, 1986) and the highest shoreline of the Bonneville 

lake cycle (14.5 ka) (Bucknam and Anderson, 1979). 

These morphometric relations and the degree of scarp 

dissection suggest that the most recent movement on the 

Fayette segment occurred between 10 and 15 ka and, 

thus, is substantially older than and clearly different in 

age from the Levan segment to the north and west. 

SUMMARY OF SEGMENTATION OF THE 

WASATCH FAULT ZONE 

Our studies show that the Wasatch fault zone has 10 

discrete fault segments, each of which is capable of 

generating large-magnitude earthquakes. Four of these 

segments (the northern three and the southern one) have 

not been active in the past 10 to 15 ka. The slip rates for 

these distal segments are lower, and recurrence intervals are at least five times longer than those of the six 

active segments along the central part of the Wasatch 

fault zone. Five of these six segments show evidence of 

multiple surface ruptures during the Holocene. Trenches 

across these segments generally reveal evidence for two, 

three, or four faulting events since the middle Holocene 

(5-6 ka). 

Schwartz and Coppersmith's (1984) studies of segmentation suggested 6 segments for the Wasatch fault zone; 

our segmentation scheme increases to 10 the number of 

segments that are likely to have discrete surface rupturing (see "Recurrence of Large Earthquakes on the 

Wasatch Fault Zone") but slightly reduces the length of 

expected surface rupturing during major earthquakes 

along the Wasatch Front. However, we cannot discount 

the possibility that major earthquakes will rupture more 

than one adjacent segment or that some earthquakes will 

rupture only part of a long segment, such as Provo or 

Weber. 

The average length of all of the Wasatch fault zone 

segments is about 37 km (table 2, fig. 4). However, the 

six Holocene segments, which comprise three-fourths of 

the fault zone, average about 48 km in length. This 

length is almost twice that suggested from paleoseismicity studies in the northeastern part of the Basin and 

Range province and in the Intermountain Seismic Belt. 

Because the prehistoric earthquakes along the Wasatch 

fault zone have typically caused about 2 m of surface 

displacement (see Schwartz and Coppersmith's (1984) 

discussion of characteristic displacement), we expect 

large-magnitude CMs 7-7.5) earthquakes to occur along 

the Wasatch fault zone in the future. 

CHRONOLOGY OF RECENT MOVEMENT ON 

THE CENTRAL WASATCH FAULT ZONE: 

A SYNOPSIS 

The Wasatch fault zone has been the focus of extensive 

trenching efforts since Cluff and others' (1970, 1973, 

1974) initial investigations in the 1970's. Including the 

trench studies that were completed in 1987, 43 trenches 

and 4 natural exposures have been logged and described 

from the 18 sites on the Holocene segments of the 

Wasatch fault zone (table 1). As such, the Wasatch fault 

zone may be the most studied normal fault zone in the 

world. 

Most of the trenches across the Wasatch fault zone 

have provided some control on the times of most recent 

faulting or provided limits for recurrence intervals and 

slip rates for faulting events. In the past 2 years, we 

have obtained 36 14C dates (both conventional and accelerator methods on charcoal and soil organic matter) and 

17 experimental TL age estimates (see table A1). The 

combination of these new ages, trenching investigations, 

and published data is the basis for our estimates of the 

timing of Holocene surface-faulting events (paleoearthquakes) and recurrence intervals on segments of the 

Wasatch fault zone (fig. 20). More importantly, differences in the timing of the most recent and older events 

along the length of the Wasatch fault zone are the most 

diagnostic criteria for determining the lengths, validity, 

and persistence of segments. 

Several interesting patterns evolve from the chronology depicted in figure 20. The most striking pattern is 

one of temporal clustering of large earthquakes-that is, 

a strong grouping of surface-rupturing earthquakes on 

the Wasatch fault zone over a geologically short time 

interval. If one assumes that the most recent event on 

the Salt Lake City segment occurred about 1.5 ka (rather 

than closer to the 1.8-ka limit shown in fig. 20), then 

faulting occurred between about 400(?) and 1,500(?) 

years ago on five of the six segments of the Wasatch that 

have Holocene movement. On the basis of the Wasatch 

fault zone's behavior since the middle Holocene, we 

estimate that a major earthquake occurred about once 

every 395 years (see table 4) on the central part of the 

Wasatch fault zone between Brigham City and Nephi. 

However, the recent episode of movement on the 

Wasatch fault zone (six faulting events during a 1,100-

year interval) indicates one major surface-rupturing 

event every 220 years, or about twice as often as we 

would expect from the longer (Holocene) record. In 

addition, we have no evidence for a major surfacerupturing earthquake on the Wasatch fault zone during 

the past 400 to 500 years (our best estimate for the most 

recent event on the Nephi segment and the youngest 

time that we allow for the Weber segment). Although 

these relations point strongly to a process of temporal 
tz 

UJ 

::1E 

(.!) 

UJ 

(/) 

0 

> 0 

a: 

0... 

PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE 

PRESENT 

Brigham City 

segment (1) 

Weber segment 

(2,3) 

Salt Lake City 

segment (4) 

American Fork 

site (2,5) 

Mapleton site 

(4) 

Nephi segment 

(6,7) 

Levan segment 

(6,7) 

I 

TODAY 

~ 

i 

~ 0 »< 0 ~ 'Xy. 'X 

'X »< 'X )< 'Xy. X 

9< 0 

~ 0 ~ 'Xy. ~ 'X D< )< 

'X 'Xy. )< 

X ;>< 

'X 

<f f 

J )? 

:>l 

~ 

~ IK 

1000 

I 

>0<~ 

'X 'X >« 

'X 'X >« 

-x>S: 

'X '>)(< 'X )<'X X" )0< X" :><? )< 

1000 

TIME, IN YEARS BEFORE PRESENT 

2000 3000 4000 

I I 

I I I 

2000 3000 4000 

SOURCES OF INFORMATION 

(1) Person ius (in press a) 

(2) Machette and others (1987), modified in this report 

(3) Swan and others (1980), modified in this report 

(4) Schwartz and others (1988), modified in this report 

(5) Forman and others (1989) 

(6) Schwartz and Coppersmith (1984) 

(7) Jackson (1988) 

A49 

5000 6000 

I 

I 

5000 6000 

FIGURE 20.-Estimated timing of major surface ruptures on 

segments of the Wasatch fault zone during the past 6,000 years. 

Dashed vertical lines indicate our estimates of the most probable 

time of faulting (queried or missing where timing is uncertain). Cross-hachure patterns indicate permissible limits for 

faulting events as determined from a combination of calibrated 14C ages and TL age estimates listed in table Al. 
REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

cluutering of large-magnitude earthquakes, the process 

seems to be intermittent through time. 

~ econd, there seems to be a significant difference in 

the average recurrence intervals for the active segments 

fro: n south to north along the Wasatch fault zone. The 

segments from Salt Lake City to Nephi have fairly long 

intt:rvals, typically 2,000 to 4,000 years, whereas the 

We ::>er and Brigham City segments are characterized by 

shorter intervals (about 1,100 years). However, the lack 

of movement on the Brigham City segment during the 

pa;;: c 3,600 years is striking (fig. 20). Personius (1991a) 

suggested a total of three faulting events between 3,600 

anc 7,000(?) years ago (an interval of approximately 

3,4')0 years). Thus, the recent 3,600-year period of 

tee ,onic quiescence stands in marked contrast to earlier 

reccirrence intervals on this segment. 

'l hird, the lack of surface faulting along the Brigham 

Cit r segment since 3. 6 ka suggests that this segment is 

a h cely candidate for future surface faulting. No other 

sep nent of the central Wasatch fault zone has been 

do:r nant as long in the late Holocene. On the other hand, 

thh segment may be in a dormant phase if its movement 

hisr ory is related (spatially) to other extensional faults to 

the northeast. Studies of Quaternary faulting in northeas~ern Utah and southern Idaho indicate that the East 

Cache fault (McCalpin, 1989; McCalpin and Forman, 

1991), the Morgan fault (Sullivan and Nelson, this volum ~), and the East Bear Lake fault (A.J. Crone and 

M.N. Machette, unpub. data, 1988) have been active 

dur;ng the Holocene. These faults form the southern part 

of ·m extensive right-stepping en echelon pattern of 

act ve faulting that extends to the Yellowstone area of 

norchwestern Wyoming and westward to the north of the 

Snake River Plain (see Smith and others, 1985; Pierce 

and Scott, 1986; Smith, 1988; Anders and others, 1989). 

Thi;; system of faults may be preferentially transferring 

ext-_msion to the northeast, thereby decoupling the 

nonhern three segments (Malad City, Clarkston Mountair , and Collinston) of the Wasatch fault zone from its 

mm·e active portions to the south (Machette and others, 

1991). 

RECURRENCEOFLARGEEARTHQUAKESON 

THE WASATCH FAULT ZONE 

8chwartz and Coppersmith (1984) estimated that a 

ma.;or surface-faulting event should occur on average 

oncB every 615 to 666 years on one of their six proposed 

segments of the Wasatch fault zone, although their 

preferred value wa~ 444 years. These values were based 

on estimates of 12 or 13 (minimum) to 18 (probable) 

fawting events in tb.e past 8,000 years. We refer to this 

inth"'val (444 years) as the composite recurrence interval 

(CRI) of faulting. The main usefulness ofCRI values is in 

estimating the influence of earthquakes from adjacent or 

nearby segments. For example, the damaging effects of 

ground shaking and liquefaction are known to occur 

hundreds of kilometers away from the epicenter of 

large-magnitude earthquakes. Thus, for hazards assessment, one must consider the effects of an earthquake on 

any one of the populated central segments of the 

Wasatch fault zone. 

Our calculations of CRI are based on the Wasatch fault 

zone segments that show recurrent Holocene movement 

and, therefore, have determinable recurrence intervals. 

We should, however, qualify the use of CRI values. 

Inspection of our data on the timing of events on the 

Wasatch fault zone will show that recurrence intervals 

vary widely, from as little as 500 to 1, 000 years on some 

segments to as much as 4,000 years on others. In 

addition, we have strong evidence that some, but not all, 

earthquakes tend to occur in clusters and that recurrence 

intervals between earthquakes on adjacent segments 

during a temporal clustering episode may be as short as 

several hundred years. Thus, values for recurrence on a 

single segment of the Wasatch fault zone and on the 

entire Wasatch fault zone may have little significance 

other than to characterize the general activity of the fault 

zone. 

There are several problems inherent in Schwartz and 

Coppersmith's (1984) method of estimating recurrence 

intervals, although these problems arise mainly from a 

lack of information on the age of faulted datums and the 

timing of the most recent event. They chose to estimate 

the number of earthquakes over a set time interval 

rather than to use intervals defined by dated earthquakes. The first problem is the result of including time 

for undefined recurrence intervals. For example, if the 

most recent event on a segment was 1.5 ka, then an 

additional 1,500 years is included in the observational 

window for calculating the recurrence interval of that 

segment. Ideally, one should discard incomplete recurrence intervals by removing the time since the most 

recent event (column B, table 4) and the time between 

the faulted datum and the oldest event (footnote 3, table 

4). Even for our calculations, the latter value is not 

always known or determinable; but, where it is known, 

we use the time of the oldest event in column A of table 

4. In such cases, a calculated recurrence interval is more 

accurate because it includes only the time between the 

oldest recorded event and the beginning of the period 

being considered (that is, the age of the faulted stratigraphic unit). Our estimate of the total time encompassed by the segments showing recurrent Holocene 

movement (17,800 years) (column C, table 4) may include 

a total of several hundred years between the oldest fault 

event and the dated materials for the Nephi segment and 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A51 

thus represents only 1 to 2 percent of potential error. 

Alternatively, one can argue that the deletion of at least 

3, 600 years for the current recurrence interval on the 

Brigham City segment leads us to underestimate the 

composite recurrence interval. If such an unconstrained 

value is included, then the minimum values should be 

included for all segments, and the method of calculation 

will be similar to that used by Schwartz and Coppersmith. Nevertheless, if we had included an additional 

3,600 years and one additional (but incomplete) recurrence interval in our calculations (table 4), the CRI value 

would increase about 35 years for a value of 430 years. 

The chronology of faulting that we have constructed so 

far (fig. 20) suggests that at least 15 faulting events 

occurred in the past 3,500 to 5,500 years on the central 

six segments of the Wasatch fault zone (table 4). Our 

model for recurrence calculations considers the five 

segments that have recurrent Holocene movement. 

These data yield a CRI of 395±60 years (this value is 

based on 15 faulting events and 9 recurrence intervals on 

a total of 5 segments of the Wasatch fault zone during 

17,800±2,800 cumulative years). 

The chronology of faulting along the central Wasatch 

fault zone (fig. 20) suggests that all the segments except 

Brigham City have been active in the past 1,500 years 

and that none have been active in the past 400 to 500 

years. The average time between faulting events on the 

whole fault zone in the most recent episode of faulting 

was about 220 years (six events between 400 and 1, 500 

years ago). The shortness of intervals between recent 

events, the recent period of fault quiescence, and the 

long average recurrence intervals for individual segments (1,980±310 years) (table 4) lead us to suspect that 

major earthquakes and surface faulting along the 

Wasatch fault zone may have occurred in clusters interspersed with less-frequent random events between clusters. 

A historical analog for the Holocene faulting on the 

Wasatch fault zone may be the Central Nevada Seismic 

Belt (Wallace and Whitney, 1984) of the western Basin 

and Range province. This belt is characterized by a 

concentration of historical large-magnitude earthquakes 

and associated surface ruptures that occurred on an 

average of once every 15 to 20 years. The pattern of 

rupturing suggests that the belt is composed of a series 

of active segments and seismic gaps, which correspond to 

inactive fault segments. Through time, these gaps have 

been filled sequentially by faulting events and, as such, 

represent a historical example of temporal clustering 

(Wallace, 1984b, 1987). If and when rupturing is completed by filling the two remaining seismic gaps in the 

belt, activity might shift to another major fault zone in 

the region. Pearthree and Demsey's (1987) morphometric analysis of prehistoric fault scarps of the Central 

Nevada Seismic Belt suggests that its individual faults 

have recurrence intervals of many thousands of years, 

much like those of the Wasatch fault zone (table 4). 

However, the Central Nevada Seismic Belt and :he 

Wasatch fault zone may differ in one major respect: the 

average times between major earthquakes during temporal clustering are 15 to 20 years and about 220 yec.rs, 

respectively. 

The process of earthquake clustering, which is physically manifested as a temporal linking of movement from 

one fault or fault segment to another, has been termed 

"contagion" by Perkins (1987), after the analogy ·with 

contagious illness. The implications of a clustering process are significant for analyses of earthquake risk al( mg 

the Wasatch fault zone and other fault zones. For example, assuming a Poisson distribution of nonclustered 

earthquakes (the commonly accepted distribution), one 

would expect a major earthquake somewhere on the 

central section of the Wasatch fault zone once every 

395±60 years on the basis of our timing model (fig. 18, 

table 4). Such a short recurrence interval poses a significant risk for the densely populated part of the Wasatch 

fault zone, where many critical structures and facilities 

have expected lifespans of 100 years or more. If the case 

of the recent temporal clustering of earthquakes on the 

Wasatch fault zone having CRI of about 220 years is valid 

and if the most recent episode ended about 400 to tiOO 

years ago, then one might argue that the recent episode 

has been completed. In this case, one may expect a long 

period of relative tectonic quiescence (perhaps 1,000 to 

2,000 years long) before the next episode of tempPral 

clustering on the Wasatch fault zone. Complicating this 

scenario is the seeming random distribution of faulting 

events that occurred before 1,500 years (fig. 20). 

Although our data present compelling evidence of a 

non-Poisson distribution of earthquakes on the Wasatch 

fault zone, especially during late Holocene time, we do 

not understand what causes the variability in earthquake 

recurrence on individual segments, what controls the 

pattern of rupturing on segments during clustered t.~pisodes of earthquakes, and what mechanisms control the 

process of temporal clustering. Proving that the process 

was or is active and representative of long-term behavior 

of the Wasatch fault zone will require an extensive and 

well-dated record that extends at least 15,000 years into 

the past. 

DISCUSSION OF SLIP-RATE DATA FORTH£ 

WASATCH FAULT ZONE 

Sediment that predates the Bonneville lake cycle 

(more than 35 ka) is rarely exposed on both sides of 

the Wasatch fault zone along its northern and central 
A 52 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

en 

a: 

w 

1-

w 

:E 

~ 

50 

40 

~ 30 w 

:E 

w 

u 

:5 

11.. 

en 

0 

'::i. 20 

u 

i= 

a: 

w 

> 

1-

w 

z 

10 

10 ~ oo m 1~ 200 250 

AGE OF FAULTED DEPOSIT, IN THOUSANDS OF YEARS 

FIGURE 21. -Some slip rates on the Wasatch fault zone for different 

time intervals as determined from deposits of Holocene to middle 

Quaternary age. Letters A through I (listed below) indicate locations 

of slip-rate determinations. Abbreviated site names (in parentheses) 

are the same as those used on cited figures. Vertical and horizontal 

bars indicate the range in probable amounts of displacement and ages 

of deposits. Dashed lines indicate different rates of slip. Note 

changes in scale at the breaks in horizontal axes (25/40 ka and 80/100 

ka). A, Gardner Creek (GC, fig. 18), Nephi segment, Wasatch fault 

zone; B, North Creek (NC, fig. 18), Nephi segment, Wasatch fault 

sections. However, south of Santaquin, Utah (fig. 18), 

the Wasatch fault zone generally is above the highest 

shoreline of the Bonneville lake cycle; here we can 

measure and compare the net slip in alluvial deposits that 

both postdate and predate the Bonneville lake cycle. In 

addition, several exposures of pre-Bonneville age deposits on fault segments to the north of Santaquin provide 

data on longer term rates of slip. We have studied these 

and other sites in detail to compare slip rates for the time 

intervals before and after the Bonneville lake cycle (fig. 

21). The slopes of the dashed lines shown in figure 21 

delineate slip rates of 0.1 to 3. 0 mm/yr for different parts 

of the time scale as recorded by faulted deposits. In order 

to plot the amount of displacement of deposits of middle 

zone; C, canyon of Santaquin Creek (S, fig. 16), Nephi segment, 

Wasatch fault zone; D, Hobble Creek (HC, fig. 15), Provo segment, 

Wasatch fault zone; E, canyon of Dry Creek (DC, fig. 15), Provo 

segment, Wasatch fault zone; F, canyons of Big and Little Cottonwood Creeks (BCC and LCC, fig. 13), Salt Lake City segment, 

Wasatch fault zone; G, Kaysville (KV, fig. 9), Weber segment, 

Wasatch fault zone; H, Jim May Canyon (JMC, fig. 8), Brigham City 

segment, Wasatch fault zone; I, Granger fault (fig. 13), West Valley 

fault zone (Keaton and others, 1987). 

Pleistocene to Holocene age on one diagram, two breaks 

were made in the time scale, one between 25 and 40 ka 

and a second between 80 and about 100 ka. Only the 

slip-rate lines for the youngest interval project through 

the axis, and the time axis has a different scale for each 

of the three intervals. However, by plotting the slip-rate 

data in this manner, we can make direct comparisons of 

average slip rates over much different time intervals. 

At Gardner Creek near Nephi, 150- to 250-ka alluvialfan deposits that have well-developed calcic soils are 

displaced about 30 m across the fault zone (Machette, 

1984). The Wasatch fault zone at this site has had an 

average slip rate of 0.12 to 0.20 mm/yr during the past 

150 to 250 ka. At the same site, middle(?) Holocene 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A 53 

sediment is displaced 3. 9 m, indicating an average slip 

rate of 0.8 to 1.0 mm/yr for the past 4 to 5 ka. These two 

averages (labeled as points A, fig. 21), which reflect an 

appropriate fivefold increase in slip rate sometime during 

the latest Quaternary, are typical of the central part of 

the Wasatch fault zone. Large temporal changes in slip 

rate (fig. 21) have been documented at sites near Santaquin (northern part of Nephi segment), at Dry Creek 

northeast of Alpine (northern part of Provo segment), at 

Big and Little Cottonwood Canyons (Salt Lake City 

segment), at Jim May Canyon (near Honeyville on the 

Brigham City segment), and on the Granger fault (West 

Valley fault zone) (Keaton and others, 1987). 

Along the central Wasatch fault zone, sediment of or 

related to the Bonneville lake cycle (10-32 ka) commonly 

has been deposited against or across preexisting scarps 

of the fault zone. This sediment and the subsequent 

Holocene deposits record relatively high rates of fault 

slip. For example, Swan and others (1980) calculated slip 

rates for three separate geologic units at the Hobble 

Creek site (point D, fig. 21). The slip rates range from a 

maximum of 1.8 to 2.5 mm/yr in sediment at the highest 

Bonneville shoreline level (17±2 ka) to 0.85 to 1.0 mm/yr 

in alluvium graded to the Provo level (13.5-14 ka) to 1 to 

2 mm/yr in middle to lower(?) Holocene stream sediment 

(4.5-9 ka). 

With the exception of the Hobble Creek site, most slip 

rates along the Wasatch fault zone (fig. 21) have been 

between 0. 5 and 1. 5 mm/yr since culmination of the 

Bonneville lake cycle. As figure 21 illustrates, the slip 

rates during the past 15 ka have been distinctly higher 

(2-3 times) than the average rates of 0.1 to 0.3 mm/yr 

recorded by deposits older than the Bonneville lake 

cycle. If we examine the average slip rates recorded 

between the time of deposition of the older deposits and 

the culmination of the Bonneville lake cycle, the difference in long- and short-term slip rates is even greater. 

For example, if 200-ka alluvium records an average slip 

rate of 0.2 mm/yr (40-m net slip) and adjacent 15-ka lake 

sediment records an average slip rate of 1.0 mm/yr (15-m 

net slip), then the apparent increase in slip rates is 

fivefold. However, the displacement recorded between 

deposition of the older unit and that of the younger unit 

is only 25m, whereas the time interval for slip is 185,000 

years . The resulting interim (185,000 years) slip rate is 

about 0.14 mm/yr, or about 30 percent less than the 

long-term (200,000 years) average slip rate. As we 

discuss later, our observation of large and systematic 

changes in slip rate through time suggests a causal 

relation between hydrologic cycles of Lake Bonneville 

and large changes in slip rates on the adjacent Wasatch 

fault zone. 

POSSIBLE CAUSAL RELATION BETWEEN 

DEEP CYCLES OF LAKE BONNEVILLE 

AND HIGH RATES OF SLIP ON THE 

WASATCH FAULT ZONE 

The pattern, magnitude, and regularity of change in 

slip rates that we have seen along the Wasatch fault zone 

are of regional extent and, thus, are probably related to 

processes acting on a regional rather than a local scale. 

The basic evidence for this conclusion comes from the 

relatively low average slip rates (0.1-0.3 mm/yr) 

recorded in alluvium deposited before the culmination of 

the transgressive phase of the most recent and deepest 

Quaternary lake cycle in the Bonneville Basin and the 

high slip rates (typically 0.4-1.8 mm/yr) recorded in 

sediment deposited since 15 ka (fig. 21). This relation 

strongly suggests that the presence of a large, deep lake 

may have influenced the timing of movement on the 

Wasatch fault zone. 

Several aspects of Lake Bonneville's history may be 

related to the recent relatively high slip rates along the 

Wasatch fault zone. During the highest stand of the lake 

(15 ka) (fig. 2), most basins in northwestern Utah were 

flooded to a maximum depth of 300 m. This depth of 

water imposes the same load as 130 to 160 m of sedimentary rock. Almost all of this load was on the down thrown 

block of the Wasatch fault zone. The lake was fairly deep 

(200-250 m) just a few kilometers west of localities where 

high slip rates have been measured. The hydrologic and 

lithostatic loads imposed by Lake Bonneville and its 

sediment may have significantly increased the confining 

pressure on the westward-dipping Wasatch fault zone 

and thereby inhibited faulting. If increased pore pressure reached seismogenic depths, the effective confining 

pressure on the fault zone could have been reduced. 

Nevertheless, if we envision the Earth's crust as a gently 

upwarped beam over a viscous mantle, then the beam 

could be shortened (compressed) during deep lake cycles 

owing to isostatic depression in the lake basin; as much as 

60 m of depression occurred at the high stand of the 

Bonneville lake cycle. In an east-west extensional regime 

such as the Basin and Range province, this compression 

would tend to increase horizontal forces across steeply 

dipping, north-striking normal faults and perhaps retard 

normal faulting (Keaton and others, 1987). 

As mentioned previously, beginning about 14.5 ka, the 

level of Lake Bonneville rapidly fell108 m (to the Provo 

level) by overflow and rapid downcutting of the lake's 

sill. An additional almost 200 m of fall occurred in the 

next several thousand years owing to the onset of the 

Holocene Epoch. These geologically rapid changes in 

Lake Bonneville were accompanied by (1) rapid isostatic 

uplift of the basin (Crittenden, 1963a, b; Currey and 

Burr, 1988), (2) deposition of thick fan-delta complexes 

and lacustrine spits on the downdropped blocks of the 
A 54 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Wasatch fault zone during the lake's regression, and (3) 

a probable decrease in the lateral compression that may 

have previously retarded faulting during the deep (transgressive) phase of the lake cycle. 

This well-documented but geologically short-lived episode of isostatic rebound represents upward arching and 

lengthening of the crust and may have allowed the 

east-west extension that formed the Basin and Range 

province to become predominant again. Thus, we speculatE.~ that several aspects of the most recent and deepest 

cycle of Lake Bonneville may have stimulated the recent 

acceleration of slip rate and (by inference) paleoseismic 

activity along the Wasatch fault zone. Similar causal 

relations have been suggested for seismicity in other 

continental regions. For example, rapid glacio-isostatic 

rebound after the retreat of late Pleistocene icecaps in 

Scotland, Finland, and Sweden was associated with both 

new and renewed tectonic deformation and (by inference) seismicity (Morner, 1978). Morner (1978, p. 45) 

concluded that "intense faulting, fracturing and seismic 

activity are shown to be linked to the degla~iation phase 

because of the initiation of changes [our emphasis] in 

stress and strain when the rate of glacio-isostatic uplift is 

near its maximum." Along the same lines, Johnston 

(1987) proposed that the weight of extensive continental 

ice sheets suppresses seismic activity in the interior 

regions of both Greenland and Antarctica. If thick (2-km) 

ice sheets can have such an effect, it seems reasonable 

that Lake Bonneville (which at its highest stand was 

nearly 1 km deep) also may have suppressed seismic 

activity and slip on the Wasatch fault zone. In addition 

rap~d. changes in water level during either filling o; 

dra1rung are known to have induced seismicity under 

moderate to large reservoirs in various parts of the world 

(see Simpson's (1986) review of induced seismicity). 

Recent studies of lake sediment that predates the 

Bonneville lake cycle (for example, Scott and others, 

1983; McCalpin, 1986; McCoy, 1987; Oviatt and others, 

1987; McCalpin and others, this volume) suggest that the 

Bonneville Basin may have been occupied by shallow to 

moderately deep pluvial lakes during most glacial episodes. Oxygen-isotope data from the deep marine record 

suggest that major glaciations occur about every 120,000 

years and minor interglaciations at intervals of 40,000 to 

50,000 years (Martinson and others, 1987). If the filling 

or presence of a deep lake adjacent to the Wasatch fault 

zone was a suppressor of strain release (slip) on the 

Wasatch fault zone, then older lake cycles of somewhat 

lesser extent (such as the Cutler Dam and Little Valley 

lake cycles shown in fig. 2) also may have influenced 

cyclic episodes of movement on the Wasatch fault zone. 

We do not mean to infer that periods of high lake level 

were times of seismic quiescence on the fault zone but 

rather that, during these periods, the Wasatch fault zone 

may have had lower slip rates-rates similar to those 

determined from late Quaternary faults in the eastern 

Basin and Range province and backvalleys of the eastern 

Wasatch Range (rates of 0.1-0.2 mm/yr are common for 

these faults). The late Cenozoic uplift rate for the central 

part of the Wasatch Range is about 0.4 to 0.8 mm/yr as 

determined by fission-track studies (N aeser and others ' 1983; Parry and Bruhn, 1986, 1987). These long-term 

rates are intermediate-slower than the Holocene-late 

Pleistocene rate and faster than the post-middle Pleistocene rate on the Wasatch fault zone. 

If this postulated causal relation between water depth 

and slip rates is correct, one might view the Wasatch 

fault zone as having four modes of slip or tectonic 

activity: (1) low rates during the maxima of deep-lake 

cycles (perhaps approaching the long-term rates), (2) 

very low (suppressed) rates during the transgressive 

phase of a lake cycle, (3) very high (accelerated) rates 

during the regressive phase of a lake cycle, and (4) high 

rates during interpluvials (shallow or dry lake levels). A 

thorough testing of the hypothesis would require long 

and detailed (150,000 years) records of paleoseismicity at 

one or more sites on the Wasatch fault zone. Trenches 

that could expose such a record would probably be tens of 

meters deep, have sloping or stair-stepped walls, be 

costly to construct, and require several man-years to log 

and interpret. In addition, dating methods whose age 

capabilities are greater than those of 14C analyses (less 

than 40 ka) would have to be employed. 

Although there have been relatively few detailed 

studies of paleoseismicity in the eastern Basin and Range 

province (Nelson and Weisser, 1985; Nelson and 

VanArsdale, 1986; Foley and others, 1986; Keaton and 

others, 1987; Sullivan and others, 1988; West, 1989; 

Nelson and Sullivan, this volume), regional studies 

(Bucknam and Algermissen, 1984; Wallace, 1984b) show 

that slip rates on other major faults in the province have 

been as much as an order of magnitude less than rates 

documented for the Wasatch fault zone during the latest 

Pleistocene and Holocene. These studies include some 

major range-bounding faults in the transition zone 

between the Colorado Plateaus and the Basin and Range 

provinces. Many of these faults are considered to be 

fundamental, deeply penetrating structures capable of 

generating large-magnitude earthquakes. Because these 

faults have lower slip rates (and, by inference, longer 

recurrence intervals) than the Wasatch fault zone and 

generally do not show evidence of vastly different shortversus long-term slip rates, the Wasatch fault zone may 

present a special case for assessment of earthquake risk. 

CONCLUSIONS 

[T]he filling of the Bonneville Basin with water added a very 

considerable weight to the valleys, and therefore to the down-
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE .'\.55 

thrown blocks, and made no corresponding addition to the 

uplifted blocks represented in the mountain ranges.... It is 

therefore theoretically conceivable that during the presence of 

the lake the process of faulting along the mountain bases was 

stimulated, and that after the evaporation of the water the 

process was correspondingly retarded [G.K. Gilbert (1890, p. 

357)]. 

We are struck by Gilbert's astute observations about 

Lake Bonneville stratigraphy and the Wasatch fault zone 

and his inference of a causal relation between the lake 

and the fault zone, made almost 100 years ago (Machette, 

1988b). In contrast, however, our investigations suggest, somewhat meagerly, that movement on the 

Wasatch fault zone may have been stimulated by the 

catastrophic overflow and lowering of Lake Bonneville, 

whereas its transgression actually may have supressed 

or slowed the faulting process. Evidence from recent 

trenching investigations and mapping of Quaternary 

geology along the Wasatch Front suggests that the 

Wasatch fault zone was very active during the regressive 

phase of the Bonneville lake cycle (15,000-10,000 years 

ago) and during the subsequent low lake stand (Holocene 

interpluvial). 

The sparse data now available on fault activity during 

the transgressive phase of the Bonneville lake cycle 

suggest suppression of faulting on the West Valley fault 

zone (Keaton and others, 1987) but inducement of faulting in Hansel Valley (McCalpin and others, this volume). 

The apparent nonsynchroneity of movement on these 

fault systems may be related to their structural and 

geographic settings. The Hansel Valley earthquake was 

associated with an intra-Bonneville Basin fault that does 

not seem to be a fundamental structure of the northern 

Basin and Range, whereas the West Valley fault zone 

may be a major basin-margin fault that is sympathetic to 

the Wasatch fault zone. 

Our recognition of 10 discrete segments of the Wasatch 

fault zone, 6 of which have demonstrable Holocene 

movement, increases the possible number of separate 

localities where earthquakes may nucleate and where 

associated surface ruptures may occur along the Wasatch 

Front. Although the average recurrence interval for the 

active segments is about 2,000 years, we think that 

major earthquakes have struck the central, heavily 

urbanized section of the Wasatch fault zone on an average of once every four centuries (395 years) during the 

past 5,500 years. This estimate is only slightly shorter 

than Schwartz and Coppersmith's (1984) preferred composite recurrence interval of 444 years. We have no firm 

evidence that large earthquakes (those that cause significant surface faulting) have occurred in the past 400 to 

500 years, as the average would demand. However, the 

abundance of faulting events (one every 220 years) 

between 400 and 1,500 years ago-at about twice .the 

average Holocene rate-suggests that a form of temporal clustering of earthquakes has been (and may still be) 

active along the Wasatch fault zone. 

One of the most important questions remaining tr be 

answered is whether the recent temporal cluster has 

been completed. The lack of movement along the 

Brigham City segment during the past 3,600 years is 

somewhat ominous in view of evidence for moverr: ent 

during the past 1,500 years on the other four segment 3 of 

the central Wasatch fault zone to the south. 

REFERENCES CITED 

Anders, M.H., Geissman, J.W., Piety, L.A., and Sullivan, J.T., 1 l89, 

Parabolic distribution of circum-eastern Snake River Plain ~ eismicity and latest Quaternary faulting-Migratory pattern and 

association with the Yellowstone hotspot: Journal of Geophy: ical 

Research, v. 94, no. B2, p. 1589-1621. 

Antevs, E., 1948, The Great Basin, with emphasis on glacial and 

postglacial times, pt. III, Climate changes and pre-white ·1an: 

University of Utah Bulletin 38, no. 20, p. 168-191. 

--1955, Geologic-climatic dating in the west: American Antiq1 ity, 

v. 20, p. 317-335. 

Birkeland, P.W., 1984, Soils and geomorphology: New York, O:x 'ord 

University Press, 372 p. 

Bissell, H.J., 1963, Lake Bonneville-Geology of southern Utah lalley, Utah: U.S. Geological Survey Professional Paper 257-J', p. 

B101-B130. 

Bonilla, M.G., Mark, R.K., and Lienkaemper, J.J., 1984, Statis ical 

relations among earthquake magnitude, surface rupture len.-rth, 

and surface fault displacement: Bulletin of the Seismological f ociety of America, v. 74, no. 6, p. 2379-2411. 

Bruhn, R.L., Gibler, P.R., and Parry, W.T., 1987, Rupture chara( teristics of normal faults-An example from the Wasatch fault z me, 

Utah, in Coward, M.P., Dewey, J.F., and Hancock, P.L., fds., 

Continental extensional tectonics: Geological Society of Lm don 

Special Publication 28, p. 337-353. 

Bucknam, R.C., 1978, Northwestern Utah seismotectonic studier, in 

Seiders, W., and Thomson, J., compilers, Summaries of techJ jcal 

reports, v. VII: Menlo Park, Calif., U.S. Geological Survey O'fice 

of Earthquake Studies, p. 64. 

Bucknam, R.C., and Algermissen, S.T., 1984, A comparison of gee logically determined rates of late Quaternary seismic activity and 

historic seismicity data in the Great Basin, western United Stc.tes, 

in A collection of papers of International Symposium on Conti- tental Seismicity and Earthquake Prediction (Sept. 8-14, 1982, Jeijing, China): Beijing, Seismological Press, p. 169-176. 

Bucknam R.C., and Anderson, R.E., 1979, Estimation of fault-sr arp 

ages from a scarp-height-slope-angle relationship: Geology, '. 7, 

p. 11-14. 

Cluff, L.S., Brogan, G.E., and Glass, C.E., 1970, Wasatch fwlt, 

northern portion-Earthquake fault investigation and evaluation, 

prepared for the Utah Geological and Mineral Survey: Oakland, 

Calif., Woodward-Clyde and Associates, 27 p., 21 maps, scale 

1:24,000. 

--1973, Wasatch fault, southern portion-Earthquake fault investigation and evaluation, prepared for the Utah Geological and 

Mineral Survey: Oakland, Calif., Woodward-Lundgren and Associates, 79 p., 23 maps, scale 1:24,000. 

Cluff, L.S., Glass, C.E., and Brogan, G.E., 1974, Investigation and 

evaluation of the Wasatch fault north of Brigham City and Cache 

Valley faults, Utah and Idaho-A guide to land-use planning with 

recommendations for seismic safety (prepared for the U.S. Geo-
A 56 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

logical Survey under contract 14-08-001-13665): Oakland, Calif., 

Woodward-Lundgren and Associates, 147 p., 35 maps, scale 

1:24,000. 

Crittenden, M.D., Jr., 1963a, New data on the isostatic deformation of 

Lake Bonneville: U.S. Geological Survey Professional Paper 

454-E, p. E1-E31. 

---1963b, Effective viscosity of the Earth derived from isostatic 

loading of Pleistocene Lake Bonneville: Journal of Geophysical 

Research, v. 68, no. B19, p. 5517-5530. 

Crittenden, M.D., Jr., and Sorenson, M.L., 1985, Geologic map of the 

North Ogden quadrangle and part of the Ogden and Plain City 

quadrangles, Box Elder and Weber Counties, Utah: U.S. Geological Survey Miscellaneous Investigations Series Map I-1606, scale 

1:24,000. 

Crone, A.J., 1983, Amount of displacement and estimated age of a 

Holocene surface faulting event, eastern Great Basin, Millard 

County, Utah, in Gurgel, K.D., ed., Geologic excursions in neotectonics and engineering geology in Utah, guidebook-Part IV: 

Utah Geological and Mineral Survey Special Studies 62, p. 49-54. 

Crone, A.J., and Haller, K.M., 1989, Segmentation of basin-and-range 

normal faults-Examples from east-central Idaho and southwestern Montana, in Schwartz, D.P., and Sibson, R.H., eds., Proceedings of Conference XLV; a workshop on fault segmentation and 

controls on rupture initiation and termination: U.S. Geological 

Survey Open-File Report 89-315, p. 110-130. 

---1991, Segmentation and the coseismic behavior of Basin-andRange normal faults-Examples from east-central Idaho and 

southwestern Montana, U.S.A., in Hancock, P.L., Yeats, R.S., 

and Sanderson, D .J., special eds., Characteristics of active faults 

(special issue): Journal of Structural Geology, v. 13, no. 2, p. 

151-164. 

Crone, A.J., Machette, M.N., Bonilla, M.G., Lienkaemper, J.J., 

Pierce, K.L., Scott, W.E., and Bucknam, R.C., 1987, Surface 

faulting accompanying the Borah Peak earthquake and segmentation of the Lost River fault, central Idaho: Bulletin of the Seismological Society of America, v. 77, no. 3, p. 739-770, 3 pis., scale 

1:24,000. 

Currey, D.R., and Burr, T.N., 1988, Linear model of thresholdcontrolled shorelines of Lake Bonneville, in Machette, M.N., ed., 

In the footsteps of G.K. Gilbert-Lake Bonneville and neotectonics of the eastern Basin and Range Province (Geological Society of 

America Guidebook to Field Trip 12): Utah Geological and Mineral 

Survey Miscellaneous Publication 88-1, p. 104-110. 

Currey, D.R., and Oviatt, C.G., 1985, Durations, average rates, and 

probable causes of Lake Bonneville expansion, still-stands, and 

contractions during the last deep-lake cycle, 32,000 to 10,000 yrs 

ago, in Kay, P.A., and Diaz, H. F., eds., Problems of and prospects 

for predicting Great Salt Lake levels-Proceedings of a NOAA 

Conference held March 26-28, 1985: Salt Lake City, Center for 

Public Affairs and Administration, University of Utah, p. 9-24. 

Currey, D.R., Oviatt, C.G., and Plyler, G.B., 1983, Lake Bonneville 

stratigraphy, geomorphology, and isostatic deformation in westcentral Utah, in Gurgel, K.D., ed., Geologic excursions in neotectonics and engineering geology in Utah, guidebook-Part IV: Utah 

Geological and Mineral Survey Special Studies 62, p. 63-82. 

Davis, F.D., 1983a, Geologic map of the central Wasatch Front, Utah: 

Utah Geological and Mineral Survey Map 54-A, scale 1:100,000. 

---1983b, Geologic map of the southern Wasatch Front, Utah: 

Utah Geological and Mineral Survey Map 55-A, scale 1:100,000. 

---1985, Geologic map of the northern Wasatch Front, Utah: Utah 

Geological and Mineral Survey Map 53-A, scale 1:100,000. 

dePolo, C.M., Clark, D.G., Slemmons, D.B., and Aymard, W.H., 1989, 

Historical basin and range province surface faulting and fault 

segmentation, in Schwartz, D.P., and Sibson, R.H., eds., Proceedings of Conference XLV; a workshop on fault segmentation 

and controls on rupture initiation and termination: U.S. Geological 

Survey Open-File Report 89-315, p. 131-162. 

dePolo, C.M., Clark, D.G., Slemmons, D.B., and Ramelli, A.R., 1991, 

Historical surface faulting in the Basin and Range province, 

Western North America-Implications for fault segmentation, in 

Hancock, P.L., Yeats, R.S., and Sanderson, D.J., special eds., 

Characteristics of active faults (special issue): Journal of Structural 

Geology, v. 13, no. 2, p. 123-136. 

Dohrenwend, J.C., 1987, Basin and Range, in Graf, W.L., ed., 

Geomorphic systems of North America: Geological Society of 

America Centennial Special Volume 2, ch. 9, p. 303-342. 

Doser, D.I., 1985, Source parameters and faulting processes of the 

1959 Hebgen Lake, Montana, earthquake sequence: Journal of 

Geophysical Research, v. 90, no. B6, p. 4537-4555. 

---1986, Earthquake processes in the Rainbow Mountain-Fairview Peak-Dixie Valley, Nevada, region 1954-1959: Journal of 

Geophysical Research, v. 91, no. B12, p. 12572-12586. 

---1989, Extensional tectonics of northern Utah-southern Idaho, 

U.S.A., and the 1934 Hansel Valley sequence: Physics of the Earth 

and Planetary Interiors, v. 54, p. 120-134. 

Doser, D.I., and Smith, R.B., 1985, Source parameters of the 28 

October, 1983, Borah Peak, Idaho, earthquake from body wave 

analysis: Bulletin of the Seismological Society of America, v. 75, p. 

1041-1051. 

Foley, L.L., Martin, R.A., Jr., and Sullivan, J.T., 1986, Seismotectonic study for Joes Valley, Scofield, and Huntington North dams, 

Emery County and Scofield projects, Utah: U.S. Bureau of 

Reclamation Seismotectonic Report 86-7, 302 p. 

Forman, S.L., Nelson, A.R., and McCalpin, J.P., 1991, Thermoluminescence dating of fault-scarp-derived colluvium-Deciphering the 

timing of paleoearthquakes on the Weber segment of the Wasatch 

fault zone, north-central Utah: Journal of Geophysical Research, v. 

96, no. B1, p. 595-606. 

Gilbert, G.K., 1890, Lake Bonneville: U.S. Geological Survey Monograph 1, 438 p. 

---1928, Studies of Basin-Range structure: U.S. Geological Survey 

Professional Paper 153, 89 p. 

Hanson, K.L., and Schwartz, D.P., 1984, Guidebook to Late Pleistocene and Holocene faulting along the Wasatch Front and vicinity-Little Cottonwood Canyon to Scipio, Utah: American Geophysical Union, Chapman Conference on Fault Behavior and the 

Earthquake Generation Process, Snowbird, Utah, 1982, 40 p., 34 

figs., 1 pl. 

Hintze, L.F., 1973, Geologic road log of western Utah and eastern 

Nevada, pt. 1: Brigham Young University Geology Studies, v. 20, 

pt. 2, p. 10. 

Hunt, C.B., ed., 1982, Pleistocene Lake Bonneville, ancestral Great 

Salt Lake, as described in the notebooks of G.K. Gilbert, 

1875-1880: Brigham Young University Geology Studies, v. 29, pt. 

1, p. 1-225. 

Hunt, C.B., Varnes, H.D., and Thomas, H.E., 1963, Lake Bonneville-Geology of northern Utah Valley, Utah: U.S. Geological 

Survey Professional Paper 257-A, 99 p. 

Izett, G.A., 1981, Volcanic ash beds-Recorders of upper Cenozoic 

silicic pyroclastic volcanism in the Western United States: Journal 

of Geophysical Research, v. 86, no. B11, p. 10,200-10,222. 

---1982, The Bishop ash bed and some older compositionally 

similar ash beds in California, Nevada, and Utah: U.S. Geological 

Survey Open-File Report 82-582, 44 p., 3 oversized sheets. 

Izett, G.A., Wilcox, R.E., Powers, H.A., and Desborough, G.A., 1970, 

The Bishop ash bed, a Pleistocene marker bed in the western 

United States: Quaternary Research, v. 1, p. 121-132. 

Jackson, M.E., 1988, Thermoluminescence dating of Holocene paleoseismic events on the Nephi and Levan segments, Wasatch fault 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A57 

zone, Utah: Boulder, University of Colorado, unpublished M.Sci. 

thesis, 149 p. 

---in press, Number and timing of paleoseismic events on the 

Nephi and Levan segments of the Wasatch fault zone, Utah, in 

Lund, W.R., ed., Paleoseismology of Utah, v. 3: Utah Geological 

and Mineral Survey Special Studies 78. 

Johnston, A. C., 1987, Suppression of earthquakes by large continental 

ice sheets: Nature, v. 330, p. 467 (December 3, 1987). 

Keaton, J.R., and Currey, D.R., 1989, Earthquake hazards evaluation 

of the West Valley fault zone, Salt Lake City urban area, Utah 

(report sponsored by U.S. Geological Survey under contract 

14-08-0001-G1397, September 15, 1989): Salt Lake City, Utah, 

Dames and Moore, 68 p. 

Keaton, J.R., Currey, D.R., and Olig, S.J., 1987, Paleoseismicity and 

earthquake hazards evaluation of the West Valley fault zone, Salt 

Lake City urban area, Utah (report sponsored by U.S. Geological 

Survey under contract 14-08-0001-22048, March 6, 1987): Salt 

Lake City, Utah, Dames and Moore, 55 p., 33-p. app. 

King, G., and N abelek, J., 1985, Role of fault bends in the initiation and 

termination of earthquake rupture: Science, v. 228, p. 984-987. 

Lund, W.R., and Schwartz, D.P., 1987, Fault behavior and earthquake 

recurrence at the Dry Creek site, Salt Lake segment, Wasatch 

fault zone, Utah: Geological Society of America Abstracts with 

Programs, v. 19, no. 5, p. 317. 

Lund, W.R., Schwartz, D.P., Mulvey, W.E., Budding, K.E., and 

Black, B.D., 1991, Fault behavior and earthquake recurrence on 

the Provo segment of the Wasatch fault zone at Mapleton, Utah 

Co., Utah, in Lund, W.R., ed., Paleoseismology of Utah, v. 1: 

Utah Geological and Mineral Survey Special Studies 75, 41 p. 

Machette, M.N., 1984, Preliminary investigations of late Quaternary 

slip rates along the southern part of the Wasatch fault zone, in 

Hays, W.W., and Gori, P.L., eds., Proceedings of Conference 

XXVI-A workshop on evaluation of regional and urban earthquake hazards and risk in Utah: U.S. Geological Survey Open-File 

Report 84-763, p. 391-406. 

---1985, Late Cenozoic geology of the Beaver basin, southwestern 

Utah: Brigham Young University Geology Studies, v. 32, pt. 1, p. 

19-37. 

---1987, Recent studies of the Wasatch fault zone, Utah-A 

review: Geological Society of America Abstracts with Programs, 

v. 19, no. 5, p. 317. 

---1988a, American Fork Canyon-Holocene faulting, the Bonneville fan-delta complex, and evidence for the Keg Mountain 

oscillation, in Machette, M.N., ed., In the footsteps of G.K. 

Gilbert-Lake Bonneville and neotectonics of the eastern Basin 

and Range province (Geological Society of America Guidebook to 

Field Trip 12): Utah Geological and Mineral Survey Miscellaneous 

Publication 88-1, p. 89-95. 

---1988b, ed., In the footsteps of G.K. Gilbert-Lake Bonneville 

and neotectonics of the eastern Basin and Range Province (Geological Society of America Guidebook to Field Trip 12): Utah 

Geological and Mineral Survey Miscellaneous Publication 88-1, 

120 p. 

---1989, Preliminary surficial geologic map of eastern Utah Valley, Utah County and parts of Salt Lake and Juab Counties, Utah: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF-2109, scale 1:50,000, 30-p. text. 

---in press, Surficial geologic map of eastern Utah Valley, Utah 

County and parts of Salt Lake and Juab Counties, Utah: U.S. 

Geological Survey Miscellaneous Investigations Map I-2095, scale 

1:50,000. 

Machette, M.N., and Lund, W.R., 1987, Trenching across the American Fork segment of the Wasatch fault zone, Utah: Geological 

Society of America Abstracts with Programs, v. 19, no. 5, p. 317. 

Machette, M.N., Personius, S.F., and Nelson, A.R., 1986, Late 

Quaternary segmentation and slip-rate history of the Wasatch 

fault zone, Utah: EOS [Transactions of the American Geophysical 

Union], v. 67, no. 44, p. 1107. 

---1987, Quaternary geology along the Wasatch fault zone-Segmentation, recent investigations, and preliminary conclusions, in 

Gori, P.L., and Hays, W.W., eds., Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. 

Geological Survey Open-File Report 87-585, p. A1-A72. 

Machette, M.N., Personius, S.F., Nelson, A.R., Schwartz, D.P., and 

Lund, W.R., 1991, The Wasatch fault zone, Utah-Segmentation 

and history of Holocene earthquakes, in Hancock, P.L., Yeats, 

R.S., and Sanderson, D.J., special eds., Characteristics of active 

faults (special issue): Journal of Structural Geology, v. 13, no. 2, p. 

137-150. 

Madsen, D.B., and Currey, D.R., 1979, Late Quaternary glacial and 

vegetation changes, Little Cottonwood Canyon area, Wasatch 

Mountains, Utah: Quaternary Research, v. 12, p. 254-270. 

Marsell, R.E., and Threet, R.L., 1964, Geologic map of Salt Lake 

County, Utah, in Crawford, A.L., ed., Geology of Salt Lake 

County: Utah Geological and Mineral Survey Bulletin 69, scale 

1:63,360. 

Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., 

Jr., and Shackleton, N.J., 1987, Age dating and the orbital theory 

of the Ice Ages-Development of a high-resolution 0 to 300,000-

year chronostratigraphy: Quaternary Research, v. 27, p. 1-29. 

McCalpin, J., 1986, Thermoluminescence (TL) dating in seismic hazard 

evaluations-An example from the Bonneville basin, Utah, in 

Symposium on Engineering Geology and Soils Engineering, 22d, 

Boise, Idaho, 1986, Proceedings: p. 156-176. 

---1989, Surficial geologic map of the Cache Valley, Cache County, 

Utah: U.S. Geological Survey Miscellaneous Field Studies Map 

MF-2107, scale 1:50,000. 

McCalpin, J., and Forman, S.L., 1991, Quaternary faulting and thermoluminescence dating of the East Cache fault zone, north-central 

Utah: Bulletin of the Seismological Society of America, v. 81, no. 

1, p. 139-161. 

McCoy, W.D., 1987, Quaternary aminostratigraphy of the Bonneville 

basin, Western United States: Geological Society of America 

Bulletin, v. 98, p. 99-112. 

Miller, R.D., 1982, Surficial geologic map along part of the Wasatch 

Front, Great Salt Lake and Utah Valleys, Utah: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1477, scale 

1:100,000. 

Morner, N.-A., 1978, Faulting, fracturing, and seismicity as functions 

of glacio-isostasy in Fennoscandia: Geology, v. 6, p. 41-45. 

Morrison, R.B., 1965, Quaternary geology of the Great Basin, in 

Wright, H.E., Jr., and Grey, D.G., eds., Quaternary geology of 

the United States: Princeton, N.J., Princeton University Press, p. 

265-285. 

Naeser, C.W., Bryant, B., Crittenden, M.D., Jr., and Sorensen, M.L., 

1983, Fission-track ages of apatite in the Wasatch Mountains, 

Utah-An uplift study, in Miller, D.M., Todd, V.R., and Howard, 

K.A., eds., Tectonic and stratigraphic studies in the eastern Great 

Basin: Geological Society of America Memoir 157, p. 29-36. 

Nelson, A.R., and Personius, S.F., 1987, A nonconservative barrier to 

Holocene rupture propagation in the northern Wasatch fault zone, 

Utah, USA: International Union for Quaternary Research International Congress, 12th, Ottawa, 1987, Programme with 

Abstracts, p. 231. 

---1990, Preliminary surficial geologic map of the Weber segment, 

Wasatch fault zone, Weber and Davis Counties, Utah: U.S. 

Geological Survey Miscellaneous Field Studies Map MF-2132, 

scale 1:50,000. 
A 58 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

---in press, Surficial geologic map of the Weber segment, Wasatch 

fault zone, Weber and Davis Counties, Utah: U.S. Geological 

Survey Miscellaneous Investigations Map I-2199, scale 1:50,000. 

Nelson, A.R., and Weisser, R.R., 1985, Quaternary faulting on 

Towanta Flat, northwestern Uinta Basin, in Picard, M.D., ed., 

Geology and energy resources, Uinta Basin, Utah: Utah Geological 

Association Publication 12, p. 147-158. 

Nelson, A.R., Klauk, R.H., Lowe, M., and Garr, J.D., 1987, Holocene 

history of displacement on the Weber segment of the Wasatch fault 

zone at Ogden, northern Utah: Geological Society of America 

Abstracts with Programs, v. 19, no. 5, p. 322. 

Oviatt, C.G., 1986a, Geologic map of the Cutler Dam quadrangle, Box 

Elder and Cache Counties, Utah: Utah Geological and Mineral 

Survey Map 91, scale 1:24,000. 

---1986b, Geologic map of the Honeyville quadrangle, Box Elder 

and Cache Counties, Utah: Utah Geological and Mineral Survey 

Map 88, scale 1:24,000. 

Oviatt, C.G., McCoy, W.D., and Reider, R.G., 1985, Quaternary 

lacustrine stratigraphy along the lower Bear River, Utah-Evidence for a shallow early Wisconsin lake in the Bonneville basin: 

Geological Society of America Abstracts with Programs, v. 17, no. 

4, p. 260. 

---1987, Evidence for a shallow early or middle Wisconsin-age lake 

in the Bonneville basin, Utah: Quaternary Research, v. 27, p. 

248-262. 

Parry, W.T., and Bruhn, R.L., 1986, Pore fluid and seismogenic 

characteristics of fault rock at depth on the Wasatch fault, Utah: 

Journal of Geophysical Research, v. 91, no. B1, p. 730-744. 

---1987, Fluid inclusion evidence for minimum 11 km vertical 

offset on the Wasatch fault, Utah: Geology, v. 15, p. 67-70. 

Pearthree, P.A., and Demsey, K., 1987, Patterns of Holocene faulting 

and the rate of extension in central Nevada: Geological Society of 

America Abstracts with Programs, v. 19, no. 7, p. 802. 

Perkins, D.M., 1987, Contagious fault rupture, probabilistic hazard, 

and contagion observability, in Crone, A.J., and Omdahl, E.M., 

eds., Proceedings of Conference XXXIX-Directions in paleoseismology: U.S. Geological Survey Open-File Report 87-673, p. 

428-439. 

Personius, S.F., 1986, The Brigham City segment-A new segment of 

the Wasatch fault zone, northern Utah: Geological Society of 

America Abstracts with Programs, v. 18, no. 5, p. 402. 

---1988a, Preliminary surficial geologic map of the Brigham City 

segment and adjacent parts of the Weber and Collinston segments, 

Wasatch fault zone, Box Elder and Weber Counties, Utah: U.S. 

Geological Survey Miscellaneous Field Studies Map MF -2042, 

scale 1:50,000. 

---1988b, A brief summary of the surficial geology along the 

Brigham City segment of the Wasatch fault zone, Utah, in 

Machette, M.N., ed., In the footsteps of G.K. Gilbert-Lake 

Bonneville and neotectonics of the eastern Basin and Range 

Province (Geological Society of America Guidebook to Field Trip 

12): Utah Geological and Mineral Survey Miscellaneous Publication 

88-1, p. 26-32. 

---1990, Surficial geologic map of the Brigham City segment and 

adjacent parts of the Weber and Collinston segments, Wasatch 

fault zone, Box Elder and Weber Counties, Utah: U.S. Geological 

Survey Miscellaneous Investigations Map I-1979, scale 1:50,000. 

---1991a, Paleoseismic analysis of the Wasatch fault zone at the 

Brigham City trench site, Brigham City, Utah, in Lund, W.R., 

ed., Paleoseismology of Utah, v. 2: Utah Geological and Mineral 

Survey Special Publication 76, p. 1-18. 

---1991b, Paleoseismic analysis of the Wasatch fault zone at the 

Pole Patch trench site, Pleasant View, Utah, in Lund, W.R., ed., 

Paleoseismology of Utah, v. 2: Utah Geological and Mineral 

Survey Special Publication 76, p. 19-39. 

Personius, S.F., and Gill, H.E., 1987, Holocene displacement on the 

Brigham City segment of the Wasatch fault zone near Brigham 

City, Utah: Geological Society of America Abstracts with Programs, v. 19, no. 5, p. 326. 

Personius, S.F., and Scott, W.E., 1990, Preliminary surficial geologic 

map of the Salt Lake City segment and parts of adjacent segments 

of the Wasatch fault zone, Davis, Salt Lake, and Utah Counties, 

Utah: U.S. Geological Survey Miscellaneous Field Studies Map 

MF-2114, scale 1:50,000. 

---in press, Surficial geologic map of the Salt Lake City segment 

and parts of adjacent segments of the Wasatch fault zone, Davis, 

Salt Lake, and Utah Counties, Utah: U.S. Geological Survey 

Miscellaneous Investigations Map I-2106, scale 1:50,000. 

Peterson, D.L., 1974, Bouguer gravity map of part of the northern 

Lake Bonneville basin, Utah and Idaho: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-627, scale 1:250,000. 

Pierce, K.L., and Colman, S.M., 1986, Effect of height and orientation 

(microclimate) on geomorphic degradation rates and processes, 

late-glacial terrace scarps in central Idaho: Geological Society of 

America Bulletin, v. 97, p. 869-885. 

Pierce, K.L., and Scott, W.E., 1986, Migration of faulting along and 

outward from the tract of thermo-tectonic activity in the eastern 

Snake River Plain region during the last 15 m.y.: EOS [Transactions of the American Geophysical Union], v. 67, no. 44, p. 1225. 

Richmond, G.M., and Fullerton, D.S., 1987, Introduction to Quaternary glaciation in the United States of America: Quaternary 

Science Reviews, v. 5, p. 3-10. 

Savage, J.C., and Hastie, L.M., 1966, Surface deformation associated 

with dip-slip faulting: Journal of Geophysical Research, v. 71, no. 

B20, p. 4897-4904. 

Schumm, S.A., 1977, The fluvial system: New York, John Wiley, 338 p. 

Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquakes-Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

Schwartz, D.P., Hanson, K.L., and Swan, F.H., III, 1983, Paleoseismic investigations along the Wasatch fault zone-An update, in 

Gurgel, K.D., ed., Geologic excursions in neotectonics and engineering geology in Utah, guidebook-Part IV: Utah Geological 

and Mineral Survey Special Studies 62, p. 45-48. 

Schwartz, D.P., Lund, W.R., Mulvey, W.E., and Buddington, K.E., 

1988, New paleoseismicity data and implications for space-time 

clustering of large earthquakes on the Wasatch fault zone, Utah 

[abs.]: Seismological Research Letters, v. 59, no. 1, p. 15. 

Scott, W.E., 1988, G.K. Gilbert's observations of post-Bonneville 

movement along the Warm Springs fault, Salt Lake County, Utah, 

in Machette, M.N., ed., In the footsteps of G.K. Gilbert-Lake 

Bonneville and neotectonics of the eastern Basin and Range 

province (Geological Society of America Guidebook to Field Trip 

12): Utah Geological and Mineral Survey Miscellaneous Publication 

88-1, p. 44-46. 

Scott, W.E., and Shroba, R.R., 1985, Surficial geologic map of an area 

along the Wasatch fault zone in the Salt Lake Valley, Utah: U.S. 

Geological Survey Open-File Report 85-448, pamphlet, 2 pls., 

scale 1:24,000. 

Scott, W.E., McCoy, W.D., Shroba, R.R., and Rubin, M., 1983, 

Reinterpretation of the exposed record of the last two cycles of 

Lake Bonneville, Western United States: Quaternary Research, v. 

20, no. 3, p. 261-285. 

Scott, W.E., Pierce, K.L., and Hait, M.H., Jr., 1985, Quaternary 

tectonic setting of the 1983 Borah Peak earthquake, central Idaho: 

Bulletin of the Seismological Society of America, v. 75, no. 4, p. 

1053-1066. 

Shenon, P.J., 1936, The Utah earthquake of March 12, 1934 (extracts 

from a report), in Neumann, F., United States earthquakes, 1934: 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A59 

Washington, D.C., U.S. Coast and Geodetic Survey Serial593, p. 

43-48. (Reissued in 1968 by National Earthquake Information 

Center as United States Earthquakes, 1928--1935.) 

Simpson, D.W., 1986, Triggered earthquakes: Annual Review of Earth 

and Planetary Science Letters, v. 14, p. 21-42. 

Slemmons, D.B., 1957, Geological effects of the Dixie Valley-Fairview 

Peak, Nevada, earthquakes of December 16, 1954: Bulletin of the 

Seismological Society of America, v. 47, p. 353-375. 

---1977, Faults and earthquake magnitude, in State-of-the-art for 

assessing earthquake hazards in the United States: U.S. Army 

Corps of Engineers, Waterways Experiment Station Miscellaneous Paper S-73-1, rept. 6, p. 1-129. 

Smith, R.B., 1988, Seismicity and earthquake hazards of the Borah 

Peak-Hebgen Lake-Yellowstone-Teton region-Implications for 

earthquakes in extensional and active volcanic regimes: Geological 

Society of America Abstracts with Programs, v. 20, no. 7, p. A12. 

Smith, R.B., and Sbar, M.L., 1974, Contemporary tectonics and 

seismicity of the western United States with emphasis on the 

Intermountain Seismic Belt: Geological Society of America Bulletin, v. 85, no. 8, p. 1205-1218. 

Smith, R.B., Richins, W.D., and Doser, D.l., 1985, The 1983 Borah 

Peak, Idaho, earthquake-Regional seismicity, kinematics of 

faulting, and tectonic mechanism, in Stein, R.S., and Bucknam, 

R. C., eds., Proceedings of Workshop XXVIII on the Borah Peak, 

Idaho, earthquake: U.S. Geological Survey Open-File Report 

85-290, p. 236-263. 

Stein, R.S., and Bucknam, R.C., 1985, Basin and Range viewed from 

Borah Peak: EOS, Transactions of the American Geophysical 

Union, v. 66, no. 34, p. 603-604. 

Stover, C.W., ed., 1987, United States earthquakes, 1983: U.S. 

Geological Survey Bulletin 1698, 196 p. 

Sullivan, J.T., Nelson, A.R., LaForge, R.C., Wood, C.K., and 

Hansen, R.A., 1988, Central Utah regional seismotectonic study 

for USBR dams in the Wasatch Mountains: U.S. Bureau of 

Reclamation Seismotectonic Report 88--5, 338 p. 

Swan, F.H., III, Schwartz, D.P., and Cluff, L.S., 1980, Recurrence of 

moderate to large magnitude earthquakes produced by surface 

faulting on the Wasatch fault zone, Utah: Bulletin of the Seismological Society of America, v. 70, p. 1431-1462. 

Wallace, R.E., 1984a, Faulting related to the 1915 earthquakes in 

Pleasant Valley, Nevada: U.S. Geological Survey Professional 

Paper 1274-A, 33 p. 

---1984b, Patterns and timing of late Quaternary faulting in the 

Great Basin province and relation to some regional tectonic 

features: Journal of Geophysical Research, v. 89, no. B7, p. 

5763-5769. 

---1987, Variations in slip rates, migration, and grouping of slip 

events on faults in the Great Basin province: Bulletin of the 

Seismological Society of America, v. 77, no. 3, p. 868--876. 

Wallace, R.E., and Whitney, R.A., 1984, Late Quaternary history of 

the Stillwater seismic gap, Nevada: Bulletin of the Seismological 

Society of America, v. 74, no. 1, p. 301-314. 

West, M.W., 1989, Neotectonics of Derby Hogsback and Absaroka 

thrust plates, Uintah County, Wyoming, and Summit County, 

Utah, with applications to earthquake hazard assessment: Golden, 

Colorado School of Mines, unpublished Ph. D. thesis, v. 1, 358 p.; v. 

2, 142-p. appendix, 17 pls. 

Wheeler, R.L., 1987, Boundaries between segments of normal faultsCriteria for recognition and interpretation, in Crone, A.J., and 

Omdahl, E.M., eds., Proceedings of Conference XXXIX-Directions in paleoseismology: U.S. Geological Survey Open-File 

Report 87-673, p. 385-398. 

---1989, Persistent segment boundaries on basin-range normal 

faults, in Schwartz, D.P., and Sibson, R.H., eds., Proceedings of 

Conference XLV; a workshop on fault segmentation and controls 

on rupture initiation and termination: U.S. Geological Survey 

Open-File Report 89-315, p. 432-444. 

Wheeler, R.L., and Krystinik, K.B., 1987, Evaluating coinciding 

anomalies along a fault trace or traverse-Simulations and statistical procedures: U.S. Geological Survey Bulletin 1802, 12 p. 

---1988, Segmentation of the Wasatch fault zone, Utah-Summaries, analyses, and interpretations of geological and geophysical 

data: U.S. Geological Survey Bulletin 1827, 47 p. 

Williams, J.S., 1958, Geologic atlas of Utah, Cache County: Utah 

Geological and Mineral Survey Bulletin 64, 104 p. 

Witkind, I.J., 1964, Reactivated faults north of Hebgen Lake, in The 

Hebgen Lake, Montana, earthquake of August 17, 1959: U.S. 

Geological Survey Professional Paper 435-G, p. G37-G50. 

Zoback, M. L., 1983, Structure and Cenozoic tectonism along the 

Wasatch fault zone, Utah: Geological Society of America Memoir 

157' p. 3-27. 

APPENDIX 

PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A63 

DATING TECHNIQUES AND METHODOLOGY 

This appendix contains a discussion of the methods 

that we used to estimate the timing of depositional or 

tectonic events from new 14C ages and thermoluminescence (TL) age estimates cited in the main body of this 

report and listed in table A1. The 14C ages are of two 

types: (1) conventional gas-proportional and acceleratormass-spectrometer (AMS) ages on charcoal samples and 

(2) apparent mean residence time (AMRT) ages (gas-

. proportional counters) on organic concentrates from the 

A horizons of soils. Although both types of age are 

measured in radiocarbon years e4

C yr B.P.) and converted to calendric (calendar year) dates, the geologic 

interpretation of AMRT ages is a complex process, and 

their associated total errors are larger than the errors on 

ages from charcoal. 

Ages derived from the TL method (Aitken, 1985; 

Forman and others, 1988, 1989) are based on the property of some minerals to emit light when heated. 

Although widely used in archeologic investigations, thermoluminescence has been applied only recently to problems in Quaternary geology (Wintle and Huntley, 1982). 

Exposing mineral grains to sunlight for periods of as 

little as 8 hours drains their inherited or acquired TL 

signal; if a mineral has been exposed to sunlight during 

transport, its TL signal is zeroed. In turn, burial allows 

acquisition of a new TL signal, which restarts the TL 

clock. Thermoluminescence directly dates the last exposure of a mineral to sunlight and, as such, reflects the 

time of its burial by any number of surficial processes-in 

this case, by colluviation in response to surface faulting. 

TL ages are reported in years (assumed to be calendar 

years), and we know of no systematic deviation of 

reported TL ages from calendric dates (Forman and 

others, 1988, 1989). However, the methodology of TL 

analysis is still evolving, and the values of several 

parameters used in calculating TL ages for most sites are 

not well known. The relation between 14C age and 

calendric date, on the other hand, is known to be 

nonlinear owing to variable rates of 14C production in the 

upper atmosphere; that is, 14C years do not necessarily 

correspond with calendar years (Stuiver and Quay, 

1979). Extensive studies on tree rings of known ages in 

Europe and North America show that 14C ages in the 

2,000-year age range can be as much as 100 years too 

young, whereas those in the 4,000- to 5,000-year range 

may be several hundreds of years too young (Stuiver and 

Kra, 1986). 14C ages of less than 1,000 years may be 

either older or younger than the corresponding calendric 

date. 

The ages in this report fall into two categories: numerical ages and calibrated ages as defined by Colman and 

others (1987). Radiocarbon and TL ages are numerical 

ages, but estimates of the times of the beginning and end 

of soil formation for A horizons are, at best, calibrated 

ages. Note that Colman and others' (1987) use of the 

term "calibrated" does not refer to the calibration of 14C 

dates, which is the topic of much of the following 

discussion. We follow the suggestions of the North 

American Commission on Stratigraphic Nomenclature 

(1983) (see also Stuiver and Kra, 1986; Colman and 

others, 1987) in reserving the noun "date" for 14C ages 

that can be accurately calibrated to the calendric time 

scale. Most of our 14C ages from charcoal can be calibrated, and the resulting dates are reported in calendar 

years (cal yr B.P.). Although we use the 14C calibration 

procedure to improve the accuracy of our AMRT ages, 

we do not consider the resulting "corrected ages" to be 

accurate enough to be termed "dates" (they represent 

intervals of time, not points in time); thus, we refer to 

them as calibrated AMRT ages. We choose to highlight 

the large uncertainties in our estimates of the ages of the 

beginning and ending times of A horizon formation by 

calling them "age estimates." This usage helps to distinguish these soil formation ages, which have large uncertainties, from "numerical ages" e4

C and TL), which are 

more precise and accurate. Because estimates of soil 

formation ages are derived from 14C ages that have been 

calibrated to the calendric time scale (explained below), 

the age estimates are measured in years. In this report, 

we refer to approximate ages, groups of ages, and 

estimates derived from such ages in terms of thousands 

of years (ka) (complete listing of ages are reported in 

table A1). This abbreviation is used only for ages, not for 

time intervals, which are spelled out (3,000 years) (North 

American Stratigraphic Code, 1983). 

RADIOCARBON CALIBRATION 

To directly compare our 14C ages with TL ages, we 

must convert the 14C ages to the calendric time scale. 

This calibration procedure allows us to estimate the 

times of soil formation and fault events and to estimate 

earthquake recurrence intervals in calendar years rather 

than in 14C years. The calibration of 14C ages is much 

simpler and more accurate than it was a few years ago, 

owing to the development of high-precision calibration 

curves for the past 9,000 years (Stuiver and Kra, 1986) 

and the distribution of computer software that performs 

the calibration calculations (Stuiver and Reimer, 1986). 

The calibration procedure requires making several 

decisions about the analyzed samples: a laboratory error 

multiplier must be estimated, the type of reference data 

(decadal or bidecadal) must be selected, and the age span 

of the carbon in the sample must be estimated. The error 

reported with 14C ages from most laboratories is based 
TABLE A 1.-Data for radiocarbon and thermoluminescence analyses cited in the text > 

[Published or unpublished data from other workers not included. -, not applicable] ~ 

~ 

Age4 

Laboratory 

number Radiocarbon age Calibrated charcoal MRC Estimated 

(field Material Geologic material (in 14C yr B.P.) or and AMRT 14C ages and age of soil 

sample sampled and map unit TL age estimate (range of one-sigma error) CAS horizon 

Trench1 number)2 (preparation)3 (sampling depth, in m) (in cal yr) [x of ages] (in yr)5 contacts6 Remarks 

Brigham City trench 1 (S.F. Personius, September 1986) 

BC-1 ..... PITT-0092 A horizon Scarp slope colluvium, 1,260±60 14C - - - Minimum time since youngest event. ~ 

t:rj 

(BC-7). (B). unit 1-2A (0.35). 0 

USGS-2535 A horizon Scarp slope colluvium, 3,430±70 14C 3,700 200 CAS 3,600 UHC Maximum time since most recent 0 

-

z (BC-5). (B). unit 3A (1.35). (3,529-3,889) 100 MRCU event. > 

USGS-2536 A horizon Soil on debris flow, 4,330±70 14C 4,910 200 CAS 4,700 UHC Maximum time since second event. t'"'l 

(BC-4). (B). unit 9-2A (2.6). (4, 759-5,079) 200 MRCU t:rj 

> USGS-2604 A horizon Scarp slope colluvium, 4,340±1oo 14C 4,915 200 CAS 4,700 UHC Maximum time since second event. ~ 

(BC-8). (B). unit 5-1A (2.6). (4,659-5,249) 200 MRCU t-3 

::c: 

.0 

Pole Patch trench 1 (S.F. Personius, September 1985) ~ 

> 

PP-1. ..... PITT-0093 Organic matter Soil debris in tectonic 4,190±125 14C ~ 

4,760 200 CAS Near time of most recent event. t:rj 

(SP-85-4-1). (B). crack; unit c (0.60). (4,419-5,029) 200 MRCU 4,600 AMRT ::c: 

> N 

Garner Canyon exposure (A.R. Nelson, July 1985) > 

~ 

DIC-3278 A horizon Scarp slope colluvium, 150±70 14C 161 200 CAS 350 LHC Modern soil on slope. tj 

w 

(ARN85-04). (B). unit 1-1A (0.25). (160-350) 200 MRCL > 

DIC-3279 A horizon Scarp slope colluvium, 1,300±80 14C 1,234 200 CAS 1,150 UHC Maximum time since most recent z 

tj 

(ARN85-05). (B). unit 3-1A (1.20). (1,050-1,370) 100 MRCU 1,450 LHC event. ~ 

200 MRCL -

w 

DIC-3237 A horizon Scarp slope colluvium, 2, 110± 100 14C 2,106 200 CAS 2,000 UHC Maximum time since second event. ~ 

(ARN85-03). (B). unit 5A (1. 90). (1,880-2,369) 100 MRCU 2,250 LHC > t'"'l 

200 MRCL 0 

z 

East Ogden trenches (A.R. Nelson, September 1986) 0 

t-3 

::c: 

E0-1 ..... PITT-0100 A horizon Scarp slope colluvium, 285±30 14C 346 200 CAS 450 LHC Modern soil near fault scarp. t:rj 

(ARN86-26). (B). unit 1-3A (0.15). (300-388) 100 MRCL :.;: 

PITT-0098 A horizon Scarp slope colluvium, 1,365±40 14C 1,294 250 CAS 1,100 UHC Maximum time since most recent > w (ARN86-19) (B). unit 3-2A (0. 72). (1,214-1,357) 200 MRCU 1,500 LHC event. > t-3 

200 MRCL 0 

ITL-113 A horizon Scarp slope colluvium, 1,200± 100 TL Shows nonuniform sedimentation ::c: - - - ~ (E01TL11). (C). unit 1-3A (0.25). rate in colluvium. ~ 

ITL-47 A horizon Distal scarp colluvium, 1,200±20 TL - - - Probably postdates most recent 0 

z (E01TL10). (C). unit 1-5A (0.40). event. t-3 

ITL-112 A horizon Distal scarp colluvium, 2,000±200 TL - - - Shows nonuniform sedimentation 

(E01TL9). (C). unit 1-5A (0.65). rate in colluvium. 

ITL-75 A horizon Distal scarp colluvium, 2,500±300 TL - - - Predates(?) most recent event. 

(E01TL8). (C). unit 1-5A (0.84). 

ITL-72 A horizon Distal scarp colluvium, 3,100±300 TL - - - Shows nonuniform sedimentation 

(E01TL7). (C). unit 3-1A (1.28). rate in colluvium. 

ITL-24 A horizon Distal scarp colluvium, 2,700±300 TL - - - Maximum time since second event. 

(E01TL6). (C). unit 3-1A (1.65). 

ITL-138 A horizon A horizon sediment, 4,600±400 TL - - - Maximum time since earliest event. 

(E01TL3). (C). unit 7-1A (1.85). 
TABLE Al.-Data for radiocarbon and thermoluminescence analyses cited in the text-Continued 

Age4 

Laboratory 

number Radiocarbon age Calibrated charcoal MRC Estimated 

(field Material Geologic material (in 14C yr B.P.) or and AMRT 14C ages and age of soil 

sample sampled and map unit TL age estimate (range of one-sigma error) CAS horizon 

Trench1 number)2 (preparation)3 (sampling depth, in m) (in cal yr) [x of ages] (in yr)5 contacts6 Remarks 

East Ogden trenches (A.R. Nelson, September 1986)-Continued 

E0-2 ..... ITL-115 A horizon Distal scarp colluvium, 1,500±200 TL - - Maximum time since most recent event. 

(E02TL3). (C). unit 3-1A (0. 75). 

ITL-114 A horizon Distal scarp colluvium, 1,800±200 TL - - - Maximum time since most recent event. 

(E02TL2). (C). unit 3-1A (1.05). 

ITL-74 A horizon Distal scarp colluvium, 3,200±300 TL - - - Probably minimum time since first 

(E02TL4). (C). unit 8-1A (1.35). (oldest) event. 

ITL-80 A horizon A horizon sediment, unit 4,000±400 TL - - - Maximum time since first event. '"I:! 

(E02TL5). (C). 7A (2.35). > t-' 

PITT-0097 A horizon Scarp slope colluvium, 122 percent - - - Top of modern soil on scarp; trj 

0 (ARN86-14). (B). unit 1-3A (0.05). modern. shows extent of burrowing. w 

trj 

PITT-0106 A horizon Scarp slope colluvium, 122 percent - - - Bottom of modern soil on scarp; ~ 

w (ARN86-15). (B). unit 1-4A (0.45). modern. shows extent of burrowing. ~ 

PITT-0096 A horizon Scarp slope colluvium, 1,065±30 TL 989 200 CAS Maximum time since most recent 0 

t-' 

(ARN86-13). (B). unit 3-1A (1.30). (941-1,032) 100 MRCU 850 UHC major event. 0 

200 MRCL 1,150 LHC 0 

~ 

PITT-0104 A horizon Scarp slope colluvium, 3,295±130 14C 3,518 100 CAS Minimum time since first event. 0 

(ARN86-06). (B). unit 5-1A (2.65). (3,259-3,859) 50 MRCU 3,450 UHC Horizon may be eroded. Maxi- '"%j 

50 MRCL 3,550 LHC mum time since second event. t-3 

:I: 

USGS-2499 Charcoal Top of soil on debris 4,100±180 14C 4,568, 4,589, - - May be close to time of first event. trj 

(ARN86-20). (A). flow, unit 7A (2.50). 4,606, 4, 773, ~ 

4,804. > w (4,240-4,972) > 

[x=4,668] t-3 

0 

PITT-0094 A horizon Soil on debris flows, 4,505±65 14C 5,131 200 CAS Maximum AMRT of first event. :I: 

(ARN86-05). (B). unit 7A (2.35). (4,939-5,349) 400 MRCU 4,750 UHC '"%j 

> 200 MRCL 5,350 LHC ~ 

AA-2270 Charcoal Scarp slope colluvium, 4, 780± 1oo 14C 5,484, 5,521, - Collected 20 em west of PITT-104; t-' 

t-3 

(ARN86-21). (A). unit 5-1A (2.67). 5,571 charcoal is reworked. N 

(5,299-5, 729) 0 

z [x=5,528] trj 

E0-3 ..... PITT-0101 A horizon Weak soil on debris flow, 290±60 14C 325 100 CAS Maximum age on one of most recent 

(ARN86-17). (B). unit 6-1A (0.60). (7-490) 50 MRCU 250 UHC debris flows across scarp. 

50 MRCL 350 LHC 

PITT-0095 A horizon Slopewash, unit 9-2A 1,340±95 14C 1,274 200 CAS Maximum time since most recent 

(ARN86-10). (B). (1.00). (1,060-1,420) 200 MRCU 1,050 UHC event. Thick soil on slopewash 

above scarp. 

PITT-0107 A horizon Base of soil on debris 2,820±65 14C 2,933 200 CAS Minimum age for extensive upper 

(ARN86-18). (B). flow, unit 10-1A (2,819-3, 119) 300 MRCU 2,650 UHC Holocene debris flow not found 

(1.80). 100 MRCL 3,050 LHC in E0-1 and E0-2. 

PITT-0105 A horizon Soil on alluvium, 112 percent - - - Shows extent of animal burrowing. 

(ARN86-08). (B). unit 5-1A (0.80). modern 14C Youngest alluvium at site. 

AA-2269 Charcoal Sandy alluvium, unit 580±70 14c 555, 617, - - Within tectonically(?) deformed 

(ARN86-43). (A). 9-1 (0.80). 622 unit. Maximum time since possible 

(510-680) small, most recent event. > [x=598] 0":\ 

Ol 
TABLE AI.-Data for radiocarbon and thermoluminescence analyses cited in the text-Continued > <:r.l 

Age4 <:r.l 

Laboratory 

number Radiocarbon age Calibrated charcoal MRC Estimated 

(field Material Geologic material (in 14C yr B.P.) or and AMRT 14C ages and age of soil 

sample sampled and map unit TL age estimate (range of one-sigma error) CAS horizon 

Trench1 number)2 (preparation)3 (sampling depth, in m) (in cal yr) [x of ages] (in yr)5 contacts6 Remarks 

East Ogden trenches (A.R. Nelson, September 1986)-Continued 

E0-4 ..... PITT-{)103 A horizon Scarp colluvium, unit 460±65 14C 514 100 CAS 450 UCH Minimum time since possible small, ~ (ARN86-{)4). (B). 2-1A (0.35). (357-570) 50 MRCU 550 LHC most recent event. Proximal M 

50 MRCL colluvium. 0 

PITT-{)102 A horizon Crotovinas in scarp 105±55 14C - - - Shows extent of animal burrowing 0 

z (ARN86-{)3). (B). colluvium, unit 4-1A in coarse-grained alluvium. > 

(0.60). t-t 

E0-5 ..... PITT-{)108 A horizon Scarp slope colluvium, 815±45 14C Modern A horizon; must include M - - - > (ARN86-24). (B). unit 1-1A (0.05). older reworked A horizon sediment. ~ 

PITT-{)099 A horizon Organic-rich distal 390±25 14C 481 50 CAS 800 LHC Organic-rich lens near base of t-3 

::c: 

(ARN86-23). (B). colluvium, unit 1-2A (439-506) 400 MRCL modern A horizon adjacent to fault. .0 

c:::: (0.30). > 

PITT-{)109 A horizon Scarp slope colluvium, 1,040±75 14C 971 250 CAS 750 UHC Maximum time since most recent ~ 

M (ARN86-25). (B). unit 3-1A (1.20). (800-1,110) 200 MRCU 1,150 LHC major event. ::c: 200 MRCL > N 

American Fork Canyon trenches (M.N. Machette, September 1986) > 

~ 

t:1 

AF-1 ..... AA-2267 Charcoal Fault-scarp colluvium, 140±120 14C 142, 214, 264 25 CAS Minimum time since most recent UJ. - > (AF-1A). (A). unit 1 (0.55). (post-1950 to 480) event. Excludes calendric z 

[x=207] dates of less than 0 and 20 t:1 

yr B.P. ~ 

ITL-23 A horizon Fault scarp colluvium, 400±100 TL - - - Near time of most recent event. UJ. 

-

~ (F86-U8). (C). unit 2 (0.55). > 

USGS-2532 A horizon Fault-scarp colluvium, 980±7o 14C 713 300 CAS 500 UHC Near time of most recent event. t-t 

0 (AF-1C). (B). unit 2 (0.80). (620-830) 200 MRCU z 

USGS-2531 A horizon Fault-scarp colluvium, 2,620±70 14C 2,777 300 CAS 2,650 UHC Near time of second event. 0 

(AF-1B). (B). unit 3 (0. 75). (2,549-2,879) 200 MRCU t-3 

::c: 

ITL-16 A horizon Fault-scarp colluvium, 2, 700±200 TL - - - Near time of second event. M 

(F86-U7). (C). unit 3 (0. 75). ~ 

AA-2266 Charcoal Burn layer in debris 4,740±90 14C 5,467, 5,534, 25 CAS 5,300 UHC Near time of first event. MRCU > UJ. 

(AF-1D). (A). flow, unit 4 (1.50). 5,553 200 MRCU includes 100 years for deposition > t-3 (5,289-5, 721) and 100 years to form weak soil. 0 

[x=5,518J ::c: 

ITL-2 Silt (C) Loess below debris flow, 6,600±700 TL - - Time near end of loess deposition; ~ - ~ 

(F86-U4) unit 9 (1.62). predates debris flow (unit 4). 0 

AF-2 ..... ITL-18 Silt(C) Modern sag-pond deposit, 250±100 TL Minimum time since most recent z - - - t-3 

(F86-U14). unit 1 (0.50). event. 

ITL-3 Silt (C) Sag-pond deposit under 500±200 TL - - - Near time of most recent event. 

(F86-U9). colluvium, unit 3 (1.00). 

USGS-2533 A horizon Fault-scarp colluvium, 620±150 14C 612 150 CAS 500 UHC Maximum time since most recent 

(AF-2A). (B). unit 2 (1.00-1.05). (360-830) 100 MRCU event. 

AF-3 ..... ITL-46 Silt(C) Loess under debris flow, 7, 700±300 TL - - - Time near end of loess deposition; 

(F86-Ull). unit 6 (2.25). predates debris-flow sequence. 

AA-2268 Charcoal Loess under debris flow, 7,290±100 14C 8,061 25 CAS - Time near end of loess deposition; 

(AF-3A). (A). unit 6 (2.80). (7,919-8,336) predates debris-flow sequence. 
TABLE A 1.-Data for radiocarbon and thermoluminescence analyses cited in the text-Continued 

Age4 

Laboratory 

number Radiocarbon age Calibrated charcoal MRC Estimated 

(field Material Geologic material (in 14C yr B.P.) or and AMRT 14C ages and age of soil 

sample sampled and map unit TL age estimate (range of one-sigma error) CAS horizon 

Trench1 number)2 (preparation)3 (sampling depth, in m) (in cal yr) [x of ages] (in yr)5 contacts6 Remarks 

Rock Canyon natural exposure (M.N. Machette, October 1986) 

RC ....... DIC-3236 Organic matter Fault-scarp colluvium, 1,110±50 14C 1,005 200 CAS - Approximate time of most recent 

(RC-1) (B). unit 3 (1.20). (950-1,150) event. 

PITT-0091 A horizon Postfault debris flow, 455±35 14C 512 150 CAS 400 UHC Minimum time since burial of 

(RC-2). (B). unit 5A (1.50). (457-552) 100 MRCU fault scarp. 

Woodland Hills trench 2 (M.N. Machette, November 1986) 

WH-2 ..... USGS-2500 A horizon Block in scarp-slope 1,190±50 14C 

(WH-2-1). (B). colluvium, unit 8A 

(90-160). 

BETA-19594 A horizon Block in scarp-slope 1,380±60 14C 

(WH-2-1). (B). colluvium, unit 8A 

(90-160). 

1

Sites are listed from north to south by trench number and then by increasing age (stratigraphic 

depth). 2AA, University of Arizona-National Science Foundation Accelerator Facility for Radioisotope 

Analysis (TAMS method); BETA, Beta Analytic, Inc.; DIC, Dicarb Radioisotope Company; PITT, 

University of Pittsburgh Applied Research Center Radiocarbon Laboratory; USGS, U.S. Geological 

Survey Radiocarbon Laboratory, Menlo Park, Calif.; ITL, thermoluminescence analyses by S.L. 

Forman, Institute for Arctic and Alpine Research, University of Colorado. 3 A, root hairs removed; B, root hairs removed, HCl and NaOH extracted, and organics concentrated 

by sedimentation or centrifugation; C, sample is sieved and split, and calcium and organic matter are 

removed in darkness. 

1,102 300 CAS 900UHC Near time of most recent event; 

(1,020-1,220) 200 MRCU same sample as BETA-19594. 

1,306 300 CAS 1,100 UHC Near time of most recent event; 

(1,180-1,400) 200 MRCU same sample as USGS-2500. 

4

yr B.P., years before present (1950); 14C, radiocarbon; TL, thermoluminescence; AMRT, apparent 

mean residence time (age) of carbon in sample. Error limits for 14C are one-sigma standard deviation, 

but we use an error multiplier of twice the reported laboratory error for calibration; error limits for TL 

are means (x) of errors (one-sigma standard deviation from each measurement). 5MRC, estimated mean residence correction to AMRT age for estimates on upper (U) and lower (L) 

soil contacts (estimated AMRT age of sample before burial); CAS, estimated span for age of carbon in 

sample at time of burial. 6UHC, upper horizon contact; LHC, lower horizon contact; AMRT, apparent mean residence time 

(age) of sample. 

""d 

> t"4 

trj 

0 

rn 

trj 

~ 

rn 

is: 

0 

t"4 

0 

Q 

t-< 

0 

~ 

t-3 

::r: 

trj 

rn 

~ 

~ 0 

::r: 

~ 

> C! 

t"4 

t-3 

N 

0 

z 

trj 

m > 

-::J 
A68 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

only on counting statistics of the sample and reference 

standards. To estimate the true laboratory error in an 

age, the reported one standard deviation should be 

multiplied by an error multiplier for samples of a particular type analyzed by a particular laboratory (International Study Group, 1982; Long, 1990). Because we have 

no error multipliers for the laboratories that dated our 

samples, we use an error multiplier of 2 for all of our 

samples (see Scott and others, 1983; Stuiver and Pearson, 1986). The long estimated age spans and large errors 

for most of our ages indicate that the bidecadal (20-year 

interval) data set is the appropriate one for our samples 

(see Stuiver and Reimer, 1986). 

The calibration program treats ages derived from 

samples containing carbon in plants that grew over short 

spans of time (less than 30 years) differently than it does 

samples containing carbon that accumulated over longer 

intervals (Stuiver and Reimer, 1986). We suspect that 

our charcoal samples from the Wasatch Front are mostly 

burned sagebrush or other shrubs that grew in less than 

30 years. For these samples, the calibration program 

uses the curve based on tree-ring data grouped into 

20-year intervals. Because the detailed curve has many 

small fluctuations, a 14C age and the two-sigma error 

limits about the age (one-sigma error times error multiplier of 2) will usually intersect the calibration curve in 

several places, yielding several possible calendric dates 

(or calendric ages for our calibrated AMRT ages) (Stuiver and Pearson, 1986). We can rarely determine which 

of these calibrated dates represents the true age of the 

sample, so we report each date and the total one-sigma 

error limits of the calibration procedure (that is, the 

error limits in table A1 cover the range from the minimum one-sigma limit on the youngest calibration date to 

the maximum one-sigma limit on the oldest date). For 

samples that are judged to contain carbon that spans 

more than 30 years, the calibration program calculates a 

moving average of the calibration curve based on the 

estimated age span of the sample (to the nearest 20 

years). Most of our AMRT samples contain soil organic 

carbon that accumulated over a span of more than 100 

years. When we use the moving-average procedure of 

the program to calibrate our AMRT ages, many of the 

fluctuations in the calibration curve are smoothed out or 

eliminated. This smoothing of the curve can result in the 

AMRT age's intersecting the curve at only one point; it 

gives a single calendric age (calibrated AMRT age) and a 

modest error for each AMRT age. Thus, even though the 

AMRT ages are much more difficult to interpret than the 

charcoal ages (discussed below), the calibration procedure yields calibrated AMRT ages that can appear more 

precise than the charcoal ages; however, the following 

discussion will show that they are not. 

APPARENT MEAN RESIDENCE 14C AGES OF 

SOILS 

The concept of an AMRT age of a soil is the most 

effective and widely used way to interpret the 

laboratory-reported 14C age of organic matter in soil 

horizons (Scharpenseel and Schiffmann, 1977). However 

widely used, the interpretation of AMRT ages is complicated by a number of factors (Matthews, 1980). The 

gradual accumulation of the many components of organic 

matter in the upper (A) horizon of a soil profile, the 

removal of less resistant components of organic matter 

through decay and oxidation in the horizon, the penetration and decay of all ages of plant roots into the horizon, 

the mixing of organics within the horizon by animals, 

both small (for example, insects and worms) and large 

(rodents), and the continual translocation of insoluble as 

well as soluble organic compounds from higher to lower 

parts of the horizon result in a diverse mixture of carbon 

of different ages throughout the horizon. 

Organic matter is a combination of humus (or humic 

substances) and other organic components (such as protein, carbohydrates, and amino acids), each of which 

responds differently upon burial. Generally, humic substances will resist decay and oxidation and are relatively 

inert with respect to biologic and chemical attack, 

whereas other organic substances decay quickly (some in 

a matter of years or decades). Hence, humic acids are 

often selected for dating the oldest carbon in a soil 

sample. These processes of carbon accumulation and 

decay, transport, and removal are highly dependent on 

the topographic setting of the soil (slope, aspect, and 

drainage), the type and rate of biologic activity in the 

soil, the inorganic sedimentation rate, the type of vegetation (for example, grassland versus forest), and climate 

(Martel and Paul, 1974; Scharpenseel and Schiffmann, 

1977). 

An AMRT age is calculated from the total 14C activity 

measured on all the carbon-containing components in a 

pretreated soil sample. The age distribution of carbon in 

the sample is a function of the turnover rate of carbon in 

the soil (O'Brien, 1984). Thus, for example, even the 

upper few centimeters of modern A horizons could yield 

an AMRT age of a few tens to a few hundreds of years, 

depending on the importance of the factors listed above. 

An AMRT age from the upper part of an A horizon is 

usually younger than the mean age of carbon in a soil 

because A horizons contain much more young carbon 

(such as the other organics discussed above) than old 

carbon (Geyh and others, 1971). For old, deeply buried A 

horizons, the alternate can be true-younger, less resistant organic components may be destroyed, and the more 

resistant, older (humic) organic components may remain. 

Thus, depending on the organic constituents, the time 

and depth of burial, and the sample's position in a soil, an 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A69 

AMR T age could theoretically be as old as the time of soil 

inception to as young as the time of burial. 

To interpret and calibrate AMRT ages, we need to 

estimate the total amount of time represented by the 

ages of the carbon-containing compounds in the soil 

sample. We term this time parameter the carbon age 

span (CAS) of a soil sample. The CAS is very dependent 

on but quite different from the length of time that it took 

for the organic matter in an A horizon sample to accumulate because of the carbon-cycling processes mentioned above. Two samples from A horizons might have 

similar AMRT ages but very different CAS values, 

depending, for example, on their position on a slope 

(Martel and Paul, 197 4). CAS is used in the calibration 

program as an estimate of the age span of the carbon in 

our samples. These estimates are based on the general 

form of AMRT age versus depth relations reported by 

others (see Matthews, 1980; Brown, 1986), thickness of 

the sampling interval, organic content of the sample, rate 

of organic matter turnover (bioturbation and organic 

cycling), slope position, sample thickness, vegetation, 

and, more importantly, our interpretation of AMRT ages 

on a small number of A horizon samples from sites in 

central Utah (discussed below). 

A further problem in interpreting AMRT ages is that 

the procedures for treatment of samples before 14C 

analysis are not standardized. The many different procedures used to isolate different components of soil organic 

matter yield different AMRT ages, depending on the 

objectives of each study (for example, see Stuckenrath 

and others, 1979; Goh and Molloy, 1978; Matthews, 

1980). No single procedure is recognized as optimal, but 

consistent pretreatment of all samples is mandatory for 

meaningful interpretation of AMRT ages. We followed 

many previous studies in using physical methods to 

isolate the fine organic fraction (less than 125 j.L) of the A 

horizon (Scharpenseel and others, 1968; Kihl, 1975) and 

then asked the dating laboratories to centrifuge this 

fraction during HCl and NaOH treatments to recover as 

much of the finest organic fraction as possible before 

analysis. Thus, although our soil concentrates were 

analyzed by three different laboratories (USGS, Dicarb, 

and University of Pittsburgh), pretreatment procedures 

were the same for all samples; thus, all AMRT ages 

should be as comparable as possible, although we cannot 

fully evaluate differences in each 14C laboratory's analytical procedures (for example, Scott and others, 1990). 

In contrast, procedures for the pretreatment of charcoal samples are standardized, and ages for samples 

analyzed by different laboratories can be compared 

directly (Stenhouse and Baxter, 1983). Major uncertainties involved in interpreting charcoal ages are (1) the 

length of time between the formation of the charcoal and 

its deposition (including possible reworking), (2) the 

relation of the charcoal to the stratigraphic unit in which 

it is found (for example, a burned root or filled animal 

burrow), and (3) contamination by rootlets. 

Because the interpretation of AMR T ages is so complex and total errors of ages are large, some authors 

argue that the basic procedure for calibrating AMRT 

ages is questionable. For example, as Geyh and others 

(1971) pointed out, the proportions of carbon of different 

ages in a typical horizon sample vary greatly, so most of 

the carbon often dates from the younger portion of the 

CAS. The dominance of younger carbon in the sample 

and the exponential decay rate of 14C cause the relations 

between AMRT ages and CAS values from the upper and 

lower parts of a soil horizon to vary significantly with the 

mean calendric age of the soil. We agree that AMRT ages 

cannot be calibrated with the same accuracy as dates 

from charcoal. However, there are systematic and geologically important deviations between the 14C time scale 

and the calendric time scale, particularly for samples less 

than 1,000 years old. Including averaged estimates of 

this deviation (through use of the smoothed curve) in our 

age corrections improves the accuracy of our estimates of 

the timing of faulting events. 

DATING THE FORMATION OF AN A HORIZON 

A major limitation of AMRT ages is that they are 

composite ages (combined activity of all 14C in the 

sample); for this reason, most AMRT ages cannot accurately date geologic events represented by depositional 

units above and below the sampled horizons. The time of 

the beginning of soil formation on a particular deposit can 

rarely be dated with AMRT methods (Geyh and others, 

1971). Similarly, an AMRT age on the upper 1 em of an 

organic-rich buried A horizon still provides only a maximum limit on the time of the soil's burial (Matthews, 

1980). 

Despite these problems, we need to estimate as accurately as possible the times of faulting events at our 

trench sites. In pursuit of this goal, we use calibrated 

AMRTages (and other parameters used to estimate CAS 

values) to derive age estimates for the upper horizon 

contact (UHC) and sometimes for the lower horizon 

contact (LHC) of selected A horizons. The estimated 

CAS for each sample and AMRT ages on modern A 

horizons from these and similar sites help us estimate 

how divergent the sample's calibrated AMRT age is from 

the time of deposition of the unit on which the A horizon 

is developed and from the date of burial of the A horizon. 

We probably could have improved the accuracy of our 

UHC and LHC age estimates though an extensive 

program of 14C analysis of various organic fractions in 

the samples of surface and buried soils that we submitted 
A70 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

for ARMT dating (for example, Matthews, 1980). However, time and budget constraints prevented this type of 

approach in a project of such broad scope. Although the 

analysis of many more samples of modern A horizons at 

different depths might have improved our age estimates, 

our results from the East Ogden trench site highlight a 

major problem in measuring AMRT ages on modern 

soils. Some AMRT ages from East Ogden (from depths of 

as much as 60 em) have greater 14C activity than modern 

samples (that is, they date in the future). Such ages 

indicate that burrowing has introduced significant 

amounts of carbon produced during thermonuclear 

explosions (bomb carbon) into the lower parts of some of 

the more permeable, sand-rich A horizons. It is difficult 

to estimate how much of the carbon in the modern A 

horizons on these fault scarps is bomb carbon, which 

horizons have been burrowed and which have not, and 

whether reworked organic sediment from older A horizons farther up the scarp has been incorporated into 

modern horizons. For these reasons, even a very extensive program of 14C dating of modern A horizons might 

not significantly improve our estimates of the AMRT age 

of modern A horizons from sites such as East Ogden. 

Our AMRT data on modern A horizons are limited to a 

small number of samples from middle to lower slope 

positions on fault scarps at trench sites along the 

Wasatch fault zone. These data suggest that samples 

from the upper 3 to 20 em of a typical modern 10- to 

40-cm-thick A horizon would yield AMRT ages of about 

100 to 400 years if no bomb carbon were present. Bomb 

carbon has clearly influenced some near-surface samples, 

but AMRT ages from upper and lower parts of thick A 

horizons buried by the last major faulting event at the 

East Ogden site also differ by about 50 to 300 years. The 

ages from these buried horizons have not been influenced 

by bomb carbon (because they are deeply buried and are 

prehistoric), and they therefore suggest that our estimates of the AMRT in modern horizons are reasonable. 

For the thicker horizons, there must be some age-depth 

effect (see for example, Matthews, 1980), but we have 

too few samples to determine the influence of this effect. 

A horizons developed under scrub oak forests probably 

give younger AMRT ages than horizons developed at 

sagebrush or grassland sites, because organic material is 

generated and accumulates much faster at wooded sites. 

AMRT ages of as much as 1.5 to 2. 7 ka have been 

obtained from a drier, higher altitude grassland site, 

which has a low rate of organic productivity, 50 km to the 

east of the Wasatch fault zone (Nelson and VanArsdale, 

1986). Samples from distal-slope positions on scarps 

probably give younger ages than those from midslope 

positions from the same depth because organic-rich sediment (for example, accretionary A horizons) accumulates fastest near the base of a slope. A horizons on steep 

scarps appear to yield younger ages than those on gentle 

scarps. Thus, because a scarp broken by repeated Holocene displacements becomes higher and steeper with 

each succeeding event, an A horizon buried during the 

first fault event at a site would generally yield an older 

AMRT age immediately after burial (MRC, mean residence correction) than would a comparably developed A 

horizon on the steepened scarp formed after the second 

or third fault event. 

We use the above generalizations, estimated values for 

CAS, the sample parameters used to estimate the CAS, 

and modern AMRT ages of 100, 150, 200, or 300 years to 

estimate UHC and, in some cases, LHC ages from the 

calibrated AMRT ages. UHC age estimates are calculated by considering horizon and sample thicknesses and 

then subtracting an appropriate MRC (based on modern 

AMRT value) from the calibrated AMRT age. Modern 

AMRT values were generally added to calibrated AMRT 

ages where minimum estimates of LHC age were 

needed. We deal with the problems inherent in determining UHC and LHC age estimates from such a limited 

database by assigning large errors to our age estimates. 

The myriad uncertainties in interpreting AMRT ages 

and the even larger errors introduced by estimating 

UHC and LHC ages suggest errors on the final age 

estimates of from ±200 to ±400 years. Regardless of the 

one-sigma error ranges listed in table A1 for AMRT and 

calibrated AMRT ages and the UHC and LHC age 

estimates, this magnitude of total error should be 

assumed for all horizon contact age estimates. For these 

reasons, we also round all calibrated soil ages to the 

nearest 50 years and report them in units of thousands of 

years (ka). The errors in 14C ages introduced by isotopic 

fractionation of 14C in terrestrial organic matter are 

small (probably less than 50 years) relative to these total 

errors. Therefore, we did not attempt to correct our ages 

for isotopic fractionation. 

The following example will illustrate our method of 

deriving an upper horizon contact (UHC) age estimate 

from a laboratory-reported AMRT age of 2,000±100 14C 

yr B.P. of the upper 10 em of a tectonically buried A 

horizon that is 35 em thick. For this example, we 

estimate that the carbon in the sampled part of the A 

horizon had a 300-year age span (CAS) and a MRC of 200 

years (AMRT at the time of burial). In addition, we apply 

a multiplier of 2 to the reported laboratory error. The 

resulting calibrated AMRT age is 1,963 cal yr B.P. (the 

one-sigma error limits are +236 and -237 years). From 

this date, we subtract the MRC of 200 years to obtain an 

age estimate of 1, 763 cal yr B.P. for the UHC (buried 

surface of the A horizon). After rounding the estimate to 

the nearest 50 years, we estimate that this A horizon was 

buried by fault-scarp colluvium about 1. 75±0.2 ka. As 

can be seen from this example, the reported AMRT age 
PALEOSEISMOLOGY OF THE WASATCH FAULT ZONE A71 

of 2,000 yr B.P. is almost 250 years (12 percent) greater 

than the estimated time of faulting. 

REFERENCES CITED 

Aitken, M.J., 1985, Thermoluminescence dating: New York, Academic 

Press, 291 p. 

Brown, R. H., 1986, 14C depth profiles as indicators of trends in climate 

and 14CP2

C ratio: Radiocarbon, v. 28, no. 2A, p. 350-357. 

Colman, S.M., Pierce, K.L., and Birkeland, P.W., 1987, Suggested 

terminology for Quaternary dating methods: Quaternary 

Research, v. 28, p. 314-319. 

Forman, S.L., Jackson, M.E., McCalpin, J., and Maat, P., 1988, The 

potential of using thermoluminescence to date buried soils developed on colluvial and fluvial sediments from Utah and Colorado, 

U.S.A.-Preliminary results: Quaternary Science Reviews, v. 7, 

no. 3/4, p. 287-294. 

Forman, S.L., Machette, M.N., Jackson, M.E., and Maat, P., 1989, An 

evaluation of thermoluminescence dating of paleoearthquakes on 

the American Fork segment, Wasatch fault zone, Utah: Journal of 

Geophysical Research, v. 94, no. B2, p. 1622-1630. 

Geyh, M.A., Benzler, J.-H., and Roeschmann, G., 1971, Problems of 

dating Pleistocene and Holocene soils by radiometric methods, in 

Yaalon, D.H., ed., Paleopedology-Origin, nature and dating of 

paleosols: Jerusalem, International Society of Soil Science, Israel 

University Press, p. 63-75. 

Goh, K.M., and Molloy, B.P.J., 1978, Radiocarbon dating of paleosols 

using soil organic matter components: Journal of Soil Science, v. 

29, p. 567-573. 

International Study Group, 1982, An inter-laboratory comparison of 

radiocarbon measurements in tree rings: Nature, v. 298, p. 

6194i23. 

Kihl, R., 1975, Physical preparation of organic matter samples for 14C 

dating (app.), in Andrews, J.T., Radiocabon date list II from 

Cumberland Peninsula, Baffin Island, N.W.T., Canada: Arctic and 

Alpine Research, v. 7, p. 90-91. 

Long, A., 1990, A quality assurance protocol for radiocarbon dating 

laboratories: Radiocarbon, v. 32, p. 393-397. 

Martel, Y.A., and Paul, E.A., 1974, The use of radiocarbon dating of 

organic matter in the study of soil genesis: Soil Science Society of 

America Proceedings, v. 38, p. 501-506. 

Matthews, J.A., 1980, Some problems and implication of 14C dates from 

a podzol buried beneath an end moraine at Haugabreen, southern 

Norway: Geografiska Annaler, v. 62A, p. 185-208. 

Nelson, A.R., and VanArsdale, R.B., 1986, Recurrent Late Quaternary movement on the Strawberry normal fault, Basin and 

Range-Colorado Plateau transition zone, Utah: Neotectonics, v. 

1, p. 7-37. 

North American Commission on Stratigraphic Nomenclature, 1983, 

North American stratigraphic code: Bulletin of the American 

Association of Petroleum Geologists, v. 67, p. 841-875. 

O'Brien, B.J., 1984, Soil organic carbon fluxes and turnover rates 

estimated from radiocarbon enrichments: Soil Biology and Biochemistry, v. 16, p. 115-116. 

Scharpenseel, H.W., and Schiffmann, H., 1977, Radiocarbon dating of 

soils-A review: Zeitschrift fur Pflanzenernahrung Dungung und 

Bodenkunde, v. 140, p. 159-174. 

Scharpenseel, H.W., Pietig, F., and Tamers, M.A., 1968, Bonn radiocarbon measurements 1: Radiocarbon, v. 10, p. 8-28. 

Scott, E.M., Baxter, M.S., and Aitchison, T.C., 1983, 14C dating 

reproducibility-Evidence from a combined experimental and statistical programme, in Mook, W.G., and Waterbolk, H.T., eds., 

Proceedings of the First International Symposium, 14C and 

Archaeology: PACT Journal of the European Study Group on 

Physical, Chemical and Mathematical Techniques Applied to 

Archaeology, v. 8, p. 133-146. 

Scott, E.M., Aitchison, T.C., Harkness, D.D., Cook, G.T., and Baxter, 

M.S., 1990, An overview of all three stages of the International 

Radiocarbon Intercomparison: Radiocarbon, v. 32, p. 309-320. 

Stenhouse, M.J., and Baxter, M.S., 1983, 14C dating reproducibility-Evidence from routine dating of archaeological samples, in 

Mook, W.G., and Waterbolk, H.T., eds., Proceedings of the First 

International Symposium, 14C and Archaeology: PACT Journal of 

the European Study Group on Physical, Chemical and Mathematical Techniques Applied to Archaeology, v. 8, p. 147-166. 

Stuckenrath, R., Miller, G.H., and Andrews, J.T., 1979, Problems of 

radiocarbon dating Holocene organic-bearing sediments, Cumberland Peninsula, Baffin Island, N.W.T., Canada: Arctic and Alpine 

Research, v. 11, p. 109-120. 

Stuiver, M., and Kra, R., eds., 1986, Calibration issue-Proceedings of 

the 12th International Radiocarbon Conference, Trondheim, Norway, 1985: Radiocarbon, v. 28, 225 p. 

Stuiver, M., and Pearson, G.W., 1986, High-precision calibration of the 

radiocarbon time scale, AD 1950-500 BC: Radiocarbon, v. 28, p. 

805-838. 

Stuiver, M., and Quay, P.D., 1979, Changes in atmospheric carbon-14 

attributed to a variable sun: Science, v. 207, p. 11-19. 

Stuiver, M., and Reimer, P.J., 1986, A computer program for radiocarbon age calibration (ver. 2.0): Radiocarbon, v. 28, no. 2B, p. 

1022-1030 (Rev 2.0). 

Wintle, A. G., and Huntley, D.J., 1982, Thermoluminescence dating of 

sediments: Quaternary Science Reviews, v. 1, p. 31-53. 

Persistent and Nonpersistent Segmentation of the 

Wasatch Fault Zone, Utah: Statistical Analysis for 

Evaluation of Seismic Hazard 

By RUSSELL L. WHEELER and KATHERINE B. KRYSTINIK 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-B 

Confirmation that the Wasatch fault zone is 

segmented into lengths that tend to rupture 

independently and implications for hazard 

evaluation in the population centers of Utah 

and in other normal fault zones 

CONTENTS 

Page Page 

Abstract ......................................................................... . B1 Results from Simulation and Statistical Testing-Continued 

Introduction .................................................................... . 1 Three-Dimensional Comparisons of Observed and 

Segmentation ............................................................ . 1 Simulated Patterns .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .... .. .. .. . B16 

Purpose ................................................................... . 2 Quantitative Comparisons of Observed and Simulated 

Acknowledgments ...................................................... . 4 Patterns.............................................................. 16 

Methods ......................................................................... . 4 Interpreting a Single Anomalous Part of the Fault 

Anomalies ................................................................ . 4 Zone................................................................... 20 

Data ........................................................................ . 4 Discussion . . . .. . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

Causes of Coincident Anomalies ................................... . 7 Segmented Ruptures................................................... 22 

Geologic Causes of Anomalies other than Segmenta- Persistent and Nonpersistent Segment Boundaries........... 24 

tion ................................................................ . 7 Bedrock Ridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . .. . . . . .. . . . . . . . .. . 25 

Simulation and Statistical Procedures ............................ . 8 Applications to Large Historical Earthquakes .. .. .. .. .. .. .. .. . 31 

Locational Uncertainties of Anomaly Edges ................... . 9 Borah Peak, Idaho-October 28, 1983 .................. .......... 31 

Descriptive Results .......................................................... . 10 Hebgen Lake, Montana-August 17, 1959....................... 33 

Observed Pattern of Anomalies .................................... . 10 Rainbow Mountain, Nevada-July 6 and August 24, 

Anomalous Sections of the Fault Zone .......................... .. 10 1954 ................................................................... 35 

Associated Data Types ............................................... . 10 Fairview Peak-Dixie Valley, Nevada-December 16, 

Diagnostic and Characteristic Data Types ...................... . 13 1954 ................................................................... 35 

Summary of Descriptive Results .................................. . 13 Pleasant Valley, Nevada-October 2, 1915...................... 39 

Results from Simulation and Statistical Testing .................... . 15 Summary . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . 42 

One-Dimensional Comparisons of Observed and Conclusions...................................................................... 43 

Simulated Patterns .............................................. . 15 References Cited .. .. .. .. .. . .. . .. .. .. .. .. .. . .. .. .. .. .. .. .. .. . .. . .. .. .. .. .. .. .. . 43 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of the Wasatch fault zone in northern central Utah and selected towns and cities along the Wasatch Front... B3 

2. Graph showing observed distribution of anomalies along the Wasatch fault zone .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 6 

3, 4. Diagrams showing: 

3. Three-dimensional geometry of comparison of the observed anomaly pattern to simulated patterns................. 9 

4. Axes of figure 3, showing a serial section.............................................................................................. 9 

5. Graphs showing observed anomaly pattern of figure 2 redrawn to give minimum and maximum widths allowed for 

each anomaly by locational uncertainties of its edges...................................................................................... 11 

6. Histograms of anomaly widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . .. . .. . .. . . . . . . . . . . . . . .. . .. . . . . . . . . . . .. . .. . .. . . . . . . . . . . .. . .. . .. . . . . .. 12 

7. Graphs showing pairs and m-tuplets of coincident anomalies in the observed anomaly pattern along the Wasatch 

fault zone................................................................................................................................................ 14 

8. Diagram representing coincidings between anomalies at km 130 of the Wasatch fault zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

9. Histograms of values of the Jaccard coefficient J for the 15 possible pairs of 6 data types .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 17 

10. Graphs showing the constancy and fidelity of the six data types in the four anomalous parts of the Wasatch 

fault zone................................................................................................................................................ 18 

11. Histograms comparing numbers of m-tuplets in observed and simulated patterns..................................................... 19 

12. Diagrams comparing numbers of m-tuplets in observed and simulated patterns........................................................ 22 

13. Graphs showing summaries of serial sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

14-20. Maps showing: 

14. Four sets of segment boundaries for the Wasatch fault zone..................................................................... 28 

15. Segment boundary at the Traverse Mountains salient.............................................................................. 29 

16. Fault network in a hypothetical nonconservative barrier on a normal fault.................................................. 30 

III 
IV CONTENTS 

Page 

17. October 28, 1983, Borah Peak, Idaho, earthquake................................................................................... B32 

18. August 17, 1959, Hebgen Lake, Mont., earthquake................................................................................. 34 

19. July-December 1954, Rainbow Mountain, Fairview Peak, and Dixie Valley, Nev., earthquakes..................... 36 

20. October 2, 1915, Pleasant Valley, Nev., earthquake................................................................................ 40 

TABLES 

Page 

TABLE 1. Locations of transverse anomalies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B5 

2. Coincidings of anomalies........................................................................................................................................ 16 

3. Constancy and fidelity for anomalous and nonanomalous parts of the Wasatch fault zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . 18 

4. Total counts of m-tuplets produced by all simulations together..................................................................................... 24 
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

PERSISTENT AND NONPERSISTENT SEGMENTATION OF THE 

WASATCH FAULT ZONE, UTAH: STATISTICAL ANALYSIS FOR 

EVALUATION OF SEISMIC HAZARD 

By RussELL L. WHEELER and KATHERINE B. KRYSTINIK 

ABSTRACT 

The Wasatch fault zone of central Utah might be divided into 

segments that tend to rupture independently of one another. If so, 

ruptures of sufficiently large earthquakes would tend to start and stop 

at segment boundaries and would tend not to cross boundaries. The 

validity of the segmentation hypothesis will affect how seismic hazard 

is estimated for the Wasatch Front urban corridor, the severity of the 

estimated hazard, and the degree to which the estimated hazard is 

concentrated in the State's population centers along the central part of 

the Wasatch fault zone. 

To test the segmentation hypothesis, we examined Bouguer gravity 

data, earthquake epicenters, fault zone geometry, and aeromagnetic, 

topographic, and structural data along the fault zone. In each of the six 

data types, we sought the kinds of anomalies (characteristic expressions) that a long-lasting segment boundary should produce. Anomalies 

in fault zone geometry are large footwall salients, at which the eastern 

(upthrown) wall of the north-striking normal fault zone projects westward into the hanging wall and the fault trace bows westward around 

the projections. Of 26 anomalies in the 6 data types, 4 are salientsfrom north to south, the Pleasant View, Salt Lake, Traverse Mountains, and Payson salients. Salients appear to be the most fundamental 

anomalies. Seventeen of the other 22 anomalies coincide with the 

salients; at each salient, anomalies in three to five other data types 

coincide; no other anomalies coincide elsewhere. Geologic processes 

other than segmentation might have produced any of the anomalies, 

but only segmentation or chance could produce such coincidings of 

anomalies in a way that is consistent with the geologic evolution of 

central Utah. By using simulation experiments and statistical tests of 

the results of those simulations, we show that the observed degree of 

anomaly coinciding should not be attributed to chance. 

Formation of the salients occupied much or all of the history of the 

Wasatch fault zone. Fault scarps of late Pleistocene and Holocene age 

tend to end or change age at the salients, an indication that scarps also 

are segmented there. We conclude that the Wasatch fault zone has 

been persistently segmented at the four salients throughout much or all 

Manuscript approved for publication November 20, 1990. 

of its 10-m.y. history and will likely continue to be segmented there 

through at least the next several millenia, which is the time span of 

interest for hazard evaluation. 

From mapping of late Quaternary deposits along the fault zone, other 

workers detected these four persistent segment boundaries, plus five 

other boundaries that are little expressed or poorly expressed in our six 

data types. The five others might be nonpersistent boundaries, alternating between active states, in which they control several successive 

ruptures, and inactive states, in which they do not. Although recently 

active, such boundaries might change state and so are less likely to 

affect the next few large ruptures than the persistent boundaries are. 

For the estimation of seismic hazard, it is fortunate that most of the 

segment boundaries in the populous central part of the Wasatch fault 

zone are persistent, so that their past behavior is a reasonable guide to 

their expected future behavior. 

From our consideration of the persistent boundaries, we conclude 

that others should exist on other normal fault zones where exposed or 

shallowly buried transverse bedrock ridges, some having salients, 

indicate accumulated slip deficits and where two or more individual 

large historical or prehistoric rupture zones have started or stopped. 

Testing this model against five historical earthquake sequences in and 

near the Basin and Range province indicates the existence of two 

persistent boundaries on the Lost River fault zone of Idaho (which 

broke in 1983), one persistent boundary between the Dixie Valley and 

Fairview faults of Nevada (December 1954), and one persistent boundary and one nonpersistent boundary in the Pleasant View rupture zone 

of Nevada (1915). The northern end of the Dixie Valley fault is not a 

segment boundary, persistent or otherwise. Both ends of the Rainbow 

Mountain rupture zone of Nevada (July-August 1954), the southern end 

of the Fairview Peak rupture zone, and the northern end of the 

Pleasant Valley rupture zone provide too little information to test the 

model. The Hebgen Lake earthquake in Montana (1959) occurred on a 

fault zone that is too young for the model to apply. 

INTRODUCTION 

SEGMENTATION 

Swan and others (1980) and Schwartz and Coppersmith (1984) hypothesized that the Wasatch fault zone is 

B1 
B2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

divided into six segments that are mostly or wholly 

independent of one another structurally and seismogenically. The proposed segments range in length from at 

least 30 to about 70 km (fig. 1). Schwartz and Coppersmith (1984) also hypothesized that seismic energy 

release on a given segment occurs mostly as earthquakes 

of a size that is characteristic of the segment. These 

characteristic earthquakes would rupture the whole segment but would not rupture across segment boundaries 

into adjacent segments. This report concentrates on the 

segmentation hypothesis. 

If the segmentation hypothesis is valid for the Wasatch 

fault zone, the consequences for hazard evaluation in 

Utah would be twofold. First, segmentation would affect 

the geographic distribution of expected large earthquakes along the Wasatch fault zone. Scarp-forming 

earthquakes over the last 8,000 years have been concentrated in the four central and most heavily populated 

proposed segments (Schwartz and Coppersmith, 1984, 

table 2). When the data of Schwartz and Coppersmith 

(1984) are compared with the frequencies that would be 

expected in the central segments if scarp-forming earthquakes were distributed uniformly along the fault zone, 

the implication is that segmentation could nearly double 

the frequency of scarp-forming earthquakes in the Salt 

Lake City and Nephi segments by maintaining the 

number of large earthquakes in the fault zone as a whole 

but concentrating them into the middle segments. 

Second, segmentation would change probabilistic 

maps of seismic hazard. Producing such maps requires 

estimates of three elements: the magnitude of the largest 

expected earthquake, the frequency distribution of 

earthquakes of various magnitudes, and the geographic 

area (seismic source zone) over which the estimates of 

magnitude and frequency apply (Algermissen and others, 1982; Thenhaus, 1983). Segmentation of the Wasatch 

fault zone would change all three estimates because each 

segment would become a separate source zone having its 

own maximum expected magnitude and frequency distribution. Estimates of magnitudes and frequencies would 

probably have larger uncertainties than they would if the 

whole Wasatch fault zone were treated as a single, 

unsegmented source zone, because each segment would 

have to be evaluated by using only the comparatively 

sparse data from within its source zone. The estimated 

maximum magnitude might decrease because earthquakes large enough to rupture more than one segment 

(multisegment earthquakes) might be precluded. Then 

the estimated frequency of single-segment earthquakes 

might increase because the strain that would have been 

released by one multisegment earthquake would have to 

be released in several smaller, single-segment earthquakes. 

PURPOSE 

Following Schwartz and Coppersmith (1984), our 

study area is the Wasatch fault zone between lat 39° N. 

and lat 42° N. (the Utah-Idaho border), a north-south 

distance of 332 km. For this area, we examined several 

types of geologic and geophysical data that might contain 

anomalies (characteristic expressions of long-lasting segment boundaries). For each kind of data, we expanded or 

contracted the study area to whatever width was needed 

to characterize pertinent aspects of the data at the fault 

zone. 

Wheeler (1984) examined the aims of hazard evaluation and analyses of temporal variations in the long 

historical record of seismicity in a tectonically analogous 

part of northeastern China. He concluded that the geologic records most likely to be useful in evaluating 

segmentation are those that formed during, or can be 

shown to bear on, the Holocene and late Pleistocene. The 

importance of Holocene and late Pleistocene records 

would increase if the degree of independence between 

segments and the level of seismic activity on single 

segments change over a period of centuries to millenia 

(Machette, 1984; Smith and Richins, 1984, p. 79; Personius, 1985; Machette and others, this volume). In fact, 

examining different parts of the Quaternary geologic 

record led Maclean (1985), Machette and others (1986, 

this volume), Mayer and Maclean (1986), Personius 

(1986), and Nelson and Personius (1987) to' delinea_t_e 

segment boundaries, some of which differ from boundaries recognized in the Holocene record by Schwartz and 

Coppersmith (1984). These differences might indicate 

that different segment boundaries have persisted for 

different lengths of time. 

Schwartz and Coppersmith's (1984) segmentation 

hypothesis was based mostly on data from trenches that 

were dug across scarps of the Wasatch fault zone at only 

four sites (fig. 1). Although the trench data are sufficient 

to suggest the segmentation hypothesis, one trench site 

per segment is not enough to evaluate that hypothesis. 

Schwartz and Coppersmith (1984) observed several 

apparent spatial associations of the proposed segment 

boundaries with anomalies in topographic, geophysical, 

and other data and suggested that these spatial associations are stronger than those that would be expected to 

occur by chance. Wheeler (1984) noted that an appropriately formulated statistical test could provide a rigorous 

evaluation of these and other spatial associations and 

therefore of the segmentation hypothesis itself. 

Thus, to evaluate the segmentation hypothesis as it 

applies to the Wasatch fault zone, we must answer two 

questions. First, do segments exist along the Wasatch 

Front, or are they artifacts of subjective perception and 

a small sample, being indistinguishable from patterns 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B3 

420 113° IDAHO 112° 111° --r---UTAH ___ \l ___________ 

! 

Great Salt Lake 

UTAH 

Index map showing 

location of study area 

EXPLANATION 

Trace of Wasatch 

fault in Utah 

--- Proposed segment 

boundary 

® Trench site 

(40 km) Length of proposed 

segment 

0 

0 25 

Collinston segment 

1 

(~30 km) 

I 

I 

~I~ ~I:E :;)10 

I~ 

Ogden 0 

Ogden segment 

(70 km) 

I 

I 

I 

SaltLak~ 0 City ~ -z. 

~ 

Nephi of 

---

25 

50 

Salt Lake City 

segment 

(35 km) 

I 

I 

L __ 

Provo segment 

(55km) 

Nephi segment 

(35km) 

Levan segment 

(40 km) 

50 MILES 

75 KILOMETERS 

FIGURE 1.-Wasatch fault zone in northern central Utah and selected towns and 

cities along the Wasatch Front. The 

Wasatch Front is usually taken to 

include the populated valleys that lie 

along and immediately west of the trace 

of the Wasatch fault zone (Mabey, this 

volume) and that include most of the 

population of Utah. The trace of the 

fault zone, proposed segment boundaries, and trench sites are those of 

Schwartz and Coppersmith (1984), from 

whose figure 1 this map is modified. 

Investigations in progress by several 

workers have added trench sites and 

natural fault exposures and have 

changed some segment boundaries and 

mapped fault traces. Inset shows the 

location of the map area within Utah. 
B4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

that could arise by chance? Second, if segments exist, 

have they affected uplift and seismicity along the fault 

zone both during the Holocene and far enough back in 

geologic time that the segments can be expected to 

continue to affect seismicity for the next decades to 

millenia? 

ACKNOWLEDGMENTS 

Because of the diversity of information that we analyzed and interpreted, we are especially indebted to 

topical and regional specialists for discussions, corrections, preprints, and sharing of unpublished materials. 

These generous colleagues include W.J. Arabasz and 

R.L. Bruhn (University of Utah), D.R. Mabey (Utah 

Geological and Mineral Survey), Larry Mayer (Miami 

University, Ohio), and A.J. Crone, S.K. Goter, M.N. 

Machette, A.R. Nelson, S.F. Personius, W.E. Scott, 

D.P. Schwartz, and R.S. Stein (U.S. Geological Survey). 

The entire manuscript benefited from comments by 

Machette, Nelson, K.M. Shedlock, and M.L. Zoback 

(U.S. Geological Survey). 

METHODS 

ANOMALIES 

Wheeler and Krystinik (1988) examined six kinds of 

data along the length of the Wasatch fault zone in Utah 

for the kinds of anomalies that a long-lasting segment 

boundary might produce. Each data type and its anomalies satisfy seven criteria (Wheeler and Krystinik, 1988) 

insuring that the anomalies (1) pertain to the identification of segment boundaries and the evaluation of seismic 

hazard, (2) are reliable, and (3) can be analyzed by the 

statistical and related methods of Wheeler and Krystinik 

(1987a). 

At several places along the fault zone, anomalies in two 

or more data types coincide. (Anomalies in the same data 

type cannot coincide because they would merge into a 

single anomaly.) Two anomalies form a coincident pair if 

either overlaps the center of the other (Wheeler and 

Krystinik, 1987a). Three anomalies can form as many as 

three coincident pairs. If all three pairs exist, the three 

anomalies form a triplet. Similarly, m anomalies define 

an m-tuplet if the anomalies overlap enough to form all of 

the possible m(m-1)/2 coincident pairs (Wheeler and 

Krystinik, 1987a). For example, 6 anomalies that form 15 

coincident pairs form a sextuplet. 

DATA 

The gravity data show transverse anomalies G 1 

through G10 (table 1, fig. 2) as groups of east-trending 

zones of gravity saddles, east-trending gradients, and 

ends of north-trending gradients and gravity lows. The 

transverse gravity anomalies match those derived independently by Zoback (1983) and include those of Mabey 

(this volume). All of us interpreted G1 through G10 to 

record changes along the normal fault zone in the depths 

to the bottoms of adjoining basins in the hanging (western) wall. Wheeler and Krystinik (1988) argued that 

basin subsidence probably occurred as a result of seismic 

slip on the fault zone, so that adjoining hanging-wall 

basins displaying different amounts of subsidence would 

abut parts of the fault zone having different seismic 

histories. Thus, a transverse gravity anomaly that separates such basins could be a segment boundary. We do 

not claim that G 1 through G 10 must represent segment 

boundaries. We merely observe that long-lasting segment boundaries or their effects should produce transverse anomalies like G1 through G10; therefore, examination of the gravity data and of G 1 through G 10 is 

justified. We make similar observations for the other five 

data types. 

The total magnetic intensity data show transverse 

anomalies M1 through M3 (table 1, fig. 2) as easttrending belts of east-trending gradients and of narrow, 

high-amplitude magnetic highs (Zietz and others, 1976). 

Mabey and others (1964) attributed M1 to uplifted and 

exposed Precambrian metamorphic rocks and M2 and M3 

to buried and exposed intrusive and volcanic rocks of 

Tertiary age and associated mineral deposits. Wheeler 

and Krystinik (1988) suggested that M1 is bounded on 

the south and perhaps on the north by cross faults along 

which the metamorphic rocks were uplifted, perhaps on 

lateral ramps in Cretaceous thrust faults. They also 

suggested that M2 and M3 might be localized by fracture 

systems that extend to depths at which magma was 

generated. Pre-existing faults or other fracture zones 

that cross the late Tertiary (N aeser and others, 1983) 

Wasatch fault zone might segment it by separating and 

decoupling adjoining blocks of one fault wall or both. 

Seismological data show anomalies Se1 through Se4 

(table 1, fig. 2) as changes along the fault zone in the 

abundance of epicenters of small earthquakes that 

occurred near the trace of the fault zone from 1962 to 

1986 (Arabasz and others, 1980; Arabasz, 1984; Smith 

and Richins, 1984; W.J. Arabasz, oral communs., 1986). 

The changes could represent differences between 

present-day rates of seismic slip on adjacent lengths of 

the fault zone. Such differences could represent different 

rates of long-term seismic slip, different ratios of seismic 

to aseismic slip, or different magnitudes and frequencies 

of the earthquakes that caused seismic slip to occur; any 

of these differences could arise from independent slip of 

two fault segments separated by a change in epicentral 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B5 

TABLE 1.-Locations of transverse anomalies 

[Study area is 332 km long, north to south, the northern end being at Utah-Idaho 

border. Values are from Wheeler and Krystinik (1988)] 

Distance south of 

Utah-Idaho border (km) 

Distance 

Northern Southern to next 

Transverse end of end of Transverse transverse 

anomaly transverse transverse anomaly anomaly 

number anomaly1 anomaly width (km)2 to south (kml 

Gravity data 

Gl ............ -10 2 2 10 

G2 ............ 12 17 5 40 

G3 ............ 57 63 6 5 

G4 ............ 68 78 10 45 

G5 ............ 123 135 12 13 

G6 ............ 148 157 9 9 

G7 ............ 166 175 9 11 

G8 ............ 186 192 6 19 

G9 ............ 211 237 26 42 

GlO ........... 279 292 13 40 

Aeromagnetic data 

Ml ............ 54 130 76 18 

M2 ............ 148 173 25 54 

M3 ............ 227 246 19 86 

Seismological data 

Sel ............ 59 78 19 42 

Se2 ............ 120 133 13 8 

Se3 ............ 141 151 10 62 

Se4 ............ 213 226 13 106 

Fault geometry (salients) 

Sal ............ 70 76 6 48 

Sa2 ............ 124 134 10 31 

Sa3 ............ 165 169 4 48 

Sa4 ............ 217 230 13 102 

Topographic data 

Tl ............. 130 140 10 107 

T2 ............. 247 254 7 78 

Structural data 

Stl ............ 112 138 26 8 

St2 ............ 146 167 21 53 

St3 ............ 220 245 25 87 

1

Zoback (1983, fig. 2) showed that transverse anomaly G1 extends 10 km north 

of the Utah-Idaho border. Data north of the border lie outside the study area and 

are not used here, so the width of G1 is taken as 2 km. 2

For each data type, the sum of these values is the length of the study area that 

is occupied by transverse anomalies. 3

For each data type, the last entry is the distance from the southernmost 

transverse anomaly to the southern end of the study area at lat 39o N. For each 

data type, the sum of the values in this column is the length of the study area not 

occupied by transverse anomalies. 

abundance. Then Sel through Se4 could be segment 

boundaries. 

The geometry of the fault zone shows anomalies Sal 

through Sa4 (table 1, fig. 2). At each anomaly, the 

footwall projects westward into the hanging wall as a 

large salient, and the mapped trace of the fault zone or 

one of its strands bends west around the tip of the salient 

(Hintze, 1980; Davis, 1983a, b, 1985). The trace of the 

fault zone also forks or steps en echelon at the salients. 

Sal, Sa2, Sa3, and Sa4 are the Pleasant View, Salt Lake, 

Traverse Mountains, and Payson salients, respectively 

(Wheeler and Krystinik, 1987b, c, 1988). If slip vectors 

change orientation across the salients, then, by definition, the salients are nonconservative barriers as 

described by King (1983). King (1983), King and Yielding 

(1984), and King and Nabelek (1985) suggested a mechanism by which large ruptures would tend to start and 

stop at nonconservative barriers. Briefly, if two adjacent 

parts of a fault slip along different vectors, the slip 

geometry at the junction of the two parts requires the 

formation of a network of many subsidiary faults surrounding the junction. This network is the process zone 

of King and Nabelek (1985). The energy of an incoming 

rupture is absorbed by the process zone and dispersed 

into small slips on many of its faults. Large ruptures that 

propagate toward the process zone tend to stop there, so 

that the zone is a barrier. The shattered process zone has 

low strength but high fracture toughness; thus, it also 

acts as an asperity, and large ruptures tend to start there 

and propagate away along the main fault. Accordingly, 

ruptures tend not to cross the process zone, so that it 

becomes a segment boundary. Bruhn and others (this 

volume, 1987) concluded from structural measurements 

and geometric arguments that slip vectors change orientation across the Salt Lake and Traverse Mountains 

salients. Nelson and Personius (1987) noted that young 

fault scarps end or change age at the Pleasant View 

salient, as would be expected of a nonconservative 

barrier. If the mechanism of King and Nabelek (1985) 

operates at the salients, they would be segment boundaries. 

Transverse topographic anomalies Tl and T2 (table 1, 

fig. 2) are formed where the height of the uplifted 

mountains or their width or both change abruptly northward or southward. Choosing anomalies is a more subjective process for topographic data than it is for the 

other five data types and the least likely to give reproducible results. To insure reproducibility, Wheeler and 

Krystinik (1988) used only those topographic anomalies 

that were recognized by a majority of 12 independent 

interpreters. Of 14 suggested topographic anomalies, 2 

were chosen by at least 9 interpreters; no others were 

chosen by more than 5. A topographic anomaly could 

mark a place along the fault zone where a footwall block 

has been uplifted more than its neighbor across the 

anomaly. Because much or all uplift has occurred seismically, a topographic anomaly could separate lengths of 
B6 

0 

ffff50 f---

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a: 

w 100 f---

Iw 41° :e f0 

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w 

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m 150 f----

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REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

~ (.,) ..J 

i= <( 

w (.,) <( z a 0 CJ 0 

~ <( ..J 

:e 0 

> 0<( :e<( 

<( a:: I- a: W<( ~~ (!) <(O eno 1---->-t--G1 

r-==- ~ 

L...-

'---- $.. ~ 

'---- Se1 

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r-- r---

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...._ - Se2 G_5 - 1--- r--

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I-- I-- Se3 

G6 ~ 

1---

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G7 

1--- ........---

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- ...._ 

Se4 

G9 - r--

r-- M3 

I--

L...-

G10 

r--

> J: 

CL en <( I- a: z CJ w 0 :::::i CL <( 

en ~ 

1

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,.......--

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-

-

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-

the fault zone that have different seismic histories and 

could be a segment boundary. 

Structural anomalies St1 through St3 (table 1, fig. 2) 

were chosen by considering the depths at which large 

normal faults of the Basin and Range province typically 

nucleate (about 15 km, at or near the base of the brittle 

upper crust) (Smith and Richins, 1984, p. 97, figs. 9, 11). 

Large cross faults that extend to this depth could decouple adjoining lengths of the Wasatch fault zone from each 

other and could act as segment boundaries. Wheeler and 

Krystinik (1988) reviewed the structural evolution of 

central Utah and found three large east-striking faults or 

fault zones (St1-St3) that could serve as structural anomalies and could extend to depths of about 15 km. Stl is a 

buried lateral ramp in thrust sheets that moved eastward 

into north-central Utah mostly in Cretaceous time (Armstrong and Oriel, 1965; Oriel and Armstrong, 1966; 

Armstrong, 1968; Royse and others, 1975). The ramp is 

nowhere exposed, but Smith and Bruhn (1984), Schirmer 

(1985), and Wheeler and Krystinik (1988) used geometric 

arguments to conclude that it must dip north. St3, a 

similar north-dipping lateral ramp at the southern end of 

a more southerly thrust sheet, also formed in Cretaceous 

time (Crittenden, 1961; Armstrong, 1968; Lawton, 1985; 

Bryant and Nichols, 1988). The top of the ramp is 

exposed in the southern end of the Wasatch Range 

northeast of Nephi (Hintze, 1980). St2 is the buried fault 

system that is inferred to bound and underlie the Uinta 

aulacogen, a long-lasting trough (Forrester, 1937) that 

extends more than 200 km eastward from the Salt Lake 

City segment (fig. 1). The aulacogen probably formed as 

an east-trending continental rift in Middle Proterozoic 

time (Hansen, 1965; Wallace and Crittenden, 1969; 

Burke and Dewey, 1973). Its existence is inferred from a 

long stratigraphic record of repeated, superimposed, 

eastward-elongated uplifts and downwarps from Middle 

Proterozoic and perhaps Archean through Oligocene 

time (Bryant, 1985; Wheeler and Krystinik, 1988) and 

from young, inward-dipping reverse faults that bound 

the inferred aulacogen along its northern and southern 

edges (Bruhn and others, 1983; Gries, 1983; Stone, 1986). 

However, we found no evidence that the reverse faults 

penetrate as deeply as 15 km or that they were active 

when the aulacogen formed during Middle Proterozoic 

time. Accordingly, Wheeler and Krystinik (1988) took 

anomaly St2 to represent the entire system of deep, 

..... FIGURE 2. -Observed distribution of anomalies along the 

Wasatch fault zone. The length of the anomaly pattern is 332 km, 

from the Utah-Idaho border at lat 42° N. to the southern edge of 

the study area at lat 39° N. Each of the six data types is 

represented by a vertical line that steps to the right at one end 

of an anomaly and back to the left at the other end. Anomaly 

locations and letter-number names are from Wheeler and Krystinik (1988). 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B7 

east-striking, unexposed faults that are inferred to 

underlie the aulacogen, from the northern edge of the 

aulacogen to the southern edge inclusive. 

CAUSES OF COINCIDENT ANOMALIES 

Segmentation and other geologic processes might each 

produce single anomalies of the kinds described above. 

Coincident anomalies might be produced by segmentation, by some other geologic process that affects more 

than one data type, or by chance in the form of the 

spatially random operation of independent geologic processes that each affect only one data type. We will 

attribute coincident anomalies to segmentation if we can 

eliminate the two alternative causes. Geologic processes 

other than segmentation must be eliminated by considering the geologic evolution and characteristics of central 

Utah. Chance must be eliminated statistically before 

coincident anomalies can be considered worth interpreting, either as segment boundaries or as manifestations of 

any other geologic processes. 

GEOLOGIC CAUSES OF ANOMALIES OTHER THAN 

SEGMENTATION 

Transverse anomalies Gl through GlO in Bouguer 

gravity data can be interpreted as separating sedimentfilled hanging-wall basins (Zoback, 1983). Such anomalies 

would reflect segment boundaries because they would 

separate parts of the fault zone that have slipped separately and perhaps independently. Overlaying an 

enlarged version of Zoback's (1983) Bouguer gravity map 

on Hintze's (1980) geologic map indicates that G5 through 

G7 can be tentatively attributed partly to large lateral 

ramps in thrust sheets. The ramps could act as segment 

boundaries by decoupling adjacent parts of the Wasatch 

fault zone so that the parts could slip independently of 

each other. Other kinds of east-striking cross faults 

might also cause transverse gravity anomalies, such as 

east-trending gravity gradients, by juxtaposing rocks of 

different densities. Deeply penetrating cross faults could 

also decouple adjacent parts of the Wasatch fault zone 

and act as segment boundaries. For example, eaststriking faults in the Uinta aulacogen or at its edges 

could cause G6 and G7. Edges of high-density igneous 

rocks could produce transverse gravity anomalies, but 

intrusive and extrusive igneous rocks at and near the 

Wasatch fault zone generally have low densities, some 

comparable to those of surrounding sedimentary rocks 

(Zoback, 1983, p. 5, 11). Also, most edges of gravity 

highs near the fault zone follow the contact between 

bedrock and basin-filling sediments more faithfully than 

they follow contacts between igneous rocks and other 

rocks (Zoback, 1983; Hintze, 1980) and so appear to 

outline shapes of basins instead of igneous bodies. Facies 

changes within the sediments that fill some basins could 

produce lateral density contrasts (Mabey, this volume), 

but these contrasts are likely to be too small to produce 

the 10 to 40 mGal of gravity relief that occurs across 

several of anomalies Gl through GlO. Thus, segment 

boundaries of one form or another are the most likely 

explanation for most of Gl through GlO. 

Mabey and others (1964) attributed transverse anomalies in the aeromagnetic field to exposed Precambrian 

rocks (Ml) and to east-trending belts of igneous rocks 

(M2, M3). The Precambrian rocks form the main mass of 

a thrust sheet, the southern edge of which is a lateral 

ramp (Wheeler and Krystinik, 1988). The magma that 

fed the igneous belts could have risen along east-trending 

systems of fractures. Both lateral ramps and deeply 

penetrating fracture systems could decouple adjacent 

parts of the Wasatch fault zone and thus cause its 

segmentation. Of the various ways to emplace magnetic 

rock into nonmagnetic surroundings, only thrusting and 

igneous activity are consistent with the geologic history 

of north-central. Utah. For example, sedimentary processes do not produce magnetic anomalies having the 

magnetic relief of Ml through M3 (200-800 'Y) (Zietz and 

others, 1976), and uplift of magnetic rock in mantled 

gneiss domes would have been restricted to the more 

westerly parts of the orogen whose eastern parts are 

preserved along the Wasatch Front. The only reasonable 

explanations for Ml through M3 are consistent with the 

notion that these transverse anomalies are expressions 

of segment boundaries. 

By definition, seismological anomalies Sel through Se4 

are segment boundaries for the small earthquakes that 

were recorded from 1962 through 1986. 

Sal through Sa4 are salients occurring where the 

Wasatch fault splits and rejoins itself to enclose a partly 

downdropped block (Sal, Sa2), bends sharply (Sal, Sa3), 

or steps en echelon to the east or west (Sa4) (Wheeler 

and Krystinik, 1988). Our inspection of fault patterns and 

bedrock lithologies near Sal through Sa4 (Hintze, 1980) 

suggests two possible causes for such splits, sharp bends, 

and steps. In the upper few kilometers of the crust, a 

massive, unlayered or irregularly layered rock mass 

might have sufficient fracture toughness that a rupture 

propagating toward the mass might go partly or entirely 

around it more easily than it would go straight through 

it. This scenario might have happened at the southern 

end of the Archean migmatites, schists, and gneisses of 

the Farmington Canyon Complex (Bryant, 1984) at Sa2 

and along the margin of the quartz monzonite of the 

Little Cottonwood stock (Crittenden, 1965) at Sa3. 

Alternatively, two ruptures that nucleate independently 

of each other and propagate toward each other are 
B8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

unlikely to meet in perfect alignment. Their tips would 

probably propagate past each other for some distance 

before each rupture curved to join the other. The result 

could be a sharp bend in the combined fault trace (Sal, 

Sa3), a fault trace that appears to split and rejoin itself 

(Sal, Sa2), or en echelon steps in the fault trace (Sa4). 

Whether a salient originates and grows at a tough rock 

mass, whether two rupture tips barely miss each other, 

or whether both occur, each of the four salients has 

hundreds to thousands of times more structural relief 

that it could have acquired during one large earthquake. 

Each salient must have grown over many recurrence 

intervals of such earthquakes. A salient that grows 

through similar behavior of many large ruptures is a 

segment boundary. 

Topographic anomalies Tl and T2 could be produced 

by (1) differences in rates or amounts of uplift across a 

segment boundary, (2) irregular fault geometry, (3) 

lithologic variation, (4) variation in the geometries of the 

thrust sheets and related folds that are exposed throughout most of the eastern wall of the Wasatch fault zone, or 

(5) variations in erosion rates and stream sizes. Factors 

2 through 4 might be related to segment boundaries but 

need not be. Later, we will show that topographic 

anomalies are poorly associated with inferred segment 

boundaries. Perhaps the plethora of possible causes of 

topographic anomalies is the reason for this poor association. Tl is a topographic low (Wheeler and Krystinik, 

1988) that coincides with Mesozoic rocks in and near the 

Parleys Canyon syncline (Crittenden, 1974). The Mesozoic rocks are bounded on the north and south by more 

resistant Precambrian rocks (Hintze, 1980). T2 is also a 

topographic low (Wheeler and Krystinik, 1988) that 

coincides with a river that cuts through easily erodible 

Mesozoic strata (Hintze, 1980). These strata pass northward under the resistant Paleozoic rocks of a thrust 

sheet, which form a tall mountain at the northern edge of 

T2 (Hintze, 1980). Thus, both topographic anomalies 

might arise from lithologic variations that need not be 

related to segment boundaries. 

Anomalies in pre-Cenozoic structures are large lateral 

ramps that dip moderately northward (Stl, St3) and an 

inferred system of east-striking faults (of unknown 

geometry) of the Uinta aulacogen (St2) (Wheeler and 

Krystinik, 1988). Stl through St3 are inferred to cut the 

upper crust. All three could act as segment boundaries 

by decoupling adjacent parts of the Wasatch fault zone 

and allowing the parts to operate as independent segments. No other similar structures are known along the 

fault zone. The best candidate for a similar structure is 

the north-dipping Willard fault (Crittenden, 1972), which 

dips too gently to be considered a segment boundary 

(Wheeler and Krystinik, 1988). 

In summary, transverse anomalies in aeromagnetic 

and seismological data, salients, and pre-Cenozoic structures can be reasonably explained only as expressions of 

segment boundaries or of faults and other fractures that 

could act as segment boundaries. Topographic anomalies 

can have causes other than segment boundaries, but they 

are the anomalies that are least involved in the inferred 

segment boundaries of the Wasatch fault zone. There are 

so many transverse gravity anomalies that one might 

expect some to have causes other than segment boundaries. Some of the other causes are geologically reasonable; in fact, as we will show later, 4 of the 10 transverse 

gravity anomalies do not coincide with other anomalies 

(Gl, G2, G8, GlO) (fig. 2). However, the striking grouping of most anomalies into a few anomalous parts of the 

fault zone (fig. 2) can be reasonably attributed only to the 

operation of segment boundaries or to chance. 

SIMULATION AND STATISTICAL PROCEDURES 

Wheeler and Krystinik (1987a) described or derived 

several procedures for summarizing and evaluating how 

anomalies are distributed along the fault zone. They 

illustrated these procedures by applying them to an 

artificial data set that resembles a simple version of the 

real data of figure 2. Among these procedures is modification and use of the Jaccard coefficient J (Cheetham and 

Hazel, 1969) to identify strongly associated data types or 

those that tend to have and lack anomalies at the same 

places along the fault zone. One nonnegative value of J 

can be calculated for each pair of data types. Values of J 

near or larger than 1 identify pairs of data types that are 

comparatively strongly associated. These data types 

might be more useful together than they are separately 

for interpreting the observed pattern of anomalies. Conversely, values of J near 0 identify pairs of data types 

that are comparatively unassociated and that tend to 

have and lack anomalies without regard to each other. 

We chose all data types and their anomalies with an eye 

to identifying segment boundaries, so that any data type 

that tends to be unassociated with the others (and 

particularly with data types of most interest for identification of segment boundaries) probably can be dropped 

from further consideration. 

Simulations and appropriately designed statistical 

tests aid in deciding whether anomalies in the observed 

pattern (fig. 2) coincide more than should be attributed to 

chance. Each simulation takes the observed number and 

widths of anomalies in each data type, locates those 

anomalies randomly along the fault zone, and produces a 

simulated anomaly pattern that resembles figure 2 to 

some degree. For the anomalies in figure 2, we performed 300 simulations. This number was chosen 

because it was large enough to smooth out the peculiar-
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B9 

(/) u.. 

0 1-

w 

...J 

0... 

::J 

1-

z 

NUMBER OF 

TRIPLETS 

FIGURE 3. -Three-dimensional geometry of comparison of the 

observed anomaly pattern to simulated patterns. Each pattern 

plots in this graph according to its numbers of triplets, quadruplets, 

and quintuplets of coincident anomalies, which are identified 

according to the rules given in the text. Axes are mutually 

perpendicular, so the quadruplet axis rises out of the page. The red 

line outlines the cloud of points, one from each simulated pattern. 

Dashed parts of axes and origin 0 lie behind the cloud. The center 

of the small ball enclosing an asterisk represents the single 

observed pattern. A statistical test determines whether this ball is 

significantly distinct from the cloud of points. 

ities of any small group of simulated patterns but small 

enough for the simulations to be computationally feasible. 

Each anomaly pattern plots in a three-dimensional 

graph according to its numbers of triplets, quadruplets, 

and quintuplets (fig. 3). Only one simulation produced a 

sextuplet, so this single simulated pattern will be treated 

individually. Wheeler and Krystinik (1987a) described a 

simple way to perform the statistical test by counting the 

numbers of patterns that plot in serial sections through 

figure 3 (fig. 4). Each observed or simulated pattern 

plots on one of a few serial sections. For example, the 

patterns for the data from the Wasatch fault zone all plot 

on the first three sections, because each pattern has k=O, 

1, or 2 quintuplets. The significance plane S' intersects 

each section in a dashed line (fig. 4), although only one 

section and one dashed line contain the point P that 

corresponds to the observed pattern. N(e) is the number 

of simulated patterns that plot on or outside (farther 

from the origin than) the dashed lines, summed over all 

serial sections. Points C and D (fig. 4) contribute to N(e). 

N(p) is the total number of simulated patterns in all the 

sections; thus, points B through D (fig. 4) all contribute 

to N(p ). The level of significance of the statistical test is 

N(e)/N(p) (Wheeler and Krystinik, 1987a). 

u.. (/) 

1- 0 w 

...J a: c.. w :::::> a:l 1- ~ z :::::> 5 z 0 

NUMBER OF 

TRIPLETS 

FIGURE 4.-Axes of figure 3, showing a serial section (red lines). The 

serial section lies parallel to the triplet and quadruplet axes and 

intercepts the quintuplet axis perpendicularly at k, so that all 

observed or simulated patterns of anomalies that contain k quintuplets plot in this serial section (for example, as points B-D). S' is 

called the significance plane (dash-dot line) and intercepts all three 

axes obliquely. There is one serial section for each value of k, k=O, 

1, 2, .... The section shown here is the one on which the observed 

pattern plots (point P, shown as an asterisk). S' intersects the 

serial section in the thin dashed line and is constructed so that the 

dashed line passes through P. Modified from Wheeler and Krystinik (1987a). 

LOCATIONAL UNCERTAINTIES OF ANOMALY EDGES 

The northern and southern edges of each anomaly have 

locational uncertainties. Most uncertainties are ± 1 to 5 

km (Wheeler and Krystinik, 1988). We investigated the 

effects of uncertainties by analyzing two observed patterns in addition to the pattern in figure 2, which shows 

the measured width of each anomaly (table 1). One of the 

other patterns, which shows the minimum anomaly 

widths that are allowed by the locational uncertainties, 

was tested against 300 simulations of the locations of 

these narrowed anomalies. The other pattern, which 

shows maximum allowable widths, was tested against 

only 100 simulations because the wider anomalies generated more and larger coincidings, and the greatly 

increased computation time made more simulations 

infeasible. Thus, we did three statistical tests instead of 

one. We tested for significance with each of these three 

sets of simulations. The principles of simultaneous infer-
BlO REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

ence (Miller, 1981, p. 8) require that each of the three 

tests use a level of significance of 0.05/3=0.0167 instead 

of the habitual 0.05. 

DESCRIPTIVE RESULTS 

OBSERVED PATTERN OF ANOMALIES 

Gravity, aeromagnetic, seismological, and topographic 

data, salients, and pre-Cenozoic structures together 

define a total of 26 anomalies along the Wasatch fault 

zone (fig. 2). Locational uncertainties of anomaly edges 

define minimum and maximum widths for each anomaly 

(fig. 5). Most anomalies occupy only a small fraction of 

the fault's length of 332 km (fig. 6). 

ANOMALOUS SECTIONS OF THE FAULT ZONE 

Most of the 26 anomalies (figs. 2, 5) fall in four narrow, 

well-demarcated, anomalous sections of the fault zone 

(fig. 7). Only five anomalies occur singly or elsewhere 

(fig. 7A). This striking concentration of anomalies into 

four distinct parts of the fault zone is robust, being 

weakened little even if each anomaly takes on its 

extreme widths (figs. 7B, C). 

Triplets, quadruplets, and quintuplets of coincident 

anomalies are built up from coincident pairs. For example, about at km 70, five anomalies in four data types are 

involved in eight pairs (fig. 7A). These 5 anomalies could 

define as many as 10 pairs. Of these 10 possible pairs, the 

one between anomalies G3 and G4 is forbidden because 2 

anomalies in the same data type cannot coincide. Anomalies G3 and Sal do not overlap and so cannot form a pair. 

The other eight possible pairs are all present. A triplet is 

present if and only if all three possible pairs between 

its three anomalies also are present. At km 70, pair 

lines define five triplets: G3-Ml-Sel, G4-Ml-Sel, 

G4-Sel-Sal, G4-Ml-Sal, and Ml-Sel-Sal. Similarly, a 

quadruplet is formed by six pairs among four anomalies. 

At km 70, the only quadruplet is G4-Ml-Sel-Sal. No 

quintuplet can link these four anomalies with G3 because 

the G3-G4 pair is forbidden and the G3-Sal pair is not 

present. If anomalies take on their maximum widths, no 

change occurs in this list of pairs and m-tuplets (fig. 7C). 

However, using minimum widths breaks the pair 

G3-Sel, which causes the triplet G3-Ml-Sel to degenerate into its two remaining pairs (fig. 7B). 

The six coincident anomalies at km 160 and the five at 

km 230 can be examined similarly. The six anomalies in 

six data types at km 130 are linked more complexly (fig. 

8). Consideration of figure 8 will reveal the 12 pairs, 11 

triplets, 5 quadruplets, and 1 quintuplet that are summarized at the right edge of figure 7 A. Narrowing 

anomalies to their minimum widths breaks the pairs 

G5-Ml, G5-Tl, Ml-Se2, and Ml-Sa2 and causes some 

m-tuplets to degenerate (fig. 7 B). Widening anomalies to 

their maximum widths produces all possible pairs except 

Ml-Tl, and some m-tuplets are added (fig. 7C). 

Some of these pairs and m-tuplets duplicate each 

other. A quintuplet comprises five quadruplets, which 

duplicate it. Similarly, a quadruplet comprises four triplets, and a triplet comprises three pairs. For measured 

widths, the observed pattern contains one quintuplet, 

one quadruplet, eight triplets, and one pair after duplicates have been omitted (table 2). For minimum widths, 

the observed pattern contains two quadruplets, two 

triplets, and seven pairs. For maximum widths, it contains two quintuplets, six quadruplets, one triplet, and 

one pair. 

Anomalies Gl, G2, G8, GlO, and T2 are singlets (figs. 

2, 7A); none coincides with any other anomaly. As widths 

decrease to their minimums, several things occur: anomalies overlap less, coincident anomalies become scarcer, 

and these five singlets are joined by G7 and Se3 (figs. 5A, 

7B). As widths increase to their maximums, the reverse 

occurs, but the original five singlets do not join any pairs 

or m-tuplets (figs. 5B, 7C). 

ASSOCIATED DATA TYPES 

The 15 possible pairings of the 6 data types give 15 

values of the Jaccard coefficient J (fig. 9). Increased 

anomaly widths will tend to increase the number of 

coincident anomalies and the degree of association 

between any two data types. Decreased anomaly widths 

will tend to produce the opposite effect. Both effects can 

be seen by comparing median values of J and histogram 

shapes in figure 9. 

The most highly associated data types overall are 

aeromagnetic data (M) and pre-Cenozoic structure (St). 

This high association also holds for maximum and minimum anomaly widths. Each structural anomaly coincides 

with one and only one aeromagnetic transverse anomaly 

and vice versa, regardless of whether anomalies take on 

their measured, minimum, or maximum widths (fig. 7). 

The data type that is most highly associated with seismological data (Se) is footwall salients (Sa). This association is also robust under the effect of width but is only 

moderately strong (J=0.60) (fig. 9A). Each seismological 

anomaly coincides with one and only one salient and vice 

versa, except Se3 and Sa3, which do not coincide with 

each other (fig. 7A). No change occurs for minimum 

widths (fig. 7 B), but for maximum widths Se2 and Se3 

both coincide with Sa2 (fig. 7C). 

Each type of data can combine with five others to 

produce five J values (fig. 9). The median of these five J 

values indicates how well the data type is associated with 

the other five types of data overall. Aeromagnetic data 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE Bll 

0 

(.) -1 

~ i= <( 

w (.) <( >- -1 z a :c <( 0 (!) 0 a.. a: 

~ <( -1 en <( :::::> 

::?! 0 I- a: I- z (!) 

> 0<( ::?!<( w 0 (.)<( 

<( a:: I- ~~ ::::i a.. :::>I- a: w<( <( 0 a:<( 

(!) <Co eno en I- tilo 

0 

(.) -1 

~ i= <( 

w (.) >- -1 

z a :c <( 0 a.. (!) 0 en <( a: 

~ <( -1 :::::> 

0 I- a: I- ::?! z (!) 

> 0<( ::?!<( w 0 (.)<( 

<( a:: I- ~~ ::::i a.. :::>I- a: W<( <( 0 a:<( 

(!) <Co eno en I- tilo t--r--G1 ~ 1- -

F===:: I- G2 -

- I-- -

G2 - - -

I- - - -

1- - - -

50 1-- - 50 - - -

'------ - I- f== 1--- - - G3 -

G3 1- -- ~ Se1 

:sil - --' 

!--"---

F== "---

- Se1 - G4 Sa1 

I- - - r--- -

r- 1--

en 

a: 

w 100 

Iw 

::?! 

0 

-1 

~ 

~ 

r:i 

w 

0 

a: 

0 

Ill 150 

0 

:c <( 

0 

± 

~ :::::> 

en 

a: 

w 100 

Iw 

41° ::?! 

0 

-1 

~ 

~ 

a:~ 

w 

0 

a: 

0 

Ill 150 

0 

:c <( 

0 

± 

~ :::::> 

1- M1 -

I-- -

1- -

-

1- - .....__ - h f--- St1 I- G5 Sa2 - r--- Se2 f--- n --- ;--'--'-- ---= 

f==::i; 1--

~ - Se3 -

- - M2 Sti ---= 

- G7 - f=::::>; -

r=-'-- Sa3 

- -

- M1 -

- 1--

- '------ -

'------

- Stl- -

G5 Se2 Sa2 - - -

1-- c::=: T1 r-- F -

1--- - '- Se3 - G6 !=== -

r- St~ - - M2 - - G7 Sa3 -

- - r-----= -

- r- u. 

0 

:c 

I- 200 :::::> 

0 

en 

u. 

0 

:c 

I- 200 :::::> 

0 

en 

- ~ r-Ga -

- -

- - .....__ 

- GB -

r--

- -

'------ '------

- - -

w 

40° (.) 

z 

~ en 

0 

w 

40° (.) 

z <( 

Ien 

0 

- ~ '---

G9 r----= Se4 Sa4 - I-- r- -

- M3 St3 

I- r-- -

- Se4 1---= 

G9 - Sa4 

- r- St3 

- M3 - - r- r- - - 250 250 1-- :::::::; -

'T2 - - T2 -

- I- - - -

I 

I- - - -

- I- - - - -

G10 G10 I- - - - -

- 1-- - 300 - - 300 

1- - - -

I- - - -

A 

FIGURE 5. -Observed anomaly pattern of figure 2 redrawn to give (A) minimum and (B) maximum widths allowed for each anomaly by 

locational uncertainties of its edges. Uncertainties are from Wheeler and Krystinik (1988). In figure 5A, this procedure would give 

anomaly Se3 a width of 0 km. Its minimum width is arbitrarily set at 1 km. In figure 5B, Se2 and Se3 overlap by 2 km, as do Stl and St2. 
B12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

(IJ 

w 

::J 

<( 

~ 

0 

z 

<( 

u. 

0 

a: 

w 

Ill 

~ 

::J 

z 

10 

5 

A 

T2 Sa2 

Sa1 Se4 

G8 Se3 

G7 Se2 

G6 G10 

Sa3 G3 G5 

G1 

ANOMALY WIDTH, IN KILOMETERS 

(IJ 

w 

:::::i 

<( 

~ 

0 

z 

<( 

u. 

0 

a: 

w 

CCI 

~ 

::J 

z 

10 

5 

8 

Sa1 T1 

Se4 Sa2 

Se3 Se1 

Se2 G10 

G8 G7 

G3 G6 

G2 

G1 

10 20 40 50 60 80 90 100 

ANOMALY WIDTH, IN KILOMETERS 

(IJ 

w 

::J 

<( 

~ 

0 

z 

<( 

u. 

0 

a: 

w 

Ill 

~ 

::J 

z 

10 

5 

c 

G1 

10 

T2 

T1 

G8 

G7 

G6 Sa 1 Se4 St3 

Se3 Se1 

30 50 60 70 80 90 100 

ANOMALY WIDTH, IN KILOMETERS 

FIGURE 6. -Anomaly widths. Each anomaly is identified by its 

letter-number name. G denotes Bouguer gravity data; M, 

aeromagnetic data; Se, seismologic data; Sa, salients; T, 

topographic data; St, pre-Cenozoic structures. Widths are 

from Wheeler and Krystinik (1988). A, Measured anomaly 

widths (fig. 2). The median of 26 widths is 11 km, about 3 

percent of the 332-km length of the fault zone in the study 

area. The widest anomaly after M1 spans 8 percent of the 

fault zone length. B, Minimum widths that are allowed by 

locational uncertainties of anomaly edges (fig. 5A). The 

median is 6 km. The widest anomaly after M1 spans 8 percent 

of the fault zone length. C, Maximum allowable widths (fig. 

5B), the median being 19 km. The widest anomaly after M1 

spans 13 percent of the fault zone length. 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE Bl3 

are the most strongly associated with other data types 

overall (median J=O. 75, 0.33, and 0.86 in fig. 9A, B, and 

C, respectively). Most of the aeromagnetic transverse 

anomalies are connected by pair lines to several other 

anomalies (fig. 7). In contrast, topographic anomaly Tl is 

sparsely connected, and T2 is not connected to other 

anomalies. Topographic data are the least strongly associated overall (median J=O.OO, 0.00, and 0.20 in fig. 9A, 

B, and C, respectively). 

DIAGNOSTIC AND CHARACTERISTIC DATA TYPES 

Does any one data type identify anomalous parts of the 

fault zone particularly well? This question can be 

answered by using techniques that are associated with 

cluster analysis. The four anomalous parts of the fault 

zone divide the zone into nine parts (fig. 7), which group 

naturally into two clusters. The anomalous cluster contains four parts (those around kms 70, 130, 160, and 230). 

The nonanomalous cluster contains five parts (the three 

intervening parts of the fault zone and the two ends, 

where anomalies are scarce and coincident anomalies are 

absent). 

The entries in a presence-absence matrix (table 3) 

allow calculation of two measures called constancy (C) 

and fidelity (F) (Hazel, 1970). Each is expressed as an 

integer between 0 and 10. Constancy expresses the 

degree to which the anomalies of a data type characterize 

a cluster. Structural anomalies characterize the anomalous cluster moderately well (C=8, table 3), because one 

or more of these anomalies are present in most of the 

fault zone parts that are in the anomalous cluster. 

Fidelity expresses the degree to which the anomalies of 

a data type are diagnostic of a cluster. Structural anomalies are perfectly diagnostic of the anomalous cluster 

(F= 10, table 3) but undiagnostic of the nonanomalous 

cluster (F=O). The occurrence of a structural anomaly in 

a particular part of the fault zone indicates that the part 

belongs in the anomalous cluster. 

For most data types, the C and F values for the 

nonanomalous cluster are lower than those for the anomalous cluster (table 3). This difference is reassuring, 

because the data types were chosen and the anomalies 

were defined to be likely to reflect the presence of 

segment boundaries (which should correspond to anomalous parts of the fault zone) instead of segment interiors 

(which should correspond to nonanomalous parts). 

We seek data types that are both diagnostic and 

characteristic of anomalous parts of the fault zone and 

that will plot at the upper right-hand corner of a graph of 

C plotted against F (fig. 10). For measured anomaly 

widths, salients (Sa) and seismological anomalies (Se) are 

perfectly characteristic and perfectly diagnostic of anomalous parts of the Wasatch fault zone (fig. lOA). When 

uncertainty in anomaly widths is considered (fig. lOB, 

C), seismological anomalies become slightly worse indicators of anomalous parts of the fault zone, but salients 

remain perfect one-to-one indicators. Aeromagnetic 

transverse anomalies (M) are perfectly characteristic 

(C=lO) but only moderately diagnostic (F=8), and this 

result is not affected by the uncertainties in anomaly 

widths. Every anomalous part of the fault zone contains 

an aeromagnetic transverse anomaly, but such an anomaly can also occur in a nonanomalous part of the fault. 

Because the unusually wide anomaly Ml spans two 

anomalous parts of the fault zone and the intervening 

nonanomalous part (fig. 7), F=8. 

Recall that the two most highly associated data types 

are aeromagnetic (M) and pre-Cenozoic structural (St) 

data. Anomalies in these two data types correspond in 

parts A2, A3, and A4 of the fault zone but not in part Al 

(table 3), producing C=8 and F=lO for coincident structural and aeromagnetic transverse anomalies in the 

anomalous cluster. These values do not change with 

uncertainty in anomaly widths. Thus, the coinciding of 

structural and aeromagnetic transverse anomalies is 

perfectly diagnostic but only moderately characteristic of 

anomalous parts of the fault zone. Coincident transverse 

anomalies in aeromagnetic and structural data identify 

parts of the fault zone as anomalous, but one anomalous 

part of the fault zone (Al) lacks such a coincident pair. 

SUMMARY OF DESCRIPTIVE RESULTS 

With one exception, the 26 anomalies found in the 

Wasatch fault zone are of similar widths, the median 

being 3 percent of the length of the fault zone. Most 

anomalies coincide with one another in four narrow, 

anomalous parts of the fault zone. 

For the fault zone as a whole, aeromagnetic transverse 

anomalies are the most highly associated with anomalies 

of other kinds, especially those in structural data. Topographic anomalies are the least well associated with 

other kinds. The data type that is most highly associated 

with seismological anomalies is salients, although only 

moderately so. 

For anomalous parts of the fault zone, salients are 

perfectly diagnostic and perfectly characteristic; seismological anomalies are the second best such one-to-one 

indicator. Aeromagnetic transverse anomalies are perfectly characteristic but only moderately diagnostic, and 

coincident structural and aeromagnetic transverse anomalies are perfectly diagnostic but only moderately characteristic. 
B14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

~ ~ < < Cl Cl ~ 

0 

u -1 < ~ F < Cl 

w u >- -1 < z a :I: Cl a.. < (.!) 0 <( a: 

~ < -1 (/l ::::> 

~ 0 1- a: 1- z (.!) u > 0 ~ w 0 ::::> < a: (/l ::J a.. a: a: w w < 0 1- (.!) < (/l (/l 1- (/l 

-

- '-- -

;--

-

-

-

50 - - -

(/l 

a: 

(8,5,1,0) 

~ :::::::::: -- - t----' 

-t------ f- r--1--- ....... .,....... 

~ - -==---- 1-- -- r- ..... 

w 

1-

w t- -

~ 100 0 t-- -

-1 

41° :i2 r- r----= ~ 

a:' 

w 

Cl 

a: 

0 

( 12, 11,5, 1) 

r- '"-b:::::= r-.:::; - -

r- - - v -

- r- - 1--- - ----::: 

OJ 

0 150 

:I: 

< Cl 

(8,3,0,0) 

- r-- ._ :--:-- - r- .... - r-- -

r--.......L t--

r-- -

_,....)~ r--- -~ :..---

-----: - r. 

~ 

r- ... ~ 

- - - -

::::> r-- - u. r-- 0 200 - - :I: 

1-

::::> f--- '-- -

0 

40° (/l ...... r-- r-- - r-- w 

u 

z 

~ 

(7,3,0,0) 

+ 

-

"" -t--I=-- -- ~ 

~ 

r-- - (/l 

0 250 (35,22,6, 1) r-- - ~ 

t-- -

1--

r- -

r- -

r- 1--- -

r- 1-- -

300 1-- -

f-- -

- -

39°-

A 

FIGURE 7. -Pairs and m-tuplets of coincident anomalies in the 

observed anomaly pattern along the Wasatch fault zone. Figures 2 

and 5 name each anomaly. Solid circles identify anomalies that are 

involved in pairs and m-tuplets. Only anomaly M1 has two solid 

circles, because only it is long enough to span two groups of 

coincident anomalies. Solid pair lines that connect these solid circles 

link anomalies that are paired according to the rule that either 

anomaly contains the center of the other. An m-tuplet is present 

wherever all m(m-1)/2 of the possible pairs of the m anomalies are 

also present (Wheeler and Krystinik, 1988). In parentheses to the 

~ < Cl 

u 

~ F 

w < z Cl (.!) 

~ < 

~ 

> 0 

< a: a: w (.!) < 0 

= 

50 -

~ '-- ......_ 

- 1---::--::::; .......-:: - ~ 

- (/l 

a: f--

w 

1-

w - ~ 100 0 -

-1 

41° :i2 

~ ... f--

ci - w - Cl - a: 

0 -

OJ 

0 150 

:I: 

< Q 

r. 

1--

~ r-

.......... f-- ...... t--

r-- - t----' 

-

~ f--

::::> 

u. f-- == 

0 200 1--

:I: 

1-

::::> r-- - 0 

40° (/l f--

w 

u t----' z 

~ 

- ..- f-- r-

(/l 

0 250 f--

f--

r--

1- -

r-- -

300 1--

1-

1-

39°-

~ < Cl 

-1 

< u >-

a :I: 

a.. 0 < -1 (/l a: 0 1-

z (.!) 

~ w 0 (/l ::J a.. w <( 0 (/l (/l 1-

..... -- f::::::: ~ 

r--=--

r---

~ / 

= 

--ISE 

::::.::: r--

r--:::::--

-

1--

Cl 

~ 

-1 

< a: 

::::> 

1-

u 

::::> 

a: 

1-

(/l 

-

-

-

-

-

-

-

-

-

C---..:: 

-

;--

-

::.--

-

-

..____ 

;--

-

-

-

-

-

-

-

-

(7,4,1,0) 

(8,4,1,0) 

(3,0,0,0) 

(6,2,0,0) 

+ 

(24,10,2,0) 

ri~t of each group of paired anomalies are the numbers of pairs, 

triplets, quadruplets, and quintuplets in the group, in that order. 

There are no sextuplets or larger m-tuplets. Below the plus sign are 

the totals of pairs and m-tuplets for the whole anomaly pattern. A, 

Each anomaly is given its measured width (fig. 2). B, Each anomaly 

is given the minimum width that is allowed by the locational 

uncertainties of its edges (fig. 5A). C, Each anomaly is given its 

maximum allowable width (fig. 5B). Numbers in square brackets to 

the right summarize the single pair between anomalies Se3 and Sa2. 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B15 

~ ~ <( <( 

0 0 ~ u ...J <( 

<( i= <( 0 

1- w u > ...J <( z a J: <( 0 a.. C!l 0 <( a: 

~ <( ...J CJ) ::J 

~ 0 1- a: 1-

> ~ z C!l u 0 w 0 ::J <( a: CJ) :::::i a.. a: a: w w <( 0 1- C!l <( CJ) CJ) 1- CJ) 

I--

f--

,...--- -

-

-

50 f- f-- -

'----- - f- ... b.- - -

= v -.....;;;:~ ~ ~ f--

I- b.._ - (8,5,1,0) 

CJ) -- a: 

w 

r- - -

- ;---

1- - - w 

~ 100 0 - '----

...J 

41° ~ f-- -

-

~ 

ci 

w 

0 

a: 

0 

co 

0 150 

J: 

<( 

0 

- - - - / ~ ~ f:-- - 1/ .....:::::: ~ -

( 14, 16,9,2) - = / r- ===:: - f.---

~ f-- [1,0,0,0] - -

-~~ ,..../ - ~ -

~ v - - b-"""' (12, 10,3,0) r- f.---

f.---v~--' - ~ 

- L--- ~ 

± 

<( 

1-

::J 

f- .. f-- f.--- 1------- If.--- f---

u.. r0 200 

-

I- - J: 

1-

::J - - f--- f- -

0 

40° CJ) 

w 

u 

z 

<( 

1-

f- - 1-- - f--

""' _... -r:::::: ~ 8:::: 

v .2. r-- ~ (9,7,2,0) r-_ 

f- - .. + f-- CJ) 

i5 250 f-- - (44,38, 15,2) 1- f-- -

f-- I- -

r- -

- 1- -

~ 

f--

300 I-- -

~ 

-

FIGURE 7. -Continued. 

RE SULTS FROM SIMULATION AND 

STATISTICAL TESTING 

served concentration of anomalies into four 

fault zone worth interpreting, or should it be 

Is the ob 

parts of the 

dismissed 

anomalies i 

as a likely result of a random scattering of 

n unrelated variables alon g the Wasatch fault 

FIGURE 8. -Coincidings between anomalies at km 130 of the Wasatch 

fault zone. Six ellipses represent anomalies that coincide there (figs. 

2, 7A). G denotes Bouguer gravity data; M, aeromagnetic data; Se, 

seismologic data; Sa, salients; T, topographic data; St, pre-Cenozoic 

structure. Lines joining ellipses represent the pair lines that identify 

coincident pairs (fig. 7A). A triplet appears here as three pair lines 

that form a triangle (G5-Ml-Se2 or Ml-Sa2-Stl). The triplet 

Sa2-Tl-Stl is absent because the pair Sa2-Tl is absent (fig. 7A). 

A quadruplet appears as four ellipses linked by six pair lines that 

form a closed quadrilateral and its two internal diagonals 

(G5-Se2-Sa2-Stl). A quintuplet appears as five ellipses, each connected to the others by four pair lines (G5-Ml-Se2-Sa2-Stl). 

zone? The most direct way to answer this question is to 

compare the observed anomaly pattern with simulated 

patterns that have been produced by such a random 

scattering of anomalies, as Wheeler and Krystinik 

(1987a) illustrated by using an artificial data set. 

ONE-DIMENSIONAL COMPARISONS OF OBSERVED AND 

SIMULATED PATTERNS 

For measured widths, few of the simulations produced 

as many triplets or quintuplets as those observed, and 

the observed pattern has more quadruplets than most 

simulated patterns do (fig. llA). For minimum widths, 

the observed pattern has unusually many quadruplets 

but not unusually many triplets, and there are no quintuplets in any simulated or observed pattern (fig. liB). 

For maximum widths, the observed pattern has unusually many quadruplets and quintuplets but unusually few 

triplets (fig. llC). Also, one simulation for maximum 

widths produced the only sextuplet encountered in any 
B16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

TABLE 2.-Coincidings of anomalies (duplicating pairs and mtuplets deleted) 

[Pairs and m-tuplets (triplets, quadruplets, and quintuplets) are among those 

shown in fig. 7. Entries are lists of anomaly names from figs. 2 and 5. -,none 

of the indicated kinds of pairs or m-tuplets is present, except as duplicates of 

parts of larger m-tuplets that might be listed farther to the left for the same 

location] 

Location1 Quintuplets Quadruplets Triplets Pairs 

Measured widths 

70 ...... G4-M1-Se1-Sa1 G3-M1-Se1 

130 ..... G5-M1-Se2- G5-T1-Stl 

Sa2-St1 

160 ..... G6-M2-St2 Se3-St2 

G7-M2-Sa3 

M2-Sa3-St2 

230 ..... G9-Se4-Sa4 

G9-M3-St3 

G9-Sa4-St3 

Minimum widths 

70 ...... G4-M1-Se1-Sa1 G3-M1 

130 ..... G5-Se2-Sa2-Stl M1-Stl 

T1-Stl 

160 ..... G6-M2 

M2-Sa3 

M2-St2 

230 ..... G9-Sa4-St3 M3-St3 

G9-Se4-Sa4 

Maximum widths 

70 ...... G4-M1-Se1-Sa1 G3-M1-Se1 

130 ..... G5-Se2-Sa2-

T1-St1 

G5-M1-Se2-

Sa2-Stl 

140 ..... 2

Se3-Sa2 

160 ..... G7-M2-Sa3-St2 

G6-M2-Sa3-St2 

G6-M2-Se3-St2 

230 ..... G9-Se4-Sa4-St3 

G9-M3-Sa4-St3 

1 Approximate distance south of the Utah-Idaho border, in kilometers. 2

This pair links two anomalous parts of the fault zone by spanning a nonanomalous part and is indicated by square brackets at the right-hand edge of figure 7C. 

anomaly pattern. Thus, these one-dimensional comparisons suggest that anomalies in the observed pattern 

coincide more than those in the simulated patterns, but 

the difference is not striking enough to distinguish 

clearly the observed degree of coinciding from the effects 

of chance. 

THREE-DIMENSIONAL COMPARISONS OF OBSERVED AND 

SIMULATED PATTERNS 

Histograms of m-tuplet frequencies (fig. 11) are easy 

to understand, but information is lost when a pattern's 

number of triplets is separated from its numbers of 

quadruplets and other m-tuplets and similarly for other 

combinations of m-tuplets. We retain this information by 

representing each pattern's triplets, quadruplets, quintuplets, and sextuplets together (fig. 12). 

For measured widths (fig. 12A), 294 of the 300 simulated patterns lack quintuplets and plot in the plane of 

the page. Five simulated patterns and the observed 

pattern have one quintuplet each and plot one unit above 

the page, and a sixth simulated pattern plots two units 

above the page. Viewed this way, the observed pattern 

plots near the outside of the group of all points that 

represent simulated patterns. Accordingly, when all 

m-tuplets are examined together the anomalies of the 

observed pattern are seen to coincide strongly when they 

are compared with the anomalies of the simulated patterns. 

For minimum and maximum anomaly widths, the 

picture is similar but less clear cut. For minimum widths, 

no pattern has m-tuplets larger than quadruplets, so all 

plot in the plane of the page (fig. 12B). The asterisk lies 

on the edge of the cloud of plotted points from the 

simulations. At least two simulated patterns have anomalies that coincide at least as strongly as is observed. For 

maximum widths, the observed pattern plots to the right 

and above the page from the envelope that encloses the 

plotted locations of all simulated patterns (fig. 12C). The 

sextuplet is much rarer than the quintuplets, so this 

simulated pattern probably should be considered to have 

anomalies that are at least as strongly coincident as those 

of the observed pattern. 

Therefore, examining all sizes of m-tuplets together 

shows that the observed pattern has anomalies that 

coincide more strongly than those of nearly all simulated 

patterns. This difference holds whether anomalies take 

on their measured, minimum, or maximum widths. 

QUANTITATIVE COMPARISONS OF OBSERVED AND 

SIMULATED PATTERNS 

For the statistical test of figures 3 and 4, a concise 

statement of the null hypothesis of Wheeler and Krystinik (1987a) is that the observed pattern was drawn 

FIGURE 9.-Values of the Jaccard coefficient J for the 15 ~ 

possible pairs of 6 data types. For each entry, the value of J is 

written above the letters identifying the two data types whose 

degree of association is summarized by J. G denotes Bouguer 

gravity data; M, aeromagnetic data; Se, seismologic data; Sa, 

salients; T, topographic data; St, pre-Cenozoic structures. For 

example, in figure 9A, structural and aeromagnetic data are 

highly associated (J=l.OO). A, Measured widths of anomalies 

(figs. 2, 7A). The median value of J is 0.40. B, Minimum widths 

that are allowed by locational uncertainties of anomaly edges 

(figs. 5A, 7B). The median decreases to 0.22. C, Maximum 

allowable widths (figs. 5B, 7C). The median increases to 0.56. 
5 

CJ) 4 w 

::J 

<( 

:2! 

~ 3 

<( 

LL 

0 

a: 2 w 

Ill 

:2! 

:J 

z 

A 

,-

I--

1-

I--

0 

0 

5 

en 4 w 

::J 

8 

-

0.09 

G,T 

0.00 

M,T 

0.00 

Se,T 

0.00 

Sa,T 

0.00 

Sa,St <( 

~ 

~ 3 

<( 

LL 

0 

a: 2 w 

Ill 

:2! 

:J 

z 

-

-

1-

0 

0 

5 

CJ) 4 w 

::J 

<( 

~ 

~ 3 

<( 

LL 

0 

a: 2 w 

Ill 

~ 

:J 

z 1 

c 

,-

I--

1--

1--

0 

0 

0.00 

Sa,T 

0.00 

M,T 

0.00 

G,T 

0.09 

G,T 

0.00 

M,T 

I 

PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B17 

0.40 

G,Se 

0.40 

G,Sa 

0.40 0.75 

M,Se M,Sa 

0.25 0.30 0.40 0.60 0.75 0.86 1.00 

T,St G,St St,Se Se,Sa Sa,St G,M M,St 

I I I I I 

0.2 0.4 0.6 0.8 1.0 1.2 

JACCARD COEFFICIENT (J) 

0.18 0.27 

G,St G,Se 

0.17 0.27 

M,Se G,Sa 

0.17 0.25 0.30 0.40 0.50 0.60 1.00 

Se,St T,St G,M M,Sa Se,T Se,Sa M,St 

I I I I I I I I 

0.2 0.4 0.6 0.8 1.0 1.2 

JACCARD COEFFICIENT (J) 

0.25 0.75 

T,St Sa,T 

0.20 0.56 0.75 1.00 

Sa,T G,Se Se,St Se,Sa 

0.20 0.44 0.56 0.75 0.86 1.00 1.33 

Se,T G,St G,Sa M,Se G,M M,St M,Sa 

I I I 

0.2 0.4 0.6 0.8 1.0 1.2 1.4 

JACCARD COEFFICIENT (J) 
B18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

TABLE 3.-Constancy and fidelity for anomalous and nonanomalous 

parts of the Wasatch fault zone (measured anomaly widths) 

[ -, column not applicable] 

Data type3 

Cluster1 Part2 G M Se Sa T St 

Presence-absence matrix4 

N ............... Nl 1 0 0 0 0 0 

A ............... Al 1 1 1 1 0 0 

N ............... N2 0 1 0 0 0 0 

A ............... A2 1 1 1 1 1 

N ............... N3 0 0 0 0 0 0 

A ............... A3 1 1 1 0 1 

N ............... N4 0 0 0 0 0 

A ............... A4 1 1 1 1 0 1 

N ............... N5 1 0 0 0 1 0 

Constancy 

A ............... 10 10 10 10 2 8 

N ............... 6 2 0 0 2 0 

Fidelity 

A ............... 6 8 10 10 5 10 

N ............... 4 2 0 0 5 0 

1

A is the cluster of four anomalous parts of the fault zone, which are about 70, 

130, 160, and 230 km south of the Utah-Idaho border (fig. 7A). N is the cluster of 

five intervening and surrounding nonanomalous parts of the fault zone, including 

the northern and southern ends. 2

Part of the fault zone. In each cluster, parts are numbered consecutively from 

north to south. 3

G, gravity data; M, aeromagnetic data; Se, seismologic data (epicenters); Sa, 

salients in fault zone geometry; T, topographic data; St, pre-Cenozoic structures. 4

1 means that the indicated data type has one or more anomalies in the 

indicated part of the fault zone. 0 means that no such anomaly is present in the 

indicated part. Values of entries were determined by inspection of figure 7 A. 

from a population in which anomalies do not tend to 

coincide more strongly than anomalies of the simulated 

patterns. Analogously, the alternative hypothesis is that 

anomalies in the population do tend to coincide more 

strongly than those in the simulations. Each serial section contains a dashed line, the position and slope of 

which are calculated from the total counts of m-tuplets in 

FIGURE 10. -Constancy and fidelity of the six data types in the ..... 

four anomalous parts of the Wasatch fault zone. High values of 

constancy C identify data types in which the presence of an 

anomaly characterizes the cluster of four anomalous parts of the 

fault zone. High values of fidelity F identify data types in which 

the presence of an anomaly is diagnostic of the cluster of 

anomalous parts of the fault zone. G denotes gravity data; M, 

aeromagnetic data; Se, seismologic data; Sa, salients; T, topographic data; St, pre-Cenozoic structures. A, Measured widths 

of anomalies (figs. 2, 7A). Plotted values are from the bottom 

two parts of table 3. B, Minimum widths that are allowed by 

locational uncertainties of anomaly edges (figs. 5A, 7B). Plotted 

values are from tables like table 3 and derived from figure 7 B. 

C, Maximum allowable widths (figs. 5B, 7C). Plotted values are 

derived by a method analogous to that used in figure lOB. 

s 

~ ::J 

w 

0 

u::: 

s 

~ ::J 

w 

0 

u::: 

A 

5 

8 

c 

5 

CONSTANCY (C) 

5 

CONSTANCY (C) 

Se • 

St • 

5 

CONSTANCY (C) 

10 

10 

10 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B19 

A 

110 

206 

294 

290 

100 

82 

80 80 

(/) 

z 

0 

~ :::> 

:2 

en 

u. 60 60 60 0 

a: 

w 

fil 51 ~ 

:::> 

z 46 

40 40 40 

26 

20 20 

12 

* * 5 

0 0 1 3 4 5 0 0 1 2 3 4 0 5 0 1 2 3 4 5 6 7 8 9 10 

NUMBER OF NUMBER OF NUMBER OF 

QUINTUPLETS QUADRUPLETS TRIPLETS 

FIGURE 11. -Comparison of numbers of m-tuplets in observed 

and simulated patterns. Histograms summarize simulated patterns. The number atop each bar shows its height, in numbers 

of simulations. Asterisks indicate the numbers of m-tuplets in 

the observed pattern of figure 7. All patterns have had duplicate 

m-tuplets removed, as they are in table 2. Pairs are ignored. 

all simulations taken together (table 4) and from the 

value of k that identifies the serial section (fig. 4) (fig. 10 

of Wheeler and Krystinik (1987a) shows equations for the 

calculations). Any simulated pattern that plots on or 

outside the dashed line in the appropriate serial section is 

taken to have anomalies that are at least as strongly 

coincident as those of the observed pattern. We perA, Measured anomaly widths. 300 simulations. No patterns 

have any sextuplets or larger m-tuplets. B, Minimum widths 

that are allowed by locational uncertainties of anomaly edges. 

300 simulations. No patterns have any quintuplets or larger 

m-tuplets. C, Maximum allowable widths. 100 simulations. No 

patterns have any m-tuplets larger than sextuplets. 

formed three tests-one each of measured, m1mmum, 

and maximum anomaly widths-so, as described previously, we will reject the null hypothesis if N(e)IN(p) is 

less than 0.0167 for all three tests. N(p) is 300, 300, and 

100 for measured, minimum, and maximum widths, 

respectively. Comparisons of figures 12 and 13 allow 

counting of N(e). Scale differences preclude combining 
B20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

C/) 

z 

0 

i= 

:5 

~ 

~ 

(j) 

~ 

0 

a: 

w 

m 

~ 

~ 

z 

8 

257 

80 

60 

40 

* 

8 

0 L-.J___L_]___L_..J_J 

012345 

NUMBER OF 

QUADRUPLETS 

110 

100 

80 

60 

40 

16 

0 1 2 3 4 5 6 7 8 9 10 

NUMBER OF 

TRIPLETS 

FIGURE 11. -Continued. 

figures 12 and 13 without sacrificing needed detail, but 

the observed pattern is represented in both figures by an 

asterisk to facilitate comparison. 

For measured widths, the upper right-hand cell of 

figure 12A has 5 quadruplets and 10 triplets and would 

plot at q=5, t=10 in figure 13A. Comparison of the two 

figures is aided by observing that no simulated patterns 

can plot above or to the right of this cell in either figure. 

For k=O (no quintuplets), all simulated patterns plot to 

the left of the appropriate dashed line and so cannot 

contribute to N(e). The same is true for the five simulated patterns having k=1, all of which have q=O. 

However, the one simulated pattern that has k=2 plots 

at (0,1) (fig. 12A), which falls to the right of the appropriate dashed line (fig. 13A). N(e) comprises the following contributions: 0 from k=O, 0 from k=1, and 1 from 

k=2. The level of significance is 1/300=0.0033. This value 

is smaller than the prechosen level of significance 0.0167; 

so, for measured widths, we reject the null hypothesis 

that the observed pattern has anomalies located by 

random scattering. 

For minimum widths, the dashed line of figure 13B 

passes through the center of the cell (2,2) in figure 12B. 

Because the dashed line slopes steeply down to the right, 

only two simulated patterns plot in cells whose centers 

lie to the right of the dashed line (fig. 12B). Since 

N(e)=2, the level of significance is 2/300=0.0067. This 

value is less than 0.0167, so, for minimum widths, we also 

reject the null hypothesis. 

For maximum widths, the upper right-hand cell of 

figure 12C would plot at q= 10, t= 15 in figure 13C. The 

point (10, 15) lies to the left of the dashed lines for k=O 

and k= 1 (fig. 13C), so, for these two serial sections, no 

simulated patterns can plot to the right of the corresponding dashed lines or contribute to N(e). For k=2, 

two simulations plot at (0, 3) and (1, 3) (fig. 12C). Both 

points lie to the left of the dashed line for k=2 (fig. 13C). 

So far, no serial section has contributed to N(e). However, we consider that the simulated pattern having the 

sextuplet (fig. 12C) has more strongly coincident anomalies than the observed pattern does. Then N(e)=1, so 

the level of significance is 1/100=0.01. This value is less 

than 0.0167, so, for maximum widths, we again reject the 

null hypothesis. 

The result of the statistical testing is that, regardless 

of uncertainties in anomaly widths, the observed pattern 

contains anomalies that coincide significantly more 

strongly than we would expect if the anomalies had been 

scattered randomly along the Wasatch fault zone. 

INTERPRETING A SINGLE ANOMALOUS PART OF THE 

FAULT ZONE 

The degree of coinciding among anomalies in the 

observed pattern is statistically significant because there 

are unusually many places where unusually many anomalies coincide. All four anomalous parts of the fault zone 

and all of their component anomalies are required to 

support this conclusion of significance. No one anomalous 

part of the fault zone can be singled out as significant in 

itself. For example, random processes occasionally produced a single quintuplet and commonly produced a 

single quadruplet (fig. 11A). Thus, our analyses allow 

interpretation of the observed pattern as a whole but not 

of any one part of it in isolation. Interpretation of a single 

anomalous part of the fault zone could be based on a 

geologic description of the history and present state of 

that part of the fault zone to demonstrate that the 

individual anomalies coinciding there are genetically 

linked instead of randomly coincident. Such description 

and demonstration lie beyond the scope of this report. 
(/) 

z 

0 

~ ...J 

::::> 

~ 

U5 

u. 

0 

a: 

w 

m 

~ 

::::> 

z 

(/) 

z 

0 

~ ::::> 

~ 

U5 

u. 

0 

a: 

w 

m 

::?! 

::::> 

z 

PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE 

c 

60 

40 

36 

20 18 

0 

0 1 2 3 4 5 

NUMBER OF 

QUADRUPLETS 

* 100 99 

40 

20 

0 L.._j_.JL___L__L____L__J 

0 1 2 3 4 5 

NUMBER OF 

SEXTUPLETS 

6 

60 

40 

22 

20 

0 0 1 2 3 4 5 6 7 8 9 

NUMBER OF 

TRIPLETS 

100 

90 88 

20 

0 L.._j__l___L__L___J___J 

0 1 2 3 4 5 

NUMBER OF 

QUINTUPLETS 

FIGURE 11. -Continued. 

10 11 12 13 14 15 

B21 
B22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

A 

10 

9 

8 1 

7 3 

en 

1-

w 6 10 ...J 

a.. 

a: 

1-

u. 5 0 20 

a: 

w 

a:l 

::?i 37 

::::> 4 

z 1 I 

3 48 

52 

2 

2 I 

25 

2 J 

0 4 

0 

1 

* I 

1 

1 1 

6 

11 2 

22 1 

26 2 

13 5 

1 1 1 

1 2 3 4 

NUMBER OF QUADRUPLETS 

5 

FIGURE 12. -Comparison of numbers of m-tuplets in observed and 

simulated patterns. Asterisks represent observed patterns. 

Numerical entries are the numbers of simulated patterns that 

have the indicated numbers of triplets and quadruplets. Cells that 

are divided into three subcells record patterns having quintuplets. Upper, lower left-hand, and lower right-hand subcells give 

the numbers of patterns having zero, one, and two quintuplets, 

respectively. Zero-valued entries are omitted for clarity. Numbers of quintuplets are plotted along a third axis that rises 

perpendicularly out of the page. For example, in figure 12A, 28 

DISCUSSION 

SEGMENTED RUPTURES 

We have shown that the Wasatch fault zone has been 

segmented persistently through much of the late Cenozoic. We still need to show that ruptures from large 

8 

10 

9 

8 

7 1 

en 

1-

w 6 ...J 

CL 

a: 

1-

u. 5 0 5 

a: 

w 

a:l 

::?i 

::::> 4 12 

z 

3 44 

2 81 

73 

0 41 

0 

3 1 

7 1 

5 * 

15 5 

5 1 

1 2 3 4 

NUMBER OF QUADRUPLETS 

5 

simulated patterns have 1 triplet but no quadruplets; of these 

patterns, 25 lack quintuplets and plot in the plane of the page, 2 

have 1 quintuplet each and plot 1 unit above the page, and 1 has 2 

quintuplets and plots 2 units above the page. A, Measured anomaly 

widths. 300 simulations. B, Minimum widths that are allowed by 

locational uncertainties of anomaly edges. 300 simulations. C, 

Maximum allowable widths. 100 simulations. Circled entry at (1, 2) 

indicates that one of these two simulated patterns also has a 

sextuplet. 

earthquakes on the fault zone are segmented at the same 

places. The Wasatch Range and San Pitch Mountains and 

the valleys that border them on the west (fig. 1) form a 

crustal block that is segmented gravitationally, magnetically, seismologically, and structurally. The Wasatch 

fault zone, which traverses this block, is itself segmented 

geometrically by salients. These expressions of segmen-
en 

1-

w 

...J 

a.. 

15 

14 

13 

12 

11 

10 

9 

a: 8 

1-

u. 

0 

ffi 7 cc 

:E 

::> 

z 

6 

5 

4 

3 

2 

1 

0 

PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B23 

c 

2 

1 1 

3 --l 

3 

I 

2 

1 4 2 

1 6 4 

3 5 

1 I 

8 

1 I 

3 1 

4 8 I 1 1 1 I 

2 I 1 i 

2 

! 1 3 11 

5 6 1 1 

0 1 

1 

1 I 

2 

1 I 

1 1 I* 

0 2 3 4 5 6 7 8 9 10 

NUMBER OF QUADRUPLETS 

FIGURE 12. -Continued. 
B24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

TABLE 4.-Total counts of m-tuplets produced by all simulations 

together 

Width Triplets Quadruplets Quintuplets Sextuplets 

Measured ......... 817 109 7 0 

Minimum ......... 517 51 0 0 

Maximum ......... 483 155 14 1 

tation coincide at salients and only there. We conclude 

that the coinciding takes place not by chance but because 

salients have the same cause as the other expressions of 

segmentation. We consider the salients to be segment 

boundaries. 

The salients have map widths and structural reliefs 

that are measured in kilometers (Davis, 1983a, b, 1985; 

Zoback, 1983, fig. 6). Such structural relief presumably 

accumulated by seismic rupture. The structural relief of 

a salient is many hundreds to thousands of times larger 

than the few meters of relief that would be produced by 

one rupture. For example, values given by Zoback (1983) 

and Schwartz and Coppersmith (1984) show the salients 

to have structural reliefs of about 2.6 to 4.0 km, which 

were produced about 2 m at a time by ruptures recurring 

at intervals of about 2,000 to 8,000 years. Such structural 

relief would have taken about 2.6 to 16 m.y. to accumulate, or much or all of the history of the Wasatch fault 

zone (the past 10 m.y. or more) (Naeser and others, 

1983). Thus, the Wasatch fault zone and the rocks 

adjacent to it are segmented, and this segmentation has 

persisted through much or all of the fault's evolution. 

However, none of the six data types of this report 

records properties of individual large seismic ruptures or 

groups of large ruptures. The moderate association of the 

four segment boundaries with seismological anomalies 

(figs. 7, 9) indicates that the salients affect the seismicity 

of the fault zone, at least for the small earthquakes that 

define the seismological anomalies. However, the segmentation hypothesis of Schwartz and Coppersmith 

(1984) states that large ruptures will tend to start, stop, 

or do both at segment boundaries and that no ruptures 

are likely to cross boundaries into adjacent segments. 

Results so far are consistent with the possibility that 

large ruptures might start at salients and propagate in 

both directions along the fault zone, so that ruptures 

would span salients and extend simultaneously into both 

adjacent segments. The nature of the association 

between the segmented fault zone and the behavior of 

large seismic ruptures cannot be inferred from information presented so far. 

The logical jump from a geometrically segmented fault 

zone to segmented ruptures requires information about 

the ruptures. Such information comes from recent U.S. 

Geological Survey mapping of the late Quaternary history of the Wasatch fault zone (Machette and others, 

1986, this volume; Nelson and Personius, 1987; Personius, 1985, 1986; Scott and Shroba, 1985). From field 

mapping, analyses of profiles of fault scarps, maps of 

trenches dug across scarps, and related chronological 

and stratigraphic information, these workers concluded 

that Holocene and latest Pleistocene surface-rupturing 

events on the fault zone have been segmented. They 

infer the existence of 12 segments and subsegments 

separated by 11 boundaries. 

Four of these segment boundaries in Holocene to latest 

Pleistocene surface ruptures occur at the Pleasant View, 

Salt Lake, Traverse Mountains, and Payson salients. At 

these four places where the fault zone is geometrically 

segmented, the surface ruptures that represent the last 

few large increments of the fault's growth caused by 

seismic activity are also segmented. If one assumes that 

earlier (Pleistocene) surface ruptures behaved like the 

younger ruptures, then the older ruptures were also 

segmented at the four salients. We make that assumption and conclude that the four salients of the Wasatch 

fault zone were persistent boundaries for individual large 

seismic ruptures through most or all of the evolution of 

the fault zone. 

PERSISTENT AND NONPERSISTENT SEGMENT 

BOUNDARIES 

Three studies have inferred segment boundaries from 

examinations of geologic records younger than those that 

we used (fig. 14). Schwartz and Coppersmith (1984) 

divided the Wasatch fault zone into six segments. Most of 

their information came from the Holocene record that 

was exposed in and near trenches at four sites along the 

fault zone (fig. 1). Maclean (1985) studied morphology 

and morphometry of 98 drainage basins that evolved 

throughout the Quaternary in the footwall of the fault 

zone. For this time span, Maclean (1985) and Mayer and 

Maclean (1986) divided the fault zone into nine segments. 

M.N. Machette and his coworkers have mapped and 

examined the late Quaternary history of the fault zone. 

Their partly published studies cover nearly all the length 

of the fault zone in Utah and record numerous aspects of 

the single- and multiple-event scarps that form the most 

striking expression of the late Quaternary seismic history of the fault zone. They divided the fault zone into 10 

segments, 8 of which and a part of a ninth are within the 

area covered by this study. The nine late Quaternary 

segment boundaries of Machette and others (this volume) represent the most complete and uniform description of segmentation of the last few large seismic ruptures on the various parts of the fault zone. 

Each of these three studies of a young geologic record 

recognized the four persistent segment boundaries, 

although Schwartz and Coppersmith (1984) did not iden-
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B25 

tify the Pleasant View boundary (fig. 14). Each study 

also interpreted several additional segment boundaries. 

Agreement on these other boundaries, however, was 

much poorer than it was on the four persistent boundaries. Other boundaries were inferred at eight or nine 

places in the study area, but only two or three of these 

boundaries were chosen by two of the three studies, and 

none was chosen by all three (fig. 14). 

Our preferred interpretation of these discrepancies is 

that most represent real changes through time instead of 

error by one or more studies, including ours. A segment 

boundary that is recognized in one young geologic record 

but not in another and not in the pre-Quaternary record 

might be nonpersistent. A nonpersistent boundary is one 

that nucleates or stops several successive seismic ruptures but that does not do so before or after the time 

spanned by these several ruptures. The location, time of 

turning on, and duration of a nonpersistent boundary 

might be controlled by older structures, but this control 

might be so subtle or complex that it cannot be distinguished from the operation of random processes. Then 

the location, appearance, and disappearance of a nonpersistent boundary could be observed but not predicted. 

A useful guide to considering these hypothetical nonpersistent boundaries is the idea of nonconservative 

barriers (King, 1983; King and Yielding, 1984; King and 

N abelek, 1985). Faults contain geometric irregularities 

such as bends, forks, and bumps. Where an irregularity 

is extreme enough that the orientation of the slip vector 

cannot be conserved across it, the irregularity is a 

nonconservative barrier. Geometric consequences of this 

change in slip vector orientation cause ruptures to tend 

to start and stop at nonconservative barriers. Bruhn and 

others (this volume, 1987) analyzed the Salt Lake and 

Traverse Mountains salients and interpreted them as 

nonconservative barriers. Nelson and Personius (1987) 

interpreted the Pleasant View boundary similarly. Geometric irregularities such as salients are large enough 

and extreme enough that they likely will endure considerable slip and likely will persist through much or all of 

the history of a fault zone like the Wasatch. Narrower 

forks, gentler bends, and smaller bumps might force the 

slip vector to change orientation across them for a few 

successive ruptures but then might be smoothed out by 

abrasion of the bumps and insides of the bends and by 

abandonment of one branch of each fork. Such lesser 

geometric irregularities could be formed where lithology, strength, and other rock properties vary along the 

fault zone. The interactions of these lesser variations 

with the rupture processes are likely to be complex 

enough that nonpersistent boundaries could appear to be 

scattered randomly along the fault zone and to form and 

disappear randomly through the history of the fault zone. 

The idea of nonconservative barriers suggests reasonable explanations for the appearance and disappearance 

of a nonpersistent boundary, for its expected duration 

over several successive ruptures, and for any inability to 

predict its location, time of appearance, or duration. 

If nonpersistent boundaries exist, as they appear to, 

they would pose a problem for hazard evaluation. The 

past behavior of a persistent boundary is a guide to its 

likely behavior in the coming decades to millenia, which 

is the time span of interest to hazard evaluation. A 

nonpersistent boundary might be the starting or ending 

point of a rupture, might limit the maximum length of a 

rupture and therefore the maximum size of the earthquake, or might do neither, without much regard for the 

recent behavior of the boundary. It is fortunate that 

most recent boundaries appear to be persistent (three of 

four) in the populous central part of the Wasatch Front 

between Brigham City and Provo (fig. 14). Most of the 

suggested boundaries that might be nonpersistent are 

farther north or south, where fewer people and engineered structures are at risk. 

BEDROCK RIDGES 

Each salient coincides with one or two transverse 

gravity anomalies (fig. 7A). Although each transverse 

gravity anomaly is built up from several components 

(gradients and their ends, the ends of elongate highs and 

lows, and the like), each also has the general form of a 

gravity saddle-a local interruption in the long gravity 

low that trends northward along the western side of the 

Wasatch fault zone (Zoback, 1983). Four of the five 

saddles having the greatest gravity relief are those that 

coincide most closely with the salients-transverse 

anomalies G4, G5, G7, and G9 (fig. 7A). Zoback (1983), 

Mabey (this volume), and we interpret each gravity 

saddle as the expression of a bedrock ridge, partially or 

wholly buried by the low-density Cenozoic fillings of the 

valley basins that underlie the north-trending gravity 

low. The bedrock ridges trend across the north-trending 

basins, so that the gravity saddles mark places where the 

basins are less deep than they are under the adjacent 

valleys to the north and south. Because the basins 

formed by slip on the Wasatch fault zone, the gravity 

saddles also mark places where structural relief and the 

accumulated slip that created the relief are less than they 

are to the north and south. 

The Traverse Mountains segment boundary is the 

clearest example of a bedrock ridge and its gravity 

expression (fig. 15). The ridge, partially exposed as the 

eastern Traverse Mountains, is at least partly bounded 

by faults that have slipped during the Quaternary, as the 

Wasatch fault zone has. The isogals show that the 

bedrock ridge extends across the valley, although the 

western part of the ridge is buried. Structural relief on 
B26 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

+t 

\ \ t0' = 132.21 

\ \ \ 

\ \ \ 

\ \ \ 

\ \ \ 

\ \ 100 \ 

\ \ \ 

\ \ \~ 

\~ \ \~ \t-) \ \i j 

\~ \ \t ,s. 

~ \ \~ \-§. (1) \ \~ 50 \~ I \ 

\ \ \ 

\ \ \ 

\ \ \ 

\ \ \ 

\ \ t,' = 15.~9 

\ .. . \ 

\ \ q0' = 17.64 

-q +q q2'= -13.54 -50 

\ \ qt' =2.07\ 50 

\ \ \ 

\ \ \ 

\ \ \ 

\ \ \ 

\ \~ \ 

\ \::: \ I 

-,50 \~ \ 

\ \~. \ 

\ \~ \ 

\ \~ \ 

\ \ \ 

\ \ \ 

\ \ \ 

\ \ \ 

-100 \ \ 

t,' = -101.221\ \ \ 

\ \ 

A \ \ 

-t 

FIGURE 13. -Summaries of serial sections (like the one shown in fig.4). 

In notation similar to that of Wheeler and Krystinik (1987a), the 

number of quadruplets per pattern is q, and the number of triplets is 

t. Dashed lines are traces of significance planeS' (fig. 4), for various 

+t 

\ 

\ 

\ 

\ 

25 

to'=22.27 

20 

\~ 

\g 

15 \t 

\~ \-g. ~ 

,~ 

10 •. t •........ 

\ 

\ 

\ 

5 \ 

\ 

\ 

-q ' q0'=2.20 

+q 

-10 -5 \ 5 10 

\ 

\ 

-5 \ 

\ 

\ 

\ 

-10 \ 

8 \ 

-t 

numbers (k) of quintuplets per anomaly pattern. Locations of intercepts qk' and tk' are shown by solid circles. To aid comparisons with 

figure 12, values of intercepts are written near these solid circles; 

asterisks represent the observed pattern, and the top and right-hand 

edges of corresponding parts of figure 12 are shown by dotted lines. 

A, Measured anomaly widths (compare with fig. 12A). Observed 

pattern has eight triplets, one quadruplet, and one quintuplet. B, 

Minimum widths that are allowed by locational uncertainties of 

anomaly edges (compare with fig.12B). The observed pattern has 

two triplets and two quadruplets. No simulated or observed pattern 

has any quintuplets, so only the dashed line for k=O is shown. C, 

Maximum allowable widths (compare with fig. 12C). The observed 

pattern has one triplet, six quadruplets, and two quintuplets. 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ to =88.70 

\ \ 

\ 80 \ 

\ \ 

\ \ 

\ \ 

\ \\\ \\\ ,..:I 

~ \"o 

~ 

~ \~ 

\~ 

\ 

\ 

\ 

\ 

\ \; 

\~ \ \~· 60 \~ 

\ \\ t,'=54.20 ~ 

\ \ 

\\\ \\ ~ 

\ 40 ~~ \\ 

\\ \ \ 

\ 

\ 

\ 

\ 

\ 

\~ \ \<;, 

\ \ 

\ \ 

\ \ 

\ \ 20 \ t2' = 19.70 \ 

\-······· \ \ \ \ \ 

\ \ \ 

\ \ \ 

q1' = 17.39 \ q0'=28.46 -q~---------+----------+-~~~~~----.---~+q 

- 40 - 20 \ 20 \ 40 

\ \ 

\ \ 

\ \ 

\ \ 

\ \ 

\ \ \ \ 

c 

-20 

-t 

FIGURE 13. -Continued. 

B27 
B28 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

113° 112° 42°,-----,-----------------~--~,----------.------------------------------------------, 

q, 

~Q 

"'& Qq 

<Q.f. 

41° ~ 

Salt Lake0 

City 

~ 

~ 

UtahLa~)o 

40° (~ 

( 5 ~~ Nephi il 0 i5 

" (% 

~ /1.4 (; 

~ 0 25 MILES 

39° 0 25 KILOMETERS 

the Wasatch fault zone is much less at the ridge than it is 

in Salt Lake or Utah Valleys. The relief between the 

salient and the bottoms of both valleys is partitioned 

among a network of extensional faults, including the 

Pre-a 

0 

Pleasant 

View 

0 

Salt 

Lake 

0 

Traverse 

Mountains 

0 

Payson 

a 

0 

0 

0 

0 

0 

0 

0 

0 

Late Q 

6. 

D. 

D. 

D. 

D. 

H 

0 

0 

0 

0 

0 

normal fault that separates the salient from the Traverse 

Mountains (fig. 15), normal faults that are mapped within 

the eastern Traverse Mountains (Davis, 1983b), normal 

faults that are mapped as separating the Traverse Moun-
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B29 

<1111111 FIGURE 14.-Four sets of segment boundaries for the Wasatch 

fault zone, derived from different parts of the geologic records. 

The right-hand side of the figure shows different symbols placed 

at the latitudes of the suggested boundaries. Four diamonds 

show names boundaries from this report. The geologic record 

that we examined to define these boundaries is mostly preQuaternary (pre-Q), although we also used earthquake epicenters from 1962 through 1986. Eight circles show segment 

boundaries proposed by Maclean (1985) and Mayer and Maclean 

(1986), which are based on the Quaternary (Q) record. Ten 

S A L T 

L A K E 

triangles locate boundaries determined by M.N. Machette, A.R. Nelson, S.F. Personius, and W.E. Scott (written communs., cited by 

Anderson, 1985, 1986), Machette and others (1986, this volume), 

Nelson and Personius (1987), Personius (1986), and Machette, Personius, Scott, and Nelson (written commun., 1986). These boundaries are 

based on the late Quaternary (late Q) record. An eleventh boundary is 

north of our study area. Five squares show Schwartz and Coppersmith's (1984) boundaries, which are based mostly on the Holocene (H) 

record. 

V A L l E Y 

0 

0 5 

FIGURE 15. -Segment boundary at the Traverse Mountains 

salient. Heavy lines represent traces of normal faults 

exhibiting Quaternary or probable Quaternary slip, simplified from Davis (1983b), Scott and Shroba (1985), 

Keaton and others (1987), and M.N. Machette (oral and 

written communs., 1987). Bar and ball identify the downthrown side of each fault. Dotted lines show contacts 

between Quaternary valley-filling sediments (Q) and preQuaternary bedrock (dots on Quaternary side of contact) 

Q 

5 

10 

UTAH 

VALLEY 

10 MILES 

15 KILOMETERS 

(simplified from Hintze (1980) and Zoback (1983, fig. 4)). 

Contour lines are selected isogals of the Bouguer gravity 

field, traced from Zoback (1983, fig. 4). The contour interval 

is 5 mGal near the northern and eastern edges of map and 10 

mGal elsewhere. Single hachures point inward on the lowest 

contours around local gravity minima. WFZ, Wasatch fault 

zone; WVFZ, southern end of West Valley fault zone of 

Keaton and others (1987). 
B30 

c 

N 

1 

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

-8 

8 

-1 

A 

-8 

tains from Utah Valley (fig. 15) and are inferred to do the 

same at Salt Lake Valley (Mattick, 1970; Houghton, 

1986, p. 52), and perhaps still other normal and normaloblique faults that underlie the east-trending gravity 

gradients at the northern end of Utah Valley and the 

southern end of Salt Lake Valley. For example, Okaya 

and Thompson (1985) described three buried normalfaults that step down into the deepest part of a graben 

and that underlie a gravity gradient in the Dixie Valley 

of Nevada. 

Two processes can explain the observed association of 

salients with bedrock ridges and small structural relief 

across the Wasatch fault zone. The first process arises 

from the requirement that slip for any earthquake must 

decrease to zero at the edges of the rupture zone. King 

(1986) postulated that slip measured along the ground 

rupture of a single earthquake will be largest in the 

middle and will taper toward the ends of the ground 

rupture, so that the ends will have a slip deficit in 

comparison with the middle portion. Measurements on 

historical ground ruptures of large, normal-faulting 

earthquakes in and near the Basin and Range province 

show much local variation along the ruptures but, in 

general, support King's postulate (Byerly, 1956; Tocher, 

1956; Slemmons, 1957; Slemmons and others, 1959; 

Myers and Hamilton, 1964, pl. 2; Crone and Machette, 

1984; Wallace, 1984, fig. 28). Large ruptures on normal 

faults will tend to have ends (that is, to start, stop, or do 

both) at persistent segment boundaries, so that, over 

time, these boundaries will accumulate large slip deficits 

in comparison with the slips in segment interiors. The 

slip deficits will be expressed as structural reliefs across 

the fault zone that are smaller at the boundaries than 

they are in segment interiors. 

The second process is implicit in the model of a 

nonconservative barrier (King and N abelek, 1985). 

Imagine a simple, planar, normal fault zone that strikes 

north, dips west, and slips an arbitrary eight units (fig. 

16). For simplicity, we ignore slip tapering toward 

rupture ends. Tapering of slip only accentuates the result 

described later. Suppose that two strands within the 

fault zone rupture toward each other, not necessarily 

simultaneously, with slightly divergent slip vectors. 

Convergent slip vectors would produce a more complex 

but otherwise similar result. The divergence causes 

.... FIGURE 16. -Fault network (process zone) in a hypothetical 

nonconservative barrier on a normal fault. Solid lines represent 

traces of faults; bar and ball are on the down thrown side of each 

fault. Arrows show trends of slip vectors. Numbers give structural elevations (in arbitrary units) with respect to zero elevation of the eastern wall of the main (north-striking) fault strand. 

A identifies a fork in the fault trace; B and C, two ends of a 

bedrock ridge; D, a small fault block (all are discussed in the 

text). 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B31 

extension in the hanging wall at the northern end of the 

southern rupture; arguments analogous to those that 

follow apply to the southern end of the northern rupture. 

The southward extension in the hanging wall can be 

accommodated if the northern end of the main fault 

strand forks at, say, 45° to the northwest (loc. A, fig. 16). 

Suppose that the two forks share equally the slip on the 

single fault strand, so that each branch of the fork slips 

four units. The geometric requirements of brittle slip 

force each branch to fork again and so on, down to sizes 

at which the rock mass no longer acts like distinct rigid 

blocks (King, 1983). After three forkings, the northern 

end of the hanging wall is broken into blocks that step 

downward to the south and west (fig. 16) and form a fault 

network that is the process zone of the nonconservative 

barrier (King and Nabelek, 1985). Structural relief of the 

blocks in the hanging wall decreases northward and 

eastward across the fault network. If a similar thing 

happens at the southern end of the northern rupture, the 

result is a ridge that plunges westward across the 

barrier, from B to C in figure 16. Slip taper will decrease 

the westward plunge of the ridge. If the barrier is a 

persistent segment boundary, then over the course of 

many ruptures, the ridge will increase its structural 

relief in comparison with adjacent parts of the footwall. 

Suppose that sediment accumulates on the downdropped blocks, as it has in Salt Lake and Utah Valleys 

(fig. 15). If the fault pattern of figure 16 were buried to 

level -0.5, halfway between the top of block D and the 

top of the footwall block, the result would be a buried 

bedrock ridge. The low-density sediments would be 

thinner over the ridge than they would be in the valleys 

to the north and south, and a gravity saddle would be 

created. If sediment cover reached only to level -1.5, 

the eastern fault-bounded end of the ridge would be 

exposed as a salient like the Pleasant View and Salt Lake 

salients or like the eastern Traverse Mountains. If cover 

were thinner still, reaching to levels between -5.5 and 

-7.5, an exposed bedrock ridge would separate the two 

valleys, as one does west of the Payson salient (Hintze, 

1980). 

Consideration of figure 16 suggests a way to estimate 

the relative persistence of some segment boundaries, 

averaged over the whole history of the fault zone. If the 

boundary is perfectly persistent, the fault must end 

there. If the boundary is so persistent that few ruptures 

cross it, block D of figure 16 will have no throughgoing 

fault of large throw on its eastern side and might be part 

of the footwall block. A bedrock ridge will be exposed 

across most or all of the valley. This situation occurs at 

the Traverse Mountains salient (fig. 15), which is cored 

by the massive Little Cottonwood stock of Oligocene age 

(Crittenden and others, 1973). If more ruptures cross the 

boundary, they will tend to drop block D down a bit more 

in comparison with the footwall, but the bedrock ridge 

might still be exposed. Both occurrences are present at 

the Payson salient (Davis, 1983b; Machette and others, 

this volume). As successively larger proportions of the 

ruptures cross the boundary, the bedrock ridge will first 

be buried (as it is at the Salt Lake salient) (Davis, 1983a), 

then the salient itself will be buried or nearly buried (as 

it is at the Pleasant View salient) (Crittenden and 

Sorenson, 1985), and, finally, the gravity saddle will 

flatten and nearly disappear at boundaries that persist 

for only a few successive ruptures. 

Thus, on the Wasatch fault zone, the most obvious 

expression of a persistent segment boundary is an 

exposed salient, but the key to estimating the relative 

persistence of several boundaries is the relative sizes of 

the structural reliefs between footwall, salient, bedrock 

ridge, and adjacent valley bottoms. For segment boundaries of intermediate persistence or for boundaries that 

have been persistent for only the most recent portion of 

a fault's history, bedrock ridges might be more diagnostic than salients. Where Bouguer gravity data are abundant, as they are along the Wasatch Front, relative 

structural relief can be estimated easily, but, along other 

fault zones where gravity coverage is sparse or nonexistent, relative structural relief must be estimated indirectly. 

APPLICATIONS TO LARGE HISTORICAL EARTHQUAKES 

The conclusion that bedrock ridges and salients, where 

coincident with the ends of large individual ruptures, 

identify persistent segment boundaries on the Wasatch 

fault zone can be generalized to other normal fault zones 

in and near the Basin and Range province. We test the 

generalization by examining the ends of ground ruptures 

from single large historical earthquakes. 

BORAH PEAK, IDAHO-OCTOBER 28, 1983 

Crone and others (1987) summarized evidence that the 

two ends of the main shock rupture zone of the 1983 

Borah Peak, Idaho, earthquake occurred at persistent 

boundaries of the Thousand Springs segment of the Lost 

River fault zone (fig. 17). At the southeastern boundary, 

here named the Elkhorn Creek segment boundary, the 

fault zone cuts the older Borah Peak horst. Geologic 

mapping and a gravity saddle indicate that the horst 

extends as a bedrock ridge southwest to the main shock 

epicenter (Skipp and Harding, 1985). Crone and others 

(1987) suggested that the horst and its faults extend to 

the depth of the main shock hypocenter (about 15 km) 

(Doser, 1985a; Dewey, 1987), because some of the faults 

have up to 3 km of throw and because a similar horst 
B32 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

44030, ~-----:-tr __ 1_14,-0

0_0_' ____________ 11_,3°30' 1985) indicate that many ruptures have had slip deficits, 

10 

10 

I IDAHO 

Index. map 

showing location 

of study area 

20 MILES 

FIGURE 17. -October 28, 1983, Borah Peak, Idaho, earthquake. Heavy 

lines show ground ruptures from the normal-faulting earthquake; bar 

and ball are on the downthrown side of each rupture. Solid lines show 

continuous surface ruptures; dashed lines show discontinuous ruptures. Red lines represent older normal faults. Thin lines show 

contacts between Quaternary sediments (Q) in valleys and preQuaternary bedrock (dots on Quaternary side of contact). Two 

crosses show the epicentral locations of the main shock and the 

locational uncertainties of two independent workers (large cross, 

Richins and others (1987, table 2, p. 704); small cross, Dewey (1987, 

table 1)). WSS, Warm Spring segment of the Lost River fault zone; 

TSS, Thousand Springs segment; MS, Mackay segment; WCH, 

Willow Creek Hills; EC, Elkhorn Creek; BPH, Borah Peak horst; 

two small arrows, boundaries of TSS. Inset shows the location of the 

map area within Idaho. Simplified from Bond (1978), Skipp and 

Harding (1985), and Crone and others (1987). 

about 25 km to the south is cored by an Eocene pluton. 

Surface faulting in 1983 followed Holocene scarps along 

the Thousand Springs segment (Crone and Machette, 

1984). A gap of 4 km separates that faulting and those 

scarps from evidence of recurrent late Quaternary surface faulting along the entire length of the Mackay 

segment, an indication that at least two pre-1983 ruptures also had ends at the Elkhorn Creek boundary. 

About 5 to 10 mGal of gravity relief across the gravity 

saddle from northwest to southeast (Skipp and Harding, 

and presumably ends, at the boundary during the evolution of the Lost River fault zone. 

At the northwestern boundary of the main rupture 

zone, here named the Willow Creek Hills segment 

boundary, a bedrock ridge is exposed most of the way 

across the valley. Zones of small, discontinuous surface 

offsets and cracks fork to extend northwestward into the 

bedrock ridge and northward along the Warm Spring 

segment. However, the main rupture was confined to the 

Thousand Springs segment, as determined from geologic 

(Crone and Machette, 1984), geodetic (Barrientos and 

others, 1987), and seismological (Doser and Smith, 1985) 

evidence. The 1983 rupture of the Thousand Springs 

segment repeated an early Holocene (Hanks and 

Schwartz, 1987) rupture of the same segment with 

striking fidelity (Salyards, 1985; Vincent, 1985; especially Schwartz and Crone, 1985). The Warm Spring 

segment ruptured at about the same time in the early 

Holocene or more recently, but neither Holocene rupture 

broke the Willow Creek Hills boundary (Crone and 

others, 1987). The exposed bedrock ridge indicates that 

many previous ruptures behaved similarly. Structural 

relief across the Lost River fault zone there is about half 

that in the interior of the Thousand Springs segment 

(Crone and others, 1987). 

Thus, both boundaries of the main 1983 rupture have 

been persistent, and both form bedrock ridges like those 

along the Wasatch fault zone. There is no fault-bounded 

salient at the Willow Creek Hills boundary. At the 

Elkhorn Creek boundary, the sharp bend in the Lost 

River fault zone is much less sharp than the bends in the 

Wasatch fault zone at the Salt Lake, Traverse Mountains 

(fig. 15), and Payson salients but resembles in shape the 

bend at the Pleasant View salient (Crittenden and 

Sorensen, 1985). The corner of the footwall block at 

Elkhorn Creek is broken by several small faults (Susong 

and Bruhn, 1986) and, in this respect, has a structural 

character between the unfaulted Traverse Mountains 

salient (fig. 15) and the moderately faulted Payson 

salient (Hintze, 1980). The Pleasant View and Salt Lake 

salients are separated from the main footwall block by 

normal faults of much larger throw. We choose not to call 

the bend at Elkhorn Creek a salient because doing so 

would stretch the term beyond the limit of usefulness. 

Both boundaries have been interpreted as barriers to 

the propagation of the rupture zone (Boatwright, 1985; 

Crone and others, 1985). We further interpret both as 

nonconservative barriers. Slip-vector orientations are 

too sparse to determine whether orientations change 

across either boundary (A.J. Crone, oral commun., 

1987), as Bruhn and others (1987, this volume) have done 

for the boundaries of the Salt Lake segment. However, 

at the Elkhorn Creek boundary, many ruptures have had 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B33 

ends at about the same place, including at least the last 

three ruptures. The northeast-striking normal faults of 

the Borah Peak horst form a network with the 

northwest-striking Lost River fault zone. Within the 

bend in the range front, the Lost River fault zone 

expands into a skein of northwest-striking fault strands 

that cut diagonally across the bend; some of those 

strands were reactivated in 1983 (Susong and Bruhn, 

1986). The 1983 rupture started at the horst and propagated unilaterally northwestward. Crone and others 

(1987, p. 763) speculated that the faults of the horst 

" ... could create irregularities or obstructions on the Lost 

River fault plane that might inhibit propagating ruptures .... " All of these characteristics are properties that 

would be expected of a nonconservative barrier (King 

and Nabelek, 1985). 

At the Willow Creek Hills boundary, at least the last 

three ruptures and many previous ones have had ends. 

Early aftershocks in 1983 clustered in the boundary and 

the northern part of the Thousand Springs segment 

(Richins and others, 1987). After November 1983, aftershocks migrated through the boundary into the Warm 

Spring segment (Zollweg and Richins, 1985). Aftershocks at and near the boundary tended to have large 

magnitudes, high stress drops, and focal mechanisms 

displaying diversely oriented nodal planes (Boatwright, 

1985; Richins and others, 1987), as though strain were 

being accommodated by breaking bit by bit through a 

network of interlocked faults. Crone and others (1987, p. 

764) speculated about intersecting faults there much as 

they did for the Elkhorn Creek boundary. Geodetic 

modeling (Barrientos and others, 1987, fig. 14) indicates 

that rupture in the boundary extended to lesser depths 

and involved less slip than rupture on the Thousand 

Springs segment did, as though the boundary at depth 

had more fracture toughness than the segment interior. 

All these characteristics are also expected properties of a 

nonconservative barrier. Thus, although we lack slip 

orientation data across the Willow Creek Hills and 

Elkhorn Creek segment boundaries, the 1983-84 earthquake sequence revealed enough characteristics of the 

two boundaries for us to conclude that they are nonconservative barriers. 

HEBGEN LAKE, MONTANA-AUGUST 17, 1959 

The main shock of the 1959 Hebgen Lake, Mont., 

earthquake consisted of two subevents (Ryall, 1962). The 

first was at a depth of about 10 km; the second, 5 s later, 

was a much larger subevent at a depth of about 15 km 

(Doser, 1985b). Epicenters are poorly constrained, but 

most teleseismic epicenters for the main shock, its subevents, and large aftershocks and the epicenters of small 

aftershocks determined from a portable network cluster 

around the southeastern half of the trace of the Red 

Canyon fault, near the southeastern end of ground 

ruptures (fig. 18) (Stewart and others, 1964; Trimble and 

Smith, 1975; Doser, 1985b). Geodetic modeling indicated 

that rupture occurred on two planes that dip southwest 

from the traces of the Hebgen and Red Canyon faults; 

the rupture downdip from the Hebgen fault extended to 

a greater depth but had about the same length and slip as 

the rupture downdip from the Red Canyon fault (Barrientos and others, 1987). However, the first subevent was 

so much smaller than the second that both planes must 

have slipped during the second subevent (Barrientos and 

others, 1987). Therefore, it is not known which plane or 

which outcropping fault slipped first to produce the first 

subevent and presumably trigger the second. 

The results described above can be interpreted in two 

ways in terms of segment boundaries and barriers. The 

first interpretation is that the fault-bounded Red Canyon 

fault block (fig. 18) is a salient and a persistent boundary 

between the Hebgen and Red Canyon segments of the 

southwest-dipping Hebgen-Red Canyon fault zone. In 

this interpretation, the Hebgen Lake earthquake-the 

largest normal-faulting shock so far recorded in or near 

the Basin and Range province-was one of the few 

earthquakes that ruptured past the salient and thus gave 

it a structural relief intermediate between that of the 

hanging wall and that of the footwall. If the salient is a 

nonconservative barrier, its expected high fracture 

toughness could require an unusually large earthquake to 

drive a rupture all the way through the process zone. 

Alternatively, if previous earthquakes had strained the 

barrier until it was ready to fail, the comparatively small 

first subevent could have triggered that failure in the 

form of the much larger second subevent, which had 

unusually large slip and stress drop (Barrientos and 

others, 1987). 

The second interpretation (which we prefer) is that the 

Hebgen-Red Canyon fault zone is young and that its 

still-evolving geometry has not yet stabilized enough to 

justify interpretation in terms of persistent segment 

boundaries and long-lasting barriers. The youth of the 

fault zone has several bases: 

1. First, Barrientos and others (1987) summarized evidence that the fault zone is part of a fault system 

about 0.6 to 2.0 m.y. old, only about 12 to 50 percent 

the estimated age of the Lost River fault zone. 

2. Witkind and others (1962) and Myers and Hamilton 

(1964, p. 85-87) observed that younger ground ruptures, including those of 1959, tend to extend into 

unbroken rock beyond the ends of older faulting. 

Barrientos and others (1987) suggested that the high 

stress drops inferred for the main shock subevents 

could reflect fresh breaks of largely intact rock. 
B34 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

111°20' 111°10' 

RED CANYON 

FAULT BLOCK 

44°50' 

'-'--, L 

Q \ 'j ,·. ) ~/ 

( 

0~,1 

J 

0 

0 2 

FIGURE 18.-August 17, 1959, Hebgen Lake, Mont., earthquake. 

Heavy lines show main ground ruptures along the Hebgen and Red 

Canyon faults and other faults from the normal-faulting earthquake; 

bar and ball are on the downthrown side of each rupture. Red lines 

show traces of reverse faults; teeth are on the up thrown sides. Light 

lines show the shoreline of Hebgen Lake and the contact between 

3. Witkind and others' (1964, pl. 5) geologic mapping 

shows that there is little cumulative slip across the 

ends of the comparatively short traces of the Hebgen 

and Red Canyon faults and that there is no deep, 

sediment-filled valley on the down thrown block, indicative of few slip events and presumably a short 

history in comparison with the Lost River or Wasatch 

fault zones. 

4. In 1959, the largest ground-level slip on the Red 

Canyon fault was 9 to 15ft (2. 7--4.6 m) after correction 

for local slumping (Myers and Hamilton, 1964, pl. 2). 

Near the same place, the fault has its largest cumulative slip of about 1,000 ft (303 m) (Witkind and 

others, 1964, pl. 5). If slip in 1959 was typical of large 

events on the Red Canyon fault, such events have 

2 3 

3 5 

4 MILES 

6 KILOMETERS 

Index map showing 

location of study area 

Quaternary glacial, landslide, and other deposits (Q) and underlying 

pre-Quaternary bedrock (dots on Quaternary side of contact). Inset 

shows the location of the map area in southwestern Montana. Simplified from Myers and Hamilton (1964, pl. 2) and Witkind and others 

(1964, pl. 5). 

occurred no more than 67 to 111 times. Lesser slips in 

additional smaller earthquakes would decrease this 

number. If the fault is 0.6 to 2.0 m.y. old, the 

recurrence interval for slips like that of 1959 is at least 

5,400 to 29,800 years, which is consistent with evidence for several late Quaternary scarp-forming 

events on the Hebgen-Red Canyon fault zone (Myers 

and Hamilton, 1964). 

5. From structural, geomorphic, and geodetic evidence, 

Myers and Hamilton (1964, p. 91-96) concluded that, 

through Quaternary time, a new system of eaststriking normal faults has been propagating eastward 

into the region near and south of Hebgen Lake, 

cutting across and reactivating parts of older, 

northwest-striking faults. Although the Hebgen and 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B35 

Red Canyon faults strike northwest, overall subsidence in 1959 formed an east-trending basin (Myers 

and Hamilton, 1964, pl. 2). 

That the geometry of the fault zone is still evolving 

would be suspected from its youth alone. Additional 

support comes from the observation that the traces of the 

Hebgen and Red Canyon faults lie near and in the 

downdip direction from the traces of older thrust faults 

(fig. 18). Myers and Hamilton (1964) thus concluded that 

the normal faults probably reactivated the thrust faults 

in the shallow subsurface. However, they also noted that 

the thrust complex is probably only a few kilometers 

thick, so that the shapes of the normal faults in map view 

are unlikely to resemble their shapes at hypocentral 

depths. In particular, the curve of the Red Canyon fault 

around the Red Canyon fault block is probably a nearsurface feature, as the block itself is. The two southwestdipping rupture planes that were inferred from geodetic 

data converge downward. We speculate that, eventually, 

the Hebgen fault will connect with the eastern half of the 

Red Canyon fault and abandon the Red Canyon fault 

block to the footwall of the fault zone. At depth, this 

evolution might have occurred already. If the Red Canyon fault block is shallow and ephemeral, interpretations 

of the Hebgen-Red Canyon fault zone in terms of persistent segment boundaries and long-lasting barriers are 

premature by at least several hundred thousand years. 

RAINBOW MOUNTAIN, NEVADA-JULY 6 AND AUGUST 24, 

1954 

Nevada's Rainbow Mountain fault zone produced discontinuous, normal-slip surface ruptures having less 

than 1 m of surface offset twice within 7 weeks in 1954 

(Tocher, 1956). The July main shock (fig. 19A, epicenter 

A) comprised two subevents about 8 s apart, the first and 

larger occurring at a depth of 10 km in right-normal slip 

and the second occurring at 7 km in mostly normal slip 

(Doser, 1986). The August main shock (fig. 19A, epicenter C) involved three subevents starting over 22 s, all at 

depths of about 12 km, of decreasing sizes and involving 

right-normal slip (Doser, 1986). From epicenters, focal 

mechanisms, and locations of ground ruptures, Doser 

(1986) inferred that the fault zone broke in two overlapping sections, the southern portion in July and the 

northern portion in August, each rupture zone propagating northward. Large aftershocks (Doser, 1986) and 

small earthquakes in 1969 and 1970 (Ryall and Malone, 

1971) clustered near epicenters A and C (fig. 19A). The 

southern part of the surface breaks passed along the 

straight eastern side of Rainbow Mountain (Slemmons, 

1956; Stewart and Carlson, 1978), but otherwise the 

rupture " ... did not in its entirety follow any previously 

recognized fault ... " (Tocher, 1956, p. 10). Sparse gravity 

data between exposed Cenozoic andesites and basalts of 

Rainbow Mountain and the southern Stillwater Range 

indicate a thin cover of sediments there but are too few 

to support inferences of the shape of the buried bedrock 

surface (fig. 19B) (Erwin and Berg, 1977). 

The Rainbow Mountain earthquakes illustrate an 

important point about inferring segmentation of a fault 

zone. Despite the geologic, gravity, and seismological 

investigations of the earthquake sequence and its surroundings, there are too few data to determine whether 

any previous earthquakes had ends where the 1954 

rupture zones started and stopped. The idea of segmentation requires that at least a few ruptures have had ends 

at about the same place. We cannot demonstrate coincident ends of successive ruptures on the Rainbow Mountain fault, so we cannot show that the 1954 breaks were 

not randomly distributed along an unsegmented fault 

zone. As shown by the discussions of other historic 

earthquakes in this report, the most useful evidence for 

such a demonstration comes from field investigations of 

scarps and other records of Holocene and late Pleistocene 

earthquakes, and from geophysical delineation of 

changes in structural relief along a fault zone. 

FAIRVIEW PEAK-DIXIE VALLEY, NEVADA-DECEMBER 16, 

1954 

Four months after the Rainbow Mountain earthquake 

sequence, another sequence on the opposite side of the 

Stillwater Range produced information from which segmentation can be inferred. Bilateral rupture near the 

middle of the graben-bounding Fairview fault in Nevada 

began at a depth of 15 km, involved as many as three 

subevents starting over 16 s, and also broke the opposite 

bounding fault of the graben (West Gate fault) and a 

third fault (Gold King fault) that is on strike north of the 

Fairview fault but dips opposite to it (fig. 19A) (Doser, 

1986). Slip was right normal at the hypocenter and at the 

surface on the Fairview and southern West Gate faults, 

but the Gold King and northern West Gate faults showed 

only normal slip at the surface (Larson, 1957; Slemmons, 

1957; Slemmons and others, 1959; Doser, 1986). However, the following summer, geodetic measurements 

detected about 2 m of right slip that had occurred since 

the summer of 1954 across the southern Gold King and 

northern West Gate faults (Whitten, 1957), so either 

coseismic or postseismic right-normal movement characterized all three faults. In 1966 and from 1969 to 1970, 

small earthquakes mostly clustered between the main 

shock hypocenter of 1954 and the southern part of the 

trace of the Fairview fault, which had right-normal slip 

(Stauder and Ryall, 1967; Westphal and Lange, 1967; 

Ryall and Malone, 1971). 
B36 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

40000' ,----------'-1_18'-r--

0

3.::_:0_' ------------,,---------------;"/---1_18,

0

0_0_' ----------, 

Index map showing 

location of study area 

~) 

~ 

0 

0 

;. ~ 

~ 

(J 

RMF 1-e' ~--· 30' 

0 

0 

Q 

A.. 

~ 

l.t.t 

0 

l.t.t 

<) 

~ 

li.J 

A.. 

~ 

~ 

......., 

......., 

...... 

A.. 

0 

10 

10 

.· 

20 

20 MILES 

30 KILOMETERS 

FIGURE 19. -July-December 1954 Rainbow Mountain, Fairview Peak, and 

Dixie Valley, Nev., earthquakes. A, 

Normal faults and epicenters. Red lines 

show main surface faulting from 1954; 

bar and ball are on the downthrown side 

of each fault. DVF, Dixie Valley fault; 

FF, Fairview fault; GKF, Gold King 

fault (hachured part also had ground 

rupture in 1903); RMF, Rainbow Mountain fault; WGF, West Gate fault. Thin 

lines with bars and balls represent other 

late Quaternary faults. Thin lines with 

dots show contact between Quaternary 

valley fillings (Q) and pre-Quaternary 

bedrock, wherever the contact is not a 

late Quaternary normal fault (dots on 

Quaternary side of contact). Solid circles locate epicenters of 1954 main 

shocks, using identifying letters of 

Doser (1986): A, Rainbow Mountain 

earthquake, M (moment magnitude) 

6.2, July 6, locational precision about 20 

km; C, Rainbow Mountain earthquake, 

M 6.5, August 24, precision also about 

20 km; F, Fairview Peak earthquake, 

December 16, M 6.9 to 7.27, precision 

about 8 km (Doser (1986) used this 

event as a master to locate the other 

three but did not include the effect of its 

locational uncertainty in the precision of 

the other locations); G, Dixie Valley 

earthquake, December 16, M 6. 7, precision about 40 km. P shows Pirouette 

Mountain; R locates Rainbow Mountain. 

Inset shows the location of the map area 

within Nevada. Simplified from Slemmons (1956, 1957), Tocher (1956), Slemmons and others (1959), Stewart and 

Carlson (1978), and Doser (1986). B, 

Bouguer gravity contours. Dashed lines 

show the Quaternary-pre-Quaternary 

contact from figure 19A, both faulted 

and unfaulted (dots on Quaternary side 

of contact). Solid lines represent isogals 

at 10-mGal contour intervals. Simplified 

from Erwin and Bittleston (1977) and 

Erwin and Berg (1977), who used a 

contour interval of 5 mGal. 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B37 

0 10 20 MILES 

0 10 20 30 KILOMETERS 

FIGURE 19. -Continued. 
B38 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Four minutes after the Fairview Peak earthquake, a 

smaller shock broke the Dixie Valley fault (fig. 19A), 

beginning at a depth of 12 to 15 km in two subevents 

starting 6 s apart and perhaps propagating bilaterally 

(Okaya and Thompson, 1985; Doser, 1986). Slip was 

normal at the hypocenter (Doser, 1986), but, at the 

surface, it was right normal northeast of a sharp bend in 

the fault trace (at the letters DVF, fig. 19A) and left 

normal south of the bend (Slemmons, 1957; Slemmons 

and others, 1969). Small earthquakes from 1969 to 1970 

were few, but most clustered between the main shock 

epicenter and the trace of the Dixie Valley fault (Ryall 

and Malone, 1971). 

The following evidence indicates that the narrow passage between Dixie and Fairview Valleys is a persistent 

segment boundary of the fault zones on both sides of the 

valleys, which we named the Pirouette Mountain boundary. On the eastern side of the valley, the FairviewWest Gate-Gold King rupture ended there in 1954. 

Although surface faulting broke the boundary along the 

Gold King fault, slip there was small and discontinuous, 

and its dip was the opposite of that on the Fairview fault 

(Slemmons, 1957), analogous to surface faulting in 1983 

in the Willow Creek Hills boundary on the Lost River 

fault (fig. 17). The Gold King fault also produced at least 

3 mi (5 km) of surface faulting in the fall of 1903 

(Slemmons and others, 1959). The 1903 rupture was 

confined to the Pirouette Mountain boundary, an indication that the boundary differs mechanically or in its 

strain history from parts of the same fault zone to the 

north and south. Slemmons (1956, fig. 2) depicted the 

fault zone on the eastern sides of Dixie and Fairview 

Valleys as having had late Quaternary slip. In both 

valleys, the fault zone separates Quaternary sediments 

from pre-Quaternary bedrock of the Clan Alpine and 

adjoining ranges (Stewart and Carlson, 1978). Accordingly, the fault zone in both valleys has probably accumulated at least a few tens of meters of structural relief. 

However, in the Pirouette Mountain boundary the fault 

zone crosses exposed bedrock as the Gold King fault, and 

there is little or no accumulated structural relief (Slemmons and others, 1959). Therefore, at least one and 

probably several pre-1903 faulting events either stopped 

at the Pirouette Mountain boundary or decreased their 

slip markedly for 5 to 10 km across the boundary. The 

latter possibility is unlikely, because maps and longitudinal profiles of slip data measured along surface ruptures of large historical earthquakes in and near the 

Basin and Range province indicate that slip from single 

earthquakes varies markedly over distances of 1 to 2 km 

and can shift from one fault strand to another where their 

tips overlap, but stretches of small slip that are 5 to 10 

km wide usually exist only at the ends of ruptures 

(Slemmons, 1957; Myers and Hamilton, 1964, pl. 2; Crone 

and Machette, 1984; Wallace, 1984, fig. 28). We conclude 

that the Pirouette Mountain boundary has been a late 

Quaternary segment boundary. 

Geophysical evidence indicates that the boundary has 

been persistent. Gravity modeling across Dixie Valley 

and seismic reflection profiles, all near the northern end 

of the 1954 ground ruptures, indicate up to 2 km of 

Quaternary sediments, thickest in the western half of the 

valley (Okaya and Thompson, 1985). Seismic refraction 

studies in Dixie Valley indicate depths to pre-Tertiary 

bedrock as great as 3.2 km in a central graben about 25 

km north of Pirouette Mountain and shallowing southward to 1.1 km in the passage between Dixie and 

Fairview Valleys (Meister, 1967). Sparse Bouguer gravity data show a poorly defined gravity saddle at the 

boundary (fig. 19B) having about 20 mGal of gravity 

relief with respect to the centers of Dixie and Fairview 

Valleys (Erwin and Berg, 1977). A local gravity high, 

perhaps attributable to exposed Cenozoic and Mesozoic 

andesites, rhyolites, and tuffs, masks any gravity relief 

that might exist between the boundary and the narrow 

valley between the Fairview and West Gate faults (fig. 

19B) (Erwin and Berg, 1977). Thus, Pirouette Mountain 

is the exposed eastern end of a largely buried bedrock 

ridge that separates Dixie and Fairview Valleys. The 

Pirouette Mountain boundary lacks a fault-bounded 

salient or a projecting spur like the persistent boundaries 

of the Wasatch fault zone have and more closely resembles the Willow Creek Hills boundary of the Lost River 

fault zone (where the bedrock ridge is exposed) and the 

Elkhorn Creek boundary (where the ridge is buried) (fig. 

17). Because pre-Tertiary bedrock is shallower at the 

bedrock ridge than it is under Dixie and Fairview 

Valleys, ruptures along the eastern sides of both valleys 

have tended to lose slip and probably stopped there 

persistently throughout the evolution of the valleys. The 

same might be true for the graben and valley between 

the Fairview and West Gate faults and was true in 1954. 

On the western sides of Dixie and Fairview Valleys, 

the 1954 Dixie Valley rupture ended at the Pirouette 

Mountain boundary. The gravity saddle indicates that 

many other ruptures also did so throughout the evolution 

of both valleys. Accordingly, the bedrock ridge at Pirouette Mountain has persistently segmented the fault zones 

on both sides of the valleys. 

The manner in which the 1954 ruptures jumped from 

the east-dipping Fairview fault across the Pirouette 

Mountain boundary to the east-dipping Dixie Valley fault 

suggests an analogy to the northern end of the 1983 

Borah Peak rupture of the Lost River fault zone (fig. 17). 

At the Willow Creek Hills boundary, the rupture broke 

discontinuously and with little slip northwestward 

through the boundary, forming or reactivating faults 

that dip opposite to the main rupture along the Thousand 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B39 

Springs segment. Previously, we interpreted results of 

rupture mapping, aftershock studies, and geodetic modeling to suggest that, at the Willow Creek Hills boundary, the 1983 rupture and its aftershocks broke, bit by 

bit, through a network of interlocked faults that form the 

process zone of a nonconservative barrier, after the 

model of King and Nabelek (1985). For the Dixie Valley 

and Fairview Peak earthquakes, Snay and others (1985) 

modeled geodetic data and suggested that slip in 1954 

also occurred on a buried fault that strikes northnorthwest, dips steeply east-northeast, and underlies 

southern Dixie Valley, the Pirouette Mountain boundary, and northern Fairview Valley. Perhaps the 1954 and 

1983 ruptures represent different expressions of the 

same phenomenon, in which most large ruptures on the 

Fairview Valley and Thousand Springs segments stop at 

the Pirouette Mountain and Willow Creek Hills boundaries, but some break through to rupture the Dixie 

Valley and Warm Spring segments. 

The northern end of the 1954 surface rupture of the 

Dixie Valley fault is not a segment boundary. The end 

overlaps by 15 to 20 km the southern ends of Holocene 

and late Pleistocene scarps (Wallace and Whitney, 1984), 

and there is no gravity saddle there (fig. 19B). Perhaps 

rupture ends in the central part of the Dixie Valley fault 

are distributed more or less randomly along the fault, the 

southern ends being bounded by the Pirouette Mountain 

boundary and the ruptures extending northward various 

distances according to the amount of strain that has 

accumulated on this part of the Dixie Valley fault and 

adjacent parts. This example shows the need to demonstrate that two or more rupture ends coincide before 

inferring the presence of a segment boundary, whether 

persistent or not. 

PLEASANT VALLEY, NEVADA-OCTOBER 2, 1915 

Because of its early date, little is known seismologically about the 1915 Pleasant Valley, Nev., earthquake, 

but geologic and gravity evidence and the more recent 

earthquakes that we have discussed previously allow 

some interpretation. The main shock, two large foreshocks, or both produced five main scarps (fig. 20A) 

(Wallace, 1984). Wallace (1984) suggested that, of these 

five, the small Stillwater scarp might have formed by 

lateral spreading of a ridge instead of by tectonic rupture. We will not consider this scarp further. 

Because of its length and large displacements, the 

Pearce scarp dominates the 1915 surface ruptures and 

provides 70 percent of the total estimated seismic 

moment (Wallace, 1984). Glass and Slemmons (1969) 

concluded from examining low-sun-angle aerial photographs that nearly all the 1915 scarps, including the 

Pearce scarp, followed still older scarps, although these 

older scarps could be of diverse ages. From diverse 

geologic observations, Wallace (1984) concluded that the 

Pearce scarp had broken in at least one late Quaternary 

earthquake, the displacement of which was similar to 

that of the 1915 event, probably between a few thousand 

and 12,000 years ago. An earthquake producing such 

displacement would probably have broken most or all of 

the 30-km length of the Pearce scarp, as happened in 

1915. From a trench dug across the northern tip of the 

Pearce scarp, Bonilla and others (1984) interpreted several Holocene and late Pleistocene displacements, each 

probably less than 1 m, and suggested that two of them 

occurred in the 5,000 years before 1915. Therefore, the 

Pearce scarp probably has broken at least twice, including in 1915, and perhaps more often in the Holocene and 

latest Pleistocene. 

To the south, Wallace and Whitney's (1984) compilation shows no Holocene or late Pleistocene scarps on the 

Sou Hills scarp or in the Sou Hills, although Wallace 

(1984, pl. 1) showed a few small pre-1915 scarps on the 

Sou Hills scarp and along strike to the north (fig. 20A). 

Individual scarps from 1915 are lower, shorter in map 

view, and less continuous along the Sou Hills scarp than 

they are along the Pearce scarp. At its southern end, the 

Pearce scarp splays to form numerous very short, low, 

east- and west-facing scarps covering an area about 5 km 

long and 2 km wide and dating from 1915 and earlier 

(Wallace, 1984, pl. 1). The area resembles the shallowly 

buried Pleasant View salient of the Wasatch fault zone 

both in size and in the diffuse style of its faulting. 

Fonseca (1985) described the Sou Hills similarly, contrasting its late Cenozoic faulting style to that of the 

Tobin and Stillwater Ranges. At locality C in figure 20B, 

a gravity saddle and the exposed bedrock ridge of the 

Sou Hills separate Pleasant Valley, where Erwin (1974) 

inferred 2,000 to 2,500 ft (0.6-0.8 km) of Cenozoic alluvial 

filling on the basis of gravity data, from northern Dixie 

Valley, where Speed (1976) estimated 6,000 ft (1.8 km) of 

Cenozoic filling on the basis of gravity modeling but 

Erwin (1974) inferred only about 2,000 ft (0.6 km). These 

observations and inferences support a suggestion that 

the 1915 rupture and one or more earlier ruptures had 

slip deficits at the Sou Hills and stopped there, analogous 

to the model in figure 16. 

By analogy to other fault zones discussed previously, 

the ending of the 1915 rupture at the Sou Hills, the 

gravity saddle, and the diffuse style of the 1915 and older 

faulting indicate that the Sou Hills is a persistent segment boundary and perhaps a nonconservative barrier 

also. Fonseca (1985) drew an analogy to the Borah Peak 

earthquake, during which low, discontinuous surface 

ruptures broke into the Willow Creek Hills segment 

boundary and, a few kilometers farther north, into the 

Warm Spring segment (fig. 17). Like the Willow Creek 
B40 

15' 

~--. ;y··· 

ss 

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Q 

·. 

.. 

DIXIE 

J 

VALLEY 

Q I 

11J030' 

~ PUMPERNICKEL 

.\ VALLEY 

j) 

/ AUGUSTA 

MOUNTAINS 

Q 

Q 

Index map showing 

location of study area 

A 40~0·~~----------~------------------~L-----------~--------------~ 

FIGURE 20. -October 2, 1915, Pleasant Valley, Nev., earthquake. A, 

Normal faults. Red lines show 

main surface faulting from 1915; 

bar and ball are on the downthrown side of each fault. CMS, 

China Mountain scarp; TS, Tobin 

scarp; PS, Pearce scarp; SHS, Sou 

Hills scarp; SS, Stillwater scarp. 

Thin lines with bars and balls represent other Quaternary faults 

(bars and balls omitted where slip 

sense is not known). Thin lines 

with dots show contact between 

Quaternary valley fillings (Q) and 

pre-Quaternary bedrock, wherever the contact is not a Quaternary fault (dots on Quaternary side 

of contact). Inset shows the location of the map area within 

Nevada. Simplified from Muller 

and others (1951), Speed (1976), 

Stewart and Carlson (1978), Wallace (1984), and Wallace and Whitney (1984). B, Bouguer gravity 

contours. Dashed lines show 

Quaternary-pre-Quaternary contact from figure 20A, both faulted 

and unfaulted (dots on Quaternary 

side of contact). Solid lines represent selected isogals. The contour 

interval is 5 mGal near gravity 

saddles A, B, and C and 10 mGal 

elsewhere. Simplified from Erwin 

(1974). 

0 5 10 MILES 

0 5 10 15 KILOMETERS 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE 

-~J 

.. ) 

0 

0 

5 

5 10 

FIGURE 20. -Continued. 

10 MILES 

I 

15 KILOMETERS 

B41 
B42 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Hills, the Sou Hills are fault bounded, juxtaposing Tertiary rocks of the bedrock ridge with the Triassic rocks of 

the adjacent Stillwater Range and the Triassic and 

Permian rocks of the southern Tobin Range (Muller and 

others, 1951). The structural relief of the Sou Hills is 

intermediate between that of the bounding ranges and 

that of Pleasant and Dixie Valleys. We follow Fonseca's 

(1985) usage in defining the Sou Hills segment boundary, 

which includes the Sou Hills scarp of Wallace (1984). 

North of the Pearce scarp, at the gap that separates it 

from the Tobin scarp, a gravity saddle indicates a buried 

bedrock ridge (locality B, fig. 20B). Erwin (1974) 

inferred from gravity data that Grass Valley contains 

about 4,000 ft (1.2 km) of Cenozoic filling at the northern 

edge of the area shown in figure 20. Throw across the 

Tobin scarp is at least 2,500 ft (0.8 km), comparable to 

Erwin's (1974) inference of 2,000 ft (0.6 km) of Cenozoic 

filling in northern Pleasant Valley. Throw on the Tobin 

scarp decreases to zero immediately south of the scarp's 

southern end (Muller and others, 1951). This evidence of 

accumulated slip deficits at the gap could support a 

speculation of a persistent segment boundary there. 

However, we found no evidence that individual ruptures have had ends there. In 1915, the Tobin scarp 

broke an older scarp as continuously as the Pearce scarp 

did and creating displacements as large (Wallace, 1984). 

Wallace and Whitney's (1984) compilation shows late 

Pleistocene scarps on both sides of the gap. The compilation shows Holocene scarps along the Pearce scarp but 

not along the Tobin scarp. However, these Holocene 

scarps are so few that the difference might be erosional 

instead of tectonic. Also, gravity relief in both Grass and 

Pleasant Valleys is about 25 mGal between the valley 

centers and the ranges to the east and about 40 mGal to 

the west, whereas relief across the saddle between the 

valleys is only 10 to 20 mGal (Erwin, 1974). If structural 

relief increases with gravity relief, then the structural 

relief across the Tobin and Pearce scarps is almost as 

large at the saddle as it is to the north or south. There is 

little accumulated slip deficit at the gap between the 

Tobin and Pearce scarps. Finally, Wallace (1984) 

observed that a piedmont surface that spans the gap 

slopes north and northwest and appears to be oversteepened. He suggested that structural relief might be 

expressed in the gap as warping. An old scarp strikes 

northeast partway across the gap (fig. 20A) and could be 

evidence of occasional surface rupture across the gap. 

Thus, because geologic evidence is insufficient to determine whether prehistoric ruptures crossed the gap, we 

conclude that the buried bedrock ridge might have been 

produced by warping and occasional throughgoing ruptures instead of by repeated rupture ends at a segment 

boundary. If there is a boundary there, it did not persist 

through 1915. 

At the northern end of the 1915 ground ruptures, 

scarps were few, low, and discontinuous in comparison 

with those on the Tobin and Pearce scarps. This situation 

occurred in 1915, probably during the Holocene, and 

during the late Pleistocene (Wallace, 1984; Wallace and 

Whitney, 1984). The China Mountain scarp is the only 

one from 1915 to form within a mountain block instead of 

along a range front. Wallace (1984) and Wallace and 

Whitney (1984) showed no scarps of any late Quaternary 

age in the gap between the China Mountain and Tobin 

scarps, and Wallace (1984) wondered why the 1915 break 

jumped across the gap to the China Mountain scarp 

instead of following the older scarps northwestward 

along the eastern side of Grass Valley. Which of the two 

paths was followed by most pre-1915 ruptures is 

unknown. A wide, gentle gravity saddle centers at 

locality A in figure 20B but is poorly defined by a few 

stations in its northwestern quarter (Erwin, 1974). The 

existence of the saddle is determined mostly by lows in 

Pumpernickel Valley north of the area shown in figure 20 

and in Grass Valley. The area of the China Mountain 

scarp, including the gap between it and the Tobin scarp, 

might be a segment boundary, perhaps persistent, but 

we cannot settle the issue without more gravity and 

geologic evidence. Prehistoric ruptures might have 

tended to end at the gap north of the Tobin scarp, might 

have continued preferentially northwest along the edge 

of Grass Valley, or might have begun to abandon Grass 

Valley in favor of the China Mountain scarp in response 

to a hypothesized clockwise rotation of the direction of 

regional extension (Wallace, 1984). Evidence supporting 

such a hypothesized rotation can be seen in map relations 

in the Augusta Mountains (fig. 20A). North-striking 

normal faults within the mountains are truncated by a 

north-northeast-striking normal fault at the range front 

(Muller and others, 1951). This range front fault forms 

the contact between bedrock and Quaternary alluvium 

and thus has been active during the Quaternary. Similar 

relations are less clearly evident in the southern Tobin 

Range. 

In conclusion, the Sou Hills are a persistent segment 

boundary, similar to the Willow Creek Hills boundary on 

the Lost River fault zone. The gap between the Tobin 

and Pearce scarps either is not a segment boundary or is 

one of low persistence, similar to the several gravity 

saddles on the Wasatch fault zone that lack salients ' exposed bedrock ridges, or other evidence of recurrent 

slip deficits. The area of the China Mountain scarp cannot 

be evaluated with available evidence. 

SUMMARY 

A segment boundary exists where two or more large 

historical or prehistoric ruptures have coincident start-
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B43 

ing or stopping ends. Analysis of the Wasatch fault zone 

produced a model in which gravity saddles or other 

evidence of buried bedrock ridges and footwall salients 

that separate hanging-wall valleys indicates accumulated 

slip deficits, which are interpreted as evidence of many 

coincident starting or stopping ends (that is, as persistent segment boundaries). 

The Borah Peak rupture of 1983 started at the Elkhorn 

Creek boundary and ended at the Willow Creek Hills 

boundary, both persistent and both probably nonconservative barriers. 

The Hebgen Lake earthquake of 1959 occurred on a 

young fault zone whose geometry is still evolving and 

which should not yet be interpreted in terms of persistent segment boundaries and long-lasting barriers. 

The Rainbow Mountain earthquakes of 1954 occurred 

on a fault zone where there are too few gravity or 

geologic data to test the Wasatch model, an indication 

that descriptions of more than one rupture zone are 

needed before segmentation can be inferred. 

The Fairview Peak-Dixie Valley rupture zones of 1954 

were separated by the persistent Pirouette Mountain 

segment boundary, which tends to control large ruptures 

on both the eastern and western sides of Fairview and 

Dixie Valleys and perhaps in the graben between the 

Fairview and West Gate faults. The Pirouette Mountain 

boundary might be a nonconservative barrier. The fact 

that the northern end of the Dixie Valley rupture zone is 

not a segment boundary, persistent or otherwise, demonstrates that segments need not rupture completely 

even in large earthquakes. There are too few data to 

allow evaluation of the southern end of the Fairview 

Peak rupture zone. 

The southern end of the 1915 Pleasant Valley earthquake was at the persistent Sou Hills boundary, which 

might be a nonconservative barrier. The gap between 

the Pearce and Tobin scarps is a nonpersistent boundary, 

which did not end the 1915 rupture zone. The northern 

end of the 1915 rupture lacks enough data for interpretation. 

CONCLUSIONS 

The Wasatch fault zone and most or all of the large 

seismic ruptures that occur on it are segmented. Large 

ruptures tend to start, stop, or do both at boundaries 

between segments, tend to be confined within single 

segments, and tend not to rupture across boundaries into 

adjacent segments. 

Persistent segment boundaries occur at the Pleasant 

View, Salt Lake, Traverse Mountains, and Payson salients. These boundaries have controlled the seismic evolution of the Wasatch fault zone for much or all of its 

history and likely will continue to do so into the near 

future. Most boundaries in the populous central parts of 

the Wasatch Front are persistent. 

Other proposed segment boundaries elsewhere along 

the fault zone might be nonpersistent, and their locations 

and times of activity might change unpredictably with 

time. If boundaries that have affected Holocene and 

older Quaternary ruptures are not persistent, then these 

boundaries might not affect the next several ruptures 

that occur on the Wasatch fault zone. However, the next 

ruptures might also be affected by nonpersistent boundaries that have not affected the young geologic record. 

Present boundaries that might be nonpersistent lie 

mostly in the comparatively sparsely populated areas 

north of Brigham City and south of Provo. 

From 1962 to 1986, epicenters of small earthquakes 

that occurred near the trace of the Wasatch fault zone 

changed abundance across the four persistent segment 

boundaries. In contrast, Arabasz and Julander (1986) 

suggested that small earthquakes elsewhere along the 

Wasatch Front can be controlled in complex ways by 

little-known structures of buried thrust sheets and therefore are unlikely to be clearly associated with exposed 

structure. The fact that seismicity near the fault zone is 

associated with the salients is encouraging for efforts to 

understand earthquake generation along the Wasatch 

Front. 

Analysis of the Wasatch fault zone produced a model 

that predicts persistent segment boundaries where 

exposed or shallowly buried transverse bedrock ridges, 

some with salients, indicate accumulated slip deficits and 

where two or more individual large rupture zones have 

started or stopped. 

Testing this model against five historical earthquake 

sequences in and near the Basin and Range province 

indicates the existence of two persistent segment boundaries on the Lost River fault zone of Idaho, one persistent boundary between the Dixie Valley and Fairview 

faults of Nevada, and one persistent boundary and one 

nonpersistent boundary in the Pleasant View rupture 

zone of Nevada. The northern end of the Dixie Valley 

fault is not a segment boundary, persistent or otherwise. 

There is too little information available on both ends of 

Nevada's Rainbow Mountain rupture zone, the southern 

end of the Fairview Peak rupture zone, and the northern 

end of the Pleasant Valley rupture zone to test the model 

there. The Hebgen Lake earthquake in Montana 

occurred on a fault zone that is too young for the model 

to apply. 

REFERENCES CITED 

Algermissen, S.T., Perkins, D.M., Thenhaus, P.C., Hanson, S.L., and 

Bender, B.L., 1982, Probabilistic estimates of maximum acceler-
B44 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

ation and velocity in rock in the contiguous United States: U.S. 

Geological Survey Open-File Report 82-1033, 99 p., 6 pls. 

Anderson, R.E., 1985, Regional and local hazards mapping in the 

eastern Great Basin, in Jacobsen, M.L., and Rodriguez, T.R., 

comps., National Earthquake Hazards Reduction Program, summaries of technical reports, v. XXI: U.S. Geological Survey 

Open-File Report 86-31, p. 472-478. 

---1986, Regional and local hazards mapping in the eastern Great 

Basin, in Jacobsen, M.L., and Rodriguez, T.R., comps., National 

Earthquake Hazards Reduction Program, summaries of technical 

reports, v. XXII: U.S. Geological Survey Open-File Report 

86-383, p. 480-483. 

Arabasz, W.J., 1984, Earthquake behavior in the Wasatch Front area: 

Association with geologic structure, space-time occurrence, and 

stress state, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, p. 310-339. 

Arabasz, W.J., and Julander, D.R., 1986, Geometry of seismically 

active faults and crustal deformation within the Basin and RangeColorado Plateau transition in Utah: Geological Society of America 

Special Paper 208, p. 43-74. 

Arabasz, W.J., Smith, R.B., and Richins, W.D., 1980, Earthquake 

studies along the Wasatch Front, Utah: Network monitoring, 

seismicity, and seismic hazards: Bulletin of the Seismological 

Society of America, v. 70, p. 1479-1499. 

Armstrong, R.C., and Oriel, S.S., 1965, Tectonic development of 

Idaho-Wyoming thrust belt: Bulletin of the American Association 

of Petroleum Geologists, v. 49, p. 1847-1866. 

Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and Utah: 

Geological Society of America Bulletin, v. 79, p. 429-458. 

Barrientos, S.E., Stein, R.S., and Ward, S.N., 1987, Comparison of 

the 1959 Hebgen Lake, Montana and the 1983 Borah Peak, Idaho 

earthquakes from geodetic observations: Bulletin of the Seismological Society of America, v. 77, p. 784-808. 

Boatwright, J., 1985, Characteristics of the aftershock sequence of the 

Borah Peak, Idaho, earthquake determined from digital recordings of the events: Bulletin of the Seismological Society of America, v. 75, p. 1265-1284. 

Bond, J.G., 1978, Geologic map of Idaho: Moscow, Idaho Bureau of 

Mines and Geology, 1 sheet, scale 1:500,000. 

Bonilla, M.G., Villalobos, H.A., and Wallace, R.E., 1984, Exploratory 

trench across the Pleasant Valley fault, Nevada: U.S. Geological 

Survey Professional Paper 1274-B, p. B1-B14, 1 pl. 

Bruhn, R.L., Picard, M.D., and Beck, S.L., 1983, Mesozoic and early 

Tertiary structure and sedimentology of the central Wasatch 

Mountains, Uinta Mountains and Uinta Basin, in Gurgel, K.D., 

ed., Geologic excursions in the overthrust belt and metamorphic 

core complexes of the Intermountain region: Utah Geological and 

Mineral Survey Special Studies 59, p. 63-105. 

Bruhn, R.L., Gibler, P.R., and Parry, W.T., 1987, Rupture characteristics of normal faults: An example from the Wasatch fault zone, 

Utah, in Coward, M.P., Dewey, J.F., and Hancock, P.L., eds., 

Continental extensional tectonics: Geological Society of London 

Special Publication 28, p. 337-353. 

Bryant, B., ·1984, Reconnaissance geologic map of the Precambrian 

Farmington Canyon Complex and surrounding rocks in the 

Wasatch Mountains between Ogden and Bountiful, Utah: U.S. 

Geological Survey Miscellaneous Investigations Map 1-1447, 1 

sheet, scale 1:50,000. 

---1985, Structural ancestry of the Uinta Mountains, in Picard, 

M.D., ed., Geology and energy resources, Uintah basin of Utah: 

Utah Geological Association Publication 12, p. 115-120. 

Bryant, B., and Nichols, D.J., 1988, Late Mesozoic and early Tertiary 

reactivation of an ancient crustal boundary along the Uinta trend 

and its interaction with the Sevier orogenic belt, in Schmidt, C.J., 

and Perry, W.J., Jr., eds., Interaction of the Rocky Mountain 

foreland and the frontal Cordilleran thrust belt: Geological Society 

of America Memoir 171, p. 411-430. 

Burke, K., and Dewey, J.F., 1973, Plume-generated triple junctions: 

Key indicators in applying plate tectonics to old rocks: Journal of 

Geology, v. 81, p. 406-433. 

Byerly, P., 1956, The Fallon-Stillwater earthquakes of July 6, 1954, 

and August 23, 1954-Historic introduction: Bulletin of the Seismological Society of America, v. 46, p. 1-3. 

Cheetham, A.H., and Hazel, J.E., 1969, Binary (presence-absence) 

similarity coefficients: Journal of Paleontology, v. 43, p. 

1130-1136. 

Crittenden, M.D., Jr., 1961, Magnitude of thrust faulting in northern 

Utah: U.S. Geological Survey Professional Paper 424-D, p. 

D128-D131. 

---1965, Geology of the Dromedary Peak quadrangle, Utah: U.S. 

Geological Survey Geologic Quadrangle Map GQ-378, 1 sheet, 

scale 1:24,000. 

---1972, Willard thrust and the Cache allochthon, Utah: Geological 

Society of America Bulletin, v. 83, p. 2871-2880. 

---1974, Regional extent and age of thrusts near Rockport Reservoir and relation to possible exploration targets in northern Utah: 

Bulletin of the American Association of Petroleum Geologists, v. 

58, p. 2428-2435. 

Crittenden, M.D., Jr., and Sorensen, M.L., 1985, Geologic map of the 

North Ogden quadrangle and part of the Ogden and Plain City 

quadrangles, Box Elder and Weber Counties, Utah: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1606, 1 

sheet, scale 1:24,000. 

Crittenden, M.D., Jr., Stuckless, J.S., Kistler, R.W., and Stern, T.W., 

1973, Radiometric dating of intrusive rocks in the Cottonwood 

area, Utah: Journal of Research of the U.S. Geological Survey, v. 

1, p. 173-178. 

Crone, A.J., and Machette, M.N., 1984, Surface faulting accompanying 

the Borah Peak earthquake, central Idaho: Geology, v. 12, p. 

664-667. 

Crone, A.J., Haller, K.M., Guo, S., Liao, Y., Li, Y., and Vincent, 

K.R., 1985, Late Quaternary faulting along the Mackay segment of 

the Lost River fault, central Idaho [abs.]: EEBS, Transactions of 

the American Geophysical Union, v. 66, no. 46, p. 1067. 

Crone, A.J., Machette, M.N., Bonilla, M.G., Lienkaemper, J.J., 

Pierce, K.L., Scott, W.E., and Bucknam, R.C., 1987, Surface 

faulting accompanying the Borah Peak earthquake and segmentation of the Lost River fault, central Idaho: Bulletin of the Seismological Society of America, v. 77, p. 739-770. 

Davis, F.D., comp., 1983a, Geologic map ofthe central Wasatch Front, 

Utah: Utah Geological and Mineral Survey Map 54-A, 2 sheets, 

scale 1:100,000. 

---1983b, Geologic map of the southern Wasatch Front, Utah: 

Utah Geological and Mineral Survey Map 55-A, 2 sheets, scale 

1:100,000. 

---1985, Geologic map of the northern Wasatch Front, Utah: Utah 

Geological and Mineral Survey Map 53-A, 2 sheets, scale 

1:100,000. 

Dewey, J.W., 1987, Instrumental seismicity of central Idaho: Bulletin 

of the Seismological Society of America, v. 77, p. 819-836. 

Doser, D.l., 1985a, The 1983 Borah Peak, Idaho, and 1959 Hebgen 

Lake, Montana, earthquakes; models for normal fault earthquakes 

in the Intermountain seismic belt, in Stein, R.S., and Bucknam, 

R.C., eds., Workshop XXVIII-On the Borah Peak, Idaho, earthquake: U.S. Geological Survey Open-File Report 85--290-A, p. 

368-384. 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B45 

---1985b, Source parameters and faulting processes of the 1959 

Hebgen Lake, Montana, earthquake sequence: Journal of Geophysical Research, v. 90, no. B6, p. 4537-4555. 

---1986, Earthquake processes in the Rainbow Mountain-Fairview 

Peak-Dixie Valley, Nevada, region 1954-1959: Journal of Geophysical Research, v. 91, no. B12, p. 12572-12586. 

Doser, D.I., and Smith, R.B., 1985, Source parameters of the 28 

October 1983 Borah Peak, Idaho, earthquake from body wave 

analysis: Bulletin of the Seismological Society of America, v. 75, p. 

1041-1051. 

Erwin, J.W., 1974, Bouguer gravity map of Nevada-Winnemucca 

sheet: Nevada Bureau of Mines and Geology Map 47, 4-p. pamphlet, 1 sheet, scale 1:250,000. 

Erwin, J.W., and Berg, J.C., 1977, Bouguer gravity map of NevadaReno sheet: Nevada Bureau of Mines and Geology Map 58, 1 sheet, 

scale 1:250,000. 

Erwin, J.W., and Bittleston, E.W., 1977, Complete Bouguer gravity 

map of Nevada-Millett sheet: Nevada Bureau of Mines and 

Geology Map 53, 1 sheet, scale 1:250,000. 

Fonseca, J.E., 1985, Barriers to faulting in the Basin-Range province-Evidence from the Sou Hills transverse block: Geological 

Society of America Abstracts with Programs, v. 17, no. 7, p. 585. 

Forrester, J.D., 1937, Structure of the Uinta Mountains: Geological 

Society of America Bulletin, v. 48, p. 631-666. 

Glass, C.E., and Slemmons, D.B., 1969, Restudy of surface faulting 

from the October 2, 1915, Pleasant Valley area earthquake, 

Nevada: Geological Society of America Abstracts with Programs, 

v. 1, no. 5, p. 28. 

Gries, R., 1983, North-south compression of Rocky Mountain foreland 

structures, in Lowell, J.D., and Gries, R., eds., Rocky Mountain 

foreland basins and uplifts: Denver, Colo., Rocky Mountain Association of Geologists, p. 9-32. 

Hanks, R.C., and Schwartz, D.P., 1987, Morphologic dating of the 

pre-1983 fault scarp on the Lost River fault at Doublespring Pass 

Road, Custer County, Idaho: Bulletin of the Seismological Society 

of America, v. 77, p. 837-846. 

Hansen, W.R., 1965, Geology of the Flaming Gorge area, UtahColorado-Wyoming: U.S. Geological Survey Professional Paper 

490, 196 p. 

Hazel, J.E., 1970, Binary coefficients and clustering in biostratigraphy: 

Geological Society of America Bulletin, v. 81, p. 3237-3252. 

Hintze, L.F., 1980, comp., Geologic map of Utah: Salt Lake City, Utah 

Geological and Mineral Survey, 2 sheets, scale 1:500,000. 

Houghton, W.P., 1986, Structural analysis of the Salt Lake-Provo 

segment boundary of the Wasatch fault zone: Salt Lake City, 

University of Utah, unpublished M.Sci. thesis, 64 p. 

Keaton, J.R., Currey, D.R., and Olig, S.J., 1987, Paleoseismicity and 

earthquake hazards evaluation of the West Valley fault zone, Salt 

Lake City urban area, Utah: Report to U.S. Geological Survey 

under contract 14-08-0001-22048, 93 p. 

King, G., 1983, The accommodation of large strains in the upper 

lithosphere of the Earth and other solids by self-similar fault 

systems: The geometrical origin of b-value: Pure and Applied 

Geophysics, v. 121, p. 761-815. 

---1986, Speculations on the geometry of the initiation and termination processes of earthquake rupture and its relation to morphology and geological structure: Pure and Applied Geophysics, v. 

124, p. 567-585. 

King, G., and Nabelek, J., 1985, Role of fault bends in the initiation and 

termination of earthquake rupture: Science, v. 228, p. 984-987. 

King, G., and Yielding, G., 1984, The evolution of a thrust fault system: 

Processes of rupture initiation, propagation and termination in the 

1980 El Asnam (Algeria) earthquake: Geophysical Journal of the 

Royal Astronomical Society, v. 77, p. 915-933. 

Larson, E.R., 1957, Minor features of the Fairview fault, Nevada: 

Bulletin of the Seismological Society of America, v. 47, p. 377-386. 

Lawton, T.F., 1985, Style and timing of frontal structures, thrust belt, 

central Utah: Bulletin of the American Association of Petroleum 

Geologists, v. 69, p. 1145-1159. 

Mabey, D.R., Crittenden, M.D., Jr., Morris, H.T., Roberts, R.J., and 

Tooker, E.W., 1964, Aeromagnetic and generalized geologic map 

of part of north-central U tab: U.S. Geological Survey Geophysical 

Investigations Map GP-422, 1 sheet, scale 1:250,000. 

Machette, M.N., 1984, Preliminary investigations of late Quaternary 

slip rates along the southern part of the Wasatch fault zone, 

central Utah, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in U tab: U.S. Geological 

Survey Open-File Report 84-763, p. 391-406. 

Machette, M.N., Personius, S.F., and Nelson, A.R., 1986, Late 

Quaternary segmentation and slip-rate history of the Wasatch 

fault zone, Utah [abs.]: EEBS, Transactions of the American 

Geophysical Union, v. 67, no. 44, p. 1107. 

---1987, Quaternary geology along the Wasatch fault zone-Segmentation, recent investigations, and preliminary conclusions, in 

Gori, P.L., and Hays, W.W., eds., Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. 

Geological Survey Open-File Report 87-585, p. Al-A72. 

Maclean, A.F., 1985, Quaternary segmentation of the Wasatch fault 

zone, Utah, as studied by morphometric discriminant analysis: 

Oxford, Ohio, Miami University, unpublished M.Sci. thesis, 200 p. 

Mattick, R. E., 1970, Thickness of unconsolidated to semiconsolidated 

sediments in Jordan Valley, U tab: U.S. Geological Survey Professional Paper 700-C, p. C119-C124. 

Mayer, L., and Maclean, A., 1986, Tectonic geomorphology of the 

Wasatch Front, Utah, using morphologic discriminant analysis-Preliminary implications for Quaternary segmentation of the 

Wasatch fault zone [abs.]: Geological Society of America Abstracts 

with Programs, v. 18, no. 2, p. 155. 

Meister, L.J., 1967, Seismic refraction study of Dixie Valley, Nevada, 

in Thompson, G.A., Meister, L.J., Herring, A.T., Smith, T.E., 

Burke, D.B., Kovach, R.L., Burford, R.O., Salehi, LA., and 

Wood, M.D., Geophysical study of Basin-Range structure, Dixie 

Valley region, Nevada: Bedford, Mass., U.S. Air Force Cambridge Research Laboratories, final scientific report under contract AF 19(628)-3867, pt. I, 80 p. 

Miller, R.G., Jr., 1981, Simultaneous statistical inference (2d ed.): New 

York, Springer-Verlag, 299 p. 

Muller, S.W., Ferguson, H.G., and Roberts, R.J., 1951, Geology of the 

Mount Tobin quadrangle, Nevada: U.S. Geological Survey Geologic Quadrangle Map GQ-7, 4-p. text, 1 sheet, scale 1:125,000. 

Myers, W.B., and Hamilton, W., 1964, Deformation accompanying the 

Hebgen Lake earthquake of August 17, 1959-The Hebgen Lake, 

Montana, earthquake of August 17, 1959: U.S. Geological Survey 

Professional Paper 435-I, p. 55-98, 2 pis. 

Naeser, C.W., Bryant, B., Crittenden, M.D., Jr., and Sorensen, M.L., 

1983, Fission-track ages of apatite in the Wasatch Mountains, 

Utah: An uplift study, in Miller, D.M., Todd, V.R., and Howard, 

K.A., eds., Tectonic and stratigraphic studies in the eastern Great 

Basin: Geological Society of America Memoir 157, p. 29-36. 

Nelson, A.R., and Personius, S.F., 1987, A nonconservative barrier to 

Holocene rupture propagation in the northern Wasatch fault zone, 

Utah [abs.]: International Union for Quaternary Research International Congress, 12th, Ottawa, Ontario, 1987, Programme with 

Abstracts, p. 231. 

Okaya, D.A., and Thompson, G.A., 1985, Geometry of Cenozoic 

extensional faulting: Dixie Valley, Nevada: Tectonics, v. 4, p. 

107-125. 
B46 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Oriel, S.S., and Armstrong, F.C., 1966, Times of thrusting in IdahoWyoming thrust belt: Reply: Bulletin of the American Association 

of Petroleum Geologists, v. 50, p. 2614-2621. 

Personius, S.F., 1985, Preliminary assessment of late Quaternary 

displacement along the Wasatch fault zone near Brigham City, 

Utah [abs.]: Geological Society of America Abstracts with Programs, v. 17, no. 4, p. 261. 

---1986, The Brigham City segment-A new segment of the 

Wasatch fault zone, northern Utah [abs.]: Geological Society of 

America Abstracts with Programs, v. 18, no. 5, p. 402. 

Richins, W.D., Pechmann, J.C., Smith, R.B., Langer, C.J., Goter, 

S.K., Zollweg, J.E., and King, J.J., 1987, The Borah Peak, Idaho, 

earthquake and its aftershocks: Bulletin of the Seismological 

Society of America, v. 77, p. 694-723. 

Royse, F., Warner, M.A., and Reese, D.L., 1975, Thrust belt structural geometry and related stratigraphic problems, WyomingIdaho-northern Utah, in Bolyard, D.W., ed., Deep drilling frontiers of the central Rocky Mountains: Denver, Colo., Rocky 

Mountain Association of Geologists, p. 41-54. 

Ryall, A., 1962, The Hebgen Lake, Montana, earthquake of August 18, 

1959-P waves: Bulletin of the Seismological Society of America, 

v. 52, p. 235-271. 

Ryall, A., and Malone, S.D., 1971, Earthquake distribution and 

mechanism of faulting in the Rainbow Mountain-Dixie ValleyFairview Peak area, central Nevada: Journal of Geophysical 

Research, v. 76, no. 29, p. 7241-7248. 

Salyards, S. L., 1985, Patterns of offset associated with the 1983 Borah 

Peak, Idaho, earthquake and previous events, in Stein, R.S., and 

Bucknam, R.C., eds., Proceedings of Workshop XXVIII-On the 

Borah Peak, Idaho, earthquake: U.S. Geological Survey Open-File 

Report 85-290-A, p. 59-75. 

Schirmer, T.W., 1985, Basement thrusting in north-central Utah: A 

model for the development of the northern Utah highland, in 

Kerns, G.J., and Kerns, R.L., Jr., eds., Orogenic patterns and 

stratigraphy of north-central Utah and southeastern Idaho: Utah 

Geological Association Publication 14, p. 129-143. 

Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquakes: Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

Schwartz, D.P., and Crone, A.J., 1985, The 1983 Borah Peak earthquake-A calibration event for quantifying earthquake recurrence 

and fault behavior on Great Basin normal faults, in Stein, R.S., 

and Bucknam, R.C., eds., Proceedings of Workshop XXVIII-On 

the Borah Peak, Idaho, earthquake: U.S. Geological Survey OpenFile Report 85-290-A, p. 153-160. 

Scott, W.E., and Shroba, R.R., 1985, Surficial geologic map of an area 

along the Wasatch fault zone in the Salt Lake Valley, Utah: U.S. 

Geological Survey Open-File Report 85-448, 18 p., 2 oversize 

sheets, scale 1:24,000. 

Skipp, B., and Harding, S.T., 1985, Preliminary report on geology of 

Borah Peak area, Idaho, including interpretation of seismic and 

gravity data, in Stein, R.S., and Bucknam, R.C., eds., Proceedings of Workshop XXVIII-On the Borah Peak, Idaho, earthquake: U.S. Geological Survey Open-File Report 85-290-A, p. 

657-671. 

Slemmons, D.B., 1956, Geologic setting for the Fallon-Stillwater 

earthquakes of 1954: Bulletin of the Seismological Society of 

America, v. 46, p. 4-9. 

---1957, Geological effects of the Dixie Valley-Fairview Peak, 

Nevada, earthquakes of December 16, 1954: Bulletin of the Seismological Society of America, v. 47, p. 353-375, 1 pl. 

Slemmons, D.B., McDonald, R.L., and Cluff, L., 1969, Surface faulting 

from the December 16, 1954, earthquake in Dixie Valley, Nevada: 

Geological Society of America Abstracts with Programs, v. 1, no. 

5, p. 73-74. 

Slemmons, D.B., Steinbrugge, K. V., Tocher, D., Oakeshott, G.B., and 

Gianella, V.P., 1959, Wonder, Nevada, earthquake of 1903: Bulletin of the Seismological Society of America, v. 49, p. 251-265, 2 

pls. 

Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional tectonics of 

the eastern Basin-Range: Inferences on structural style from 

seismic reflection data, regional tectonics, and thermal-mechanical 

models of brittle-ductile deformation: Journal of Geophysical 

Research, v. 89, no. B7, p. 5733-5762. 

Smith, R.B., and Richins, W.D., 1984, Seismicity and earthquake 

hazards of Utah and the Wasatch Front: Paradigm and paradox, in 

Hays, W.W., and Gori, P.L., eds., Proceedings of Conference 

XXVI; a workshop on Evaluation of regional and urban earthquake hazards and risk in Utah: U.S. Geological Survey Open-File 

Report 84-763, p. 73-112. 

Snay, R.A., Cline, M.W., and Timmerman, E.L., 1985, Dislocation 

models for the 1954 earthquake sequence in Nevada, in Stein, 

R.S., and Bucknam, R.C., eds., Proceedings of Workshop 

XXVIII-On the Borah Peak, Idaho, earthquake: U.S. Geological 

Survey Open-File Report 85-290-A, p. 531-555. 

Speed, R.C., 1976, Geologic map of the Humboldt lopolith and surrounding terrane, Nevada: Geological Society of America Map and 

Chart Series MC-14, 6-p. pamphlet, 1 sheet, scale 1:81,000. 

Stauder, W., and Ryall, A., 1967, Spatial distribution and source 

mechanism of microearthquakes in central Nevada: Bulletin of the 

Seismological Society of America, v. 57, p. 1317-1345. 

Stewart, J.H., and Carlson, J.E., 1978, Geologic map of Nevada: U.S. 

Geological Survey and Nevada Bureau of Mines and Geology, 3 

sheets, scale 1:500,000. 

Stewart, S.W., Hofmann, R.B., and Diment, W.H., 1964, Some 

aftershocks of the Hebgen Lake earthquake-The Hebgen Lake, 

Montana, earthquake of August 17, 1959: U.S. Geological Survey 

Professional Paper 435-D, p. 19-24. 

Stone, D.S., 1986, Seismic and borehole evidence for important preLaramide faulting along the Axial Arch in northwest Colorado, in 

Stone, D.S., ed., New interpretations of northwest Colorado 

geology: Denver, Colo., Rocky Mountain Association of Geologists, p. 19-36. 

Susong, D.D., and Bruhn, R.L., 1986, Structure of an earthquake 

rupture segment boundary in the Lost River fault zone, Idaho-Implications for rupture propagation during the 1983 Borah 

Peak earthquake [abs. ]: EEBS, Transactions of the American 

Geophysical Union, v. 67, no. 44, p. 1107. 

Swan, F.H., III, Schwartz, D.P., and Cluff, L.S., 1980, Recurrence of 

moderate to large magnitude earthquakes produced by surface 

faulting on the Wasatch fault zone, Utah: Bulletin of the Seismological Society of America, v. 70, p. 1431-1462. 

Thenhaus, P. C., ed., 1983, Summary of workshops concerning regional 

seismic source zones of parts of the conterminous United States, 

convened by the U.S. Geological Survey 1979-1980, Golden, 

Colorado: U.S. Geological Survey Circular 898, 36 p. 

Tocher, D., 1956, Movement on the Rainbow Mountain fault: Bulletin 

of the Seismological Society of America, v. 46, p. 10-14, 3 pls. 

Trimble, A.B., and Smith, R.B., 1975, Seismicity and contemporary 

tectonics of the Hebgen Lake-Yellowstone Park region: Journal of 

Geophysical Research, v. 80, no. B5, p. 733-741. 

Vincent, K. R., 1985, Measurement of vertical tectonic offset using 

longitudinal profiles of faulted geomorphic surfaces near Borah 

Peak, Idaho-A preliminary report, in Stein, R.S., and Bucknam, 

R.C., eds., Proceedings of Workshop XXVIII-On the Borah 

Peak, Idaho, earthquake: U.S. Geological Survey Open-File 

Report 85-290-A, p. 76-96. 
PERSISTENT AND NONPERSISTENT SEGMENTATION OF WASATCH FAULT ZONE B47 

Wallace, C.A., and Crittenden, M.D., Jr., 1969, The stratigraphy, 

depositional environment and correlation of the Precambrian 

Uinta Mountain Group, western Uinta Mountains, Utah, in Lindsay, J.B., ed., Geologic guidebook of the Uinta Mountains, Utah's 

maverick range: Intermountain Association of Geologists annual 

field conference, 16th, Evanston, Wyo., 1969, Guidebook, p. 

127-141. 

Wallace, R.E., 1984, Fault scarps formed during the earthquakes of 

October 2, 1915, in Pleasant Valley, Nevada, and some tectonic 

implications: U.S. Geological Survey Professional Paper 1274-A, 

p. Al-A33, 1 pl. 

Wallace, R.E., and Whitney, R.A., 1984, Late Quaternary history of 

the Stillwater seismic gap, Nevada: Bulletin of the Seismological 

Society of America, v. 74, p. 301-314. 

Westphal, W.H., and Lange, A.L., 1967, Local seismic monitoringFairview Peak area, Nevada: Bulletin of the Seismological Society 

of America, v. 57, p. 1279-1298. 

Wheeler, R.L., 1984, A plan for evaluating hypothesized segmentation 

of the Wasatch fault, in Hays, W.W., and Gori, P.L., eds., 

Proceedings of Conference XXVI; a workshop on Evaluation of 

regional and urban earthquake hazards and risk in Utah: U.S. 

Geological Survey Open-File Report 84-763, p. 576-605. 

Wheeler, R.L., and Krystinik, K.B., 1987a, Evaluating coinciding 

anomalies along a fault trace or other traverse-Simulations and 

statistical procedures: U.S. Geological Survey Bulletin 1802, 12 p. 

---1987b, Persistent and nonpersistent segment boundaries on the 

Wasatch fault, central Utah [abs.]: Seismological Research Letters, v. 58, no. 1, p. 31. 

---1987c, Persistent and nonpersistent seismic segmentation of 

the Wasatch fault zone, Utah [abs.]: Geological Society of America 

Abstracts with Programs, v. 19, no. 5, p. 342. 

---1988, Segmentation of the Wasatch fault zone, Utah-Summaries, analyses, and interpretations of geological and geophysical 

data: U.S. Geological Survey Bulletin 1827, 47 p. 

Whitten, C.A., 1957, Geodetic measurements in the Dixie Valley area: 

Bulletin of the Seismological Society of America, v. 47, p. 321-325. 

Witkind, I.J., Myers, W.B., Hadley, J.B., Hamilton, W., and Fraser, 

G.D., 1962, Geologic features of the earthquake at Hebgen Lake, 

Montana, August 17, 1959: Bulletin of the Seismological Society of 

America, v. 52, p. 163-180. 

Witkind, I.J., Hadley, J.B., and Nelson, W.H., 1964, Pre-Tertiary 

stratigraphy and structure of the Hebgen Lake area-The Hebgen 

Lake, Montana, earthquake of August 17, 1959: U.S. Geological 

Survey Professional Paper 435-R, p. 199-207, 1 pl. 

Zietz, 1., Shuey, R., and Kirby, J.R., Jr., 1976, Aeromagnetic map of 

Utah: U.S. Geological Survey Geophysical Investigations Map 

GP-907, 1 sheet, scale 1:1,000,000. 

Zoback, M. L., 1983, Structure and Cenozoic tectonism along the 

Wasatch fault zone, Utah, in Miller, D.M., Todd, V.R., and 

Howard, K.A., eds., Tectonics and stratigraphy of the eastern 

Great Basin: Geological Society of America Memoir 157, p. 3-27. 

Zollweg, J.E., and Richins, W.D., 1985, Later aftershocks of the 1983 

Borah Peak, Idaho, earthquake and related activity in central 

Idaho, in Stein, R.S., and Bucknam, R.C., eds., Proceedings of 

Workshop XXVIII-On the Borah Peak, Idaho, earthquake: U.S. 

Geological Survey Open-File Report 85-290-A, p. 345-384. 

Subsurface Geology Along the Wasatch Front 

By DON R. MABEY, UTAH GEOLOGICAL AND MINERAL SuRVEY, SALT LAKE CITY, 

UTAH 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-C 

CONTENTS 

Page Page 

Abstract ......................................................................... . C 1 Interpretations-Continued 

Introduction .................................................................... . 1 Displacement Along the Wasatch Fault Zone.................. Cll 

Geophysical Data ............................................................. . 2 Dip of the Wasatch Fault............................................. 11 

Gravity Data ............................................................. . 2 Spurs of the Wasatch Range......................................... 12 

Magnetic Data ........................................................... . 6 Fault Segments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

Interpretations ................................................................ . 6 Conclusions...................................................................... 15 

Cenozoic Sediments .................................................... . 6 References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

ILLUSTRATIONS 

Page 

FIGURES 1-4. Maps showing: 

1. Complete Bouguer gravity anomalies of the Wasatch Front area and the surface traces of the Wasatch and 

East Cache fault zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C4 

2. Residual total magnetic intensities of the Wasatch Front area and the surface traces of the Wasatch and East 

Cache fault zones.......................................................................................................................... 8 

3. Thickness of low-density sedimentary rocks in Wasatch Front valleys and the Cache Valley, inferred from 

gravity data................................................................................................................................. 10 

4. Blocks of contrasting lithology inferred from magnetic anomalies, boundaries of areas of contrasting gravity 

anomalies, and fault-rupture segment boundaries from Schwartz and Coppersmith (1984) and additional 

boundaries inferred from geophysical data for the Wasatch Front......................................................... 14 

III 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

SUBSURFACE GEOLOGY ALONG THE WASATCH FRONT 

By DoN R. MABEY1 

ABSTRACT 

Most of the life and property at risk from an earthquake on the 

Wasatch fault is in the valley areas west of the fault, where a complete 

cover of Quaternary sediments obscures the pre-Quaternary geology. 

A compilation of data from drill holes that provide significant information on the subsurface geology in the valley has been used with regional 

gravity and magnetic data, supplemented locally by more detailed 

geophysical surveys, to study the subsurface geology of the Wasatch 

Front area. Bouguer gravity lows are produced by low-density sediments in a string of Cenozoic basins along the Wasatch Front. These 

sediments, which are primarily Miocene and Pliocene in age, are up to 

4 km thick in grabens that underlie the valleys. Major normal faults are 

reflected by large linear gravity gradients. Detailed quantitative 

interpretations of the gravity anomalies require information on the 

lateral and vertical variations in the density of the Cenozoic sediments. 

The large eastward decrease in regional Bouguer gravity anomalies 

across the Wasatch Front reflects isostatic compensation for the high 

elevations of the Wasatch Range and areas to the east. Individual 

basins and ranges to the west are not in isostatic equilibrium. The 

segmentation of the gravity lows and the blocks of contrasting basement lithology indicated by the magnetic data suggest segment boundaries for the Wasatch fault zone that are generally consistent with 

segment boundaries inferred from surface mapping of the fault zone. 

INTRODUCTION 

The Wasatch fault zone lies between the northtrending Wasatch Range on the east and a nearcontinuous string of valleys on the west. At two locations, rocks of Tertiary age and older occur in transverse 

zones extending between the Wasatch Range and parallel ranges to the west. Elsewhere, Quaternary sediments 

cover the valley areas. Most of the population and 

property at risk from an earthquake on the Wasatch fault 

zone are in these valleys. Knowledge of the subsurface 

structure of these valleys and the physical properties of 

Manuscript approved for publication November 20, 1990. 1

Now at Atwood and Mabey, Inc., CVSR Box 2408, Moab, UT 84532. 

the rocks underlying the valley floors is important to 

both understanding the geologic structure of the 

Wasatch Front and predicting the ground response and 

deformation that will occur with an earthquake along the 

Wasatch fault zone. Most pre-Cenozoic rocks that are 

exposed in the Wasatch Range and in the parallel ranges 

to the west are in thrust sheets that moved tens of 

kilometers eastward relative to underlying rocks during 

the Sevier orogeny. Exposures of autochthonous preCenozoic rocks are not adequate to reveal the deep 

subsurface geology or to define major crustal blocks 

along the Wasatch Front. The primary purposes of the 

investigations reported here are to examine the data 

relating to the subsurface geology of the Wasatch Front, 

to interpret those data in terms of regional subsurface 

geology related to the earthquake hazard of the area, and 

to indicate how geophysical data can be used in more 

detailed studies. 

The name Wasatch Front as generally used by the 

residents of Utah refers to a loosely defined area along 

the western side of the Wasatch Range, where most of 

Utah's population resides. In 1983, the National Board of 

Geographic Names (NBGN), on the recommendation of 

the Utah State Committee on Place Names, defined the 

Wasatch Range as extending from Collinston on the 

north to Nephi on the south and as being the westernmost of the several north-trending mountain masses in 

the Middle Rocky Mountain physiographic province. 

Wasatch Mountains is not recognized as a geographic 

name by the NBGN. Stokes (1977) defined the "Wasatch 

Front valleys" physiographic subdivision as the valley 

area west of the Wasatch Range (as later defined by the 

NBGN), excluding the area of maximum historical 

extent of the Great Salt Lake but including Utah Lake 

and including the valley area to the north on the western 

side of Clarkston Mountain. In this study, the eastern 

part of the Great Salt Lake is included as part of the 

Wasatch Front valleys. Cache Valley and the other 

Cl 
C2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

valleys lying immediately east of the Wasatch Range are 

not normally considered part of the Wasatch Front. 

Local concentrations of population and development 

exist in Cache Valley, as does a significant earthquake 

hazard. Several lifelines into the Wasatch Front valleys 

extend through the Wasatch Range, and water storage 

reservoirs in the Wasatch Range are upstream from 

major population centers of the Wasatch Front. For 

these reasons, the Wasatch Range and Cache Valley are 

included in this study. 

Information on the subsurface geology along the 

Wasatch Front can be obtained from geophysical and 

drill-hole data. As part of the Wasatch Front Earthquake 

Hazards Reduction Program, the Utah Geological and 

Mineral Survey (UGMS) has compiled geophysical data 

along the Wasatch Front and assembled information on 

drill holes in the valley areas along the front. The 

primary sets of geophysical data are regional gravity and 

magnetic surveys that cover the entire study area and a 

few refraction and reflection seismic profiles. Thousands 

of wells have been drilled in the valleys, but only a small 

percentage provides geologic data of use in this study. 

All available drill-hole logs were examined, and a computer file was developed from 267 drill holes that either 

(1) were over 300 m deep, (2) encountered bedrock 

(usually pre-Tertiary rock or Tertiary volcanic rocks), or 

(3) had complete and accurate lithologic logs (Case, 

1985). The geophysical data, drill-hole data, and surface 

geology were then interpreted to define (1) the thickness, structure, and composition of the Cenozoic rocks 

underlying the valley areas, (2) blocks of crust displaying 

contrasting lithology, and (3) other regional crustal 

anomalies that might relate to current seismicity. 

This study is built on numerous other studies that 

usually involved only one of the data sets used or that 

examined only part of the Wasatch Front. The earliest 

published regional geophysical studies of a major part of 

the Wasatch Front were the gravity survey by Cook and 

Berg (1961) and the aeromagnetic survey by Mabey and 

others (1964). These two studies defined the major 

regional gravity and magnetic anomalies along the 

Wasatch Front from Nephi to Farmington and established the sources of the larger regional anomalies. 

Zoback (1983) used gravity data as the foundation for a 

study of structure and Cenozoic tectonism in northcentral Utah. Arnow and Mattick (1968) reported on a 

seismic refraction profile to determine the thickness of 

valley fill in Salt Lake Valley. (Several reports by the 

U.S. Geological Survey (USGS) and others before 1983 

used the name Jordan Valley for the valley area between 

the Oquirrh Mountains and the Wasatch Range. In 1983, 

the NBGN rejected the name Jordan Valley in favor of 

Salt Lake Valley.) Mattick (1970) used seismic refraction 

data along with gravity, magnetic, and drill-hole data to 

study the thickness of unconsolidated to semiconsolidated sediments in Salt Lake Valley. Resistivity soundings have been used locally to determine the thickness 

and resistivity of sediments in the valleys (Zohdy and 

Jackson, 1969). Seismic reflection profiling has been used 

extensively by commercial companies to explore for 

petroleum in the Cenozoic basins and underlying older 

rocks, but most of these data are not available to the 

public. Cook and others (1967) presented an interpretive 

geologic cross section along an east-west profile between 

Little Mountain and the Wasatch Range in the Ogden 

area based on a seismic refraction and reflection survey 

and detailed gravity data. Smith and Bruhn (1984) presented data from several reflection profiles obtained in 

petroleum exploration, and Glenn and others (1980) 

reported on one profile across Hill Air Force Base. 

Recently, high-resolution seismic refraction profiles 

have been used to study near-surface fault geometry 

(Crone and Harding, 1984). Several other investigators 

have used geophysical data, most commonly gravity 

data, in studies of the subsurface geology of individual 

valleys or parts of the valleys. 

The surface geology of the Wasatch Front has been 

studied by numerous investigators for over a century. 

Regional geologic maps that were particularly useful to 

this study were the geologic map of Utah (Hintze, 1980), 

geologic maps of the central, southern, and northern 

Wasatch Front (Davis, 1983a, b, 1985), and the geologic 

map of the Farmington Canyon complex and surrounding 

rocks (Bryant, 1984). 

GEOPHYSICAL DATA 

GRAVITY DATA 

Zoback (1983) compiled a gravity map of the Wasatch 

Front region based on data in USGS files. This data set 

includes gravity stations from numerous surveys done by 

the USGS and others. Zoback's compilation was used to 

prepare a gravity map for this study (fig. 1). After the 

study reported here was completed, a new complete 

Bouguer gravity anomaly map of Utah was prepared by 

the USGS and the UGMS (Cook and others, 1989). This 

map, which was published by the UGMS at a scale of 

1:500,000, is based on a new data set, which includes 

many gravity stations from a 30-year compilation made 

by K.L. Cook and his students at the University of Utah 

and not previously available to the public. The new data 

set better defines the gravity field throughout Utah and 

provides data for detailed interpretation of gravity 

anomalies in the area of the study reported here. The 

new data do not significantly alter the 5-mGal contours 

presented in figure 1 or the regional interpretation based 

on the data used to prepare that map. 
SUBSURFACE GEOLOGY ALONG THE WASATCH FRONT C3 

The Wasatch Front is coincident with a steep regional 

Bouguer gravity anomaly gradient between the regional 

gravity high over the topographic low of the Lake 

Bonneville basin and the higher terrain of the Middle 

Rocky Mountain province to the east. At its maximum 

east of the Great Salt Lake, this gradient averages over 

1 mGal/km for a distance of 100 km. This 100-mGal 

regional Bouguer gravity relief correlates well with the 

inverse of the regional topography, which involves a 

change in average elevation of about 800 to 1,100 m 

(depending on how the average elevation is determined). 

This correlation of 0. 09 to 0.13 mGaVm is approximately 

equal to the Bouguer correction of 0.11 mGaVm used to 

compute the Bouguer anomaly and indicates that the 

regional topography across the Wasatch Front is in 

approximate isostatic equilibrium. A major part of the 

mass deficiency under the Wasatch Range and the area 

to the east relative to the Basin and Range province to 

the west is likely due to a thicker crust, but part of the 

deficiency may be due to a less dense crust. The steepest 

part of the regional gravity gradient is coincident with 

the western front of the Wasatch Range and indicates 

that the relief across the range front is, at least in part, 

an isostatic response. This gravity feature is in marked 

contrast to that of most ranges in the Basin and Range 

province to the west, where ranges of average size 

appear to be supported by the strength of the crust 

(Eaton and others, 1978). Any explanation of the structure of the western front of the Wasatch Range must 

involve both the extension associated with basin and 

range structure and the isostatic response to the contrasting crust on the two sides of the front. 

Superimposed on the large regional gravity gradient 

along the Wasatch Front are local anomalies typical of 

the Basin and Range province that are produced by the 

relatively low density rocks of Cenozoic age that underlie 

the valley areas and by generally smaller amplitude 

anomalies produced by density variations within preCenozoic rocks. Before a quantitative interpretation of 

the local anomalies can be made, they must be isolated 

from the regional anomaly. 

In the Wasatch Front valleys is a zone of northtrending gravity lows produced by the Cenozoic rocks. 

The range in amplitude and form of these anomalies 

indicates the complexity of the structure underlying the 

valleys. Although the gravity data have been used by 

several investigators to infer the structure of the valleys 

and the thickness of the Cenozoic rocks, important 

uncertainties exist relative to the interpretation of the 

gravity data. The two most important uncertainties are 

(1) the amplitude and form of the regional gravity 

anomaly produced by deeper sources, which must be 

inferred before the anomaly caused by the Cenozoic 

rocks can be analyzed, and (2) the variations in the 

density of the Cenozoic rocks. 

Several methods can be used to isolate the gravity 

effect of the Cenozoic rocks. A regional gravity anomaly 

can be computed by fitting a relatively low order surface 

to the Bouguer gravity anomaly values and removing 

that regional anomaly from the Bouguer gravity anomaly. The application of this technique is compromised 

because the local anomalies of interest are so large in 

both extent and amplitude that they are an important 

factor in controlling the computed surface. The most 

commonly used method has been to assume a regional 

anomaly based on the anomaly values for stations on 

pre-Cenozoic rocks. This technique is often highly subjective, particularly in the area immediately west of the 

Wasatch Range. A regional gravity anomaly that correlates with regional elevation can be computed by assuming either an isostatic model or a correlation function 

between the regional gravity anomaly and the average 

elevation. The computed regional gravity anomaly is 

strongly influenced by the isostatic model or correlation 

function assumed. A technique that works well for 

two-dimensional modeling of profiles is to model the total 

Bouguer gravity anomaly, including the regional component. This technique allows all mass anomalies that 

contribute to the Bouguer gravity profile to be considered in one model, and the uncertainties of all assumptions are apparent to the interpreter. 

In computing the Bouguer gravity anomaly and in 

interpreting the gravity anomalies, the rocks of preCenozoic age in the Wasatch Front area are generally 

assumed to have a density of 2.67 g/cm3

• The density of 

the Cenozoic rocks ranges from less than 2.0 g/cm3 for 

some of the unconsolidated, fine, well-sorted sediments 

to over 2.5 g/cm3 for some of the volcanic rocks and the 

well-consolidated sedimentary rocks. In general, the 

older, more deeply buried Cenozoic sedimentary rocks 

and the volcanic rocks are more dense than the younger 

sedimentary rocks, but another important factor determining density is sedimentary rock type. The coarse, 

poorly sorted, fluvial sediments along the margins of the 

local basins are significantly more dense than the fine, 

well-sorted, lacustrine sediments in the central parts of 

the basins. This systematic decrease in density toward 

the centers of Cenozoic basins in the Basin and Range 

province was described by Kane and Pakiser (1961) and 

Mabey (1960) for California basins and by Mabey and 

Morris (1967) for a Utah basin, but most interpretations 

of gravity anomalies along the Wasatch Front have 

assumed a single density contrast for sediments of the 

same age or depth within a basin. Although this assumption is valid when the gravity data are used only to 

indicate the general configuration and thickness of the 

Cenozoic basins, it can lead to incorrect conclusions in 
C4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 
0 10 20 30 MILES I I I I [ I I I 

0 10 20 30 40 KILOMETERS 

FIGURE 1. -Complete Bouguer gravity anomalies of the Wasatch Front area and the surface traces of the Wasatch and East Cache fault 

zones. Contour interval is 5 mGal. Gravity map is modified from Zoback (1983). 

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C6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

modeling details of basin margins, including location and 

dip of normal faults bounding the basins. 

MAGNETIC DATA 

The magnetic map of Utah (Zietz and others, 1976) was 

prepared by mosaicking magnetic contour maps based on 

several independent surveys. A new magnetic map of 

Utah based on merging digital data sets to a common 

datum and projecting the data to a common level is being 

prepared and will be published by the UGMS at a scale of 

1:500,000. As with the new gravity data set, the new 

magnetic data set will facilitate a more detailed quantitative analysis of the data than is attempted here. The 

magnetic map of the Wasatch Front area presented here 

(fig. 2) is modified only slightly from that of Zietz and 

others (1976). The two surveys that were used to compile 

this part of the State map were both flown at 3, 700 m 

above sea level; thus, no flight-level problem exists 

within the data. The survey in the northern part of the 

area is based on north-south flight lines 8 km apart; the 

survey to the south is based on east-west flight lines 3.2 

km apart. 

The regional residual magnetic field in the Wasatch 

Front area is dominated by magnetic anomalies from two 

principal sources: Tertiary igneous rocks and Precambrian crystalline rocks (Mabey and others, 1964). Two 

west-trending zones of Tertiary igneous rocks are indicated by bands of magnetic highs south of Salt Lake City. 

The northern zone is here called the Bingham-Park City 

magnetic zone, and the southern is the Tintic magnetic 

zone. The major cause of these magnetic highs is Tertiary plutons, although Tertiary volcanic rocks locally 

contribute to the magnetic anomalies. The magnetic 

anomalies over Antelope Island and the Wasatch Range 

north of Salt Lake City are produced by rocks in the 

Archean Farmington Canyon Complex. Magnetic highs 

over these Archean rocks are associated with the quartz 

monzonite granite gneiss and migmatite mapped by 

Bryant (1984). There is little magnetic expression of the 

schist and gneiss as mapped by Bryant. A broad magnetic high in Cache Valley and over the range to the west 

is also probably produced primarily by Precambrian 

rocks, but Cenozoic volcanic rocks in northeast Cache 

Valley appear to be contributing to the anomaly (Scheu, 

1985). 

INTERPRETATIONS 

CENOZOIC SEDIMENTS 

The Wasatch Formation of Eocene and Paleocene age 

is exposed in the central Wasatch Range and areas to the 

east but does not crop out west of the Wasatch fault zone. 

The sedimentary rocks exposed over much of the Salt 

Lake salient were once thought to be Wasatch Formation 

but now appear to be younger (Bryant, 1984). The 

Wasatch Formation is over 1,000 m thick in the Wasatch 

Range and may extend west under the Wasatch Front 

valleys. Sedimentary rocks encountered in several wells 

in valleys west of the Wasatch fault have been identified 

as Wasatch Formation primarily on the basis of color 

(Case, 1985). Some of these rocks may be younger or 

older than the Wasatch Formation. Paleocene and 

Eocene rocks of the North Horn Formation, the Flagstaff Limestone, and the Colton Formation occur in the 

southern Wasatch Range and may occur under adjacent 

valleys. 

Volcanic rocks of late Eocene and Oligocene age are 

widespread in the ranges over the Wasatch Front area 

and are reported in some drill holes in the valleys (Case, 

1985). The Norwood Tuff, which includes tuff, tuffaceous 

sandstone, conglomerate, and clay, is over 1,000 m thick 

in the central Wasatch Range and underlies some of the 

valley areas. Latite, andesite, and rhyolite flows and 

tuffs were erupted in the igneous belts producing the 

Bingham-Park City and Tintic magnetic zones. These 

rocks are exposed in the ranges and are known to extend 

under the valleys in the area of these belts. 

The pattern of basin and range topography in the 

Wasatch Front region appears to have developed in 

Miocene time in response to regional extension (Stewart, 

1978). In the Wasatch Front area, Miocene and Pliocene 

rocks are extensively exposed only around Cache Valley 

and the City Creek spur; however, they are encountered 

in most deeper wells in the valleys (Case, 1985) and 

appear to be the major low-density rock causing the 

gravity lows in the valleys. 

The Miocene and Pliocene history of the Wasatch 

Front valleys is poorly known, but it appears that local 

basins of deposition began to subside in Miocene time and 

have continued subsiding to the present. As the basins 

evolved, the area, location of maximum subsidence, style 

of deformation, climate, and source of sediments likely 

changed, producing a complex distribution of sedimentary rock types in the basins. Igneous flows and sills 

occur interbedded with Miocene and Pliocene sedimentary rocks in northern Cache Valley (Scheu, 1985) and in 

the area of the Great Salt Lake. 

The subsurface contact between the Tertiary and 

Quaternary sediments under the valleys often cannot be 

identified with confidence. In northern Salt Lake 

County, Arnow and Mattick (1968) correlated a downward increase in resistivity and an increase in seismic 

velocity with the Tertiary-Quaternary contact. P-wave 

velocities above this contact were less than 1.8 km/s. 

They attributed P-wave velocities as high as 3.8 km/s to 

Tertiary rocks and velocities greater than 4. 6 km/s to 
SUBSURFACE GEOLOGY ALONG THE WASATCH FRONT C7 

consolidated rocks of several ages. They interpreted a 

seismic refraction profile across the valley north of Salt 

Lake City and data from three drill holes as indicating 

that the thickness of the Quaternary sediments ranges 

from 180 to 770 m and the thickness of Tertiary rock from 

0 to 1,040 m. The maximum thickness of the combined 

Cenozoic units along the profiles is 1,470 m. Arnow and 

Mattick reported that one well penetrated volcanic 

rocks, possibly andesites, between 700 and 850 m below 

the surface. Arnow and others (1970) prepared a contour 

map (at 500-ft contour intervals) of the base of the 

Quaternary deposits in Salt Lake Valley south of this 

profile. This map was based on drillers' logs, and the top 

of the Tertiary deposits was selected at the first reported 

beds of lava, conglomerate, hardpan, or various types of 

cemented or hard materials that were more than 1 m 

thick. Now here did they find Quaternary deposits 

thicker than those along the seismic profile; over most 

the valley, Quaternary sediments were less than 300 m 

thick. 

Hunt and others (1953) interpreted well logs in northern Utah Valley as indicating that the Quaternary sediments thickened southward to the Geneva Steel plant, 

where they were about 150 m thick. Dustin and Merritt 

(1980), also using well logs, concluded that the Quaternary deposits were approximately 50 to 60 m thick at the 

northern end of Utah Valley and 135 to 150 m thick at the 

southern end. Feth and others (1966), in a study of the 

Weber and Davis County area, reported that a well near 

Farmington appears to have penetrated the base of the 

Quaternary at 248 m but that Quaternary sediments are 

less than 60 m thick at most places in their study area. 

Williams (1962) reported that most wells in southern 

Cache Valley do not penetrate the base of the Quaternary sediments but that one well did at about 150 m. In 

northern Cache Valley, Stanley (1971) found an increase 

in resistivity at depths ranging from 90 to 150 m. He 

concluded that this horizon was probably the top of 

pre-Lake Bonneville sediments of Pleistocene age; however, this horizon could also be interpreted as the base of 

the Quaternary sediments. Bjorklund and McGreevy 

(1971) examined the logs of five deep wells in Cache 

Valley; although they concluded that it was difficult to 

distinguish the Quaternary-Tertiary contact, they placed 

the contact at the top of a thick conglomerate at depths 

of about 335 to 400 m. 

Only in one local area of the Wasatch Front valleysnorthern Salt Lake County- are Quaternary sediments 

in excess of 400 m thick indicated; as will be discussed 

later, this area is characterized by anomalous structure. 

In most of the valley areas, the Quaternary sediments 

are reported to be less than 200 m thick. Where seismic 

velocity and resistivity data are available, the base of the 

assumed Quaternary deposits appears to correlate 

with a significant increase in seismic velocity and resistivity. The inferred subsurface boundary between Tertiary and Quaternary sediments in the Wasatch Front 

valleys has not been dated in most areas. The horizon at 

which the degree of consolidation of the sediments 

increases may not everywhere correspond to a time 

boundary. It is, however, a boundary significant to 

predicting ground response in an earthquake. Either 

seismic refraction or resistivity soundings have proven to 

be practical geophysical techniques for mapping this 

boundary. 

Paleogene and Quaternary rocks do contribute to the 

large gravity lows under the Wasatch Front valleys, but, 

in areas where borehole data are available, the largest 

contribution is from the Miocene and Pliocene sedimentary rocks. Locally, these rocks are very thick. Nearly 

4,000 m of these rocks were penetrated in the Gulf 

Energy and Minerals well in southern Utah Valley, and 

the base still was not reached (Davis and Cook, 1983). 

If the assumption is made that all of the residual 

Bouguer gravity lows over the Wasatch Front valleys 

are produced by the density contrast between sediments 

of Miocene age and younger and the more dense, older 

rocks, then the gravity anomaly can be analyzed to 

determine the thickness of the low-density rocks 

required to produce the gravity low. A computer program was used to determine the thickness of low-density 

sediments that would produce the Bouguer gravity low 

along 26 profiles across the Wasatch Front valleys. The 

computation assumes that the prism of sediments is 

normal to the profile and extends to considerable distances on each side of the profile. Because of the elongation of the gravity lows and the locations and orientations 

of the profiles, the assumption does not induce large 

errors. A density contrast of -0.4 g/cm3 was assumed. 

The resulting inversion of the gravity data is shown as a 

contour map of sediment thickness in figure 3. Zoback 

(1983), Peterson (1974), Peterson and Oriel (1970), Davis 

and Cook (1983), and others have interpreted gravity 

profiles to infer the thickness of low-density sediments 

along profiles in the same area. Their procedures involved assuming a density model and modifying it until a 

model was produced that satisfied their chosen geologic 

constraints and that also produced an acceptable match 

to the gravity anomaly. The technique used here differs; 

its only geologic constraint is that the surface extent of 

the low-density sediments must correspond to the known 

geology. It is thus an inversion of the gravity data. The 

thickness of sediments indicated in figure 3 is generally 

greater than that inferred by Zoback (1983), Peterson 

(1974), and Peterson and Oriel (1970); they assumed a 

density contrast of -0.5 g/cm3

, and here a contrast of 

-0.4 g/cm3 was assumed. The thicknesses are in general 

agreement with those of Davis and Cook (1983), who 
C8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 
N 

~ 

u 

a.. 

I 

[0 

N 

~ 

1-

0 10 20 30 MILES I I I I I I I I 

0 10 20 30 40 KI LOMETERS 

FIGURE 2.-Residual total magnetic intensities of the Wasatch Front area and the surface traces of the Wasatch and East Cache fault zones. 

B-PCMZ is the Bingham-Park City magnetic zone, and TMZ is the Tintic magnetic zone. Contour interval is 20 -y. Magnetic map is modified 

from Zietz and others (1976). 

Ul 

c:::: 

1::0 

Ul 

c:::: 

~ 

"':1 

> 0 

t:rj 

0 

t:rj 

0 

t'" 

0 

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> t'" 

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z 

0 

>-3 

::I1 

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w 

~ 

~ 0 

::I1 

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>-3 

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CIO REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

.,_ 

D 

i . 

I 

'· 

: ·~ 

/ 

/ 

\ 

/ 

/ 

/ 

' 

[• 

\ 

\ 

\ •. 

0 10 20 30 MILES 

-~L_I .------,-L- 1 ---,--1 I I I I 

0 10 20 30 40 KILOMETERS 

\ 

'· 

I· 

.· >. 

'( '.: 

( R 

s 

i I 

I 

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1110 

FIGURE 3.-Thickness of low-density sedimentary rocks in Wasatch Front valleys and the Cache Valley, inferred from gravity 

data. Contour interval is 1 km. 
SUBSURFACE GEOLOGY ALONG THE WASATCH FRONT C11 

assumed a contrast decreasing with depth from -0.62 to 

-0.22 g/cm3

• The density of the Cenozoic sediments 

normally increases with depth; at depths greater than 2 

km, the density contrast with the older rocks is likely to 

be substantially less than the -0.4 g/cm3 assumed in the 

inversion of the gravity data. The result is that, in the 

area of thickest sediments, the calculated thickness may 

be substantially less than the actual thickness. This 

situation is known to occur at the southern end of Utah 

Valley, where a drill hole is reported to have bottomed at 

4,000 min Cenozoic sediments at a location where the 

inversion of the gravity data indicates that the sediments 

are less than 4,000 m thick. 

Throughout the Wasatch Front valleys, the axis of the 

gravity low (and, presumably, the axis of the structural 

low) is east of the center of the valley. The topographic 

low is generally west of the axis of the valley. Thus, 

movement along the Wasatch fault zone on the eastern 

side of the valley tilts the valley floor eastward, but 

debris derived from the Wasatch Range and the highlands to the east is then deposited along the eastern side 

of the valley and displaces the topographic low to the 

west. This scenario is well illustrated in Utah Valley, 

where the axis of the gravity low lies along the eastern 

shore of Utah Lake, and on the eastern shore of the 

Great Salt Lake, where the axis of the gravity low is 

generally east of the lake. The gravity data support the 

conclusion that the gross structure of the valleys is 

eastward-tilting blocks. 

DISPLACEMENT ALONG THE WASATCH FAULT ZONE 

The crest of the Wasatch Range is generally 1. 5 to 2 

km above the valley floor to the west. This range of 

values is often accepted as the minimum vertical displacement of the Wasatch fault. The maximum vertical 

displacement can be estimated by adding to the range 

height the maximum depth to the base of the Miocene 

sediments under the valley (about 4 km) and the estimated thickness of rock removed from the range crest in 

Miocene and post-Miocene time, which is difficult to 

estimate but might be as much as 2 km. This process 

yields a maximum vertical displacement across the 

Wasatch fault zone of about 8 km. These calculations of 

maximum and minimum fault displacement both assume 

that warping has not made a major contribution to the 

structural relief across the Wasatch Front. 

Although local evidence of lateral displacement along 

the Wasatch fault zone has been identified, there is no 

evidence that lateral displacement is an important component of total displacement, and the character of the 

surface trace of the fault is inconsistent with major 

lateral movement. Displacement of magnetic features 

that extend across the fault can be used to estimate 

lateral movement. The Bingham-Park City magnetic 

zone is nearly normal to the Wasatch fault. If the 

Oligocene intrusive rocks that are the major cause of the 

magnetic anomaly were once a continuous linear-sided 

mass, any post-Oligocene offset of that mass should be 

apparent in the magnetic data. No major offset is apparent. The approximately 5-km left-lateral offset of the 

crest of the magnetic high across the fault can be 

explained by the present topography of the top of the 

intrusive mass. The Tintic magnetic zone is best defined 

from the Tintic Mountains west, and it is questionable 

whether the large volumes of Oligocene intrusive rocks 

that produce the higher amplitude magnetic highs in the 

zone occur east of the Wasatch fault. Local magnetic 

highs in the Wasatch Range east of Nephi appear to be 

produced by outcropping Tertiary volcanic rocks. Therefore, this magnetic zone is of limited use in detecting 

lateral offset. 

The magnetic anomalies north of Salt Lake City are 

not continuous but appear to define two zones, one 

trending north and one trending north-northwest. Three 

high magnetic closures form a north-northwest-trending 

zone about 50 km long over Antelope Island and the 

adjacent valley area to the east. A magnetic low over the 

structurally low part of the valley separates these anomalies from a magnetic high zone of similar length over the 

Wasatch Range. The left-lateral offset of these two zones 

is about 40 km. This evidence of lateral offset of the 

Precambrian rocks is weak. Even if it is real, the offset 

could have occurred long before the development of the 

Wasatch fault zone. 

DIP OF THE WASATCH FAULT 

Gilbert (1928) measured the dip of the Wasatch fault at 

several locations and, excluding the dips on spurs and 

salients, determined an average dip of 33.4°. Zoback 

(1983) questioned whether some of the dips that Gilbert 

measured were of the fault surface, and she reported 

four new measurements for the dip of the fault in 

bedrock, at locations not on spurs; her values of dip 

ranged from 44° to 68°. Gilbert's measurements and 

others in the vicinity of the spurs and the Transverse 

Range indicate that individual faults of the Wasatch fault 

zone in near-surface bedrock dip about 70° to the west. 

Away from the spurs, the near-surface dips vary considerably but are generally less than 70° and in some areas 

are less than 30°. 

A Bouguer gravity anomaly gradient parallels the 

front of the Wasatch Range and the Wasatch fault zone 

for most of their lengths. Locally, the steepest gradient 

is coincident with the active surface trace of the fault, 

but, along portions of the fault, the gravity data suggest 

that pre-Tertiary bedrock occurs at relatively shallow 
C12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

depths on the downthrown side of the mapped traces of 

the fault. In the Bountiful area, for example, the mapped 

fault traces are in the area of a gravity high that suggests 

the presence of bedrock at relatively shallow depths on 

both sides of the fault. A large gravity gradient that is 

suggestive of faulting along the northern extension of the 

Warm Springs fault (Cook and Berg, 1961, pl. 13) lies 

about 3 km west of the mapped traces. In some places 

where a gravity gradient occurs in the area of the fault 

zone, the gradient does not indicate a high-angle fault 

interface between contrasting density units but can be 

modeled with an interface sloping about 20° to the west. 

An example is the East Bench fault in the eastern part of 

Salt Lake City. These relatively low gravity gradients 

can be interpreted as reflecting a series of step faults. 

The decrease in density of basin fill toward the center of 

the valley will also tend to extend the gravity gradient 

associated with the edge of a prism of valley fill over a 

greater distance than it would if fill were one density 

unit. If a single density contrast is assumed in interpreting such an anomaly, the inferred dip of the density 

interface will be lower than the actual dip. In some 

locations, such as at the western end of spurs and in parts 

of the range front, steep gravity gradients approximately coincident with the surface fault trace indicate 

considerable concealed displacement along the exposed 

traces of the Wasatch fault zone. Because the gravity 

lows in the valleys west of the fault zone are in large part 

reflecting the distribution of Miocene and Pliocene sediments, the lack of correlation between some of the 

structures indicated by the gravity data and the surface 

traces of the Wasatch fault may reflect changes of basin 

structure with time. The relatively low dip of most of the 

faults in the Wasatch fault zone and the lateral gradation 

of density in the basin fill are serious handicaps in using 

the gravity data to infer the geometry of the Wasatch 

fault zone. 

Using seismic reflection data, Smith and Bruhn (1984) 

estimated the dip of the Wasatch fault at five locations. 

At depths from 0.8 to 2.0 km, they estimated apparent 

dips ranging from 6° to 54° to the west, the average 

estimate being 30°. At two locations, the apparent dips at 

depths greater than 2.0 km were estimated to be 4° and 

10°. 

The geophysical evidence available on the dip of the 

Wasatch fault zone indicates that the zone and major 

faults within the zone dip to the west. In the vicinity of 

the spurs and the Transverse Range, individual faults 

dip steeply at least near the surface. Elsewhere, the dips 

are considered to be less and commonly appear to 

decrease with increasing depth. Accurately determining 

the dip of the Wasatch fault zone or that of individual 

faults in the zone at depths of more than a few tens of 

meters is difficult. The most useful data have been 

obtained by seismic reflection profiling. Seismic reflection surveys designed to determine fault geometry and 

supplemented by other geophysical data and perhaps 

drilling appear to offer the most promise of providing 

detailed information on subsurface geology of the 

Wasatch fault zone. When the fault ruptures, measurements of deformation before, during, and after the 

events, focal mechanism analysis, and the distribution in 

space of related earthquakes will likely provide information on fault geometry that cannot be obtained before the 

event. 

Linear zones of steep gravity gradients occur on the 

western sides of most of the major gravity lows in the 

Wasatch Front valleys. These gradients are generally 

interpreted as indicating normal faults or fault zones that 

are the western sides of grabens within the valleys. 

These fault zones are usually not along mountain fronts 

but from 4 to 20 km east of the nearest mountains to the 

west. These gradients cannot usually be correlated with 

mapped surface faults, but thermal springs coincident 

with these gradients suggest that they may reflect active 

fault zones. These gradients are in areas inundated by 

Lake Bonneville within the last 15,000 years, and evidence of older faulting is difficult to identify in the 

unconsolidated sediments involved. The areas of lowest 

Bouguer gravity anomaly values in the Wasatch Front 

valleys can be interpreted as indicating grabens, but the 

individual grabens are generally significantly narrower 

than the valley. Only in a few locations, such as at the 

ends of some of the spurs of the Wasatch Range, does the 

inferred location of the faults bounding the grabens 

correspond to the mapped location of the surface trace of 

active faults. 

In the Cache Valley area, the area of thick, lowdensity rocks extends beyond the valley over outcropping Tertiary sedimentary rocks. The gravity data suggest that all of southern Cache Valley is underlain by 

thick, low-density sediments, but, in the north, the area 

of thick sediments is confined to the western side of the 

valley. An appended area of thick sediments extends 

from Cache Valley into Clarkston Valley. 

SPURS OF THE WASATCH RANGE 

Gilbert (1928) studied the Wasatch fault and named the 

major spurs of the Wasatch Range; his names are used 

here. The gravity data provide some indication of the 

subsurface geology that was not accessible to Gilbert, 

and geologic mapping since Gilbert's work has better 

defined the surface geology. Gilbert concluded that the 

spurs were separated from the main mass of the Wasatch 

Range and from the valleys by faults. He further concluded that the faults along the valley side of the spurs 

were likely to dip more steeply than those along the 
SUBSURFACE GEOLOGY ALONG THE WASATCH FRONT C13 

range front. He noted that hot springs occur along each 

of the major spurs and that only at two other locations 

along the Wasatch fault did hot springs occur. He 

(Gilbert, 1928, p. 32) concluded, "It may be that the outer 

faults of the spurs are peculiarly favorable for the 

conveyance of deep penetrating circulation because of 

the less perfect adjustment there of the fault walls." 

Geophysical data and recent geologic mapping generally 

support Gilbert's conclusion, except as they relate to his 

Traverse spur. 

Steep local gravity gradients have been measured 

along the Wasatch fault zone at the western edges of the 

Madsen, Pleasant View, and City Creek spurs. This 

occurrence supports Gilbert's conclusion that the faults 

here dip more steeply than is normal for the Wasatch 

fault zone. The faults on the western side of each of these 

spurs trend a few degrees west of north. Only at the City 

Creek spur are there data on the thickness of Quaternary 

sediments under the valley west of the spur, but the 

770-m thickness inferred here by Arnow and Mattick 

(1968) is the thickest reported anywhere in the Wasatch 

Front valleys. The Tertiary sedimentary rocks in the 

City Creek spur that had earlier been considered to be of 

Eocene age are now mapped as Miocene and Pliocene age 

(Bryant, 1984). If these sediments were deposited in the 

valley, their presence on the spur supports Gilbert's 

conclusion that the spur blocks were once so associated 

with the valley block as to receive the same deposit and 

have since been lifted with reference to the valley block 

(Gilbert, 1928, p. 31). In addition, Gilbert (1928, p. 32) 

noted that the diversities in the history of each spur 

"show that the conditions determining adhesion to the 

[Wasatch] range or to the sinking valley were local and 

variable." There is no evidence in the gravity data of a 

fault displaying a major vertical displacement on the 

western side of the Traverse spur as defined by Gilbert. 

A concealed fault trending a few degrees north of east, at 

the western end, has been postulated along the hot 

springs in the area (Gilbert, 1928, p. 29; Cook and Berg, 

1961, p. 81). The Tertiary rocks exposed on the spur are 

Oligocene volcanic rocks and do not provide evidence 

that the spur is an uplifted part of the valley. The 

Traverse spur as defined by Gilbert appears to be a part 

of the Traverse Range and not a spur of the Wasatch 

Range in the sense that the Madsen, Pleasant View, and 

City Creek spurs are. 

FAULT SEGMENTS 

The Wasatch fault zone consists of several segments, 

each behaving with some degree of independence from 

adjacent segments. Schwartz and Coppersmith (1984) 

defined six major rupture segments and proposed that a 

rupture event will likely be confined to one segment. The 

gravity and magnetic data were analyzed independently 

to identify features that might relate to segment boundaries. These maps were then compared with the segments defined by Schwartz and Coppersmith (fig. 4). 

Because Schwartz and Coppersmith used Zoback's (1983) 

gravity data along with other data to define their segments, their segments are not completely independent of 

the geophysical data. 

Five blocks of contrasting magnetic properties were 

identified (fig. 4A). Block I is the eastern end of the 

Tintic magnetic zone. To the west, the zone is dominated 

by magnetic highs produced by Tertiary intrusive rocks, 

but, in the Wasatch Range, the amplitude of the anomalies is lower, and most individual magnetic highs relate 

to Tertiary volcanic rocks. Block III is the Bingham-Park 

City magnetic zone. Tertiary intrusive rocks appear to 

make up much of the upper crust. In both of these blocks, 

but particularly in Block III, Tertiary igneous activity 

likely modified the crust significantly, so that its 

response to regional strain is now different from that of 

the adjacent blocks. Blocks II and IV are characterized 

by generally low magnetic relief. In these blocks, the 

Phanerozoic sedimentary rocks appear to be very thick, 

and there is no evidence of an underlying, strongly 

magnetized magnetic basement. Block V is characterized 

by magnetic highs produced by Precambrian crystalline 

rocks of the Farmington Canyon Complex. There is no 

indication in the magnetic data that these rocks are 

allochthonous. Block VI is a region of relatively low 

magnetic relief, except for two areas of high magnetic 

intensity in the Cache Valley area. The maximum intensity of the northern magnetic high in Cache Valley occurs 

a few kilometers north of the Utah-Idaho border in an 

area where intrusive and extrusive basalts crop out 

(Scheu, 1985). However, there is no evidence that these 

rocks underlie the central and southern parts of Cache 

Valley, and it seems unlikely that the entire area of high 

magnetic intensity is produced by Tertiary igneous 

rocks. This zone of high magnetic intensity continues 

north to the Pocatello area, where Precambrian rocks 

appear to be the primary cause of the anomaly. The 

magnetic data suggest that a block of anomalous magnetic basement rocks underlies much of Cache Valley. 

The gravity lows along the western side of the 

Wasatch fault are segmented, and these segments reflect 

the total accumulation of sediment since the start of the 

subsidence of the structural lows, presumably in Miocene 

time (fig. 4B). The southern segment (A) is defined by a 

low-amplitude gravity low between Nephi and Santaquin 

in Juab Valley, which is adjacent to the highest part 

of the Wasatch Range and is the valley area of thinnest sediment west of the range. 1Segment B is Utah 

Valley, which contains the largest gravity low of any of 

the Wasatch Front valleys and probably the thickest 
C14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

0 

I 

0 

I 

10 

10 

I I 

20 

20 

I I 

30 

I 

40' 

30 

I I 

50 

I 

40 MILES 

I 

60 KILOMETERS 

FIGURE 4. -A, Blocks of contrasting lithology (I-V) inferred from magnetic anomalies; B, boundaries of areas of contrasting gravity anomalies 

(A-H); C, fault-rupture segment boundaries from Schwartz and Coppersmith (1984) (solid and dashed bars) and additional boundaries 

inferred from geophysical data (dotted lines) for the Wasatch Front. 
SUBSURFACE GEOLOGY ALONG THE WASATCH FRONT C15 

ments. This segment is bounded on the north by a high 

gravity trend approximately coincident with the 

Traverse Range. Segment C is Salt Lake Valley. The 

southern part of Salt Lake Valley (designated C-2) 

contains an approximately equidimensional gravity low 

that is different in shape from any other lows immediately west of the Wasatch fault zone. This low may be in 

part produced by rocks of Oligocene age or older. In 

northern Salt Lake Valley (C-1), there is a lowamplitude gravity low east of the axis of the valley. The 

two gravity segments of Salt Lake Valley are separated 

by a major gravity gradient that extends across the 

valley and into the ranges on each side of the valley. The 

northern edge of segment C-1 is defined by a highgravity trend coincident with the City Creek spur. 

Segment D is a pronounced gravity low in the eastern 

part of the valley area extending from Bountiful to a 

gravity high over the Pleasant View spur north of 

Ogden. Segment E is flanked by a relatively narrow 

gravity low along the front of the range between the 

Pleasant View spur and the largely concealed spur near 

Honeyville (Madsen spur). The northern segment (F) is a 

moderately complex low on the eastern side of the valley. 

Two segments of the East Cache fault zone on the 

eastern side of Cache Valley are suggested by the 

gravity data. In Cache Valley south of Smithfield, segment G is along a pronounced gravity low approximately 

coincident with the valley. North of Smithfield in segment H, the gravity low is confined to the western side 

of the valley. 

Combining the segments inferred from the gravity and 

magnetic data suggests six segments of the Wasatch 

fault zone between Nephi and the Utah-Idaho border. 

The three southern segments agree well with the segments defined by Schwartz and Coppersmith (1984) (fig. 

4C). The geophysical data suggest that the Ogden segment as defined by Schwartz and Coppersmith may be 

terminated on the north by the Pleasant View spur. A 

division of the fault zone north of the Pleasant View spur 

into two segments is suggested by relatively subtle 

features in the gravity and magnetic anomalies. Rather 

than segment boundaries near Brigham City (dashed 

bar, fig. 4C) as Schwartz and Coppersmith concluded, 

interpretations of the geophysical data suggest a segment boundary at the largely concealed Madsen spur 

near Honeyville. The boundaries proposed here reduce 

the length of the Ogden segment from over 80 km to less 

than 60 km and leave the Provo segment, at about 60 km, 

as the longest segment of the Wasatch fault zone. 

CONCLUSIONS 

Geophysical data combined with borehole data and 

surface geologic mapping can make substantial contributions to understanding the Wasatch fault zone. Gravity 

data in the Wasatch Front valleys indicate that the 

valleys are underlain by low-density sediments that are 

commonly more than 1, 000 m thick and locally more than 

3, 000 m thick. Data from drill holes and from other 

geophysical surveys indicate that most of these rocks are 

Miocene or Pliocene in age. The thickest accumulations of 

these rocks are in grabens that are usually centered east 

of the centers of the valleys. The geophysical data are 

generally consistent with the Wasatch fault's dipping 

rather gently to the west, except at the ends of spurs, 

where the faults dip steeply to the west. Faults on the 

western sides of the grabens within the valleys generally 

dip steeply to the east. The gravity and magnetic data 

can be used to define crustal blocks of contrasting 

lithology along the Wasatch fault, and the boundaries of 

these blocks may correspond to segment boundaries of 

the fault. The Wasatch Range and adjacent highlands to 

the east are in approximate isostatic balance with the 

lower areas to the west. This contrast with normal 

ranges in the Basin and Range province suggests that 

isostatic forces are partly responsible for the uplift of the 

Wasatch Range. 

REFERENCES CITED 

Arnow, T., and Mattick, R.E., 1968, Thickness of valley fill in the 

Jordan Valley east of the Great Salt Lake, Utah, in Geological 

Survey research 1968: U.S. Geological Survey Professional Paper 

600-B, p. B79-B82. 

Arnow, T., Van Horn, R., and LaPray, R., 1970, The pre-Quaternary 

surface in the Jordan Valley, Utah, in Geological Survey research 

1970: U.S. Geological Survey Professional Paper 700-D, p. 

D257-D261. 

Bjorklund, L.J., and McGreevy, L.J., 1971, Ground-water resources of 

Cache Valley, Utah and Idaho: Utah Department of Natural 

Resources Technical Publication 36, 72 p. 

Bryant, B., 1984, Reconnaissance geologic map of the Precambrian 

Farmington Canyon complex and surrounding rocks in the 

Wasatch Mountains between Ogden and Bountiful, Utah: U.S. 

Geological Survey Miscellaneous Investigations Map 1-1447, scale 

1:50,000. 

Case, W.F., 1985, Significant drill holes of the Wasatch Front Valleys 

including Cache Valley and Tooele Valley: Utah Geological and 

Mineral Survey Open-File Report 82, 78 p. 

Cook, K.L., and Berg, J.W., Jr., 1961, Regional gravity survey along 

the central and southern Wasatch Front, Utah: U.S. Geological 

Survey Professional Paper 316-E, p. E75-E89. 

Cook, K.L., Berg, J.W., Jr., and Lum, D., 1967, Seismic and gravity 

profile across the northern Wasatch trench, Utah, in Musgrave, 

A.W., ed., Seismic refraction prospecting: Tulsa, Okla., Society of 

Exploration Geophysicists, p. 539-549. 

Cook, K.L., Bankey, V., Mabey, D.R., and DePangher, M., 1989, 

Complete Bouguer gravity anomaly map of Utah: Utah Geological 

and Mineral Survey Map 122, scale 1:500,000. 

Crone, A., and Harding, S., 1984, Near-surface faulting associated 

with Holocene fault scarps, Wasatch fault zone, Utah, in Hays, 

W.W., and Gori, P.L., eds., Proceedings of Conference XXVI; a 
C16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

workshop on Evaluation of regional and urban earthquake hazards 

and risk in Utah: U.S. Geological Survey Open-File Report 

84-763, p. 242-268. 

Davis, D.A., and Cook, K.L., 1983, Evaluation of low-temperature 

geothermal potential in Utah and Goshen Valleys and adjacent 

areas, Utah, pt. I, Gravity survey: Utah Geological and Mineral 

Survey Report of Investigation RI-179, 139 p. 

Davis, F.D., 1983a, Geologic map of the southern Wasatch Front, 

Utah: Utah Geological and Mineral Survey Map 55-A, scale 

1:100,000. 

---1983b, Geologic map of the central Wasatch Front, Utah: Utah 

Geological and Mineral Survey Map 54-A, scale 1:100,000. 

---1985, Geologic map of the northern Wasatch Front, Utah: Utah 

Geological and Mineral Survey Map 53-A, scale 1:100,000. 

Dustin, J.D., and Merritt, L.B., 1980, Hydrogeology of Utah Lake 

with emphasis on Goshen Bay: U tab Geological and Mineral 

Survey Bulletin 23, 50 p. 

Eaton, G.P., Wahl, R.R., Prostka, H.J., Mabey, D.R. and Kleinkopf, 

M.D., 1978, Regional gravity and tectonic patterns: Their relation 

to the late Cenozoic epeirogeny and lateral spreading in the 

western Cordillera, in Smith, R.B., and Eaton, J.P. eds., Cenozoic 

tectonics and regional geophysics of the western Cordillera: Geological Society of America Memoir 152, p. 51-91. 

Feth, J.H., Barker, D.A., Brown, R.J., Moore, L.G., and Viers, C.E., 

1966, Lake Bonneville: Geology and hydrology of the Weber delta 

district, including Ogden, Utah: U.S. Geological Survey Professional Paper 518, 76 p. 

Gilbert, G.K., 1928, Studies of basin-range structure: U.S. Geological 

Survey Professional Paper 153, 89 p. 

Glenn, W.E., Chapman, D.S., Foley, D., Capuano, R.M., Cole, D.R., 

Sibbett, B., and Ward, S.H., 1980, Geothermal exploration program, Hill Air Force Base, Davis and Weber Counties, Utah: 

Earth Science Laboratory, University of Utah Research Institute 

Report 34, 77 p. 

Hintze, L.F., 1980, Geologic map of Utah: Salt Lake City, Utah 

Geological and Mineral Survey, scale 1:500,000. 

Hunt, C.B., Varnes, H.D., and Thomas, H.E., 1953, Lake Bonneville: 

Geology of northern Utah Valley, Utah: U.S. Geological Survey 

Professional Paper 257-A, 99 p. 

Kane, M.F., and Pakiser, L.C., 1961, Geophysical study of subsurface 

structure in southern Owens Valley, California: Geophysics, v. 26, 

p. 12-26. 

Mabey, D.R., 1960, Gravity survey of the western Mojave Desert, 

California: U.S. Geological Survey Professional Paper 316-D, p. 

D51-D73. 

Mabey, D.R., and Morris, H.T., 1967, Geologic interpretation of 

gravity and aeromagnetic maps of Tintic Valley and adjacent 

areas, Tooele and Juab Counties, Utah: U.S. Geological Survey 

Professional Paper 516-D, p. D1-D10. 

Mabey, D.R., Crittenden, M.D., Jr., Morris, H.T., Roberts, R.J., and 

Tooker, E.W., 1964, Aeromagnetic and generalized geologic map 

of part of north-central U tab: U.S. Geological Survey Geophysical 

Investigations Map GP-422, scale 1:250,000. 

Mattick, R.E., 1970, Thickness of unconsolidated to semiconsolidated 

sediments in Jordan Valley, Utah, in Geological Survey research 

1970: U.S. Geological Survey Professional Paper 700-C, p. 

Cl19-C124. 

Peterson, D.L., 1974, Bouguer gravity map of part of northern Lake 

Bonneville basin, Utah and Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-627, scale 1:250,000. 

Peterson, D.L., and Oriel, S.S., 1970, Gravity anomalies in Cache 

Valley, Cache and Box Elder Counties, Utah, and Bannock and 

Franklin Counties, Idaho, in Geological Survey research 1970: 

U.S. Geological Survey Professional Paper 700-C, p. C114-C118. 

Scheu, S.R., 1985, A gravity and magnetic investigation east of 

Preston, Idaho: Pocatello, Idaho State University unpublished 

M.Sc. thesis, 77 p. 

Schwartz, D.P. and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquakes: Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional tectonics of 

the eastern Basin-Range: Inferences on structural style from 

seismic reflection data, regional tectonics, and thermal-mechanical 

models of brittle-ductile deformation: Journal of Geophysical 

Research, v. 89, no. B7, p. 5733-5762. 

Stanley, W.D., 1971, An integrated geophysical study related to 

ground-water conditions in Cache Valley, Utah and Idaho: Salt 

Lake City, University of Utah, unpublished Ph.D. thesis, 142 p. 

Stewart, J.H., 1978, Basin-range structure in western North America: 

A review, in Smith, R.B., and Eaton, J.P. eds., Cenozoic tectonics 

and regional geophysics of the western Cordillera: Geological 

Society of America Memoir 152, p. 1-32. 

Stokes, W.L., 1977, Subdivisions of the major physiographic provinces 

in Utah: Utah Geology, v. 4, no. 1, p. 1-17. 

Williams, J.S., 1962, Lake Bonneville: Geology of southern Cache 

Valley, Utah: U.S. Geological Survey Professional Paper 257-C, p. 

C131-C152. 

Zoback, M.L., 1983, Structure and Cenozoic tectonism along the 

Wasatch fault zone, Utah: Geological Society of America Memoir 

157, p. 3-27. 

Zohdy, A.A.R., and Jackson, D.B., 1969, Electrical resistivity profile 

east of the Jordan Narrows, Utah, in U.S. Geological Survey 

Professional Paper 650-C, p. C83-C88. 

Zietz, I., Shuey, R., and Kirby, J.R., Jr., 1976, Aeromagnetic map of 

Utah: U.S. Geological Survey Geophysical Investigations Map 

GP-907, scale 1:1,000,000. 
Observational Seismology and the Evaluation of 

Earthquake Hazards and Risk in the 

Wasatch Front Area, Utah 

By W.J. ARABASZ, ].C. PECHMANN, and E.D. BROWN, 

DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF UTAH 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-D 

CONTENTS 

Page Page 

Abstract ......................................................................... . D1 Seismotectonic Framework-Continued 

Introduction .................................................................... . 1 Earthquake Focal Depths . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . ... D13 

Acknowledgments ...................................................... . 3 Problematic Correlation of Seismicity with Geologic 

Earthquake Data Base ..................................................... . 3 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

Seismotectonic Framework ................................................ . 5 Seismic Source Zones and Seismicity Parameters................... 20 

General Setting and Characteristics .............................. . 5 General Remarks........................................................ 20 

Late Quaternary Faulting ........................................... . 8 Recurrence Modeling................................................... 20 

The 1959 Hebgen Lake and 1983 Borah Peak Fault-Specific Sources . . . .. . . . . . . . . . . . . . .. . . . . .. . . . . . . .. . . .. . . . . . . . .. . . 24 

Earthquakes ....................................................... . 10 Ground-Shaking Hazard..................................................... 26 

Threshold of Surface Faulting ...................................... . 10 Current Seismicity and the Wasatch Fault............................ 28 

Seismicity ................................................................. . 11 References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . 33 

ILLUSTRATIONS 

Page 

FIGURE 1. Flow chart outlining steps in a formalized earthquake hazard analysis and interrelated aspects of observational 

seismology . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . .. . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . .. . . . . . . . .. .. . . . . . D2 

2, 3. Maps showing: 

2. Distribution of seismograph stations in the Utah region at four different times between 1955 and 1985............ 4 

3. Setting of the Wasatch Front study area with respect to the Intermountain Seismic Belt and epicenters 

of historical earthquakes of magnitude 6.0 and greater...................................................................... 6 

4. Schematic block diagram illustrating selected aspects of the seismotectonic framework of the Wasatch Front area ........ 7 

5. Index map of the Wasatch Front study area showing traces of late Quaternary faulting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

6-9. Epicenter maps of: 

6. Utah region, showing all independent main shocks of ML 4.0 or greater, 1850-1986, and Quaternary faults...... 12 

7. All earthquakes located by the University of Utah Seismograph Stations in the Utah region, July 1, 1962, 

to December 31, 1986............................................................................................... ... . . . . . . . . . .. . . . . . . 14 

8. All earthquakes located by the University of Utah Seismograph Stations in the Wasatch Front area, 

October 1, 1974, to June 30, 1978 ................................................................................................... 16 

9. All earthquakes located by the University of Utah Seismograph Stations in the Wasatch Front area, 

July 1, 1978, to December 31, 1986................................................................................................. 17 

10. Epicenter map and corresponding vertical sections of earthquakes in the Wasatch Front area located with reliable 

focal depths, October 1974 through December 1986........................................................................................ 18 

11. Schematic geologic cross section illustrating the complex association of seismicity in the upper crust with geological 

structure in the Intermountain Seismic Belt................................................................................................. 19 

12. Map showing epicenters of independent main shocks in the Wasatch Front area from July 1, 1962, through 

December 31, 1985 ............................................................................................ ~...................................... 22 

13, 14. Graphs showing: 

13. Recurrence data for independent main shocks in the Wasatch Front area from July 1962 through 

December 1985............................................................................................................................ 23 

14. Probability of exceedance per year for peak horizontal ground accelerations on soil at the intersection of 

I-15 and I-80 in South Salt Lake at approximately 40°43.1' N., 111°54.2' W. ........................................ 27 

15. Strip map showing the Wasatch fault, segment boundaries and names, and all earthquakes located by 

the University of Utah Seismograph Stations during the period July 1962 through December 1986........................ 29 

16, 17. Composite figures showing: 

16. Cross sections keyed to the boxes in figure 15........................................................................................ 30 

17. Space-time diagrams of earthquake occurrence keyed to figure 15 ............................................................. 32 

III 
IV CONTENTS 

TABLES 

Page 

TABLE 1. Largest earthquakes in the Utah region, 1850 through December 1986.......................................................................... D13 

2. Independent main shocks of magnitude (ML) 4.0 or greater in the Utah region, July 1962 through December 1986 ............... 15 

3. Average recurrence intervals for earthquakes in the Wasatch Front region.................................................................... 23 

4. Information for selected fault-specific sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF 

EARTHQUAKE HAZARDS AND RISK IN THE 

WASATCH FRONT AREA, UTAH 

By W.J. ARABASZ, J.C. PECHMANN, and E.D. BROWN 

ABSTRACT 

This chapter presents and considers in a systematic way up-to-date 

information from observational seismology that is basic to evaluating 

earthquake hazards and risk in the Wasatch Front area. We present 

fundamental information relating to (1) the earthquake data base, (2) 

the seismotectonic framework, (3) seismic source zones and seismicity 

parameters, (4) the ground-shaking hazard, and (5) current seismicity 

and the Wasatch fault. 

Important features of the seismotectonic framework of the Wasatch 

Front include (1) a threshold magnitude for surface faulting of ML 6.0 

to 6.5, (2) a maximum magnitude ofM8 7.5 to 7. 7, (3) the absence of any 

large surface-faulting earthquakes and the notable paucity of smaller 

earthquakes on the Wasatch fault during historical time, and (4) the 

problematic correlation of background seismicity with mapped Cenozoic faulting. In light of this framework, we consider seismic hazards in 

the Wasatch Front region to arise from two fundamental sources: (1) 

the occurrence of infrequent large (M8 6.3±0.2-7.5±0.2) surfacefaulting earthquakes on identifiable faults having evidence of late 

Quaternary displacement and (2) small- to moderate-sized (up to ML 

6.5) earthquakes that are not constrained in location to mapped faults 

and may occur anywhere throughout the region. The small to moderate 

earthquakes dominate the historical earthquake record and, at most 

localities, are the largest contributor to the probabilistic groundshaking hazard for exposure periods of 50 years or less. Recurrence 

modeling using independent main shocks from the 23.5-year instrumental catalog from July 1962 through 1985 predicts an average recurrence 

interval of 24~~g years for potentially damaging earthquakes of magnitude 5.5 or greater in the Wasatch Front area. The occurrence of 

eight such shocks from 1850 through 1987 gives an observed average 

recurrence interval of 17 years. 

INTRODUCTION 

The record of earthquake activity since 1850, together 

with geologic observations dating from more than a 

century ago, firmly establishes the danger of earthManuscript approved for publication November 20, 1990. 

quakes in Utah's Wasatch Front area. This chapter 

presents and considers in a systematic way information 

from observational seismology that is basic to evaluating 

earthquake hazards and risk in the Wasatch Front area. 

We follow usage urged by the Earthquake Engineering Research Institute and distinguish a seismic hazard 

from seismic risk in the following way. A seismic hazard 

is "any physical phenomenon ... associated with an earthquake that may produce adverse effects on human activities," whereas seismic risk is "the probability that social 

or economic consequences of earthquakes will equal or 

exceed specified values at [one or more sites] during a 

specified exposure time" (Earthquake Engineering 

Research Institute Committee on Seismic Risk, 1984, p. 

38). The most notable seismic hazards are the geologic 

processes of ground shaking, ground failure, surface 

faulting, tectonic deformation, and inundation. 

Insofar as the evaluation of earthquake hazards 

involves recognition of the location, frequency, and 

severity of these hazards, observational seismology is 

generally relied upon to characterize the space, time, and 

size distribution of the earthquakes giving rise to them. 

This procedure might involve little more than a qualitative consideration of the available earthquake record and 

the spatial pattern of earthquake occurrence. On the 

other hand, more rigorous evaluations of seismic hazard 

and risk rely on observational seismology to specify 

quantitative models of earthquake behavior and mechanics, so that either the level of a hazard (for example, a 

nonexceedance value of ground motion) or the level of 

risk can be computed at one or more sites for some 

exposure time. Either a deterministic or a probabilistic 

approach may be used. In the former case, each independent variable has a single value, and a model predicts 

a specific value for the dependent variable. For example, 

D1 
D2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

I. Compilation of earthquake catalog (earthquake 

times, locations, and sizes) 

2. Characterization of seismotectonic framework 

(crustal structure, contemporary deformation 

[stress state and strain rate], location and 

geometry of active faults, correlation of seismicity with geologic structure, and fault 

mechanics) 

3. Definition and geometric depiction of seismic 

source zones 

4. Estimation of seismicity parameters (pretesting of catalog for uniformity of size 

estimates, dependency of events, and sample 

completeness; assessment of maximum magnitude; recurrence modeling and computations) 

5. Modeling and estimation of ground motion 

for engineering applications-including 

description of source spectrum (and its scaling 

with earthquake size), wave propagation, and 

attenuation 

-

--

-

-

6. Studies of pre-instrumental, historical 

earthquakes 

7. Instrumental seismic monitoring and 

routine data analysis (earthquake locations, 

sizes, focal depths) 

8. Network and portable-array seismology 

(special studies of hypocentral resolution, 

focal mechanisms, seismic moment, stress 

drop, and other source parameters; also 

crustal structure) 

9. Analysis and interpretation of space-time 

patterns of earthquake behavior for recurrence 

modeling and earthquake forecasting 

I 0. Analysis and interpretation of earthquake 

source mechanics and wave propagation 

effects, including attenuation 

FIGURE 1. -Steps in a formalized earthquake hazard analysis (left column) and interrelated aspects of observational seismology (right column). 

the maximum ground shaking expected at a site might be 

estimated from the comparative effects of nearby earthquake source zones, each of which is assigned a 

maximum-size event and a minimum distance to the site, 

along with a corresponding upper limit of predicted 

ground motion. In the case of a probabilistic approach, 

uncertainties arising from natural variations or incomplete knowledge are taken into account, and probability 

theory is used for the analysis (for example, Youngs and 

others, in press; Algermissen and others, this volume). 

The flowchart shown in figure 1 outlines the basic 

elements of a modern seismic hazard analysis (for example, Savy and others, 1986; Electric Power Research 

Institute, 1988), using as an example the objective of 

estimating the hazard of ground motion. (Seismologic 

input to a risk analysis would be similar.) The sequence 

of necessary procedures is shown by steps 1 through 5 in 

the left-hand column; interrelated aspects of observational seismology are shown in the right-hand column. 

Figure 1 is a useful guide to our presentation as we give 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D3 

an overview of fundamental information from observational seismology in the Wasatch Front area contributing 

to steps 1 through 4. Sequentially, we consider (1) the 

earthquake data base, (2) the seismotectonic framework, 

and (3) seismic source zones and seismicity parameters. 

We then discuss a selected aspect of the ground-shaking 

hazard, referring the reader to Chapter M (in the second 

volume of this report) by R. R. Youngs and others for an 

example of the fully completed process of step 5 in which 

probabilistic estimates of ground motion are made. In the 

final section, we address the question, "What does 

observational seismology tell us about the behavior of the 

Wasatch fault?" 

ACKNOWLEDGMENTS 

The data in this manuscript implicitly reflect substantial efforts by numerous individuals associated with the 

University of Utah Seismograph Stations. We thank D. 

Veneziano of the Massachusetts Institute of Technology 

for providing data on independent main shocks for the 

recurrence modeling and for assisting with the probabilistic hazard methodology. We also thank J.A. Barlow, 

L.B. Burnett, P.A. Onstott, K.J. Quigley, and J.E. 

Shemeta, all of the University of Utah, for their assistance in the manuscript preparation. Critical reviews 

were provided by R.L. Wheeler and C.J. Langer of the 

U.S. Geological Survey. The research arises from ongoing support by the U.S. Geological Survey, Department 

of the Interior, currently under award numbers 

14-08-0001-G1349 and 14-08-0001-A0265, and from support by the State of Utah. 

EARTHQUAKE DATA BASE 

The catalog of documented earthquakes in the Utah 

region1 (step 1, fig. 1) is, as it is elsewhere in the 

Western United States, a mixed one, relying initially on 

reports and newspaper accounts of felt earthquakes and 

later on seismographic recordings made during several 

stages of evolving instrumental coverage (Arabasz and 

others, 1979, 1980) (see fig. 2). The historical earthquake 

record effectively dates from 1850, when the first newspaper was published in the region, shortly after Mormon 

pioneers began to settle the area in 1847. Instrumental 

earthquake locations in the region, based on regional 

seismographic recordings in the Western United States, 

date from about 1938, with the exception of the 1934 Ms 

6. 6 Hansel Valley earthquake and its larger aftershocks. 

The most prominent sources of earthquake data for the 

Utah region are compilations made by the University of 

1

0ur usage of the term "Utah region" is defined in figure 2. 

Utah Seismograph Stations (UUSS) (Arabasz and others, 1979; Richins and others, 1981, 1984; Brown and 

others, 1986; unpublished data of various workers) and 

data files of the National Geophysical Data Center, 

National Oceanic and Atmospheric Administration 

(NOAA). Other earthquake summaries include a recent 

compilation for the State of Utah by Stover and others 

(1986), 2 a data file compiled by Askew and Algermissen 

(1983) for the Basin and Range province, a Western U.S. 

data file produced by Eddington and others (1987) for a 

study of Basin and Range geodynamics, and a 

continental-scale data file produced by Engdahl and 

Rinehart (1988) for the "Decade of North American 

Geology" publication series. 

Ideally, the establishment of a master "consensus" 

catalog would involve the coordinated and formalized 

efforts of numerous individuals having relevant data and 

expertise, as was recently done for the entire Central 

and Eastern United States (Electric Power Research 

Institute, 1988). The required efforts relate to establishing catalog completeness, uniform estimates of earthquake size, and preferred epicentrallocations and origin 

times and to documenting uncertainties. Similarly rigorous and formal scrutiny of a data base for the Utah region 

remains to be made. In this chapter, we rely upon the 

University of Utah's data base, which represents the 

primary source of instrumental earthquake data since 

mid-1962 for the Utah region and which includes a 

comprehensive listing of historical seismicity for which 

only minor variance from other catalogs should be found. 

For 1962 through 1986, the UUSS catalog contains an 

order-of-magnitude more earthquakes than the NOAA 

data file for the Utah region (for example, 6,994 versus 

342 earthquakes for the Wasatch Front area alone). 

Although seismographs were first installed on the 

University of Utah campus in 1907, contributions to the 

instrumental location of regional earthquakes postdate 

1939, when photographic records from modern seismographs began to be routinely forwarded from Salt Lake 

City to the U.S. Coast and Geodetic Survey. Systematic 

computerized locations based on local seismographic coverage in the Utah region by the University of Utah date 

from mid-1962. From mid-1962 through late 1974, a 

skeletal statewide network of several widely spaced 

stations was in operation (fig. 2, upper right). Instrumental earthquake data for the period 1962 through 1974 

were systematically reanalyzed and revised in a major 

study by the UUSS (Arabasz and others, 1979). Since 

late 1974, the University of Utah has operated a modern 

telemetered network of high-gain short-period seismo2

This compilation omits significant earthquakes in Utah that occurred on 

November 10, 1884, July 21, 1959, and August 30, 1962 (table 1) (Coffman and 

others, 1982). 
D4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

"Utah region" 

~6 ~ 

~ 6 SLC 

(Salt Lake City) 

u T A H 

"Utah region" 

6 A 6 

0 6 

~~kc 61'66 

6 6 6 

6 

6 

u T A H6 

6 

6 

6 

0 

I 

0 

SEISMOGRAPH STATIONS 

1955 

0 

1975 

I 

100 

100 

I 

I 

200 

6 

6 

"Utah region" 

%6 ~ 

~ 6 SLC 

6 

6 

u T A H 

Q 

"Utah region" 

6 A 6 

~~0 6 

6 

~SLC 

6 6 6 

6 66 

6 

6 

66 

u T 

6 6 6 

6 

6 

6 6 6 

6 

6 

I 

200 MILES 

I 

y 

300 KILOMETERS 

A 6 H 

6 

6 

6 

6 

1965 

1985 

FIGURE 2. -Distribution of seismograph stations (triangles) in the Utah region at four different times between 1955 

and 1985. The Utah region, corresponding to a specific domain of the University of Utah's earthquake catalog, 

extends from latitude 36°45' N. to 42°30' N. and from longitude 108°45' W. to 114°15' W. 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D5 

graphic stations in the Intermountain region. About 

two-thirds of the current network's 85 stations lie within 

the Utah region (fig. 2, lower right). The seismographic 

data are centrally recorded at the University of Utah in 

Salt Lake City. From 1974 through 1980, data were 

recorded in analog form; since January 1, 1981, recordings have been in digital form. 

The UUSS catalog for the Utah region currently 

contains 9,561 earthquakes for the period from 1850 

through 1986, including 413 (chiefly noninstrumental) 

earthquake locations from 1850 through June 1962 and 

9,148 (instrumental) locations from July 1, 1962, to 

December 31, 1986. Magnitudes given in the UUSS 

catalog are estimates of local Richter magnitude (M L) 

based on systematic procedures described, for example, 

by Brown and others (1986). Such estimates may differ 

significantly from values of magnitude in the NOAA 

catalog, which are typically body-wave magnitude (mb) 

and known to be commonly as much as one magnitude 

unit larger than ML for small (mb::S4.0) earthquakes 

(Dewey, 1987). We refer the reader to special publications of the UUSS (for example, Arabasz and others, 

1979; Brown and others, 1986) for other details of the 

UUSS data base. 

SEISMOTECTONIC FRAMEWORK 

The characterization of the seismotectonic framework 

of any seismically active region (step 2, fig. 1) basically 

involves an understanding of its geologic and geophysical 

makeup, its earthquake-generating faults, and the operative deformational processes that lead to earthquake 

occurrence. Previous studies that have developed such a 

framework for the Wasatch Front area and its surrounding region include those by Smith and Sbar (1974), Smith 

(1978), Arabasz and others (1980), Zoback (1983), Smith 

and Bruhn (1984), and Arabasz and Julander (1986). We 

will not attempt a complete review of previous studies 

here. Our intent is (1) to summarize some essential 

characteristics of the seismotectonic framework of the 

Wasatch Front area and (2) to emphasize new information acquired from observational seismology by researchers at the University of Utah. Information from network 

and portable-array seismology (element 8, fig. 1) 

includes precise earthquake locations, earthquake focal 

mechanisms and source properties, strain rate tensors 

from seismic moment release, and models of crustal 

structure. 

GENERAL SETTING AND CHARACTERISTICS 

The Wasatch Front area, synonymous herein with the 

rectangular area outlined in figure 3, is located along the 

eastern boundary of the Basin and Range province. This 

boundary coincides with a prominent west-facing topographic escarpment that follows the 380-km-long 

Wasatch normal fault zone. The Wasatch Front area is 

traversed by the Intermountain Seismic Belt, a coherent 

belt of intraplate earthquake activity extending more 

than 1,500 km from southern Nevada and northern 

Arizona to northwestern Montana (Smith and Sbar, 

1974; Smith, 1978; Stickney and Bartholomew, 1987; 

Smith and Arabasz, 1991). In general, the Intermountain 

Seismic Belt is characterized by late Quaternary normal 

faulting, diffuse shallow seismicity (focal depths of less 

than 15-20 km), and episodic scarp-forming earthquakes 

(M ~6.5-7.5), all associated with intraplate deformation 

within the western North American plate. 

Since 1850, at least 16 independent earthquakes (aftershocks excluded) of magnitude 6.0 or greater have 

occurred within the Intermountain Seismic Belt (fig. 3). 

Three of these historical earthquakes were associated 

with documented surface faulting. Normal fault scarps 

having maximum surface displacements of 0.5, 5.5±0.3, 

and 2. 7 m, respectively, were produced by the M8 6.6 

Hansel Valley, Utah, earthquake of March 1934 (Shenon, 

1936), the M8 7.5 Hebgen Lake, Mont., earthquake of 

August 1959 (Bonilla and others, 1984), and the M8 7.3 

Borah Peak, Idaho, earthquake of October 1983 (Crone 

and others, 1987). 

The Intermountain Seismic Belt within the Utah 

region is notably characterized by (1) a general predominance of normal faulting, which reflects an extensional 

stress regime (although young strike-slip deformation 

has recently been recognized from seismologic and geologic studies in central Utah (Arabasz and J ulander, 

1986; Anderson and Barnhard, this volume)); (2) moderate background seismicity, which is lower by about a 

factor of 4 in comparison with that along the western 

North American plate boundary (Smith and Arabasz, 

1981); (3) diffuse seismicity having weak correlation with 

major active faults and with focal depths almost exclusively shallower than 15 to 20 km; (4) relatively long 

(approximately 1,000 years or more), and perhaps temporally variable, average recurrence intervals for surface faulting on individual fault segments (Schwartz and 

Coppersmith, 1984; Wallace, 1987; Machette and others, 

this volume); (5) slip rates of late Quaternary faulting of 

about 1 mm/yr or less, one to two orders of magnitude 

lower than those on major plate-boundary faults 

(Schwartz, 1987); and (6) the historical absence of any 

surface-faulting earthquake larger than the M8 6. 6 

Hansel Valley earthquake of 1934, despite the presence 

of abundant late Quaternary and Holocene fault scarps. 

Figure 4 illustrates some important aspects of the 

seismotectonic framework of the Wasatch Front area. 

(Letters in parentheses here are keyed to the figure.) 
D6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

48° 

46° 

44° 

42° 

40° 

38° 

116° 

1945 (6.0) 

1944 (6.1). • 

0 

Boise 

Basin and Range 

Province 

114° 112° 

/ ' 

/ / .1901 (6%) 

~ // . 

11 oo 

/ // Colorado 

1_../r/ -1921 (6) // _4 

/ I Plateau 

108° 

• .1902 (6) /1 

I /-v- /~----U~T~A=H~-+-=CO=L~O~R~AD~O~ 1966 (5.6:6.D( I/ / ,;1 ARIZONA NEW MEXICO 

Las Vegas 360 0 

/ Y"' (/ /.A ~ / / ·~ 

.J (/ 

0 

I 

0 

(/ 

,.... 

I 

100 

100 

I 

I 

200 

I 

200 MILES 

I 

300 KILOMETERS 

FIGURE 3. -Setting of the Wasatch Front study area with respect to the Intermountain 

Seismic Belt (hachured zone) and the epicenters of historical earthquakes of magnitude 

6.0 and greater (solid circles) (adapted and updated from Arabasz and Smith, 1981). 

Year and magnitude are labeled for each earthquake. 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D7 

,,,,,,,,,,, ,,,,,,,,, SfY!>--... 

D.,,,,,,,; 'Z~ U.c ,,,,,,,, 

lfl 11111 

c...~ ,,,,,,,,,, 

''''e 

FIGURE 4. -Selected aspects of the seismotectonic framework of the 

Wasatch Front area (vertical exaggeration approximately 1.0-1.5). 

Heavy lines with arrows (directions of displacement) represent fault 

traces. Lowercase letters indicate elements discussed in the text: a, 

the base of the crust defined by the Moho; b, lower crustal material; 

Total crustal thickness (a) increases eastward from about 

36 to 44 km across the transitional boundary between the 

eastern Basin and Range and the Middle Rocky MountainColorado Plateau provinces (Loeb and Pechmann, 1986; 

Loeb, 1986). Total lithospheric thickness similarly 

increases from about 65 km beneath the Basin and Range 

to more than 80 km beneath the Colorado Plateau (Smith 

and others, 1987). A wedge of material (b) having a 

P-wave velocity of approximately 7.5 km/s, formerly 

thought to be upwarped mantle, has been mapped by 

WASATCH FAULT 

g 

c, low-angle detachments; d, listric fault geometry; e, the brittleductile transition; f, the nucleation zone of a large earthquake; g, 

surface-fault scarps; h, a fault segment boundary; i, the direction of 

crustal extension; j, the orientation of principal stresses (<T1 , <T2 , <T3), 

compression being positive. 

J.C. Pechmann and D.T. Loeb (Pechmann and others, 

1984; Loeb, 1986) as lying above the 7.9-km/s Moho, on 

the basis of an analysis of traveltimes from local earthquakes and blasts recorded by the University of Utah's 

regional seismic network. The importance of low-angle 

detachments (c) and listric fault geometries (d) in upper 

crustal structure beneath the eastern Basin and Range 

appears well established from data from the Consortium 

for Continental Reflection Profiling (Allmendinger and 

others, 1983, 1987) and from interpretation of industry 
D8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

seismic reflection data at the University of Utah (Smith 

and Bruhn, 1984) (see also Smith and others (1989) for a 

summary of multiple studies). 

The brittle-ductile transition (e) marking the base of 

the seismogenic layer has been modeled rheologically by 

Smith and Bruhn (1984) to be transitional, perhaps as 

shallow as approximately 8 km but probably about 10 to 

15 km deep beneath the Wasatch Front area. Large 

surface-faulting earthquakes in the Wasatch Front area 

are expected to nucleate (f) at that depth on penetrative 

planar faults of moderate dip (Smith and Richins, 1984; 

Doser, 1985a) somehow connected to surface fault scarps 

(g). As we will discuss later, most of the small- to 

moderate-sized background seismicity in the area is not 

directly associated with the first-order faults. Variability 

in slip, rupture velocity, and fault orientation during 

large normal-faulting earthquakes can be expected to 

produce locally complex ground motions (Benz and 

Smith, 1987). Segmentation (h) of first-order faults such 

as the Wasatch fault (Swan and others, 1980; Schwartz 

and Coppersmith, 1984; Machette and others, 1991, this 

volume) places important constraints on rupture length, 

maximum earthquake size, and rupture dynamics. 

The Wasatch Front area is characterized by roughly 

east-west extensional deformation (i), as earthquake 

focal mechanisms and slickenside data indicate (Smith 

and Lindh, 1978; Arabasz and others, 1980; Zoback and 

Zoback, 1980; Zoback, 1983). Twenty-four single-event 

focal mechanisms recently determined by Bjarnason and 

Pechmann (1989), representing the best focal mechanism 

data to date for the Wasatch Front area, have an average 

tension axis azimuth of96°±12° s.d. Extension calculated 

by Eddington and others (1987) from moment tensors of 

historical earthquakes (see also Doser and Smith, 1982) 

implies an average strain rate of about 2x 10-16/s for the 

Wasatch Front area and a total extension rate of 1 to 2 

mm/yr across the study area. 

The stress tensor in the upper crust (j) implied by 

earthquake focal mechanisms and other stress indicator 

data (for example, Zoback, 1983) has a vertical maximum 

principal stress axis and intermediate and minimum 

principal stress axes lying in the horizontal plane, the 

latter being oriented in an east-west (±20°) direction. 

Zoback (1984) has argued that both horizontal principal 

stresses are approximately equal in magnitude along the 

Wasatch Front, implying the potential for slip on normal 

faults of varying strike. Earthquake focal mechanisms, 

however, display strong clustering of tension axes in an 

east-west direction (Arabasz and Julander, 1986; Bjarnason and Pechmann, 1989). Focal mechanisms for earthquakes in the Wasatch Front region south of about 40° N. 

show a mixture of normal, oblique-normal, and strikeslip faulting, in contrast to those north of 40° N., which 

show predominantly normal faulting. This observation 

suggests that, in the southern Wasatch Front region, the 

maximum and intermediate principal stresses may be of 

similar magnitude (Bjarnason and Pechmann, 1989). 

LATE QUATERNARY FAULTING 

In the absence of a definitive map of known and 

suspected active faults throughout the Wasatch Front 

area, efforts were made to compile a base map of late 

Quaternary faulting for comparison with instrumental 

seismicity. The resulting map (fig. 5), which relies chiefly 

on published sources, includes the traces of fault displacements of late to middle Pleistocene (10,000 to about 

500,000 yr B.P.) and Holocene (less than 10,000 yr B.P.) 

age. 

A number of sources were used to compile the digitized 

base map in figure 5. The trace of the Wasatch fault is 

based chiefly on detailed mapping done by Cluff and 

others (1970, 1973, 1974) and was taken, in part, from 

subsequent compilations by Davis (1983a, b, 1985). 

Depiction of the West Valley fault zone near Salt Lake 

City is from Keaton and others (1986). Faulting to the 

east of the Wasatch fault in Utah is from detailed maps 

by Sullivan and others (1988) and Foley and others 

(1986); that in Wyoming is from Gibbons and Dickey 

(1983). The trace of the East Great Salt Lake fault, lying 

within the bounds of the Great Salt Lake, was taken from 

Cook and others (1980) and Viveiros (1986). West of 112° 

W., we relied heavily on maps of fault scarps in unconsolidated sediments published by Bucknam (1977) and 

Bucknam and Anderson (1979). Some additional faults in 

the ''Western Desert" region of Utah included in the 

compilation of Anderson and Miller (1979) were added for 

completeness. Finally, faulting to the north of 42° N. in 

Idaho, other than the northern extension of the Wasatch 

fault, was taken from Witkind (1975). 

There is some inhomogeneity in figure 5 in that some of 

the fault traces to the west of the Wasatch fault within 

the Basin and Range province reflect only the extent of 

mapped fault scarps in unconsolidated deposits and not 

necessarily the entire length of a seismogenic range-front 

fault. Fluctuation of ancient Lake Bonneville, especially 

between about 25,000 and 13,000 years ago (Currey and 

others, 1984), could have obliterated evidence of late 

Quaternary surface faulting in many places. 

The Wasatch fault is by far the best-studied fault 

depicted in figure 5. We refer the reader to other papers 

in this volume for summaries of up-to-date paleoseismologic information on its segmentation and slip rate and on 

the timing and size of prehistoric earthquakes. A major 

active fault to which little attention has heretofore been 

paid is one beneath the Great Salt Lake that Cook and 

others (1980) named the "East Great Salt Lake fault 

zone" and whose slip rate and earthquake potential have 
40° 

39° 

OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D9 

113° 112° 

Malad Cityd ~ l 

IDAHO 

UTAH 

Dugway0 

I 

f~M ~ \ Delta 0 

\-\ 

( 

Tooele \ 

orNO 

,r. 

sv', 

(• 

PR\ \ 

0 

I 

0 

I 

} 

I 

'') 

\t 

0 Manti 

I 

,'j 

/JV 

., 

111° 

Kemmerer 0 

' 0 ' Evanston 

50 MILES 

I 

50 KILOMETERS 

FIGURE 5. -Traces of late Quaternary 

faulting in the Wasatch Front study 

area (see text for sources). Large 

arrows delimit the extent of the 

Wasatch fault zone. Other faults labeled 

for general orientation include BL, Bear 

Lake; BR, Bear River Range; CL, 

Clear Lake; CM, Crawford Mountains; 

DM, Drum Mountains.; EC, East 

Cache; ECN, East Canyon; EGSL, 

East Great Salt Lake (dashes indicate 

approximate); HV, Hansel Valley; JV, 

J oes Valley; LD, Little Diamond Creek; 

ME, Mercur; MO, Morgan; NO, Northern Oquirrh; OV, Ogden Valley; PR, 

Pavant Range; PV, Puddle Valley; RV, 

Round Valley; SC, Sulphur Creek; SH, 

Sheeprock Mountains; ST, Stansbury 

Mountains; STW, Strawberry Valley; 

SV, Scipio Valley; TH, Topliff Hill; WV, 

West Valley. The diamond near Salt 

Lake City marks the site for which we 

performed a probabilistic calculation of 

the ground-shaking hazard (see fig. 14). 
D10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

been investigated recently by one of us (J.C. Pechmann, 

unpub. report, 1987; Pechmann and others, 1987). This 

fault zone can be seen clearly in seismic reflection profiles 

across the lake (Mikulich and Smith, 1974; Smith and 

Bruhn, 1984; Bortz and others, 1985; Viveiros, 1986). 

Reflection data and well data indicate that the sedimentary basin underlying the lake deepens eastward and is 

bounded on the east by the East Great Salt Lake fault. 

The deepest part of the basin contains more than 3,048 m 

of post-Miocene sedimentary rocks (Mikulich and Smith, 

1974; Bortz and others, 1985; Viveiros, 1986), an indication that major subsidence has occurred during the past 

24m.y. 

The East Great Salt Lake fault cuts sediments identified as Quaternary on the basis of well data (Mikulich and 

Smith, 1974; Viveiros, 1986) and must be considered 

active. Seismic reflection data (Mikulich and Smith, 197 4; 

Viveiros, 1986) indicate that the East Great Salt Lake 

fault appears to offset sediments to within at least 0.015 

to 0. 025 s two-way traveltime beneath the lake bottom, 

corresponding to an approximate depth of less than 10 to 

20 m, which implies that slip has occurred in the recent 

geologic past. Viveiros (1986, p. 72) estimated fault slip 

rates on the East Great Salt Lake fault of 0.96 mm/yr 

during the Pliocene and 1.48 mm/yr during the Quaternary from the thicknesses of sedimentary deposits 

(dependent on an interpreted geometry of faulting). 

Pechmann and others (1987) interpreted average Quaternary slip rates of 0.4 to 0. 7 mm/yr, taking subsurface 

fault dip into account and assuming that rates of sedimentation adjacent to the fault are controlled by subsidence on the fault. These slip rates are about half the 

recent slip rates along central segments of the Wasatch 

fault (Schwartz and Coppersmith, 1984). 

THE 1959 HEBGEN LAKE AND 1983 BORAH PEAK 

EARTHQUAKES 

The only historical earthquake in the Utah region 

known to have produced surface faulting occurred on 

March 12, 1934, in Hansel Valley just north of the Great 

Salt Lake (figs. 3, 5). This earthquake was assigned a 

magnitude of 6. 6 by Gutenberg and Richter (1954) and is 

the largest earthquake to have occurred in the Utah 

region since 1850. Geologic evidence indicates that earthquakes having surface displacements much larger than 

the 0.5 m of the 1934 earthquake are expected to occur in 

Utah in the future, so information from large surfacefaulting earthquakes elsewhere in the Intermountain 

Seismic Belt and in the Basin and Range province is 

important for evaluating their likely magnitudes and 

other characteristics. 

In the Intermountain Seismic Belt, there have been 

two large, historical normal-faulting earthquakes 

greater than magnitude 7.0, both of which produced 

surface rupture: the October 28, 1983, Ms 7.3 (U.S. 

Geological Survey determination) Borah Peak earthquake in central Idaho and the August 18, 1959 (GMT), 

Ms 7.5 Hebgen Lake earthquake in southern Montana 

(fig. 3). These two earthquakes are generally considered 

to be good models for future large earthquakes on the 

Wasatch fault and other major faults in Utah (Smith and 

Richins, 1984; Doser, 1985a). The surface-wave magnitude (Ms) of 7.5 for the Hebgen Lake event is from Abe 

(1981) and is probably more accurate than a previous 

estimate of 7.1, attributed by Murphy and Brazee (1964) 

to Pasadena. The Hebgen Lake earthquake was accompanied by 35 km of surface faulting along the Red Canyon 

and Hebgen faults, vertical displacements of up to 

5.5±0.3 m (Bonilla and others, 1984; Witkind, 1964), and 

an average displacement of 2.1 m (Hall and Sablock, 

1985). The Borah Peak earthquake produced 36 km of 

surface faulting along the Lost River and Arentson Gulch 

faults, vertical displacements of up to 2. 7 m, and an 

average displacement of 0.8 m (Crone and Machette, 

1984; Crone and others, 1987). Both earthquakes nucleated at depths of about 15 to 16 km and ruptured upward 

along faults dipping at 45° to 60° (Doser, 1985a, b; Doser 

and Smith, 1985). 

The most reliable and physically meaningful measurement of earthquake size is the seismic moment, M0 (Aki, 

1966), given by 

where f.L is the shear modulus, Sis the area of the rupture 

surface, and d is the average displacement along the 

rupture surface. From the seismic moment, a moment 

magnitude, Mw (Kanamori, 1977; Hanks and Kanamori, 

1979), can be calculated from the definition 

Mw should be comparable to Ms for earthquakes of 

5.0::;Mw::;7.5 (Hanks and Kanamori, 1979). The Hebgen 

Lake earthquake had a seismic moment of l.Ox 1027 

dyne-em (Doser, 1985b), which converts to a moment 

magnitude of 7.3. The Borah Peak earthquake had a 

moment of 2.1 x 1026 (Doser and Smith, 1985) to 3.1 x 1026 

(Ekstrom and Dziewonski, 1985), which gives a moment 

magnitude of 6.8 to 7.0. 

THRESHOLD OF SURFACE FAULTING 

Various authors (for example, Arabasz, 1984; Doser, 

1985a; Arabasz and Julander, 1986) have suggested that 

the threshold magnitude for surface faulting in Utah is 

approximately 6. 0 to 6. 5. On the basis of the historical 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D11 

record of earthquakes in the Intermountain Seismic Belt 

and in the Basin and Range province, this conclusion 

appears to be well founded. Bucknam and others (1980) 

noted that all seven historical earthquakes of ML>6.3 in 

the Great Basin have produced surface faulting, including the 1934 M8 6.6 Hansel Valley, Utah, event. In their 

tabulation of 11 earthquakes that produced historical 

surface faulting in the Basin and Range province, all five 

events of ML <6.8 had maximum displacements of less 

than 1 m. The tabulation includes one California normalfaulting earthquake of ML 5.6 in 1950 that had 0.2 m 

maximum displacement and a Nevada earthquake of ML 

6.3 in 1934 that had 0.1 m maximum displacement. In the 

Intermountain Seismic Belt, Doser (1985a) pointed out 

that neither the 1975 ML 6.0 Pocatello Valley earthquake 

nor the 1975 ML 6.1 Yellowstone Park earthquake (fig. 3) 

had identifiable surface faulting, although both earthquakes were accompanied by an apparently coseismic 

subsidence of up to 12 to 13 em (Bucknam, 1976; Pitt and 

others, 1979). On the basis of the historical record, we 

adopt ML 6.3±0.2 as a reasonable estimate for the 

threshold of surface faulting in the Utah region. The 

implication of this threshold is that one can argue that 

earthquakes up to this size could occur anywhere in the 

Wasatch Front region within the main seismic belt, even 

where there is no geologic evidence for Quaternary 

surface faulting. We will elaborate in a later section. 

SEISMICITY 

Figure 6 shows the distribution of all historical main 

shocks of estimated Richter magnitude 4.0 or greater (or 

maximum Modified Mercalli intensity V or greater) in 

the Utah region. The historical sample includes at least 

15 independent main shocks that have had an estimated 

Richter magnitude of 5. 5 or greater (or a Modified 

Mercalli intensity VII or greater). These earthquakes 

are listed in table 1, and their epicenters are shown as 

solid circles in figure 6. At the scale of figure 6, apparent 

correlations of historical seismicity with specific faults 

must be considered with care. For example, many of the 

epicenters located along the Wasatch fault are noninstrumental and correspond to locations where effects were 

felt most strongly for a particular shock, typically in an 

established city or town. Hence, the coincidence of 

historical epicenters with the Wasatch fault reflects the 

locations of settlements along the Wasatch Front and is 

not necessarily indicative of earthquake activity on the 

Wasatch fault itself. It is arguable whether the earthquakes of about magnitude 5.5 in 1910 near Salt Lake 

City and in 1914 near Ogden occurred directly on the 

Wasatch fault (Arabasz and others, 1980). Thus, as many 

as two-or perhaps no-earthquakes of magnitude 5 or 

greater have occurred on the Wasatch fault in historical 

time. The average interevent time since 1884 of the 

moderate to large main shocks listed in table 1 is about 

6.5 years. There was, however, one unusually long 

25-year interval between 1934 and 1959 in which no 

earthquake of magnitude 5. 5 or greater occurred. 

Figure 7 shows the pattern of instrumental seismicity 

for the Utah region determined by University of Utah 

monitoring since mid-1962 (element 7, fig. 1). The epicentral distribution of small- to moderate-sized background earthquakes is generally similar to that of figure 

6. The sample shown in figure 7 includes 9,148 earthquakes of all sizes up to magnitude 6.0 and 2,152 earthquakes of magnitude 2.0 or greater. Table 2 lists independent main shocks of magnitude 4.0 and greater in the 

sample. Focusing attention on the Wasatch Front study 

area, figures 8 and 9 show the patterns of instrumental 

seismicity from October 1, 1974, through June 30, 1978 

(2,480 events), and from July 1, 1978, through December 

31, 1986 (3,936 events), respectively. The first sample is 

the same as that described in detail by Arabasz and 

others (1980) for the initial 3. 75 years of detailed monitoring by the University of Utah's telemetered seismic 

network. The second sample allows an updated comparison for the subsequent 8.5-year period. The earthquake 

samples in figures 8 and 9 are probably complete above 

about magnitude 2.0 (discussed later). The extent of 

seismographic coverage, relevant to inferences of epicentral precision, is shown in figure 2. 

The patterns of seismicity shown in figures 8 and 9 are 

remarkably similar, and comparison with the pattern for 

1962 through 1974 (Arabasz and others, 1980, fig. 4) 

indicates general stability in the pattern of seismicity 

throughout the period of instrumental monitoring. One 

notable difference in the seismicity before and after 1974 

is the aftershock activity that followed the 1975 ML 6.0 

Pocatello Valley earthquake on the Idaho-Utah border 

(upper left-hand parts, figs. 8, 9). The pattern of most 

recent seismicity in the Wasatch Front area illustrated in 

figure 9 includes several notable features, described 

from north to south: 

1. At the northern extremity of figure 9, earthquake 

clusters are part of a northeast-trending belt of seismicity that continues to the Jackson-Yellowstone 

Park region. This trend includes episodic swarm 

seismicity in the Soda Springs area of Idaho (approximately 42°30' N., 111°30' W.). A swarm beginning 

as early as December 1981 peaked with an ML 4. 7 

earthquake in October 1982 (Richins and others, 

1983), and seismicity continues in that area. 

2. Earthquake activity in the Idaho-Utah border area 

west of the Wasatch fault has been prominent ever 

since the 1975 ML 6.0 Pocatello Valley earthquake 

(Arabasz and others, 1981). Small- to moderate-sized 

earthquakes extending southward from the State 
D12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

0 })IDAHO ---.---.---

1 ., 1934 

I 

I 

b 

I 

0 

~I ( ~: 

~~· ;\ 

I 

I 

I 

I 

I 1\ 

I \ 

0 

{ 

I 

'· I ~ 0 

• l_/_· -FVOI\!I~Q_ - - -.-

UTAH 1 

Salt Lake City 

1910 __ _.....----

,~ 

Price 0 t9~ ·' [] ~o'\~ l .. \ \. 0 .. , , .. · 1901 ~ ' 

I 

I 

~ I 

I 

I 

t~O 1921 (2) 'ts~,Moab 

, 

1 

... ,Richfield .. __ .. ~' _ 

(j ~ \~, ( '~ I 

(§) ! '"'!, ,.'" I ', I 

'o , ~"ol o / 0 l 

I ~ 01 I I 

1 ~ O CedarCity ( ' 

96·6· Ia 0 ~ 1902 o 1887 I - 1~·~'::.._, ~~·2 -ARIZoNA----------------~- 1959 

0 

I 

0 

I 

100 MILES 

I 

100 KILOMETERS 

EXPLANATION 

Magnitudes 

04.0+ Os.o+ 06.0+ 

FIGURE 6.-Epicenters of all independent main shocks ofML 4.0 or greater (or Modified Mercalli intensity Vor 

greater) in the Utah region, 1850-1986, and Quaternary faults. Earthquakes of estimated ML 5.5 or greater 

are indicated by solid circles, labeled with date. Data from University of Utah Seismograph Stations. 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D13 

TABLE 1.-Largest earthquakes in the Utah region, 1850 through December 1986 

[Modified from Arabasz and others (1979). Includes earthquakes of maximum Modified Mercalli intensity VII or greater or of Richter magnitude 5.5 or greater. 

Aftershocks excluded. Magnitudes in parentheses are estimated from intensity. Sample area (Utah region): 36.75° N.-42.50° N., 108.75° W.-114.25° W. (fig. 

6). Italics denote earthquakes within the Wasatch Front study area: 38.92° N.-42.50° N., 110.42° W.-113.17° W. (fig. 3)] 

Modified Mercalli Magnitude Moment' 

Local date Lat, 0

N. Long, ow. Location intensity (ML) ( x 1024 dyne-em) 

1881,, Nov. 10 . ........... 1,2.0 111.3 Bear Lake Valley 8 (6) 

1887, Dec. 5. . ........... 37.1 112.5 Kanab 7 (5Y2) 

1900, Aug. 1 ............ 40.0 112.1 Eureka 7 (51fz) 

1901, Nov. 13 ........... 38.8 112.1 Richfield 9 (61/2+) 

1902, Nov. 17 ........... 37.4 113.5 Pine Valley 8 (6) 

1909, Oct. 5 ............. 41.8 112.7 Hansel Valley 8 (6) 

1910, May 22 . ........... 1,0.8 111.9 Salt Lake City 7 (51fz) 

1911,, May 13 . ........... 41.2 112.0 Ogden 7 (Siffl) 

1921, Sept. 29 ........... 38.7 112.2 Elsinore 8 (6) 

1921, Oct. 1 ............. 38.7 112.2 Elsinore 8 (6) 

1934, Mar. 12 ........... 41.7 112.8 Hansel Valley 9 26.6 77.0 

1959, July 21 ............ 37.0 112.5 Utah-Arizona border 6 5.5+ 

1962, Aug. 30 ........... 42.04 111.74 Cache Valley 7 5.7 7.0 

1966, Aug. 16 ........... 37.46 114.15 Nevada-Utah border 6 5.6 1.1 

1975, Mar. 27 ........... 42.06 112.52 Pocatello Valley 8 6.0 18.6 

1

Doser and Smith (1982). 2

Richter (1935, p. 24) estimated an ML value of 7.0. The value of 6.6 comes from Gutenberg and Richter (1954) and appears to be a surface-wave magnitude. 

border form an inverted Y pattern, which began to 

form several months after the March 1975 main shock 

and in which seismicity persists to the present. At the 

southwestern extremity of the Y pattern, scattered 

small earthquakes beneath the northern part of the 

Great Salt Lake previously have appeared to be part 

of a broad northeast-trending belt. The sample in 

figure 9, however, suggests a northwest-trending 

pattern of epicenters in the vicinity of the East Great 

Salt Lake fault. 

3. Densely clustered earthquakes occurring roughly 10 

to 40 km east of the Wasatch fault define a linear belt 

extending southward from about 41 °50' N. to at least 

41 o N. and perhaps as far south as 40° N. Earthquakes 

in the northern part of this belt lie east of the 

west-dipping East Cache and Wasatch faults and 

occur within a volumetric zone beneath the Bear 

River Range, whose western boundary is formed by 

the East Cache fault. To the south of 41 o N., earthquake clusters within this belt to the east of the 

Wasatch fault follow a zone of northerly trending 

valleys within the so-called Wasatch Hinterland (Sullivan and others, 1988). 

4. Clusters of seismicity appear close to or just west of 

the trace of the Wasatch fault-near Honeyville at 

about 41 °40' N., in the vicinity of Salt Lake City at 

about 40°45' N., at the northern end of Utah Valley at 

about 40°20'N., in the vicinity of Goshen Valley at 

about 40°00' N., and in a broadly scattered zone at the 

southern end of the Wasatch fault. We will return to 

these observations when we consider the Wasatch 

fault in greater detail in a later section. 

5. In the lower right-hand part of figure 9, a prominent 

feature of the seismicity of east-central Utah is an 

inverted U-shaped pattern of persistent shallow seismicity that corresponds to underground coal mining 

along the eastern side of the Wasatch Plateau 

(between about 39°15' N. and 39°35' N.) and along the 

arcuate Book Cliffs escarpment (east of about 39°45' 

N., 111°00' W.). The seismicity is well known to be 

mining related and appears to correlate with mining 

areas where annual rates of extraction exceed 500,000 

tons (see summary by Arabasz and Julander, 1986). 

Results of a recent study of mining-related seismicity 

in the eastern Wasatch Plateau have been reported by 

Williams and Arabasz (1989). 

EARTHQUAKE FOCAL DEPTHS 

This section examines seismic network data from the 

Wasatch Front region for focal-depth information. It is 

an unfortunate fact that the computed focal depths of 

most earthquakes located with the regional seismic network are unreliable because of the large station spacings 

of 15 to 35 km in the immediate Wasatch Front area and 

35 to 100 km elsewhere (fig. 2). As a partial solution to 

this problem, temporary arrays of portable seismographs are routinely deployed by the University of Utah 

in selected target areas (element 8, fig. 1) and have 

provided some of the best data for correlating seismicity 

with structure (for example, Arabasz and Julander, 

1986). It remains useful, nonetheless, to evaluate the 

regional network data for focal-depth information. 
D14 

42° 

39° 

37" 

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

0 

114° 113° 

0 0 0 

IDAHO 

0 

0 

0 

Ci 

0 0 

0 0 

0 

0 

~0 0 

0 ~ 0 

Ooo o 0 

0 0 0 

0 00 0 

8 

0 

o. 0 ' 

0 008 

o j~O 

~oo 0 

oo • o 

oOO 

.0 0 ~ 

~0~ 0 

0 

I 

0 

0 ~0 

~:k 0 8 cJ; oo 

0 0 

0 

0 

00 

oo 

Q ~ 

0 0 

~ o4 o>o 

0 ~Q:> 8 

0 

I 

~00 0 

0 

co 

100 MILES 

I 

100 KILOMETERS 

EXPLANATION 

Magnitudes 

11 oo 109° 

0 

0 

0 0 

0 

0 

'\vvoMING 

UTAH 0 0 

OJ 0 

Oooo 

0 

0 

0 

~ 

0 

00 

0 

0 

0 

oO.O+ o2.0+ 03.0+ 04.0+ Os.O+ 

FIGURE 7. -Epicenters of all earthquakes located by the University of Utah Seismograph Stations in the 

Utah region, July 1, 1962, to December 31, 1986. 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D15 

TABLE 2.-Independent main shocks of magnitude (MJ 4.0 or greater in the Utah region, july 1962 through 

December 1986 

[Italics denote earthquakes within the Was~tch Front study area (see table 1 for boundaries of the Utah region and Wasatch Front 

area)] 

Magnitude 

Local date Lat, 0

N. Long, ow. (ML) Location 

1962, Aug. 30 ........... 42.04 111.74 5.7 Cache Valley (Logan)-damage. 

1962, Sept. 5 ............ 40.72 112.09 5.2 Magna-damage. 

1963, July 7 ............. 39.53 111.91 4-4 Juab Valley (Levan/Nephi)-damage. 

1963, July 9 . ............ 40.03 111.19 4.0 Soldier Summit area. 

1963, Sept. -30 ........... 38.10 111.22 4.3 Capitol Reef area. 

1964, Oct. 18 ............ 41.73 111.73 4.1 Cache Valley. 

1966, Mar. 17 ........... 41.66 111.56 4.6 Logan. 

1966, May 20 ............ 37.98 111.85 4.1 Aquarius Plateau area. 

1966, Aug. 16 ........... 37.46 114.15 5.6 Nevada-Utah border. 

1966, Oct. 21 ............ 38.20 113.16 4.2 Escalante Valley. 

1967, Feb. 14 ............ 40.11 109.05 4.0 Utah-Colorado border. 

1967, Feb. 16 ............ 41.27 113.33 4.0 Newfoundland Mountains area. 

1967, Oct. 4 ............. 38.54 112.16 5.2 Marysvale-damage. 

1970, Mar. 29 ............ 41.66 113.84 4.7 Grouse Creek area. 

1970, Apr. 21. ........... 40.06 109.01 4.0 Colorado-Utah border. 

1971, Dec. 3 ............. 42.50 110.34 4.1 Southwestern Wyoming. 

1972, Jan. 3 ............. 38.65 112.17 4.4 Elsinore-damage. 

1972, June 1. ............ 38.67 112.07 4.0 Southwestern Sevier County. 

1972, Oct. 1 ............. 40.51 111.35 4.3 Heber City-damage. 

1973, Apr. 13 ............ 42.04 112.63 4.2 Pocatello Valley. 

1975, Mar. 27 ........... 42.06 112.52 6.0 Pocatello Valley-damage. 

1976, Nov. 4 . ............ 41.81 112.70 4.0 Hansel Valley1

• 

1977, Sept. 30 ........... 40.46 110.48 4.5 Northwestern Duchesne County-damage. 

1978, Nov. 29 . ........... 42.10 112.49 4.6 Pocatello Valley. 

1979, Mar. 19 ............ 40.04 108.86 4.1 Colorado-Utah border. 

1980, May 24 . ........... 39.94 111.96 4.4 Goshen Valley. 

1981, Apr. 4 ............. 37.59 113.30 4.6 Kanarraville. 

1982, May 24 ............ 38.71 112.04 4.0 Sevier Valley (Annabella). 

1983, Oct. 8 ............. 40.75 111.99 4.3 Northern Salt Lake Valley. 

1986, Mar. 24 ........... 39.24 112.01 4.4 Japanese Valley. 

1986, Aug. 22 ........... 37.45 110.53 4.0 Southeastern Utah (near Bullfrog Basin). 

1

ldentified by Shimizu (1987) as a dependent event related to the March 1975 Pocatello Valley earthquake and included here for the 

sake of argument. 

It is commonly assumed that the presence of a recording station within one focal depth of an earthquake's 

epicenter provides good depth control. To test this 

assumption, K.J. Quigley (unpub. report, 1986) performed numerical experiments on synthetic P-wave arrivaltime data for the Wasatch Front seismic network to 

determine statistical criteria for focal-depth reliability. 

On the basis of his results, 485 earthquakes out of 6,416 

from the 197 4-86 UUSS catalog for the Wasatch Front 

area were judged to have reliable focal depths. The 

criteria were as follows: (1) distance to the nearest 

station was less than or equal to the focal depth or 5 km, 

whichever was larger, and (2) standard vertical hypocentral error (ERZ) was 2 km or less, as calculated by 

the location program HYPOELLIPSE (1974-80) or 

HYPOINVERSE (1981-86). Quigley's results verified 

theoretical expectations that there is a 68 percent probability that the computed focal depth for such events is 

within 1. 0 E RZ of the true focal depth and a 98 percent 

probability that it is within 2.4 ERZ. 

Figure 10 shows the best-resolved foci in map and 

cross-section view. The selection criterion of having a 

close station may filter out shallow events in some areas, 

but deeper events will be less affected. In general, the 

east-west cross sections do not appear to be particularly 

informative in terms of significant variations in the 

east-west distribution of maximum focal depths or spatial 

association with the Wasatch fault. The north-south 

cross section, however, suggests that there may be 

variations in maximum earthquake depth along the 

Wasatch Front. For example, there are no earthquakes 

deeper than 17 km in the central part of the cross section, 

whereas there are earthquakes deeper than 17 km and as 

deep as 25 km at the northern and southern ends. We 

caution, however, that the apparent variation in maximum focal depth may be an artifact of sampling because 
D16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

0 

• ( 

• 

, 

• 0 

.' 

I 

~. 0 

• 

.. 39° 

0 0.0+ 

• 

0 

I 

0 

111° 

.I; 

~ ">; .) r"" -~o 

~. I 

' J • 0 

\t~a· • • 0 

• • 0 0 

•• ' 0 0 fjof 0: . • o q; •• 0 0 

\ . oO 

• ~·;&~·· .. "· • ' • " If;-0 : .JL.. • •• • • 

.. . ·~· . r. \0. -~,- • 

. -~·\ ~, :·:I I 

·) l.i . '~ . . . . 40.: j . c: .0. 0 •• 0. • 

C:tYo• 

• , • ''"' 8· • o Cfi~O 

• • ~ 0 • 

• o'i11 • oo. 

• • i &• 0 •• • 

.. .,tt ... 

I 

• • 

50 MILES 

I 

50 KILOMETERS 

EXPLANATION 

Magnitudes 

• 

0 2.0+ 0 3.0+ 0 4.0+ 0 5.0+ 0 6.0+ 

FIGURE 8.-Epicenters of all earthquakes located by the University of Utah Seismograph 

Stations in the Wasatch Front area, October 1, 1974, to June 30, 1978. Base map is as shown in figure 5. 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK 

113° 112° 111° 

•t_' 0 o; G 

~,.: 0 0 

~ l;o,,:l 0 0 0 

. ·~t· ' 00 

~ o,.. ,p 0 

0 

"-~ • 0 

0 

0 

•• .... '' -~ .. ~ ' 0 

' • , .. o .o 0 .. • ,. c. J 

\ i 0

,) Jo .. r. r ( ' • • ' 0 ( . . 0 t J.. ·.J • ~ 0 0 0 

0 0 •t • o• ···~ • 

. •d•. • 

.' :,; til'. .,. • ~~ o.. • 0 0 .0 

~ 

!. ~~. ~~·= ~,. . • ~- ,. • • ":Of. • 0 

• ~ ... 0 • ., . i • •• • • ' •• 0 00 

\ • •••o .o• • .. , 0 • ~--:• • 0 • 

0 50 MILES 

I I 

I 

0 50 KILOMETERS 

EXPLANATION 

0 0.0+ 0 2.0+ 

Magnitudes 

0 3.0+ 04.0+ 0 5.0+ 0 6.0+ 

FIGURE 9. -Epicenters of all earthquakes located by the University of Utah Seismograph 

Stations in the Wasatch Front area, July 1, 1978, to December 31, 1986. Base map is as 

shown in figure 5. 

D17 
D18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

NORTH 

I 

42° 

0 

0 

Oo 

§r 

cti 0 

oCD 

I 

41° 

0 0 

0 

Wasatch 

fault 

0 

g~ 0 0 

40° 15' 

39°45' 

39°15' 

0 

40° 

FIGURE 10.-Epicenters (right) and corresponding vertical sections 

(left) of earthquakes in the Wasatch Front area located with 

reliable focal depths, October 1974 through December 1986. Sample includes only earthquakes located with a standard vertical 

there are relatively few events in the central and southern parts of the section. The larger any sample of 

earthquakes, the more likely it is to contain extreme 

values of focal depths. To evaluate the influence of 

north-south variations in the number of earthquakes 

sampled on the observed maximum focal depths, we 

determined, for various latitude bands, the focal depth 

above which 90 percent of the earthquakes lie. These 

90th-percentile depths are as follows: 17 km between 

I 

39° 

41° 

40° 

39° 

113° 

0 

I 

0 

112° 

01 

0 ~ \ 

0 0 

I 

111° 

' 0 

0 0 <:9 

50 MILES 

I 

0 

50 KILOMETERS 

0 

EXPLANATION 

Magnitudes 

0 0.0+ 

0 2.0+ 

0 3.0+ 

0 4.0+ 

Qs.o+ 

Q6.0+ 

hypocentral error of 2 km or less and with the distance to the 

nearest recording station being less than or equal to the focal depth 

or 5 km, whichever is larger. Each vertical section includes foci 

whose epicenters lie within 15' latitude of the line of section. 

38°55' N. and 40° N. latitude, 13 km between 40° N. and 

41° N. latitude, 15 km between 41° N. and 42° N. 

latitude, and 11 km from 42° N. to 42°30' N. latitude 

(where the sampling is dominated by aftershocks of the 

ML 6.0 Pocatello Valley earthquake of 1975). These 

results suggest the possibility of variations in maximum 

focal depth on the order of 2 to 6 km. 

Maximum earthquake focal depths bear directly on 

crustal rheology, the depth of nucleation of large earth-
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D19 

quakes, and maximum fault-rupture dimensions. Accordingly, the north-south variations of maximum focal depth 

suggested in figure 10 should be investigated further. 

Figure 10 emphasizes the sparseness of threedimensional information available from the thousands of 

earthquakes located by the permanent regional network. 

Consequently, only limited information is available to 

address the association of seismicity with subsurface 

structure, as we discuss next. 

PROBLEMATIC CORRELATION OF SEISMICITY WITH 

GEOLOGIC STRUCTURE 

Fundamental problems in correlating diffuse seismicity with mapped Cenozoic faulting and subsurface geologic structure in the Utah region have been discussed at 

length by Arabasz (1984) and by Arabasz and Julander 

(1986). These discussions are repeated here because of 

their continued relevance. Problems include (1) uncertain subsurface structure, which typically is more complex along the main seismic belt than surface geology 

would indicate, (2) observations of discordance between 

surface fault patterns and seismic fault slip at depth 

(Arabasz and others, 1981; Zoback, 1983), (3) a paucity of 

historical surface faulting, and (4) inadequate focal-depth 

resolution from regional seismic monitoring. 

Crustal structure along the eastern Great Basin is 

known to involve vertically stacked plates separated by 

low-angle detachments resulting from relict preNeogene thrust-belt structure and (or) Neogene extension (Allmendinger and others, 1983; Smith and Bruhn, 

1984; Smith and others, 1987). Smith and Bruhn (1984) 

presented a summary of seismic reflection data that 

indicate the widespread presence of low-angle and 

downward-flattening faults in the subsurface and an 

intimate relationship between pre-Neogene thrust-belt 

structure and young normal faults along the eastern 

Basin and Range margin. Seismologic evidence to date 

indicates that, at least for small to moderate earthquakes, seismic slip in this region predominates on faults 

having moderate (~30°) to high-angle dip (Zoback, 1983; 

Arabasz and Julander, 1986; Bjarnason and Pechmann, 

1989). 

Given the relatively high magnitude threshold of surface faulting and observations of discordance between 

surface-fault patterns and seismic slip at depth, one can 

argue that-with the sole exception of the 1934 Hansel 

Valley earthquake-no other of Utah's 15 historical 

earthquakes of ML 5.5 or greater (table 1) can be 

confidently associated with a mapped surface fault. 

Within the domain of Utah's main seismic belt, future 

seismicity below the threshold of surface faulting (M L 

6.3±0.2) thus cannot be confidently precluded by knowledge of the surface geology alone. Where subsurface 

Base of 

seismogenic layer 

Surface 

fault scarp 

~ 

FIGURE 11. -Schematic geologic cross section illustrating the complex 

association of seismicity in the upper crust with geological structure 

in the Intermountain Seismic Belt (from Arabasz, 1984; Arabasz and 

Julander, 1986). Starbursts indicate foci of moderate to large earthquakes; solid circles, microseismicity; lines in subsurface, faults. 

Arrows indicate sense of slip on faults; two-directional arrows show 

extensional backsliding on preexisting low-angle faults possibly 

formed as thrust faults. The base of the seismogenic layer is at a 

depth of approximately 10 to 15 km. Lowercase letters identify 

aspects (not exhaustive) of observations and a working hypothesis 

relating seismicity to structure: a, local predominance of seismicity 

within a lower plate; b, nucleation of a large normal-faulting earthquake near the base of the seismogenic layer, hypothetically on an 

old thrust ramp, with linkage or an established rupture pathway to a 

major surface fault; c, occurrence of a moderate-sized earthquake 

within a lower plate, without linkage to a shallow structure; d, 

occurrence of a moderate-sized earthquake and aftershocks on a 

secondary fault where an underlying detachment restricts deformation to the upper plate; e, diffuse block-interior microseismicity 

predominating within an upper plate, perhaps responding to extension enhanced by gravitational backsliding on an underlying detachment; f, diffuse block-interior microseismicity within a lower plate 

where frequency of occurrence is markedly lower than it is in the 

overlying plate. 

structure is complex, moderate-sized earthquakes may 

occur on blind subsurface structures that have no direct 

surface expression. 

On the basis of special earthquake studies in the 

southern Wasatch Front area, neighboring parts of central Utah, and southeastern Idaho, the following working 

hypothesis was offered by Arabasz (1984) (see also 

Arabasz and Julander, 1986) to explain observations of 

diffuse background seismicity. Background seismicity, it 

was suggested, is fundamentally controlled by variable 

mechanical behavior and the internal structure of individual horizontal plates within the seismogenic upper 

crust. Diffuse epicentral patterns may then result from 

the superposition of seismicity occurring within individual plates and also perhaps from favorable conditions for 

block-interior rather than block-boundary microseismic 

slip. Figure 11 schematically shows some aspects of this 

working hypothesis. 
D20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

SEISMIC SOURCE ZONES AND SEISMICITY 

PARAMETERS 

GENERAL REMARKS 

Two key steps in any analysis of earthquake hazards or 

risks are (1) the identification and geometric depiction of 

seismic source zones within which earthquakes are likely 

to originate (step 3, fig. 1) and (2) the estimation of 

seismicity parameters for these source zones (step 4, fig. 

1). The important seismicity parameters are the maximum magnitude, the expected earthquake distribution 

as a function of size, and the rate of activity. Both 

earthquake catalog data and geologic data on active 

faults can be used to identify and characterize seismic 

source zones. 

Our current understanding of the seismotectonic 

framework of the Wasatch Front region, as summarized 

above, leads to the position that seismic hazards arise 

from two fundamental classes of earthquakes: (1) infrequent, large (Ms~6.3±0.2) surface-faulting earthquakes 

on identifiable faults showing evidence of late Quaternary displacement and (2) small- to moderate-sized (up to 

ML 6.5) earthquakes, below the threshold of surface 

faulting, that are not constrained in location to mapped 

faults and may occur randomly in space throughout 

broadly defined regions. This position is a consequence of 

detailed studies that show clear evidence for large prehistoric earthquakes on faults in the region (for example, 

Swan and others, 1980; Schwartz and others, 1984; 

Machette and others, 1991, this volume), the diffuse 

scatter of small- to moderate-sized earthquakes (illustrated in figs. 7, 8, 9), and the problematic correlation of 

seismicity and geologic structure discussed above. It 

therefore seems reasonable to distinguish two different 

types of seismic source zones: (1) fault-specific sources 

for which the evidence is primarily geologic and (2) areal 

source zones that are based on the historical and instrumental earthquake record and have a maximum magnitude of 6.5. These two types of source zones have been 

used in several studies of seismic hazard in the Wasatch 

Front region, including the elaborate probabilistic analysis of the ground-shaking hazard by Youngs and others 

(in press), a study of the ''Wasatch Hinterland" by 

Sullivan and others (1988), and a study by Arabasz and 

others (1987) of the region surrounding the sites that 

Utah proposed for the Superconducting Supercollider, 

which are located southwest of the Great Salt Lake along 

the western boundary of the Wasatch Front region. 

In this section, we treat the entire Wasatch Front 

study area as one areal source zone and use the instrumental earthquake record to model the recurrence of 

earthquakes within the area of up to magnitude 6.5. 

Although the available data suggest that seismicity rates 

may vary somewhat within this region, there are not 

enough earthquakes in the record to reliably estimate 

seismicity parameters for subsets of this region. Furthermore, the 25-year record of instrumental seismicity 

is simply too short to allow extrapolation into the future 

of spatial variations of the seismicity rate within the 

Wasatch Front area. We first present results from 

recurrence modeling for the Wasatch Front area and 

then briefly discuss the fault-specific sources in this 

region and the maximum earthquake size that can be 

expected to occur on them. 

RECURRENCE MODELING 

Earthquake catalog data provide the only means of 

determining the rate of occurrence and the size distribution of earthquakes smaller than the threshold of surface 

faulting. A standard way to characterize seismicity in 

any seismically active region is with the GutenbergRichter exponential frequency-magnitude relationship, 

given by 

where N is the average number of independent events 

per year of magnitude M or greater and a and b are 

constants appropriate for the particular region. The a 

value is a measure of the overall rate of earthquake 

activity. The b value is a measure of the relative proportion of small events to large events; higher b values 

indicate relatively more small events. If we define A as 

the average number of events per year ofM~3.0 (that is, 

A= 10a-3

.

0 b), then the above equation can be rewritten as 

N = A 10-b(M-3.0) 

To obtain meaningful estimates for the seismicity 

parameters a (or A) and b from earthquake catalog data, 

it is first necessary to remove from the catalog all 

dependent events such as aftershocks, foreshocks, and 

secondary events in swarm sequences. Shimizu (1987) 

identified dependent and independent events in the 

University of Utah catalog for the period July 1962 

through December 1985 by applying the local clustering 

method of Veneziano and Van Dyck (1985, 1986). This 

method uses statistical tests to identify earthquake clusters, which Veneziano and Van Dyck defined as spacetime windows in which the rate of earthquake activity is 

significantly greater than the estimated local background 

rate. One or more of the earthquakes within each cluster 

are then classified as main shocks, and the rest are 

classified as dependent events. This procedure is 

repeated until no additional events are identified as 

dependent. The primary advantage of the local clustering 

method in comparison with other methods of identifying 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D21 

dependent events is that it allows for variations in the 

spatial and temporal extents of clusters of dependent 

events associated with different main shocks of the same 

magnitude. 

Shimizu's listing of independent events of ML2::2.0 in 

the catalog for this period was supplied to us on computer 

tape by D. Veneziano. This listing contained 571 main 

shocks within the study area. The epicenters of these 

events are plotted in figure 12. For purposes of recurrence modeling, we eliminated the 126 events in the 

study area south of 39°50' N. and east of 111°15' W. 

(dashed box, fig. 12), where the seismicity is predominantly mining related. This elimination left 445 independent main shocks of 2.0~ML~6.0 in the remaining 85,000 

km2 of the study area. These main shocks form the basic 

data set for our recurrence modeling of the Wasatch 

Front area. 

Figure 13 is a cumulative recurrence plot (log N versus 

M) for the 445 independent main shocks. The observed 

number ofML2::2.0 events per year is equal to or greater 

than the number expected from a linear extrapolation of 

the data for the larger magnitude cutoffs. In fact, the 

recurrence curve formed by connecting the data points in 

figure 13 even appears to have a steeper slope for 

ML~3.0. This observation suggests that the University 

of Utah catalog contains a reasonably complete listing of 

main shocks of ML2::2.0 in the Wasatch Front region 

after July 1962. However, the precise threshold magnitude of completeness for the catalog varies with time and 

with location within the region, depending on station 

coverage (fig. 2). 

The apparent change in slope of the recurrence curve 

near magnitude 3 may be an artifact of the way that 

magnitudes are assigned in the University of Utah 

catalog. The catalog magnitudes for most earthquakes in 

the Wasatch Front region over magnitude 3 are true 

local magnitudes calculated from peak amplitudes measured on seismograms from low-gain Wood-Andersontype instruments. The magnitudes listed for most of the 

smaller earthquakes are coda magnitudes calculated 

from signal durations measured on records from various 

types of higher gain instruments (for details, see Arabasz 

and others, 1979; Richins and others, 1981, 1984; Brown 

and others, 1986). The coda magnitude scales were 

calibrated against Wood-Anderson ML measurements 

and should therefore give compatible results. N evertheless, there may be some small, systematic differences 

between the two types of estimates. Research on refined 

methods of magnitude determination using digital waveform data is currently under way at the University of 

Utah. 

The heavy line in figure 13 shows a straight-line fit to 

the recurrence data above ML 3.0, determined by using 

the maximum-likelihood technique of Weichert (1980) 

and an assumed maximum magnitude of7.5. The best-fit 

line has a slope of b=0.71±0.09. The lighter lines are 

drawn with lower bound and upper bound slopes of 0. 62 

and 0.80, respectively. All three lines are constrained to 

pass through the data point for the minimum magnitude 

of 3.0. The lighter lines appear to bound the range of 

possible slopes quite well if a magnitude cutoff of 3. 0 is 

used. However, the b value is unfortunately very sensitive to the cutoff magnitude. Cutoff magnitudes of 2.0 

and 2.5 give b values of 0.82±0.04 and 0.88±0.07, 

respectively. We chose the cutoff magnitude of 3.0 

because of possible problems with the smaller magnitudes discussed above and because the lines calculated by 

using this cutoff provide a better fit to the data for the 

larger earthquakes, which are of primary concern for 

seismic hazards. 

The recurrence relationship given by our preferred 

maximum likelihood fit to the data shown in figure 13 is 

N = (2.51)10-(0.71±0.09)(ML -3.0) 

or, alternatively, 

log N = 2.53-0.71 ML 

Table 3 lists average recurrence intervals for earthquakes in the Wasatch Front region calculated from the 

first of these equations. 

As a check on table 3, consider the number of earthquakes of estimated magnitude 5.5 or greater in the 

Wasatch Front region from 1850 to 1987. The occurrence 

of eight such earthquakes (table 1, fig. 6) gives an 

average recurrence interval of 17 years. This average 

falls within the lower end of our estimated range of 

recurrence intervals for ML;:::5.5 earthquakes, which is 

14 to 40 years. If, following Arabasz and others (1980), 

we assume that the catalog is complete at the ML;:::5.5 

level only since 1878, the average recurrence interval 

shortens to 14 years. The interevent times for the eight 

ML2::5.5 earthquakes range from 0.6 to 28.5 years, the 

mean being 12.9 years. 

The recurrence intervals calculated for earthquakes 

greater than magnitude 6 represent an extrapolation 

beyond the limits of the data shown in figure 13. Nevertheless, it is interesting to compare our predicted recurrence interval for ML2::7.0 events with estimates of the 

recurrence interval for ML2::7.0 events on the Wasatch 

fault, given that the Wasatch fault is the dominant source 

of earthquakes of magnitude 7.0 and greater in the 

region. The average recurrence interval for ML2::7.0 

surface-faulting earthquakes on the Wasatch fault has 

been estimated to be 400 to 666 years by Schwartz and 

Coppersmith (1984), 330±90 years by Youngs and others 

(in press), and 220 to 395 years by Machette and others 
D22 

113° 

0 

0 

I 

f 

~. 0 

39° .. \ 

0 0.0+ 

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

0 

I 

0 

112° 111° 

I 

0 

0 

50 MILES 

I 

50 KILOMETERS 

EXPLANATION 

Magnitudes 

0 

0 0 

0 

0 

0 

0 2.0+ 03.0+ 0 4.0+ 0 5.0+ 0 6.0+ 

FIGURE 12.-Epicenters of independent main 

shocks in the Wasatch Front area from July 1, 

1962, through December 31, 1985, plotted on the 

map of late Quaternary faulting shown in figure 5. 

The dashed box in the lower right-hand corner 

indicates an area of mining-induced seismicity that 

was excluded from the recurrence analysis. 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D23 

100 

(/) 

w 

~ 

<( 

:::> 

0 10 

I 

1-

a: • 

<( • w 

LL.a;: 1.0 

0<( 

a:w 

w>-

ala: 

~w 

::JCL 0.1 

z 

w ......... > 

t= ......... 

<( 0.01 ', -.......0 ...J "'- '6""-o ·6< :::> 

~ "o ·>~. ', :::> ·c9o ' u " ', 

0.001 " 2.0 3.0 4.0 5.0 6.0 7.0 

MAGNITUDE, ML 

FIGURE 13.-Recurrence data for independent main shocks in the 

Wasatch Front area from July 1962 through December 1985. Solid 

circles show the cumulative number of earthquakes per year greater 

than or equal to the local magnitude (ML), given on the horizontal 

axis. The circles are spaced 0.1 magnitude unit apart. The heavy line 

through the circles for ML:2:3.0 indicates the preferred maximumlikelihood fit having a slope b of 0. 71, calculated by using the method 

of Weichert (1980). The lighter lines indicate the lower and upper 

bound slopes of 0.62 and 0.80, respectively. The area used for the 

recurrence modeling is 85,000 km2 (area shown in fig. 12, minus the 

dashed box). 

(1991). These estimates are consistent with our predicted 

average recurrence interval of 120 to 630 years (the 

preferred value being 280 years) for ML2:7.0 events in 

the Wasatch Front region as a whole. 

Arabasz and others (1980) modeled earthquake recurrence for the Wasatch Front region by using independent 

main shocks from the University of Utah catalog of 

Modified Mercalli intensity V or greater (approximately 

ML2:4) from 1850 through 1978. They obtained the 

equation 

log N = 2.98- 0.72ML 

Their b value of 0. 72 is essentially the same as our b value 

of 0. 71±0.09. However, their a value of 2.98 is larger 

than our value of 2. 53. This difference corresponds to a 

difference in seismicity rates of a factor of 2.8. Two 

possible explanations for this disparity are that (1) the 

seismicity from 1962 through 1985 was low in comparison 

with the seismicity from 1850 through 1961 or that (2) the 

magnitudes of the pre-1962 events were overestimated 

by about 0.6 magnitude unit by Arabasz and others 

(1980). 

Our preferred b value of 0. 71 is lower than the b values 

obtained in most previous studies of earthquake recurTABLE 3.-Average recurrence intervals for earthquakes in the Wasatch Front region 

[38°55' N.-42°30' N., ll0°25'W.-l13°10' W., excluding miningrelated seismicity in the southeastern corner of the area. 

Area equals 85,000 km2

] 

Average recurrence interval (years) 

Preferred Range of 

Magnitude range estimate1 estimates2 

ML:2:3.0 ................ . 

ML:2:3.5 ................ . 

ML:2:4.0 ................ . 

ML:2:4.5 ................ . 

ML:2:5.0 ................ . 

ML:2:5.5 ................ . 

ML2:6.0 ................ . 

ML:2:6.5 ................ . 

ML:2:7.0 ................ . 

0.40 

.90 

2.0 

4.6 

10 

24 

54 

120 

280 

3.0~ML~3.5............ 71 

3.5~ML~4.0 .. .. .. .. .. .. 1.6 

4.0~ML~4.5 . . . . . . . . . . . . 3. 7 

4.5~ML~5.0 .. .. .. .. . .. . 8.3 

5.0~ML~5.5 . . . . . . . . . . . . 19 

5.5~ML~6.0 . . . . . . . . . . . . 42 

6.0~ML~6.5 .. .. .. .. .. .. 96 

6.5~ML~7.0 . . . . . . . . . . . . 220 

7.0~ML~7.5 .. . .. .. .. .. . 490 

1

Calculated by using a b value of 0. 71. 

0.81-1.0 

1. 7-2.5 

3.4-6.3 

7-16 

14-40 

29-100 

60-250 

120-630 

.66-.78 

1.6-1.7 

3.3-4.2 

6.6-10 

14-26 

28-66 

57-170 

120-420 

240-1,000 

2

Lower and upper limits calculated by using the limiting b 

values of 0.62 and 0.80, respectively. 

renee along the Wasatch Front that made major or 

exclusive use of the post-1962 instrumental University of 

Utah catalog. Arabasz and others (1980) determined a b 

value of 0.95±0.15 by using all magnitude 2.3 and 

greater earthquakes in the catalog from July 1962 

through June 1978 within a subarea of the Wasatch Front 

region between 111°15' W. and 112°15' W. Youngs and 

others (in press) obtained b values ranging from 

0. 75±0.03 to 0.83±0.03 (depending on the criteria used 

to remove dependent events) by using catalog data from 

1850 through March 1986 above the uniform detection 

thresholds that they had estimated. The north-south 

extent of the study area of Youngs and his co-workers is 

from 39° N. to 42°30' N., nearly the same as that of our 

study area, but their area has a narrower east-west 

extent. Finally, Shimizu (1987) obtained a b value of 1.1 

by using independent main shocks from July 1962 

through December 1985 for a region extending from 39° 

N. to 42.5° N. and 108°45' W. to 114°15' W. (excluding 

mining-related seismicity in east-central Utah), which he 

calls the "Central-Northern Utah" region. 

The higher b value obtained by Arabasz and others 

(1980) for the post-1962 instrumental catalog can be 

attributed to the fact that they did not remove dependent 

events from the catalog before calculating their b value. 

The differences between the b values of 0. 75 to 0.83 

determined by Youngs and others (in press) and our 
D24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

value of 0. 71 appear to result primarily from differences 

in the methods employed to remove dependent events. 

The higher b value of 1.1 obtained by Shimizu (1987) 

cannot be explained in this way, since we used his listing 

of independent events for our recurrence modeling. The 

higher b value obtained by Shimizu results from his use 

of a minimum magnitude of 2.5 in his recurrence analysis 

and, to a lesser extent, from his use of the maximumlikelihood method of Utsu (1965) to fit a straight line to 

the data. In the method of Utsu (1965), the b value is 

calculated from the equation 

where M is the mean magnitude of all events of magnitude greater than or equal to M0 • The b values given by 

Utsu's method tend to be higher than those given by the 

more elaborate method of Weichert (1980), which was 

used both by us and by Youngs and others (in press). 

Applying Utsu's method to the data in figure 13 for 

ML~2.5 gives a b value of 0.98, which is in reasonable 

agreement with Shimizu's value, considering that his 

calculation included data from a larger area. 

FAULT-SPECIFIC SOURCES 

Although the 1934 Ms 6. 6 Hansel Valley earthquake is 

the largest historical earthquake to have occurred in the 

Wasatch Front region, there is good geologic evidence 

that larger earthquakes can be expected to occur on the 

Wasatch fault and other active faults in the region. All of 

the faults showing evidence of late Quaternary (less than 

500,000 years) movement (fig. 5) are considered to be 

potential sources of earthquakes larger than ML 6.3±0.2, 

the likely threshold for surface faulting. Maximum earthquake magnitudes for these fault-specific sources can be 

assessed from estimates of the rupture length, area, and 

displacement for the maximum size event. Average 

recurrence intervals for surface-faulting events can, at 

least in some cases, be determined from geomorphic 

observations or from the stratigraphy of deposits near 

the fault trace exposed by trenching (see Allen (1986) and 

Schwartz and Coppersmith (1986) for reviews of these 

techniques). A detailed description of all the faultspecific sources shown in figure 6 is beyond the scope of 

this paper. We refer the reader to Chapter M of this 

report (Youngs and others, in press) for a thorough 

summary of available information on the major faults in 

the Wasatch Front region and a determination of seismicity parameters for these faults. Here we present 

information on four fault-specific sources that we use 

below in some probabilistic hazard calculations, consider 

the maximum probable earthquake for the whole 

Wasatch Front area, and briefly discuss the issue of 

frequency-magnitude relationships for individual faults. 

Table 4 presents basic information for the four most 

prominent fault-specific sources near Salt Lake City: the 

Wasatch fault, the West Valley fault zone, the East 

Great Salt Lake fault, and the North Oquirrh Mountains 

fault (fig. 5). The faulting parameters for the Wasatch 

fault are taken from segmentation model B of Youngs 

and others (in press). (Minor differences with subsequent 

segmentation models-for example, that of Machette 

and others (1991)-are unimportant for our purposes 

here.) The maximum magnitude listed for each segment 

is a mean estimate calculated from the set of possible 

values and their assigned weights listed in table 1 of 

Youngs and others (in press). The recurrence intervals 

for maximum earthquakes are mean estimates taken 

directly from table 3 of Youngs and others (in press). For 

the West Valley fault zone, the maximum magnitude of 

6.5 is also a mean value calculated from table 1 of Youngs 

and others (in press). The average recurrence interval of 

2,000 years is inferred from the observation of six to 

seven discrete faulting events on the fault zone within 

the last 13,000 years (S.J. Olig, written commun., 1987). 

The segmentation of the East Great Salt Lake fault is 

very uncertain, but it appears to be divided into at least 

two segments, each about 50 km long (fig. 5) (Pechmann 

and others, 1987). The maximum magnitude of 7.2 

assigned to each segment is based on a simple analogy 

with the Weber and Salt Lake City segments of the 

Wasatch fault, which are of comparable length. The 

average recurrence interval for maximum earthquakes 

on each segment of the East Great Salt Lake fault is 

judged to be about 4,000 years, on the basis of the fact 

that the observed slip rates are about half of those 

measured for the central segments of the Wasatch fault, 

which have recurrence intervals of about 2,000 years 

(Pechmann and others, 1987). For the North Oquirrh 

Mountains fault, the maximum magnitude of 7.0 is from 

table 1 of Youngs and others (in press). The average 

recurrence interval of about 10,000 years or more is 

inferred from the observation that the most recent 

surface-faulting event on this fault occurred between 

8,000 and 13,500 years ago (Youngs and others, in press), 

together with generic arguments made by Arabasz and 

others (1987). For all of the faults in table 4, the annual 

probability of the maximum earthquake is taken to be the 

inverse of the average time interval between maximum 

earthquakes, since, in most cases, the time elapsed since 

the last large earthquake is poorly known. 

Estimates of the maximum earthquake size on any 

particular fault are subject to the uncertainties inherent 

in both the prediction of future rupture characteristics 

and the conversion of those rupture characteristics to 

magnitude estimates. For this reason, it is useful to 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D25 

TABLE 4.-Information for selected fault-specific sources 

Approximate length Maximum magnitude 

Mimimum distance 

to site1 

Interval for 

maximum earthquake 

(years) 

Annual probability of 

maximum earthquake 

(104 Fault source (km) (M /year) 8 ) (km) 

Wasatch fault segments:2 •••••••••••••••• 

Collinston ......................... . 60 7.3 110 

70 

15 

8,800 

7,250 

1,350 

2,550 

1,900 

1,900 

2,050 

1,750 

7,100 

1.1 

1.4 

7.4 

3.9 

5.3 

5.3 

4.9 

5.7 

1.4 

1.0 

5.0 

Brigham City ...................... . 40 7.1 

Weber ............................ . 57 7.2 

Salt Lake City ..................... . 45 7.2 0 

25 

40 

60 

80 

American Fork .................... . 18 6.8 

Provo ............................. . 19 6.8 

Spanish Fork ...................... . 32 7.0 

Nephi ............................ . 40 7.1 

Levan ............................ . 35 7.1 120 

155 

0 

Fayette ........................... . 14 6.6 10,000 

West Valley fault zone 2,000 3 ••••••••••.••••••• 

East Great Salt Lake fault segments:4 •••• 

18 6.5 

Promontory ......................... . 50 7.2 70 

20 

30 

4,000 

4,000 

10,000 

2.5 

2.5 

1.0 

Antelope Island ...................... . 50 7.2 

Northern Oquirrh Mountains fault5 .••••.• 35 7.0 

1

The site used for the calculation of the hazard curves is the intersection ofl-15 

and l-80 in South Salt Lake, at approximately 40°43.1' N., 111°54.2' W. 

(diamond, fig. 5). 2

Youngs and others (in press), Machette and others (1986). 

consider, from a historical point of view, the maximum 

earthquake size that is likely to occur anywhere within 

the Wasatch Front region. 

The 1959 Ms 7.5 Hebgen Lake earthquake is considered by some to represent the maximum earthquake for 

the Intermountain Seismic Belt (for example, Doser, 

1985a). However, larger earthquakes than this have 

occurred in the western Basin and Range province. 

Slemmons (1980) listed 13 historical surface-faulting 

earthquakes in the western Great Basin, including two 

events having magnitudes greater than 7.5: the March 

26, 1872, Owens Valley, Calif., earthquake of estimated 

magnitude 8.0 and the October 3, 1915, Pleasant Valley, 

Nev., earthquake of magnitude 7. 75. Slemmons (1980) 

listed a rupture length of 110 km and a maximum 

displacement of 6.44 m for the Owens Valley event and a 

rupture length of 62 km and a maximum displacement of 

5. 6 m for the Pleasant Valley event. Thus, both of these 

earthquakes have a maximum displacement comparable 

to that of the Hebgen Lake event but significantly longer 

rupture lengths. The magnitude of 7. 75 for the Pleasant 

Valley event is from Gutenberg and Richter (1954) and is 

nearly identical to the surface-wave magnitude of 7. 7 

calculated by Abe (1981). The magnitude of the Owens 

Valley event is more controversial, owing to the lack of 

seismographic instruments at the time. Magnitude estimates for this earthquake, based on felt reports, range 

from 8.3 (Oakeshott and others, 1972) to 7.2 (Evernden, 

1975). From the surface faulting, Beanland and Clark 

(1987) estimated a moment magnitude of 7.5 to 7. 7. 

3

Young and others (in press), S.J. Olig (written commun., 1987). 4

Pechmann and others (1987), Viveiros (1986). 5

Youngs and others (in press), T. Barnhard (personal commun., 1987). 

The slip that occurred during the 1872 Owens Valley 

earthquake had a large strike-slip component to it (Richter, 1958; Oakeshott and others, 1972) and may even 

have been dominantly strike slip (Beanland and Clark, 

1987). Faults in the Wasatch Front region show predominantly normal slip, although there is abundant evidence 

of strike-slip faulting in south-central Utah (Anderson 

and Barnhard, 1984, this volume). Thus, the Pleasant 

Valley and Hebgen Lake earthquakes may be better 

models for the maximum credible Wasatch Front earthquake than the Owens Valley earthquake. The characteristics of the Pleasant Valley and Hebgen Lake earthquakes suggest that future Wasatch Front earthquakes 

could have surface-wave magnitudes of up to 7.5 to 7. 7, 

rupture lengths of up to 35 to 65 km, and maximum 

vertical displacements of up to about 6 m. 

One interesting question concerning fault-specific 

sources is the extent to which they act as sources for 

small- to moderate-sized earthquakes, excluding foreshocks and aftershocks of large events. The GutenbergRichter exponential frequency-magnitude relationship 

usually fits the size distribution of earthquakes over 

large regions quite well. However, in general, this 

relationship does not apply to individual faults (W esnousky and others, 1983; Singh and others, 1983; Youngs 

and Coppersmith, 1985). Youngs and others (in press) 

modeled the magnitude distribution of independent 

earthquakes on individual faults in the Wasatch Front 

region in two different ways: (1) by using a standard 

exponential magnitude distribution and (2) by using the 
D26 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

"characteristic" magnitude distribution proposed by 

Youngs and Coppersmith (1985). In both cases, only 

earthquakes of ML2:::6.0 were included in their analysis. 

The characteristic distribution is a modification of the 

exponential distribution that incorporates a decrease in b 

value at magnitudes approaching the maximum magnitude for the fault. Thus, the characteristic magnitude 

distribution includes relatively more larger earthquakes. 

A third possible model to consider is the "maximum 

magnitude" model, which holds that each fault or fault 

segment produces only maximum-size earthquakes, 

together with their associated aftershocks and possible 

foreshocks (Wesnousky and others, 1983; Wesnousky, 

1986). In tectonic regions where the repeat time for large 

earthquakes is relatively short, the available data favor 

either the characteristic or the maximum magnitude 

model. In the Wasatch Front region, major faults such as 

the Wasatch fault have not been important sources of 

small to moderate background earthquakes over the 

25-year record of instrumental seismicity. This observation tends to favor the characteristic and maximum 

magnitude earthquake models, but the period of observation is too short relative to the repeat times for large 

earthquakes to allow any definite conclusions to be 

drawn. 

GROUND-SHAKING HAZARD 

The final step in the seismic hazard analysis outlined in 

figure 1 is the estimation of ground motion for engineering applications. Predictions of ground motions from 

future earthquakes can be arrived at in many different 

ways and presented in a variety of different forms. One 

especially popular approach to the problem is the probabilistic methodology for estimating peak ground 

motions first outlined by Cornell (1968). Algermissen and 

others (1982) used this basic methodology to derive maps 

of probabilistic ground-motion estimates for the contiguous United States, including Utah. Youngs and others 

(in press) used a similar methodology to carry out a 

detailed probabilistic analysis of the ground-shaking hazard for the Wasatch Front region. In this section, we use 

Cornell's method to perform some simple probabilistic 

calculations of maximum ground acceleration for a representative site in the Salt Lake Valley, halfway 

between the Wasatch and West Valley faults. Our intent 

here is not to duplicate the extensive work of Youngs and 

others (in press) but rather to illustrate the relative 

contributions of various seismic sources to the groundshaking hazard. In particular, we investigate the relative 

importance to seismic hazard of small to moderate 

(ML:s;6.5) earthquakes, which dominate the historical 

earthquake record. 

The site that we chose for our calculation is the 

intersection of Interstates 15 and 80 in the city of South 

Salt Lake at approximately 40°43.1' N., 111°54.2' W. 

(diamond, fig. 5). Our calculation incorporated two fundamental types of earthquake sources, as discussed 

above: (1) small to moderate (3.0:s;ML:s;6.5) earthquakes 

within a circular source area having a 100-km radius 

centered on the site and (2) the fault-specific sources 

listed in table 4, which have maximum magnitudes of 6.5 

to 7.3. Although we have not included all of the possible 

earthquake sources in the Wasatch Front region in our 

calculations, we believe that we have included all of the 

major contributors to the ground-shaking hazard in the 

Salt Lake Valley. Earthquakes within the circular source 

area are assumed to be independent in size and location, 

to have a spatially uniform probability of occurrence, and 

to have an exponential magnitude distribution. We used 

the seismicity parameters estimated in the section on 

recurrence modeling to characterize this source area. 

For the fault-specific sources, only the contribution of 

the maximum earthquake to the hazard was included in 

the computation. For these maximum earthquakes, the 

contribution to the annual probability of exceedance of a 

given peak acceleration value is the product of two 

factors: (1) the annual probability of occurrence of the 

earthquake and (2) the probability that the given peak 

acceleration level will be exceeded if an earthquake of the 

specified magnitude and distance occurs. 

Peak horizontal acceleration as a function of magnitude 

and distance was calculated by using the constrained 

relationship of Campbell (in press). Campbell actually 

presented a range of median acceleration estimates to 

reflect uncertainties in how stress regime, fault type, 

anelastic attenuation, and local site conditions could 

affect ground motion in Utah. We performed our calculations by using both his upper bound curves and his 

lower bound curves, which differ in their predictions of 

peak horizontal accelerations by about a factor of 2. In 

both cases, the natural log of the peak acceleration was 

assumed to be normally distributed about the predicted 

median value, the standard deviation being 0.3 (the 

standard error given by Campbell). 

Some assumption about the depths of the earthquakes 

was necessary, since Campbell defines source-to-site 

distance as the shortest three-dimensional distance 

between the site and the zone of seismogenic rupture. 

For the earthquakes in his data set, Campbell identified 

this zone from the aftershock distribution, when possible. We assumed that the zone of seismogenic rupture 

penetrates to within 4 km of the surface for all earthquakes included in the computation, on the basis of the 

observation that the tops of the aftershock zones for the 

1962 Cache Valley earthquake (Smith and Sbar, 1974), 

the 1975 Pocatello Valley earthquake (Arabasz and oth-
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D27 

Background Wasatch 

0.01 seismicity fault 100 

90 percent probability of 

nonexceedance in 50 years 475 

0.001 1000 

a: 90 percent probability of 

<( nonexceedance in 250 years 

w 2373 

>-

a: 

w 

a.. 0.0001 w 

u 

(/) 

a: 10,000 <( 

w 

>-

z 

<( ~ 0 

w 

w 0.1 0.3 0.5 0.7 0.9 0.1 0.3 0.5 

u 

X 

w 

6 

0.7 0.9 0 

a: 

w 

a.. u. 

0 Other 

>- faults 

t- 0.01 

z 

a: 

::::l 

100 t-

::J 

iiJ 

w 

a: 

<( 

CXJ 

0 

a: 475 a.. 

0.001 1000 

2373 

0.0001 10,000 

0.1 0.3 0.5 0.7 0.9 0.1 0.3 0.5 0.7 0.9 

PEAK HORIZONTAL ACCELERATION, g 

FIGURE 14.-Probability of exceedance per year for peak horizontal 

ground accelerations on soil at the intersection of I-15 and I-80 in 

South Salt Lake at approximately 40°43.1' N., 111°54.2' W. (diamond, fig. 5). The hachured zones represent the range of values 

calculated by using the range of median attenuation curves for peak 

horizontal acceleration presented by Campbell (in press). The results 

of separate calculations for the background seismicity, the Wasatch 

fault, other faults (the West Valley, East Great Salt Lake, and North 

Oquirrh Mountains faults), and all of these sources together are 

ers, 1981) and the 1983 Borah Peak earthquake (Richins 

and others, 1987) were all at depths of 3 to 4 km. 

However, the assumed minimum depth is important only 

for earthquakes very near the site. The site is located 

about 4 km east of the surface trace of the West Valley 

fault and 4 km west of the surface trace of the Wasatch 

fault. Because the West Valley fault dips to the east and 

the Wasatch fault dips to the west, both faults probably 

extend directly beneath the site. If fault dips of 45° to 70° 

are assumed, the distance to the seismogenic rupture for 

a surface-faulting event on either fault would be about 3 

shown. On the graph for background seismicity, the dashed 

curves bound the range of values obtained by using the preferred 

b value of 0. 71. The lower solid curve was calculated by using the 

upper bound b value of 0.80 and Campbell's lower bound acceleration estimates, whereas the upper solid curve was calculated by 

using the lower bound b value of0.62 and Campbell's upper bound 

acceleration estimates. The vertical axes at the right show the 

average return period in years (the inverse of the annual probability). 

to 4 km, which is consistent with the assumption that we 

made about seismogenic depth. 

Figure 14 presents four graphs showing the probability of exceedance per year versus peak horizontal ground 

acceleration at our representative Salt Lake Valley site. 

The four graphs illustrate contributions to the groundshaking hazard from (1) background seismicity, (2) earthquakes on the Wasatch fault, (3) earthquakes on the 

other three faults listed in table 4, and (4) all of these 

sources together. The solid curves bordering the shaded 

regions on each graph were calculated by using Camp-
D28 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

bell's lower and upper bound peak acceleration relationships. The shaded regions between these curves indicate 

the range of annual probabilities for any given peak 

horizontal acceleration. On the graph for the background 

seismicity, the two dashed curves were calculated by 

using the preferred b value of 0. 71, whereas the upper 

and lower solid curves were calculated by using the lower 

and upper limit b values of 0.62 and 0.80, respectively. 

If earthquake occurrence is assumed to be a Poisson 

process, peak acceleration values having a 90 percent 

probability of nonexceedance in 50 years have an annual 

probability of 0.0021 or a return period of 475 years 

(upper horizontal lines, fig. 14). On the graph that 

includes all of the sources, the acceleration value corresponding to this return period lies between 0.17 and 0.40 

g, as the plot indicates. For comparison, the value 

obtained by Algermissen and others (1982) is 0.20 to 0.28 

g, and the value obtained by Youngs and others (in press) 

is 0.3 g. Peak acceleration values having a 90 percent 

probability of nonexceedance in 250 years have an annual 

probability of 0.00042 or a return period of 2,373 years 

(lower horizontal lines, fig. 14). The acceleration value 

corresponding to this return period is 0.49 to 1. 05 g 

(fig.14). For comparison, the value obtained by Algermissen and others (1982) is 0.60 to 0. 70 g, and the value 

obtained by Youngs and others (in press) is 0. 7 g. Thus, 

the results of Algermissen and others (1982) and Youngs 

and others (in press) fall within the range of probabilistic 

acceleration values that we have calculated for both the 

475- and the 2,373-year return periods. 

Figure 14 implies that the background source is the 

largest contributor to the ground-shaking hazard at the 

site for return periods shorter than 500 years and 

accelerations less than 0.1 to 0.2 g, depending on the 

ground-motion model used. The Wasatch fault is the 

largest contributor to the hazard at longer return periods 

and higher accelerations. However, other faults, particularly the West Valley fault, are also quite important. 

Our calculation assumes that earthquakes on the West 

Valley fault are independent earthquakes, even though 

there is some possibility that they represent subsidiary 

fault movements associated with earthquakes on the 

Wasatch fault (S.J. Olig, personal commun., 1987). At 

locations farther away from the West Valley and 

Wasatch faults, the contribution of these faults to the 

ground-shaking hazard will be less than figure 14 indicates. At locations less than 4 km from the Wasatch fault 

or near segments whose recurrence intervals are shorter 

than the recurrence interval of the Salt Lake City 

segment (table 4), the contribution of the Wasatch fault 

to the ground-shaking hazard will be slightly greater 

than figure 14 indicates. The hazard curves for the 

background source, on the other hand, are applicable 

throughout most of the interior of the Wasatch Front 

region. Thus, we can infer from figure 14 that, at most 

localities, the background source will be the largest 

contributor to the ground-shaking hazard for return 

periods of 4 75 years or less (exposure periods of 50 years 

or less). 

CURRENT SEISMICITY AND THE WASATCH 

FAULT 

In this final section, we wish to consider the question, 

separate from the systematics of a hazard or risk analysis, "What does observational seismology tell us about 

the behavior of the Wasatch fault?" We then summarize 

our perspective on evaluating earthquake hazards and 

risk in the Wasatch Front area from the viewpoint of 

observational seismology. 

Let us first consider instrumental earthquake locations. Figure 15 shows a detailed plot of instrumental 

seismicity that might, on the basis of epicentrallocations 

alone, be associated with the Wasatch fault zone. The 

data include 1,538 earthquake locations from the UUSS 

catalog for the period July 1962 through December 1986. 

The largest earthquake included is the ML 5.2 Magna, 

Utah, earthquake of September 1962, located in the 

northwestern corner of box B-B'. Earthquakes were 

sampled from 30-km-wide zones extending 10 km east of 

the surface trace of the Wasatch fault to allow for 

epicentral errors and 20 km west of the fault to allow for 

both epicentral errors and reasonable downdip projection of the fault. Figure 16 shows complementary crosssection views for those foci in figure 15 that meet the 

rigorous criteria for focal-depth reliability specified in 

figure 10. Epicentral precision of ±5 km would be 

conservative for the data in figure 15, but errors as large 

as ± 10 km cannot be ruled out for a very minor fraction 

of the sample. Because the vast majority of the data 

postdate 1974, when modern network recording began, 

most of the epicenters probably have a precision of ±2 to 

3 km (Brown and others, 1986; Arabasz and Julander, 

1986). 

The cross sections in figure 16 suggest that very few of 

the well-located foci sampled from the vicinity of the 

Wasatch fault could be interpreted to lie on the fault-if 

one believes that the fault is a planar penetrative structure of moderate dip. Although the subsurface geometry 

of the Wasatch fault is not generally well known, Zoback 

(this volume) interpreted a planar, relatively steeply 

dipping (50°-55° W.) subsurface geometry for the 

Wasatch fault near Nephi. Smith and Bruhn (1984) 

interpreted a subsurface fault dip of ~34° for the 

Wasatch fault near Levan. 

The best available data for determining whether small 

earthquakes might be occurring on a listric Wasatch fault 
41° 

40° 

OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D29 

113° 

Collinston 

Nephi 

Levan 

0 

I 

0 

112° 

I 

Salt Lake City 

Nephi 

50 MILES 

I 

50 KILOMETERS 

FIGURE 15.-Wasatch fault, segment 

boundaries and names [according to 

Machette and others (this volume), 

right-hand side; Schwartz and Coppersmith (1984), left-hand side], and all 

earthquakes located by the University 

of Utah Seismograph Stations during 

the period July 1962 through December 

1986. Circle sizes indicate the relative 

magnitudes of events. 
D30 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

WEST EAST WEST EAST 

0 0 

0 / 

oO / 

0 

0 

0 ~ .... /0 0 

-10 -10 Oa> 'r ~ Cf:' (/) (/) 

~'?>~~ 0 a: a: w w t- t- )I' w w ~ ~ 0 { 0 ....J ....J Q -20 Q -20 

~ ~ 

:C :c' t- t- a_ a_ w w 0 0 

-30 -30 

0 10 20 30 0 10 20 30 

DISTANCE, IN KILOMETERS DISTANCE, IN KILOMETERS 

WEST EAST WEST EAST 

0 0 

/ / 

/o ~"'-/ 0 ~ 00 o/ 0 '-..'?> 0 

-10 c-7 o ~ o 0 o -10 4' (/) ~r (/) 

~ 0 a: 

~ a: w w ~ 0 00 0 t--

w t- / w ~ 0 ~ / 0 0 / 0 ....J ....J Q -20 Q -20 

~ ~ 

:c' :c' t- t-- a_ a_ w w 0 0 0 

-30 -30 

0 10 20 0 10 20 30 

DISTANCE, IN KILOMETERS DISTANCE, IN KILOMETERS 

FIGURE 16. -Cross sections keyed to the boxes in figure 15. The Wasatch fault is arbitrarily depicted by a plane dipping 45° to the west 

from its surface trace. Only foci meeting criteria for reliable focal depths (see fig. 10) are included. 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D31 

are those from portable-array studies reported by Arabasz and Julander (1986). At the southern end of the 

Wasatch fault, in both the northern and the southern 

parts of box D-D', these data show that earthquake foci 

and corresponding focal mechanisms are incompatible 

with seismic slip on either a listric or a simple planar 

projection of the Wasatch fault. Rather, the background 

seismicity appears to be occurring on secondary structures of moderate (greater than 30°) to high-angle dip. 

Elsewhere along the Wasatch fault, existing data for 

earthquakes downdip of the fault are either inadequate 

or ambiguous (for example, Pechmann and Thorbjarnardottir, 1984) for interpreting subsurface association with 

a listric projection of the fault. 

Comparing figure 15 with figures 8 and 9 indicates a 

remarkable paucity of microseismicity along the Wasatch 

fault within the broadly active earthquake belt of the 

Wasatch Front. Some specific features of smallearthquake occurrence in the vicinity of the fault are 

targets of ongoing investigation, such as epicentral clustering along the fault trace at the northern end of the 

Brigham City segment and clustering to the west of the 

Collinston and Salt Lake City segments. The episodic 

nature of the earthquake activity, however, handicaps 

the studies. 

Figure 17 shows the space-time distribution of the 

earthquake sample included in figure 15. Such data might 

appear attractive for relating seismicity to the segmentation of the Wasatch fault, but caution is clearly appropriate, given the problematic relationship between the 

seismicity and the fault. 

The following general observations can be made from 

figure 17. Seismicity north of Brigham City provides a 

clear contrast with seismicity between Brigham City and 

Bountiful. Ironically, the northernmost segments of the 

Wasatch fault show no evidence of latest Quaternary 

movement (Machette and others, 1991, this volume). 

There is a clear onset of the northern seismicity at about 

1975, which corresponds both to the timing of the 1975 

ML 6.0 Pocatello Valley earthquake and the approximate 

beginning of operation of the UUSS telemetered seismic 

network. The case for seismic quiescence before the 1975 

earthquake was made by Arabasz and Smith (1981). 

Activation of regional seismicity west of the Wasatch 

fault after the Pocatello Valley earthquake (discussed 

earlier) would be consistent with the interpretation that 

most of the northern seismicity in figure 17 is not on the 

Wasatch fault itself, although the episodic clustering of 

earthquakes near Honeyville at the boundary between 

the Brigham City and Collinston segments (fig. 15) is 

indeed suggestive of at least partial association. 

Seismicity near the northern end of the Salt Lake City 

segment (northern end of box B-B'), both west and east 

of the Wasatch fault, has long been recognized, as has 

quiescence to the south. Other apparent features of the 

space-time diagrams in figure 17 are the post-197 4 

appearance of scattered earthquakes near the general 

latitude of American Fork and temporally persistent 

seismicity near Santaquin and along the southernmost 

Wasatch fault. There is minimal microseismicity along a 

30-km-long section of the Wasatch fault north of Levan, 

including at least the southern half of the Nephi segment, 

which has one of the youngest (~400 years) surface 

ruptures on the Wasatch fault (Machette and others, 

1991). Depending on the structural association of earthquakes west of the Wasatch fault along much of its 

course, the Wasatch fault itself could conceivably be 

aseismic throughout most of the space-time diagram in 

figure 17 (see Arabasz and others, 1980). Thus, figure 17 

may not provide simply a side view of seismicity on the 

Wasatch fault. To what extent seismicity in the vicinity 

of the fault might be reflecting segmented behavior of 

the fault must be investigated further (see Wheeler and 

Krystinik, this volume). 

As Arabasz and Smith (1981) argued, apparent seismicity gaps along the Wasatch fault are not necessarily 

indicative of a late (that is, pre-earthquake) stage of a 

seismic cycle. Temporal decreases in seismicity ambiguously characterize both the early post-aftershock stage 

and the late pre-main-shock stage of a seismic cycle (for 

example, Ellsworth and others, 1981). Veneziano and 

others (1987) have recently analyzed the UUSS earthquake catalog between 1962 and 1985 to identify independent main events and the hierarchical clustering of 

secondary events. One result of their statistical analysis 

was documentation that between roughly 40° N. and 

41.5° N. -along an area encompassing most of the 

Wasatch fault-dependent events are systematically 

suppressed in comparison with earthquake behavior in 

neighboring areas. The significance of this suppressed 

earthquake clustering is the subject of current study. 

The Wasatch Front area is a classic example of a 

seismically active region in which the historical and 

instrumental earthquake record provides an inadequate 

guide to assessing earthquake potential. The absence of 

any surface-faulting earthquakes on the Wasatch fault in 

historical time, the problematic correlation of background seismicity with mapped Cenozoic faulting, and 

the notable paucity of contemporary earthquakes along 

the Wasatch fault all combine to enhance the importance 

of paleoseismologic information. The potential for large 

surface-faulting earthquakes effectively must be considered independently of observed seismicity. The occurrence of the 1983 M8 7.3 Borah Peak, Idaho, surfacefaulting earthquake in an area of low historical seismicity 

(Richins and others, 1987; Dewey, 1987)-but in an area 

of clearly recognizable late Quaternary faulting-is a 

case in point. 
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FIGURE 17.-Space-time diagrams of earthquake occurrence keyed to figure 150 Circle sizes indicate relative magnitudes of events. Shown for reference are the locations 

of selected cities and towns along the Wasatch fault (above) and the boundaries and names of segments of the Wasatch fault delineated by Machette and others (1991, 

this volume) (below). 

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OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D33 

Our closing perspective is that observational seismology contributes essential information-much of it still to 

be refined-for accurate estimations of earthquake hazards and risk in the Wasatch Front area. Background 

seismicity currently predominates on second-order faults 

in the Wasatch Front area and is the largest contributor 

to the probabilistic ground-shaking hazard for exposure 

periods of 50 years or less. From a seismotectonic point 

of view, background seismicity poses a major challenge 

in terms of understanding its association with geologic 

structure and its information content about deformation 

on individual segments of the Wasatch fault. Beyond 

providing essential input to analyses of hazards and risk 

for engineering applications and decisionmaking, observational seismology remains a critical tool for probing the 

deformational state of segments of the Wasatch fault and 

for monitoring changes in their behavior. 

REFERENCES CITED 

Abe, K., 1981, Magnitudes of large shallow earthquakes from 1904 to 

1980: Physics of the Earth and Planetary Interiors, v. 27, p. 72-92. 

Aki, K., 1966, Generation and propagation of G waves from the Niigata 

earthquake of June 16, 1964: Bulletin of the Earthquake Research 

Institute, Tokyo University, v. 44, p. 23-88. 

Algermissen, S.T., Perkins, D.M., Thenhaus, P.C., Hanson, S.L., and 

Bender, B.L., 1982, Probabilistic estimates of maximum acceleration and velocity in rock in the contiguous United States: U.S. 

Geological Survey Open-File Report 82-1033, 107 p. 

Allen, C.R., 1986, Seismological and paleoseismological techniques of 

research in active tectonics, in Active tectonics: Washington, 

D.C., National Academy Press, p. 148-154. 

Allmendinger, R.W., Hauge, T.A., Hauser, E.C., Potter, C.J., 

Knuepfer, P., Nelson, K.D., and Oliver, J.E., 1987, Overview of 

the COCORP 40° N Transect western U.S.A.: The fabric of an 

orogenic belt: Geological Society of America Bulletin, v. 98, p. 

308-319. 

Allmendinger, R.W., Sharp, J.W., Von Tish, D., Serpa, L., Brown, 

L., Kaufman, S., Oliver, J., and Smith, R.B., 1983, Cenozoic and 

Mesozoic structure of the eastern Basin and Range province, 

Utah, from COCORP seismic reflection data: Geology, v. 11, p. 

532-536. 

Anderson, L.W., and Miller, D.G., 1979, Quaternary fault map of 

Utah: Long Beach, Calif., Fugro, Inc., technical report, 35 p., 1 pl. 

Arabasz, W.J., 1984, Earthquake behavior in the Wasatch Front area: 

Association with geologic structure, space-time occurrence, and 

stress state, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, p. 310-339. 

Arabasz, W.J., and Julander, D.R., 1986, Geometry of seismically 

active faults and crustal deformation within the Basin and RangeColorado Plateau transition in Utah, in Mayer, L., ed., Extensional tectonics of the southwestern United States: A perspective 

on processes and kinematics: Geological Society of America Special 

Paper 208, p. 43-74. 

Arabasz, W.J., and Smith, R.B., 1981, Earthquake prediction in the 

Intermountain seismic belt-An intraplate extensional regime, in 

Simpson, D.W., and Richards, P.G., eds., Earthquake prediction-An international review: American Geophysical Union Maurice Ewing Series, no. 4, p. 248-258. 

Arabasz, W.J., Smith, R.B., and Richins, W.D., eds., 1979, Earthquake studies in Utah, 1850 to 1978: Salt Lake City, University of 

Utah Seismograph Stations Special Publication, 552 p. 

---1980, Earthquake studies along the Wasatch Front, Utah: 

Network monitoring, seismicity, and seismic hazards: Bulletin of 

the Seismological Society of America, v. 70, p. 1479-1499. 

Arabasz, W.J., Richins, W.D., and Langer, C.J., 1981, The Pocatello 

Valley (Idaho-Utah border) earthquake sequence of March to April 

1975: Bulletin of the Seismological Society of America, v. 71, p. 

803-826. 

Arabasz, W.J., Pechmann, J.C., and Brown, E.D., 1987, Evaluation of 

seismicity relevant to the proposed siting of a Superconducting 

Supercollider (SSC) in Tooele County, Utah: Salt Lake City, Utah, 

technical report prepared for Dames and Moore, Utah, University 

of Utah Seismograph Stations, SSC Proposal Team, 107 p. 

(Reprinted in 1989 by Utah Geological and Mineral Survey as 

Miscellaneous Publication 89-1, 107 p.) 

Askew, B., and Algermissen, S. T., 1983, An earthquake catalog for the 

Basin and Range province, 1803-1977: U.S. Geological Survey 

Open-File Report 83-86, 42 p. 

Beanland, S., and Clark, M.M., 1987, The Owens Valley fault zone, 

eastern California, and surface rupture associated with the 1872 

earthquake: Seismological Research Letters, v. 58, p. 32. 

Benz, H.M., and Smith, R.B., 1987, Kinematic source modelling of 

normal-faulting earthquakes using the finite element method: 

Geophysical Journal of the Royal Astronomical Society, v. 90, p. 

305-325. 

Bjarnason, I. T., and Pechmann, J. C., 1988, Contemporary tectonics of 

the Wasatch front region, Utah, from earthquake focal mechanisms: Bulletin of the Seismological Society of America, v. 79, no. 

3, p. 731-755. 

Bonilla, M.G., Mark, R.K., and Lienkaemper, J.J., 1984, Statistical 

relations among earthquake magnitude, surface rupture length, 

and surface fault displacement: Bulletin of the Seismological Society of America, v. 74, p. 2379-2411. 

Bortz, L.C., Cook, S.A., and Morrison, O.J., 1985, Great Salt Lake 

area, Utah, in Gries, R.R., and Dyer, R.C., eds., Seismic exploration of the Rocky Mountain region: Denver, Colo., Rocky 

Mountain Association of Geologists and the Denver Geophysical 

Society, p. 275-281. 

Brown, E.D., Arabasz, W.J., Pechmann, J.C., McPherson, E., Hall, 

L.L., Oehmich, P.J., and Hathaway, G.M., 1986, Earthquake data 

for the Utah region-January 1, 1984 to December 31, 1985: Salt 

Lake City, University of Utah Seismograph Stations Special 

Publication, 83 p. 

Bucknam, R.C.f 1976, Leveling data from the epicentral area of the 

March 27, 1975, earthquake in Pocatello Valley, Idaho: U.S. 

Geological Survey Open-File Report 76-52, 6 p. 

---1977, Map of suspected fault scarps in unconsolidated deposits, 

Tooele 1°X2° sheet, Utah: U.S. Geological Survey Open-File 

Report 77-495, 2 p., 1 pl. 

Bucknam, R.C., and Anderson, R.E., 1979, Map of fault scarps on 

unconsolidated sediments, Delta 1°X2° sheet, Utah: U.S. Geological Survey Open-File Report 79-366, 21 p. 

Bucknam, R.C., Algermissen, S.T., and Anderson, R.E., 1980, Patterns of late Quaternary faulting in western Utah and an application in earthquake hazard evaluation, in Evernden, J.F., comp., 

Proceedings of Conference X; Earthquake hazards along the 

Wasatch and Sierra Nevada frontal fault zones: U.S. Geological 

Survey Open-File Report 80-801, p. 299-314. 

Campbell, K.W., in press, Predicting strong ground motion in Utah, in 

Chap. L,, Gori, P.L., and Hays, W.W., eds., Assessment of 
D34 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

regional earthquake hazards and risk along the Wasatch Front, 

Utah: U.S. Geological Survey Professional Paper 1500. 

Cluff, L.S., Brogan, G.E., and Glass, C.E., 1970, Wasatch fault, 

northern portion: Earthquake fault investigation and evaluation: 

Oakland, Calif., Woodward-Clyde and Associates, technical report 

prepared for Utah Geological and Mineral Survey, 27 p. 

---1973, Wasatch fault, southern portion: Earthquake fault investigation and evaluation: Oakland, Calif., Woodward-Lundgren and 

Associates, technical report prepared for Utah Geological and 

Mineral Survey, 79 p. 

---1974, Investigation and evaluation of the Wasatch fault north of 

Brigham City and Cache Valley faults, Utah and Idaho: A guide to 

land-use planning with recommendations for seismic safety: Oakland, Calif., Woodward-Lundgren and Associates, technical report 

prepared for U.S. Geological Survey under contract 

14-08-001-13665, 147 p. 

Coffman, J.L., von Hake, C.A., and Stover, C.W., 1982, Earthquake 

history of the United States: Boulder, Colo., National Oceanic and 

Atmospheric Administration and U.S. Geological Survey Publication 41-1, 208 p. (with supplement). 

Cook, K.L., Gray, E.F., Iverson, R.M., and Strohmeier, M.T., 1980, 

Bottom gravity meter regional survey of the Great Salt Lake, 

Utah, in Gwynn, J.W., ed., Great Salt Lake, a scientific, historical, and economic overview: Utah Geological and Mineral Survey 

Bulletin 116, p. 125-143. 

Cornell, C.A., 1968, Engineering seismic risk analysis: Bulletin of the 

Seismological Society of America, v. 58, p. 1583-1606. 

Crone, A.J., and Machette, M.N., 1984, Surface faulting accompanying 

the Borah Peak earthquake, central Idaho: Geology, v. 12, p. 

664-667. 

Crone, A.J., Machette, M.N., Bonilla, M.G., Lienkaemper, J.J., 

Pierce, K.L., Scott, W.E., and Bucknam, R.C., 1987, Surface 

faulting accompanying the Borah Peak earthquake and segmentation of the Lost River fault, central Idaho: Bulletin of the Seismological Society of America, v. 77, p. 739-770. 

Currey, D.R., Atwood, G., and Mabey, D.R., 1984, Major levels of 

Great Salt Lake and Lake Bonneville: Utah Geological and Mineral 

Survey Map 73, scale 1:750,000. 

Davis, F.D., 1983a, Geologic map of the southern Wasatch Front, 

Utah: Utah Geological and Mineral Survey Map 55-A, scale 

1:100,000. 

---1983b, Geologic map of the central Wasatch Front, Utah: Utah 

Geological and Mineral Survey Map 54-A, scale 1:100,000. 

---1985, Geologic map of the northern Wasatch Front, Utah: Utah 

Geological and Mineral Survey Map 53-A, scale 1:100,000. 

Dewey, J.W., 1987, Instrumental seismicity of central Idaho: Bulletin 

of the Seismological Society of America, v. 77, p. 819-836. 

Doser, D.l., 1985a, The 1983 Borah Peak, Idaho, and 1959 Hebgen 

Lake, Montana, earthquakes: Models for normal fault earthquakes 

in the Intermountain Seismic Belt, in Stein, R. S., and Bucknam, 

R.C., eds., Proceedings of Workshop XXVIII-On the Borah 

Peak, Idaho, earthquake: U.S. Geological Survey Open-File 

Report 85-290-A, p. 368-384. 

---1985b, Source parameters and faulting processes of the 1959 

Hebgen Lake, Montana, earthquake sequence: Journal of Geophysical Research, v. 90, no. B6, p. 4537-4555. 

Doser, D.l., and Smith, R.B., 1982, Seismic moment rates in the Utah 

region: Bulletin of the Seismological Society of America, v. 72, p. 

525-551. 

---1985, Source parameters of the 28 October 1983 Borah Peak, 

Idaho, earthquake from body wave analysis: Bulletin of the 

Seismological Society of America, v. 75, p. 1041-1051. 

Earthquake Engineering Research Institute Committee on Seismic 

Risk, 1984, Glossary of terms for probabilistic seismic-risk and 

hazard analysis: Earthquake Spectra, v. 1, p. 33-40. 

Eddington, P.K., Smith, R.B., and Renggli, C., 1987, Kinematics of 

Basin-Range intraplate extension, in Coward, M.P., Dewey, J.F., 

and Hancock, P.L., eds., Continental extensional tectonics: Geological Society of London Special Publication 28, p. 371-392. 

Ekstrom, G., and Dziewonski, A.M., 1985, Centroid-moment tensor 

solutions for 35 earthquakes in western North America 

(1977-1983): Bulletin of the Seismological Society of America, v. 

75, p. 23-39. 

Electric Power Research Institute, 1988, Seismic hazard methodology 

for the central and eastern United States, v. 1, pt. 2, Methodology 

(revision 1): Palo Alto, Calif., technical report NP-4726 prepared 

for Seismicity Owners Group and Electric Power Research Institute under Research Projects P101-38, P101-45, P101-46, 2356, 

and 2256-14, 194 p. 

Ellsworth, W.L., Lindh, A.G., Prescott, W.H., and Herd, D.G., 1981, 

The 1906 San Francisco earthquake and the seismic cycle, in 

Simpson, D.W., and Richards, P.G., eds., Earthquake prediction-An international review: American Geophysical Union Maurice Ewing Series, no. 4, p. 209-216. 

Engdahl, E.R., and Rinehart, W.A., 1988, Seismicity map of North 

America: Geological Society of America Continent-Scale Map--004, 

scale 1:5,000,000. 

Evernden, J.F., 1975, Seismic intensities, "size" of earthquakes, and 

related parameters: Bulletin of the Seismological Society of America, v. 65, p. 1287-1313. 

Foley, L.L., Martin, R.A., and Sullivan, J.T., 1986, Seismotectonic 

study for Joes Valley, Scofield, and Huntington North Dams, 

Emery County and Scofield Projects, Utah: U.S. Bureau of 

Reclamation Seismotectonic Report 86-7, 132 p. 

Gibbons, A.B., and Dickey, D.D., 1983, Quaternary faults in Lincoln 

and Uinta Counties, Wyoming, and Rich County, Utah: U.S. 

Geological Survey Open-File Report 83-288, scale 1:100,000. 

Gutenberg, B., and Richter, C.F., 1954, Seismicity of the earth and 

associated phenomena (2d ed.): Princeton, N.J., Princeton University Press, 310 p. 

Hall, W.B., and Sablock, P.E., 1985, Comparison of the geomorphic 

and surficial fracturing effects of the 1983 Borah Peak, Idaho, 

earthquake with those of the 1959 Hebgen Lake, Montana, earthquake, in Stein, R.S., and Bucknam, R.C., eds., Proceedings of 

Workshop XXVIII-On the Borah Peak, Idaho, earthquake: U.S. 

Geological Survey Open-File Report 85-290, p. 141-152. 

Hanks, T.C., and Kanamori, H., 1979, A moment magnitude scale: 

Journal of Geophysical Research, v. 84, no. B5, p. 2348-2350. 

Kanamori, H., 1977, The energy release in great earthquakes: Journal 

of Geophysical Research, v. 82, no. 20, p. 2981-2987. 

Keaton, J., Currey, D.R., and Olig, S.J., 1986, Paleoseismicity and 

earthquake hazards evaluation of the West Valley fault zone, Salt 

Lake City urban area: Technical report prepared for U.S. Geological Survey under contract 14--08-0001-22048, 18 p. 

Loeb, D.T., 1986, The P-wave velocity structure of the crust-mantle 

boundary beneath Utah: Salt Lake City, University of Utah, 

unpublished M.Sci. thesis, 126 p. 

Loeb, D.T., and Pechmann, J.C., 1986, The P-wave velocity structure 

of the crust-mantle boundary beneath Utah from network travel 

time measurements [abs.]: Earthquake Notes, v. 57, p. 10. 

Machette, M.N, Personius, S.F., and Nelson, A.R., 1986, Late Quaternary segmentation and slip-rate history of the Wasatch fault 

zone, Utah [abs.]: EEBS, Transactions of the American Geophysical 

Union, v. 67, p. 1107. 

Machette, M.N., Personius, S.F., Nelson, A.R., Schwartz, D.P., and 

Lund, W.R., 1991, The Wasatch fault zone, Utah-Segmentation 

and history of Holocene earthquakes: Journal of Structural Geology, v. 13, p. 137-149. 
OBSERVATIONAL SEISMOLOGY AND THE EVALUATION OF EARTHQUAKE HAZARDS AND RISK D35 

Mikulich, M., and Smith, R.B., 1974, Seismic reflection and aeromagnetic surveys of the Great Salt Lake, Utah: Geological Society of 

America Bulletin, v. 85, p. 991-1002. 

Murphy, L. M., and Brazee, R.J., 1964, Seismological investigations of 

the Hebgen Lake earthquake: U.S. Geological Survey Professional 

Paper 435-C, p. C13-C17. 

Oakeshott, G.B., Greensfelder, R.W., and Kahle, J.E., 1972, The great 

Owens Valley earthquake of 1872 ... one hundred years later: 

California Geology, v. 25, p. 55-62. 

Pechmann, J.C., and Thorbjarnardottir, B., 1984, Investigations of an 

ML 4.3 earthquake in the western Salt Lake Valley using digital 

seismic data, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Repert 84-763, p. 340-365. 

Pechmann, J.C., Richins, W.D., and Smith, R.B., 1984, Evidence for a 

double Moho beneath the Wasatch Front, Utah [abs.]: EEBS, 

Transactions of the American Geophysical Union, v. 54, p. 988. 

Pechmann, J.C., Nash, W.P., Viveiros, J.J., and Smith, R.B., 1987, 

Slip rate and earthquake potential of the East Great Salt Lake 

fault, Utah [abs.]: EEBS, Transactions of the American Geophysical 

Union, v. 68, p. 1369. 

Pitt, A.M., Weaver, C.S., and Spence, W., 1979, The Yellowstone 

Park earthquake of June 30, 1975: Bulletin of the Seismological 

Society of America, v. 69, p. 187-205. 

Richins, W.D., Arabasz, W.J., Hathaway, G.M., Oehmich, P.J., Sells, 

L.L., and Zandt, G., 1981, Earthquake data for the Utah regionJanuary 1, 1978 to December 31, 1980: Salt Lake City, University 

of Utah Seismograph Stations Special Publication, 125 p. 

Richins, W.D., Arabasz, W.J., and Langer, C.J., 1983, Episodic 

earthquake swarms (ML:54. 7) near Soda Springs, Idaho, 1981-82: 

Correlation with local structure and regional tectonics [abs.]: 

Earthquake Notes, v. 54, p. 99. 

Richins, W.D., Arabasz, W.J., Hathaway, G.M., McPherson, E., 

Oehmich, P.J., and Sells, L.L., 1984, Earthquake data for the 

Utah region-January 1, 1981 to December 3, 1983: Salt Lake 

City, University of Utah Seismograph Stations Special Publication, 111 p. 

Richins, W.D., Pechmann, J.C., Smith, R.B., Langer, C.J., Goter, 

S.K., Zollweg, J.E., and King, J.J., 1987, The 1983 Borah Peak, 

Idaho, earthquake and its aftershocks: Bulletin of the Seismological Society of America, v. 77, p. 694-723. 

Richter, C.F., 1935, An instrumental earthquake magnitude scale: 

Bulletin of the Seismological Society of America, v. 25, p. 1-32. 

---1958, Elementary seismology: San Francisco, W.H. Freeman, 

768 p. 

Savy, J.B., Bernreuter, D.L., and Mensing, R.W., 1986, Seismic 

hazard characterization for the eastern United States: Nuclear 

Safety, v. 27, no. 4, p. 476-487. 

Schwartz, D.P., 1987, Earthquakes of the Holocene: Reviews of 

Geophysics, v. 25., p. 1197-1202. 

Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquakes: Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

---1986, Seismic hazards: New trends in analysis using geologic 

data, in Active tectonics: Washington, D.C., National Academy 

Press, p. 215-230. 

Schwartz, D.P., Swan, F.H., III, and Cluff, L.S., 1984, Fault behavior 

and earthquake recurrence along the Wasatch fault, in Hays, 

W.W., and Gori, P.L., eds., Proceedings of Conference XXVI; a 

workshop on Evaluation of regional and urban earthquake hazards 

and risk in Utah: U.S. Geological Survey Open-File Report 

84-763, p. 113-125. 

Shenon, P.J., 1936, The Utah earthquake of March 12, 1934 (extracts 

from unpublished report), in Neumann, F., United States earthquakes, 1943: U.S. Coast and Geodetic Survey Serial593, p. 43-48. 

Shimizu, Y., 1987, Earthquake clustering in the Utah region: Cambridge, Massachusetts Institute of Technology, unpublished 

M.Sci. thesis, 149 p. 

Singh, S.K., Rodriguez, M., and Esteva, L., 1983, Statistics of small 

earthquakes and frequency of occurrence of large earthquakes 

along the Mexican subduction zone: Bulletin of the Seismological 

Society of America, v. 73, p. 1779-1796. 

Slemmons, D.B., 1980, Design earthquake magnitudes for the western 

Great Basin, in Evernden, J.F., comp., Proceedings of Conference 

X; Earthquake hazards along the Wasatch and Sierra Nevada 

frontal fault zones: U.S. Geological Survey Open-File Report 

80-801, p. 62-85. 

Smith, R.B., 1978, Seismicity, crustal structure, and intraplate tectonics of the interior of the western Cordillera, in Smith, R.B., and 

Eaton, G.P., eds., Cenozoic tectonics and regional geophysics of 

the western cordillera: Geological Society of America Memoir 152, 

p. 111-144. 

Smith, R.B., and Arabasz, W.J., in press, Seismicity of the Intermountain Seismic Belt, in Slemmons, D.B., Blackwell, D., Engdahl, 

E.R., and Zoback, M., eds., Neotectonics of North America: 

Boulder, Colo., Geological Society of America Decade Map Volume. 

Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional tectonics of 

the eastern Basin-Range: Inferences on structural style from 

seismic reflection data, regional tectonics, and thermal-mechanical 

models of brittle-ductile deformation: Journal of Geophysical 

Research, v. 89, no. B7, p. 5733-5762. 

Smith, R. B., and Lindh, A., 1978, A compilation of fault plane solutions 

of the western United States, in Smith, R.B., and Eaton, G.P., 

eds., Cenozoic tectonics and regional geophysics of the western 

cordillera: Geological Society of America Memoir 152, p. 107-110. 

Smith, R.B., and Richins, W.D., 1984, Seismicity and earthquake 

hazards of Utah and the Wasatch Front: Paradigm and paradox, in 

Hays, W.W., and Gori, P.L., eds., Proceedings of Conference 

XXVI; a workshop on Evaluation of regional and urban earthquake hazards and risk in Utah: U.S. Geological Survey Open-File 

Report 84-763, p. 73-112. 

Smith, R.B., and Sbar, M.L., 1974, Contemporary tectonics and 

seismicity of the western United States with emphasis on the 

Intermountain Seismic Belt: Geological Society of America Bulletin, v. 85, p. 1205-1218. 

Smith, R.B., Nagy, W.C., Julander, K.A., Viveiros, J.J., Barker, 

C.A., Bashore, W.W., and Gants, D.G., 1989, Geophysical and 

tectonic framework of the Basin Range-Colorado Plateau-Rocky 

Mountain transition, in Pakiser, L.C., and Mooney, W.D., eds., 

Geophysical framework of the continental United States: Geological Society of America Memoir 172, p. 205-233. 

Stickney, M.C., and Bartholomew, M.S., 1987, Seismicity and late 

Quaternary faulting of the northern Basin and Range province, 

Montana and Idaho: Bulletin of the Seismological Society of 

America, v. 77, p. 1602-1625. 

Stover, C.W., Reagor, B.G., and Algermissen, S.T., 1986, Seismicity 

map of the State of Utah: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1856, scale 1:1,000,000. 

Sullivan, J.T., Nelson, A.R., LaForge, R.C., Wood, C.K., and 

Hansen, R.A., 1988, Central Utah regional seismotectonic study 

for USBR dams in the Wasatch Mountains: U.S. Bureau of 

Reclamation Seismotectonic Report 88-5, 365 p. 

Swan, F.H., III, Schwartz, D.P., and Cluff, L.S., 1980, Recurrence of 

moderate to large magnitude earthquakes produced by surface 

faulting on the Wasatch fault zone, Utah: Bulletin of the Seismological Society of America, v. 70, p. 1431-1462. 
D36 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Utsu, T., 1965, A method for determining the value of bin the formula 

log n=a-bM showing the magnitude-frequency relation for earthquakes: Hokkaido University Geophysical Bulletin, v. 13, p. 

99-103. 

Veneziano, D., and Van Dyck, J., 1985, Statistical discrimination of 

"aftershocks" and their contribution to seismic hazard, appendix 

A-4, in Seismic hazard methodology for nuclear facilities in the 

eastern United States: Golden, Colo., report prepared for Electric 

Power Research Institute under Research Project P101-29, 

Dames and Moore, v. 2, p. A121-A186. 

---1986, Statistical analysis of earthquake catalogs for seismic 

hazard, in International Symposium on Engineering Geology 

Problems in Seismic Areas, Bari, Italy, 1986, Proceedings: 43 p. 

Veneziano, D., Shimizu, Y., and Arabasz, W.J., 1987, Suppressed 

earthquake clustering in the Wasatch Front region, Utah [abs.]: 

EEBS, Transactions of the American Geophysical Union, v. 68, p. 

1368-1369. 

Viveiros, J.J., 1986, Cenozoic tectonics of the Great Salt Lake from 

seismic reflection data: Salt Lake City, University of Utah, 

unpublished M.Sci. thesis, 81 p. 

Wallace, R.E., 1981, Active faults, paleoseismology, and earthquake 

hazards in the western United States, in Simpson, D.W., and 

Richards, P.G., eds., Earthquake prediction-An international 

review: American Geophysical Union Maurice Ewing Series, no. 4, 

p. 209-216. 

---1987, Grouping and migration of surface faulting and variations 

in slip rates on faults in the Great Basin Province: Bulletin of the 

Seismological Society of America, v. 77, p. 868-876. 

Weichert, D. H., 1980, Estimation ofthe earthquake recurrence parameters for unequal observation periods for different magnitudes: 

Bulletin of the Seismological Society of America, v. 70, p. 

1337-1346. 

Wesnousky, S.G., 1986, Earthquakes, Quaternary faults, and seismic 

hazard in California: Journal of Geophysical Research, v. 91, no. 

B12, p. 12587-12631. 

Wesnousky, S.G., Scholz, C.H., Shimazaki, K., and Matsuda, T., 

1983, Earthquake frequency distribution and the mechanics of 

faulting: Journal of Geophysical Research, v. 88, no. Bll, p. 

9331-9340. 

Williams, D.J., and Arabasz, W.J., 1989, Mining-related and tectonic 

seismicity in the east mountain area, Wasatch Plateau, Utah, in 

Gibowicz, S.J., ed., Seismicity in mines: PAGEOPH (Journal of 

Pure and Applied Geophysics), v. 129, nos. 3/4, p. 345-368. 

Witkind, I.J., 1964, Reactivated faults north of Hebgen Lake: U.S. 

Geological Survey Professional Paper 435-G, p. G37-G50. 

---1975, Preliminary map showing known and suspected active 

faults in Idaho: U.S. Geological Survey Open-File Report 75-278, 

71 p. 

Youngs, R.R., and Coppersmith, K.J., 1985, Implications of fault slip 

rates and earthquake recurrence models to probabilistic seismic 

hazards estimates: Bulletin of the Seismological Society of America, v. 75, p. 939-964. 

Youngs, R.R., Swan, F.H., Power, M.S., Schwartz, D.P., and Green, 

R.K., in press, Probabilistic analysis of the earthquake groundshaking hazard along the Wasatch Front, Utah, Chap. M o/Gori, 

P.L., and Hays, W.W., eds., Assessment of regional earthquake 

hazards and risk along the Wasatch Front, Utah: U.S. Geological 

Survey Professional Paper 1500. 

Zoback, M. L., 1983, Structure and Cenozoic tectonism along the 

Wasatch fault zone, Utah: Geological Society of America Memoir 

157, p. 3-27. 

---1984, Constraints on the in-situ stress field along the Wasatch 

Front, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, p. 286-310. 

Zoback, M.L., and Zoback, M.D., 1980, State of stress in the conterminous United States: Journal of Geophysical Research, v. 85, no. 

Bll, p. 6113-6156. 
Superimposed Late Cenozoic, Mesozoic, and 

Possible Proterozoic Deformation Along the 

Wasatch Fault Zone in Central Utah 

By MARY LOU ZOBACK 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-E 

CONTENTS 

Page 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E 1 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Acknowledgments................................................................................................ 1 

Local Geologic Setting................................................................................................ 1 

Gravity and Density Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Seismic Reflection and Velocity Data............................................................................ 5 

Interpreted Subsurface Structure................................................................................. 10 

Basin Geometry and Shallow Structure of the Wasatch Fault Zone............................. 10 

Deep Structure.................................................................................................. 14 

Geologic Interpretation of Deep Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. 16 

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

References Cited....................................................................................................... 19 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of central Utah showing late Cenozoic normal faults and Sevier frontal thrust faults................................... E2 

2, 3. Maps showing: 

2. Generalized geology of the study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

3. Complete Bouguer gravity data for the Nephi area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

4. Seismic reflection profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

5. Line drawing of the seismic reflection profile and true-depth migrated cross section of selected reflectors . . . . . . . . . . . . . . . . . . . . . 8 

6. Detail of seismic reflection profile in Juab Valley showing basin fill and true-depth cross section of prominent reflectors 

in basin................................................................................................................................................... 11 

7. Diagram showing gravity models of basin fill in Juab Valley.................................................................................. 13 

8. Cross section showing geologic interpretation of the western half of the true-depth line drawing shown in figure 5B....... 16 

TABLES 

Page 

TABLE 1. Log-determined densities and sonic velocities of significant wells in the region of the Wasatch fault zone . . . . . . . . . . . . . . . . . . . . . . . . .. . .. E9 

III 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

SUPERIMPOSED LATE CENOZOIC, MESOZOIC, AND 

POSSIBLE PROTEROZOIC DEFORMATION ALONG THE 

WASATCH FAULT ZONE IN CENTRAL UTAH 

By MARY Lou ZoBACK 

ABSTRACT 

Surface geology, gravity data, and information from petroleum 

exploration wells have been used to constrain the subsurface structure 

along a 30-km-long seismic reflection profile across the Wasatch fault 

zone in central Utah near the town of Nephi. In this region, late 

Cenozoic normal faulting along the Wasatch fault zone is superimposed 

on the frontal portion of the Mesozoic Sevier fold and thrust belt. Juab 

Valley, which is bounded on the east by the Wasatch fault zone, is 

interpreted as an asymmetric sag; beds on both sides tilt toward the 

axis of the valley, and a central graben defines the deepest part of the 

basin. Maximum valley thickness is about 1.6 km, based on a combined 

interpretation of the gravity and seismic reflection data. Possible 

displacement of a buried thrust(s), truncation of deep reflectors, and 

the absence of reverse drag in the basin sediments suggest a planar, 

relatively steeply dipping (50°-55°) geometry for the Wasatch fault 

zone at this locality. Because of complex subsurface structure, however, the lack of reverse drag in the basin sediments is probably the 

strongest evidence against a listric geometry for the Wasatch. Within 

the study area, both a major Sevier frontal thrust (the Nebo thrust) 

and possible deeper thrust(s) appear to ramp up in the vicinity of the 

Wasatch fault zone. A similar correlation of major thrust ramps 

occurring along several other segments of the Wasatch fault zone was 

noted by R.B. Smith and R.L. Bruhn in 1984. It is suggested here that 

both the thrust ramping and the late Cenozoic normal faulting may be 

localized by a major west-dipping normal fault zone formed during the 

early phases (pre-drift) of late Precambrian rifting of the western 

Cordillera. 

INTRODUCTION 

In central Utah, late Cenozoic extensional faulting 

along the Wasatch fault zone and other normal fault 

zones is superimposed on the frontal portion of the 

Mesozoic Sevier fold and thrust belt (fig. 1), which 

deforms sediments deposited on a passive margin formed 

by late Precambrian rifting (Stewart, 1972; Stewart and 

Manuscript approved for publication November 20, 1990. 

Poole, 1974). Many of the rocks exposed in the modern 

range blocks are remnants of laterally extensive thrust 

sheets transported 100 km or more from the west during 

the Sevier orogeny (Crittenden, 1961). 

This chapter used gravity data and well data, together 

with a 30-km-long seismic reflection profile across the 

Wasatch fault zone near its intersection with one of the 

frontal sole thrust faults, to detail the structural style of 

late Cenozoic extension superimposed on Mesozoic compressional deformation. Particular emphasis is placed on 

understanding the subsurface geometry of Cenozoic normal faulting and its relationship to earlier low-angle 

thrust faults, ramps, and decollement surfaces, as well as 

to possible normal faults in the crystalline basement 

formed during Precambrian rifting. 

ACKNOWLEDGMENTS 

This manuscript was substantially improved by critical, thoughtful reviews by Anthony Crone and Thomas 

Brocher (U.S. Geological Survey). Discussion with Douglas Sprinkel (Utah Geological and Mineral Survey), Timothy Lawton (New Mexico State University), William 

Jefferson (Amoco Production Company), and Hal Morris 

(U.S. Geological Survey) greatly increased my understanding of the structure and tectonics of the region. 

However, any shortcomings of the interpretations presented here are my own responsibility. 

LOCAL GEOLOGIC SETTING 

The area of study is centered on the town of Nephi in 

central Utah, a region of complex, superimposed compressional and extensional deformation of several ages. 

El 
E2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Nielson-Sieger #1 

CEDAR 

HILLS 

FIGURE 1.-Late Cenozoic normal faults (heavy lines, bar and ball on down thrown side) and Sevier frontal thrust faults (barbed) 

in central Utah. Ranges are shaded. The Leamington tear fault is shown as a heavy solid line (inferred buried extension 

shown as a dotted line). Locations of petroleum exploration wells are also indicated. Box shows location of figure 2. 
LATE CENOZOIC, MESOZOIC, AND POSSIBLE PROTEROZOIC DEFORMATION, WASATCH FAULT ZONE E3 

0 

l 

5 

I 

I 

10 MILES 

I 

D 

• rn . 

. 

~ 

~ 

• m 

~ 

......... 

~ 

70.>-

-+-

4-

E9 

EXPLANATION 

Quaternary alluvium 

Tertiary sedimentary, volcanic, and 

intrusive rocks 

Cretaceous synorogenic conglomerates 

(Indianola Group) 

Jurassic Arapien rocks 

Lower Jurassic-Triassic sedimentary 

rocks 

Paleozoic sedimentary rocks 

Normal fault 

Thrust fault 

High-angle reverse fault 

Strike and dip of beds 

Vertical beds 

Overturned beds 

Petroleum exploration well 

0 

I 

5 

I 

10 15 KILOMETERS 

FIGURE 2. -Generalized geology of the study area (see fig. 1 for location). The inferred buried extension of the Leamington tear fault is shown 

by a heavy dotted line. Locations of seismic reflection profiles near Nephi and to the south (Smith and Bruhn, 1984) are indicated by dashed 

lines. 

The Wasatch fault zone separates the southern Wasatch 

Mountains and Gunnison Plateau on the east from Juab 

Valley on the west (fig. 2). Approximately 5 km wide, 

Juab Valley is bordered on the west by Long Ridge and 

West Hills. 

A late Precambrian (younger than 850 Ma) rifting 

event (Stewart, 1972) formed a north-trending, westfacing passive margin through western Utah and eastern 

Nevada (Stewart and Poole, 197 4). The study area lies 

along the inferred hinge line (boundary between shelf 

and miogeocline) of this margin. In central Utah, deposition on the passive margin continued until the inception 

of Sevier compressional deformation, which began in 

latest Jurassic or possibly middle Cretaceous and continued to at least Paleocene time (Standlee, 1982; Lawton, 

1982; Heller and others, 1986). During this time, widespread low-angle eastward-directed thrusting occurred 

in central Utah, concurrent with deposition of synorogenic clastic sediments. The Sevier frontal thrust system 

is considered the basal and most far-traveled thrust 

sheet and typically places Paleozoic rocks on Mesozoic 

rocks. Thrust relations interpreted as part of the frontal 

thrust zone have been mapped in the Mount N ebo area of 

the southern Wasatch Mountains, 3.5 km north of the 

seismic line (fig. 2). The Nebo thrust has been correlated 

with the Charleston thrust to the north (Crittenden, 

1961) and occupies a structural level similar to that of the 

Pavant thrust to the south (Burchfield and Hickcox, 

1972); however, Morris (1983) inferred a major discontinuity in thrust sheet geometry between the N ebo and 

the Pavant thrusts. Morris interpreted a major tear 

fault-the Leamington fault-as separating these two 

frontal thrust systems. 

The southern edge of the N ebo thrust, which is 

exposed in the Mount Nebo area, contains steeply dipping Paleozoic through early Mesozoic rocks deformed 

into an overturned anticline (Black, 1965). The Nebo 

allochthon is thrust over the Jurassic Arapien Shale, a 
E4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

sequence of highly disturbed calcareous mudstone and 

siltstone units that locally contain significant quantities 

of salt and anhydrite. A lively controversy exists over 

the stratigraphic definition and nomenclature of Jurassic 

marine rocks in central Utah (Spieker, 1946; Hardy, 

1952; Witkind, 1982, 1983; Sprinkel, 1982; Lawton, 

1985). These disputes over nomenclature are of little 

importance here; hence, an informal usage describing all 

the Jurassic shale, silt, and limestone rocks above the 

Jurassic-Triassic Navajo Sandstone as Arapien rocks (or 

simply Arapien) has been adopted. This usage includes 

all of the Arapien Shale as originally named and 

described by Spieker (1946), including his upper Twist 

Gulch Member and the underlying Twelve-Mile Canyon 

Member. Sprinkel (1982) subdivided the Jurassic rocks 

into two units on the basis of well-log data-the Arapien 

Shale (calcareous mudstone, shaly siltstone, arenaceous 

limestone, and evaporites) and the underlying Twin 

Creek Limestone (limestone and minor amounts of siltstone, sandstone, gypsum, and anhydrite), which rests 

directly on the Navajo Sandstone. The Twin Creek 

Limestone has not been identified in surface exposure in 

central Utah. Twin Creek Limestone is used in this 

report as the lowermost part of Arapien rocks, as 

identified in well data. 

The Wasatch fault zone truncates the Nebo allocthon 

along the western edge of the southern Wasatch Mountains and presumably has downdropped the thrust sheet 

beneath Juab Valley. Exposures of overturned Paleozoic 

and early Mesozoic rocks in the vicinity of and just south 

of where the seismic line crosses West Hills and Long 

Ridge probably represent the western continuation of 

the N ebo thrust (fig. 2). 

The probable southern boundary of the N ebo thrust, 

theN. 60° E.-trending Leamington tear fault, is exposed 

22 km southwest of the end of the seismic line (fig. 1). 

Where exposed, the fault dips steeply, and the dominant 

sense of motion is strike slip (Morris, 1983). Its projected 

trace through the study area is constrained by the 

exposures of overturned Paleozoic and early Mesozoic 

rocks in the northern part of West Hills and by two wells 

in West Hills drilled 8 km south of the seismic line (Placid 

Oil Howard #1-A and Howard #2) (see figs. 1, 2). Both 

wells penetrate a normal stratigraphic section of Tertiary through late Mesozoic sedimentary rocks and a 

section of Jurassic Arapien rocks and bottom in the 

Jurassic-Triassic Navajo Sandstone (Howard #2, total 

depth of 3,282 m) or the Triassic Chinle(?) (Howard 

#1-A, total depth of 3,704 m). 

The eastern half of the seismic line crosses the Gunnison Plateau, which is composed primarily of Arapien 

rocks and overlying synorogenic conglomerates (the Cretaceous Indianola Group), both of which are deformed 

into a broad asymmetric syncline whose axis generally 

parallels the modern range. Deformation of these synorogenic conglomerates, particularly along the steep eastern limb of this syncline, has been attributed to simple 

folding (Spieker, 1946), Laramide compression (Gilliland, 

1963), and extensive salt diapirism (Witkind, 1983). 

However, recent mapping combined with extensive well 

data and seismic profiling indicates that much of the 

geometry of deformation within the Gunnison Plateau 

can be attributed to late-stage Sevier compression and 

superimposed Tertiary normal faulting. Specifically, the 

deformation has been related to thrusting beneath the 

Gunnison Plateau that culminates either in a complex 

system of west- and east-verging thrusts (Standlee, 

1982) or in a triangular zone (Lawton, 1985), both along 

the eastern margin of the Gunnison Plateau. Well data 

indicate that the Arapien rocks beneath both the Gunnison Plateau and areas to the west are complexly 

deformed and tectonically thickened, probably by 

bedding-plane thrusts (Standlee, 1982; Lawton, 1985). 

The thrusting beneath the Gunnison Plateau probably 

has minor displacement and may not have reached the 

synorogenic erosion surface; Lawton (1985) estimated 

only 5 to 6 km of shortening in his interpreted triangular 

zone along the eastern margin of the Gunnison Plateau. 

On the basis of repeated and overturned stratigraphic 

sections in wells along the western margin of the Gunnison Plateau, both Standlee (1982) and Lawton (1985) 

concluded that bedding-plane thrusts within the Arapien 

rocks ramped upward in the vicinity of the modern 

Wasatch fault. An increase in the westward dip of the 

N ebo basal thrust from an essentially flat attitude on the 

eastern edge of its exposure in the southern Wasatch 

Mountains (fig. 2) to a dip of 23° W. near the range-front 

fault also suggests a ramp structure near the Wasatch 

fault. Evidence of similar structural styles within thrust 

sheets to the north presented by Smith and Bruhn (1984) 

suggests that the modern Wasatch fault is located near 

the sites of major ramps in Sevier-age thrust faults. 

In summary, the seismic line is located in a complex 

structural setting. The eastern part of the line crosses 

Arapien rocks that have been tectonically thickened by 

bedding-plane thrusts and folding and locally disturbed 

by salt diapirism. These bedding-plane thrusts are below 

the ''basal" N ebo thrust and probably involve only minor 

transport (approximately 5 km). Upper plate rocks of the 

N ebo thrust are exposed on the westernmost part of the 

line; thus, both the N ebo and the lower minor thrust(s) 

are likely present in the subsurface in this area. The line 

probably obliquely cuts the southern edge of the N ebo 

thrust beneath Juab Valley. Geologic and well data 

suggest a ramp in both the Nebo and the deeper, minor 

thrust(s) near the modern location of the Wasatch fault 

zone. Because of the complex subsurface structure, 
LATE CENOZOIC, MESOZOIC, AND POSSIBLE PROTEROZOIC DEFORMATION, WASATCH FAULT ZONE E5 

additional geophysical data were collected to aid in the 

interpretation of the seismic profile. 

GRAVITY AND DENSITY DATA 

Existing gravity data in the study area were supplemented by over 200 new stations (fig. 3). As expected, a 

broad gravity low is associated with Juab Valley. The 

data suggest a maximum anomaly of about -20 mGal in 

the valley south of the town of Nephi. 

Quantitative modeling of the observed gravity field 

requires information on the subsurface geometry of the 

basin and also on the rock densities. Constraints on the 

subsurface geometry come from the seismic reflection 

profile and are described later in the interpretive section, 

together with a quantitative model of valley fill in Juab 

Valley. Density data from density logs of significant 

wells in the region are summarized in table 1. In general, 

rock density increases with age. Despite the complex 

internal structure of the Arapien rocks and reported 

large lateral variations in lithology (particularly in saltanhydrite content), this unit has only a moderate variation in density (2.54-2.64 g/cm3

). However, these values 

are substantially greater than the values of the density of 

Arapien rocks reported by Brown and Cook (1982) from 

logs of the Phillips U.S. D #1 well located 90 km south of 

the seismic line (see fig. 1). Brown and Cook (1982) 

recognized three distinct lithologic units within the 

Arapien-age rocks having mean densities of 2.11, 2.54, 

and 2. 01 g/cm3 and a weighted mean density of 2.41 

g/cm3

• For this study, a mean density for the Arapienage rocks (including the Twin Creek Limestone of 

Sprinkel (1982)) of 2.55 g/cm3 is preferred because it is 

more representative of the values given in table 1 for 

wells located close to the seismic profile, particularly 

Howard #1-A, State #1, and State #2 (figs. 1, 2). 

The Jurassic-Triassic Navajo Sandstone underlying 

the Arapien rocks has approximately the same mean 

density (2.55 g/cm3

) as the Arapien rocks, whereas the 

Triassic rocks are slightly more dense (2. 63 g/cm3). Rock 

units within the Paleozoic have primarily limestone 

lithologies and a mean density of about 2. 73 g/cm3

, the 

result being a density contrast of +0.10 to +0.18 g/cm3 

with the overlying Triassic and Jurassic rocks. 

In addition to the main basin fill anomaly, the gravity 

data also reveal a relative gravity high within the basin 

that separates the closed low directly southwest of Nephi 

from the closed low about 15 km to the north (fig. 3). The 

source of this relative gravity high may be a transverse 

structure separating a shallow basin to the north from a 

deeper basin to the south. Similar transverse structures 

have been inferred from gravity and geologic data for 

many of the basins bounded by the Wasatch fault zone 

(Zoback, 1983). Alternatively, because the east-westtrending gravity high separating the two gravity lows 

generally coincides with the southern termination of the 

Nebo allochthon in the range to the east, the gravity high 

may be caused in part by lateral variations in bedrock 

density. In this case, the gradient may define the buried 

extension of the Lemington tear fault, which forms the 

southern edge of the Nebo thrust plate. The decrease in 

gravity values south of this gravity high would thus be 

attributed to the +0.10- to +0.18-g/cm3 density contrast 

between the primarily limestone lithologies of Paleozoic 

rocks within the thrust plate and the predominantly 

shale and siltstone lithologies of the Arapien rocks on 

which it rests. This interpretation supports an inferred 

buried connection between the overturned Paleozoic 

rocks forming the N ebo allochthon and the overturned 

Paleozoic and Mesozoic rocks exposed in and just south of 

the pass between Long Ridge and West Hills (figs. 2, 3). 

The gentle slope of the gravity gradient may indicate 

that, in this area, the tear fault may have a shallow dip to 

the north. Wallace and Morris (1979) reported that tear 

faults commonly flatten in dip to merge with the bounding thrust. The shape of the gravity gradient, particularly the closed low southwest of Nephi, suggests that 

this transverse relative gravity high may be related to 

both effects described above, in which case the southern 

edge of the Nebo allochthon (Leamington tear fault) may 

have acted to segment the extensional fault structure in 

the hanging wall and thereby produce a variation in fill 

thickness between the northern and southern portions of 

the basin. 

SEISMIC REFLECTION AND VELOCITY DATA 

A standard 24-fold Vibroseis (trademark of Conoco, 

Inc.) profile was shot across the Wasatch fault zone 

through the town of Nephi in 1975 (fig. 2). The data were 

collected by a private contractor, United Geophysical, 

and purchased by the U.S. Geological Survey. Group and 

shot point intervals were 220 ft, and the data were 

recorded on a split spread having a near offset of 880 ft 

and a far offset of 5, 940 ft. The standard processing 

sequence included demultiplexing, velocity analysis, elevation and residual statics, stacking, and filtering. The 

automatic residual statics produced a very poor quality 

record section in which there was little continuity of 

reflectors. The contractor then used an "in-house" statics 

routine and hand picked some refraction arrivals, a 

procedure that greatly improved the data quality and 

yielded the section shown in figure 4. A floating datum 

used to display the data is generally 30 to 70 m below the 

ground surface and has a constant value in the basin. 

The 30-km-long profile begins in San Pete Valley on 

the east and follows the divide separating the southern 
E6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

... 

. . . . . . . . 

FIGURE 3. -Complete Bouguer gravity data for the Nephi area (contour interval2 mGal). Gravity stations are shown by 

plus signs. Ranges are shaded. Rocks inferred to be part of the Nebo thrust sheet are hachured. The dashed line is the 

location of the reflection profile. The Wasatch fault is indicated by a heavy solid line (bar and ball on downthrown side). 
~ 1 

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KILOMETERS 

FIGURE 4. -Seismic reflection profile (see fig. 2 for location). Vibrator points are numbered across the top of the profile. 

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E8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

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FIGURE 5. -A, Seismic reflection profile. Reflectors discussed in the text are lettered. B, Selected reflectors on line drawing (true-depth migrated 

cross section). V and W indicate limits (discussed in text) on the surface projection of the Wasatch fault. The star indicates the axis of a 

proposed major diapiric fold crossing the seismic line. 

Wasatch Mountains from the Gunnison Plateau (fig. 2). It 

crosses the Wasatch fault zone at Nephi, continues 

westward across Juab Valley, through the pass between 

Long Ridge and West Hills, and ends in Dog Valley. 

There are several large data gaps on the section; the one 

in the western half of the section is caused by a stream 

and the one near the center of the section by a major 

highway and Nephi city streets. Despite folded and 

steeply dipping to overturned beds exposed in the 

ranges, the profile is dominated by horizontal and subhorizontal reflectors beneath both the basin and the 

ranges. 

A line drawing of the prominent reflectors on the 

section is given in figure 5A. A discussion of the lettered 

reflectors and their geologic interpretation will be presented later. First, velocity data are needed to estimate 

the depth and apparent dip of reflectors on the time 

section. Some velocity information is derived from the 

processing of the reflection data; these "stacking velocities" are best determined in areas of strong subhorizontal 

continuous reflectors. An additional source of velocity 

data is sonic logs of exploration wells. Sonic velocities 

and rock densities obtained from logs of nearby wells are 

summarized in table 1. 
TABLE 1.-Log-determined densities and sonic velocities of significant wells in the region of the Wasatch fault zone 

[Locations of wells shown in fig. 1. -, no data] 

Howard #1-A Nielson-Singer #1 Gunnison #1 State #1 State #2 Barton #1 

Density Velocity Density Velocity Density Velocity Density Velocity Density Velocity 

Tertiary volcanic rocks ....................................................................................................... . 

Eocene Flagstaff Limestone.......................... - 1

3.05 2.55 13.93 

Cretaceous Indianola Group...... . . 1

2.57 14.88 2.55 14.47 12.68 15.65 2.54 14.97 

Jurassic Arapien rocks . . . . . . . . . . . . 2.54 5.04 1

2.59 4.97 1

•

3

2.60 1

•

3

5. 72 

Ja2 ............................. . 2.54 4.95 

Jtc2 ••••••••••••••••••••••••••••• 2.54 5.40 

Jurassic-Triassic Navajo 2.60 5.12 

Sandstone ..................... . 

Triassic shales ................... . 1

2.63 1

5.44 

1

2.58 15.23 

'~.64 1

•

4

5.61 3 3

5.63 

1

2.60 

1

'

4

5.23 1

'

3

5.95 

1

6.25 

1

6.16 

Permian limestones ........................................................................................ . 

Paleozoic shelf section 

1

2.63 

2.56 

2.60 

1

2.69 

1

4.27 

4.97 

5.03 

1

5.35 

Density Velocity 

1

2.93 

4.82 

4.70 

1

5.54 

5.09 

1

5.21 

(Permian-Cambrian) .......................................................................................................................... . 

1

0nly part of the unit was sampled owing to logging difficulties or structure encountered in the well. 2Division of Arapien-age rocks into Arapien (Ja) and Twin Creek Limestone (Jtc) equivalents (after Sprinkel, 1982). 3 Above a thrust fault believed to have been encountered in the well. 4

Below a thrust fault believed to have been encountered in the well. 

U.S. E #1 

Density Velocity 

1

4.76 

2.53 

2.65 

2.75 5.94 

1

2.73 15.95 

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E10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

The well log velocities show a significant amount of 

scatter for individual units; this scatter may reflect real 

variations in lithology between wells or may be because 

of poor log quality caused by borehole irregularities or 

geologic structure in the well, which led to incomplete 

sampling. The velocity values in table 1 represent reasonable ranges for the lithologies of the units of interest. 

The velocity ranges for the Mesozoic have considerable 

overlap among units (Indianola, 4.47-5.65 km/s; Arapien, 

4. 70-6.25 km/s; Navajo, 4.97-6.16 km/s; Triassic rocks, 

5.03-5.44 km/s). In those wells where Sprinkel (1982) 

subdivided Arapien-age rocks into Arapien and Twin 

Creek equivalents, there is generally a large velocity 

increase between the calcareous mudstones and shaly 

siltstones of the Arapien rocks and the limestones that 

dominate the underlying Twin Creek equivalents. 

Since seismic reflectance depends on impedance contrasts (velocity times density), the density and velocity 

data in table 1 indicate numerous potential reflecting 

horizons throughout the section. Clearly, the MesozoicPaleozoic boundary has a large impedance contrast. 

There is also a considerable impedance contrast between 

the Arapien Shale and the Twin Creek Limestone in 

wells where these units are differentiated (Sprinkel, 

1982). This impedance contrast within Arapien-age rocks 

is greater than that between the base of the Twin Creek 

Limestone and the underlying Navajo Sandstone. The 

velocity and density data from the Paleozoic shelf rocks 

drilled in the U.S. E # 1 well on the Wasatch Plateau (fig. 

1) also indicate significant impedance contrasts between 

individual units in the Paleozoic section. Thus, numerous 

potential reflectors occur in the Mesozoic and Paleozoic 

rocks owing to stratigraphic and lithologic contrasts. 

In addition, structural features, including thrustjuxtaposed units of high impedance contrast as well as 

low-angle fault zones (Jones and Nur, 1984), may produce some of the reflections. 

Comparison of these well log velocities with stacking 

velocities obtained during processing of the seismic section indicates that the stacking velocities in general are 

unrealistically low, except for those in the basin fill 

region, where the reflectors are strong and very continuous. In particular, the stacking velocities from a velocity analysis near the center of the Gunnison Plateau, 

where Arapien rocks are exposed and are probably at 

least 2,000 m thick, indicate a velocity of 3.8 km/s at 1.0 

s two-way traveltime and a velocity of 4.27 km/s at 2.0 s 

two-way traveltime. In contrast, the sonic-logdetermined velocities for Arapien rocks, although variable, range between 4. 70 and 6.25 km/s (table 1). Similarly, stacking velocities on the western end of the line, 

where overturned Paleozoic limestones are exposed at 

the surface, are low in comparison with sonic log velocities. The log velocity of the Paleozoic rocks is quite high 

(approximately 5. 95 km/s) in comparison with several 

stacking velocities between 3.66 and 3.81 km/s for the 

prominent reflector (E-E') at about 1.9 s two-way traveltime. The Paleozoic rocks at the surface are presumed 

to be part of the N ebo thrust, which probably rests on 

rocks of Jurassic Arapien age or older; nonetheless, these 

stacking velocities are unrealistic, even if there is only a 

thin plate of Paleozoic rocks and much of the section is 

lower plate Mesozoic rocks. Low stacking velocities may 

sometimes indicate the presence of multiple reflections in 

the section. The simplest multiple path predicts a second 

reflection at twice the two-way traveltime and twice the 

dip and a stacking velocity of one-half the true rootmean-square velocity. However, there are no obvious 

shallower reflectors that may have generated multiples, 

particularly in the case of the laterally continuous reflector E-E'. Because of the inconsistencies between the 

sonic log velocities and the stacking velocities, the interpretation of the seismic reflection time section in the next 

section generally relies on the well log velocities (except 

within the basin fill section) to calculate depth to 

observed reflectors. 

INTERPRETED SUBSURFACE STRUCTURE 

BASIN GEOMETRY AND SHALLOW STRUCTURE OF THE 

WASATCH FAULT ZONE 

The most prominent and continuous reflectors on the 

seismic profile are from the late Cenozoic basin fill 

section (figs. 4, 5). Figure 6A presents a detailed view of 

this part of the seismic line and a possible interpretation 

of reflectors across minor faults within the basin. Figure 

6B is a one-to-one schematic depth cross section of 

several prominent reflectors showing the depth and dip 

of these reflectors as inferred from stacking velocities. 

Stacking velocities within the basin fill section compare 

quite favorably with velocity data from other basins (for 

example, Okaya and Thompson, 1985); because the 

reflectors are so strong and continuous, the stacking 

velocities were considered valid for this part of the line. 

The best-constrained reflectors are marked X and Y; 

three separate velocity analyses detail their depths. 

Depths of other reflectors shown on figure 6B were 

obtained by interpolation. 

The Wasatch fault zone bounds the basin on the east, 

and the internal structure of the basin is a gentle 

synclinal sag displaying dips of 7° to 20° on the western 

flank and 5° to 7o on the eastern flank. On the time 

section (fig. 6A), the basinward dips on the eastern flank 

are exaggerated because of a velocity increase (suggested by stacking velocities) eastward toward the 

Wasatch fault zone. 

The base of the valley fill section is not marked by a 

single well-defined reflector. In the western half of the 
LATE CENOZOIC, MESOZOIC, AND POSSIBLE PROTEROZOIC DEFORMATION, WASATCH FAULT ZONE Ell 

A 

w 

(/) 

0 

z 

0 

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315 

I 

w 

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0 

305 

I 

KILOMETERS 

295 

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285 

I 

2 

2 

FIGURE 6. -A, Detail of seismic reflection profile in Juab Valley 

showing basin fill. V and W mark possible surface locations of the 

Wasatch fault zone described in text. Vibrator points are numbered 

across the top of the section. A major fault on the western side of the 

basin is labeled a. Reflectors X and Yare discussed in the text. Heavy 

basin, the base of the valley fill is inferred to lie between 

the discontinuous basal synclinal sag (reflectors) and the 

deeper discordant reflectors (fig. 5A). The deepest part 

of the basin has a grabenlike structure, bounded on the 

west by a normal fault (fault a, fig. 6) having about 700 m 

of displacement. There are no obvious subjacent discord265 

I 

KILOMETERS 

3 

255 

I 

60" 

245 

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v 

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v W E 

solid lines are interpreted faults. B, Prominent reflectors in basin 

(true-depth cross section). Projections of planar faults having dips 

of 45° and 60° and reaching the surface at locations V and W are 

shown. Vertical and horizontal scales are equal. 

0.5 

1.0 

1.5 

ant reflectors directly beneath the eastern half of the 

basin, so only a minimum thickness of valley fill can be 

determined there. Mean velocities for the basin fill 

obtained from three stacking velocity analyses on reflectors near the base of the synclinal sag vary between 2. 7 4 

and 2. 97 km/s, the lower velocity being from the center of 
E12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

the valley and the slightly higher values being near the 

margins. Interval velocities between reflectors in the 

lower part of the basin fill exceed 3.2 km/s and may 

represent early Tertiary volcanic or sedimentary rocks 

(see table 1). The thickest section of basin fill is interpreted to lie beneath VP 285, where the fill is estimated 

to be 1. 60 km thick. 

The eastern margin of the basin is marked by a broad 

zone of little or no reflectance. Interpretation of this part 

of the section is hampered by a data gap caused by 

cultural noise associated with the town of Nephi. The 

eastward termination of the basin reflectors in this 

region may be owing to the data gap and the reduction in 

fold as the gap is approached but may also be owing in 

part to a general decay in reflector continuity in the 

poorly sorted, poorly stratified coarse clastics that comprise the alluvial fans adjacent to major range-front 

faults. This decay of coherent reflectors within the fan 

region (as opposed to the uniform reflectors from the 

finer grainer basinal facies) is typical of many basins 

within the Basin and Range province (Anderson and 

others, 1983; Zoback and Anderson, 1983; Okaya and 

Thompson, 1985). 

Prominent multiple-event Holocene scarps mark the 

trace of the Wasatch fault zone directly north of the 

study area (Schwartz and Coppersmith, 1984) (see fig. 2); 

the scarps extend southward to within about 2 km of the 

seismic line. A curved scarp, located basinward of the 

main zone of range-front scarps, has been identified by 

Cluff and others (1973) just north of the seismic profile 

(fig. 2). The surface location of the main Wasatch fault 

zone along the profile is believed to lie between the 

southerly projection of the large Holocene scarps to the 

north (W, fig. 6) (Schwartz and Coppersmith, 1984) and 

young faulting exhibiting a net vertical displacement of 

11m at depths of about 150m (V, fig. 6) inferred from a 

shallow high-resolution seismic reflection profile run 

along the same road as the industry reflection profile 

(Crone and Harding, 1984). The zone of shallow faulting 

at V is near the southern end of the curved scarp of Cluff 

and others (1973). However, minor fault displacements 

were also observed by the high-resolution seismic survey 

near W, the southward projection of the range-front 

scarp. 

Projections of faults that dip between 45° and 60° are 

shown on figure 5B at localities V and W. The eastward 

extent of reflector Y places a minimum dip of about 52° 

for a bounding fault having a surface trace at V. Locating 

the major bounding fault trace at V assumes that some 

coherent basin reflectors extend through any fan wedge 

that might have built up along such a bounding fault; it 

also assumes that there was no decay of the reflector 

signal by the data gap. Interestingly, high-quality reflection data possessing good surface-faulting control in a 

comparable basin (Dixie Valley, Nevada) indicate a 1. 0-

to 1.25-km fan wedge adjacent to the range-bounding 

fault (Okaya and Thompson, 1985); this hypothesis would 

favor location W as the major bounding-fault zone. Of 

course, there may well be a broad fault zone having 

displacement distributed across it, the youngest offset 

apparently being near V, as the high-resolution seismic 

data (Crone and Harding, 1984) indicate. 

Gravity data can help to constrain the geometry of the 

basin margin. As described earlier, gravity data were 

collected at more than 200 stations in the vicinity of and 

along the seismic profile. Gravity measurements were 

made approximately every 170 m along most of the 

reflection line. These data are plotted in cross section in 

figure 7. Several two-dimensional gravity models were 

computed for the basin fill assuming a -0.4 g/cm3 density 

contrast with adjacent bedrock. The only difference 

between the three models in figure 7 is the location of the 

eastern margin of the basin. The maximum fill thickness 

in the center of the basin for all models is about 1. 6 km, 

consistent with the interpretation of the reflection data. 

A notable feature of the models is a central graben, 

inferred from the reflection data. The configuration of 

and vertical displacement on the fault bounding the 

western side of the graben are taken directly from the 

reflection data. The modeled base of the basin closely 

matches the base inferred from the seismic data, except 

along the eastern margin of the basin. 

In model1 (fig. 7), the Wasatch fault zone is located at 

W, along the southern projection of the range-front fault, 

and has a dip of 60° W. This model matches the gravity 

gradient directly above the fault quite well but greatly 

overestimates the gravity values east of the fault. Model 

2 places the main Wasatch fault zone at V, the most 

basinward position of young faulting inferred from the 

high-resolution seismic reflection described by Crone 

and Harding (1984). This position for the bounding fault 

clearly provides a very poor fit to the observed gravity 

data for almost the entire eastern part of the basin. 

In an attempt to model the low gravity values east of 

the fault zone, an easternmost limit for faulting (model3) 

was generated from the surface location of bedrock in the 

range and from the western truncation of reflector R-R' 

on the footwall block (fig. 5B). This model overpredicts 

the anomaly to the west of the fault (although the fit 

would improve if a higher density alluvial wedge were 

added to the model). Once again, although the fit to the 

gravity data east of the fault is improved, the computed 

values are still too large in a 2-km-wide zone east of the 

easternmost possible fault location. This poor fit suggests that the source of the observed gravity low east of 

the fault is the result of less dense bedrock (Arapien 

rocks) of the footwall. 
LATE CENOZOIC, MESOZOIC, AND POSSIBLE PROTEROZOIC DEFORMATION, WASATCH FAULT ZONE E13 

A 

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EXPLANATION 

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EXPLANATION 

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FIGURE 7.-A, Gravity models of basin fill in Juab Valley. Open 

circles are observed gravity values; solid, dashed, and dotted 

lines give computed values based on the geometries indicated. V 

and W mark possible surface locations of the Wasatch fault zone 

described in text. A constant density contrast Ap of -0.4 g/cm3 

between basin fill and surrounding bedrock was assumed for all 

models. Reflectors X and Y from figure 6 are shown for reference. B, Same as modell in A, except a low-density tabular body 

(Ap=-0.07) has been added to the footwall of the Wasatch zone. 
E14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

In all three models in figure 7 A, the inferred base of 

the basin contradicts the position of the intra basin reflector Y whose depth was determined from an Advanced 

Interpretative Modeling Systems (AIMS, Geoquest) 

migration using stacking velocities on the seismic section. One explanation of this discrepancy between the 

gravity-inferred and seismically inferred basin geometries just west of the bounding-fault zone may be the 

assumption of a single density contrast for the entire 

basin. As previously mentioned, alluvial fans along major 

basin-bounding normal faults produce a wedge of higher 

velocity, higher density sediment adjacent to the fault 

zone. Okaya and Thompson (1985) used a fan wedge 

having a density 0.2 to 0.3 g/cm3 higher than that of the 

rest of the basin to reconcile gravity and reflection data 

from Dixie Valley, Nev. Although using a higher density 

fan wedge would increase the gravity values west of the 

fault (and thus improve the fit in model3), the fan wedge 

was not included in these simple models, since it would 

not explain the low gravity values east of the fault zone. 

Because the initial objective in modeling the gravity data 

was to constrain both the location and the shallow 

geometry of the Wasatch fault zone, several possible 

surface traces for the main bounding-fault zone (using 

the simple but probably unrealistic assumption of a 

constant density for the basin) were examined to determine the effects of different fault locations on the gravity 

field east of the fault. 

Figure 7 B shows a modified simple model that is 

generally consistent with the observed gravity values 

east of the fault. Here the main Wasatch fault zone is 

positioned at W, and a tabular low-density body roughly 

2 km wide has been included east of the fault zone. The 

geometry of this low-density body was chosen simply to 

provide a reasonable fit to the gravity data. Inclusion of 

a higher density clastic wedge west of the fault zone 

(discussed above) could provide a near-perfect fit to the 

data; however, in the absence of additional constraints on 

the geometries of these bodies, any near-perfect fit 

would be nonunique and not particularly instructive. The 

presence of a low-density body just east of the Wasatch 

fault zone is supported by the geology along the western 

edge of the Wasatch Range in the area of the seismic line. 

Here the seismic line passes an open-pit mine where 

large quantities of gypsum have been removed from the 

Arapien rocks. Possibly, the low-density body in the 

model represents a concentration of diapiric gypsum that 

rose along or just east of the Wasatch fault zone when the 

major bedrock offset occurred. 

Thus, because of complicated structure within the 

footwall, the gravity data cannot tightly constrain the 

near-surface location of the Wasatch fault zone. The 

gravity data do appear to exclude location V as the 

position of the main Wasatch fault. On the basis of the 

geology and the high-resolution seismic work, location W 

seems to be the most likely position for the main fault 

zone at the time of the major bedrock offset. 

DEEP STRUCTURE 

Prominent reflectors on the seismic profile are shown 

on the line drawing in figure 5A. Each of the lettered 

reflectors was represented by a straight-line segment or 

a series of straight-line segments and was migrated by 

means of an AIMS migration scheme and a very simple 

velocity model. Velocities in Juab Valley were obtained 

from the stacking velocities; elsewhere, a constant velocity of 5 km/s was assumed, because, as discussed earlier, 

stacking velocities were generally unrealistically low. A 

velocity of 5 km/s is probably somewhat low for the 

deeper portions of the section where Paleozoic rocks are 

probably present; thus, the depths given for the deep 

events are only minimum depths. 

Figure 5B shows the approximate true-depth positions 

(obtained from the AIMS migration) of the lettered 

reflectors in figure 5A. The outline of the inferred basin 

structure (from the gravity and seismic interpretation in 

fig. 6B) is indicated, as are projections of planar faults 

dipping 45° and 60° from the preferred near-surface fault 

location W. The relationship and position of the lettered 

reflectors are discussed below, and their geologic interpretation is described in the next section. 

Subhorizontal reflectors are common throughout the 

profile to depths of 2.5 to 3.0 s, and there are several 

strong and relatively continuous reflectors (marked P 

and Q) at two-way traveltimes of 3.3 to 3. 7 s on the 

western half of the profile. If the velocity model 

described above is used, the depth to the top of the P 

reflectors is greater than 8.3 km. Relatively shallow 

reflectors in the western half of the seismic profile 

include the events marked A, 8-B', C-C', and D-D'. 

These events are beneath the pass between Long Ridge 

and West Hills, where overturned, steeply dipping Paleozoic rocks believed to be part of the upper plate of the 

N ebo thrust are exposed at the surface. The complex 

band of reflectors marked A, B-B', and C-C' lacks 

significant lateral continuity. Events A and B-B' are at a 

depth of 1.65 to 2.0 km (if a mean near-surface velocity of 

5.0 km/s is assumed). Event C-C' occurs at a depth of 

about 2.3 km, and a slightly deeper zone of subhorizontal 

reflectors (D-D'), laterally continuous for 6 km, is at a 

depth of about 3. 0 km. 

Where the Nebo thrust is exposed in the southern 

Wasatch Mountains on the eastern side of the basin, 

Paleozoic limestones in the upper plate of the N ebo 

thrust are emplaced on Arapien rocks, which, on the 

basis of the velocities and densities given in table 1, 

should produce a significant impedance contrast. How-
LATE CENOZOIC, MESOZOIC, AND POSSIBLE PROTEROZOIC DEFORMATION, WASATCH FAULT ZONE E15 

ever, the lack of a single continuous, well-defined reflector marking the N ebo thrust fault on the western half of 

the profile is not surprising and is probably due to the 

complex structure of the overturned anticline forming 

the upper plate. The Paleozoic rocks are overturned at 

the surface and dip to the north 50° to 75°. An additional 

complicating structural feature along this portion of the 

line is the apparent eastward tilt of the entire Long 

Ridge-West Hills range complex. Oligocene volcanic and 

volcanic-sedimentary rocks capping these two ranges are 

rather uniformly tilted 15° to 30° to the east. The steeply 

dipping, overturned Paleozoic limestones (inferred to be 

part of the Nebo thrust) that are exposed in the pass 

between these two ranges presumably also have been 

eastward tilted. However, none of the subhorizontal 

reflectors A, B-B1

, C-C1, or D-D1 have eastward dips. 

Either structural accommodation of the tilting seen at 

the surface occurs at depths of less than 1. 5 to 2. 0 km, or 

else those shallow reflectors had a westward dip before 

tilting. Thus, because of the complex internal structure 

of the thrust sheet and post-thrust tilting of the range, as 

well as the possibility of a ramp in the thrust near the 

Wasatch fault (discussed previously), not even an 

approximate thickness of the N ebo plate can be used to 

constrain which of the reflections A, B-B 1

, C-C 1, or D-D 1 

might be from the actual N ebo thrust plane. 

The most continuous zone of deep reflectors is E-E 1

, a 

nearly horizontal band of reflectors that extends across 

the western half of the section at 1.8 to 2.0 s two-way 

traveltime (fig. 5A). These reflectors are laterally continuous for at least 10 km and possibly for as much as 15 

km (to G-G 1). An average (root mean square) velocity of 

about 5. 0 km/s for this part of the profile yields a depth 

of 4. 7 km for this prominent reflector. The eastward 

continuation of reflector E-E 1

, which appears to exhibit 

velocity "pulldown" beneath the basin, is obscured by 

crosscutting events, including west-dipping events K and 

F-F 1

• However, on the migrated depth section shown in 

figure 5B, the eastern end of event E-E 1

, when corrected 

for velocity pulldown, actually dips to the west beneath 

the western part of the basin. 

On the time section (fig. 5A), it appears that events 

E-E 1 and G-G 1 might be laterally continuous. However, 

on the migrated depth section, the events appear unrelated. Reflector G-G 1 becomes subhorizontal, shifts eastward, and would be located in the footwall block of a fault 

dipping 60° and having a surface trace at W. Significantly, west-dipping events L-L1 and F-F1 (which appear 

to cut events E-E1 and G-G1 on the time section and could 

be interpreted as possible reflections from a listric fault 

plane) migrate, according to the simple velocity model 

described above, updip and are located, along with G-G 1

, 

in the footwall block of a 60° (or shallower) fault plane. 

The apparent dips, in the plane of the profile, of reflectors L-L1 and F-F1 are 11° W. and 30° W., respectively 

(true dips may be steeper). Event M also appears to be 

in the footwall block of a 60° fault, whereas the top of the 

deep events marked Q would be within the footwall block 

of a west-dipping planar fault of 55° or less. Similarly, the 

top of the subhorizontal deep event P constrains a planar 

fault dip to be greater than 47°. 

The reflectors deeper than E-E1

, including those 

marked H-H 1

, 1-1 1, J-J 1, and K, are rather complex, 

consisting of west-dipping, east-dipping, and crosscutting events. Westward-dipping event J-J 1

, beneath 

the western side of the basin, moves considerably upon 

migration and has an apparent dip of22° W. East-dipping 

event N migrates updip and appears to be truncated by 

event J-J'. Similarly, event 1-1 1 also migrates to event 

J-j 1 and appears truncated. The migrated position and 

geometry of event J-J 1 are highly suggestive of a reflection from a listric fault plane possibly flattening and 

merging with event H-H 1

• However, there is no suggestion on the actual reflection data of continuity between 

J-j 1 and H-H1

• 

The general character of the seismic profile changes 

near the Wasatch fault zone. The eastern half of the 

profile, beneath the Gunnison Plateau, is characterized 

by a thick sequence of short, discontinuous subhorizontal 

reflectors. With the exception of the reflectors marked 

R-R1

, S, T, and U on figure 5A, no attempt has been made 

to interpret these numerous discontinuous reflectors. 

Seismic reflection and well data along the western margin of the Gunnison Plateau 15 to 20 km south of the 

profile indicate a complicated structure within the Arapien rocks, including multiple thrust faults and overturned to recumbent folds (Standlee, 1982; Lawton, 

1985). The west-dipping reflector R-R1 on the time 

section is one of the stronger reflectors east of the 

Wasatch fault zone and appears to truncate overlying 

horizontal reflectors. Upon migration, the western portion of event R-R1 becomes subhorizontal and appears to 

be cut by the west-dipping eastern limb of R-R1

• This 

eastern limb may also truncate subhorizontal reflectors 

S, T, and U. 

Prominent subhorizontal reflectors beneath the Gun~ 

nison Plateau argue against large-scale deep-seated salt 

diapirism along the axis of the Gunnison Plateau, as 

Witkind (1982, 1983) suggested. In fact, the axis of a 

proposed major diapiric fold interpreted to extend for at 

least 38 km along strike (Witkind, 1983) crosses the 

seismic line approximately at the location of the star on 

figure 5A and B. Reflectors in the shallowest part 

of the time record (fig. 5A) here are slightly upwarped; 

however, the deeper reflectors (S, T, and U), at approximate depths of 1.5, 2.0, and 2. 75 km, respectively 

(assuming a velocity of 5. 0 km/s), are subhorizontal 

beneath this presumed anticlinal axis and thus suggest 
E16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

0 

Highly deformed Lower Mesozoic-Paleozoic rocks 

10 

KILOMETERS v w 

ii:l 

]b!d 

A~~NE~B~O~TH~R~U~S~T--~~~~~~--~~--~ 

Jurassic Arapien rocks 8---- ----R'--S 

2 - -----T 

en 3 a: 

w 

.... 

C----C' 

Twin Creek Limestone 

D D' 

Navajo Sandstone 

---U 

L-L' 

State #2 F' Series of bedding plane thrusts, 

thrust 0 A o ~ 4 

0 

...J 

~ 5 

z 

~ 6 

t 

~ 7 

F"' duplexing, and ramping involving 

Permian Kirk\an Limestone? ~ E' b y , pr~ma~ily rocks of Jurassic and 

aAr~;ien rocks? AJ' : M G Tnassrc age 

Lower Mesozoic-Paleozoic 

Lower Mesozoic rocks? , autochthonous rocks 

.....___..,_~H' a w8 -=:= ~J~I/ .// 

8 7~ ~--a 9 

Autochthonous 

Paleozoic rocks 

-P--

Proterozoic rift 

basin sediment? 

EXPLANATION 

Thrust fault-Teeth 

on upper plate 

Normal fault 

FIGURE 8. -Geologic interpretation of the western half of the true-depth line drawing shown in figure 5B. Heavy lines are seismic reflections. 

Inferred stratigraphic contacts are indicated. 

that the diapirism may be a localized shallow phenomena 

rather than a major deep-seated feature. 

GEOLOGIC INTERPRETATION OF DEEP STRUCTURE 

One possible geologic interpretation of the subsurface 

structure on the western half of the seismic profile 

consistent with subsurface data and regional geologic 

structure is shown on the true-depth line drawing in 

figure 8 and described below. Reflector A and short 

west-dipping reflectors located just west of graben fault 

a (fig. 6) are interpreted to be the base of the Nebo 

thrust, which rests on the lower part of the Arapien 

section. The seismic profile is interpreted to obliquely cut 

the southern edge of the N ebo thrust near a ramp in the 

thrust inferred from surface exposures of the thrust just 

east of the Wasatch fault zone. As discussed earlier, none 

of the reflectors in this area display the westward dip 

expected for a thrust fault ramping up to the east; 

however, the younger eastward tilt of the Long RidgeWest Hills range no doubt complicates and obscures the 

earlier thrust geometry. 

Reflectors B-B', C-C', and D-D' are interpreted as 

stratigraphic boundaries, possibly within Arapien rocks 

or from the base of Arapien rocks resting on JurassicTriassic Navajo Sandstone. This interpretation is based 

in part on well data from the Howard #1-A and Howard 

#2 wells drilled 8 km south of the seismic line (figs. 1, 2). 

Both wells were positioned south of the southern edge of 

the N ebo thrust (Leamington tear fault) and were spud 

in Tertiary sedimentary rocks, drilled through Cretaceous synorogenic conglomerates into the Jurassic Arapien rocks (thicknesses of 2,082 and 1,977 m, respectively), and bottomed in Jurassic and Triassic rocks. The 

absolute depth range at which the Arapien rocks were 

encountered in these two wells is indicated on the 

left-hand side of figure 8. Sprinkel (1982) designated the 

lower 431 m of the Arapien section of Howard # 1-A as 

the limestones of the Jurassic Twin Creek; he did not 

report on the data from Howard #2. Thus, interpretation 

of B-B', C-C', and D-D' as being from within or from the 

base of the Jurassic rocks is consistent with the depth 

levels of Arapien rocks south of the N ebo thrust. 

The State #2 well, located approximately 28 km south 

of the seismic line (fig. 1), was drilled through a thin 

section of Arapien rocks and apparently a complete 

section (1,383 m thick) of Jurassic-Triassic (Navajo Sandstone) and Triassic rocks testing on 672 m of Permian 

rocks (Diamond Creek Sandstone and Kirkman Limestone) (D.A. Sprinkel, oral commun., 1984). The Permian rocks were truncated by a thrust fault that 

emplaced the overlying rocks on the Navajo Sandstone. 

The absolute depth of this thrust of Permian over 

Jurassic-Triassic rocks was 2,542 m below sea level, 

which corresponds roughly to a depth (from the surface) 

along the seismic profile of about 4.15 km (see left-hand 
LATE CENOZOIC, MESOZOIC, AND POSSIBLE PROTEROZOIC DEFORMATION, WASATCH FAULT ZONE E17 

side of fig. 8). Thus, if 0-01 is interpreted as the top of 

the Navajo Sandstone, then the prominent, nearly horizontal reflector E-E 1 is in a similar position with respect 

to the Permian limestones encountered just above the 

thrust in State #2. Beneath the Cenozoic basin, reflector 

E-E 1 begins to dip westward and is interpreted to be part 

of a ramp anticline above this inferred blind thrust fault. 

The blind thrust is shown as emplacing Permian on early 

Mesozoic, possibly Arapien rocks. The section between 

0-01 and E-E1 is about 1.58 km thick, comparable to the 

1.38-km-thick section of Jurassic-Triassic rocks beneath 

the Arapien rocks and above the Permian in State #2. 

Reflectors H-H 1 are interpreted as a deeper blind 

thrust that ramps up (J-J') and possibly splays (K). 

Events 1-1 1 and N appear truncated by J-J 1

• The section 

beneath E-E1 and above H-H1 probably represents Jurassic and Triassic rocks; the section is probably not a 

complete section, however, because Standlee (1982) estimated a 1, 702-m thickness of Arapien rocks alone 

beneath the Gunnison Plateau, and the State #2 well 

encountered an additional 1. 38 km of Jurassic and Triassic rocks beneath the Arapien rocks. The impedance 

contrast at H-H 1 is interpreted to be due to the thrust 

juxtaposition of lower Mesozoic rocks on Paleozoic limestones. The 1.8-km-thick section between H-H 1 and P is 

interpreted as autochthonous Paleozoic rocks. A nearly 

complete section of autochthonous Paleozoic rocks 

(approximately 1,280 m thick) was drilled in U.S. E #1, 

located approximately 30 km east of the western end of 

the seismic line. Along most of its length, the Wasatch 

fault zone is close to and generally parallels the "Wasatch 

line" (Kay, 1951), the hinge line that marks the transition 

from shelf to basinal facies of the Cordillera geosyncline 

(that is, the zone of dramatic westward thickening of 

strata deposited on the late Precambrian rifted margin 

(Stewart, 1972). Thus, the 1,280 m encountered in U.S. 

E # 1 probably represent a minimum thickness for the 

autochthonous Paleozoic rocks. H-H 1 may mark a thrust 

occurring near the top of the Paleozoic section. Both of 

the inferred blind thrust faults (below E-E1 and H-H1

) 

may be cut and offset by the Wasatch fault zone and may 

be correlative with the bedding plane thrust(s) inferred 

beneath the southern Wasatch Mountains just north of 

the seismic profile by Jefferson (1982) and Lawton 

(1985). 

The band of reflectors defined by P are interpreted as 

late Proterozoic sediments. Thrust ramps have been 

interpreted near the location of the modern Wasatch 

fault in several localities (for example, Smith and Bruhn, 

1984). These ramps may have developed over abrupt 

changes in basement geometry (that is, preexisting 

structural discontinuities). Specifically, it is proposed 

that the thrust ramps may have developed over westdipping normal faults that formed during the initial 

Precambrian rifting event in the Precambrian crystalline 

basement. In this scenario, the reflectors marked by P 

may be sediments deposited in such a rift basin. 

Well data along the westernmost edge of the Gunnison 

Plateau 25 km to the south (Gunnison #1, fig. 1, and 

other wells not shown) indicate a very complicated 

shallow structure beneath the plateau, including recumbent and overturned folds and numerous bedding-plane 

thrusts involving Arapien rocks and Jurassic-Triassic 

rocks (Standlee, 1982; Lawton, 1985). The possible 

thrust splay K and reflectors R-R1 and F-F1 may be 

related to such deformation. 

Two possible subsurface geometries for the Wasatch 

fault zone are indicated in figure 8. The position and 

westward dip of reflectors E-E1 and J-J 1 beneath the 

Cenozoic basin strongly suggest a listric geometry for 

the main Wasatch fault zone. In particular, reflection 

J-j 1

, which appears to flatten and merge with inferred 

blind thrust H-H 1

, is an excellent candidate for listric 

backsliding on a preexisting thrust. The symmetric, 

sag like geometry of the Cenozoic basin and, specifically, 

the lack of growth faulting and reverse drag (for example, Anderson and others, 1983) argue against such a 

sharply curved listric geometry (flattening at depths of 

only 4.5-6.5 km). The alternate interpretation-a planar 

dip of 54 o for the main fault zone-is constrained by the 

eastern end of reflectors J-J 1 and the western truncation 

of reflector Q. The synclinal sag like geometry of the 

basin is produced by beds dipping away from the boundary fault zone and appears related to the development of 

the small central graben. Analysis of a number of seismic 

reflection profiles in the Great Basin area led Anderson 

and others (1983) to suggest that the saglike geometry is 

commonly associated with a steep, relatively planar 

bounding-fault zone. Dixie Valley, Nev., is a welldocumented example of such a basin and steep planar 

fault geometry (Okaya and Thompson, 1985). Microearthquake studies in central U tab also support a highangle fault interpretation, indicative of seismic strain 

release on relatively steep (approximately 33°-90°) 

planes (Arabasz, 1983). 

Distinguishing between the two alternate models for 

the Wasatch fault geometry is complicated by a possible 

earlier episode of extensional tectonism responsible for 

the 15° to 30° eastward tilt of the Oligocene volcanic 

and sedimentary rocks capping Long Ridge and West 

Hills. An inferred angular unconformity between these 

rocks and the oldest basin fill sediments on the western 

end of the seismic line (maximum dips of 20° E.) suggests possible listric or block-tilt faulting before the 

development of the modern basin. As mentioned earlier, 

the lack of eastward-tilted reflectors at depth may indicate a relatively shallow depth or a complex geometry for 

accommodation of this earlier tilting. 
E18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Both the listric and the planar fault models suggest a 

markedly asymmetric fault pattern for the development 

of Juab Valley. The west-dipping Wasatch fault zone is 

the master fault, and internal basin structure is related 

to formation of a central graben. There is a net topographic decrease across the Wasatch fault zone, as range 

heights on either side of Juab Valley indicate (range 

height is also moderated by bedrock lithology of the 

ranges). The elevation of Long Ridge (1,800-2,130 m) 

and West Hills (1,800-1,900 m) are 500 to 800 m lower 

than those of the southern Wasatch Mountains 

(2,440-3,600 m) and the Gunnison Plateau (2,440-2, 700 

m) east of the Wasatch fault zone. This net topographic 

offset could be accommodated by either the listric or the 

planar fault model. 

DISCUSSION 

It is apparent that there is no unique interpretation of 

the reflection section, despite constraints imposed by the 

gravity data and the high-resolution seismic profiling of 

Crone and Harding (1984). The complex and superposed 

Mesozoic and Cenozoic deformation as well as the lack of 

any drill-hole data along the profile permits only inferences on the subsurface thrust geometry. As noted 

above, if several of the reflectors are interpreted as 

thrust faults, simple listric backsliding on these faults 

fails to explain the basin fill geometry. 

The steep-fault interpretation presented here contrasts with Smith and Bruhn's (1984) interpretation for 

the Wasatch fault zone 18 km to the south near Levan. 

From a Vibroseis profile across the fault near Levan (see 

fig. 2), Smith and Bruhn interpreted a planar fault zone 

having a 34° dip and penetrating only 5 km deep. Reverse 

drag and growth faulting are observed in the subsurface 

structure of the basin along their profile. Schwartz and 

Coppersmith (1984) inferred a segment boundary along 

the Wasatch fault zone between the two reflection profiles, allowing the possibility that both subsurface geometries might be correct and applicable to different major 

fault segments. 

As described in the section on geologic setting, the 

Nebo thrust and a deeper thrust(s) within the study area 

may ramp up in the vicinity of the Wasatch fault zone. 

Smith and Bruhn (1984) concluded that, along several of 

the segments (including the segment in the study area), 

the Wasatch fault zone is located above inferred westdipping Sevier thrust fault ramps. They suggested that, 

along several segments, the Wasatch fault may overlie 

and utilize the thrust fault ramps and merge with subhorizontal thrusts at depth. This conceptual model 

strongly influenced their and Standlee's (1982) interpretations of the subsurface geometry of the Wasatch fault 

zone for the Levan seismic profile mentioned above. 

As Smith and Bruhn suggested, the general correlation between the Wasatch fault zone and the inferred 

thrust ramps in the subsurface can be used to argue for 

a relationship between the location of late Cenozoic 

normal faults and the geometry of older thrust faults. 

Alternatively, an even older structure may have controlled the development of both the thrust ramps and the 

Wasatch fault zone. The location of thrust fault ramps 

can be controlled by facies changes along the decollement 

horizon or by interruptions of the decollement by basement offsets such as normal fault zones (Suppe, 1985, p. 

282). It is suggested here that west-dipping normal fault 

zones that formed in the Precambrian crystalline basement during the early phases (pre-drift) of Precambrian 

rifting (Stewart, 1972) may have provided the structural 

relief responsible for the formation of the thrust ramps. 

The modern passive margin along the Atlantic coast of 

the United States contains numerous examples of major 

Triassic basins that formed on the shelf near the contemporary hinge line (for example, Klitgord and Behrendt, 

1979; Hutchinson and others, 1986). Since the Wasatch 

fault zone developed near the Paleozoic hinge line, similar Precambrian rift basins may be located at depth near 

the trace of the modern Wasatch fault. Interestingly, the 

lengths of these Atlantic shelf basins are similar to the 

length of the basin structure along the modern Wasatch 

fault. Hence, it is suggested here that the location of both 

the thrust ramps and the modern Wasatch fault ultimately may have been controlled by the location of a 

Precambrian normal fault zone bounding a basin or series 

of rift basins along the developing continental shelf. 

Wheeler and Krystinik (1988) independently suggested that Precambrian extensional faults may help 

localize the Wasatch fault zone and control its geometry. 

They discussed geologic and stratigraphic data that 

suggest the presence of these late Precambrian normal 

faults and cited several examples of reactivation of such 

subthrust faults. The critical test of this hypothesis 

would be the identification of the sedimentary sections 

filling the basins of Late Precambrian-early Paleozoic 

age that were bounded by these faults. Unfortunately, 

the thick cover of Mesozoic thrust sheets has deeply 

buried these basins, and, as is obvious from the present 

study, the complex superposed Mesozoic and late Cenozoic structure makes it difficult to identify these older, 

deeper structures. The identification here of reflector P 

as a sequence of Proterozoic sediments is purely speculative and intended to provoke further studies. 

CONCLUSIONS 

Seismic reflection and gravity data suggest that the 

overall structure of Juab Valley near the town of Nephi 
LATE CENOZOIC, MESOZOIC, AND POSSIBLE PROTEROZOIC DEFORMATION, WASATCH FAULT ZONE E19 

is an asymmetric sag, in which beds on both sides of the 

valley tilt toward the axis of the valley. A central graben 

defines the deepest part of the basin. Anderson and 

others (1983) and Zoback and Anderson (1983) have 

identified this type of basin as commonly being bounded 

by one or more major, planar, steep normal faults. The 

maximum valley fill thickness is about 1. 60 km, on the 

basis of interpretation of both the reflection and gravity 

data (if a density contrast for basin sediments of -0.4 

g/cm3 is assumed). Possible displacement of a buried 

thrust(s), truncation of deep reflectors, and the absence 

of reverse drag in basin sediments suggest that the 

Wasatch fault zone has a planar, high-angle dip of 50° 

to 55° (as opposed to a listric or low-angle geometry). 

However, the lack of reverse drag within basin 

sediments is the strongest evidence against a listric 

geometry. 

On a more regional note, structural data suggest the 

presence of major ramps in the Mesozoic thrust system 

near the trace of the modern Wasatch fault at several 

localities along its length. One possible source for this 

ramping may be major basement displacements on normal faults formed along a north-trending, west-facing 

passive margin that developed through the region in 

Late Proterozoic time (Stewart, 1972). Thus, the 

Wasatch fault zone may be localized by a zone of preexisting extensional faulting in Precambrian crystalline 

rocks. 

REFERENCES CITED 

Anderson, R.E., Zoback, M.L., and Thompson, G.A., 1983, Implication of selected subsurface data on the structural form and 

evolution of some basins in the northern Basin and Range Province, Nevada and Utah: Geological Society of America Bulletin, v. 

94, p. 1055-1072. 

Arabasz, W .J., 1983, Geometry of active faults and seismic deformation 

within the Basin and Range-Colorado Plateau transition, central 

and SW Utah: Earthquake Notes, v. 54, no. 1, p. 48. 

Black, B.A., 1965, Nebo overthrust, southern Wasatch Mountains, 

Utah: Brigham Young University Geology Studies, v. 12, p. 55-89. 

Brown, R.P., and Cook, K.L., 1982, A regional gravity of the SanpeteSevier Valley and adjacent areas in Utah, in Nielson, D.L., ed., 

Overthrust belt of Utah: Utah Geological Association Publication 

10, p. 121-136. 

Burchfiel, B. C., and Hickcox, C. W., 1972, Structural development of 

central Utah, in Baer, J.L., and Callaghan, E., eds., PlateauBasin and Range transition zone, central Utah: Utah Geological 

Association Publication 2, p. 55-66. 

Cluff, L.S., Brogan, G.E., and Glass, E.E., 1973, Wasatch fault, 

southern portion, earthquake fault investigation and evaluation-A guide for land use planning: Oakland, Calif., report 

prepared by Woodward-Lundgren Associates for Utah Geological 

and Mineralogical Survey under Project G-12069A, 33 p. 

Crittenden, M.D., Jr., 1961, Magnitude of thrust faulting in northern 

Utah: U.S. Geological Survey Professional Paper 424-D, p. 

D128-D131. 

Crone, A.J., and Harding, S.T., 1984, Near-surface faulting associated 

with Holocene fault scarps, Wasatch fault zone, Utah-A preliminary report, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, p. 241-265. 

Gilliland, W.N., 1963, Sanpete-Sevier Valley anticline of central Utah: 

Geological Society of America Bulletin, v. 74, p. 115-124. 

Hardy, C.T., 1952, Eastern Sevier Valley, Sevier and Sanpete Counties, Utah, with reference to formations of Jurassic age: Utah 

Geological and Mineral Survey Bulletin 43, 98 p. 

Heller, P.L., Bowdler, S.S., Chambers, H.P., Coogan, J.C., Hagen, 

E.S., Shuster, M.W., Winslow, N.S., and Lawton, T.F., 1986, 

Time of initial thrusting in the Sevier orogenic belt, IdahoWyoming and Utah: Geology, v. 14, p. 388-391. 

Hutchinson, D.R., Klitgord, K.D., and Detrick, R.S., 1986, Rift basins 

of the Long Island platform: Geological Society of America Bulletin, v. 97, p. 688-702. 

Jefferson, W.S., 1982, Structural and stratigraphic relations of Upper 

Cretaceous to Lower Tertiary orogenic sediments in the Cedar 

Hills, Utah: Utah Geological Association Publication 10, p. 65-80. 

Jones, T.D., and Nur, A., 1984, The nature of seismic reflections from 

deep crustal fault zones: Journal of Geophysical Research, v. 89, 

no. B5, p. 3153-3171. 

Kay, M., 1951, North American geosynclines: Geological Society of 

America Memoir 48, 143 p. 

Klitgord, K.D., and Behrendt, J.C., 1979, Basin structure of the U.S. 

Atlantic margin, in Watkins, J.S., Montadert, L., and Dickerson, 

P. W., eds., Geological and geophysical investigations of continental margins: American Association of Petroleum Geologists Memoir 29, p. 85-112. 

Lawton, T.F., 1982, Lithofacies correlations within the Upper Cretaceous Indianola Group, central Utah, in Nielson, D.L., ed., 

Overthrust belt of Utah: Utah Geological Association Publication 

10, p. 199-213. 

---1985, Style and timing of frontal structures, thrust belt, central 

Utah: Bulletin of the American Association of Petroleum Geologists Bulletin, v. 69, p. 1145-1159. 

Morris, H.T., 1983, Interrelations of thrust and transcurrent faults in 

the central Sevier orogenic belt near Leamington, Utah, in Miller, 

D.M., Todd, V.R., and Howard, K.A., eds., Tectonic and stratigraphic studies in the Eastern Great Basin: Geological Society of 

America Memoir 157, p. 75-82. 

Okaya, D.A., and Thompson, G.A., 1985, Geometry of Cenozoic 

extensional faulting: Dixie Valley, Nevada: Tectonics, v. 4, p. 

107-126. 

Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquakes: Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional tectonics of 

the eastern Basin-Range: Inferences on structural style from 

seismic reflection data, regional tectonics, and thermal-mechanical 

models of brittle-ductile deformation: Journal of Geophysical 

Research, v. 89, no. B7, p. 5733-5762. 

Spieker, E.M., 1946, Late Mesozoic and early Cenozoic history of 

central Utah: U.S. Geological Survey Professional Paper 205-D, p. 

D117-D161. 

Sprinkel, D.A., 1982, Twin Creek Limestone-Arapien Shale relations 

in central Utah, in Nielson, D.L., ed., Overthrust belt of Utah: 

Utah Geological Association Publication 10, p. 169-180. 

Standlee, L.A., 1982, Structure and stratigraphy of Jurassic rocks in 

central Utah: Their influence on tectonic development of the 

Cordilleran foreland thrust belt, in Powers, R.B., ed., Geologic 
E20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

studies of the Cordilleran thrust belt: Denver, Colo., Rocky 

Mountain Association of Geologists, p. 357-382. 

Stewart, J.H., 1972, Initial deposits in the Cordilleran geosyncline: Evidence of a Late Precambrian (850 m.y.) continental 

separation: Geological Society of American Bulletin, v. 83, p. 

1345-1360. 

Stewart, J.H., and Poole, F.G., 1974, Lower Paleozoic and uppermost 

Precambrian Cordilleran Miogeocline, Great Basin, western 

United States, in Dickinson, W.R., ed., Tectonics and sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, p. 28-57. 

Suppe, J., 1985, Principles of structural geology: Englewood Cliffs, 

N.J., Prentice-Hall, 537 p. 

Wallace, R.E., and Morris, H.T., 1979, Characteristics of fault and 

shear zones as seen in mines at depths as much as 2.5 km below the 

surface, in Speed, R., and Sharp, R., co-organizers, and 

Evernden, J.F., convenor, Proceedings of Conference VIIIAnalysis of actual fault zones in bedrock: U.S. Geological Survey 

Open-File Report 79-1239, 599 p. 

Wheeler, R.L., and Krystinik, K.B., 1988, Segmentation of the 

Wasatch fault zone, Utah-Summaries, analyses, and interpretations of geological and geophysical data: U.S. Geological Survey 

Bulletin 1827, 47 p. 

Witkind, I.J., 1982, Salt diapirism in central Utah, in Nielson, D.L., 

ed., Overthrust belt of Utah: Utah Geological Association Publication 10, p. 13-30. 

---1983, Overthrusts and salt diapirs, central Utah: Geological 

Society of America Memoir 157, p. 45-59. 

Zoback, M.L., 1983, Structure and Cenozoic tectonism along the 

Wasatch fault zone, Utah: Geological Society of America Memoir 

157, p. 3-27. 

Zoback, M.L., and Anderson, R.E., 1983, Style of Basin-Range 

faulting as inferred from seismic reflection data in the Great Basin, 

Nevada and Utah: Geothermal Resources Council Special Report 

13, p. 363-381. 
Neotectonic Framework of the Central 

Sevier Valley Area, Utah, and 

Its Relationship to Seismicity 

By R. ERNEST ANDERSON and THEODORE P. BARNHARD 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-F 

CONTENTS 

Page Page 

Abstract ......................................................................... . F1 Results from Fault-Slip Sample Areas-Continued 

Introduction .................................................................... . 2 Sevier Fault-Continued 

Acknowledgments ...................................................... . 6 Annabella Segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F25 

Procedure ....................................................................... . 6 Anticline Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

Structural History and Setting ........................................... . 7 Northern Antelope Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Results from Fault-Slip Sample Areas ................................. . 10 Seismicity and Late Quaternary Deformation . . . . . . . . . . . . . . . . . . . . . . . . 32 

Clear Creek Downwarp .............................................. . 10 Northeast of Annabella................................................ 34 

Pavant Range ........................................................... . 18 Southwest of Elsinore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

Joseph Flats to Richfield ....................................... . 18 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

Richfield to Willow Creek ..................................... . 20 Role of Diapirism........................................................ 44 

Sevier Fault ............................................................. . 21 Speculations on Crustal-Scale Deformation...................... 44 

Monroe Segment ................................................. . 23 References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map showing location of the central Sevier Valley study area relative to the structural transition zone and 

bordering provinces.................................................................................................................................. F2 

2. Map showing distribution of earthquake epicenters in Utah for the period July 1962 through September 1984 .............. 3 

3. Geologic map and section of the central Sevier Valley area................................................................................... 4 

4. Generalized stratigraphic column for the central Sevier Valley area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

5-8. Photographs showing: 

5. Dry Wash fault ......... ................. .... ... .... ... . .. . .. ...... ... ... . ........... ............ ... ... .. ....... ... .. ... . ..... .... .. . .. . .. ...... 11 

6. View to the northwest from Dry Wash fault across Clear Creek to the trace of the sinistral strike-slip fault . . . . 12 

7. Northeast-trending sinistral-slip fault shown in figure 6 ........................................................................... 13 

8. Well-exposed, large, smooth, striated surface of small sinistral-slip fault in the Joe Lot Tuff Member of the 

Mount Belknap Volcanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

9. Schematic representation in block diagram form showing a fold cut by a sinistral fault and two dextral faults . . . . . . . . . . . . . . . 14 

10. Histograms of strike, dip, and rake and lower hemisphere stereographic plots of computed paleostress axes for faults 

from the Clear Creek sample area in the easternmost part of the Clear Creek downwarp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

11. Photograph looking northwest at the excavated surface of a steep planar fault that cuts transversely through 

northeast-tilted beds of the Sevier River Formation at locality A in figure 3...................................................... 15 

12. Lower hemisphere stereographic plot of the main fault and 16 subsidiary northeast-trending faults at locality A in 

figure 3 .................................................................................................................................................. 16 

13. Photograph showing view looking north along the well-exposed Joe Lott Tuff Member of the Mount Belknap Volcanics 

in the footwall of a north-northwest-striking normal fault at locality Bin figure 3............................................... 16 

14. Lower hemisphere plots of fault-slip and bedding data at localities along Interstates 70 and 15................................... 17 

15. Histograms of strike, dip, rake, and sense of fault slip for faults along the southeastern margin of the Pavant Range 

between Joseph Flats and Richfield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

16. Photograph showing west-looking view in Flat Canyon north of Elsinore showing the offset contact between smoothand rough-weathering beds of gently southwest-dipping sandstone, siltstone, and claystone belonging to the 

formation of Aurora of late Eocene age........................................................................................................ 20 

17. Lower hemisphere stereographic plot of poles to bedding along the flank of the Pavant Range between Joseph Flats 

and Richfield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

18. Map showing generalized geology of the northwestern part of the Aurora 7V2-min quadrangle................................... 22 

19. Cross-sectional sketch showing the typical fault offsets seen in the monoclinally flexed margin of the Pavant Range . . . . . . 23 

III 
IV 

FIGURE 20. 

21. 

22. 

23. 

24. 

25. 

26. 

27. 

28. 

29. 

30. 

31. 

32. 

33. 

34. 

35. 

36. 

37. 

CONTENTS 

Histograms of strike, dip, and rake, lower hemisphere stereographic plot of faults, striae, and computed paleostress 

axes, and plot of slip sense for faults along the eastern margin of the Pavant Range between Richfield and Willow 

Page 

Creek..................................................................................................................................................... F24 

Histograms of strike, dip, and rake, lower hemisphere stereographic plots of computed paleostress axes, and plot of 

dip-slip versus strike-slip components of fault slip for faults in the western margin of the Sevier Plateau in the 

Monroe area............................................................................................................................................ 25 

Lower hemisphere plot of poles to bedding in bedrock along the Annabella segment of the Sevier fault . . . . . . . . . . . . . . . . . . . . . . . 26 

Geologic map and cross section of the Annabella-Glenwood area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

Histograms of faults and striations from Tertiary volcanic and sedimentary rocks along the Annabella segment of the 

Sevier fault (Annabella sample area) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

Lower hemisphere stereographic plot of a subsample of predominantly strike-slip faults derived from the sample shown 

in figure 24A........................................................................................................................................... 29 

Histograms of faults and striations from the Tertiary cover rocks of the Sanpete-Sevier anticline (anticline sample)....... 31 

Stereographic plots of faults that cut Tertiary rocks at or above the contact with the mechanically weak Arapien 

Formation along the southern part of the Sanpete-Sevier anticline................................................................... 32 

Histograms of strike, dip, and rake and plot of dip-slip versus strike-slip components of fault slip for faults in the 

northern part of the Antelope Range........................................................................................................... 33 

Photograph showing west-looking view at the surface trace of a high-angle fault in the Antelope Range at locality F in 

figure 3 .................................................................................................................................................. 33 

Map and diagrams showing aspects of seismicity in the transition zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 

Photograph looking south across the northeast-trending main strand of the Sevier fault toward the Glenwood Mountain 

part of the western Sevier Plateau.............................................................................................................. 36 

Profiles across scarps formed on late Quaternary alluvium and debris flows in the Annabella area............................... 37 

Diagrams showing spatial distributions of aftershock foci in the Annabella area and focal mechanisms for the 1982 

magnitude 4.0 main shock and magnitude 2. 7 aftershock................................................................................. 38 

Geologic map showing south-central part of the Monroe Northwest 71/2-min quadrangle showing geology by 

Steven (1979), modified and augmented in the present study........................................................................... 39 

Photograph looking south and showing terraces that have progressively greater height and greater eastward tilts 

with greater age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

Drawing showing channel pattern of the Sevier River between Joseph and Elsinore in relationship to an axis of relative 

uplift and hypothetical model of the time-sequential response of the slope and channel pattern to uplift across the 

course of a mixed-load meandering river...................................................................................................... 41 

Map showing structural characterizations of the central Sevier Valley area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . 42 

TABLE 

Page 

TABLE 1. Fault-slip data and computational results.................................................................................................................. F8 
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY 

AREA, UTAH, AND ITS RELATIONSHIP TO SEISMICITY 

By R. ERNEST ANDERSON and THEODORE P. BARNHARD 

ABSTRACT 

The central Sevier Valley is one of the most seismically active parts 

of the Intermountain Seismic Belt. Two areas of concentrated seismicity in the valley coinciding with anomalous concentrations of late 

Quaternary deformation suggest a genetic relationship. Both areas 

contain late Quaternary fault scarps suggestive of normal faulting and 

microseismicity suggestive of strike-slip faulting, and each is broadly 

consistent with east-west extension. Both areas are located at complex 

structural junctures where the geometries and vergence directions of 

major structures show dramatic on-strike contrast. Transverse zones 

of structural accommodation in which stress is likely to be concentrated 

are required in both areas and probably explain the concentration of 

seismicity and late Quaternary deformation. Strain rates, faulting 

histories, rupture lengths, and other deformational aspects of these 

zones that might be relevant to earthquake hazards assessments are 

not characteristic of the major structures that extend away from them 

and are therefore of little value in regional assessments. Geologic 

mapping and detailed fault-slip studies of areas directly adjacent to the 

areas of concentrated seismicity show that the youngest deformation is 

dominated by strike-slip faulting and associated folding of apparent 

compressional origin in one area, by extreme extension and structural 

attenuation in another area, and by mixtures of strike-slip and normal 

faulting with or without monoclinal flexuring in still other areas. Thus, 

the areas of concentrated seismicity not only are localized at complex 

structural junctures but also are surrounded by complexly variable 

neotectonic structures. 

Major features of the seismicity include (1) a mixture of strike-slip 

and normal faulting focal mechanisms in the transition zone; (2) a 

predominance of strike-slip microseismal focal mechanisms having a 

conspicuous element of slip incompatibility, the P axes of some mechanisms being parallel to the T axes of others; and (3) hypocenter 

frequencies that fall off sharply at depths of 5 to 6 km and thus suggest 

deformation above planes of structural compensation. To explain the 

mechanics and kinematics of neotectonic deformation in the central 

Sevier Valley and the way in which that deformation relates to the 

neotectonic framework of the region requires not only the integration 

of these major features of the seismicity but also the integration of an 

extremely varied and complex group of structures, including folds of 

contrasting orientation and conjugate, nonconjugate, and incompatible 

types of strike-slip faults, normal faults, mixtures of normal and 

Manuscript approved for publication November 20, 1990. 

strike-slip faults, and known or inferred attenuation-type detachment 

faults. These structures must be understood in the contextual framework of an extensional tectonic regime superposed on an area characterized by a long and complex history of earlier deformation and a 

strong preexisting northeast-trending structural fabric. Although it 

would appear impossible to develop a tectonic model capable of 

explaining all these complexities, one model does seem to satisfy most. 

The five critical elements of this model are as follows: 

1. Extensional deformation in which cr 3 is oriented approximately 

east-west is distributed very unevenly in space and ranges in 

magnitude from extreme local attenuation to mountain-sized blocks 

having less than 5 percent internal extension. 

2. Either complementary to or in competition with the extension is an 

element of lateral tectonic transport resulting either from interchanges in the position of cr1 and cr2 or from shear tractions applied 

to the base of structural blocks at a high angle to the direction of 

extension (aproximately north-south). 

3. There is a strong element of strain gathering by preexisting steeply 

dipping northeast-trending zones of structural weakness and by 

gently dipping mechanically weak clay-rich or evaporite-rich strata 

or preexisting thrust faults. The former results in zones of concentrated extension or strike-slip faulting and the latter in detachment 

or decoupling of the internally deformed plates. 

4. Mechanically resistant structural entities such as the Marysvale 

volcanic pile and its plutonic root interfere with and produce internal 

deformation within laterally migrating thin-skinned structural 

plates. 

5. Facing monoclinal folds of the type bordering the Sevier Valley 

result from extension superposed on lateral shear and resemble 

divergent strike-slip and oblique-slip mobile zones. 

The inferred structural style of extension applied to laterally displaced blocks bounded by strike-slip faults and driven by tractions or 

drag on their basal detachments is consistent with seismic evidence for 

strike-slip faulting, slip incompatibility, and hypocenters above distinct 

planes of structural compensation. Because the deformational style is 

basically thin-skinned detachment, the potential for large earthquakes 

on faults in the upper plate is low. Our recognition that some of the 

largest and youngest exposed faults in the area are strike-slip faults 

does not significantly increase seismic hazard above what would be 

estimated for normal faults. Normal faults such as the Sevier fault, 

because they bound 15- to 20-km-wide blocks that are uniformly tilted, 

probably cut one or more levels of potential structural detachment and 

penetrate to the base of the seismogenic part of the crust. Such faults 

F1 
F2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

are more likely to be the source of infrequent large earthquakes than 

are faults in the complex structural junctures where late Quaternary 

deformation is concentrated. 

INTRODUCTION 

The central Sevier Valley study area is located in the 

High Plateaus of the Utah section of the Colorado 

Plateaus in a structural and physiographic transition 

zone between the Basin and Range on the west and the 

Colorado Plateaus on the east (fig. 1). The area straddles 

the Intermountain Seismic Belt, as figure 2 shows. 

Although most of the study is focused in the central 

Sevier Valley, the study extends westward to Dog 

Valley (fig. 3). 

This report has a threefold purpose: (1) to present the 

results of a study of fault-slip characteristics and associated folding in the central Sevier Valley area, (2) to 

interpret those results in the context of the diverse 

structural history and historical earthquake record of the 

region, and (3) to evaluate their relevance to the neotectonic framework and their impact on earthquake hazards. The initial intent of the investigation was to search 

for geologic evidence of strike-slip faulting in areas 

where W.J. Arabasz and his students from the UniverBASIN AND 

RANGE 

PROVINCE 

COLORADO PLATEAUS 

PROVINCE 

50 

I 

I 

100 

100 MILES 

I 

I 

150 KILOMETERS 

sity of Utah had provided us with advance reports of 37 0LL........JL__.:::J1;---L-::::L....~L-...L_ ___ ....L-___ _,__ __ _ 

earthquakes having a strike-slip mode. The search was 

successful, and it is now clear that the geologic record 

contains abundant evidence of Neogene and Quaternary 

strike-slip faulting in the region. The success of our 

search (Anderson and Barnhard, 1984a, b) led us to 

expand the scope of our investigation to include a synthesis of Pliocene and Pleistocene deformation in the 

area. The range margins along the central Sevier Valley 

were examined for geologic evidence of late Tertiary and 

Quaternary deformation, and we herein integrate those 

findings with our earlier results. 

Fault-slip data were collected from parts of the Pavant 

Range, the Tushar Mountains, the Antelope Range, the 

Sevier Plateau, and the Sanpete-Sevier anticline, all of 

which flank the Sevier Valley (fig. 3). The comprehensiveness of our investigation varies greatly from area to 

area. Accordingly, only a general description of fault slip 

is given for some areas, quantitative fault orientation and 

fault-slip data are given for other areas, and a comprehensive account of paleoslip and, where the data are 

sufficiently coherent, the results of paleostress analysis 

are given for still other areas. Only from the Clear Creek 

area do the fault-slip data provide paleostress results 

that can be confidently related to mapped structural 

patterns. However, data from the other areas provide 

valuable insight into the distribution of strike-slip faulting as well as an opportunity for qualitative comparison 

with the results from the Clear Creek area. 

HURRICANE FAULT 

FIGURE I.-Location of the central Sevier Valley study area relative 

to the structural transition zone and bordering provinces. The 

Hurricane and Sevier faults are shown by hachures on the downthrown sides. 

A long and complex structural history has been recognized in the study area. It includes compressional thrusting and folding and related foreland deposition ranging in 

age from latest Jurassic to Paleocene (Standlee, 1982), 

block uplift and basin formation of Paleocene and Eocene 

age (Stanley and Collinson, 1979), igneous activity and 

associated intense volcano-tectonic activity of Oligocene 

and Miocene age (Rowley and others, 1979), basin development and associated sedimentation of late Miocene age 

(Callaghan and Parker, 1962; Rowley and others, 1979), 

and Pliocene and Pleistocene deformation, volcanism, 

and landsliding that have shaped some of the most 

conspicuous structural and physiographic features of the 

area. Although it is the last part of this structural history 

that is of central importance to this study, the possible 

influence of old structures on young structures must be 

considered. We therefore include a section on structural 

history and setting in which we review what is known 

about the early deformations, especially those events 

that left a major imprint on the structural fabric of the 

area. We do not discuss the stratigraphy or provide 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH 

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EXPLANATION 

Magnitude scale (ML) 

• 6 4 • 2.5 •

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100 MILES 

I 

150 KILOMETERS 

• 

• 

.. _ [!)Vernal 

• 

• 

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109° 

FIGURE 2.-Distribution of earthquake epicenters in Utah for the period July 1962 through September 1984 

(from Smith and Richins, 1984). The central north-south zone of abundant seismicity is called the 

Intermountain Seismic Belt. 

F3 
F4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

A 

sw TUSHAR MOUNTAINS zz 

-o PAVANT RANGE 

A' 

NE Tv Ts Cli= 

::~~~~~~~,~~F~~~~~~~.·~~~:iTis::1::=~~~~~~~f:~~~~;ir~v~~~~~::~~~~;:~ Form lines show trace of 

bedding or foliation 

Section not to scale 

FIGURE 3.-Geology and section of the central Sevier Valley area, 

after Cunningham and others (1983), Steven and Morris (1983b), 

and Williams and Hackman (1971). BV, Black Rock volcanic field; 

CD, Clear Creek downwarp; CG, Cove Fort graben; DF, Dry 

Wash fault; MC, Monroe Peak caldera; PT, Pavant thrust; SA, 

Sanpete-Sevier Valley anticline; EF, Elsinore fault; SF, Sevier 

fault; TC, Three Creeks caldera; J, Joseph; BCH, Bull Claim 

Hills; TL, Thompson Creek landslide. Circled letters are localities referred to in the text. 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F5 

Monroe () 

SF 

VANT RANGE ELSINORE 

~~~~~~~~~~~T~KisiiiFAULT 1500 

1000 

Form lines show trace 

bedding or foliation 

Section not to scale 

Tv 

Sevier Valley 

Ts--

FIGURE 3. -Continued. 

~ 

j•.ciTv.j 

EXPLANATION 

Alluvium and colluvium of Quaternary ageIs, landslide 

Volcanic rocks of the Black Rock volcanic 

field-Mostly basalt and rhyolite of Quaternary and latest Tertiary age 

Sevier River Formation of Pliocene and Miocene age-Mostly basin-fill clastic strata 

Volcanic rocks-Mostly lava flows, breccia, and 

ash-flow tuff of Miocene and Oligocene 

age 

Sedimentary rocks-Includes the Price River, 

North Horn, Flagstaff, Green River, Crazy 

Hollow, formation of Aurora, and Dipping 

Vat Formations of Oligocene to Late Cretaceous age 

Sedimentary rocks of Mesozoic and Paleozoic 

age 

---- Contact 

---... ......... -.- Fault-Mostly steep dipping, hachured where 

expressed as scarp in alluvium, dotted 

where concealed; dashed where indefinite 

or inferred 

f t Thrust fault-Teeth on upper plate 

--+--- Anticline 

--.;..--Syncline 

50 

70 

-dStrike and dip of bedding or foliation 

Overturned bedding or foliation 

A A' -----=-- Line of cross section 

® Locality referred to in text 

0 10 MILES 

1-----------,----L--_,----rJ 

0 5 10 15 KILOMETERS 

B' 

SE FEET 

7000 

6000 

5000 

4000 
F6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

detailed information about the lithology of the deformed 

rocks. The stratigraphic sequence is summarized in 

figure 4. 

ACKNOWLEDGMENTS 

We are grateful to Walter Arabasz (University of 

Utah) for providing us with prepublication drafts of 

reports in which the ocurrence of strike-slip focal mechanisms in cental Sevier Valley was highlighted and for 

taking the time to visit us in the field. Field visits by 

Grant Willis (Utah Geological and Mineral Survey) and 

Peter Rowley (U.S. Geological Survey) helped us to 

understand some stratigraphic relationships. We thank 

Jacques Angelier (University of Paris) for providing us 

with computer programs for paleostress analysis, Mary 

Lou Zoback (U.S. Geological Survey) for help in using 

the programs, and Steve Harmsen (U.S. Geological 

Survey) for help in developing computer graphics. The 

report was improved substantally as a result of critical 

reviews by Dwight Schmidt and Peter Rowley (U.S. 

Geological Survey) and Grant Willis (Utah Geological 

Survey). 

PROCEDURE 

Field procedure for this study consisted of measuring 

the orientations of fault surfaces and movement-related 

striae on those surfaces and determining the sense of slip 

for motions parallel to those striae. These fault measurements were made in specific small sample areas within 

the Sevier Valley study area. Each sample area is either 

named for a nearby geographic locality or identified by a 

locality letter (fig. 3). In the absence of evidence to the 

contrary, the striae are assumed to represent the youngest deformation on a fault. On some faults, polyphase slip 

is indicated by two or more sets of striae; for some of 

these sets, it is possible to determine their sequence of 

development. The sequential aspects of faulting also can 

be determined by studying fault intersections or by 

inferring the angular relationship between faults of contrasting orientation and slip and the directions of bedding 

tilt. The data are classified according to the displacement 

amount and the certainty with which the sense of slip 

was determined. Data analysis consists of a preliminary 

separation into faulting mode; faults whose rake angles 

exceed 45° are separated into a dip-slip mode, and those 

whose rake angles are less than 45° are separated into a 

strike-slip mode. For most data sets, oblique-slip faults 

are relatively uncommon, so computations can be performed without separate treatment. Paleostress orientations and various paleostress parameters are computed 

by using methods similar to those described by Angelier 

(1979) and are performed on data representing both 

strike-slip and dip-slip modes. 

~ 

LJ.J ~ (/) :c LJ.J 

!;{ I- LJ.J 

(/) a: STRATIGRAPHIC UNIT 0: >- LJ.J 

LJ.J (/) (/) 

~ ctl 

c: Alluvium, colluvium, landslide, 2 

ctl basalt, and rhyolite ::J 

0 

Q.l 

c: 

Q.l 

(.) Sevier River Formation Q.l .Q c: 

Q.l a: Cl 

0 f--- Q.l 

z Q.l Joe Lott Tuff Member c: 

Q.l 

(.) 

(.) 0 

·c; 

N ~ Volcanic and volcaniclastic rocks 0 

c: 

Q.l (includes volcanic rocks of Dog u Q.l Valley) 

~ c: 

Q.l 

ctl (.) 

·-e 0 

~ .!:? 

6 Dipping Vat Formation 

Q.l Formation of Aurora c: Q.l Q.l c: Cl Q.l Crazy Hollow Formation 0 (.) 

.S2 0 Green River Formation ctl LJ.J 

c... ~Flagstaff Limestone 

- Q.l 

c: 

Q.l 

(.) 

0 

Q.l 

ro 

c... 

North Horn Formation 

(/) 

Price River Formation ::J 

0 

Q.l 

(.) 

~ Q.l 

u Indianola Group 

- (.) Morrison Formation 

·c; (.) 

N 'iii Twist Gulch Formation 0 (/) (/) ctl Q.l Arapien Formation ~ ...., ~ 

Navajo Sandstone 

-

(.) Chinle Formation ·u; 

(/) 

ctl 

~ Moenkopi Formation 

(.) 

·c; 

N 

0 Cambrian to Permian rocks Q.l 

ro 

c... 

FIGURE 4. -Generalized stratigraphic column for the central Sevier 

Valley area. 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F7 

Displacement amount in this report is classified as 

follows. Small-displacement faults have known or estimated offsets ranging from a few centimeters to a few 

meters, and large-displacement faults have known or 

inferred displacements greater than 30 m; the remainder 

are referred to as intermediate-displacement faults. 

Many of the large- and intermediate-displacement faults 

are shown in figure 3; small-displacement faults cannot 

be shown at the scale of figure 3. In each of the sample 

areas investigated, we attempted to determine the orientation and sense of slip on mapped faults. This strategy 

leads inevitably to gathering large amounts of data from 

small faults having small displacement; this report is 

based in large part on data from such faults. 

Three large-displacement north- to northeast-striking 

faults were previously named and interpreted as normal 

faults-the Sevier, Dry Wash, and Elsinore faults (Callaghan and Parker, 1961, 1962; Cunningham and others, 

1983). Of these, only the Sevier and Dry Wash faults are 

shown in figure 3. Except for the southern part of the 

Dry Wash fault, these two faults are buried by rangeflanking alluvium. The strategy used to evaluate the slip 

characteristics of the large-displacement faults was to 

search for clues on small- and intermediate-scale faults in 

the bedrock adjacent to the large faults. 

Large-displacement faults in a sample area possibly 

reflect stress states of a different vintage, a more 

fundamental rank or order, more regional extent, or a 

more deeply penetrating tectonic realm than do smalldisplacement faults. Because of these possibilities, in 

each sample area where paleostress studies are conducted, small-displacement faults must be determined to 

have orientations and slip characteristics similar to those 

of large-displacement faults. To make this determination, two separate computations are generally performed-one on the raw data and one on data that are 

weighted according to the amount of displacement on 

each fault as described by Angelier and others (1985). 

The numerical weights given in table 1 represent a 

crudely logarithmic scale from 1 to 9, category 1 displacements being less than 10 em or unknown and category 9 

displacements being greater than 100m. Because exposures of large faults are uncommon in nature, most data 

sets reported herein consist only of small-displacement 

faults or contain a very small sample of larger faults; 

thus, a direct evaluation of the effect of fault size on 

paleostress is precluded. As an exception, however, slip 

data gathered from many moderate- and largedisplacement faults in the Clear Creek area provide an 

excellent opportunity to test for the effect of fault size on 

paleostress. The test shows no significant shift in the 

position of the paleostress axes or the quality of the 

limited tensor (table 1). Paleostress computations using 

data from sample areas where only small faults were 

found yield results similar to those obtained by using the 

Clear Creek data, the suggestion being that fault size is 

not an important factor in determining paleostress in the 

central Sevier Valley study area. 

STRUCTURAL HISTORY AND SETTING 

The oldest structural feature in the study area is the 

Cordilleran hinge line that separates upper Proterozoic 

to Triassic Cordilleran miogeoclinal rocks on the west 

from platform facies rocks on the east (Armstrong, 1968; 

Burchfiel and Hickcox, 1972). Although the hinge, probably initially a zone several tens of kilometers wide, is 

tectonically and depositionally covered in the study area 

by thrusts and sediments, it probably trended northeast 

through the area and could have exerted a control on 

subsequent fold and thrust structures. Deposition in the 

miogeocline was interrupted by compressional deformation of the Sevier orogeny possibly as early as latest 

Jurassic and extending through Late Cretaceous (Campanian) (Armstrong, 1968). The northeast-trending 

Pavant thrusts shown in the western part of the Pavant 

Range (fig. 3) were formed during this compressional 

deformation. The principal compressional structure is a 

thrust-faulted recumbent fold formed in the Paleozoic 

miogeoclinal or shelf carbonate rocks and Mesozoic clastic rocks. Associated tear faults suggest thrusting to the 

southeast (Davis, 1982). A total of 10 km of displacement 

is inferred for the exposed thrusts (Baer and others, 

1982). A buried thrust of unknown displacement beneath 

the Pavant thrust at a depth of 4 to 5 km is inferred from 

seismic reflection data referred to by Arabasz and 

Julander (1986). On the basis of drill-hole data, thrusts 

and folds that developed during the Sevier orogeny are 

interpreted to extend east of the Pavant Range beneath 

the Sevier Valley and probably beneath the northernmost Sevier Plateau (Standlee, 1982). They may also 

extend southward beneath the thick volcanic cover of the 

Tushar Mountains, but that possibility is less certain 

(Richard Kennedy, oral commun., 1984; T.A. Steven, 

oral commun., 1985). 

The folds and thrusts of the Sevier orogeny were 

erosionally truncated and depositionally overlain by 

thick continental clastic and chemical (lacustrine) strata 

and some marine strata ranging in age from latest 

Cretaceous (Price River Formation) (Spieker, 1946) 

through late middle Eocene (Green River Formation) 

(Willis, 1985) to late Eocene (Crazy Hollow Formation 

and formation of Aurora) (Willis, 1985, 1986). The distribution and depositional history of the uppermost Cretaceous through lower Eocene strata in the region reflect 

the trend and location of the Cordilleran hinge line and 

the Sevier fold and thrust belt but are considered by 
Number of samples (N) ................................ . 

Principal compressional stress: 

Maximum (rr 1) •••.......•••.•••.. · •• • • · • • · · · · · • • • • • 

Intermediate (rr2) •••••••••••••••••••••••••••••••• • • 

Minimum (rr3) ••••••••••••••••.•••••••••.•••••••••• 

Stress ratio (<I>) .•••••••••••••••••••••••••••••••.••••••• 

Mean angular difference (d), 1 in deg .................... . 

Discordant data2 ••••••••••••••••••••••••••••••••••••••• 

Weight3 ••••••••••••••••••••••••••••••••••••••••••••••• 

Sense of slip4

'

5 

••••••••••••••••••••••••••••••••••••••••• 

Strike ranges 6 ••••••••••••••••••••••••••••••••••••••••• 

Degree of certainty 7 ••••••••••••••••••••••••••••••••••• 

TABLE 1.-Fault-slip data and computational results 

[ -, no data in these weight categories] 

Clear Creek area 

Strike slip Dip slip 

149 60 

4, 3 265, 84 

163, 87 170, 1 

274, 1 80, 6 

0.16 0.04 

23 23 

5 4 

1 (116) 1 (59) 

3 (25) 3(1) 

5 (3) -

9 (5) -

D=58 

S=91 

D=275°-000o 

S=001°-85° 

C=57 C=15 

P=75 P=40 

S=17 S=4 

Monroe segment, Sevier fault Annabella segment, 

Richfield to Sevier fault 

Willow Creek 

Strike slip Dip slip Dip slip 

33 36 13 90 

147, 88 173, 3 183, 73 166, 86 

45, 1 24,87 347, 17 13, 4 

315, 2 263,2 79,4 282,2 

0.06 0.12 0.26 0.03 

12 19 15 20 

1 1 1 2 

1 (25) 1 (34) 1 (10) 1 (86) 

2 (1) 5 (2) 5 (3) 4(1) 

3 (1) - 5 (2) 

5 (3) - 9 (1) 

7 (3) - - -

D=20 

S=16 

D=279°-352o 

S=358°-065° 

C=12 C=2 C=4 C=7 

P=21 P=O P=8 P=60 

S=O S=34 S=1 S=23 

5 Anticline sample is too mixed (heterogeneous) for this categorization to have meaning. 6Dextral (D) and sinistral (S) faults. 

Anticline 

sample 

49 

170, 87 

12, 3 

282, 1 

0.05 

15 

0 

1 (31) 

2 (4) 

3 (3) 

7 (1) 

9 (1) 

C=24 

P=24 

S=1 

1

Difference between actual striae and the theoretical maximum shear stress on the fault plane. 2 Identified as striae that form an angle greater than 45° with the theoretical maximum shear stress. 3

Crudely logarithmic function of fault displacement. 1, less than 50 em; 3, 2 to 5 m; 5, 10 to 20 m; 7, 

50 to 75 m; 9, 100 to 200m. Number of faults in each weight category is given in parentheses. 4Number of dextral strike-slip faults (D) and sinistral strike-slip faults (S). 

7

Determination of the sense of slip. C, certain; P, probable; S, supposed. Number is number of 

faults in each category. 

~ 

00 

~ 

trj 

0 

0 

-

z 

> t"'l 

trj 

> 

~ 

1-3 

::c: 

.0 

d 

> 

~ 

::c: 

~ > 

~ 

t:J 

w 

> z 

t:J 

~ 

w 

~ 

> t"'l 

0 

z 

0 

1-3 

::c: 

trj 

w 

~ 

~ 0 

::c: 

~ 

~ 

0 

z 

1-3 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F9 

some to be post-orogenic (Stanley and Collinson, 1979). 

Conglomeratic and other coarse clastic facies in these 

strata are related, instead, to localized uplifts resembling 

those of the Laramide orogeny. In central Utah, and 

especially along the Sanpete and Sevier Valleys, many 

deformational events, some perhaps very localized, were 

spread over much of the latest Cretaceous through late 

Eocene depositional interval (Spieker, 1949; Gilliland, 

1963; Standlee, 1982). The regional state of stress accompanying this "post-orogenic" deformation is not obvious. 

A long-standing controversy concerns the relative roles 

of vertical tectonics versus compressional tectonics in 

shaping early Tertiary structures in much of central 

Utah (see Witkind (1982), Standlee (1982), and Lawton 

(1985) for dramatically differing interpretations). The 

area of controversy extends southward into the central 

Sevier Valley at least as far south as Richfield (fig. 3). 

Structural trends involved are predominantly northeast 

and thus parallel the Cordilleran hinge line as well as the 

Sevier thrust front-foreland boundary. The potential for 

influencing young structural patterns through inheritance is high in this area. 

The late Eocene Crazy Hollow Formation and the 

formation of Aurora (fig. 4) contain lithologic evidence of 

syndepositional volcanism probably to the north of the 

area of figure 3 (Willis, 1985). As part of the provincewide southerly progression of middle Tertiary volcanism 

(Stewart and others, 1977), the Tushar Mountains area 

became the locus of major volcanic activity beginning 

about 35 Ma (the Marysvale volcanic field of Rowley and 

others (1979)). Airfall tuffs in the Dipping Vat Formation 

in the central Sevier Valley dated isotopically at 34.2 Ma 

apparently record volcanism that occurred in the Marysvale field in the southern part of and south of the area of 

figure 3 (Willis, 1985). The calc-alkaline volcanic rocks of 

the Marysvale field-mostly lava flows, volcaniclastic 

sedimentary rocks, and ash-flow tuffs-thin northward 

through the area and lap onto pre- and lower Tertiary 

sedimentary rocks (fig. 3). They range to as late as 16 Ma 

(Rowley and others, 1979). Volcanotectonic structures 

such as domes, sags, and caldera walls related to their 

eruption are recognized in the southern part of and south 

of the area covered in figure 3 (Cunningham and others, 

1983). Those structures are largely or completely earlier 

than the faults that we studied, and we therefore do not 

show their outlines in figure 3. 

Neogene deformation that included faulting and folding occurred along the flanks of the Sanpete-Sevier 

anticline from Glenwood northeastward (Spieker, 1949; 

Gilliland, 1963) contemporaneous with and outlasting the 

volcanism. The paleostress conditions that existed during this Neogene deformation are as uncertain as those 

that deformed the same area on similar trends during the 

early Tertiary. Gilliland (1963) studied the part of the 

anticline that extends into the area that we studied (from 

Glenwood to Salina, fig. 3). He suggested that it formed 

no later than during Late Cretaceous time under lateral 

compressional stresses and that those stresses played a 

role in its long history of deformation, presumably 

extending well into the Neogene. 

The southern Pavant Range is separated from the 

structurally elevated volcanic edifice of the Tushar 

Mountains to the south by a broad, open, east-trending 

synclinal sag named the Clear Creek downwarp (fig. 3) 

by Callaghan and Parker (1962). The latest stratigraphic 

unit that appears to be fully involved with the formation 

of the downwarp is a 19-Ma ash-flow tuff, the Joe Lott 

Tuff Member of the Mount Belknap Volcanics (Callaghan 

and Parker, 1962; Steven and others, 1979). That unit 

crosses the downwarp but exhibits no conspicuous thickening in the axial part of the downwarp. It exhibits 

almost 2 km of structural relief where it crosses the 

downwarp (Callaghan and Parker, 1962), and we assume 

that relief to be an approximation of the magnitude of 

downwarping. 

Between 16 and 14 Ma, conditions in the southern part 

of the area changed from predominantly volcanic to 

basin-fill sedimentation (Callaghan and Parker, 1962; 

Steven and others, 1979). Callaghan and Parker referred 

to the sediments as Sevier River Formation, and their 

general distribution is shown in figure 3. These strata 

range from basin-margin coarse conglomeratic facies to 

basin-medial silt and clay and are as young as 5. 6 Ma on 

the basis of a K-Ar age from a tuff bed exposed east of 

Annabella (H.H. Mehnert and P.D. Rowley, written 

commun., 1982). 

The Clear Creek downwarp may have influenced the 

local depositional history and distribution of the Sevier 

River Formation, but, to a large extent, those strata are 

involved in the downwarping and in subsidiary parallel 

folding on the southern flank of the downwarp (fig. 3). 

Over a stratigraphic interval of several tens of meters, 

the middle part of the Sevier River Formation directly 

southwest of Joseph Flats contains an upward transition 

from pale salmon, fluvial, coarse- to medium-grained 

sand and gravel to a sequence of pastel yellowish-gray 

and yellowish-green fluviolacustrine beds. A strikingly 

similar transitional sequence exposed 30 km northeast in 

the area south of Glenwood suggests deposition in a 

single basin of unknown size or shape. In neither area 

does the lithology of the transitional sequence suggest 

close proximity to a rising mountain flank. The paleogeographic implication is that neither the Pavant Range, 

the Sevier Plateau, nor the Sevier Valley existed when 

those beds were deposited. The tectonic implication is 

that the present system of structurally controlled mountains and valleys had not yet formed, and, by our 

definition, the neotectonic realm had not yet begun. 
FlO REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Because the Sevier River Formation represents the 

youngest well-exposed deformed strata in the area, we 

concentrated much of our study on the neotectonic 

structures that deform them. We emphasize that the 

basalt-rhyolite Black Rock volcanics in the western part 

of the study area are generally younger than the Sevier 

River Formation and are highly faulted (Clark, 1977), 

but they provide little opportunity for gathering faultslip data and have not been studied for this report. 

The study area is located in the structural transition 

zone between the Basin and Range and Colorado Plateaus provinces-a location that we believe is fundamental to any understanding of neotectonic deformation. 

Three major transition-zone faults are reported in the 

area. The Sevier fault, which has a conspicuous zigzag 

trace and an overall northerly trend, separates the 

Sevier Valley from the uplifted Sevier Plateau. The fault 

is one of several major plateau-margin faults between 

which structural blocks approximately 30 km wide are 

tilted gently eastward within the regional extent of the 

transition zone. The Dry Wash fault strikes northeast 

and is located at the northwestern base of the topographically subdued northern part of the Antelope Range, 

from where it extends southwestward into the northern 

flank of the Tushar Mountains. Callaghan and Parker 

(1961, 1962) and Cunningham and others (1983) mapped 

it as a normal fault in that area. As it is traced to the 

southwest, striations on it show that it is chiefly a 

sinistral strike-slip fault. The third major fault, the 

Elsinore fault, is essentially a northeastern extension of 

the Dry Wash fault. Callaghan and Parker (1961) mapped 

it as a major buried northeast-trending normal fault 

separating the Sevier Valley from the Pavant Range in 

the Elsinore area. 

Rowley and others (1979) concluded that the major 

faulting in the Clear Creek downwarp took place after 7 

Ma, and Rowley and others (1981a, b) concluded that the 

main uplift of the Sevier Plateau (about 1.5 km) took 

place in less than 2.5 m.y. between 7.6 and 4.8 Ma. 

Anderson and Bucknam (1979) noted fault scarps of 

probable late Pleistocene age along the Sevier fault near 

Anabella and along the Elsinore fault near Elsinore. 

Callaghan and Parker (1961) noted that Quaternary river 

terraces northeast of Joseph appear to be tilted eastward 

by displacement on the Dry Wash fault. Thus, the major 

faulting probably extends from latest Miocene through 

Pleistocene. 

RESULTS FROM FAULT-SLIP SAMPLE AREAS 

We present the results of our fault-slip studies geographically by sample areas, beginning in the Clear 

Creek downwarp, where we collected our most homogeneous and interpretable data. The next sample areas 

discussed are located northward along the eastern margin of the Pavant Range, where we have evidence of 

diachronous Neogene deformation, then eastward to the 

margin of the Sevier Plateau, where our study relies 

heavily on minor structures, and from there northeastward along the Sanpete-Sevier anticline, where Neogene 

rocks are mildly to strongly tilted. We present last the 

results from a reconnaissance in the northern Antelope 

Range, where we collected our most inhomogeneous and 

uninterpretable data. 

CLEAR CREEK DOWNWARP 

Field studies of exposed structures were made in four 

sample areas along the Clear Creek downwarp. The 

easternmost area is referred to as the Clear Creek 

sample area, which contains the specific locality A (fig. 

3). Westward, other sample areas are designated B 

through D (fig. 3). 

In the Clear Creek sample area, deep erosion by Clear 

Creek and its tributaries has produced excellent exposures of the Joe Lott Tuff Member and the overlying 

Sevier River Formation. Callaghan and Parker (1962) 

mapped several upright, open, east-trending folds in the 

rocks on the southern flank of the downwarp. These folds 

have very narrow hinge zones, amplitudes of about 200 

m, and wavelengths of about 2 km (cross section A-A', 

fig. 3). Callaghan and Parker (1961, 1962) also mapped 

several northeast-striking faults, the largest of which is 

the Dry Wash fault. The Dry Wash fault is a critical 

structure because the Clear Creek downwarp and the 

subsidiary folds on its southern flank terminate against 

it, showing a genetic and mechanical interrelationship 

between folding and faulting. Our study shows that the 

Dry Wash fault has major sinistral slip where it cuts the 

axis of the Clear Creek downwarp, as indicated by 

common subhorizontal striations and corrugations and 

the geometry and stratigraphic position of rock slabs that 

are distributed along it (fig. 5). Net displacement on the 

fault in that area could be several kilometers on the basis 

of structural and stratigraphic terminations (Cunningham and others, 1983) and stratigraphic contrasts across 

it (T.A. Steven, oral commun., 1983). In the vicinity of 

Joseph Flats, it has important dip-slip displacement. 

Directly northwest of the Dry Wash fault, two additional 

northeast-striking map-scale faults display abundant evidence of sinistral slip and have net slip of as much as 1 km 

(figs. 6, 7). Some of their displacement is probably 

accommodated by the east-trending folds that are 

located near their southwestern terminations (fig. 3). 

The east-trending folds also are cut by many smalldisplacement steep strike-slip faults, many of which are 

well exposed in cliffs held up by the Joe Lott Tuff 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH Fll 

FIGURE 5.-Dry Wash fault. A, View looking northeast along the trace 

of the fault showing slivers of the Joe Lott Tuff Member of the Mount 

Belknap Volcanics (at and beyond the geologist at arrow) displaced to 

exotic positions and separating younger coarse clastic sediments of 

the Sevier River Formation (exposed beds to left of the geologist and 

unexposed beds in covered slope to his right). The clastic strata dip 

about 35° away from the camera, so that the slivers of tuff that mark 

the trace of the fault resemble dikes that crosscut the sediments. The 

dikelike juxtapositioning results from strike-slip displacement of a 

previously fault-tilted and fault-repeated sequence of strata. B, View 

looking southeast at a location marked by the geologist in A showing 

a horizontally striated fault surface on the Joe Lott Tuff Member 

where the cemented gravel of the Sevier River Formation of the 

hanging wall (lower part of photograph) has spalled away along the 

fault plane. Compass for scale. C, View looking southwest along 

the main trace of the Dry Wash fault showing gently plunging 

striations developed on brecciated Joe Lott Tuff Member. Note the 

geologist at the arrow to the left and below the center of the 

photograph for scale. 

Member (fig. 8). In the welded ash-flow tuffs of the Joe 

Lott, many of these faults utilized preexisting cooling 

joints and represent displacements of a few centimeters 

or less. Detailed study of 81 of these faults in axial and 

opposed-limb positions in the folded Joe Lott Tuff and 

Sevier River Formation shows that they comprise 

northeast-striking sinistral (N=33) and northweststriking dextral (N=48) sets that (1) offset one another, 

(2) have broad ranges in strike and no overlap, and (3) 

contain striations whose rake angles are independent of 

position on the fold (fig. 9). These relationships indicate 

that the sinistral and dextral sets are conjugate. MohrCoulomb theory (Anderson, 1951) would predict that 

they formed under conditions of north-south compression. The relationship of their striae to position on the 

folds (fig. 9) suggests that the last motion on them 

occurred late in the folding history or perhaps after 

folding. 

We also identified sparse small-displacement dextral 

faults adjacent to the sinistral Dry Wash fault. These 

faults have the same range of strike as those that cut the 

FIGURE 5. -Continued. 

folds and exhibit mutually offsetting characteristics similar to those of the larger displacement sinistral faults of 

the Dry Wash system. These small dextral faults are 

clearly synthetic to the Dry Wash fault. 

Fault-slip data were gathered from 209 faults in the 

Clear Creek sample area. The determination of sense of 

slip is classified as certain or probable on 89 percent of 

the faults, and 16 percent have displacements of 1 m or 

more. The sample is large but by no means represents all 

faults in the sample area. We arbitrarily omit data from 

a northeast-striking oblique dextral fault zone southwest 

of Joseph Flats (specifically at locality A, fig. 3) because 

the slip sense is obviously discordant to that of the other 

northeast-trending faults in the area. The possible structural significance of the oblique-dextral fault at locality A 

is discussed below. For the 209 faults, dips are distrib-
F12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FIGURE 6.-View to the northwest from the Dry Wash fault across 

Clear Creek (foreground) to the trace (horizontal scar at X) of the 

sinistral strike-slip fault (FT) (middle distance). The fault parallels 

the Dry Wash fault and juxtaposes south-dipping rocks of the 

northern flank of the Clear Creek downwarp beyond the trace 

against north-dipping rocks of the southern flank of the downwarp on 

the photographer's side of the trace. Tj, Joe Lott Tuff Member of the 

uted in a remarkably tight, steep pattern (fig. 10), and 

rakes show a strong bimodal distribution in which low 

values (strike-slip faulting) predominate. Fault strikes 

are mostly north-northwest, north-northeast, and northeast. There is a complete separation of dextral and 

sinistral fault-strike fields. The separation would, of 

course, be incomplete if data from the omitted dextral 

fault were included. The bimodal rake distribution provides a rationale for separating the data into strike-slip 

and dip-slip subsets for computational purposes. For the 

strike-slip subset, north-south maximum compressive 

stress is indicated (fig. 10) consistent with the stress 

state inferred for the east-west folds and the small faults 

that cut them. The value for <I> of 0.16 suggests that a 1 is 

well constrained and that a2 and a3 are moderately well 

constrained. Mapped structures show that the deformation is strongly asymmetric and dominated by sinistral 

slip on northeast-striking faults and by folding. The 

analysis of small-displacement faults does not explain the 

strain asymmetry but shows that it is consistent with 

north-south compression. 

Dip-slip striations were measured on 60 smalldisplacement faults in the Clear Creek sample area (fig. 

10). Five of these faults show evidence of polydirectional 

slip. The dip slip is younger than the strike slip on three 

of those five faults and older on two. Thus, no evidence is 

provided for a temporal separation of strike-slip and 

dip-slip faulting. 

Dip-slip faults that dip greater than 80° and less than 

20° are not as compatible with a normal faulting regime 

Mount Belknap Formation; Ts, Sevier River Formation. Heavy 

irregular line is the contact between Tj and Ts; light dashed lines 

accentuate bedding traces. Linear skyline elements and vegetation 

stripes below the skyline show the extent of the south-dipping flank 

of the downwarp. Figure 7 shows closeup views of the fault, which is 

very well exposed in the area of the scar. 

as are those that dip between 80° and 20°. Also, faults 

exhibiting moderate and shallow rake angles are not 

likely to contribute as much to extensional deformation 

as are faults displaying steep rakes. To test the importance of normal faulting in the Clear Creek sample area, 

we eliminated steep- and shallow-dipping faults and 

those displaying moderate and shallow rakes and computed the paleostress characteristics from the remaining 

data (31 of the original 60 faults). We do not report the 

computational results because they do not reflect a 

lessening of angular discordance between the measured 

striae and the theoretical maximum shear stress, nor 

does the value of <I> (0.05) indicate a significantly better 

constrained a 3 axis than the one computed from all 

dip-slip data (0.04, table 1). We conclude from these 

relationships, together with the overwhelming evidence 

that the dominant faulting mode in the Clear Creek area 

is strike slip, that dip-slip faulting in the area is not of 

first-order structural significance. Alternatively, the 

dip-slip faulting may provide weak evidence of a temporal interchanging of a 1 and a2 in a stress field dominated 

by north-south compression and a3 oriented approximately east-west. 

The northeast-trending oblique-dextral fault at locality A (fig. 3) that is eliminated from the Clear Creek area 

computations dips 80° to 85° southeast, is sharp and 

planar (fig. 11), and juxtaposes contrasting facies of 

basin-fill sediments of the Miocene and Pliocene Sevier 

River Formation (Callaghan and Parker, 1962). It 

strikes transverse to bedding, which dips 20° to 30° 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F13 

FIGURE 7. -Northeast-trending sinistral-slip fault shown in figure 6. 

A, View looking southwest showing large striated grooves on the Joe 

Lott Tuff Member of the Mount Belknap Volcanics, where coarse 

cemented gravels of the Sevier River Formation (visible in the lower 

part of the scarp) have spalled away from the fault surface. B, 

Closeup view of striations on a large groove shown in A. The arrow 

drawn on the fault surface in the center of the view is 3 em long and 

shows the motion direction of the block as required by the streamlining of gouge deposits to the right of the small, mechanically 

resistant lithic inclusions in the tuff above the arrow. C, View looking 

down at the fault surface (in shade) separating the clayey sands of the 

Sevier River Formation in the foreground from the Joe Lott Tuff 

Member. The mechanically weak clayey sands are squeezed like 

toothpaste into a conspicuous tension gash about 20 em wide in the 

mechanically resistant Joe Lott Tuff. Although they are not visible, 

the conspicuous striations on the shaded fault surface show that the 

Joe Lott block moved to the left (sinistral). 

northeast (figs. 11, 12). Stratigraphic offset and sense of 

drag show that throw is down to the southeast, and the 

net displacement is more than 100 m. Fault-slip data 

were collected from the main fault and 16 additional 

northeast-striking surfaces having displacements as little as 20 em. The main fault and the adjacent smalldisplacement faults presumably constitute a single fault 

zone. Twelve of the 17 faults have a dextral-slip component, and 11 of those 12 plot withi1. 30° of the pole to 

bedding (fig. 12). This predominantly oblique dextral slip 

probably represents the principal displacement on this 

fault zone. Because the slip direction is consistently 

about parallel to the pole to bedding, the striae on the 

fault planes presumably were caused by rotation of a 

fault block containing the tilted beds. For this fault zone, 

the data set does not meet the criterion of independence 

required for the paleostress inversion procedure (Angelier, 1979). However, not all of the data are consistent 

with such an interpretation. Five of the northeaststriking minor faults have sinistral-slip components that 

suggest an incompatible element of slip in the fault zone. 

Similar slip incompatibility is seen in other fault-slip data 

FIGURE 7. -Continued. 

samples from the central Sevier Valley study area and 

suggests that slip incompatibility is a common feature. It 

could result either from block motions driven by trac-
F14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FIGURE 8. - Well-exposed, large, smooth, striated surface of a small 

sinistral-slip fault in the Joe Lott Tuff Member of the Mount Belknap 

Volcanics. Rough surfaces (near the top of the photograph) that 

extend out from and offset the large, smooth surface also contain 

gently plunging striae (not obvious in the photograph). Dextral 

displacement on the rough-surface fractures shows that they are 

conjugate to the large, smooth surface. Two-meter geologist for 

scale. 

tional forces applied to the base of the blocks or from 

transient stress states generated during recovery phases 

following major faulting events. 

At sample area B (fig. 3), sedimentary strata of the 

Sevier River Formation are downfaulted to the east 

against Miocene ash-flow tuffs on a well-exposed striated 

surface having a local attitude of 345°, 72° NE. (fig. 13). 

An average rake of about 90° indicates normal faulting. 

Over a distance of 5 km to the north along the same fault 

zone, striae on 19 subparallel subsidiary faults have rake 

angles that range from 68° to 90° and average 82°, 

providing confirmation that the last major movement on 

this fault zone is normal dip slip. South of locality B, 

bedrock in the footwall block is cut by numerous striated 

FIGURE 9. - Schematic representation of a fold cut by a sinistral fault 

(A) and two dextral faults (B, C). The short, heavy lines symbolize 

striae on fault surfaces. Fault C offsets fault A, and fault A offsets 

fault B, an indication of conjugate behavior. Also, the rake of striae 

on the faults is independent of position in the fold. 

small-displacement faults exhibiting dextral slip and 

rake angles as low as 3°. One of those fault surfaces 

contains intersecting striae indicating both dextral and 

normal displacements, of which the normal motion is the 

youngest. These relationships suggest that rocks in this 

fault zone experienced an early episode of Neogene 

strike-slip displacement but that the latest and principal 

displacement is dip slip. 

At sample area C, several roadcuts along a 1.5-km 

stretch of Interstate 70 provide good exposures of transversely faulted Oligocene and Miocene volcanic and sedimentary rocks. The faulted rocks consist of tuffaceous 

sandstone and gravel, sandstone and cobbly sandstone of 

probable fluvial origin, ash-flow tuff, and dacite and 

andesite flows. They dip east to southeast 15° to 30° and 

are cut by numerous steep, northeast-striking faults. At 

and around locality C, bedding repetition and physiography suggest simple normal faulting (Cunningham and 

others, 1983), but the fault-plane striae tell a much 

different story. Slip orientations and senses were measured on 20 faults ranging in displacement from about 20 

em to more than 100 m. One fault strikes north and is 

dextral. The remaining 19 faults strike northeast; 17 are 

sinistral, and 2 are normal (fig. 14A). The degree of 

certainty of the sense of slip was determined as certain 

on 14 and as probable for the remainder of the faults. The 

large-displacement faults have gouge zones as wide as 50 

em. The gouge has a strong internal shear fabric whose 

geometry and orientation are consistent with the orientation and sense of displacement measured on the planar 

walls of the shear zone, suggesting an unusually high 

degree of uniformity of fault motion. Eight of the 20 

faults have measured or estimated displacements of 10m 

or more. The cumulative displacement is not known but 

could easily be more than 1 km. These excellent expo-
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F15 

90 

STRIKE SLIP 

N 

Strike 

N=209 

N=149 

d=5 

E ~=23 

90 

<1>=0.16 

Rake 

N=60 

d=4 

~=23 

<1>=0.04 

FIGURE 10.-Histograms of strike, dip, and rake and lower hemisphere stereographic plots of computed paleostress axes for faults 

from the Clear Creek sample area in the easternmost part of the 

Clear Creek downwarp. N, sample size; R, radius of the histogram, 

in number of measurements; D, azimuthal range of dextral faults 

(number of faults in parentheses); S, azimuthal range of sinistral 

faults (number of faults in parentheses); a 1, a 2, and a3 , maximum, 

intermediate, and minimum principal compressional stresses, respectively; d, number of striae that form an angle greater than 45° with 

the theoretical maximum shear stress; A, mean angular difference 

between the actual striae and the theoretical maximum shear stress 

on the fault plane; <l>=(a2 -a3)/(a1 -a3 ). 

sures show that fault deformation along the 1. 5 km of 

cross-strike distance is predominantly on northeaststriking sinistral faults and that no significant difference 

exists between the slip on large- and small-displacement 

faults. The contrast between the paleoslip of the youngest observed striae in sample area B and that in sample 

area C is dramatic. 

Sample area D is along the northbound and southbound lanes of Interstate 15 about 6 km north of the 

junction with Interstate 70; this area is a few kilometers 

west of the Basin and Range boundary. Roadcuts along 

Interstate 70 expose strongly fractured Oligocene volcanic rocks that strike west-northwest and dip gently 

south-southwest. The average strike of faults in the area 

is 015° (Cunningham and others, 1983). Because the 

FIGURE 11.-View to the northwest of the excavated surface of a steep 

planar fault that cuts transversely through northeast-tilted beds of 

the Sevier River Formation at locality A in figure 3. Figure 12 shows 

a stereographic plot of the fault and other data. The photographer is 

standing on clayey and sandy sediments (not visible) that are 

downthrown by oblique dextral slip against coarse clastic gravel and 

conglomerate across the fault. Hammer for scale. 

faults strike at a high angle to the strike of the beds, the 

faults are either unrelated to and presumably younger 

than the stratal tilting or are transverse block-bounding 

structures that were active during stratal tilting. In 

either case, we infer that the latest movement on these 

faults is late Quaternary, because the faults are coextensive with faults that cut nearby volcanic rocks dated 

radiometrically at about 0.5 Ma (Steven and Morris, 

1983; Best and others, 1980). The amount of displacement on an individual fault is not known but is assumed, 

on the basis of fracture appearances, to range from a few 

centimeters to a few meters. Steven and Morris (1983) 

mapped a fault striking 045° along Interstate 15 between 

the two roadcuts, and unpublished mapping by one of us 

(R.E. Anderson, 1982) showed that the rocks in the 

eastern cut are displaced in an oblique sinistral sense at 

least 150 m relative to the rocks in the western cut. The 

orientation and sense of slip were measured on six faults 

in the eastern cut and nine in the western cut. These 

faults, which range in strike from 345° through oo to 085° 

and in dip from 28° to 85°, are probably representative of 

the range of principal fault orientations near the cuts. 

Polyphase slip is indicated on 4 of the 15 faults that we 

studied. Of the 15 faults, 3 northward-striking ones are 

predominantly dextral, 9 northeastward-striking ones 

are predominantly sinistral, and the remaining 3 show a 

predominance of dip slip (fig. 14B). These faults, inferred 

by us to reflect neotectonic deformation, are similar to 

those in the Clear Creek sample area because they 

display a mixture of strike slip and dip slip, showing a 

preference for sinistral strike slip on northeasterly strik-
F16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

N 

w 

s 

EXPLANATION 

• Dextral 

• Sinistral 

/ 

/ 

/ 

• / 

E 

FIGURE 12. - Lower hemisphere stereographic plot of the main 

fault (great-circle arc) and 16 subsidiary northeast-trending 

faults (segments of great circles) at locality A in figure 3. Striae 

on fault planes are distinguished according to the type of slip. 

Poles to bedding are indicated by the letter B. The circle shows 

a 30° cone having an axis near the poles to bedding. The 

concentration of striae within the cone is consistent with an 

interpretation that most of the faulting is caused by transverse 

block-boundary displacements related to stratal tilting. 

ing planes. They indicate that the style of faulting in the 

Clear Creek area extends westward into the Basin and 

Range. A limited stress tensor (<r1, <r2 , and <r3) computed 

from the 15 faults (using the youngest motions on those 

having two sets of striae) is shown on figure 14B. 

In summary, our study of four sample areas along the 

Clear Creek downwarp on both sides of the boundary 

between the Basin and Range province and the transition 

zone illustrates that the neotectonic deformational mode 

differs greatly from the simple extensional mode generally assumed for the late Cenozoic deformation in this 

part of Utah. In two of the four sample areas that we 

studied, highway-related excavations revealed both the 

presence and the predominance of late compressionrelated structures, about which nothing would be known 

were it not for the excavations. One of these areas is, at 

first glance, an excellent structural and physiographic 

candidate for simple late Cenozoic extension. A high 

probability of error exists in determining the sense of slip 

of faults shown on geologic maps of this region unless 

specific data on the orientation and sense of slip are 

FIGURE 13.-View looking north along the well-exposed Joe Lott Tuff 

Member of the Mount Belknap Volcanics in the footwall (left and 

center) of a north-northwest-striking normal fault at locality B in 

figure 3. The hanging-wall block (right-hand edge of photograph) 

consists of poorly exposed Sevier River Formation. Two-meter 

geologist for scale. 

available. In particular, we suspect that several faults 

mapped as scissors faults in nearby parts of the Pavant 

Range, the Tushar Mountains, and the Sevier Plateau 

(see Cunningham and others (1983), for example) are 

actually faults exhibiting a high proportion of strike-slip 

displacement and that the scissors mode is an apparent 

feature arising from the juxtapositioning of strata that 

have been gently to moderately deformed before strikeslip faulting. Although strike-slip faulting and associated 

folding predominate in the Clear Creek area, we emphasize that late Cenozoic dip-slip faulting does exist, and we 

describe a good example at and north of sample area B. 

Also, in a highway excavation near Cove Fort, at least 20 

m of late Pleistocene dip slip is indicated on the principal 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F17 

east-bounding normal fault of the Cove Fort graben 

(Anderson, 1980). The presence of dip-slip faults, some of 

large displacement, intermixed with strike-slip faults of 

similar orientation suggests complex fault linkages of the 

type proposed by Gibbs (1987). 

Studies along the Clear Creek downwarp show that 

strike-slip faulting is genetically and mechanically assoN 

w E 

s 

A 

N 

w E 

s 

B 

EXPLANATION 

e Sinistral • Dextral A Normal 

ciated with east-trending folds whose axes parallel the 

axis of the downwarp. Faults and folds in this area give 

the appearance of having formed in response to late 

Miocene and (or) later subhorizontal maximum compressive stress oriented approximately north-south. Because 

sinistral slip on northeast-striking faults dominates, the 

overall displacement of blocks bounded by these faults is 

southwesterly. If the entire fault system could be imagined to be simultaneously active, the result would be a 

positive velocity gradient from east to west. The integrated fault-fold system in the eastern part of the Clear 

Creek downwarp probably reflects a resistance to this 

southerly motion by the buttressing effect of the enormous Marysvale volcanic field and its associated plutonic 

root inferred from gravity modeling (Halliday and Cook, 

1980). Structures produced by such a resistance could 

appear to result from simple, local north-south compression. 

Strike-slip faults cutting the Clear Creek downwarp 

may have resulted from a thin-skinned deformational 

mode, because available geologic maps provide no evidence that they can be projected northeast along strike 

into the pre-Cenozoic rocks of the Pavant Range. This 

dramatic Neogene strain contrast between the two areas 

of contrasting rock types requires some form of structural accommodation or detachment at a stratigraphic 

level near the base of the Neogene volcanogenic sedimentary rocks. Detachment would also explain the strong 

contrast in faulting mode northeast and southwest of 

Richfield (noted in a subsequent section). The surface 

trace of the low-angle fault detachment would be largely 

masked by the large areas mapped as landslide at or near 

the contact between Neogene volcanic rocks and older 

rocks in the Pavant Range (fig. 3). Within the landslides 

southwest of Richfield, we recognize major attenuation 

of stratigraphic sequences above the Flagstaff Lime-

~ FIGURE 14.-Lower hemisphere plots of fault-slip and bedding data 

at localities along Interstates 70 and 15. Faults are shown as 

great-circle arcs, and the striae in those faults are distinguished 

according to the type of slip. Poles to bedding are shown by the 

letter B. A, Faults at locality C (fig. 3). Most are sinistral, except 

for one north-trending dextral fault and two dip-slip faults. The 

poles to bedding plot near the normal-faulting striae and thus 

suggest an association between the stratal tilting and the dip-slip 

components of faulting. B, Faults at locality D (fig. 3). Those that 

strike between 015° and 085° are all sinistral or normal sinistral. 

The remaining are dextral, except for a normal fault exhibiting a 

steep rake. The faults do not appear to have any simple genetic 

relationship to stratal tilting. The computed stress axes <Tv <T2, 

and <T3 (maximum, intermediate, and minimum principal compressional stresses, respectively) are poorly constrained because of 

the small sample size and the predominance of sinistral slip. 

Nevertheless, the position of <T3 is reasonable in comparison with 

other computed orientations reported herein (table 1). The moderate inclination of <T 1 could result from late-stage rotation of the 

faults and bedding. 
F18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

stone. We suspect that much of the attenuation there and 

to the southwest results from tectonic processes that left 

the area highly susceptible to landsliding or is simply the 

direct result oflandsliding. One possible interpretation is 

that the Clear Creek downwarp, which has a structural 

relief of about 2 km, provided the gravitational potential 

for producing the detachment and southward sliding, 

whereas the Tushar Mountains served to buttress the 

sliding. Deformation could result from updip attachment 

in the updip area and from downdip buttressing analogous to similar attributes of landslides in the downdip 

area (Kehle, 1970). This interpretation embodies the 

unlikely gravitational transport of rocks across the axis 

of the downwarp, where they crumpled into folds on the 

northern flank of the Tushar Mountains (cross section 

A-A', fig. 3). In a more favored interpretation, the 

compressional structures (folds and strike-slip faults) 

result from the scraping off of the volcanic cover rocks 

from a south-moving structural block that thrust under 

the northern flank of the Tushar Mountains. 

PAVANT RANGE 

The folds and thrusts of the Sevier orogeny in the 

southern Pavant Range are onlapped along a northeasttrending unconformity by southeast-dipping fluvial, 

lacustrine, and volcanic rocks ranging in age from Paleocene to Oligocene (Callaghan and Parker, 1962; Hickcox, 1971). The southeastward dip of the unconformity, 

of the strata above the unconformity, and of the Pavant 

thrust beneath the unconformity is caused by southeastward rotational uplift of the Pavant Range block during 

Neogene time. Concentration of maximum uplift along 

the western side of the range may result from structural 

rebound associated with tectonic unloading in the breakaway zone of the west-vergent late Cenozoic Sevier 

Desert detachment fault (Allmendinger and others, 1983; 

Wernicke, 1985). Map-scale high-angle faults are conspicuously sparse in the western part of the range (fig. 

3), suggesting that the block remained intact during 

uplift and tilting. We conducted a reconnaissance study 

of the slip characteristics of large- to small-displacement 

high-angle faults and measured numerous stratal attitudes along the eastern flank of the range. For purposes 

of discussion, results from the northern and southern 

parts of the eastern flank are presented separately, the 

division being made approximately at the latitude of 

Richfield (fig. 3). This division has structural significance 

because vertical displacement of the Pavant Range 

north-northeast of Richfield relative to that of the Sevier 

Valley was accomplished through a combination of monoclinal flexing and normal faulting, whereas we recognize 

no evidence of range-bounding normal faulting southsouthwest of Richfield. 

JOSEPH FLATS TO RICHFIELD 

The Pavant Range between Joseph Flats and Richfield 

consists of ash-flow tuff and lava flows of Oligocene age 

sandwiched between variegated sandstone, siltstone, 

shale, and limestone below and basin-fill clastic strata 

belonging to the Sevier River Formation above. 

Although structural complications make it difficult to 

assess stratigraphic thicknesses, the volcanic rocks are 

at least 400 m thick. Their stratigraphic relationship to 

regionally distributed volcanic rocks has been shown by 

Cunningham and others (1983). Callaghan and Parker 

(1961) showed that the structure of this part of the range 

is a northeast-trending syncline-anticline pair in the area 

south of the latitude of Elsinore grading toward Richfield 

into a single east-southeast-facing monocline. In detail, 

the structure is complex and includes a few small-scale 

folds and many previously unmapped faults. To document it adequately would require detailed mapping at a 

scale of no less than 1:12,000. Our approach, instead, was 

to measure many stratal, fault, and striation orientations, record them on aerial photographs, and use the 

photogeologic compilation as an adjunct to interpreting 

the published geologic maps. The abundant small structures are too small to depict in figure 3. Instead, figure 3 

emphasizes the anticline-syncline pair and southeastern 

dips in the monocline. Figure 3 shows, as Callaghan and 

Parker's (1961) map did, that the Miocene-Pliocene 

Sevier River Formation is involved in the folding. 

The complex structure of the Joseph Flats to Richfield 

segment of the range flank led Cunningham and others 

(1983) to depict it as dominated by the effects of landsliding. Although landsliding was certainly a factor in 

shaping the complexity, we conclude that most of it is 

caused by at least two superposed episodes of deformation, each including faulting and stratal tilting. 

Data gathered from 59 intermediate- and smalldisplacement faults (fig. 15) show that most have moderate to steep dips and strike north to northwest. Their 

strikes are transverse or acute to that of the range 

margin, which trends about N. 35° E. Directly west and 

northwest of Elsinore, ash-flow tuffs that correlate with 

the Needles Range Group of Oligocene age (Steven, 

1979) strike chiefly northwest and are repeated by 

numerous northwest-striking faults. The average angle 

between bedding and faults of about 90° suggests that 

the faults formed as tension fractures before the strata 

were tilted. Most fault-plane striae show dip slip. The 

tension fractures were probably reactivated as normal 

faults and were rotated as the beds rotated, a structural 

history similar to that described by Angelier and Colletta 

(1983). The exposed bedding and fault pattern suggests 

northeast-southwest extension. Some striae on these 

same faults or on parallel faults have low-angle rakes, 

possibly suggestive of late-stage reactivation as strike-
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F19 

slip faults. In Flat Canyon directly north of Elsinore 

(locality G, fig. 3), the formation of Aurora oflate Eocene 

age is cut by conjugate sets of northwest-trending normal faults having a symmetry axis normal to bedding. 

These faults also suggest northeast-southwest extension. These strata are also cut by steep north- and 

northeast-trending strike-slip faults (fig. 16), some of 

which have large displacements, and by smalldisplacement reverse faults that offset the conjugate 

"' 

90 

1 

1...J 

§ - 0 

z 

Q) 

~ Q) 

> 

Q) 

a: 

11 I I I I I 

Sinestral 

Strike 

N 

90 

Sense of fault slip 

0 

<> 

<> 

1 

L 

I I I I I r 

Dextral 

normal faults. The reverse faults are apparently associated with eastward tilting of the early conjugate normal 

faults. We infer that the north- to northeast-striking 

strike-slip faults formed in response to the same forces 

that produced similar faulting in the Clear Creek sample 

area and at sample area C described earlier (see also fig. 

16). We also infer that these faults postdate the early 

normal faulting in the area. This inference is strongly 

supported at a few localities between Joseph Flats and 

Richfield where fault-tilted northwest-striking strata 

can be traced to the edge of the range, where they bend 

sharply into the southeast-dipping monocline and strike 

northeast, indicating that the monocline is superposed 

on the previously block-faulted strata. This superposed deformation is reflected in figure 17 by the concentration of northwest-plunging poles to bedding. Also, 

the northeast- and southwest-plunging poles to bedding 

do not lie along a vertical northeast-striking plane (fig. 

17). Instead, they are displaced slightly to the northwest, probably as a result of superposed moderate southeastward tilting in the monocline. 

Several faults in the Joseph Flats to Richfield sample 

area have striae exhibiting a bewildering variety of rake 

angles and polyphase slip. Qualitative analysis of the 

oblique-slip and strike-slip motions on similarly oriented 

faults shows slip incompatibility. The data are not 

uniquely consistent with either a landslide interpretation 

or an interpretation based on a compressional stress field 

analogous to the one that controlled late Cenozoic structural development in the Clear Creek sample area. 

Although the cause of the slip complexity and incompatibility is not known, we suspect that much of it results 

from polyphase deformations in a tectonic regime where 

uplift caused by tectonic unloading competes with lateral 

shearing caused either by north-south compression, 

wrench faulting, or tectonic rafting of structural blocks in 

a highly mobile shear zone. It is unfortunate that the 

fault-slip data that we gathered are too few to use as a 

basis for understanding polyphase deformation. 

<IIIII FIGURE 15. -Histograms of strike, dip, rake, and sense offault slip 

for faults along the southeastern margin of the Pavant Range 

between Joseph Flats and Richfield. R, radius of the histogram, 

in number of measurements; N, sample size. The contrast 

between distributions of strike, dip, and rake in this area and 

those in the nearby Clear Creek area is strong (fig. 10). In the 

diagram showing the sense of fault slip, the ratio of strike slip to 

net slip is plotted along the abscissa, and the ratio of the dip-slip 

component to net slip is plotted along the ordinate. This plot 

reveals the complex mixture of strike-slip, dip-slip, and obliqueslip faulting that characterizes this sample area. Compare this 

fault-slip diagram with the simple pattern of dip-slip faulting 

along the same range front to the northeast of Richfield (fig. 20). 

The patterned sector on the strike plot shows the average trend, 

35°±15°, of the Pavant Range front. Very few faults have that 

trend. Symbols are the same as for figure 10. 
F20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FIGURE 16.-View looking west in Flat Canyon north of Elsinore 

showing the offset contact between smooth- and rough-weathering 

beds of gently southwest-dipping sandstone, siltstone, and claystone 

belonging to the formation of Aurora of late Eocene age (Willis, 

1986). The fault that separates the offset sections is approximately 

vertical, strikes 005°, and has an estimated down-to-the-east throw 

of 10 m. At first glance, this fault and another nearby fault of similar 

orientation and displacement but out of camera view appear to be 

steep dip-slip faults that repeat the strata by down-to-the-east (lower 

right) displacements. However, as is the case with the faults at 

locality C (fig. 3) along the Clear Creek downwarp, these faults are 

sinistral strike-slip faults having rakes of 3° and 13° to the south. 

Of the 59 small- and intermediate-displacement faults 

observed along the range front between Joseph Flats and 

Richfield, only 6 strike within ±20° of theN. 35° E. trend 

of the range front; of these 6, only 3 dip southeast-the 

dip direction that one would expect if the faults are part 

of a range-front fault system. Of the three, two show 

sinistral strike-slip striae, and one shows normal slip. 

Thus, only 1 of 59 faults has an orientation and slip sense 

similar to what would be expected for a range-front 

normal fault system that might be inferred from the 

geology and present-day physiography. 

Previous workers have inferred a major normal fault, 

the Elsinore fault, to be the range-front structure along 

the Joseph Flats to Richfield segment of the range front 

(Callaghan and Parker, 1961; Cunningham and others, 

1983). Our identification of the chief range-front structure as a monocline and, more specifically, our failure to 

identify minor structures related to a range-front normal 

fault suggest that, if a fault exists, it is either (1) buried 

beneath the monocline and genetically related to it, (2) 

buried beneath alluvium at some distance east of the 

range front, or (3) a continuation of the sinistral-slip Dry 

Wash fault, which, if projected northeastward, should lie 

along the range front (fig. 3). 

N 

E w 

s 

FIGURE 17.-Lower hemisphere stereographic plot of poles to bedding along the flank of the Pavant Range between Joseph Flats and 

Richfield. The dark shaded areas embrace poles that represent 

first-phase stratal tilting to the northeast and southwest on generally northwest-trending faults. The light shaded areas embrace 

poles representing second-phase monoclinal tilting to the southeast. 

Locally, the northeast-dipping block-faulted strata in the range can 

be traced continuously into the monocline. 

RICHFIELD TO WILLOW CREEK 

The eastern part of the Pavant Range between a line 

extending southwest from Richfield northward to Willow 

Creek (fig. 3) is underlain by lower Tertiary sedimentary 

rocks. In that area, the eastern margin of the main range 

block extends northeast from Richfield to approximately 

the locality marked by H in figure 3. From there, it 

bends sharply northwest toward Willow Creek, where it 

is marked by a fault scarp of probable Holocene age 

(Anderson and Bucknam, 1979). The range margin structure is complex because of (1) the sharp bend, (2) possible 

diachronous deformation, (3) a mixture of faulting and 

monoclinal flexing, and (4) a mixture of down-to-valley 

and down-to-mountain faulting. 

The lobe of bedrock that extends toward Aurora from 

the main range block is dominated by a series of 

northeast-tilted fault-repeated blocks of Green River 

Formation but also contains a major and controversial 

fold structure that wraps around the northeastern 

extreme of the range and involves the Green River and 

younger Tertiary rocks (fig. 18). Our early reconnaissance mapping of that area led us to interpret the 

faulting and tilting on northwestward trends as an early 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F21 

episode of deformation related to northeast-southwest 

extension and the wraparound fold as a subsequent 

episode of monoclinal fold deformation related to formation of the Sevier Valley-a structural history very 

analogous to that described in the previous section for 

the Elsinore area. On the basis of 1:24,000-scale quadrangle mapping, Willis (1988) interpreted most major 

folding and faulting in that area to be genetically and 

temporally related to a single deformational episode and 

suggested that the deformation could be as late as 

Pliocene. His interpretation was strongly influenced by 

mapping indicating that most of the northwest-striking 

faults terminate southeastward at the Elsinore fault and 

thus are contemporaneous with or younger than the 

Elsinore fault. We maintain that the quality of exposures 

and lack of marker beds east of the northeast-striking 

Elsinore fault do not preclude small faults extending into 

that area and suggest that some large faults do, on the 

basis of Willis' mapping (fig. 18). We therefore adhere to 

our initial interpretation of diachronous deformation. 

Along the Sevier Valley between Richfield and 

Aurora, a few hills of dark-colored volcanic rocks of 

Oligocene age protrude through the alluvium. Most of 

these volcanic rocks dip moderately to steeply toward 

the valley, showing that Neogene monoclinal flexing 

plays an important role in producing the vertical structural relief of that part of the range front. Displacements 

on subvertical northeast-striking faults also contribute to 

or, in some places, are the dominant elements in producing the vertical structural relief. Mapping by Willis 

(1988) showed that these faults can be traced northeast 

to the latitude of Aurora, northeast of which displacement transfers completely to a southeast-facing monocline (fig. 18). We interpret the entire wraparound fold as 

part of that monocline and, therefore, as part of the 

range-flank structure. 

Directly west of Richfield, well-exposed monoclinally 

flexed rocks belonging to the Flagstaff Limestone and 

the Colton and Green River Formations in the upthrown 

block of the range-front fault are cut by numerous smallto intermediate-displacement down-to-the-mountain normal faults. About 6 km north of Richfield, similarly 

flexed equivalent strata dipping valleyward from 15° to 

60° on the downthrown side of the range-front fault are 

cut by intermediate- to large-displacement down-to-themountain normal faults. Directly southwest of Aurora, 

we infer from an anomalously large breadth of outcrop 

and steep valleyward dips reported by Willis (1988) that 

the Tertiary volcanic rocks are displaced by as-yetunrecognized down-to-the-mountain faults (fig. 18). The 

exposures west of Richfield reveal that the down-tothe-mountain faults tend to cut the tilted strata at 

approximately 90° to bedding, whereas faults of the main 

range-front system are subvertical (fig. 19). 

Rocks of the Flagstaff Limestone in the upthrown 

block of the northwest-striking main range front in the 

vicinity of Willow Creek form a faulted monocline very 

similar in cross sectional form to the monocline west of 

Richfield (fig. 19) but having a dramatically different 

orientation. Despite their different orientation, we infer 

that these structures are coeval and that, together with 

the structures of the down thrown block, they adequately 

characterize the style of range-margin deformation. Little, if any, net extension can be attributed to the 

near-vertical down-to-valley faults. Stratal flexing tends 

to subtract from-and may completely negate-extension on down-to-range faults. Therefore, there may be 

little or no net extension associated with the uplift across 

these range-margin structures. If so, the range margins 

are very different from the bedrock lobe west of Aurora, 

where fault repetition suggests moderate northeastsouthwest extension that we believe predates formation 

of the Pavant Range. 

Fault-slip data were gathered from 33 small-, 

intermediate-, and large-displacement faults belonging 

to the range-margin system, 20 from the northeasttrending segment west of Richfield and 13 from the 

northwest-trending segment near Willow Creek. Strike 

distributions presented in figure 20 show the contrasting 

northeastern and northwestern trends of the rangemargin segments. 

A paleostress computation using the 20 faults from the 

northeast-trending segment yields almost the same orientation for a 3 (316°) as a computation using all33 faults 

(315°). At first, this coincidence seems inconsistent with 

the contrasting strikes of the range-margin structures. 

We note, however, that most of the northwest-striking 

faults in the Willow Creek area, instead of being pure 

dip-slip faults, are actually oblique-slip faults exhibiting 

rakes consistently to the northwest, as the stereoplot in 

figure 20 shows. 

We have no direct evidence of strike-slip faulting along 

the Richfield to Aurora segment of the Pavent Range 

margin. This lack contrasts greatly with the abundant 

evidence of strike-slip faults in the volcanic rocks from 

Richfield southwest. This dramatic contrast may result 

from structural decoupling or detachment of the volcanic 

rocks from the underlying sediments, as outlined in 

"Summary and Discussion." 

SEVIER FAULT 

The Sevier fault is one of the major structures of the 

transition zone. It forms the western boundary of the 25-

to 30-km-wide east-tilted Sevier Plateau block and locally 

has at least 1.5 km of down-to-the-west throw. In most 

places, uplift of the Sevier Plateau is probably distributed over a broad zone of faults that we collectively refer 
F22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Oa 

0 1 MILE 

0 1 KILOMETER 

111°52'30" 

Oa 

EXPlANATION 

Quaternary alluvial fill of Sevier Valley 

Tertiary-Quaternary alluvium and Quaternary pedi· 

ment gravel 

Sevier River Formation 

Oligocene and Miocene volcanic rocks 

Eocene and Oligocene sedimentary rocks-Mostly 

Crazy Hollow Formation and formation of Aurora 

~ Eocene sedimentary rocks-Mostly Flagstaff and 

Green River Formations 

----Contact 

--,....-- Fault-Dashed where buried or inferred, dotted 

where concealed; ball and bar on downthrown side 

--1---.. 11> Anticline-Showing " lunge 

--1--_. . Syncline-Showing plunge 

---;-- Monocline 

25 

-'- Strike and dip of bedding or foliation 

@ Locality discussed in text 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F23 

to as the Sevier fault. The Sevier fault extends from near 

Annabella south about 120 km to near Panguitch, Utah, 

but consists of several north-, northeast-, and northweststriking segments that give the fault a conspicuous 

zigzag trace (Rowley, 196g; Rowley and others, 1979; 

Cunningham and others, 19g3). The northern Sevier 

fault is well defined, although buried, south of Monroe, 

where it apparently controls the N. 5° E. trend of the 

range for about 6 km (fig. 3). Its trace north of Monroe is 

conjectural because the range-front escarpment bends 

sharply eastward, and it is uncertain whether the main 

trace of the Sevier fault follows that bend or, as Callaghan and Parker (1961) indicated, strikes 035° beneath 

a thick mantle of landslide and alluvial debris to 

reemerge at the western base of the Bull Claim Hills 

northeast of Annabella. Whether its main trace north of 

Monroe strikes northeast or east-northeast (fig. 3), its 

contrast in strike north and south of Monroe serves as a 

basis for discussing those segments separately. 

MONROE SEGMENT 

The Monroe segment of the Sevier fault is defined as 

the 6-km-long north-trending part south of Monroe (fig. 

3). Although modeling of detailed geophysical data and 

interpretation of drill-hole data at the Monroe geothermal area yield conflicting results, together they indicate 

that the Sevier fault is a zone of several major northtrending step faults that dip valleyward goo (Mase and 

others, 197g; Chapman and Harrison, 197g). In our 

study, the Sevier fault was observed at only one exposure (directly northeast of locality E, fig. 3). At this 

locality, a fault having an attitude 007°, 70° W. cuts 

sharply through conspicuously unbrecciated and unfrac-

<1111111 FIGURE 18. -Generalized geology of the northwestern part of the 

Aurora 71/z-min quadrangle. With the exception of the highly 

interpretive fault-and-fold pattern in volcanic rocks southwest of 

Aurora, the data are taken from Willis (1988). The Elsinore fault 

terminates near a point marked by A, where its displacement is 

transferred to a southeast-facing monocline that, in turn, bends 

westward and loses expression near a point marked B. Loss of 

expression is in large part due to its mergence with the homoclinally northeast-tilted Eocene strata. To the east of the Elsinore 

fault and its coextensive monocline, the Eocene and Oligocene 

sedimentary rocks and overlying volcanic rocks wrap neatly 

around the northwest-trending series of fault-repeated blocks of 

Eocene sedimentary rocks. This wrap-around structure is herein 

interpreted to postdate the faulting and tilting on northwestern 

trends. The down-to-the-northwest series of inferred faults 

shown cutting the volcanic rocks southwest of Aurora represents 

a stylized attempt to resolve an anomalously great stratal thickness of volcanic rocks suggested by the moderate to steep dips 

over a cross-strike distance of more than 1 km. The faulting style 

follows that seen to the southwest in the down thrown block of the 

Elsinore fault. The folds are based on stratal strike variations 

mapped by Willis (1988). 

FIGURE 19. -Typical fault offsets seen in the monoclinally flexed 

margin of the Pavant Range. Fault offsets are similar to those 

described and analyzed by Thompson and White (1984); patterned 

beds are hypothetical. Down-to-range faults cut bedding at approximately right angles, and bedding is not rotated by the steep 

down-to-valley faults. 

tured Oligocene andesite and is marked by a 1- to 

2-m-wide rib of massive silica that has been prospected 

locally. Striations on and within the secondary silica have 

gentle northern plunges indicating late-stage strike-slip 

motion. Neither the sense nor the amount of displacement is known. Directly northwest of locality E (fig. 3), 

the mapped trace of a fault that separates calc-alkalic 

andesite of Oligocene age from downdropped rhyolitic 

ash-flow tuffs of Miocene age strikes N. 55° E., dips 

steeply northwestward, and has a stratigraphic separation of at least 1 km (Cunningham and others, 19g3). Six 

steeply dipping gouge-coated surfaces hand excavated in 

fractured andesite breccia of the footwall block strike 

within ± 15° of the mapped fault and show normal dip 

slip. Although this fault is not part of the Monroe 

segment as defined herein, it is a main strand of the 

Sevier fault. On the basis of these limited data, we infer 

that the Sevier fault, although predominantly dip slip, 

has some strike-slip components of displacement along 

north-striking segments. This inference is consistent 

with observations of bimodal dip-slip and strike-slip 

striae seen on the southernmost part of the Sevier fault 

(R. Krantz, written commun., 19g6). 

Stratal strikes in Oligocene volcanic rocks along the 

005° -striking Monroe segment are varied, and dips average only about 10°. Study of 62 small- and intermediatedisplacement faults in these gently tilted and faulted 

rocks shows fault strikes having northeastern and northwestern maxima and no tendency for fault strikes to 

parallel the range front (fig. 21). Of 14 faults whose 

strikes fall within 20° of the range-front trend, only 3 dip 

toward the valley and show normal displacement. Of the 

62 faults, 36 have rakes less than 45°, and 26 have rakes 

greater than 45°. The data indicate heterogenous slip on 

faults that have attitudes conspicuously different from 

the attitude of the Monroe segment of the Sevier fault. 

Because they contribute little to extension, dip-slip faults 

having dips greater than goo were eliminated for computational purposes. The remaining subset of dip-slip faults 
F24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Strike 

N 

N=33 

w 

90 

Fault, striae, and paleostress axes 

N 

s 

N=33 ~=12° 

d=1 <1>=0.06 

(N = 13, table 1) is of marginally small size for paleostress 

computation. As expected, the dip-slip and strike-slip 

subsets yield paleostress axes having subvertical <T 1 and 

<T2 , respectively (fig. 21). Subhorizontal <T3 axes from the 

dip-slip and strike-slip subsets are within 12° of each 

90 

ro 

E 

0 

z 

a.. 

::::i 

(/) 

a.. E 0 

(]) 

IJ) 

Ci3 

> (]) 

a: 

1 

1...J 

0 

1, 

Rake 

Sinestral 

Sense of fault slip 

0 

I I I I I I L 

I I I I I r 

Dextral 

other, suggesting that the two modes of faulting are 

parts of a single deformational process. The computational results as well as the data distributions are quantitatively similar to those from the Clear Creek sample 

area. We find this similarity astonishing in view of the 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH 

..... FIGURE 20.-Histograms of strike, dip, and rake, lower hemisphere stereographic plot of faults, striae, and computed paleostress axes, and plot of slip sense for faults along the eastern 

margin of the Pavant Range between Richfield and Willow 

Creek. R, radius of the histogram, in number of measurements; 

N, sample size; d, number of striae that form an angle greater 

than 45° with the theoretical maximum shear stress; ~' mean 

angular difference between the actual striae and the theoretical 

maximum shear stress on the fault plane; <l>=(rr2 -rr1)/(rr3 -rr1); <Tb 

rr2 , and rr3 , maximum, intermediate, and minimum principal 

compressional stress, respectively. The contrast between strike 

and rake distributions for these data and for those shown in figure 

15 is strong. The dip distribution shows two maxima; the 85° to 

90° spike represents mostly down-to-valley faults, and the 65° to 

70° node represents mostly down-to-mountain faults. Two types 

of striae are differentiated in the stereographic plot of the fault 

planes (lower left); squares indicate normal sinistral, and triangles indicate normal dextral. The fault-slip plot shows a preference for oblique dextral slip. The oblique dextral separations are 

mostly on northwest-striking faults believed to have been early 

dip-slip faults reactivated in the stress field shown on the 

stereographic plot. The coordinates in the slip-sense plot are the 

same as those in figure 15. 

very strong contrast between the local structural settings of the two areas. However, we emphasize that the 

determination of slip sense for strike-slip faults along the 

Monroe segment is very poorly constrained (table 1). 

ANNABELLA SEGMENT 

The Annabella segment of the Sevier fault is herein 

defined as the range-front fault extending east-northeast 

from the Thompson Creek landslide (fig. 3). For 2.2 km 

east of the landslide, the trace of the Annabella segment 

is clearly marked by a precipitous straight bedrock 

escarpment trending 075°. Stratal attitudes of volcanic 

and sedimentary bedrock in the escarpment are highly 

variable, and dips are mostly less than 30° (figs. 22, 23). 

The escarpment projects eastward toward the lower 

reaches of Cliff Canyon, where excellent exposures 

reveal that volcanic rocks in the hanging wall block are 

FIGURE 21. -Histograms of strike, dip, and rake, lower hemi- ..... 

sphere stereographic plots of computed paleostress axes, and plot 

of dip-slip versus strike-slip components of fault slip for faults in 

the western margin of the Sevier Plateau in the Monroe area. The 

patterned sector on the strike plot is the average range-front 

trend, 005°±15°. R, radius of the histogram, in number of 

measurements; N, sample size; D, azimuthal range of dextral 

faults (number of faults in parentheses); S, azimuthal range of 

sinistral faults (number of faults in parentheses); d, number of 

striae that form an angle greater than 45o with the theoretical 

maximum shear stress; ~' mean angular difference between the 

actual striae and the theoretical maximum shear stress on the 

fault plane; <l>=(rr2 -rraf(rr3-<T1); <Th rr2, and rr3 , maximum, intermediate, and minimum principal compressional stresses, respectively. The coordinates in the slip-sense plot are the same as those 

in figure 15. 

co 

E 

0 

z 

a.. 

90 

STRIKE SLIP 

N 

1 J I I I I I 

:::::i 

(/)0 

a.. 

0 

Q) 

en 

Q; 

> 

Q) 

a: 

1 I I I I I I 

Sinistral 

Rake 

90 

DIP SLIP 

N 

N=13 

d=1 

Sense of fault slip 

0 

STRIKE SLIP 

I I I I I r 

Dextral 

F25 
F26 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

N varied strikes and a strongly skewed pattern of dip 

directions toward the southern hemisphere. Of particular significance is the paucity of dip directions toward the 

330° to 360° sector (that is, toward the downdropped 

block of the Annabella segment). This paucity cannot be 

ascribed to post-faulting rotation in the draped or folded 

hanging-wall block because much of the data is from the 

uplifted footwall block, which shows no consistent pattern of stratal rotation (fig. 22). We did not observe the 

main fault of the Annabella segment and therefore do not 

w 

s 

E know its true dip. We emphasize that many of our 

measurements were taken from faulted rocks within a 

few meters of the main fault. The data suggest that the 

main fault is a sharp break and that normal faults 

associated with it drop strata down toward the uplifted 

plateau. The data show that most of the dip-slip faults in 

this sample area are not part of a range-front fault 

system and may not contribute significantly to plateau 

uplift. In this respect, the Annabella segment is similar 

to the Monroe segment as well as to the eastern margin 

of the Pavant Range. 

FIGURE 22.-Lower hemisphere plot of poles to bedding in bedrock 

along the Annabella segment of the Sevier fault. Poles that plot in 

the southeastern sector mostly represent rocks in the monoclinally 

flexed or fault-dragged beds of the hanging-wall block, where flexing 

is away from the range trend (dashed line). 

bent down toward the north, suggestive of a large drape 

fold or possibly normal drag (fig. 23). The Annabella 

segment extends up Cliff Canyon, exhibiting possibly as 

much as 400 m of down-to-the-north stratigraphic offset 

of Oligocene volcanic rocks (Rowley and others, 1981a). 

A thick sequence of well-exposed Oligocene volcanic 

rocks in the plateau block south of that part of the fault 

is very mildly deformed. The rocks are horizontal to 

gently southeastward dipping and are cut by sparse 

small-displacement faults (fig. 23). 

A total of 235 fault-slip measurements were made on 

moderately to steeply dipping faults in the upthrown and 

downthrown blocks along the approximately 4-km-long 

trace of the Annabella segment (fig. 24). Fault strikes in 

this sample area are varied, and the histogram of strike 

distributions shows a minor concentration in the 30° 

sector centered on the 075° average trend of the Annabella segment. Dip directions for these faults, however, 

are skewed to the southern hemisphere (that is, toward 

the uplifted plateau block). Oblique slip is common. The 

histogram of rake distributions shows a clearly defined 

minimum at 30°. Dip-slip and oblique-slip faults predominate over strike-slip faults. 

To evaluate the role of dip-slip faulting in plateau 

uplift, we present data-distribution histograms for dipslip faults having rakes greater than 60° (90 faults) (fig. 

24). As did the entire data set, this subsample shows 

The sense of slip was determined with certainty or 

reasonable probability on only about 15 percent of the 

oblique-slip and strike-slip faults along the Annabella 

segment. A lower hemisphere stereographic plot of 

faults in this 15-percent subsample is shown in figure 25. 

For predominantly strike-slip faults, the subsample 

shows an unusually wide range of attitudes (especially 

dip values). Despite the attitude variability, slip lines 

show a strong preference for dispersion around a northsouth azimuth, but sinistral and dextral faults are not 

concentrated in separate strike sectors, as they are in the 

Clear Creek sample area. We infer from dip variability, 

north-south dispersed slip lines, and lack of sinistraldextral partitioning that faulting in a predominantly 

strike-slip mode does not represent a conjugate-shear 

response to remote stress. Therefore, quantitative 

attempts to resolve paleostress characteristics by using 

strike-slip faults should not be made. 

We computed a paleostress orientation from the dipslip faults exhibiting rakes greater than 60° (N=90). It 

shows cr3 at 282°, but the low value of <I> (0.03, table 1) 

suggests that moderately to steeply dipping faults of any 

strike are appropriately oriented for dip-slip movement. 

Although the computed cr3 orientation is clearly not a 

reflection of principal extension normal to the Annabella 

segment of the range front, the low value of <I> and the 

fact that oblique-slip and strike-slip faults are excluded in 

its computation lessens its paleostress significance. The 

dip-slip faulting may be analogous to the strike-slip 

faulting and may not represent a response to applied 

remote stresses. Alternatively, a complex and unresolvable history of polyphase deformation could be recorded 

by the striae that we measured. 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F27 

EXPLANATION 

~ Alluvium and colluvium of Quaternary age-Is, 

landslide 

U·::-Cld:·:< .. l Older alluvium and debris flow of Quaternary age 

Unconformity 

~ 

DJ 

Sevier River Formation of Miocene and Pliocene 

age-Mostly basin·fill clastic strata 

Volcanic and volcaniclastic rocks of Oligocene and 

Miocene age 

Intrusive rocks 

Sedimentary rocks of Eocene(?) age-Mostly silt· 

stone, sandstone, gravel, conglomerate, and 

limestone; upper part contains truck· and house· 

size blocks of volcanic rock. Possibly correlative 

with the Crazy Hollow Formation and formation 

Unconformity of Aurora (Willis, 1986) 

- Sedimentary rocks of Paleocene(?) age-Mostly 

brick-red sandstone, gravel, and conglomerate; 

possibly correlative with the Flagstaff formation 

Arapien Formation of Jurassic age-Mostly shale, 

siltstone, and sandstone; commonly gypsiferous 

---- Contact 

45 40 __j_ ___..__ 

Fault-Dashed where indefinite or inferred, dotted 

where concealed, teeth where gently dipping, 

bar and ball on downthrown side 

Strike and dip of bedding or foliation 

Horizontal bedding or foliation 

"T"""T'""1r-r-.- Fault scarp in alluvium, 

hachures on down side 

MS Main scarp of Sevier fault 

METERS 

2000 

1000 

West 

0 2 4 KILOMETERS 

fEET 

6000 

5000 

4000 

3000 

500 2000 

1_------~===:=~~~~~~JI~t 1000 

FIGURE 23. -Geology and cross section of the Annabella-Glenwood 

area. Low-angle faults in the Arapien Formation in the section 

schematically depict semiductile "flowage" of that mechanically 

incompetent unit out from beneath the Sevier Plateau. A large 

down-to-the-west fault analogous to the Sevier fault at the latitude of 

the cross section is not required. Except for beneath the Sevier 

Valley, areas underlain by Quaternary deposits are not shown on the 

cross section. 
F28 

A 

B 

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Strike 

N 

Dip direction 

N 

Strike 

N 

E 

90 

90 

Rake 

Dip direction 

N 

E 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F29 

ANTICLINE SAMPLE 

The southern part of the Sanpete-Sevier Valley anticline, cored by mechanically weak rocks of the Arapien 

Formation, is an important structural element in the 

central Sevier Valley area. For about 20 km southsouthwest of Salina, a 5-km-wide core zone of Arapien 

rocks is exposed, and the adjacent Tertiary rocks tend to 

dip moderately to steeply (locally overturned) away from 

the core rocks. By contrast, in the area east of Glenwood 

and northeast of Annabella, Arapien rocks are exposed 

only locally in a narrow band, and Tertiary rocks dip 

uniformly westward, as depicted in cross section (fig. 

23). We conducted reconnaissance fault-slip studies in 

Tertiary rocks at and above the contact with the Arapien. Those rocks contain abundant evidence of extensional deformation and structural attenuation at widely 

ranging scales and magnitudes. In many places, structural attenuation is so extreme that hundreds of meters 

of strata are reduced to broken or shattered layers a few 

meters thick. The ultimate contact with the Arapien is 

shown on published maps as depositional. We recognize 

it as a fault in most places. Although it ranges widely in 

dip direction and magnitude, it dips gently and is discordant to bedding in overlying rocks in many places and 

over wide areas. 

Although we gathered fault-slip data as far north as 

Salina, the area of greatest interest is to the south, 

where it is clear that the upper Miocene and Pliocene 

basin-fill sediments of the Sevier River Formation are 

intensely involved in the deformation. Our studies in that 

area demonstrate that extensional deformation, locally of 

large magnitude and including severe structural attenuation, dominates the late Cenozoic structural record. Our 

results preclude a major role for the type of compressional tectonics suggested by Gilliland (1963). 

Seventy-two fault-slip measurements were made on 69 

faults in rocks at and above the contact between Arapien 

rocks and cover rocks. Although most faults strike 

northeast, parallel to the Sanpete-Sevier anticline, many 

have a more northerly strike (fig. 26A). Rake angles are 

<1111111 FIGURE 24. -Histograms of faults and striations from Tertiary 

volcanic and sedimentary rocks along the Annabella segment of 

the Sevier fault (Annabella sample area). R, radius of the 

histogram, in number of measurements. Patterned sectors on the 

strike plots and the dip-direction plots show the average trend of 

the range front and the facing direction of the range front, 

respectively ( ± 15°). A, Strike, dip, rake, and dip-direction distributions for 235 faults. Very few faults strike parallel to the 

range front, and there is no tendency for faults to dip toward the 

down thrown block of the Sevier fault. B, Strike and dip-direction 

distributions for a subsample of 90 normal faults derived from the 

sample represented in A. Even the normal faults show no 

tendency to dip toward the down thrown block of the Sevier fault. 

The arrow shows the computed azimuth of 0', the minimum principal compressional stress. 

N 

w E 

EXPLANATION 

~ Certain _. Probable 

FIGURE 25.-Lower hemisphere stereographic plot of a subsample of 

predominantly strike-slip faults derived from the sample shown in 

figure 24A. Only faults displaying certain and probable slip-sense 

determinations are shown, and they are distinguished by different 

types of arrows. The bases of the arrow shafts mark lineations, and 

arrows point in the directions of motion of hanging-wall blocks. Solid 

lines are dextral faults, and dashed lines are sinistral faults. 

varied but mostly steep, and the faults dip more moderately than those in any of the other areas that we 

sampled. The moderate dips probably result from rotation of faults during extension. On faults displaying 

polyphase slip, striae having the shallowest rake angles 

are the youngest on 2 of the 69 faults; data are thus 

insufficient to establish whether dip-slip faulting is older 

or younger than strike-slip faulting. 

To evaluate the significance of dip-slip faulting separately from that of strike-slip faulting, we present strikeand dip-direction histograms for a subsample of 49 normal faults taken from the entire 69-fault sample (fig. 

26B). The data distributions are similar to those of the 

entire sample and show that many normal faults strike 

more northerly than the anticline and that there is a 

strong tendency for the faults to dip easterly, consistent 

with our qualitative field observation that Tertiary rocks 

dip uniformly westward and are downdropped uniformly 

eastward across the southernmost part of the anticline. 

Taken together, these relationships suggest that some 

process other than anticlinal folding was active during 
F30 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

extension and that the anticline did not exert a strong 

influence on fault kinematics during the extension. This 

suggestion is supported by a weakly constrained least 

principal stress direction of 282° computed from the 

subsample of 49 normal faults (fig. 26B, table 1). This 

orientation is closer to the orientations computed from 

other sample areas than it is to a direction perpendicular 

to the anticline. It is almost identical to the orientation 

computed for the Annabella segment to the south (fig. 

24) (also weakly constrained), even though the two 

sample areas have dramatically differing structural settings. We conclude that regionally significant extensional 

faulting and associated structural attenuation are the 

dominant deformational mode in the cover rocks of the 

southern part of the anticline. Where the Sevier River 

Formation is exposed, it is involved in the full range of 

this deformation and thus leads us to conclude that the 

deformation is neotectonic. 

To evaluate the significance of strike-slip faulting 

separately from dip-slip faulting, we present a lower 

hemisphere stereographic plot of those faults and striations (fig. 27A). This subsample of 29 faults shows 

orientation diversity similar to that seen in the strike-slip 

subsample of faults from the Annabella segment directly 

to the south (fig. 25). This similarity is remarkable in 

light of the highly contrasting structural settings of the 

two areas. In particular, there is no tendency of the 

diversely dipping north- to northeast-striking faults to 

divide into sinistral and dextral fields, as one would 

expect if they were conjugate faults. Data gathered from 

individual exposures (fig. 27B, C) indicate that faults 

having generally east-west strikes are probably blockbounding structures genetically linked to systems of 

predominantly normal faults whose motions are a 

response to generally east-west extension. Neither these 

faults (because of their lack of slip independance (Angelier, 1979)) nor the north- to northeast-striking ones 

(because of their slip incompatibility) are candidates for 

paleostress analysis. Perhaps the components of strikeslip motion on north- and northeast-trending faults are a 

response to north-south shortening or to tractions 

imparted to the base of an extending slab by lateral 

flowage of subjacent rock. Either possibility is consistent 

with a strong component of anticline-parallel shortening 

suggested in north-trending cross sections drawn by 

Hardy (1952). The possibility of basal tractions is 

explored more fully in a subsequent section on seismicity 

and Quaternary deformation. 

NORTHERN ANTELOPE RANGE 

The part of the Antelope Range adjacent to the Dry 

Wash fault consists of a fault-repeated sequence of 

Oligocene and Miocene flows and tuffs displaying east to 

northeast dips ranging from 5o to 45° (Cunningham and 

others, 1983). Mapped faults strike mostly north to 

northeast and drop strata down to the west. No evidence 

was found for the east-trending folds that are common in 

the Clear Creek area across the Dry Wash fault. Because 

the northern Antelope Range consists of a more orderly 

arrangement of fault-tilted blocks than most other areas 

in the region, it was hoped that a straightforward 

relationship between fault slip, block tilting, and extensional deformation consistent with the mapped structural 

pattern could be discerned. Instead, a reconnaissance of 

fault slip reveals a complex mixture of strike slip, oblique 

slip, and dip slip that cannot be understood in terms of a 

single deformational event or any simple structural pattern. 

Paleoslip was measured on 135 faults. Displacement on 

108 of them is known or estimated to be less than 1 m; on 

the remaining 27, it ranges from 1 to more than 100m. 

Most measurements were made within 3 km of the trace 

of the Dry Wash fault. Of the 135 faults, 17 contain 

polydirectional striae; on all but 1, the oldest slip event 

has the lowest rake. This indication that dip-slip faulting 

is consistently younger than strike-slip faulting agrees 

with observations near locality B (fig. 3) in the Clear 

Creek downwarp but is not a feature recognized in all 

other areas that we studied. 

Some major dip-slip faults in the northern Antelope 

Range displace the sparsely preserved Sevier River 

Formation by at least 100 m. We assume that the 

strike-slip displacements also postdate the Sevier River 

Formation because they do so in the adjacent Clear 

Creek area. 

Fault strikes are mostly north and northeast, and 

sinistral slip predominates over dextral slip in the strikeslip subset, similar to the faults in the Clear Creek 

sample area (figs. 10, 28). The proportion of faults having 

rake angles greater than 30° and a component of reverse 

slip is larger in this sample area than it is in the others. 

As is the case with a major, steeply north-dipping, 

FIGURE 26.-Histograms of faults and striations from the Tertiary ~ 

cover rocks of the Sanpete-Sevier anticline (anticline sample). R, 

radius of the histogram, in number of measurements. A, Strike, 

dip, rake, and dip-direction distributions for 72 faults; the solid 

arrow marks the approximate trend of the axis of the SanpeteSevier anticline. B, Strike and dip-direction distributions for a 

subset of 49 normal faults derived from the sample represented in 

A. The open arrow shows the computed azimuth of 0'3, the 

minimum principal compressional stress. The anticline trend 

approximately bisects the dispersion pattern of fault strikes. This 

occurrence, together with the moderate fault dip, computed 0'3 , 

and abundant field evidence for large-magnitude fault-related 

attenuation, shows important extension at a high angle to the 

trend of the anticline. Some of the strike-slip faults (eliminated 

from B) are block-boundary faults coupled to the normal faults as 

shown in figure 27. 
A 

B 

NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH 

90 

E 

90 

Rake 

Dip direction 

N 

F31 

E 
F32 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

N N N 

w E E W 

s s s 

A B c 

EXPLANATION 

-.. Certain 

FIGURE 27. - Stereographic plots of faults that cut Tertiary rocks at or 

above the contact with the mechanically weak Arapien Formation 

along the southern part of the Sanpete-Sevier anticline. Solid lines 

are dextral faults; dashed lines are sinistral faults. A, Subsample of 

29 predominantly strike-slip faults derived from the sample shown in 

figure 26A. B, Faults and striae from a group of closely spaced 

exposures of highly fractured welded ash-flow tuffs on the eastern 

flank of the anticline. The bases of arrow shafts mark lineations, and 

east-striking sinistral and reverse-sinistral fault at locality F (near the southern boundary of fig. 3) (fig. 29), it is 

generally not possible to distinguish between true 

reverse faults and subsequent rotation by later faulting. 

Of the 92 measurements made on faults having rake 

angles greater than 30°, the slip sense on 17 indicates 

reverse slip. Neither these 17 measurements nor the 56 

measurements of strike-slip striae having rake angles of 

30° or less are qualitatively consistent with the mapped 

pattern of fault-repeated and fault-tilted blocks. 

In summary, a reconnaissance investigation of fault 

slip in the northern Antelope Range reveals complex 

data distributions, many aspects of which are qualitatively inconsistent with what would be expected from the 

mapped structural patterns. The structural significance 

of the complexities is not understood. Also, the data 

provide no basis for meaningful paleostress computations. Nevertheless, the common occurrence of reverseslip and strike-slip displacements represented by the 

striae that we observed indicates that the faults clearly 

did not result from straightforward application of extensional stresses in which rr 1 was in an approximate vertical 

position. As is the case with the other areas that we 

studied, a horizontal maximum compression or a lateral 

tectonic transport of blocks, possibly caused by tractional 

forces applied from below, or both are required to 

produce some of the deformation. 

- Probable 

arrows point in directions of motion of hanging-wall blocks. C, Faults 

and striae from an exposure on the western flank of the anticline. The 

bases of arrow shafts mark lineations, and arrows point in directions 

of motion of hanging-wall blocks. Strike-slip faults in Band Care also 

shown in A. That group of strike-slip faults is interpreted as 

block-boundary structures genetically and mechanically related to 

the dip-slip and oblique-slip motions represented by the easterly 

directed clusters of arrows in Band C. 

SEISMICITY AND LATE QUATERNARY 

DEFORMATION 

Seismicity studies in the Intermountain Seismic Belt 

by W.J. Arabasz and his students at the University of 

Utah, using a combination of permanent regional and 

temporary local seismic networks, have established the 

distribution of earthquake epicenters and have yielded 

an extensive sample of earthquake hypocenters and 

fault-plane solutions (Arabasz and Julander, 1986). 

Earthquake epicenters in western Utah tend to form 

clusters, and there is, in general, no clear-cut spatial 

association between those clusters and fault scarps 

formed on Quaternary alluvium. In the central Sevier 

Valley, however, two areas of concentrated seismicity do 

coincide with areas of late Quaternary surface deformation. One, a magnitude 4. 0 main shock and aftershock 

sequence, occurred in 1982 in the area northeast of 

Annabella, where late Quaternary fault scarps are common. (All magnitudes reported herein are local Richter 

magnitudes taken from Arabasz and Julander (1985).) 

The other is directly southwest of Elsinore, where we 

recognize tilted Quaternary geomorphic surfaces, late 

Quaternary fault scarps, and possible deformationrelated anomalies in the channel pattern of the Sevier 

River. In both areas, complex junctures between major 

mapped structures are logical candidates for localization 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F33 

90 

E "' 

0 

z 

1 

1_) 

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<> ~ <> <> <> ~o<> <> <><). 

o<> 

L 

1 I I I I I I 

Sinistral 

I I I I I 

Dextral 

FIGURE 28.-Histograms of strike, dip, and rake and plot of dip-slip 

versus strike-slip components of fault slip for faults in the northern 

part of the Antelope Range. R, radius of the histogram, in number 

of measurements; N, sample size. Note the wide distribution of 

rake angles, the significant number of reverse-slip faults, and the 

strong bias for sinistral slip. Only in the distribution of strike and 

the bias for sinistral slip do these data distributions resemble those 

from the adjacent Clear Creek sample area (fig. 10). 

FIGURE 29.-View looking west at the surface trace of a high-angle 

fault in the Antelope Range at locality F in figure 3. Hydrothermally 

altered and fractured rocks of the Bullion Canyon volcanics form the 

upthrown hanging wall on the right, and stratigraphically higher, 

unaltered volcanic sediments in the lower part of the Mount Belknap 

Volcanics (Cunningham and others, 1983) form the footwall block. 

Bedding in the footwall dips about 40° south, whereas the fault dips 

60° to 80° north. Striations and stratigraphic separation indicate 

reverse and reverse sinistral movement on the fault. However, it is 

not known whether the bedding and the fault were tilted after 

faulting, an occurrence that would make the structure a tilted 

normal-sinistral fault. 

of the seismicity-the Sevier fault system east of Annabella and the northeasternmost extent of the Dry Wash 

fault in the Elsinore area. These two coincidences are 

very unusual in the Intermountain Seismic Belt, where 

generally an association between seismicity and known 

mapped structures is not possible (Smith, 1978; Arabasz 

and Smith, 1981). Because the coincidence of seismicity 

with mapped faults offers an opportunity for improving 

understanding of the relationship between mapped 

faults, surface deformation, and seismicity, we will discuss each area in some detail. But first, we will make 

some general comments on the seismicity of the region. 

Analysis of 53 fault-plane solutions by Arabasz and 

Julander (1986) shows that seismic slip in the transition 

zone is characterized by a mixture of strike-slip and 

normal faulting, as the summary of those results shows 

in figure 30. They suggested that this mixture results 

from interchanges in the positions of a 1 and a 2 , whereas 

a 3 remains stationary and approximately horizontal at 

about 102°. 

In three areas in the Intermountain Seismic Belt, 

Arabasz and Julander (1986) recognized that horizontally 

bounded zones of microearthquake hypocenters correlate 

with known or inferred subhorizontal thrust faults. On 

the basis of this correlation, they hypothesized that the 

depth distribution of background seismicity is controlled 

by the internal structure and variable mechanical behavior of horizontal plates bounded by major low-angle faults 

within the seismogenic upper crust (fig. 30). They stated 
F34 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

A 8 

42! 1r4_o --,--....:1..:,.:12:.... o,---, NW 

~ \ 110° 

Pavant Range Sevier Valley 

SE 

Sevier Plateau 

#1 Paxton 

Elsinore fault 

Great Sal~ o Salt LaLk-e --,-------., 2 Richfield --T---------------~ 

VP(km/s) 

Lake City 

100 MILES 

100 KILOMETERS 

0 

2 (/) 

s 

(/) 

c: 

LU 

1-

L.U 

2 

0 

_J 

~ 

z 

z 

0 

i= 

~ LU 

_J 

LU 

JKT 

Detachment , 1111, ,....,.....,

1 ~ 

rfll#l'/ 

Pz 

Seismic 

discontinuity 

Seismic 

discontinuity 

p£ 

HORIZONTAL= VERTICAL SCALES 

N NW 

0 r---t--·,-~:. ·:t• (/) 

. 2 c: 

LU 

3.8 

4.5-5.2 

5.7 

SE 

c: 

LU 4 ;~{ 

... .. ,\ . -I-

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-I- ~ ., .. 1-2 ::CLU 

1-2 

~0 6 

. ·.· . .·--. .. : .. · 0:::! 

~0 6 

0:::! ~ 8 

~ 

10 

12 

Elsinore area 

FIGURE 30. -Aspects of seismicity in the transition zone. A, Map 

showing sample of current stress orientations inferred from earthquake focal mechanisms for the transition zone. The single outwarddirected arrows are least-compressive-stress directions inferred 

from dip-slip focal mechanisms; the combined inward- and outwarddirected arrows are maximum- and least-compressive-stress directions inferred from strike-slip focal mechanisms. B, The lower part 

shows depth distribution of microseisms in the Elsinore and Annathat this heterogeneity, which influences the depth distribution of earthquakes, was produced by vertical stacking of thrust plates during Sevier-age overthrusting. 

Seismic slip on the bounding low-angle faults in any of 

the three areas or elsewhere in the Intermountain Seismic Belt is not evident and led Arabasz and J ulander to 

speculate that such faults may move aseismically. The 

central Sevier Valley, including the concentrated seismicity in the Annabella and Elsinore areas, is one of the 

three areas in which Arabasz and Julander (1986) sug-

~ 

z 8 

• 10 

Annabella area 

bella areas, and the upper part shows the correlation in schematic 

cross section between the seismic discontinuities suggested by the 

depth distributions and detachment faults inferred from reflection 

seismicity. Vp, P-wave velocity. p£, Precambrian rocks;~. Paleozoic rocks; JK, Jurassic and Cretaceous rocks; JKT, Juras13ic, Cretaceous, and Tertiary rocks; KT, Cretaceous and Tertiary rocks; Tv, 

Tertiary volcanic rocks; QT, Tertiary and Quaternary basin-fill 

sedimentary rocks. 

gested the correlation between horizontal seismicity 

zones and low-angle faults (fig. 30). 

NORTHEAST OF ANNABELLA 

The part of the Sevier Plateau bounded on the west by 

the Monroe segment and on the north by the Annabella 

segment of the Sevier fault is best characterized as a 

weakly deformed, generally east-tilted, relatively intact 

structural block of Tertiary igneous rocks uplifted 0.5 to 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F35 

2 km above its surroundings. In sharp contrast, the part 

of the Sevier Plateau north of the Annabella segment, 

including the Bull Claim Hills, is best characterized as a 

highly deformed series of west-tilted blocks of volcanic 

and sedimentary rocks that are repeated eastward by 

faults that drop strata down toward the east and southeast (toward the uplifted Sevier Plateau) (fig. 23). That 

area is also characterized by strong structural attenuation of the stratigraphic section, especially in the vicinity 

of the exposed Arapien Formation. The tilting, stratal 

repetition, and attenuation involve strata of the upper 

Tertiary Sevier River Formation to the same extent that 

they involve older Tertiary rocks, the suggestion being 

that the deformation is neotectonic. Between the two 

areas of highly contrasting internal structure and neotectonic structural vergence and tucked into the large 

physiographic embayment in the plateau margin is a 

northeast-trending graben that we refer to as the Annabella graben (fig. 23). Downdropped strata of the Sevier 

River Formation are preserved in the graben and are 

overlain unconformably by Quaternary landslides, debris 

flows, and coarse alluvial deposits that accumulated 

during graben formation. The Sevier River Formation in 

this area predates graben formation and plateau uplift. A 

K-Ar age of 5.6±0.4 Ma was determined by H.H. 

Mehnert on a sample of volcanic ash from the Sevier 

River Formation east of Annabella (H.H. Mehnert and 

P.D. Rowley, written commun., 1982). 

An anomalous concentration of northeast-trending 

fault scarps ranging in height from less than 1 to more 

than 100 m is formed on the Quaternary deposits of the 

Annabella graben. These scarps are concentrated in an 

area of only 15 km2

• Individual fault-scarp lengths are 

less than 5 km. Some of the scarps are high and contain 

evidence for recurrent movement as late as the time of 

the Pleistocene-Holocene boundary (Anderson and Bucknam, 1979). The main fault in this area appears to be the 

fault labeled MS in figure 23. This fault bounds the 

graben on the southeast. The surface scarp that marks 

the trace of this fault not only reveals strong evidence of 

recurrent late Quaternary movement, but it also has a 

large cumulative displacement of upper(?) Quaternary 

deposits of about 109m; it is thus one of the highest fault 

scarps in alluvial deposits in Utah (figs. 31, 32). Its 

midslope section is 90 m long and has a slope of about 35°. 

To maintain such a large, steep slope resulting from slip 

events equivalent in size to the youngest event (fig. 31) 

requires about 25 small slip events spaced closely in time. 

Owing to complexities resulting from landsliding, it is 

uncertain how this fault connects with the Annabella 

segment of the Sevier fault (fig. 23). 

A magnitude 4.0 main shock and an extensive aftershock sequence in 1982 are centered on the Annabella 

graben (Arabasz and Julander, 1986). First-motion data 

for the main shock are inconsistent with the focal mechanism characteristics of and planar distribution of the 

aftershock sequence (fig. 33). Of several nodal planes 

allowed by first-motion data of the main shock, a 

northeast-striking southeast-dipping plane displaying 

predominantly normal slip would correspond well with 

the fault that bounds the Annabella graben on the 

northwest. Alternatively, hypocenter distributions 

resulting from detailed aftershock studies define a 

northeast-trending envelope dipping about 75° NW. (fig. 

33). The envelope is only about 5 km long by 3 km wide 

and 5 km deep (fig. 33). Although such an envelope 

agrees well with any of several similarly oriented faults 

marked by Quaternary scarps in the Annabella graben, 

the aftershocks that define it yield focal mechanisms 

indicating predominantly strike slip on steeply dipping 

faults, and the envelope does not match any of the main 

shock's nodal planes. Thus, the faulting kinematics indicated by this seismicity are internally enigmatic, and 

their relationship to mapped faults is unresolved. N evertheless, the spatial coincidence of this seismicity and 

the area of anomalously disrupted upper Quaternary 

deposits suggests to us a genetic relationship between 

the seismicity and scarp-producing faults. 

The aftershocks of the 1982 seismicity define two types 

of focal mechanisms having approximately interchangeable pressure (P) and tension (T) axes (Arabasz and 

Julander, 1986). The preferred orientation for the Taxes 

is approximately east-west, similar to the orientation 

indicated by the main shock (Arabasz and Julander, 

1986). If the T axes are inferred to be colinear with the 

extension direction, a simple interchange between a 1 and 

a 2 satisfies most of the data. The internally incompatible 

strike-slip events can be related to the oblique-slip main 

shock by assuming that release of extensional stress 

creates a secondary compensatory motion of blocks parallel to the preexisting northeast-trending dominant 

structural fabric. In the case of the 1982 aftershocks, we 

would choose the northeast-striking dextral and sinistral 

nodal planes and would assume a southwest-directed 

tectonic transport of blocks bounded by faults analogous 

to these nodal planes. Such deformation would have to 

result from some secondary system of basal tractions or 

horizontally layered stress gradients felt by the blocks as 

extension progresses. This conjectured deformation is 

consistent with our fault-slip data, which indicate a 

mixture of dip slip and strike slip, and, in particular, with 

the tectonic transport parallel to the axis of the SanpeteSevier anticline indicated by the incompatible strike-slip 

striae from the Annabella segment and the southern 

Sanpete-Sevier Valley anticline. It is also consistent with 

the pattern of north-south shortening shown by folds 
F36 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FIGURE 31.-View to the south across the northeast-trending main 

strand of the Sevier fault toward the Glenwood Mountain part of the 

western Sevier Plateau. Geomorphic surfaces labeled A, B, and C 

terminate to the northwest against conspicuous scarps that mark the 

trace of the Sevier fault. Lettered lines A, B, and C below the 

respective surfaces point to localities where the scarp profiles shown 

in figure 32 are measured. The irregularly forested area beneath the 

scarps is part of a structurally complex graben containing clastic 

sediments of Miocene to Quaternary age. In contrast, linear elements 

between Glenwood and Salina (Hardy, 1952). We return 

to this notion of basal tractions in "Summary and Discussion." 

Late Quaternary faulting and seismicity may be concentrated in the Annabella graben, because a structural 

juncture exists here between the down-to-the-west and 

down-to-the-northwest faults related to the Sevier fault 

and the dramatically different set of down-to-the-east 

structures along the northern and northeastern projection of the Sevier fault. An east-northeast-trending 

transverse zone of structural accommodation seems to be 

required beneath the graben. Concentration of stress in 

such a zone would be expected. The short length of fault 

scarps, the fact that they do not rupture contemporaneously, and the small size of the area containing them does 

not portend large-magnitude (6.5-7.5) earthquakes associated with their development. In particular, there is no 

justification for assuming that the concentrated young 

deformation is characteristic of the Sevier fault. Slip 

events on the Sevier fault from the graben southward are 

likely to be associated with larger earthquakes having 

much longer return periods than the earthquakes that 

produced the scarps. 

Witkind (1982) noted the colinearity of the SanpeteSevier Valley anticline and the Sevier fault and sugof the Glenwood Mountain skyline as well as crudely banded vegetation patterns beneath the skyline reflect the structural simplicity of 

that structural block. The uplifted remnants of surface C that are 

found on both sides of Water Creek Canyon are interpreted to be 

parts of a once-continuous surface that predates cutting of the 

canyon. That surface is smoothly graded across the alluvium-bedrock 

contact high in the plateau margin, and there is no suggestion there 

of displacements by young faults. 

gested a genetic relationship between the two. Within 

this same context, we suggest an alternative interpretation for the concentration of seismicity and young deformation. The concentration could be related to lateral 

movement of the relatively ductile rocks of the Arapien 

Formation. The southernmost exposures of the Arapien 

are directly northeast of the graben, and equivalent 

rocks could exist in the shallow subsurface beneath the 

graben. Perhaps Arapien rocks have flowed into the 

extensional regime of the northeasterly projection of the 

Sevier fault where, because of a reduction in confining 

pressure, they tend to spread laterally. As Witkind 

(1982) suggested, movement of the Arapien could be 

stimulated by displacements on buried faults. Arapien 

rocks could be supplied to the fault zone from beneath the 

Sevier Plateau. This possibility is consistent with our 

observations in the vicinity of exposed Arapien rocks, 

where there is an indication that these rocks have been 

drawn out from beneath the Sevier Plateau (fig. 23). 

Such flowage would tend to remove most or all surface 

and near-surface traces of the Sevier fault. Although this 

hypothesis is an attractive alternative for explaining the 

thin-skinned extensional attenuation in rocks above the 

Arapien, it does not explain fault-slip and seismicity data 

indicating that strike-slip motions are common on north-
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F37 

90 

80 

70 

Ul 

a: 

UJ 60 IUJ 

2 50 

z 

t-' 40 I 

S2 30 UJ 

I 

20 

o~~--~~--~~--~~---L--~-L--~~~ 

0 10 20 30 40 50 60 70 80 90 100 110 120 130 

DISTANCE, IN METERS 

FIGURE 32.-Profiles across scarps formed on late Quaternary alluvium and debris flows in the Annabella area. Profiles A, B, and C 

cross the main strand of the Sevier fault at localities identified by 

corresponding letters in figure 31. CSO, cumulative surface offset; 

YSO, young surface offset; MSA, maximum slope angle. Profiles A 

and B were measured with a clinometer. At profile A, the fault 

offsets the youngest terrace above the modern stream bed at the 

mouth of Water Creek Canyon, whereas the same fault at profile B 

offsets an older and higher terrace a short distance to the south of A. 

The young surface-faulting event represented by profile A produced 

a surface offset of 5.2 m. That surface-faulting event, which probably 

occurred close in time to the Pleistocene-Holocene boundary, is 

shown in profile B by a 20-m-long scarp midsegment displaying an 

average slope near the angle of repose at 35°. This oversteepened 

part of the compound scarp yields a young surface offset of about 4. 7 

m, similar to that of the single-event scarp at A. This young faulting 

event is probably also expressed in the high compound scarp at 

locality C (note the tenfold scale difference), but it cannot be 

discriminated, because that profile was constructed from data on the 

1:24,000-scale topographic map (Water Creek Canyon, Utah) and 

therefore only approximates the true surface profile. A remarkably 

large planar part of the scarp at C stands approximately at the angle 

of repose (compare profile C with the photograph in fig. 31), 

indicating numerous late Quaternary faulting events. With a surface 

offset of about 130 m, it is probably the highest late Quaternary fault 

scarp in alluvial deposits in Utah. Profile D was measured with a 

clinometer on a northeast-striking fault scarp 1.5 km northwest of 

the scarp at locality C. It is included to show that other scarps in the 

Annabella area do not reflect the same high rates of activity that are 

found on the main strand of the Sevier fault. 

and northeast-trending faults and that lateral motions 

have occurred parallel to the principal fault-fold trend. 

These factors introduce complexities that seem to 

require competing deformational processes. 

SOUTHWEST OF ELSINORE 

The Elsinore area is one of the most seismically active 

areas in Utah. Five earthquakes of magnitude 5 or 

greater are assigned to the area-two in 1910 and three 

in 1921 (Williams and Tapper, 1953; Arabasz and McKee, 

1979). An intense cluster of microseisms (M::::::l.9) 

recorded in 1981 about 4 km southwest of Elsinore 

(Julander, 1983) is centered at the alluviated gap connecting Joseph Flats with the broad Sevier Valley. The 

envelope containing the densest part of the cluster dips 

steeply and has an east-west strike. Eight focal mechanisms all indicate predominantly strike-slip faulting 

(Julander, 1983). As is the case with the aftershocks 

northeast of Annabella, the focal mechanisms divide into 

two types having approximately interchangeable P and T 

axes and are indicative of strong slip inhomogeneity and 

incompatibility. 

North of the alluvial gap at Joseph Flats, at a locality 

marked X on figure 34, faulted and tilted Sevier River 

Formation is unconformably overlain by Quaternary 

alluvium. A fault at this locality having an attitude of 

054°, 76° SE. and an estimated displacement of 30 m 

contains striated clay suggestive of a 34° rake to the 

southwest. The predominately strike-slip component is 

dextral. Although we have no direct evidence of the 

direction and sense of slip on the Dry Wash fault in the 

vicinity of the alluvial gap, its strike-slip component 

elsewhere is known to be sinistral. Therefore, the geologic record suggests incompatible strike slip on 

northeast-striking faults; this suggestion is geometrically and mechanically consistent with the microseismic 

data. Our preference is to interpret such incompatible 

slip as an indication of lateral block motion, as we have 

done with the faults northeast of Annabella. 

A northwest-southeast gravity profile across Joseph 

Flats and the Dry Wash fault shows no significant 

anomaly at the fault (Halliday and Cook, 1978). Instead, 

a 10-mGal gravity low is located beneath Joseph Flats, 

its axis being about 2 km northwest of the fault trace. 

These data are consistent with our interpretation that 

the Dry Wash is primarily a strike-slip fault and that the 

alluvial basin beneath Joseph Flats results mainly from 

the southerly projection of an open south-southwestplunging syncline (fig. 3). 

Steven (1979) mapped north- to northeast-striking 

faults whose traces are well marked by scarps in Quaternary alluvium on the northern and southern flanks of 

the alluviated gap separating Joseph Flats from the 

Sevier Valley. A northeast-trending fault scarp about 3 

m high is formed on a gently inclined river terrace 

(terrace 4, fig. 34), south of which are remnants of three 

additional geomorphic surfaces showing southwardincreasing erosional dissection and southward-increasing 

eastward inclinations (fig. 35). The relationships suggest 

increasing eastward tilts on increasingly older Quaternary surfaces. The faulting, which appears to be mostly 

normal sense, and the tilting, which may be on the 

eastern flank of an anticline, paired with the syncline 

beneath Joseph Flats, seem to represent rates of Qua-
F38 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

s N w E 

·:=~;:· ·:~::- .. 2 ·1. :i 0 f·.'f.: 2 ~··· 

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HORIZONTAL=VERTICAL SCALE s 

A 

FIGURE 33. -A, Spatial distributions of aftershock foci in the Annabella area (lettered triangles represent seismograph stations). B, 

Focal mechanisms for the 1982 magnitude 4.0 main shock (upper 

stereogram) and magnitude 2. 7 aftershock (lower stereogram) (from 

Julander, 1983). The earthquake foci are shown in map view (upper 

left-hand square) and cross section (center and right-hand squares). 

ternary deformation higher than those seen along major 

structures extending away from the alluviated gap. 

Within the alluviated gap, the channel pattern of the 

Sevier River appears to have responded to young deformation. Along the 3.5-km reach downstream from 

Joseph, the channel pattern changes from sinuous to 

reticulate. Where the river crosses the projection of the 

Dry Wash fault (fig. 36A), the pattern changes abruptly 

from reticulate to a sinuous or island-braided pattern. 

This abrupt change is analogous to an experimentally 

supported model of a meandering river carrying a combination of suspended- and bed-load sediment (like the 

Sevier River) that has had to adjust to an anticlinal uplift 

across its course (fig. 36B) (Ouchi, 1985). The eastside-up fault displacement in the alluviated gap is analogous to an uplift axis at the fault trace. The reticulate 

pattern results from the damming effects of the uplift, 

which enhances flood-plain deposition upstream. The 

sinuous or island-braided pattern downstream of the 

uplift axis is a feature that results from the cutoff of 

intensified meander loops that develop first as the river 

attemps to lengthen its course to maintain its gradient 

across the oversteepened terrain. Although sufficient 

time has presumably lapsed since deformation for the 

Sevier River to reach this stage, the lapsed time is 

insufficient for the river to reestablish an equilibrium 

channel pattern analogous to the final stage shown in 

B 

The block diagram (lower left-hand square) shows the planar orientation of clustered foci and slip direction from the main shock. For 

focal mechanisms, the solid circles represent compression, and open 

circles represent tension. P and T are pressure and tension axes. &, 

dip of fault; shaded area, fault surface. 

figure 36B. Although it is not possible to estimate these 

lapsed times, the fluvial patterns clearly support late 

Quaternary displacement on the Dry Wash fault system 

in this area. About 90 km northwest of Elsinore, we 

recognize an analogous anomaly in the channel pattern of 

the Sevier River where it flows southwest across the 

southern projection of the Drum Mountains fault scarp 

system of Holocene age southwest of Delta, Utah. The 

FIGURE 34. - South-central part of the Monroe Northwest 71/2- ~ 

min quadrangle showing geology by Steven (1979), modified and 

augmented in the present study. Shaded and numbered areas are 

the four terraces first identified by Callaghan and Parker (1961) 

within which we estimate the average slope directions and values 

(arrows with numbers). The slope values represent best-fit planes 

through a series of highest-possible (ridge crests in dissected 

areas) points (number of points in parentheses) within the patterned areas. A computer-assisted stereometric procedure was 

used. The northernmost terrace (number 4) is represented by two 

areas separated by a fault scarp. Ts, Sevier River Formation; Tv, 

volcanic rocks of Tertiary age. Heavy lines, faults (dashed where 

buried, ball and bar on downthrown sides). A representative 

sample of attitudes in the Sevier River Formation shows that the 

gravels forming the geomorphic surfaces overlie unconformably 

tilted and erosionally truncated Sevier River Formation. The 

three high terraces (1 through 3), fault scarp, and two lowterrace elements (4) are identified in a south-looking photograph 

in figure 35. Young deformation in this area is probably responsible for an anomaly in the channel pattern of the Sevier River, as 

figure 36 shows. X indicates the locality referred to in the text. 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F39 

young deformation there appears to have affected a 35-

km-long reach of the river and is suggestive of a basinrange-scale structural disturbance. 

The displacement sense inferred from the streampattern anomaly near Elsinore is consistent with the 

pattern of eastern to northeastern tilting of geomorphic 

surfaces and the underlying Sevier River Formation in 

\ . '·· · 

. 2'• /)--· 

·'---

11 

···~~---" "' 0 

) 

the area east of the northernmost Dry Wash fault (figs. 

34, 35). If a range of ages is assigned to these east-tilted 

elements and if a specific size and geometry are assigned 

to the structural block containing them, it is possible to 

compute a range of uplift rates for the terrace-bearing 

block. The computed range can then be compared with 

uplift rates on faults in the eastern Great Basin region. 

_.k.· ---

---· •·i 

T 24 S 

--);'"'=~"""'-"'1 

I T 25 S 

\ 

<>· . 

\ V: 

- ---------r -·----- ----··--/Iff ·------:'c____ ..... .:t,:;!;'"':/--4.,.-'-;fii'O"~----'If':\::::;o."' -~""'=--\------7'"----------:7'j 

I 

j 

i 

I 

-- I 

iJoseph 

l 

I KILOMETER 

C1E3~~E3~~~~~~~ CONTOUR INTERVAL 40 FE ET 

DDTIED LINES PRESENT 20 -FDOT CONTOURS 

NATIONAL GEODETIC VERTICAL DATUM OF 1929 
F40 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FIGURE 35.-View looking south showing terraces that have progressively greater height and greater eastward tilts with greater age. 

Age is inferred from the height above the grade of the Sevier River 

and the degree of erosional dissection. The conspicuous shaded 

scarps below terrace 3 are excavations for an irrigation ditch; those 

We assume that the slope angles shown in figure 32 

represent structural tilts of a block 2 km wide and that 

the block is bounded by the gravity step beneath Joseph 

Flats on the west and is hinged along its eastern edge. 

We assign age ranges of 2 to 5 x 104 years for the 

0.5°-tilted surface, 2 to 5x105 years for the 5.5°-tilted 

surface, and 1 to 5x 106 years for the duration of tilting of 

the 15°-tilted Sevier River strata. The rate of uplift 

required to produce those tilts in the assumed time slots 

ranges from about 0.4 to 1.0 mm/yr for the two geomorphic surfaces and from 0.1 to 0.6 mm/yr for the Sevier 

River Formation. The range of lower rates (0.1-0.4 

mm/yr) seems to us to be more reasonable than the range 

of higher rates because it falls below the approximately 

0.4-mm/yr long-term rates of uplift on the Wasatch and 

Hurricane faults (Anderson and Mehnert, 1979). 

Because of large uncertainties in the epicentral locations of the magnitude 5 or greater 1910 and 1921 

earthquakes near Elsinore, no direct association can be 

made between known Quaternary surface deformation 

and those earthquakes. Also, only a spatial association 

can be made between known Quaternary surface deformation and the microseismic record. 

The geologic setting of the surface scarps and seismicity near Elsinore is complex. The northeastward projection of the Dry Wash fault (and the inferred flexure cut 

by it) beyond the alluvial gap between Joseph Flats and 

the Sevier Valley encounters the monoclinal flexure 

along the southeastern base of the Pavant Range. The 

fact that these two structures have opposite senses of 

vertical displacement suggests that such a projection is 

not reasonable in terms of dip-slip components. If both 

structures terminate in the vicinity of the alluvial gap, 

there is a need for a buried transverse structural zone to 

accommodate the dramatically contrasting along-strike 

deformational styles. Strain is likely to be concentrated 

below terrace 4 result from erosional trimming by the Sevier River. 

Slopes of the terraces are given in figure 34. Access was denied to the 

area where fault scarp could be studied. Only the relative ages of the 

terraces are known. 

in such a transverse zone. We infer that the concentration of seismicity and Quaternary surface deformation is 

related to this zone. If the predominantly sinistral-slip 

Dry Wash fault terminates at the alluvial gap, sagging 

might be expected in an area of extension northwest of 

the terminal segment, and uplift might be expected in an 

area of compression southeast of the terminal segment 

(Segall and Pollard, 1980). Uplift of the block containing 

the tilted terraces and depression of the block containing 

Joseph Flats could represent a highly imperfect distribution of extensional and compressional deformation 

related to the fault termination. 

In summary, concentrations of seismicity and Quaternary deformation in the Elsinore and Annabella areas 

appear to be related to strong along-strike disparities in 

the geometry and vergence of major structures extending away from those areas. These disparities are probably sites of transverse zones of structural accommodation 

and strain concentration. They are located on opposite 

sides of the Sevier Valley and could, in fact, be parts of 

a single transverse zone. Strain rates, displacement 

histories, or other features that are relevant to earthquake hazards in those zones should not be assumed to be 

typical of the major structures extending from them or of 

the central Sevier Valley in general. 

SUMMARY AND DISCUSSION 

Multiphase deformation, possibly beginning with 

events related to the evolution of the Paleozoic miogeocline and extending through the compressional events of 

the Mesozoic and early Cenozoic Sevier and Laramide 

orogenies, fashioned a strong northeast-trending structural fabric in the central Sevier Valley study area. The 

compressional events probably also left the crust of the 

area structurally layered as a result of thrusting. The 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F41 

N 

l 

Joseph 

A 

FIGURE 36.-A, Channel pattern of the Sevier River 

between Joseph and Elsinore (from the Monroe 15-min 

quadrangle, 1:62,500 scale, 1940) in relationship to an 

axis of relative uplift. B, Hypothetical model of the 

time-sequential response (age increases downward) of 

the slope and channel pattern to uplift across the 

control that these early structures exerted on the location, trend, and form of late Cenozoic structures is least 

in the southern part of the study area, probably because 

of crustal welding associated with large middle and late 

Cenozoic volcanic centers and their inferred subjacent 

plutons. Subsequent to the igneous activity and a phase 

of basin-fill sedimentation, the entire area became the 

structural transition zone between the Basin and Range 

and the Colorado Plateaus-an event that we identify as 

ushering in the neotectonic regime. Our structural studies, chiefly in the part of the area where the potential for 

control by preexisting structure is high, reveal the 

previously unappreciated importance of late Cenozoic 

folding, strike-slip faulting, and, locally, structural 

attenuation within this part of the transition zone. Thus, 

B 

Mixed-load meandering river 

Uplift -fr 

Slope deformation 

and adjustment 

Channel pattern 

Ret~ - Cutoff-

~: · .. ~ 

::,_) ; · .. -: 

'stnuous braided 

or island 

·,~ 

~ 

course of a mixed-load (bed-load and suspended-load 

sediments) meandering river (from Ouchi, 1985). Note 

the similarity between the channel pattern of the 

Sevier River and the third stage of channel-pattern 

response. 

the fundamentally extensional neotectonic framework of 

the central Sevier Valley area must be examined not only 

in terms of the long and complex and sometimes controversial history of early events occurring in diverse 

tectonic settings but also in terms of the newly revealed 

evidence for neotectonic folding, strike-slip faulting, and 

detachment faulting. The magnitude and style of neotectonic deformation vary dramatically and portend strong 

strain inhomogeneity that introduces additional complications into analyses of neotectonic deformation. The 

main elements of the variations are shown in figure 37. 

They include sharp constrasts in (1) the amount of 

extension and structural attenuation, (2) the amount of 

strike-slip faulting, (3) the amount of monoclinal flexing 

associated with block-bounding structures, and (4) the 
F42 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

BASIN-RANGE EXTENSION 

IN UPPER PLATE OF 

SEVIER DESERT DETACHMENT 

Inferred surface trace 

of Sevier Desert detachment 

,,.-- / ...... / 

/

1./ , ............ .... __ _ .,.,..,.,./ 

/ 

/"'\ ("'· / SOUTH- TO SOUTHEAST-

\/ 'f. / TRANSPORTED STRUCTURALLY-----t 

~ / INTACTAREA 

"\. 

AREA OF 

BASIN-RANGE 

EXTENSION 

y ..... 

I 

I 

I 

I 

I 

I 

I 

AREA OF SINISTRAL 

SHEAR AND APPARENT 

NORTH-SOUTH COMPRESSION 

(UTILE EXTENSION) 

TILTED BLOCKS 

CUT BY STRIKESLIP AND DIPSLIP FAULTS 

HIGH-STANDING GENTLY 

EAST-TIL TED SEVIER 

PLATEAU (VERY MILD 

INTERNAL EXTENSION) 

38°30' '---------'------'-----------'--------_.._ ______ __._ ______ _, 

0 10 

0 10 

FIGURE 37. -Structural characterizations of the central Sevier Valley 

area. Resolved paleostress directions (outward-pointing doubleheaded arrows show <13 for dip-slip solutions, inward- and outwardpointing arrows show <11 and <13 , respectively, for strike-slip solution); directions of principal tectonic transport (large open arrows, 

inferred for Sevier Valley); possible cross-valley transverse strucvergence direction of on-strike range-bounding structures. 

Seismic hazards assessments in the central Sevier 

Valley area are closely linked to our understanding of the 

depth of penetration of faults. The main Pavant Range 

block is divided into a northern part, which lacks evidence of significant internal deformation, and a deformed 

southern part, which is further divided into a northern 

(updip) area of structural(?) attenuation and landsliding 

and a southern (downdip) area deformed in apparent 

north-south compression that produced mechanically 

coupled neotectonic folds and strike-slip faults (fig. 37). 

Horizontal deformation is concentrated on northeaststriking sinistral-slip faults on which displacements 

20 

20 MILES 

30 KILOMETERS 

ture (dotted line); faulted monoclines bordering uplifted blocks (cross 

hatching); area of major extension and structural attenuation (stipple); areas of northwest-striking faults interpreted to represent a 

preneotectonic stage of northeast-southwest extension (shaded); and 

sites of coincident earthquake clustering and anomalously intense 

late Quaternary surface deformation (stars). 

range from a few centimeters to a few kilometers. The 

abundance of these faults in excavations and excellent 

natural exposures suggests localized sinistral shear 

strain approaching unity, much more than in any other 

part of the study area. The net sinistral-slip displacement 

could approach 10 km. This concentration of horizontal 

displacement and compressional structures could reflect 

a tendency for the south-moving Pavant Range block to 

buttress against and slip southeastward around the 

igneous "spot weld" represented by the huge Marysvale 

volcanic-plutonic complex. Alternatives to this interpretation of thick-skinned denudation include (1) gravitydriven thin-skinned tectonic denudation of the uplifted 

Pavant Range block and (2) scraping off from a 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F43 

southward-moving structural block that thust under the 

Tushar Mountains. Either alternative requires shallow 

detachment, albeit displaying an opposite sense of slip. 

Either can explain why northeast-striking sinistral-slip 

faults are not found in the main part of the Pavant Range 

to the northeast. Choosing between a thick-skinned 

interpretation and a thin-skinned one would seriously 

impact estimates of the depth to which faults extend and, 

in turn, estimates of earthquake hazards. In separate 

reports (Anderson and Barnhard, 1984a, 1987), we have 

favored separate interpretations. We choose here to 

straddle the fence. 

The chief element of structural consistency in the 

Pavant Range is the lack of evidence of significant 

neotectonic extensional deformation even in the 

deformed southern part and along the range-bounding 

structures on its eastern flank. Evidence for an earlier 

episode of extension is found in restricted areas west of 

Aurora and near Elsinore (fig. 37). As does the main 

Pavant Range structural block, the part of the Sevier 

Plateau that we studied south of the Annabella segment 

of the Sevier fault lacks evidence of significant extensional deformation, even in areas within a few meters of 

the main "extensional" plateau-bounding faults. These 

faults are, by definition, neotectonic structures. Because 

they bound broad, evenly tilted blocks displaying little 

internal deformation, we assume that they extend to the 

base of the seismogenic crust. 

In sharp contrast to the large blocks of weak internal 

extension, rocks adjoining and lying atop the Arapiencored Sanpete-Sevier anticline are highly extended and, 

locally, severely attenuated. The southernmost exposures reveal persistent down-to-the-east throw across 

the anticline and a strong tendency for the extensional 

strain to be absorbed at the contact zone with the 

mechanically weak underlying Arapien Formation. 

Deformational kinematics suggest that Arapien rocks 

are withdrawn from beneath the northernmost part of 

the Sevier Plateau as an integral part of the extensional 

and attenuation process. As with the deformed southern 

part of the Pavant Range, this deformation has strong 

thin-skinned attributes. The Arapien apparently serves 

as a decollement zone characterized by relatively lowviscosity rock flowage below a cover-rock sequence that 

deforms by brittle failure. Such a zone is capable of 

vertically partitioning deformation and providing the 

kind of mechanical layering in the upper crust that gives 

rise to depth partitioning of small and moderate earthquakes (Arabasz, 1984; Arabasz and Julander, 1986). 

The north-trending Monroe segment of the Sevier 

fault appears to break cleanly through gently east-tilted 

Tertiary strata, giving no suggestion of monoclinal flexing toward the Sevier Valley. Apart from this example, 

our studies show that the margins of uplifted blocks in 

the central Sevier Valley area are mostly or partly 

monoclinal flexures cut by one class of faults that displace 

rocks down toward the uplifted block and another class of 

very steep faults that displace rocks down toward the 

valley. The net extension may be small in some monoclines. Where monoclines are well developed in rangemargin structural settings, such as in the Pavant Range, 

the possibility exists that this deformational style 

reflects draping over major buried faults that dip toward 

the valley at steep or moderate angles. There may be a 

component of lateral spreading or mushrooming as 

blocks lose lateral confinement during uplift. Such 

spreading may be a factor in the large-scale extension 

and structural thinning seen in the southernmost part of 

the Sanpete-Sevier anticline. This deformational style is 

dominated by faults that drop strata down toward the 

axes of the monoclines. It qualifies as thin skinned, 

although it may portend deeply penetrating buried structures. Because it is common, it is important that this 

style be understood and that this understanding be 

integrated with our understanding of seismicity. This 

topic is vital for future research. Currently, we believe 

that very few mapped faults in the central Sevier Valley 

area penetrate more than a few kilometers and that only 

range-bounding faults such as the Sevier fault and 

inferred faults beneath range-front monoclines have the 

potential for large damaging earthquakes related to their 

probable penetration to the base of the seismogenic 

crust. 

The coincidence of clustered seismicity and surface 

deformation in the central Sevier Valley area can be 

explained in terms of localized zones of high strain that 

develop where there is a need to compensate for strongly 

contrasting along-strike style and vergence of rangefront structures. We do not claim to understand the 

dynamics of faulting and seismicity in these zones. We 

assert, however, that fault recurrence times and slip 

rates estimated from the high-strain zones do not characterize the major structures that extend away from 

them and therefore should not be used to characterize 

earthquake hazards beyond the zones themselves. 

In the central Sevier Valley area, geologic fault-slip 

and earthquake focal-mechanism data are generally similar in (1) the approximate east-west orientation of 

inferred least compressive stress (figs. 30, 37), (2) the 

widespread distribution of mixed-mode dip-slip and 

strike-slip faulting (figs. 30, 37), and (3) the common 

presence of slip incompatibility for strike-slip faults. The 

slip incompatibility indicates significant amounts of 

block-boundary faulting that cannot be interpreted in 

terms of applied remote stress. Unfortunately, we know 

of no straightforward way to separate block-bounding 

strike-slip faults from other strike-slip faults. In addition 

to the slip incompatibility, there is a paradox inherent in 
F44 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

the results of our fault-slip studies. Four of the five <I> 

values for dip-slip faulting (table 1) are very low (less 

than 0.1), and the other is low (0.26). If, for the purpose 

of discussion, strike-slip faulting is disregarded, the low 

<I> values suggest that rr 2 and rr 3 are close in value and 

that moderately dipping faults of any orientation are 

favorably oriented for dip slip. If strike-slip faulting is 

regarded, its intermixture with dip-slip faulting suggests 

that rr1 and rr2 are capable of interchanging their positions and are therefore close in value. We make no 

attempt to resolve this paradox. 

ROLE OF DIAPIRISM 

Witkind (1982) suggested that Cenozoic folding 

resulted from episodes of salt diapirism triggered by 

movement on deep-seated faults. Diapirism has great 

appeal because either compressional or extensional 

regional stresses acting on deep-seated faults will produce the same deformational style associated with diapirism. It explains how, in an extensional environment, 

Neogene folds can form parallel to or superposed on and 

having geometry similar to that of earlier folds resulting 

from compression. It does not seem adequate to explain 

the uniform down-to-the-plateau faulting in the southern 

part of the anticline unless the diapiric process is 

extended to include the lateral withdrawal of rock from 

beneath the northern Sevier Plateau. Also, the fault 

contact between the Arapien Formation and superjacent 

rocks in that area is approximately planar and dips at 

moderate to low angles over large areas. Diapiric upwelling would be expected to produce a highly irregular and 

steep to overturned contact (Witkind, 1982). 

Geologic mapping (Willis, 1986) and an unpublished 

cross-valley reflection profile by S.T. Harding (written 

commun., 1986) have shown that the part of the Sevier 

Valley bordering the Sanpete-Sevier anticline from Salina to Annabella is underlain by a relatively symmetrical 

Neogene syncline. Diapirism does not seem adequate to 

explain the northeast-trending folding and faulting along 

the Pavant Range-Sevier Valley margin, where the 

folded rocks are far above the Arapien Formation, which 

is the causative rock that has been identified for the 

diapiric process. Because fold-fault patterns involving 

Neogene rocks are so similar on both limbs of the Sevier 

Valley syncline (fig. 37), we believe that they result from 

a single process, which is not likely to be diapirism. 

SPECULATIONS ON CRUSTAL-SCALE DEFORMATION 

With special reference to thrusts and thrust-related 

structures, Standlee (1982) emphasized that major blind 

(unexposed) structures exist in the region and that 

tectonic interpretations based on surface geology alone 

can be very equivocal. In areas where deformation is thin 

skinned and dominated either by lateral spreading of 

rock at the margins of or from beneath uplifted blocks or 

by detachment faulting, we further recognize that the 

location, geometry, and kinematics of major deeply penetrating neotectonic structures could be masked. The 

alluvial fill of the Sevier Valley precludes knowledge of 

deep structure from surface studies. Even though these 

conditions foster equivocation regarding deep structure, 

it is worthwhile to speculate how the earthquake record, 

the subsurface geology, and the surface geology, especially that pertaining to widespread neotectonic folding 

and strike-slip faulting, factor into the regional picture of 

crustal-·scale deformation. 

In the Basin and Range, the principal deformational 

mode is reasonably assumed to be extensional, and the 

strike-slip faulting either results from extension or is 

superposed on it from a competing deformational process. In the western part of the Basin and Range, 

right-lateral shear associated with the plate-margin 

transform fault system (San Andreas) has been suggested as a deformational process that competes with 

east-west extension (Atwater, 1970). 

The results of this present study and of other studies 

to the southwest (Anderson, 1973, 1980a; Bohannon, 

1983) indicate that the principal horizontal shearing along 

the southeastern margin of the northern Basin and 

Range is sinistral on northeast-striking faults. Because 

such shear cannot be caused by distributed plate-margin 

transform interaction, other explanations must be 

sought. The orientation of active extension in the Basin 

and Range and in the transition zone of west-central and 

central Utah is interpreted to be about 285° (Zoback and 

Zoback, 1980; Arabasz and Julander, 1986). Anderson 

and Barnhard (1986) determined an identical neotectonic 

paleostress orientation from the eastern part of the 

transition zone, and similar poorly constrained average 

paleostress is inferred from the present study (fig. 37) for 

the western part of the transition zone. These orientations are all approximately normal to the Basin and 

Range-Colorado Plateau boundary. Extension and 

accompanying crustal thinning lead to lateral transfer of 

material at depth either by simple shearing or by ductile 

flow. For the central Utah transition zone, it is reasonable to assume east-to-west transfer of material within 

the semiductile or quasiplastic zone of upper crustal 

(7-15 km) low velocity (Smith and others, 1975; Gants 

and Smith, 1983) and possibly also at deeper levels. 

Rocks at these levels are warm and thermally weakened 

relative to the equivalent-depth lithosphere of the adjacent Colorado Plateau (Thompson and Zoback, 1979). 

Lateral transfer of thermally weakened material could 

be significantly retarded by the mechanical resistance of 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F45 

the thicker, cooler lithosphere beneath the plateau interior. The retardation could translate some of the outward 

(west-to-east) material transfer into movement parallel 

to the boundary in the direction of least resistance to 

such movement (Anderson, 1973). If an avenue of escape 

exists along the trend of the transition zone, the material 

would tend to flow or be translated in that direction (that 

is, subparallel to the province boundary). Specific directions of such translation could be strongly influenced by 

nonuniform preexisting structural strengths and weaknesses. The result is a local rafting of surficial crustal 

blocks parallel to the flow direction. If coeval extension 

on the Basin and Range side of the boundary is uniformly 

distributed over extensive areas, the rate and magnitude 

of tectonic transport would increase in the direction of 

escape because the required compensation for retardation would be cumulative. Very large and episodic 

motions, such as the 65 km of southwesterly tectonic 

transport of the Frenchman Mountain block near Las 

Vegas (Anderson, 1973; Bohannon, 1979), could be predicted as one progressed southwestward from the central Sevier Valley. This tectonic model combines a fundamental aspect of approximately east-west extension 

with tectonic rafting imposed by a system of basal 

tractions propagated upward from deep lateral flow of 

material. 

Direct evidence for large-scale strike-slip faulting 

beneath the Sevier Valley does not exist. The presence of 

folds that strike approximately parallel to a principal set 

of strike-slip faults such as the Dry Wash fault and 

subparallel faults is not an uncommon feature in strikeslip structural settings, although it eludes understanding 

through structural modeling. For example, Aydin and 

Page (1984) studied the distribution, geometry, and 

orientation of folds and other structures related to strikeslip faults in the San Francisco Bay region. They found 

that the diverse structures collectively are more compatible with a simple shear model than they are with a 

Mohr-Coulomb model of remote compressive stress. 

However, even after allowances for structural inhomogeneities, transient influences, and varied degrees of 

rotation are made, the simple shear model fails to 

account for folds that commonly parallel shear direction 

(Aydin and Page, 1984). In the Mercury 71/2-min quadrangle of Nevada, Barnes and others (1982) mapped 

Neogene folds that parallel the principal northeasttrending sinstral-shear direction. In southern Nevada, 

Bohannon (1983) mapped Neogene folds that parallel the 

sinistral-slip Lake Mead shear zone. In the Death Valley 

area, Cemen and others (1984) reported folds that developed parallel to late Cenozoic northwest-striking dextral 

faults. Those maps and reports show that such Neogene 

folds exist along strike-slip faults in the Basin and Range 

and lend confidence to the assumption that a similar 

association exists in the central Sevier Valley. In the 

Andaman Sea area, Harding (1985) showed that drape 

folds, interpreted to be caused by dip-slip components of 

displacement on predominantly strike-slip faults, have 

axes that parallel the main wrench faults. In addition, he 

noted that divergent wrench faults commonly have a pair 

of drape folds that face each other and parallel the main 

wrench fault. On the basis of seismic reflection profiles, 

Howell and others (1980) described synclinal troughs 

paralleling major faults in the divergent wrench-fault 

regime off coastal California. Basin margin unconformities attest to repeated tectonism during basin development, possibly analogous to the late-stage unconformities described along the Sevier Valley by Spieker (1946). 

We interpret the facing monoclinal folds bordering the 

Sevier Valley as resulting from the superpositioning on 

the system of east-west extension of a competing system 

of southwest-directed basal tractions and associated 

strike-slip faults. Although this model is highly speculative, it is consistent with the slip incompatibility seen in 

the earthquake and geologic fault-slip records, as well as 

with observed major components of strike slip on some of 

the deepest earthquakes of the region (Arabasz and 

Julander, 1986). 

Future geologic studies in this area should focus on 

evaluating the role of detachment faults in the localization and dynamics of lateral block motions, compressional 

folding, and shear folding. Because the style of deformation in the central Sevier Valley is dramatically different 

from that along the Wasatch Front, an effort should be 

made to locate and characterize the deformational boundary between these two on-strike terrains. 

REFERENCES CITED 

Allmendinger, R.W., Sharp, J.W., Von Tish, D., Serpa, L., Brown, 

L., Kaufman, S., Oliver, J., and Smith, R.B., 1983, Cenozoic and 

Mesozoic structure of the eastern Basin and Range province, 

Utah, from COCORP seismic-reflection data: Geology, v. 11, p. 

532-536. 

Anderson, E.M., 1951, The dynamics of faulting (2d ed.): Edinburgh, 

Oliver and Boyd, 206 p. 

Anderson, R.E., 1973, Large-magnitude late Tertiary strike-slip faulting north of Lake Mead, Nevada: U.S. Geological Survey Professional Paper 794, 18 p. 

--1980, The status of seismotectonic studies of northwestern 

Utah: U.S. Geological Survey Open-File Report 80-801, p. 

519-547. 

Anderson, R.E., and Barnhard, T.P., 1984a, Extensional and compressional paleostress and their relationship to paleoseismicity and 

seismicity, central Sevier Valley, Utah: U.S. Geological Survey 

Open-File Report 84-763, p. 515-546. 

--1984b, Late Cenozoic fault and fold patterns in Sevier County, 

Utah, and their relationships to seismicity in the area [abs.]: 

Geological Society of America Abstracts with Programs, v. 16, no. 

6, p. 430. 
F46 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

---1986, Genetic relationship between faults and folds and determination of Laramide and neotectonic paleostress, western Colorado Plateau transition zone, central Utah: Tectonics, v. 5, no. 2, 

p. 335-357. 

---1987, Neotectonic framework of the central Sevier Valley area, 

Utah, and its relationship to seismicity, in Gori, P.L., and Hays, 

W.W., eds., Assessment of regional earthquake hazards and risk 

along the Wasatch Front, Utah: U.S. Geological Survey Open-File 

Report 87-585-F, p. F1-F134. 

Anderson, R.E., and Bucknam, R.C., 1979, Map of fault scarps on 

unconsolidated sediments, Richfield J0X2° quadrangle, Utah: U.S. 

Geological Survey Open-File Report 79-1239, 1 sheet, scale 

1:250,000, 15-p. text. 

Anderson, R.E., and Mehnert, H., 1979, Reinterpretation of the 

history of the Hurricane fault, in Newman, G.W., and Goode, 

H.D., eds., 1979 Basin and Range Symposium: Denver, Colo., 

Rocky Mountain Association of Geologists, p. 145-173. 

Angelier, J., 1979, Determination of the mean principal stress for a 

given fault population: Tectonophysics, v. 56, p. T17-T26. 

Angelier, J., and Colletta, B., 1983, Tension fractures and extensional 

tectonics: Nature, v. 301, no. 5895, p. 49-51. 

Angelier, J., Colletta, B., and Anderson, R.E., 1985, Neogene paleostress changes in the Basin and Range-A case study at Hoover 

Dam, Nevada-Arizona: Geological Society of America Bulletin, v. 

96, p. 347-361. 

Arabasz, W.J., 1984, Earthquake behavior in the Wasatch Front area: 

· Association with geologic structure, space-time occurrence, and 

stress state, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, p. 310-339. 

Arabasz, W.J., and Julander, D.R., 1986, Geometry of seismically 

active faults and crustal deformation within the Basin and RangeColorado Plateau transition in Utah: Geological Society of America 

Special Paper 208, p. 43-73. 

Arabasz, W.J., and McKee, M.E., 1979, Utah earthquake catalog 

1850--June 1962, in Arabasz, W.J., Smith, R.B., and Richins, 

W.D., eds., Earthquake studies in Utah 1850 to 1978: Salt Lake 

City, University of Utah Seismograph Stations, Department of 

Geology and Geophysics, p. 548. 

Arabasz, W.J., and Smith, R.B., 1981, Earthquake prediction in the 

intermountain seismic belt-An intraplate extensional regime, in 

Simpson, D.W., and Richards, P.G., eds., Earthquake prediction-An international review: American Geophysical Union Maurice Ewing Series, v. 4, p. 238-258. 

Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and Utah: 

Geological Society of America Bulletin, v. 79, p. 429-458. 

Atwater, T., 1970, Implications of plate tectonics for the Cenozoic 

tectonic evolution of western North America: Geological Society of 

America Bulletin, v. 81, p. 3513-3536. 

Aydin, A., and Page, B.M., 1984, Diverse Pliocene-Quaternary tectonics in a transform environment, San Francisco Bay region, California: Geological Society of America Bulletin, v. 95, p. 1303-1317. 

Baer, J.L., Davis, R.L., and George, S.E., 1982, Structure and 

stratigraphy of the Pavant Range, central Utah, in Nelson, D.L., 

ed., Overthrust belt of Utah: Utah Geological Association Publication 10, p. 31-48. 

Barnes, H., Ekren, E.B., Rodgers, C.L., and Hedlund, D.C., 1982, 

Geologic and tectonic maps of the Mercury quadrangle, Nye and 

Clark Counties, Nevada: U.S. Geological Survey Miscellaneous 

Investigations Map I-1197, scale 1:24,000. 

Best, M.G., McKee, E.H., and Damon, P.E., 1980, Space-time composition patterns of late Cenozoic mafic volcanism, southwestern 

Utah and adjoining areas: American Journal of Science, v. 280, p. 

1035-1050. 

Bohannon, R.G., 1979, Strike-slip faults of the Lake Mead region of 

southern Nevada, in Armentrout, J.M., Cole, M.R., and TerBest, 

H., eds., Cenozoic paleogeography of the Western United States: 

Pacific Coast Paleogeography Symposium, 3d, Los Angeles, 1979, 

Pacific Section, Society of Economic Paleontologists and Mineralogists, p. 129-139. 

---1983, Geologic map, tectonic map and structure sections of the 

Muddy and northern Black Mountains, Clark County, Nevada: 

U.S. Geological Survey Miscellaneous Investigations Map I-1406, 

scale 1:62,500. 

Burchfiel, B.C., and Hickcox, C.W., 1972, Structural development of 

central Utah, in Baer, J.L., and Callaghan, E., eds., PlateauBasin and Range transition zone, central Utah, 1972: Utah Geological Association Publication 2, p. 55-66. 

Callaghan, E., and Parker, R.L., 1961, Geology of the Monroe quadrangle, Utah: U.S. Geological Survey Geologic Quadrangle Map 

GQ-155, scale 1:62,500. 

---1962, Geology of the Sevier quadrangle, Utah: U.S. Geological 

Survey Geologic Quadrangle Map GQ-156, scale 1:62,500. 

Cemen, I., Pickens, T.B., Wright, L.A., Drake, R.E., and Johnson, 

F. C., 1984, Cenozoic deformation and sedimentation, southeasternmost part of the Furnace Creek fault zone, Death Valley, 

California: Geological Society of America Abstracts with Program, 

v. 16, no. 6, p. 466. 

Chapman, D.S., and Harrison, R., 1978, Monroe, Utah, hydrothermal system: Results from the drilling of test wells MC-1 

and MC-2: Salt Lake City, University of Utah, Department of 

Geology and Geophysics Topical Report, DOE/DGE contract 

EY-76-S-07-1601, 26 p. 

Clark, E.E., 1977, Late Cenozoic volcanic and tectonic activity along 

the eastern margin of the Great Basin, in the proximity of Cove 

Fort, Utah: Brigham Young University Geology Studies, v. 24, pt. 

1, p. 87-114. 

Cunningham, C.G., Steven, T.A., Rowley, P.D., Glassgold, L.B., and 

Anderson, J.J., 1983, Geologic map of the Tushar Mountains and 

adjoining areas, Marysvale volcanic field, Utah: U.S. Geological 

Survey Miscellaneous Investigations Map I-1430-A, scale 

1:50,000. 

Davis, R.L., 1982, Geology of the Dog Valley-Red Ridge area, Southern Pavant Mountains, Millard County, Utah: Brigham Young 

University Geology Studies, v. 30, no. 1, p. 19-36. 

Gants, D.L., and Smith, R.B., 1983, Joint interpretation of refraction 

and reflection data for an area of low-angle faulting in the eastern 

Basin-Range: EOS, Transactions of the American Geophysical 

Union, v. 64, p. 763. 

Gibbs, A., 1987, Development of extension and mixed-mode sedimentary basins, in Coward, M.P., Dewey, J.F., and Hancock, P.L., 

eds., Continental extensional tectonics: Geological Society of 

America Special Publication 28, p. 19-33. 

Gilliland, W.N., 1963, Sanpete-Sevier Valley anticline of central Utah: 

Geological Society of America Bulletin, v. 74, p. 15-124. 

Halliday, M.E., and Cook, K.L., 1978, Gravity and ground magnetic 

surveys in the Monroe and Joseph KGRA's and surrounding 

region, south-central Utah: Salt Lake City, University of Utah, 

Department of Geology and Geophysics Report, DOE/DGE contract EY-76-S-07-1601, 164 p. 

---1980, Regional gravity survey, northern Marysvale volcanic 

field, south-central Utah: Geological Society of America Bulletin, 

v. 91, p. 502-508. 

Harding, T.P., 1985, Seismic characteristics and identification of 

negative flower structures, positive flower structures, and positive structural inversion: Bulletin of the American Association of 

Petroleum Geologists, v. 69, no. 4, p. 582-600. 
NEOTECTONIC FRAMEWORK OF THE CENTRAL SEVIER VALLEY AREA, UTAH F47 

Hardy, C.T., 1952, Eastern Sevier Valley, Sevier and Sanpete Counties, Utah: Utah Geological and Mineralogical Survey Bulletin 43, 

93 p. 

Hickcox, C.W., 1971, The geology of a portion of the Pavant Range 

allochthon, Millard County, Utah: Houston, Tex., Rice University, 

unpublished Ph.D. dissertation, 67 p. 

Howell, D.G., Crouch, J.K., Greene, H.G., McCulloch, D.S., and 

Vedder, J.G., 1980, Basin development along the late Mesozoic 

and Cenozoic California margin: A plate tectonic margin of subduction, oblique subduction and transform tectonics, in Ballance, 

P.F., and Reading, H.G., eds., Sedimentation in oblique-slip 

mobile zones: International Association of Sedimentologists Special Publication 4, p. 43-62. 

Julander, D.R., 1983, Seismicity and correlation with fine structures in 

the Sevier Valley area of the Basin and Range-Colorado Plateau 

transition, south-central Utah: Salt Lake City, University of Utah, 

unpublished M.Sci. dissertation, 142 p. 

Kehle, R.O., 1970, Analysis of gravity sliding and orogenic translation: 

Geological Society of America Bulletin, v. 81, p. 1641-1664. 

Lawton, T.F., 1985, Style and timing of frontal structures, thrust belt, 

central Utah: Bulletin of the American Asociation of Petroleum 

Geologists, v. 69, no. 7, p. 1145-1159. 

Mase, C.W., Chapman, D.S., and Ward, S.H., 1978, Geophysical study 

of the Monroe-Red Hill geothermal system: Salt Lake City, 

University of Utah, Department of Geology and Geophysics Topical Report, DOE/DGE contract 78-C-07-1701, 99 p. 

Ouchi, S., 1985, Response of alluvial rivers to slow active tectonic 

movement: Geological Society of America Bulletin, v. 96, no. 4, p. 

504-515. 

Rowley, P.D., 1968, Geology of the southern Sevier Plateau, Utah: 

Austin, University of Texas, unpublished Ph.D. dissertation, 

385 p. 

Rowley, P.D., Steven, T.A., Anderson, J.J., and Cunningham, C.G., 

1979, Cenozoic stratigraphic and structural framework of southwestern Utah: U.S. Geological Survey Professional Paper 1149, 

22 p. 

Rowley, P.D., Steven, T.A., and Kaplan, A.M., 1981a, Geologic map of 

the Monroe NE quadrangle, Sevier County, Utah: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1330, 1 sheet, scale 

1:24,000. 

Rowley, P.D., Steven, T.A., and Mehnert, H.H., 1981b, Origin and 

structural implications of upper Miocene rhyolites in Kingston 

Canyon, Piute County, Utah: Geological Society of America Bulletin, v. 92, p. 590-602. 

Segall, P., and Pollard, D. D., 1980, Mechanics of discontinuous faults: 

Journal of Geophysical Research, v. 85, no. B8, p. 4337-4350. 

Smith, R. B., 1978, Seismicity, crustal structure, and intraplate tectonics of the Western Cordillera: Geological Society of America 

Memoir 152, p. 111-144. 

Smith, R.B., and Richins, W.D., 1984, Seismicity and earthquake 

hazards of Utah and the Wasatch Front: Paradigm and paradox, in 

Hays, W.W., and Gori, P.L., eds., Proceedings of Conference 

XXVI; a workshop on Evaluation of regional and urban earthquake hazards and risk in Utah: U.S. Geological Survey Open-File 

Report 84-763, p. 73-112. 

Smith, R.B., Braile, L.W., and Keller, G.R., 1975, Crustal low velocity 

layers: Possible implications of high temperature at the Basin and 

Range-Colorado Plateau transition: Earth and Planetary Science 

Letters, v. 28, p. 197-204. 

Spieker, E.M., 1946, Late Mesozoic and early Cenozoic history of 

central Utah: U.S. Geological Survey Professional Paper 205-D, p. 

D117-D161. 

---1949, The transition between the Colorado Plateaus and the 

Great Basin in central Utah: Utah Geological Society Guidebook to 

the Geology of Utah, no. 4, 106 p. 

Standlee, L.A., 1982, Structure and stratigraphy of Jurassic rocks in 

central Utah: Their influence on tectonic development of the 

Cordilleran foreland and thrust belt, in Powers, R.B., ed., Geologic studies of the Cordilleran thrust belt: Denver, Rocky Mountain Association of Geologists, v. 1, p. 357-382. 

Stanley, K.O., and Collinson, J.W., 1979, Depositional history of 

Paleocene-lower Eocene Flagstaff Limestone and coeval rocks, 

central Utah: Bulletin of the American Association of Petroleum 

Geologists, v. 63, no. 3, p. 311-323. 

Steven, T.A., 1979, Geologic map of the Monroe NW quadrangle, 

west-central Utah: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-107, scale 1:24,000. 

Steven, T.A., and Morris, H.T., 1983a, Geologic map of the Cove Fort 

quadrangle, west-central Utah: U.S. Geological Survey Miscellaneous Investigations Map 1-1481, 1 sheet, scale 1:50,000. 

---1983b, Geologic map of the Richfield 1°X2o quadrangle, westcentral Utah: U.S. Geological Survey Open-File Report 83-583, 

scale 1:250,000. 

Steven, T.A., Cunningham, C.G., Naeser, C.W., and Mehnert, H.H., 

1979, Revised stratigraphy and radiometric ages of volcanic rocks 

and mineral deposits in the Marysvale area, west-central Utah: 

U.S. Geological Survey Bulletin 1469, p. 1-40. 

Stewart, J.H., Moore, W.J., and Zietz, 1., 1977, East-west patterns of 

Cenozoic igneous rocks, aeromagnetic anomalies, and mineral 

deposits, Nevada and Utah: Geological Society of America Bulletin, v. 88, p. 67-77. 

Thompson, G.A., and White, D.E., 1964, Regional geology of the 

Steamboat Springs area, Washoe County, Nevada: U.S. Geological Survey Professional Paper 458-A, 52 p. 

Thompson, G.A., and Zoback, M.L., 1979, Regional geophysics of the 

Colorado Plateau: Tectonophysics, v. 61, no. 1/2, p. 149-181. 

Wernicke, B., 1985, Uniform-sense normal simple shear of the continental lithosphere: Canadian Journal of Earth Sciences, v. 22, no. 

1, p. 108-125. 

Williams, J.S., and Tapper, M.L., 1953, Earthquake history of Utah, 

1850-1949: Bulletin of the Seismological Society of America, v. 43, 

p. 191-218. 

Williams, P.L., and Hackman, R.J., 1971, Geology, structure, and 

uranium deposits of the Salina quadrangle, Utah: U.S. Geological 

Survey Miscellaneous Geologic Investigations Map 1-591, scale 

1:250,000. 

Willis, G. C., 1985, Revisions to the geochronology and source areas of 

Early Tertiary formations in the Salina area of Sevier Valley, 

central Utah [abs.]: Geological Society of America Abstracts with 

Program, v. 17, no. 4, p. 272. 

---1986, Geology of the Salina quadrangle, Sevier County, Utah: 

Utah Geological and Mineral Survey Map 83, scale 1:24,000. 

---1988, Geology of the Aurora. quadrangle, Sevier County, Utah: 

Utah Geological and Mineral Survey Map 112, scale 1:24,000. 

Witkind, I.J., 1982, Salt diapirism in central Utah, in Nielson, D.L., 

ed., Overthrust belt of Utah: Utah Geological Association Publication 10, p. 13-30. 

Zoback, M.L., and Zoback, M.D., 1980, State of stress in the conterminous United States: Journal of Geophysical Research, v. 85, no. 

B22, p. 6113-6156. 

Neotectonics of the Hansel Valley-Pocatello 

Valley Corridor, Northern Utah and 

Southern Idaho 

By JAMES McCALPIN, DEPARTMENT OF GEOLOGY, UTAH STATE UNIVERSITY, 

ROBERT M. ROBISON, UTAH CoUNTY PLANNING DEPARTMENT, and 

JOHN D. GARR, EARTHFAX ENGINEERING, INC. 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-G 

CONTENTS 

Abstract ......................................................................... . 

Introduction ................................................................... .. 

Acknowledgments ...................................................... .. 

Neotectonic Features in Pocatello Valley ............................. . 

Tectonic Setting ......................................................... . 

Eastern Margin Fault ................................................. . 

Stratigraphy at the Range Front ............................ . 

Warped Shorelines ............................................... . 

Western Margin Fault ................................................. . 

Page 

G1 

1 

3 

4 

4 

4 

5 

7 

9 

Neotectonic Features in Hansel Valley ................................ . 

Tectonic Setting ....................................................... .. 

Eastern Margin Fault ................................................. . 

Northwestern Margin Fault ........................................ . 

Southwestern Margin Fault ......................................... . 

Late Quaternary Tectonic History ...................................... . 

Pocatello Valley ......................................................... . 

Hansel Valley ........................................................... . 

Conclusions ..................................................................... . 

References Cited ............................................................. . 

ILLUSTRATIONS 

FIGURES 1-3. Maps showing: 

1. Earthquake epicenters of the Hansel Valley-Pocatello Valley corridor for the period March 28, 1975, to 

Page 

G9 

9 

10 

11 

11 

15 

15 

16 

16 

17 

Page 

June 30, 1985.............................................................................................................................. G2 

2. Location and general geology of the Hansel Valley-Pocatello Valley corridor............................................... 3 

3. Bouguer gravity anomalies of the Hansel Valley-Pocatello Valley corridor and adjacent areas........................ 3 

4. Index map of the Pocatello Valley area, Idaho-Utah............................................................................................ 4 

5, 6. Logs of: 

5. Trench 1, excavated across the Lake Utaho shoreline on the eastern valley margin...................................... 5 

6. Trench 2, located at the base of a faceted spur on the eastern valley margin .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 6 

7, 8. Diagrams showing: 

7. Comparison of soil profile properties..................................................................................................... 7 

8. Method used in estimating actual elevations of pluvial lake shorelines covered by Holocene colluvium .. .. .. .. .. .. . 8 

9. Profiles of Lake Utaho shoreline elevations........................................................................................................ 10 

10. Graph of scarp surface offset plotted against the tangent of the maximum scarp slope angle for Hansel Valley fault 

scarps and other dated scarps within Utah................................................................................................... 11 

11. Sketch showing the fault exposed on the southern side of the Rattlesnake Pass (Interstate 84) roadcut........................ 11 

12. Map showing profile locations, scarp heights, and surface offsets on the Hansel Valley southwestern margin 

fault scarp .. . . . . .. . . . .. . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . .. . .. . .. . .. . .. . . . . . . . .. . .. . .. . .. . .. . . . . . 12 

13. Conceptual model showing how fault scarps on the southwestern margin fault might be heightened by a coseismic 

rotational slump....................................................................................................................................... 13 

14. Sketches showing the geomorphic setting of a fault scarp formed during a lacustrine transgression and regression........ 13 

15. Wall log of the West Gully showing fault patterns............................................................................................... 14 

16. Composite stratigraphic section of Quaternary deposits exposed in the West Gully................................................... 15 

TABLE 

Page 

TABLE 1. Surveyed elevation data for the Lake Utaho shoreline, Pocatello Valley, Idaho--------------------------------------------------------------- G9 

III 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY 

CORRIDOR, NORTHERN UTAH AND SOUTHERN IDAHO 

By jAMES McCALPIN, RoBERT M. RoBISON, 1 and JoHN D. GARR 

ABSTRACT 

The Hansel Valley-Pocatello Valley corridor on the Utah-Idaho 

border has experienced intense historical earthquake activity, including damaging earthquakes in 1934 (ML 6.6) and in 1975 (ML 6.0). Range 

fronts flanking both valleys display youthful morphology, yet valleys 

contain few well-preserved fault scarps in Quaternary deposits that 

might allow reconstruction of fault histories. Range-front trenching, 

pluvial lake shoreline surveying, and arroyo wall mapping were used to 

reconstruct the neotectonic history of each valley. In Pocatello Valley, 

15-ka shorelines are locally warped vertically as much as 6.4 m where 

they cross en echelon splays of the eastern range front fault, but no 

scarps in unconsolidated deposits are found. A trench at the range front 

exposes unfaulted colluvial wedges up to 95 ka old that may have been 

rotated down toward the valley center and locally warped. The absence 

of either fault scarps or liquefaction features in late Quaternary 

deposits implies that displacement on the range-front fault in eastern 

Pocatello Valley occurs by creep or by small displacements that do not 

propagate as discrete fractures to the surface. 

Two short fault scarps on the eastern range-front fault of Hansel 

Valley indicate that four to five surface-faulting events have occurred 

in the last 65 to 140 ka. A splay fault in an Interstate 84 roadcut shows 

only one recent 2.6-m displacement in the last 100 ka, not compatible 

with the scarp evidence. Along the southwestern valley margin, one of 

several scarps created by the 1934 earthquake merges with an 8-

km-long prehistoric scarp up to 9 m high that displaces Lake Bonneville 

shoreline gravels. Scarp profiles and geomorphic relations with shorelines indicate that the scarp was formed by one or two subaqueous 

displacements totaling 1.3 to 2.5 m between 26 and 12 ka. Supporting 

evidence of recurrent faulting based on gully exposures and five 

thermoluminescence dates show that deposits have been multiply 

faulted between 138 and 72 ka; no events occurred from 72 to 58 ka, one 

event occurred between 58 and 26 ka (nearer the latter), one event 

occurred around 15 to 21 ka, and a possible event occurred at 13 ka. 

Displacements of up to 2.5 m seem to be clustered in time when large 

pluvial lakes existed in the basin (oxygen isotope stages 6 and 2) rather 

than when either no lake or only shallow lakes existed (oxygen isotope 

stages 5, 4, and 3). We suggest that the 1934 ML 6.6 earthquake may 

be a typical interpluvial maximum event (long recurrence time, small 

Manuscript approved for publication November 20, 1990. 1

Now at Sergent, Hauskins, and Beckwith Engineers, 4030 South 500 West, 

Salt Lake City, UT 84123. 

displacement) in comparison with the larger, more frequent surfacefaulting events presumably triggered by pluvial lake water loading. 

INTRODUCTION 

The Hansel Valley-Pocatello Valley corridor on the 

Utah-Idaho border has been the site of intense historical 

seismicity, including the 1934 ML 6.6 and the 1975 ML 6.0 

earthquakes (fig. 1). The 1934 event, the largest in 

Utah's history, is one of only three historical earthquakes 

in the Intermountain Seismic Belt to rupture the surface 

and produce a fault scarp. The high rate of historical 

seismicity in the corridor contrasts with a scarcity of 

prehistoric fault scarps offsetting late Quaternary deposits, in spite of the youthful range-front morphology of 

bounding mountain ranges. The primary objective of this 

research was to determine why such a seismically active 

area lacked abundant geologic evidence of late Quaternary surface faulting. A secondary objective was to 

compare earthquake frequency-magnitude relations 

deduced from geologic evidence with those from historical time to help assess the current potential of this region 

to generate large, damaging earthquakes. Finally, if 

damaging earthquakes are found to have occurred without leaving significant geologic evidence, do the methods 

currently used on the Wasatch fault need to be modified 

for estimating earthquake potential? 

The study area is located within the Hansel MountainsWest Hills part (Stokes, 1977) of the Great Basin section 

of the Basin and Range physiographic province (Stokes, 

1977) and is composed of north- to northeast-trending 

valleys separated by linear mountain ranges (fig. 2). The 

Hansel, North Hansel, and North Promontory Mountains are primarily composed of the PennsylvanianPermian Oquirrh Formation (Adams, 1962; Allmendinger, 1983; Jordan, 1985), while Samaria Mountain 

Gl 
G2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

0 

0 

0 e 0 0 

0 0 

0 0 

0 

0 

0 'bo 0 

N 0 

1 

0 

0 

0 

0 0 0 

ooo 0 

0 Q 0 

0 

MAGNITUDE SCALE (Md 

0 0 0 0 0 

6 4 2 <1 

5 10 MILES 

5 10 15 KILOMETERS 
NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY CORRIDOR, UTAH AND IDAHO G3 

~ FIGURE 1.-Earthquake epicenters of the Hansel Valley-Pocatello 

Valley corridor for the period March 28, 1975, to June 30, 1985. 

Data provided by the University of Utah Seismograph Stations. 

Diagonal patterns represent mountain blocks; stippling represents Lake Bonneville shoreline gravels. Normal faults displaying Quaternary displacement are shown by heavy solid lines (bar 

and ball on downthrown side). Dashed lines show inferred fault 

locations at the base of steep range fronts. 

contains carbonates and clastic rocks of Ordovician 

through Pennsylvanian age (Beus, 1968; Platt, 1977). In 

the valleys, Lake Bonneville sediments occur below 

about 1,585 m; higher valley margins are covered by 

alluvial fans. The valley fill beneath Lake Bonneville 

sediments is as much as 570 m thick in Pocatello Valley 

(estimated from gravity data by Harr and Mabey (1976, 

0 0 

N 

1 

0 

0 

I 

0 

I 

5 

5 

MAGNITUDE SCALE ( Md: 0 0 0 0 

I 

6 4 

10 MILES 

I 

2 

15 KILOMETERS 

FIGURE 2. -Location and general geology of the Hansel Valley-Pocatello Valley corridor (rectangle). Marginal normal faults discussed in 

text are shown by heavy lines (dashed where inferred, bar and ball 

on downthrown side). Earthquake epicenters from 1850 through 

June 1962 are shown by heavy circles; epicenters from June 1962 

through March 27, 1975, are shown by lighter circles. Paleozoic 

rocks, diagonal pattern; Tertiary volcanic rocks, small v's; Quaternary shoreline gravels, stippling; Quaternary lake-bottom deposits 

and alluvium, no pattern. A through D locate faults discussed in text. 

0 

I 

0 

I 

5 

5 

I 

I 

10 

I 

10 MILES 

I 

15 KILOMETERS 

FIGURE 3.-Bouguer gravity anomalies of the Hansel Valley-Pocatello 

Valley corridor (rectangle) and adjacent areas (from Peterson, 1974). 

Gravity values are in milligals; contour interval 5 mGal. Stippling 

represents mountain blocks; heavy lines represent Quaternary faults 

(dashed where inferred, ball and bar on downthrown side). The 

gravity low of Pocatello Valley seems to continue southward to Blue 

Creek Valley rather than southwestward to Hansel Valley. 

p. 6-7)), presumably composed mainly of pre-Bonneville 

Quaternary deposits and of the Tertiary Salt Lake 

Formation, which crops out on valley margins. Fill in 

Hansel Valley is considerably thinner, as shown by 

smaller gravity anomalies (fig. 3) (from Peterson, 197 4) 

and by outcrops of Paleozoic rock within the valley 

(Hood, 1971, p. 5). 

ACKNOWLEDGMENTS 

Richard Forester (U.S. Geological Survey) interpreted the ostracode faunas from the West Gully. Jerry 

Stipp (Alpha Analytic, Inc.) provided the thermoluminescence dates and valuable advice, and William McCoy 

(University of Massachusetts) contributed thermoluminescence dates from type localities at Promontory Point. 

The manuscript benefited from reviews by Alan Nelson 

and Robert Bucknam (U.S. Geological Survey). Field-
G4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

work was supported by U.S. Geological Survey contract 

13-08-001-21899. 

NEOTECTONIC FEATURES IN POCATELLO 

VALLEY 

TECTONIC SETTING 

Pocatello Valley is a topographic and structural 

depression bounded on the east by Samaria Mountain, on 

the west by the North Hansel Mountains, and on the 

north and south by low hills (fig. 4). Cenozoic normal 

faults bound the eastern side of the valley (Beus, 1968; 

Platt, 1977) and part of the western side (Allmendinger, 

1983). Platt (1975, 1976) inferred late Quaternary activity on the eastern margin fault from indirect geomorphic 

evidence such as steepness of faceted spurs. Both gravity 

data and tilt of strata suggest that this fault forms the 

boundary between two east-tilted blocks. Another 

inferred north-trending fault may bound the western 

side of a gravity saddle in the center of the valley (fig. 3). 

Cenozoic displacement on the eastern margin fault may 

be up to 1,420 m, as shown by the elevation difference 

between the top of Samaria Mountain (2,440 m) and the 

bottom of inferred valley fill (1,020 m) (Harr and Mabey, 

1976). A shorter (8 km long) normal fault of lesser 

displacement bounds the central western margin of Pocatello Valley. Allmendinger (1983, section C-C') showed 

a down-to-the-east displacement of about 122 m where it 

offsets Paleozoic rocks on its northern end. The smooth 

gravity gradients and the lack of large range-front spurs 

suggest that the western valley margin is dominantly a 

dip slope of an east-tilted block (Allmendinger and Platt, 

1983, fig. 3) marked by a few short, low-displacement 

normal faults. In late Quaternary time, this topographic 

depression was occupied by pluvial Lake Utaho (Currey, 

1981), a small body of water contemporaneous with but 

physically separate from Lake Bonneville. 

EASTERN MARGIN FAULT 

The youthful range-front morphology cited by Platt 

(1977) as evidence of late Quaternary activity was quantified by using mountain-front sinuosity ratios (S) and 

ratios of valley floor width to valley height (Vf) (after 

Bull and McFadden, 1977). S is the ratio of the length of 

the mountain-piedmont junction to the overall length of 

the mountain front. Tectonically inactive fronts have 

high (3.0-7.0) S values caused by extensive embayment 

and pedimentation, whereas tectonically active fronts 

have lower values (1.2-1.6). The eastern margin range 

front has a sinuosity ratio of 1. 7, suggestive of slightly 

active to active tectonism. The ratio of valley floor width 

to valley height (Vf) gives a general indication of whether 

112°23'00" 

FIGURE 4. -Pocatello Valley area, Idaho-Utah. Diagonal pattern 

indicates highlands above the Lake Utaho and Lake Bonneville 

shorelines. Open circles are shoreline elevation data points; solid 

circles are data points and (or) endpoints of profiles shown in figure 

9. Dotted lines indicate the position of shoreline elevation data points 

used in transect A-A' (fig. 9). Normal faults are shown by heavy 

lines (dashed where inferred, bar and ball on downthrown side). Tl 

and T2 indicate the locations of trench 1 and trench 2, respectively. 

streams draining the mountain block are engaged in 

channel downcutting (high rate of tectonic base-level fall, 

low value of Vf) or lateral erosion (low rate of base-level 

fall, high value of Vf). The width of a canyon floor is 

compared with the mean height of the canyon at a given 

distance (0.5 km in this study) upstream from the mountain front. Canyons and valleys in several ranges in the 

northern Mojave desert have Vf values ranging from 

0.055 to 47.0 (Bull and McFadden, 1977). Only two 

canyons along the eastern margin of Pocatello Valley 

have stream channels long enough to use in calculating 

ratios of valley floor width to valley height; they yielded 

low values of 0.22 and 0.24 that indicated active tectonism. The mountain-front sinuosity ratio and the ratios of 

valley floor width to valley height quantify what can be 

seen in the field: the mountain front is steep, the canyons 

are deeply incised and strongly V shaped, and embayment and pedimentation are minimal, all suggestive of a 

young, active mountain front. In addition, Platt (1976) 

inferred that subsidence of the valley floor near the base 
NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY CORRIDOR, UTAH AND IDAHO G5 

WEST EAST 

16 

Units 14,15 

Units 12,13 

Units 11 

Units 1, 10 

15 

EXPLANATION 

Finer colluvium } Holocene 

Coarse colluvium 

Lake Utaho beach gravel 

Pleistocene fan alluvium 

14 13 12 11 10 9 8 7 6 5 4 3 2 0 

0 

2 

3 

(/) 

a: 

4 ~ w 

~ 

5 

6 

7 

METERS 

FIGURE 5.-Log of trench 1, excavated across the Lake Utaho shoreline on the eastern valley margin (T1, fig. 4). Lenticular late 

Pleistocene alluvial-fan gravels (units 1-10) are truncated by an 

erosional scarp, at the base of which lies Lake Utaho highstand beach 

gravel (unit 11). Bouldery scarp-derived colluvium (unit 12) is 

overlain by a wedge of gravelly colluvium (unit 13), the 35o W.-

of Samaria Mountain has resulted in alluvial fans of much 

smaller volume than one would expect, given the size of 

the canyons from which the alluvium originated. According to gravity data (Harr and Mabey, 1976), the deepest 

depocenter within the valley (570 m of fill) is adjacent to 

the southern portion of the Samaria Mountain front. The 

gravity gradient from the mountain to the depocenter is 

approximately 3 mGal/km, almost double the gradient 

around most of the valley margin. Despite this indirect 

evidence for Quaternary fault activity, no fault scarps 

displacing Quaternary deposits could be located at the 

range front, either by earlier workers (Rogers and 

others, 1975) or in this study. 

STRATIGRAPHY AT THE RANGE FRONT 

Two trenches were excavated by a large trackmounted backhoe to look for evidence of Quaternary 

faulting at the range front. Trench 1 was cut across the 

most prominent scarp in Quaternary deposits along the 

range front (T1, fig. 4). This scarp was located at the 

same elevation as the Lake U taho shoreline and could 

have been (1) a fault scarp, (2) an abnormally well 

developed pluvial lake shoreline scarp, or (3) a fault scarp 

dipping fabric of which suggests a growing colluvial wedge. Finer 

colluvial unit 14 appears to be mixed with loess (early Holocene?), 

and unit 15 contains a weak A/Cca soil profile. Alluvial-fan strata 

beneath the buried scarp face (units 1-4) are not displaced, proof that 

no surface faulting has occurred at this location in the last 15 ka. 

later occupied and reworked by a shoreline. Trench 

logging revealed a thin lens of beach gravels at the base 

of the buried scarp free face, overlain by a fining- upward 

colluvial wedge (fig. 5). Moderately well stratified alluvial fan gravels beneath the beach gravel were continuous across the entire length of the trench, indicating no 

tectonic offset beneath the scarp and thus disproving a 

tectonic origin. 

Because trench 1 yielded no tectonic information, 

trench 2 (T2, fig. 4) was cut through a colluvial apron at 

the base of a faceted spur approximately 400 m south of 

trench 1. Excavation was begun at the base of the spur 

directly on an outcrop of Pennsylvanian-Permian 

Oquirrh Formation limestone, which was encountered at 

depths ranging from 0 to 1.5 min the upslope 15 m of the 

trench (fig. 6). The bedrock contact beneath colluvium 

then abruptly dropped 3 m into the trench floor at an 

angle of 63°, beyond the limit of extension of the backhoe. 

The limestone in and around the inferred range-front 

fault zone is intensely fractured and locally hydrothermally altered. The oldest colluvial deposit (unit I) carries 

a paleosol displaying well-developed argillic horizons. 

Comparison of this soil with a dated buried soil from 
G6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

EXPLANATION 

Units Age 

NE 

0 

V Gravelly, silty colluvium 

IV Silty massive colluvium 

Soils 

A/Bca 

C2ca 

Holocene SW 

10-15 ka? 

Ill Imbricated, clean pebble gravel 

(Utaho beach) 

15-17 ka 

II Silty massive colluvium C1ca 32-17 ka? 

<Soil 63±15 ka 

B1,B2,83,84,Cox 95±15 ka? 

5 

(/) 

c::: 

w 

tw PIPo ~ 

10 

( N. 63° E.---

30 25 20 15 

METERS 

10 5 0 

FIGURE 6.-Log of trench 2, located at the base of a faceted spur on the 

eastern valley margin (T2, fig. 4). Heavy lines separate lithologic 

units I through V; light lines separate soil horizons. Dips of units II 

through IV increase downslope of the hinge line by 4o to 8°; dips in 

unit I soil horizons increase up to 49°. Ages of units are estimated by 

Jordan Valley, Utah (Scott and others, 1982, p. 42), 

suggests that this soil represents about 63±15 ka of 

weathering (fig. 7). Overlying stone-free (loessial?) colluviums (units II and IV) both underlie (unit II) and 

overlie (unit IV) Lake Utaho highstand beach gravel 

(unit III), which is assumed to be contemporaneous with 

the Bonneville shoreline (15.5-17 ka) (Scott and others, 

1983). The modern colluvium (unit V) truncates all 

underlying units. 

Although the overall geometry of older, steeper 

wedges overlain by successively younger, gentler layers 

is expectable, the very steep tilt of unit I paleosols (up to 

49°) is anomalous. Colluvial wedges below fault scarp 

free faces typically have initial slopes of 35° (Wallace, 

1977); this slope is maintained only while deposition is 

rapid. By the time the slope is declining slowly enough to 

allow soil formation on the colluvial wedge, the wedge 

slope is considerably lower (8°-25°) (Wallace, 1977, fig. 3, 

stage E). Buried soil horizons within unit I must have 

required slope stability to form, but now dip 35° to 49° 

west downslope of the hinge line shown in figure 6. The 

unit II-unit IV contact is bent about 4° along this hinge 

line;, whereas younger units are unbent. The amount of 

correlating the Lake Utaho shoreline gravels (unit III) with the Lake 

Bonneville shoreline and by assessing the degree of soil development 

(fig. 7). The block pattern represents limestone bedrock (dashed 

where bedrock has been fractured and hydrothermally(?) altered to a 

variegated clay). 

angular change on the unit 1-unit II contact (8°, age ~32 

ka if deposition occurred early during the Utaho transgression) versus that on the unconformity between units 

II and IV (4°, age 15-17 ka) suggests progressive monoclinal flexure with time (0.25°/ka). If this rate is applied 

to the even more steeply dipping buried soil horizons B3b 

and B4b, an age of approximately 88 ka is obtained. This 

date is roughly compatible with an age of 95± 15 ka for 

unit I estimated by adding the time of soil formation 

(63±15 ka) to the inferred age of the unit 1-unit II 

unconformity (fig. 7). There are no discrete faults or 

shears within the colluvial wedge. 

Monoclinal flexure of surficial materials in narrow (2-5 

m wide) zones overlying fault traces has been previously 

described by Clark and others (1972, p. 118) and by 

Bonilla (1982, p. 18). Although the evidence in trench 2 is 

not conclusive, it suggests that some kind of progressive 

flexure has occurred over the last 95 ka within 20 m 

downslope of the inferred range-front fault in bedrock, 

unaccompanied by any surface faulting. It is unknown 

whether the warping resulted from coseismic deformation or aseismic creep, but the absence of fractures 

supports the latter explanation. 
NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY CORRIDOR, UTAH AND IDAHO G7 

(/) 

c:: 

w 

1-

w 

~ 

i= 

~ 160 u 

200 

240 

280 

TRENCH 2 

Percent clay 

0 1 0 20 30 40 50 

A 

Horizons 

l:l." 

81b 

.p 

82b 

A 83b 

--A 

A· 

84b 

.4 

A. 

-

Coxb 

~ 

-

FIGURE 7. -Comparison of soil profile properties between (A) the 

trench 2 paleosol developed on unit I and (B) a buried soil developed 

on pre-Bonneville lacustrine gravel in Jordan Valley, Utah (Scott and 

others, 1982, table 5, p. 42). Total pedogenic clay in each horizon is 

shown by the diagonal pattern, computed by multiplying the percentage of pedogenic clay by weight in each soil horizon (heavy lines) 

by the horizon thickness. Two assumptions are made in the comparison: (1) the original clay content of both parent materials (dashed 

lines) was about 20 percent for loess and colluvium and 5 percent for 

lacustrine gravels, and (2) bulk densities of comparable horizons in 

both soils are similar. Numbers in the patterned areas show dimensionless values for comparing the amount of clay in each soil. The soil 

WARPED SHORELINES 

The elevation of the Lake U taho shoreline was determined at 46 points around the margin of Pocatello Valley 

to detect indirect evidence of tectonic deformation. Survey points were concentrated along suspected fault zones 

at the base of steep mountain fronts to look for anomalous warps or tilts in the originally horizontal shorelines. 

Surveying was performed with a Leitz TM10E theodolite 

and Leitz RED-2 electronic distance meter. The meter 

and the theodolite were set up over a benchmark or spot 

elevation on the valley floor, and a tripod-mounted 

(/) 

c:: 

w 

1-

w 

~ 

i= 

z 

w 

u 

40 

80 

120 

160 

200 

280 

JORDAN VALLEY 

Percent clay 

0 10 20 30 40 50 

B 

Parent materials 

and horizons 

821tb 

--

~--

II B22tb 

II B23tb 

Ill 83tb 

IV C1oxb 

shown in A contains roughly 67 percent as much pedogenic clay as 

the soil shown in B. If clay formation rates are similar for the soil in 

A (Pocatello Valley) and the soil shown in B (Jordan Valley), the soil 

in A should represent roughly 67 percent of the time of soil formation 

represented by the soil in B. Scott and others (1982) calculated the 

formation time of the calcareous facies of the soil in Bas 94±22 ka. 

Comparison of pedogenic clay amounts suggests that the soil in A in 

trench 2 developed over a period equivalent to 67 percent of 94±22 

ka or roughly 63±15 ka. Horizontal dashes, silt; dots, sand; triangles, 

angular pebbles and small cobbles; open circles, rounded pebbles and 

cobbles. Horizon abbreviations follow U.S. Department of Agriculture conventions (b indicates buried horizons). 

reflecting prism was placed at the base of the shoreline 

scarp. While measurements were taken at the instrument station, a profile of the shoreline scarp was made by 

using a 4.5-m rod and Abney level (after Bucknam and 

Anderson, 1979, p. 12). In most instances, the profile 

measurement was made from 30 m above to 30 m below 

the shoreline scarp. Typically, three distinct slope components were evident: a beach platform (3°-4°), a colluvial 

wedge (8°-12°), and a wave-cut scarp (18°-28°) (fig. 8). 

Because the Pleistocene lake shorelines were developed in unconsolidated deposits in all but a few locations 
G8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Beach 

platform 

......... ----...... ____ _ 

...&.-~::.-::.___ 40 

Base of bedrock 

faceted spur 

(wave-cut scarp) 

Colluvial /" ............. 

wedge Surveyed ,/ --, ___ _ 

''''''

,/'-, elevation / ,.. ...... ~ 

------- 5,148 ft ,/ / ............ ,l 

, ...... ,' 

/ 5.

1

25 m >I}' Elevation 

// 6.0 m .16o.· · .· correction 

, .......................... · 2.1 m (7 ft) 110 

Corrected elevation = 5,141 ft 

FIGURE 8.-Method used in estimating actual elevations of pluvial lake 

shorelines covered by Holocene colluvium. The beach platform and 

wave-cut scarp components are projected beneath the colluvial 

wedge. The resulting intersection is presumed to be a more accurate 

representation of the actual shoreline angle. 

around the valley margin, the shoreline angle has been 

covered subsequently by Holocene colluvium. The 

amount of colluvial cover varies depending on slope, 

aspect, deposit grain size, and local drainage patterns. 

To eliminate the effect of colluvial deposition from the 

surveyed shoreline scarp elevations, the beach platform 

and wave-cut scarp angles were projected under the 

colluvial wedge component on the profiles (fig. 8). The 

intersection of the two angles is assumed to be a more 

accurate approximation of the shoreline angle. Because 

trenches 1 and 2 were excavated after the shoreline 

surveying phase of the study, they provided a means of 

checking the accuracy of the method; in both instances, 

the projected beach platform and wave-cut scarp angles 

intersected within 30 em of the tops of Lake U taho beach 

gravel lenses. 

Although the water surface at the Lake Utaho highstand was horizontal, shorelines could have been superelevated, coincident, or subelevated with respect to the 

formative water plane (Currey, 1982, p. 21-22). All 

survey points along the eastern range front were in 

similar drift-aligned gravel beaches; data from Rose 

(1981, table 5.6) showed that the shoreline angle of such 

beaches is usually within 0.5 m of the mean water 

surface. Large waves that might deposit gravel far above 

the mean water surface would be hard to generate in 

Lake Utaho, which was only 10 km in diameter at its 

maximum extent. 

The precision of the instrumentation used in this study 

was calculated to be ±1.31 em elevation per kilometer of 

horizontal distance. The longest survey shot made was 

1. 78 km, and most shots were less than 0.48 km long. The 

combined uncertainties of surveyed elevation (13 em), 

projected shoreline angle (1.0 ft), and relation to water 

surface (50 em) yield a ±93-cm uncertainty for each 

measured shoreline point (table 1). 

Figure 9 (transect A-A') shows shoreline elevations 

that deviate as much as 6.4 m from one another and up 

to 4.5 m from elevations predicted by Crittenden (1963) 

on the basis of basin-scale isostatic rebound. The most 

significant deviations from a smooth profile occur where 

the north-northwest-trending shoreline crosses the more 

northerly trending faults at the base of Samaria Mountain. The shoreline at the southern end of the transect 

(point 17) is cut onto a faceted spur on the upthrown side 

of the fault, whereas, at points 4, 5, and 6, the shoreline 

is clearly on the downthrown side of the two parallel 

range-front faults (fig. 4). Point 17 falls at almost the 

exact elevation predicted for the Bonneville shoreline by 

Crittenden (1963, fig. 3), whereas points 4, 5, and 6 on 

the downthrown block plot 4.2 to 4.5 m, too low in 

relation to their expected post-rebound elevations. 

Highly variable shoreline elevations between points 16 

and 7 probably result from the sinuous shoreline's 

repeatedly crossing from the upthrown to the downthrown side of a more linear but buried fault or fold hinge 

line. For example, point 9 is a shoreline cut onto bedrock 

at the base of a spur, whereas the shoreline at points 8 

and 10 is cut into colluvium farther valleyward. The 

position of a linear fault paralleling the range front would 

cross between point 9 and points 8 and 10. Farther north, 

a parallel branch of the north-trending range-front fault 

intersects the north-northwest-trending Utaho shoreline 

somewhere between points 6 and 9 (fig. 9). Within this 

area, the shoreline loses 6.4 m of elevation in a horizontal 

distance of 900 m. The great similarity of all shoreline 

profiles suggests that they were formed with a constant 

relation to the mean water plane. Importantly, there are 

no fault scarps visible in this area. 

East-west profiles suggest that the Utaho shoreline 

has been tilted eastward from 1.8 m (fig. 9, transect 

B-B') to as much as 7.4 m (fig. 9, transect C-C') in the 

most active-appearing range-front area. Crittenden 

(1963, fig. 3), showed an expected west-to-east shoreline 

elevation drop of approximately 1 m across Pocatello 

Valley resulting from differential isostatic rebound. The 

small residual displacement on transect B-B' (1.8 m-1.0 

m=0.8 m) is within the range of surveying error and 

suggests that no significant tectonic eastward rotation 

has occurred north of the mapped eastern margin fault. 

The 6.4-m residual down-to-the-east displacment along 

transect C-C' strongly argues for post-Utaho slip along 

the central range front. 

The overall implication is that late Quaternary slip on 

the range-front fault has resulted in very little absolute 

elevation change of the upthrown block, but points on the 

downthrown block have been lowered 4.5 to 6.4 m, 

presumably by subsidence of the Pocatello Valley block. 

Geodetic data collected by Bucknam (1976) after the 1975 

M L 6. 0 earthquake showed no absolute uplift of surrounding mountain blocks but a maximum of 13 em of 

valley floor subsidence near the epicenter in western 
NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY CORRIDOR, UTAH AND IDAHO G9 

TABLE 1.-Surveyed elevation data for the Lake Utaho shoreline, Pocatello Valley, Idaho 

Reference elevation1 

(ft a.s.l.) 

Shoreline elevation Height of shoreline Calculated shoreline Colluvial wedge Corrected shoreline 

point2 above reference elevation3 elevation4 correction5 elevation6 

(ft) (ft) (ft) (ft a.s.l.) 

5,032 (a) ..................... . 1 110 5,142 -3 5,139 

5.022 (b) ..................... . 2 111 5,133 +3 5,136 

3 105 5,127 +2 5,129 

4 113 5,135 -2 5,133 

4,977 (c) ..................... . 5 157 5,134 0 5,134 

6 161 5,138 -3 5,135 

7 168 5,145 -3 5,142 

8 166 5,143 -2 5,141 

9 182 5,159 -3 5,156 

10 164 5,141 0 5,141 

11 174 5,151 0 5,151 

12 176 5,153 -3 5,150 

13 172 5,149 0 5,149 

4,994 (d) ..................... . 14 148.5 5,142.5 -1.5 5,141 

15 149 5,143 -1.5 5,141.5 

16 146.5 5,140.5 -4 5,136.5 

5,063 (e) ..................... . 17 88 5,151 -2 5,149 

18 98.5 5,159.5 -13 5,148.5 

19 96.5 5,159.5 -10 5,149.5 

5,006 (f) ...................... . 20 127 

4,997 (g) ..................... . 21 160 

1 Reference points of known elevation where the surveying base station was 

located: a, SWV4 sec. 16, T. 15 S., R. 34 E.; b, benchmark O'FNL, sec. 26, T. 15 

S., R. 34 E.; c, SWlf4 sec. 1, T. 16 S., R. 34 E.; d, SWV4 sec. 13, T. 16 S., R. 34 

E.; e, SWV4 sec. 30, T. 16 S., R. 34 E.; f, benchmark SEV4 sec. 20, T. 15 S., R. 

34 E.; g, NWV4 sec. 17, T. 16 S., R. 34 E. 2

Shoreline points located on figures 4 and 9. 3

Calculated by trigonometric leveling from reference elevation; precision 0.5 ft. 

Pocatello Valley. The closed topography of the valley 

suggests that subsidence has been the dominant longterm tectonic trend during the Quaternary, as Harr and 

Mabey (1976) previously suggested for the late Cenozoic. 

WESTERN MARGIN FAULT 

Over 30 normal faults have been mapped by Allmendinger (1983) in the portion of the North Hansel Mountains in our study area. The longest fault strikes N. 20° 

W., is 8 km long (fig. 4), and displaces older Quaternary 

pediment gravels by at least 55 m down to the east. The 

S value for the southern portion of the fault is 2.33, 

indicative of moderate to slightly active tectonism (Bull 

and McFadden, 1977). The V1 ratios of two gullies along 

the western margin fault are 5.0 and 4.0, values which 

are approximately 20 times those calculated for the 

eastern margin fault. The difference in V1 values suggests that the western margin fault is less active than the 

eastern margin fault. 

5,133 0 5,133 

5,157 +9.5 5,166.5 

4

Calculated by adding the third column to the first column. 

&rhe elevation difference between the projected shoreline angle (see fig. 8) 

and the point where the shoreline elevation was measured; precision 1.0 ft. 6

Calculated by adding or subtracting the fifth column from the fourth column. 

This elevation should be within 1.6 ft of the mean elevation of the formative water 

plane (Rose, 1981, table 5.6). 

NEOTECTONIC FEATURES IN HANSEL VALLEY 

TECTONIC SETTING 

Hansel Valley, Utah, lies southeast of Pocatello Valley 

and is bounded on the east by the North Promontory 

Mountains and on the west by the Hansel (or Summer 

Ranch) Mountains (fig. 2). The fault at the western base 

of the North Promontory Mountains has created an 

extremely linear north-trending range front, complete 

with faceted spurs, about 25 km long. The eastward tilt 

of Oquirrh Formation strata on either side of the fault 

near Bull's Pass suggests eastward rotation of fault 

blocks (Jordan, 1985). The inferred Cenozoic throw on 

this fault near Bull's Pass (425 m) (Jordan, 1985, section 

A-A') is considerably less than the 1,420 m inferred for 

the eastern fault of Pocatello Valley. The linear range 

front at the eastern base of the Hansel Mountains is 

inferred to be underlain by a normal fault (Adams, 1962, 

pl. 1; Doelling, 1980, pls. 1, 3, section G-G'). However, 

the fault does not offset Quaternary deposits and has not 
GlO REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

A' Isostatic deflection 

~~~~~~~~~~~~~~~~~~~~~~~~~-----------~rittenden,1963) 1' --------------------------

A 

19 18 

13 12 11 

8 8' 

I 

20 

c 

t 

21 

FIGURE 9.-Lake Utaho shoreline elevations. Locations of profiles are 

shown in figure 4. The amount of error owing to instrument 

inaccuracies and projecting of the shoreline angle is approximately 

represented by the width of the profile line (1.4 ft). On profile A-A', 

created the sharp faceted spurs that exist on the eastern 

valley margin. Outcrops of Oquirrh Formation on the 

valley floor suggest that the throw on this fault is no 

greater than that on the eastern margin fault; it may be 

a minor fault on a hinged valley side, similar to the 

western margin fault in Pocatello Valley. The southwestern margin of Hansel Valley is a subdued and irregular 

range front veneered by Lake Bonneville deposits. The 

east-facing 1934 fault scarp and a prehistoric scarp up to 

9 m high trend northerly across this zone. Very low 

gravity gradients in this region (fig. 3) and the lack of 

appreciable topographic relief suggest that this fault has 

a low Cenozoic slip rate. Tertiary and early Pleistocene 

sediments exposed in arroyos on the valley floor are 

extensively faulted. These faults usually have displacements of less than 5 m and do not displace overlying Lake 

Bonneville bottom sediments. The segmented linear 

nature of arroyos in northern Hansel Valley may be 

controlled by these north- to northeast-trending faults in 

underlying Tertiary sandstones (Salt Lake Formation). 

Normal faults of similar strike offset Tertiary basalts 

immediately north of Interstate 84 and tilt them into a 

series of east-dipping blocks (Adams, 1962). 

EASTERN MARGIN FAULT 

Fault scarps occur at only two locations along the 

eastern margin fault, where the trace is not covered by 

Holocene talus. At the northern location (A, fig. 2), a 

13-m-high scarp offsets a delta graded to the Bonneville 

3 

1 

3 2 

6 

~,:L 0 1 

MILES 

C' 

~20L ~ 10 

~ 0 

0 1 

l KILOMETERS 

VERTICAL EXAGGERATION 35X 

14 

significant deviations from the expected elevation occur where the 

shoreline crosses inferred branches of the range-front fault (points 

17-16, points 9-6). Profiles B-B' and C-C' show slight tilting of 

shorelines toward the eastern range-bounding fault. 

shoreline. Farther south, a branch fault (B, fig. 2) 

diverges southwesterly from the range front and creates 

a scarp up to 12.9 m high across pre-Bonneville alluvial 

fans. Both scarps are probably the result of multiple 

events; because each is limited to a single geomorphic 

surface and displays no multiple crests, reliable evidence 

of recurrent movement is lacking. Two profiles across 

the northern scarp and three across the southern scarp 

yield scarp surface offsets and maximum scarp slope 

angles that can be compared with those of dated singleevent scarps elsewhere in Utah (fig. 10). The sparse data 

suggest that the scarps are roughly comparable in age to 

the Bonneville shoreline (15.5-17 ka) (Scott and others, 

1983). 

A graben bounded by two high-angle faults in Quaternary deposits is exposed on the southern roadcut of 

Interstate 84 across Rattlesnake Pass (C, fig. 2). The 

faults offset seven distinct units of locally derived alluvium and colluvium deposited in a broad swale cut into 

Tertiary basalts (fig. 11). Of the four paleosols within the 

sequence, the lowest one is better developed than 140-

ka-old noncalcareous soils developed elsewhere in the 

Bonneville Basin (Scott and other, 1982, p. 42), whereas 

soils on units II, IV, and VI plus VIII each resemble 

calcareous soils developed elsewhere on 17-ka deposits 

(Scott and others, 1982, p. 36). Units I through V are 

clearly faulted; unit VI may be faulted, whereas soil 

horizon Kl is clearly superposed across the fault, an 

indication that faulting occurred between the deposition 

of unit V and the formation of soil Kl (latest Pleistoceneearly Holocene?). 
NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY CORRIDOR, UTAH AND IDAHO G11 

0.8 

CJ) 

0.7 35° w 

w w a: .....J (.!) (.!) 0.6 w z 0 <( 30° 

w ~ Cl... 0.5 

0 w' 25° .....J .....J 

CJ) 0.4 (.!) 

u.. z 

0 20° <( 

1- 0.3 w z Cl... w 15° 0 

(.!) 

0.2 .....J 

z CJ) 

<( 100 cl. 

1- a: 0.1 50 <( 

u 

CJ) 

0 

0 2 4 6 8 10 

SURFACE OFFSET, IN METERS 

FIGURE 10. -Scarp surface offset plotted against the tangent of the 

maximum scarp slope angle for Hansel Valley fault scarps and other 

dated scarps within Utah (modified from Hanks and others, 1984). 

Solid lines show the relationship between offset and slope angle for 

scarps at Fish Springs (FS), the Drum Mountains (DM), the Bonneville shoreline (BS, many geographic locations), and the Oquirrh 

Mountains (OM). Hansel Valley data are shown by open triangles 

marked B (from scarp area Bin fig. 2), an open square marked A 

(from scarp area A in fig. 2), and numbered solid circles (from the 

southwestern margin scarp, marked Don fig. 2). Numbers refer to 

profile locations shown in figure 12. Comparison of Hansel Valley 

scarp data with other Utah data suggests that (1) the scarp at 

location A is roughly contemporaneous with the Bonneville shoreline 

(because this scarp offsets a delta graded to the Bonneville shoreline, 

faulting must have occurred immediately after construction of the 

delta); (2) scarps at location B are older than the Bonneville shoreline, which is consistent with their displacing pre-Bonneville alluvial 

fans above the Bonneville shoreline; and (3) the scarp at location D is 

slightly younger than the Bonneville shoreline (this age is consistent 

with age estimates based on geomorphic data (fig. 14) that faulting 

occurred during the regression from the Provo shoreline before 

about 12 ka). 

NORTHWESTERN MARGIN FAULT 

The northwestern margin fault flanks the eastern 

margin on the Hansel Mountains (fig. 2). The range front 

is linear and exhibits subdued talus-covered faceted 

spurs. Bonneville-cycle shoreline deposits abut the range 

front, but no evidence of surface faulting exists in these 

sediments. Any fault scarps that may have been formed 

before the transgression of Lake Bonneville were presumably completely eroded. A shorter fault, about 4 km 

in length, parallels the northwestern margin fault along 

the east side of a small valley at the southern end of the 

Hansel Mountains (fig. 2) (Adams, 1962; Doelling, 1980). 

This fault forms a horst at the southern end of the 

mountain and decreases in displacement toward the 

north. This smaller fault is aligned with the trace of the 

1934 scarp and may be an extension of that fault zone. 

7 

6 

5 

CJ) 

a: 4 w 

tw 3 :2 

2 

0 

E 

Main fault 

2.6-m displacement 

2 3 4 5 6 7 8 9 10 11 12 

METERS 

w 

FIGURE 11. -Fault exposed on the southern side of the Rattlesnake 

Pass (Interstate 84) roadcut (C, fig. 2). Four paleosols are exposed in 

the eastern two-thirds of the cut, developed on parent materials of 

unit I (horizons B21t, B22t, and B3t), unit II (horizons Bt and K3), 

unit IV (horizons K2 and Cca), and unit VII (horizon A/C) plus unit 

VI (horizon K1). The main fault displaces units I through III by 2.6 

m. Units IV and V have evidently been eroded from the upthrown 

block west of the main fault, and unit VI is considerably thinner on 

the upthrown block, probably the result of scarp relief. Unit VIII 

(Holocene? colluvium) is not faulted. Pattern intensity indicates the 

degree of secondary carbonate or clay enrichment in soil horizons. 

SOUTHWESTERN MARGIN FAULT 

The fault on the southwestern margin of Hansel Valley 

is markedly different from the valley-bounding faults 

previously described. Here the mountain block is located 

3 to 4 km west of an 8-km-long east-facing fault scarp 

offsetting Bonneville-cycle shoreline gravels. Eleven 

profiles across the scarp (fig. 12) reveal an increase in 

scarp height from 1. 7 m to 9.0 m along the central part of 

the scarp, where it ascends a 60-m-high bedrock-cored 

escarpment. The increased scarp heights probably result 

from the addition of a fault scarp plus a landslide scarp 

produced by syntectonic sliding (fig. 13), as the arcuate 

plan shape of the scarp suggests. Because the material 

involved in the sliding (beach gravel) is not particularly 

susceptible to landsliding when it is unsaturated, we 

infer that pore pressures were very high at the time that 

faulting and sliding occurred, either from ground-water 

saturation or from submersion under Lake Bonneville. 

Geomorphic relations (described below) between the 

fault scarp and the shorelines indicate that the scarp 

must have formed after the Bonneville transgression 

rose above 1,402 m (about 26 ka) (Scott and others, 1983, 

fig. 5) but before the post-Provo regression fell below 

1,326 m (about 12 ka). Our reasoning is as follows. 

During a pluvial lake cycle, surface-faulting events can 
G12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FIGURE 12.-Profile locations, scarp 

heights, and surface offsets on the 

Hansel Valley southwestern margin 

fault scarp (scarp D, fig. 2). The 1934 

scarp appears on the southern end of 

the prehistoric scarp. Scarp heights 

increase as the scarp ascends a larger 

escarpment; the arcuate scarp plan 

shape strongly indicates syntectonic 

slumping (fig. 13). If slump effects are 

disregarded, most surface offsets are in 

the 1.0- to 2.0-m range. 
NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY CORRIDOR, UTAH AND IDAHO G13 

Fault .......... 

displacement- T,l ............... I ..... _J_ ..... 

Scarp 

displacement---

B 

Scarp height {T1l'',, 

resulting from + ' 

both processes T ~---'' ........ , 

c 

FIGURE 13. -Conceptual model showing how fault scarps on the 

southwestern margin might be heightened by a coseismic rotational 

slump. A, Initial fault displacement creates a scarp of height T1• B, 

During seismic shaking, a rotational slump begins to form on the 

downthrown block. Slumping would be likely if surface materials 

were saturated (see text). C, After slumping, the total scarp height 

includes components from fault displacement (T 1) and from the slump 

headscarp (T2). The dashed and queried fault inC indicates that the 

causative fault may actually underlie the slump, in which case the 

scarp preserved today is entirely of slump origin. Because slumps 

were large and slump toes indistinct, profiles used to calculate 

surface offsets in figure 12 were not extended downslope to include 

the entire slump deposit. 

occur (1) before the lake rose, (2) during the transgression, (3) during the lake highstand, (4) during the 

regression, or (5) after the lake had completely receded. 

Small fault scarps (1-2 m high) that formed before a 

major transgression would be reworked and destroyed, 

judging from the thickness (>5 m) and coarseness of 

Bonneville transgressive bars. Surface faulting during a 

transgression would displace submerged shorelines 

already formed; the scarp might be preserved if it were 

below the effective wave base. That part of the scarp 

above the shoreline would be destroyed during the 

continued transgression. A scarp formed during a transgression therefore should displace all shorelines below 

the one occupied at the time of faulting and terminate at 

Future 

shorelines 

destroy and 

bury fault 

Submarine 

shoreline 

features are 

faulted and 

preserved 

Existing 

regressive 

shorelines are 

faulted 

Future 

regressive 

shorelines are 

controlled by 

fault scarp 

A 

B 

FIGURE 14. -Geomorphic setting of a fault scarp formed during a 

lacustrine transgression (A) and regression (B). Time of shoreline 

occupation is shown by t and progresses from t1 to t5. In both 

instances, hypothetical faulting occurs when the shoreline is at t3. 

that shoreline, being obscured at higher elevations by 

wave reworking and longshore deposition (fig. 14A). Any 

fault scarp formed during the lake highstand would 

displace all transgressive shorelines that it intersected. 

If faulting occurred during a regression, small regressive 

shorelines already formed as the lake receded should be 

displaced. The position of later (lower elevation) regressive shorelines would be controlled by the position of the 

fault scarp, because only minor deposition (less than 1 m) 

accompanies regression (except for the Provo shoreline) 

in the Bonneville Basin. In this case, a scarp would 

displace regressive shorelines above a certain elevation; 

below that elevation, shorelines would wrap around or 

abut a modified scarp of similar height (fig. 14B). The 
G14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

w 

6 

(/) 

a: 

---------

---f --------------11 

w 

..... 

w 

~ 

0 

E 

------------- _...,~--- ........ ....... _____________ ..... _______ ...... ...,. _______________ _ 

5 10 15 20 25 

METERS 

30 35 40 45 

FIGURE 15.-Walllog of the West Gully showing fault patterns. Numbered units 1 through 6 refer to the stratigraphic section in figure 

16; units 10 and 11 (not shown in fig. 16) are Holocene alluvium. The zero point on the horizontal scale is the section 10-section 11 

boundary, which crosses the arroyo (fig. 12). The chronology of faulting deduced from this log is discussed in the text. 

elevation of this transition in morphology marks the 

position of the shoreline at the time of surface faulting. 

Surface faulting after the lake had completely receded 

would result in displacement of all transgressive and 

regressive shorelines. 

Most fault scarps in the Bonneville Basin trend parallel 

to shorelines near range fronts and therefore are not 

favorably oriented for intersecting multiple shorelines. 

The Hansel Valley scarp, however, displaces numerous 

shorelines as it ascends from the valley-floor (1,286 m) to 

1,402 m (fig. 12). The displacement of all transgressive 

shorelines up to 1,402 m indicates that surface faulting 

occurred after the Bonneville transgression had risen to 

this elevation (about 26 ka) (Scott and others, 1983, fig. 

5). Small regressive bars at 1,326 m terminate against 

the scarp, an indication that the scarp was already 

formed by the time the post-Provo regression reached 

that elevation (about 12 ka). Therefore, the scarp must 

have formed after 26 ka but before 12 ka. Scarp surface 

offset versus tangent of slope angle data suggest that the 

latest surface-rupture event occurred between roughly 

15 and 5.6 ka (closer to the older date) (fig. 10), although 

scarps formed underwater are not strictly comparable to 

subaerial scarps. The overlap between these age estimates (12-15 ka) probably closely dates the latest faulting event. If these dates are correct, then the entire 

southwestern margin scarp was produced by subaqueous 

displacement of the lake floor. 

Near its northern end, the prehistoric scarp is intersected by a 10- to 15-m-deep arroyo (informally termed 

the West Gully) (figs. 2, 12). Two deformation zones are 

well exposed on arroyo walls: (1) the main fault zone, 

consisting of 11 normal faults defining a 90-m-wide 

graben (fig. 15), and (2) a subsidiary fault zone approximately 100 m to the east, composed of three major 

normal faults. Detailed logging and sampling of 200 m of 

arroyo walls reveal that the faults offset a stratigraphic 

section 19 m thick composed of nine lacustrine units (fig. 

16) from three lake cycles. Physical stratigraphy, sedimentology, ostracode assemblages, and thermoluminescence (TL) dating (described by McCalpin, 1986) indicate 

that deposits of two pre- Bonneville ages are present. 

The older deposit (unit 1, fig. 16) appears discontinuously 

in arroyo bottoms, consists of very compact, laminated 

silt and clay beds, and is disrupted by numerous highangle fractures and faults of small displacement. Ostracode fauna and a TL date of 138 ka suggest that this 

lower unit is correlative with the Little Valley deposits 

described by Scott and others (1983). Unconformably 

overlying this unit are shoreline gravels and lakemarginal marsh sediments (units 2 and 3), which are also 

faulted (40-45 m E., fig. 15). TL dates of 76 and 82 ka 

indicate that these units were deposited during a previously undated lake cycle (informally termed the Hansel 

Valley cycle) in late oxygen isotope stage 5 or early stage 

4 (Morley and Hays, 1981). A lake cycle of similar 

inferred age has been postulated recently by Oviatt and 

others (1987). Deposits of the Bonneville cycle (units 4-9) 

include transgressive gravel and lake-bottom silts and 

clays. Lake-bottom beds (unit 6) approximately 1.5 m 

above the transgressive gravel are dated at 14 ka by 

amino acids and at 15.5 ka by 14C on gastropod shells. 

Strata 4 m below the top of the stratigraphic section 

(upper unit 8) are dated at 13 ka by TL. These dates are 
NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY CORRIDOR, UTAH AND IDAHO G15 

roughly correlative with the Bonneville-cycle lake elevation curve of Scott and others (1983, fig. 5). 

Although they are not shown on figure 15, faults 

extend through unit 6 but cannot be traced through units 

7 or 8. Both units 6 and 8 are internally deformed, the 

former by diapirs, roll structures, and convolutions and 

the latter by rotational faulting of intact blocks along 

listric faults (lateral spreading?). Although such deformation could be caused by nontectonic mechanisms, the 

coincidence of discrete deformed beds with a known 

multiple-event Quaternary fault suggests earthquakeinduced liquefaction. The difference in deformation 

styles may be owing to great difference in water depths 

(and pore pressures) between the time when unit 6 was 

deformed (at roughly 245 m) and the time when unit 8 

was deformed (at less than 60 m). 

Post-Bonneville alluvium (units 10 and 11) truncates all 

faults exposed in the West Gully, demonstrating that no 

Holocene events (not even the 1934 earthquake) have 

induced surface rupture at this location. Although the 

prehistoric scarp cannot be traced to the edge of the 

West Gully (because of agricultural disturbances), the 

net displacement (1.3 m, down to the east) of the 

transgressive gravel (unit 4) across the graben is similar 

to scarp heights north and south of the arroyo. 

LATE QUATERNARY TECTONIC HISTORY 

POCATELLO VALLEY 

Three lines of indirect evidence suggest a small but 

measurable amount of late Quaternary displacement on 

the eastern margin of Pocatello Valley: (1) lack of fault 

scarps in 15-ka deposits, (2) rapid elevation changes (up 

to 6. 4 m) of a 15-ka shoreline, and (3) tilted but unfaulted 

Quaternary colluvium at the range front. Shoreline elevation data further suggest that deformation has been 

dominated by valley subsidence along the 7-km-long 

central segment of the eastern margin fault. Compilations by Bonilla (1982) and Slemmons (1982) showed that 

the threshold of surface rupture for shallow-focus earthquakes on normal faults is roughly ML 6.2 to 6.3. 

Therefore, it appears that no earthquakes of this magnitude or larger have occurred anywhere along the rangefront fault in the last 15 ka. Within 20 m of the inferred 

range-front fault, the absence of tectonic displacements 

disrupting 95-ka colluvium suggests that no earthquakes 

of ML>6.2 to 6.3 have occurred since that time. If this 

7-km-long fault segment is capable of generating surfacefaulting earthquakes, then recurrence times between 

:them must be at least 15 ka and possibly as much as 95 

ka. 

The cause of the rapid 4.5- to 6.4-m vertical changes in 

shoreline elevation is not fully understood. The magni-

(/) 

a: 

~ 10 UJ 

:E 

0 

Upper laminated silt 

Upper convoluted silt 

Lower laminated silt 

Lower convoluted silt 

Bonneville gravel 

Green silt 

Brown silt 

Compact silt 

AGE LAKE 

(KA) CYCLE 

13 

BONNEVILLE 

(14-15.5) 

82 HANSEL 

VALLEY 

>76 

138 UTILE 

VALLEY 

FIGURE 16.-Quaternary deposits exposed in the West Gully. Numbers correspond to those in figure 15, except that post-Bonneville 

alluvium (units 10 and 11) is not shown. Ages are from thermoluminescence dates on bulk sediment (McCalpin, 1986) or from 14C dates 

on gastropod shells (in parentheses). Sample locations are shown by 

solid circles. 

tude of change appears to be significantly greater than 

that of the combined surveying errors, whereas the 

locations of greatest change coincide with shoreline-fault 

intersections. However, the calculated post-Utaho subsidence rate (4.5 m in 15 ka, or 0.3 mm/yr) seems 

excessive in light of the overall structural relief and age 

of the valley. At that rate, the 1,420-m inferred displacement across the fault could have been created in only 4 75 

ka. The presence of Salt Lake Formation (MiocenePliocene) within the downthrown block shows that inception of block faulting must be at least Miocene, so the 

inferred late Quaternary rate must be much higher than 

the post-Miocene average. This inference is compatible 

with the observed high rates of historical seismicity. 

The 1975 ML 6.0 earthquake we tentatively assign to 

the western margin fault, on the basis of epicentral 

location and focal mechanisms derived by Arabasz and 

McKee (1979). Like the eastern margin fault, this fault 

does not displace any Lake Utaho deposits, an indication 

that no ML>6.2 to 6.3 events have occurred in the last 15 

ka. An early(?) Quaternary pediment (500 ka?) is displaced at least 55 m down to the east, yielding a slip rate 

of 0.11 mm/yr. This rate is roughly one-third of the slip 

rate calculated for the eastern margin fault, compatible 

with the differences in their tectonic geomorphology 

values quoted earlier. 
G16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

HANSEL VALLEY 

Paleoseismic recurrence and magnitude data from the 

eastern margin fault are scanty and somewhat contradictory. The 8-m surface offset of the scarp across a 15-ka 

delta implies multiple ruptures (three to four if displacements per event were 2.0-2.5 m) in the last 15 ka. The 

resulting recurrence intervals (3. 7-5.0 ka) seem too 

short in view of the total range-front relief and tectonic 

geomorphology in comparison with those of nearby normal faults such as the Wasatch fault (Machette and 

others, this volume). Evidence of only a single recent 

2. 6-m displacement on the Rattlesnake Pass splay fault 

within the last 100 ka is not compatible with three to four 

events on the main trace in 15 ka, unless the main fault 

has ruptured many times without involving this particular splay. However, multiple post-Bonneville displacements are seen nowhere else along the length of the 

eastern margin, although the inferred fault trace 

emerges from under its talus cover for several long 

stretches. Only a single branch fault farther south offsets 

pre-Bonneville (oxygen isotope stage 4 or 6) alluvial fans 

9.5 m. Depending on the age assigned to these fans 

(roughly 65 or 140 ka), the recurrence intervals for 2.0- to 

2.5-m events are between 16 and 35 ka, although not 

every main trace rupture may have involved this branch. 

Given the poor geomorphic evidence, all that can be 

stated with certainty is that this fault has sustained 

surface rupture at least once since Bonneville time and 

several times since either oxygen isotope stage 4 or 6. 

Although temporal control is better for the wellexposed faulted strata in the West Gully, the inability to 

trace any subsurface faults to the surface (and thus 

connect them to the surface scarp) poses a large obstacle 

to fault-history reconstruction. The older history of 

faulting is ambiguous; multiple events of unknown displacement occurred along many traces, some of which 

were truncated by the oxygen isotope stage 4 transgression about 72 ka. On blocks rotated by post-26-ka faulting, the lack of angular unconformity between oxygen 

isotope stage 6 and 4 deposits indicates that no tectonic 

rotation occurred there from 72 to 58 ka (35m E., fig. 

15). The 26-ka Bonneville transgression truncates many 

small-displacement fractures of unknown age in unit 1 

but also fills in an open tectonic fissure (42 m W., fig. 15) 

that could not have remained open for too long after its 

creation; this fact implies an event not far in advance of 

26 ka. Although units 4, 5, and 6 are displaced along 

major faults, these faults cannot be traced through 

overlying units owing to poor exposures. The only later 

evidence of tectonism is the rotated blocks of unit 8, 

which could be a result of either tectonic or nontectonic 

lateral spreading. Taken together, the evidence argues 

for multiple events between roughly 140 and 72 ka, no 

events from 72 to 58 ka, at least one event between 58 

and 26 ka (nearer the latter), an event around 14 to 15 ka 

(the age of unit 6) and possibly another at 13 ka (unit 8), 

and a ML 6.6 event in 1934. 

This history implies widely varying recurrence intervals (RI's), from one event from 72 to 26 ka (RI=46 ka) 

to two events at 14 to 15 ka and 13 ka (RI=1-2 ka). 

Although the field evidence is ambiguous, it appears that 

recurrence intervals have been shorter when large lakes 

existed in the Bonneville Basin (oxygen isotope stages 6 

and 2) than they have been during times of small or no 

lakes (oxygen isotope stages 5, 4, 3). An exception is the 

1934 ML 6.6 earthquake, which occurred in the middle of 

an interpluvial episode. However, its maximum displacement (0.5 m) is small in comparison with the displacements seen in stage 2 time (2. 2-2.6 m, in either one or 

two events) and may indicate that interpluvial earthquakes are smaller than pluvial earthquakes. Crustal 

stresses imposed by the rapid filling and draining of 

pluvial lakes (especially the catastrophic drop from the 

Bonneville shoreline to the Provo shoreline) may be the 

main triggering mechanism for the Quaternary surfacefaulting events experienced by the southwestern margin 

fault, as others (Swan and others, 1983; Machette and 

others, this volume) have suggested for the nearby 

Wasatch fault. 

CONCLUSIONS 

Quaternary geologic mapping, shoreline surveying, 

arroyo wall logging, and trenching indicate that late 

Quaternary displacements have occurred on both margin 

faults in Pocatello and Hansel Valleys. The lack of fault 

scarps in pluvial lake deposits in Pocatello Valley suggests that no surface faulting has occurred in the last 15 

ka, yet shorelines and colluvium at the range front 

appear warped. The 6.4-m maximum vertical shoreline 

warping may be caused by monoclinal flexure over a 

buried fault trace; examples of similar warping of surficial deposits near faults have been given by Bonilla (1982, 

p. 18), although none are for normal faults. Colluvium in 

a range-front trench appeared warped but unfractlired, 

suggestive of creep flexure rather than flexure around an 

upward-propagating rupture. Recent trenching on the 

West Valley fault zone in Salt Lake County, Utah, by 

Keaton and others (1986) has exposed monoclinal warps 

along Quaternary normal faults. Given the limited data 

on hand, it appears that surface flexure could arise from 

creep or from earthquakes occurring slightly below the 

threshold magnitude necessary to create a fault scarp. 

Geomorphic evidence from Pocatello Valley suggests 

that significant vertical displacements may occur without 

the creation of fault scarps, over horizontal distances of 
NEOTECTONICS OF THE HANSEL VALLEY-POCATELLO VALLEY CORRIDOR, UTAH AND IDAHO Gl7 

hundreds of meters, and that these displacements are not 

detectable in the field without careful surveying. Such a 

conclusion has implications for displacement studies on 

the Wasatch fault, where it has been assumed that most 

(if not all) cumulative net vertical tectonic displacement 

can be measured solely by fault-scarp profiling. 

Marginal faults bounding Hansel Valley have generated multiple surface-faulting earthquakes in the late 

Quaternary, despite the seemingly small structural relief 

of this valley in comparison with that of the Pocatello 

Valley. Although a few short scarps occur on the steep 

eastern margin, the best record of activity is on the 

southwestern margin, where an 8-km-long scarp is intersected by a 15-m-deep arroyo. The multiple events 

deduced from logging a 90-m-wide complex graben that 

displaces lacustrine sediments 138 to 13 ka old show that 

even faults exhibiting minimal topographic and structural expression can have complex recurrence histories. 

TL dating of silty lacustrine deposits in the graben 

yielded ages compatible with stratigraphic and paleoecologic evidence and showed that this technique holds 

promise for dating noncarbonaceous, nonfossiliferous 

lake beds. Finally, the concentration of deformation in 

the oldest lacustrine deposit (oxygen isotope stage 6) and 

in the deposits of the Bonneville cycle (oxygen isotope 

stage 2) suggests that surface faulting was enhanced 

when deep pluvial lakes occupied Hansel Valley, as 

opposed to when no lakes (oxygen isotope stages 3 and 5) 

or shallow lakes (oxygen isotope stage 4) existed. This 

temporal clustering has been described by others (Swan 

and others, 1983) and may result from increased pore 

pressures and water loading of the crust. 

REFERENCES CITED 

Adams, O.C., 1962, Geology of the Summer Ranch and North Promontory Mountains, Utah: Logan, Utah State University, unpublished M.Sci. thesis, 57 p. 

Allmendinger, R.W., 1983, Geologic map of the North Hansel Mountains, Idaho and Utah: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-1643, scale 1:24,000. 

Allmendinger, R.W., and Platt, L.B., 1983, Stratigraphic variation and 

low-angle faulting in the North Hansel Mountains and Samaria 

Mountain, southern Idaho: Geological Society of America Memoir 

157, p. 149-163. 

Arabasz, W.J., and McKee, M.E., compilers, 1979, Utah earthquake 

catalog, 1850-June 1962, in Arabasz, W.J., Smith, R.B., and 

Richins, W.D., eds., Earthquake studies in Utah, 1850-1978: 

University of Utah Seismograph Stations Special Publication, p. 

552. 

Beus, S.S., 1968, Paleozoic stratigraphy of Samaria Mountain, IdahoUtah: Bulletin of the American Association of Petroleum Geologists, v. 52, no. 5, p. 782-808. 

Bonilla, M.G., 1982, Evaluation of potential surface faulting and other 

tectonic deformation: U.S. Geological Survey Open-File Report 

82-0732, 91 p. 

Bucknam, R.C., 1976, Leveling data from the epicentral area of the 

March 27, 1975, earthquake in Pocatello Valley, Idaho: U.S. 

Geological Survey Open-File Report 76-52, 6 p. 

Bucknam, R.C., and Anderson, R.E., 1979, Estimation of fault-scarp 

ages from a scarp height-slope angle relationship: Geology, v. 7, p. 

11-14. 

Bull, W.B., and McFadden, L.D., 1977, Tectonic geomorphology, 

north and south of the Garlock Fault, California, in Doehring, 

D. 0., ed., Geomorphology in arid regions: Annual Geomorphology 

Symposium, 8th, Binghampton, N.Y., 1977: London, Allen and 

Unwin, p. 115-138. 

Clark, M. M., Grantz, A., and Rubin, M., 1972, Holocene activity of the 

Coyote Creek fault as recorded in sediments of Lake Cahuilla: 

U.S. Geological Survey Professional Paper 787, p. 112-130. 

Crittenden, M.D., Jr., 1963, New data on the isostatic deformation of 

Lake Bonneville: U.S. Geological Survey Professional Paper 

454-E, 31 p. 

Currey, D.R., 1981, Late Quaternary Lake Utaho in southern Idahonorthern Utah: Denver, Colo., Association of American Geographers, Program Abstracts 1981, p. 166-167. 

---1982, Lake Bonneville: Selected features of relevance to neotectonic analysis: U.S. Geological Survey Open-File Report 

82-1070, 30 p. 

Doelling, H.H., 1980, Geology and mineral resources of Box Elder 

County, Utah: Utah Geological and Mineralogical Survey Bulletin 

115, 251 p. 

Hanks, T.C., Bucknam, R.C., LaJoie, K.R., and Wallace, R.E., 1984, 

Modification of wave-cut and faulting-controlled landforms: Journal of Geophysical Research, v. 89, no. B7, p. 5771-5790. 

Harr, C.J., and Mabey, D.R., 1976, Gravity survey of Pocatello Valley, 

Idaho and Utah: U.S. Geological Survey Open-File Report 76-766, 

12 p. 

Hood, J.W., 1971, Hydrologic reconnaissance of Hansel Valley and 

Northern Rozel Flat, Box Elder County, Utah: Utah Department 

of Natural Resources Technical Publication 33, 41 p. 

Jordan, T.E., 1985 Geologic map of the Bull's Pass quadrangle, Box 

Elder County, Utah: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-1491, scale 1:24,000. 

Keaton, J., Currey, D.R., and Olig, S.J., 1986, Paleoseismicity and 

earthquake hazards evaluation of the West Valley fault zone, Salt 

Lake City urban area: Contract report prepared for U.S. Geological Survey under contract 14-08-0001-22048, 18 p. 

McCalpin, J., 1986, Thermoluminescence (TL) dating in seismic hazard 

evaluations; an example from the Bonneville Basin, Utah: Annual 

Engineering Geology and Soils Engineering Symposium, 22d, 

Boise, Idaho, 1986, Proceedings, p. 156-176. 

Morley, J.J., and Hays, J.D., 1981, Towards a high-resolution, global, 

deep sea chronology for the last 750,000 years: Earth and Planetary Science Letters, v. 15, p. 279-295. 

Oviatt, C.G., McCoy, W.D., and Reider, R.G., 1987, Evidence for a 

shallow early or middle Wisconsin-age lake in the Bonneville 

Basin, Utah: Quaternary Research, v. 27, p. 248-262. 

Peterson, D.L., 1974, Bouguer gravity map of part of the northern 

Lake Bonneville Basin, Utah and Idaho: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-627, scale 1:250,000. 

Platt, L.B., 1975, Recent faulting at Samaria Mountain, southeastern 

Idaho: Geological Society of America Abstracts with Programs, v. 

7, p. 1224-1230. 

---1976, Geologic mapping and earthquakes in southeastern Idaho: 

Earthquake Information Bulletin, v. 8, p. 8-12. 

---1977, Geologic map of the Ireland Springs-Samaria area, southeastern Idaho and northern Utah: U.S. Geological Survey Miscellaneous Field Studies Map MF-890, scale 1:48,000. 
GIS REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Rogers, A.M., Langer, C.J., and Bucknam, R.C., 1975, The Pocatello 

Valley, Idaho, earthquake; Earthquake Information Bulletin, v. 7, 

no. 3, p. 16-18. 

Rose, J., 1981, Raised shorelines, in Goudie, A., ed., Geomorphological 

techniques: London, Allen and Unwin, p. 327-341. 

Scott, W.E., Machette, M.N., Shroba, R.R., and McCoy, W.D., 1982, 

Guidebook for the 1982 Friends of the Pleistocene Rocky Mountain 

Cell Field Trip to Central Utah, pt. I, Little Valley and Jordan 

Valley, Utah: U.S. Geological Survey Open-File Report 82-845, 58 

p. 

Scott, W.E., McCoy, W.D., Shroba, R.R., and Rubin, M., 1983, 

Reinterpretation of the exposed record of the last two cycles of 

Lake Bonneville, Western United States: Quaternary Research, v. 

20, p. 241-265. 

Slemmons, D.B., 1982, Determination of design earthquakes magnitudes for microzonation, in Proceedings of the Third International 

Earthquake Microzonation Conference, Seattle, WA: Berkeley, 

Calif., Earthquake Engineering Research Institute, v. 1, p. 

110-130. 

Stokes, W.L., 1977, Subdivisions of the major physiographic provinces 

in Utah: Utah Geology, v. 4, no. 1, p. 1-17. 

Swan, F.H., III, Hanson, K.L., Schwartz, D.P., and Black, J.H., 1983, 

Study of earthquake recurrence intervals on the Wasatch fault, 

Utah: 8th semiannual technical report to U.S. Geological Survey 

under contract 14-08-0001-19842, 20 p. 

Wallace, R.E., 1977, Profiles and ages of young fault scarps, north 

central Utah: Geological Society of America Bulletin, v. 88, no. 10, 

p. 1267-1281. 
Structure of the Salt Lake Segment, 

Wasatch Normal Fault Zone: 

Implications for Rupture Propagation 

During Normal Faulting 

By R.L. BRUHN, P.R. GIBLER, W. HOUGHTON, and W.T. PARRY, 

DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF UTAH 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-H 

CONTENTS 

Page Page 

Abstract.......................................................................... H1 Fault Zone Structure-Continued 

Introduction .................................................................... . 1 

2 

2 

4 

4 

Southern Fault Sections............................................... H14 

Acknowledgments ...................................................... . Northern Fault Sections.............................................. 14 

Tectonics of the Wasatch Fault Zone .................................. . Salt Lake Salient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

Structure of the Salt Lake Segment .................................... . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Paleostress Analysis ......................................................... . Geometry of Fault Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 16 

Directional Characteristics of Rupture Propagation . . . . . . . . . . 17 

Fault Zone Structure ....................................................... . 10 

10 

Conclusions...................................................................... 23 

Traverse Mountains Barrier ........................................ . References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

ILLUSTRATIONS 

FIGURES 1, 2. Maps showing: Page 

1. Tectonics of the Salt Lake rupture segment, Wasatch normal fault zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H3 

2. Geology and cross section of part of the Wasatch Mountains and the Salt Lake rupture segment . . . . . . . . . . . . . . . . . . . . 5 

3, 4. Diagrams showing: 

3. Contoured orientations of poles to fault planes and slickenlines for fault populations in each structural domain 

in figure 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

4. Paleoprincipal deviatoric stress orientations from each of the four structural domains in figure 1 based on 

inversion of fault-plane and slickenline data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . 10 

5, 6. Maps showing: 

5. Geometry of faulting at the Traverse Mountains barrier and block diagram of the tripartite fault array........... 11 

6. Geology of the Fort Canyon fault zone in the Fort Canyon structural domain shown in figure 1 . . . . . . . . . . . . . . . . . . . . . 12 

7. Stereoprojection showing the orientation of the Fort Canyon fault, the preferred orientation of the Draper fault 

section at the southern end of the Salt Lake segment, and slip directions for various 4> values of the paleostress 

field....................................................................................................................................................... 13 

8. Map showing surface fault traces in the Salt Lake segment and preferred dip angles based on geometric modeling........ 14 

9. Stereoplot of north-striking faults having dips between 20° and 80° W. and sliplines for the paleostress field in the 

Red Butte structural domain...................................................................................................................... 15 

10, 11. Maps showing: 

10. Trace of the Salt Lake fault segment and estimated dimensions Db of barriers modeled as equivalent 

''plastic" deformation zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

11. Normal faults in the northern half of the Red Butte structural domain and cross section 

illustrating the conjugate geometry of faulting in the footwall............................................................. 20 

12. Schematic cross sections illustrating possible structure of the branched fault zone in the northern half of the Salt Lake 

segment................................................................................................................................................. 21 

13. Map showing rupture traces of the Borah Peak, Idaho, and Hebgen Lake, Mont., earthquake rupture zones in 

comparison with the Quaternary trace of the Salt Lake rupture segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

TABLES 

Page 

TABLE 1. Barrier classifications............................................................................................................................................ H6 

2. Principal stress data.............................................................................................................................................. 10 

III 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL 

FAULT ZONE: IMPLICATIONS FOR RUPTURE PROPAGATION 

DURING NORMAL FAULTING 

By R.L. BRUHN, P.R. GIBLER, W. HouGHTON, and W.T. PARRY 

ABSTRACT 

The Salt Lake fault segment is one of the six independent earthquake 

rupture segments in the Wasatch normal fault zone of Utah originally 

proposed by D.P. Schwartz and K.J. Coppersmith in 1984. The Salt 

Lake segment is about 35 km long and capable of generating earthquakes of M 7.0 to 7.5. The fault segment is defined by a series of 

Quaternary scarps that form a branched fault trace; a narrow single 

rupture zone bifurcates into two subparallel rupture zones in the 

northern half of the segment. The surface rupture trace is subdivided 

into several linear to curvilinear sections approximately 3 to 12 km in 

length. The dips of these fault sections are modeled by using information about characteristics of the paleoslip directions and the paleostress 

field. Estimated dips vary from 45° to 90° depending on the strike of 

individual fault sections. 

A nonconservative rupture barrier is located at each end of the 

segment, and the zone of fault bifurcation forms a third nonconservative barrier located in the center of the segment. The relative fracture 

toughness of barriers has been estimated solely on the basis of their 

cross-sectional dimensions and on the assumption that the barriers are 

mechanically equivalent to a "plastic tip" deformation !one in fracture 

mechanics theory. 

Two potential sites of rupture initiation for large earthquakes are 

tentatively proposed on the basis of the structural history and inferred 

mechanical characteristics of the Salt Lake segment. One site is the 

nonconservative barrier in the central part of the segment. A rupture 

initiating there would propagate bilaterally to the north and the south, 

possibly arresting in the two nonconservative barriers at the ends of 

the segment. The other likely rupture initiation site is within the 

nonconservative barrier at the southern end of the fault segment. An 

earthquake initiating there would rupture unilaterally toward the 

north, propagating into the bifurcated northern half of the fault 

segment in a manner similar to the propagations of the Hebgen Lake, 

Mont. (M8 7.3), and the Borah Peak, Idaho (M8 7.3), earthquake 

ruptures. Evidence of significant lateral variations in the long-term 

deformation rate within the segment suggests that this latter site may 

have been the most common position for repetitive rupture initiation 

during roughly the last 17 m.y. 

Manuscript approved for publication November 20, 1990. 

INTRODUCTION 

Large fault zones are divided into sections that vary in 

geometry and rupture history. The intersections of adjacent fault sections may form geometric barriers that 

strongly influence the history of rupturing within the 

fault zone and play a fundamental role in determining the 

seismogenic characteristics of the crust (Aki, 1979; King, 

1983; King and Yielding, 1984; King and Nabelek, 1985). 

A propagating rupture may exhibit several different 

types of behavior upon encountering a barrier. The 

rupture front may slow down or pause temporarily for 

periods of seconds or minutes or become permanently 

arrested. In the latter case, new ruptures may nucleate 

and propagate into adjacent parts of the fault zone at 

later times. Consequently, geometric barriers must be 

considered as potential sites of both rupture arrest and 

initiation (King, 1983; King and Yielding, 1984; King and 

Nabelek, 1985). 

King and Yielding (1984) defined two fundamental 

types of geometric barriers-the conservative barrier 

and the nonconservative barrier. Geometric barriers 

represent jogs or bends in the main fault zone caused by 

offset in the fault trace and changes in strike and dip. A 

conservative barrier is one in which the fault-slip vector 

is constant on both sides of the barrier and parallels the 

intersection line between the primary fault planes comprising the adjacent sections of the fault zone. That is, 

the adjacent sections form a cylindrical surface that can 

be generated by moving the slip vector parallel to itself. 

Conversely, a nonconservative barrier develops in 

regions where the slip vectors differ between two adjacent sections of the fault zone. The structures of these 

two types of barriers differ significantly. Faulting in the 

conservative barrier is accommodated within a narrow 

fault zone and does not require extensive fracturing of 

H1 
H2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

the adjacent country rock to transfer motion between 

two adjacent sections of the fault zone. Alternatively, 

rupturing through a nonconservative barrier requires 

the development of at least one new direction of primary 

faulting in addition to the directions of the two intersecting fault sections comprising the main fault zone. This 

process will result in the formation of numerous subsidiary faults that must be activated to allow the transfer of 

motion between the fault sections. This network of 

subsidiary faults results in a broad, diffuse zone of 

deformation in the nonconservative barrier, which has 

the effect of "blunting'' the tip zone of the propagating 

rupture and diffusing energy into a large volume of rock. 

Consequently, nonconservative barriers are likely sites 

for the permanent arrest of a propagating rupture. This 

process, in turn, may set the stage for the initiation of a 

subsequent rupture that will propagate into an adjacent 

section of the primary fault zone (King and N abelek, 

1985). The mutual interference and interlocking of 

numerous subsidiary faults in the barrier result in the 

development of asperities, which must be broken in 

order to establish the full length of the next rupture 

segment. The breaking of these asperities must be a 

time-dependent process that is related to (1) rates of 

deformation that increase stress level through time, (2) 

fluid chemistry and pressure in the barrier, and (3) 

material properties in the barrier. 

Several criteria exist for identifying geometric barriers in fault zones (King, 19983; King and Yielding, 1984; 

King and Nabelek, 1985). An abrupt offset, bend, or 

geometric branching in the trace of a primary fault zone 

is sufficient to identify the presence of a barrier. The 

presence of a nonconservative barrier is indicated by a 

change in the amount or directions of slip on two adjacent 

sections of the primary fault zone. This phenomenon 

leads to (1) development of a third direction of faulting in 

the barrier, (2) breakup of the hanging wall above the 

barrier in normal fault zones, (3) abrupt changes in the 

amount of throw across the fault zone between the two 

primary sections, and (4) the possibility of diffuse seismicity associated with the failure of asperities within the 

barrier. 

This paper presents our study of the geometry and 

paleostress field of the Salt Lake rupture segment (fig. 1) 

of the Wasatch fault zone. The Salt Lake segment is one 

of six large rupture segments within the Wasatch fault 

zone that were proposed by Schwartz and Coppersmith 

(1984). This segment extends for about 35 km through 

the urban corridor of Salt Lake City, along the eastern 

edge of the Basin and Range province. The fault zone is 

marked by prominent scarps in Quaternary deposits and 

has undergone at least two, and probably four, earthquakes of M 7.0 to 7.5 that generated surface scarps 

during the last 8,000 years (Schwartz and Coppersmith, 

1984). Our goal is to determine the geometric and 

mechanical characteristics of the segment in order to 

better understand the processes of crustal segmentation 

and the seismogenic characteristics of the Wasatch fault 

zone. 

ACKNOWLEDGMENTS 

This work was part of a program of structural and 

geochemical studies of the Wasatch fault zone undertaken by R. L. Bruhn and W. T. Parry. P.R. Gibler wrote 

the stress tensor inversion programs and collected field 

data for the paleostress analysis. W. Houghton mapped 

the Fort Canyon fault zone and studied the petrofabrics 

of the fault rock. This work represents part of Gibler's 

and Houghton's M.Sci. theses at the University of Utah. 

Financial support was provided by the U.S. Geological Survey Earthquake Hazards Program grant 

14-08-0001-G-886 to Bruhn and Parry, American Chemical Society grant 14179AC to Bruhn, and the Integrated 

Tectonics and Seismology Program of the University of 

Utah. We thank W.J. Arabasz and J.C. Pechmann 

(University of Utah), R.B. Smith (University of Utah), 

and D.P. Schwartz (U.S. Geological Survey) for discussions concerning the structural and seismogenic characteristics of normal faulting. Thorough reviews of a draft 

version of the manuscript by P.C. Thenhaus and R.L. 

Wheeler (U.S. Geological Survey) were very helpful. 

TECTONICS OF THE WASATCH FAULT ZONE 

The Wasatch normal fault zone extends for 370 km 

along the eastern edge of the Basin and Range province 

in southern Idaho and northern Utah (fig. 1). This major 

fault zone has a pronounced footwall escarpment, displaying up to 3 km of relief, that separates alluvial-filled 

valleys of the Basin and Range to the west from the 

Wasatch Mountains to the east (Gilbert, 1928). Total 

throw across the Wasatch fault zone varies along strike. 

Estimates from gravity modeling range from 2.6 to 4.0 

km (Zoback, 1983). Throw across the fault zone may 

reach 11 km just south of Salt Lake City, Utah (Parry 

and Bruhn, 1984, 1986). 

Schwartz and Coppersmith (1984) proposed that the 

Wasatch fault zone is divided into six structural segments defined by differences in the geometry of the fault 

trace, variations in footwall topography, and earthquake 

rupture history. Individual segments have strike lengths 

of 35 to 70 km, and their ends are marked by easttrending faults and basement ridges that subdivide 

basins in the hanging wall of the fault zone (Zoback, 1983; 

Schwartz and Coppersmith, 1984; Smith and Bruhn, 

1984). This segmentation has been refined further by 

Machette and others (this volume). Each segment has 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H3 

Great Salt Lake 

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FIGURE 1.-Tectonics of the Salt Lake rupture segment, 

Wasatch normal fault zone, Utah. Structural domains 

for paleostress analysis are indicated by hachures. 

Normal fault is indicated by line with ball and bar on 

the hanging wall (solid where scarp is exposed, dashed 

undergone multiple episodes of surface faulting during 

the Pleistocene and Holocene and is presumed capable of 

generating characteristic earthquakes of M 7.0 to 7.5 on 

the basis of fault displacements measured in trenches 

where buried or inferred). Circled numbers are sections of the Salt Lake rupture segment presented in 

figure 8 and discussed in the text for purposes of 

inferring the positions of geometric barriers and fault 

zone orientation. 

placed across Quaternary scarps. The recurrence interval of surface rupturing on individual segments is 

estimated to be as short as 4,580 years; an average 

recurrence interval for characteristic earthquakes is 400 
H4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

to 600 years for the entire Wasatch fault zone (Swan and 

others, 1980; Schwartz and others, 1983; Schwartz and 

Coppersmith, 1984). 

The Wasatch fault zone is associated with a northsouth zone of diffuse seismicity that generally dos not 

show a strong spatial correlation with the Quaternary 

fault scarps (Arabasz and others, 1980; Zoback, 1983). 

Earthquake focal mechanisms, geodetic measurements, 

and fault-slip vector data taken from Quaternary scarps 

indicate that, regionally, the crust is extending in a 

west-southwest-east-northeast direction, at an average 

orientation of the least principal stress (maximum extension axis) of 073°± 15° (Zoback, 1983). 

STRUCTURE OF THE SALT LAKE SEGMENT 

The Salt Lake segment of the Wasatch fault zone 

extends for 35 km from the Salt Lake salient on the north 

to the Traverse Mountain spur on the south (fig. 2) 

(Gilbert, 1928; Schwartz and Coppersmith, 1984). The 

footwall of the Salt Lake segment is composed of Proterozoic through Tertiary sedimentary strata, Precambrian schist and amphibolite, and Oligocene quartz monzonite. Precambrian, Paleozoic, and Mesozoic strata 

generally strike eastward and are folded in the easttrending Uinta arch, a large anticlinorium that developed 

during the latest Cretaceous and earliest Tertiary (Crittenden, 1976). Quartz monzonite of the Little Cottonwood stock intruded Precambrian and Paleozoic rock 

along the southern part of the footwall during the 

Oligocene (Crittenden and others, 1973). The Wasatch 

fault cuts across these earlier structures at high angle to 

their strike. 

The hanging wall of the Salt Lake segment contains a 

large Neogene basin containing over 1 km of sedimentary 

fill. The fill consists of Miocene through Quaternary 

strata and volcanics (Cook and Berg, 1961; Zoback, 

1983). The hanging-wall basin is structurally segmented 

by an east-trending basement step that separates the 

deeper southern part of the basin, which is about 1 to 1.5 

km deep, from the northern section, where the thickness 

of the basin fill is generally less than 300 m, according to 

Zoback (1983). 

The history of faulting on the Salt Lake segment is 

poorly constrained. Normal faulting postdated intrusion 

of the Little Cottonwood stock, which was intruded 

31.1±0.9 m.y. ago (Crittenden and others, 1973). A 

muscovite concentrate from exhumed fault rock in the 

Wasatch fault zone along the southwestern margin of the 

stock was dated as 17.3±0. 7 m.y. and represents the age 

of the earliest observed syntectonic hydrothermal alteration in the fault zone at that locale (Parry and Bruhn, 

1986). Consequently, normal faulting in the Salt Lake 

segment began by latest early Miocene. Relative uplift of 

the footwall at the southern end of the Salt Lake segment 

is about 11 km on the basis of fluid pressure estimates 

from fluid inclusions trapped in cataclasite in the fault 

zone (Parry and Bruhn, 1984, 1986). A minimum estimate for throw at the northern end of the segment is 2.5 

km, based on the geology of the footwall and an estimated 300 m of fill in the hanging-wall basin (Zoback, 

1983). 

The Quaternary surface trace of the Salt Lake segment consists of several linear to curvilinear fault sections that vary in orientation and range in length from as 

little as 3 km to as much as 12 km (fig. 2). These sections 

are separated by significant bends in the trace of the 

fault zone that define geometric barriers. The bends are 

classified as either conservative or nonconservative barriers on the basis of the criteria listed in table 1. Several 

barriers can be readily identified; of these, three are 

classified as nonconservative barriers-one at each end 

of the Salt Lake segment (nos. 1 and 9, table 1) and a 

third composite barrier associated with a branch point in 

the fault trace located in the central part of the segment 

(nos. 4, 5a, and 5b, table 1). 

The fundamental characteristics of a nonconservative 

barrier are that the slip directions on the adjacent 

sections of the primary fault zone are different in orientation or magnitude or both and that at least a third 

direction of faulting is required to accommodate displacement within the primary fault zone (King, 1983; King and 

Yielding, 1984). The direction of slip on a series of 

variably oriented fault sections will depend on the geometry of the sections and on the orientations and relative 

magnitudes of the principal stresses operating across the 

fault planes. We first consider the nature of the paleostress and contemporary stress fields in the Salt Lake 

segment and then discuss the geometric characteristics 

of the individual fault sections and barriers. Finally, we 

speculate on the rupture characteristics of the Salt Lake 

segment. 

PALEOSTRESS ANALYSIS 

The orientation of the paleostress field in the Salt Lake 

segment was determined on the basis of a stress tensor 

inversion analysis of mesoscopic fault populations in the 

Wasatch fault zone and adjacent parts of the footwall. 

Four structural domains were chosen for analysis (fig. 1). 

Each domain was chosen because of its excellent exposure and its structural position within the fault segment. 

The Red Butte and Olympus Cove domains were located 

in Paleozoic and Mesozoic strata in the northern and 

central parts of the segment. The Draper and Fort 

Canyon structural domains were located in exhumed 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H5 

A 

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barrier 

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5 

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BREAK IN SECTION 

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NO VERTICAL EXAGGERATION 

EXPLANATION 

~ Tertiary sedimentary and volcanic rock 

I Tqm I Tertiary igneous intrusive rock 

D Jurassic strata 

B Jurassic and Triassic strata 

G Triassic strata 

G Upper Paleozoic strata 

0 Lower Paleozoic strata 

8 Precambrian strata 

I P£m I Precambrian metamorphic rock 

---

--~ 

Lithologic contact-Dashed in the subsurface 

Thrust fault-Dashed in the subsurface; 

(Ton upper plate) 

Normal fault-Dashed in the subsurface 

Normal fault-Ball and bar on hanging 

wall, dashed where buried or inferred 

Projections of barrier (bend in fault zone) on 

cross section 

Axial trace of anticline 

Axial trace of syncline 

FIGURE 2. -Geology and cross section of part of the Wasatch Mountains and the Salt Lake rupture segment. Barriers in the fault zone are 

numbered and projected onto the footwall cross section. Geology from Davis (1983a, b); Wasatch fault trace from Marsell (1969). 

fault rock within the Wasatch fault zone on the western 

and southern margins of the Little Cottonwood stock. 

The latter two domains were chosen to provide information about paleostress orientations in the Traverse 

Mountains barrier at the southern end of the Salt Lake 

segment. 
H6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

TABLE 1.-Barrier classifications 

Barrier no. 1 Structural characteristics2 

1 ........... TF, HB, TH, SE, OF 

2 ........... B 

3 ........... B 

4 ........... BR, TF, HB 

5a .......... B, ID 

5b .......... B, TF(?), ID 

6 ........... B 

7 ........... B 

8 ........... B 

9 ........... TF, BR, HB, SE, OF 

Barrier type 

N onconservative. 

Conservative. 

...... do. 

N onconservative. 

....... do. 

....... do. 

Conservative(?). 

....... do. 

....... do. 

N onconservative. 

1 Branch point 4 and bends 5a and 5b are considered one 

composite nonconservative barrier. 2

B, bend in trace; BR, branching of fault trace; OF, lateral 

offset of two primary rupture traces; TF, three directions of 

large-scale faulting; HB, hanging-wall breakup or basement 

ridge; TH, abrupt difference in primary fault throw; ID, dip 

incompatible with conservation of slip on adjacent fault section; 

SE, diffuse seismicity. 

Faults within each domain form several sets that 

permeate the rock on the meter to centimeter scale. The 

age of faulting is not well constrained. The Draper and 

Fort Canyon domains are located in a structural carapace 

of cataclastic and phyllonitic quartz monzonite that varies from a few meters to about a hundred meters in 

thickness. This fault rock formed at depth within the 

Wasatch fault zone beginning about 17 m.y. ago and 

continued to form during uplift of the footwall relative to 

the hanging wall of the fault zone (Parry and Bruhn, 

1984, 1986). 

Faults in the Red Butte and Olympus Cove domains 

cut Paleozoic and Mesozoic strata in the footwall of the 

Wasatch fault zone (figs. 1, 2). They are part of a large 

population of normal to normal-oblique slip faults that 

occur in the footwall block and increase in number as the 

Holocene scarps of the Wasatch fault zone are 

approached. The Red Butte and Olympus Cove domains 

are both located about 2.5 km east of the youngest 

Holocene scarp system but lie directly adjacent to an 

older system of Quaternary scarps along the Wasatch 

Mountain front (fig. 1) (Van Horn, 1972). 

Slickenlines for fault populations from each domain are 

plotted in figure 3, and, with the exception of those in the 

Fort Canyon domain, the slickenlines generally form two 

or more maxima. In the Fort Canyon domain, the 

slickenlines form a broad partial girdle in the stereoplot, 

indicative of a diffuse spatial distribution of slip directions. The dominant spatial concentrations in the other 

domains trend between 230° and 250°; subsidiary maxima 

plunge at moderate angles into either the northern or the 

southern quadrant. The fault populations are composed 

of a mixture of fault types, and slip varies from purely 

normal through oblique-slip to almost pure strike-slip 

motion on faults of various orientations. 

Slip directions on Quaternary fault surfaces in the Salt 

Lake segment were reported by Pavlis and Smith (1980) 

from two faults exposed at the northern end of the 

segment in the Salt Lake salient (fig. 2). Exposures of 

the Warm Springs fault, which strikes N. 45° W. and dips 

70° W., contained two sets of slickenlines. The older set 

trended 288°, and the younger trended 239°. The Virginia 

Street fault located a few kilometers to the southeast 

strikes N. 80° E. and dips 68° S. Slickenlines on this fault 

formed two maxima plunging towards the westsouthwest. The average trend of the northern maximum 

was 255°, and that of the southern maximum was 233°. 

The relative age of the two slickenline sets on the 

Virginia Street fault could not be determined from field 

observations. 

The slip directions determined by Pavlis and Smith 

(1980) for Quaternary faulting at the northernmost end of 

the Salt Lake segment are consistent with the directions 

of the most prominent slickenline maxima from the 

paleofault populations in the Red Butte, Olympus Cove, 

and Draper domains. This observation does not imply 

that the paleofault populations are Quaternary in age 

but, rather, raises the contention that extension across 

the Salt Lake segment may have been oriented consistently toward the west-southwest over an extended 

period of time. The oldest set of slickenlines on the Warm 

Springs fault provides an e4ception, in that this set 

trends about 20° north of west, whereas the prominent 

maxima in the paleofault populations trend south of west. 

However, the remainder of the Quaternary slickenline 

sets lie within the general trend of the prominent set in 

each of the paleofault population domains. 

The paleostress directions in each domain were determined by using a linear stress tensor inversion method 

like the one discussed by Michael (1984). This technique 

provides an estimate of the trend and plunge of the 

paleoprincipal deviatoric stresses (81 ?:82 ?:83) and the 

stress parameter <P=(S2 -S3)/(S1 -S3) for a population of 

faults. The data input into the inversion routine include 

the orientation of each fault and the fault's slip direction 

and sense of slip. The following assumptions are required 

to generate a system of linear equations for solution: 

1. The paleostress tensor remained constant in orientation and magnitude during the deformation event. 

2. Slip on an individual fault plane was parallel to the 

maximum resolved shear stress vector (T), which 

paralleled the slickenline directions. 

3. Each fault slipped when T reached a specific magnitude, which was assumed constant for each fault in the 

population. 

These assumptions allow construction of a system of 

linear equations of the form 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H7 

where Ti is the unit shear stress vector parallel to T on 

fault i, S is the normalized deviatoric paleostress tensor, 

and ni is the unit normal vector to the ith fault plane. 

This set of i linear equations is then solved for S by using 

a singular value decomposition procedure described by 

Lawson and Hanson (1974, subroutine SVA). The normalized principal deviatoric stresses (S1 2=.S2 2=.S3) and 

their orientations are then found as the eigenvalues and 

eigenvectors of S. The procedure has been discussed in 

detail by Gibler (1985). 

The principal stress directions determined for each 

domain indicate that the maximum principal stress (S1) 

dipped steeply and that the least principal stress (S3) 

plunged gently and trended east-northeast (fig. 4, table 

2). The preferred trend of S3 between the four domains 

varied only 11 o, and the average for the set of measurements was 050°±5°. The <1> values for the domains varied 

from a high of 0. 54 in Red Butte to a low of 0. 21 at Fort 

Canyon (table 2). 

The solutions for the paleoprincipal deviatoric stress 

directions and relative magnitudes must be considered as 

an approximation of the true characteristics of the stress 

tensor because of assumptions concerning the temporal 

and mechanical characteristics of faulting and potential 

errors in field measurement. The assumption that faults 

within the population slipped at the same resolved shear 

stress is a fundamental mechanical constraint on fault 

behavior that may not be justified, particularly in a 

volume of rock occupied by numerous intersecting and 

potentially interlocking faults. However, Michael (1984) 

compared the results of the linear inversion technique 

with the nonlinear inversion method of Angelier (1979) 

and Angelier and others (1982) and found the results to 

be similar. The nonlinear inversion method does not 

require the assumption of equal shear strength on each 

fault plane in the population. 

Potential measurement errors include the following: 

1. Inadvertently including faults of several different 

deformational events in the fault population was a 

potential problem, particularly in the Red Butte and 

Olympus Cove domains, where Paleozoic and Mesozoic strata were deformed in folds and thrust sheets 

during the Mesozoic and early Tertiary. However, 

measurements were confined to faults within zones of 

extensive north-south-trending normal faulting adjacent to the Wasatch fault zone. This sampling problem 

was considered less serious for the southern domains, 

which were located in the Little Cottonwood stock. 

There, the mesoscopic faults slipped either during or 

subsequent to the high-temperature hydrothermal 

alteration event dated at 17.3±0. 7 m.y. ago (Parry 

and Bruhn, 1986). Consequently, faults in these latter 

two domains must reflect movement in the Wasatch 

fault zone, although the history of faulting could be 

protracted, and there is no a priori reason to assume 

that the stress tensor remained constant in either 

magnitude or orientation. 

2. Random errors in fault plane and slickenline orientation may have been introduced as the result of field 

measurement. Our measurements were reproducible 

to within ±5°, and such errors should not affect the 

results of the analysis because of their random nature. 

3. The stress tensor was implicitly assumed to be constant in direction and magnitude throughout the volume of rock occupying a domain. This assumption was 

verified in a subjective manner only, by noting that 

faults formed consistently oriented sets displaying 

consistent slip directions throughout the domain as 

the measurements were made in the field. 

The results of the inversion analysis appear reasonable 

on the basis of several independent lines of evidence. 

First, the steeply dipping S1 attitude is to be expected in 

the vicinity of a normal fault zone, and the attitude of the 

least principal stress (S3 ), which trends to the westsouthwest, is consistent with the trend of the prominent 

slickenline maxima in each domain except Fort Canyon, 

where no strong maxima occur in the slickenline orientation data. The trend of S3 is also consistent with the 

Quaternary slip directions determined by Pavlis and 

Smith (1980) on the Warm Spring and Virginia Street 

faults at the northern end of the Salt Lake segment. 

Finally, paleostress orientations are very similar in each 

of the four domains, even though the fault populations 

vary considerably in their overall geometric characteristics and are located in rocks of different ages and 

lithologies. Qualitatively, the mixture of normal, oblique, 

and strike slip faults in the populations from each domain 

is consistent with <!>>0 and supports the results of the 

inversion analysis. If <!>=0, then S2=S3 , and the slickenlines should plunge downdip on all faults within the 

population, given a nearly vertical S1• Clearly, these 

conditions are not met in each of the domains. However, 

other than this qualitative evaluation based on the geometric characteristics of fault populations, we have no 

independent means of quantitatively evaluating the accuracy of <1> as determined in the paleostress inversion 

analysis. 

The average paleo-S3 trend determined from the 

stress tensor inversion of fault populations is about 23° 

north (clockwise) of the regional, contemporary S3 orientation of 073°±15° determined for the Wasatch Front 

region by Zoback (1983) on the basis of her analysis of 

earthquake focal mechanisms, geodetic measurements, 

in situ stress measurements, and Quaternary fault surfaces. The <1> value for the contemporary stresses is near 

0, according to Zoback (1985), on the basis of stress 

magnitudes and directions determined from hydrofracture stress tests and well-bore breakouts in areas south 
H8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

FAULTS SLICKENLINES 

Red Butte Canyon 

A 

N 

w E 

s 

w 

u 

/1 

II 

" " " II 

II 

~ i 

B 

N 

··'····· !Ill II ······'''···· I. ,1//11.,,, 

tlf't!l/1!/,//lllt ••• '.'"XlXYll///1//////,,, 

.. :,: .:~=~~~~:''~~~~~~j~ji:: ::::: ... u ·tnnuu'"""'"t . ..... . ,,///.'i:lX'XllXlllX/l//////,,,,,,,, 

.,,/ti'.''YXIJXXXllJ:/l////,,,,,,,,,, 

,,,1//IU.U':O:XXlXX/////,,,,,,, 

:~:~~~;~~a;:~~~~~~~~ i~::::::. 

:: ~ ~~~{;~HU~H~}~~~~:: · · ••• 11//TXU'UXU!,,,,,, ,,///1/l:'r'XXXI/,,,,,,, 

••• l/!f.'/')' 1 1'111 •••••• 

••••• 1111.' 1 '11! •••••• • 

••• ,,//.'!': ••• ; ••••••• •••• ,,. 1 1'1 •••••••••••• ••••••• '111' ••••••••••• ·······'''············· 

•• ·····'··'··········· !!/! ••••••••••••• .. • /.1 1 1/1.,, •• ,,,,,,,,, 

••••• '11 ••••••••••• ••• //,,//11 •••• 1 •• 1 •• , ••• //,!!,1' •••••••••••••• 

~;;~~~~;i··.~ if;i ::: ::· .' 1 /1,''1'.''{)',' 1 !'1.',,,, 

tl/ 1 l..'I.'X'f' 1 1.'1 ••••••• 

j~~ ~~~ '~~~~ ;~ :~ .; :::::. 

J;;qggHa~i:.i:::: ... ~ nqHa;.~:' ~~ ·:u~·:; ~:::: .. 

iif::mmli~!Wi!m~ ~ ~ ~ ~) i i ;:,~ ~UHiH:IJBHH~~~~; :::::: //.'lXJUIY'{VtJX'fKUI/1/////,,,, 

/I If XXII XU VJUXHJY/ 1/1//1,,,, J//IIXUXn.,lti'IJYI/fl/11/, •• , , 1 1••rn:xn•xtt!flXIII!Itlfr ••• 

. ·:~ ;~~~~g~~~~~ ·;~ ~~ :~ ~~~~~::::: .,,.'/YX)'I'/,/.,, •••••••••••• , 

························· 

s 

E 

Olympus Cove 

c 

w 

; ~ ~; ~ ~ 

~~;~~~ 

: ~ ; ; ~ ~ 1 1'1'1 

1 ,'1/1, 

'I'll/ 

'J'/1, ·''·'· 

N 

.... ................. ············ 

................... 

.................... .... ·············· .................... ···························· 

s 

E 

FIGURE 3. -Contoured orientations of poles to fault planes and slickenlines for fault populations in each structural domain in figure 1. 

Red Butte, 129 measurements of faults (A) and slickenlines (B); 

Olympus Cove, 105 measurements of faults (C) and slickenlines (D); 

Draper, 110 measurements of faults (E) and slickenlines (F); Fort 

w 

D 

N 'Ill/ 

f~ ~~ :' ': /1 '/!' ,, I Iff 111111'1 1 1 I .'l'l/11/ff/1 1 1 

: 111111/.'llfl/ 11/fll/f!'l 

I;~~~ Ill ; ; j l!!.'f 1 .'' 1 '''1'11 ff/11 .''1 !fill 1!1/f'll'!f/!11 l//1/.'fl ''/ 

!/ll!tl!ll/1/.'!l'lt/lli!lll/f/ll/ /'/ 11/!l.'lll''i''l'l//!!f//lf/1/l/11 ,,, 

J ~ ~ j~ J:. ~j ~~ .; ·; ·~ :. ·:~ ~~ ~ :: ~~ ;·~~j~j'.~ '·~ ~ ~~·: ~ j lfl/lllf/ 1 ff.''l'll/ll!lfl/lltii/IJII 

f/flllltll!/.'1//.'1 1 /llff/11111!11 f//1.' JII'I'I'.'IY'I'!If/1'11111 I ll/111!/11/!1/f/'{.'I'IY.IIfl/1/lf/ I lllfflf///f{/fl':t!llff!l//l{flf 

fi/JIIIII/IIfll'tYY.tlVJI!Y//1/II/ 

f!/111/11/YYYY.'t'lYY. .'YVYY.UY. If I /I /11/IIIIYUJY.Y'I'HH't"'XU'fX/IIff/ 

;);~;;mnm:ma:m:;~;;;;;,, 

'lf/1//YlXll)J)'l)'XJYYXY%.'/fl.'/,'///,'1' 

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I.' I 'II! I) ll I~ )' 7l X l/., J: .I X )'!.'I 1 I.' I II !II f //llUIXHnt/ 1 (rKX fl/11111 

/fffffl'f/IUUXHJY'XOXtttflll 

II fIll 1 I IX I IX lX ~"'~~·~ J X XX 1' .'I Ill 

~ i ~::: ~~~~~;~::~ ~~~:~;: H~;.: ~~ 1'1 1 f'JJXXXY>ItGI':tlJYYXYXYX.'I// 1 / 

' Ill I 

f/11/1 "" I IIIII/I If"" 1!111111 11/f/111 ,'ff/1//1 111111111 

'!Ifill/ 1/1 1 1/111/ 'fll/111 1'1'1111/1 I 

lf/J/1/1 II I !I IIIII f/1 !lft/1/f / 1 /fl/1/1//111 

'1!11/fl f'flfl/1/1 IXJ/1 'Ill fIll IIIII filii Ill/ If 1 1111/11 ,' 1 ///11/!ll//lll/ 1111/111 11/f/11/11 '""" .'1!11111 l/1111111 111111/J; '!IIIII/ !f/1111//1 1!!1 1U'H /!.'Ill f Jliiiii/JI/111111111 " 1111/11111 111/11 I /IIIII 1/XIII/IfJ/ IIIII/ /II'U/111/1 /IIIII II! Kl f/1/11 111/lll/lk'/1/111 

11/1/1 /XJ.IYI/1111 11//IIIIJ.I.II,I/111 

11!111 fiJ:U/fJIJJ. 111111 tlfnJnr.u 

~~~~~ ~~~~~:~~~a /fill fllf/f/1111 

11/1 1/!11111111 II/ II 111!1111 "1//ll/11/11 

I l~lj~~~~j~~ 

1 fJ.'IIU'!PI~"I"~YXJYYII'.t:lUIX///1! IIIII' I 

f ~~ I )i /'.'!/1'JlllttMriXJJ'fYJXXYI'tl///f/ I' tlffJif'f 

'll J: lll J: XI J: !'IX~"t"'"' ~v 1 X l I •II: Y .'I I I I .' f I ! I I I! I II.' I I '/1! l l X J )II tJ Y ~ i' J.,. .; 'l )'I X I J .'II f I/ 'I ' '! 'I ,I I ' ! ' I . I II I '~II' .1~(111\f( II X 'f 1 ·:fl A~ Ill JY.l 'IIIII I til '1 I ' I I ! I I I r If,';/ I /Ill f 'IYJ~tltll"le!llltt:'1'1•11U:l'l /!/l/f/UI/fl'.''" 'I!IU'I 1 !'11fl//f'l 

': !illli! i!i!!!1!~1!1!!:lll!i il :::;: ! :; : il 1:1 :I Ill\! ll!lllili! i li'F 

s 

E 

Canyon, 106 measurements of faults (G) and slickenlines (H). Contour intervals are 0, 2, 4, 6, and 8 standard deviations above a 

random distribution plotted on the lower hemisphere, equal-area 

projections. 
E 

w 

G 

w 

STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE 

FAULTS 

N 

=~;;;~;~~;~:; :::::: . • 'IIJ!!III/f//llf'l, 

·· .. :: :::rn::::: i:;.H:~~~~~jj~j~~~~i 

·:::.:. ::~:;~~;~;; ~,;~:j~~~~~~~~:~~~~~·: ..... __ ,., .. ,," ~;;·;~~~~~HHH~k~~~ ••• It 11\11 1 1/111 11/1.1/UlHHlU/1/ ••• 11 1 1 ".'J./1111 1'/'IIA.I..UUJ.H:UA/k , I J: / ,' 'I 7. ,_XI l I .1. A Ill 1 I A.kll~lAl.l.l }.l.l.ll IIIII ~~'l~,I;,IA~Al.l. llAffAllll~XlkA1t;Xl~ 

1/fiA A/,~HUHl UHUUlUXlllUUl /IAU UPU:UH/ UHl/lllllllUtHUl 

(:iH ggggg~ ~gnnHnHH~BB /l/!1. H'OX'I:XJIU UU~/UIIXU'JXYAUl 

l'i'' An.lXUllU l/UUXIJU'(JJXHUl f.'/'A.! UP.UKUU HXXX/U:IIXUIXUUI ,.1:.1. J.Jl"))'if.XUl XlCX~XlXIHXXIY:IHUX 

•• ,ft 1\U.i!lii:UlH UX~/1UUXXIJI'Xlll 

,,,/1 ,'llUXlU/ Ul't'IIIIX'OUYil'IX 

::;(; )~HHHB! ~u;mllllll!llPH 

::::~ ~;~ .. ;~~a~{~ ~·;7~HHBH:~~;:~~: ,, •. , tr;•n.nfll 1'1!/U'I.UA~IIt!l •.• ••••• ,,/,'f/l.'f/1 !ltlf.'flf!fll/1, •••• 

••• ,, ,'.''f!/ffl/ J'J!J//1,,!/f,/,,,,, 

••• ,'1'''11/11 / 1 / 1 /f//,,,,/,1,,,,, 

• • 't.'fllf/l/ /fl'llll •••••••••••• ••• •• .'.'lllli!X 1!1/f/!,,,,,,,,,,, 

:::: ::.;~:;.~~~~~~ ~·~~~~~~~:::::::::· •••• ••••• f,lf'' 1/ffl,, •••••••• 

:::: :::::~::~~~ ~~:;:::::::::: ••••• ··········' /,'!', ••••••• 

s 

N 

:::;:: 

~~ ~;jjmn;~~::. .. '''''''''!.' .. ! •• •• 11/f//ffllll.'t •• •• '11!111/IJ./11.'1/, .. 'f!l!fl/lUtt!/11 ,! 1/f/lfHXXJ.tO.!f.' 

,' f/lfllfAXl!OXVflf ,, !t/1/llllf,,A,XJ.l/'X ,• .'fl//fi/IJXX'fllJ/)'( ,f 'ffii/IUlUHUXJU 

I! .'I I II I A I). I< J. A J.J.ll ~ l ~ .' 1.' !I!HIUUU1.0J.U~AI 

f/ .'1/IIIAP'AH.'!r..IIH'I.ll r• .'1/Hii/AUlPXU/nU. 

~~ ~;;~H~~UH~lHlHB /! /~/HHXAI<H~HJ.UXH 

Draper 

E w 

Fort Canyon 

~~ i!iiHiiiiif~iiiiiit E w 

s 

~~ ~HHB~H~~unnn 

~.~ ~H~~H~;~}nHf~''~ 

~·; '·~~~~~~~~~~~~~~;·;~~:. ,' !JIIIItlf/lft,l/lff, •• • !.'1/lf.fff/1,.,(,{. 

•• ••• !11 ••••• /1,,,,,. .................... , 

FIGURE 3. -Continued. 

SLICKENLINES 

F 

N 

If/, ••• '''· ..... . •r •• ••••••• 

·:: :: :.:::;: •••• ,/(,,f, 

··' ,, 1'1'11. ,,'If.'//.'.'// 

.. 1 11 nfl;tr 'II IJ_ llll/Xf 'II 0 UKJ:lXX ·u n nnnx 0:: 'X XUUU X X 'f ,' .'X X X X ~ l lX U /UU'U <t u: xnu:o: ~ lX XIXUU X) O:~UH 

XX XllUXX 'X lJI:lHU X llUXII 

~~r;; ~ 

H 

s 

N ....... / ...... ········'··''''···· ... : :; ::;:H~~H~::~H: ::: •••••• 1/111/1111111!1111 •••• ••••••••• JII/1111111111111 •••• 

• . ::: ;: :~ :; ;: ::~:~~'~::::::::: ii: ..... fl., •• •• IIIII If! 1111111111111. 

..... ·"'·· .. ·"" llllllll/111111 /1, .... • II II t!l.l/111111111111111 ••• 1 .. 

::: ;:: 1:::: i i~ ~::::::: ::~::~::: ::: •• JIIIIIIIft ••• IJIII ...... / ........ . 

:: ~ ~:: ~~ ~:::: ~::: ~: i i:::::::::::: : 111!1/IJII.l/ll.t.t,lll ...... ,ll ...... • •••• 

:~~:::H ~:B:;~;~:: ::~ ::;;: ~: ::::: :::~~: H ~:H HH:f~~ ~:; :f;:: :::::: ••••• f/flfll/111 •• 111111111 ••••••• •••• IIII'IIJIII/11! ••• !111//,.,,.,.,.,, ••••• 111111!11/l/ll.l.llllll •••••••• •••• 1/f!tiiiii/II •••• J.IIII ........... . 

:;;;HHHH:::HfiiH;::::::::::::::. ·. :H HH: ~{~: :;:::::~~~~~~~: :::::::::: ;H:::H"H~:~iittllfllff;;;:::::::: .. :::::::: ~:::,tu~u~~~~:::~:::: :::::: · :;: ::: :; ~: H :'~~lBBlB:::::: ;;: ::::. 

:::::: :~ ~~~~~~HHH~~H~~:~H::; ::::: .. •••• • ! • • • tlll/1/,.,l/l/lll/1111./ •• •••••••••• JIII.I •• JI/111111111 ...... ........... llll ••• l/11111111111 ••••• 

·::::···::::::~H::::HHfH;;;;;: • ••• ,/1/fllll/JIIfl/11111 .... 11.1111111111/ •• IJ/1 •••••• 111111111 ••• I! II ...... IJ/111/ .... 111 ••• 1111111 •••• 11 

••••• ···""""····' JJIIIJ ..... .. ••••• IIIII •••••• ••••••• IIIII •••• ·········''······ ·······'·········· ......................... 

................... 

............... 

................ 

.................. 

s 

H9 

E 

E 
H10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

w 

N 

~~--- s, ~ -........ \ ........ , \ -to \ 

" o\ \ 6 I 

\ ./ 

s 

EXPLANATION 

0 Red Butte 

1::,. Olympus Cove 

D Draper 

.& Fort Canyon 

• Average 

E 

FIGURE 4.-Paleo principal deviatoric stress orientations from each of 

the four structural domains in figure 1 based on inversion of 

fault-plane and slickenline data. S1 is the maximum principal compressive stress, s2 is the intermediate principal stress, and s3 is the 

least principal stress. The trend and plunge of each stress axis are 

shown in table 2. 

of the Salt Lake segment in the region surrounding the 

three southernmost rupture segments defined by 

Schwartz and Coppersmith (1984). Alternatively, Arabasz (1985) and Arabasz and Julander (1986) concluded 

that <!> is close to 1 on the basis of their analysis of 

earthquake focal mechanisms and hydrofracture tests 

from the southernmost segment of the Wasatch fault 

zone and the adjacent Basin and Range-Colorado Plateaus transition zone. Consequently, the contemporary<!> 

parameter of the regional stress field remains controversial and poorly defined. 

TABLE 2.-Principal stress data 

Trend/plunge 

Structural domain <1>1 No. of faults 

Red Butte ........ 095°/83° 324°/07° 054°/00° 0.54 130 

Olympus Cove ..... 180°/82° 314°/06° 044°/07° .50105 

Draper ........... 113°/77° 317°/12° 227°/04° .42 110 

Fort Canyon ...... 301°/66° 146°/21° 056°/10° .21 106 

1

<1>= (82 - 83)/(81- 83). 

FAULT ZONE STRUCTURE 

TRAVERSE MOUNTAINS BARRIER 

A complex zone of faulting forms a nonconservative 

barrier at the southern end of the Salt Lake fault 

segment that we have informally named the Traverse 

Mountains barrier (barrier 1, table 1). The Traverse 

Mountains are located at the intersection of the Provo 

and Salt Lake rupture segments as defined by Schwartz 

and Coppersmith (1984). Here we consider only the 

northern part of the Traverse Mountains structure. 

Further work is required to establish the structural 

relations between the Traverse Mountains and the 

Ogden segment. The Traverse Mountains barrier is 

composed of three primary fault sections that form a 

triple junction comprised of the southern end of the Salt 

Lake segment, the Fort Canyon section of the Wasatch 

fault zone, and the west-southwest-trending fault zone 

along the northern edge of the Traverse Mountains (fig. 

5). The southern end of the Salt Lake segment is marked 

by a major jog in the trace of the Wasatch fault zone, 

where the Salt Lake and Provo segments are offset about 

7.5 km and connected by an east-trending fault zone 

along the southern margins of the Little Cottonwood 

stock. We informally refer to this east-trending section of 

the Wasatch fault zone as the Fort Canyon fault section. 

The Fort Canyon fault zone lies along the westward 

projection of the Deer Creek normal fault, which is 

located in the footwall of the Wasatch fault zone and can 

be traced eastward through the Wasatch Mountains for a 

distance of at least 20 km (Baker and Crittenden, 1976). 

The eastern part of the Traverse Mountains is in the 

hanging wall of the southward-dipping Fort Canyon fault 

section, whereas the western part of the mountains 

forms the footwall of a north-dipping west-southwesttrending normal fault zone at the southern end of Jordan 

Valley, informally named the Traverse Mountains fault 

section. 

The southern part of the Salt Lake segment (Draper 

fault section) can be traced as a series of southsouthwest-trending scarps that strike 035° in unconsolidated deposits (Gilbert, 1928). These scarps project 

southward into the exhumed bedrock fault zone along the 

western edge of the Little Cottonwood stock (quartz 

monzonite). Here the fault zone curves southward to join 

the east-trending Fort Canyon normal fault section along 

the southern margins of the stock and intersects the 

southwest-striking normal fault zone along the northwestern edge of the Traverse Mountains. 

The exhumed bedrock fault zone consists of a cataclastic carapace of faulted quartz monzonite that extends 

along the western and southern edges of the Little 

Cottonwood stock (fig. 1) (Gilbert, 1928; Bullock, 1954). 

The carapace varies in thickness from several tens of 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H11 

0 5 

0 5 10 

N 

t 

PROVO 

SEGMENT 

10 MILES 

15 KILOMETERS 

® 

FIGURE 5. -Geometry of faulting at the Traverse Mountains barrier 

(top) and the tripartite fault array (bottom). Fault A, Draper fault 

section; fault B, Fort Canyon fault section; fault C, Traverse 

Mountains fault section. 8 is the fault dip angle. Ball and bar are on 

the hanging wall of normal-oblique slip faults. 

meters to hundreds of meters. Progressive uplift of the 

footwall relative to the hanging wall resulted in overprinting of higher temperature and pressure alteration 

mineral assemblages by lower temperature and pressure 

assemblages during faulting (Parry and Bruhn, 1984, 

1986). The oldest and highest temperature alteration 

mineral assemblage in the faulted carapace consists of 

chlorite, epidote, and muscovite, which occur in relict 

patches of phyllonite and younger, more broadly distributed cataclasite. Fluid inclusions trapped in microcracks 

within deformed quartz grains in the cataclasite yield 

paleofluid temperatures as high as 350 oc and pressures 

up to 280 MPa, which indicate that this faulted rock 

originated at depths of 11 km or more in the crust in a 

fault zone where transient fluid pressures were nearly 

lithostatic (Parry and Bruhn, 1984, 1986). The formation 

of this high-temperature alteration in the cataclasite 

about 17 m.y. ago is indicated by a K-Ar date on 

hydrothermal muscovite from the fault zone (Parry and 

Bruhn, 1986). The high-temperature mineral alteration 

assemblage was subsequently faulted and overprinted by 

a younger, lower temperature laumontite-prehnite 

assemblage that is also faulted, suggestive of a decrease 

in fluid temperature and pressure during progressive 

uplift and exhumation of the footwall. 

The attitude of the Fort Canyon fault section has been 

determined by detailed mapping of the fault zone along 

the southern margins of the Little Cottonwood stock (fig. 

6). Here Paleocene strata and Oligocene volcanics in the 

hanging wall are faulted directly against the cataclastic 

carapace on the margin of the igneous stock, where the 

fault zone dips 30° to 40° S. Direct measurement of the 

dip of the cataclastic carapace in the footwall can be made 

in Fort Canyon, where there is an abrupt transition from 

essentially undeformed quartz monzonite in the interior 

of the Little Cottonwood stock into overlying phyllonite 

and cataclasite in the fault zone. Here we measured the 

dip as 30° to 35° S., and Gilbert (1928) estimated the dip 

as 29° S. We will use a strike of N. 80° W. and a dip of 35° 

S. as a best estimate for the attitude of the Fort Canyon 

fault section based on the mapping and direct field 

measurements. 

The paleoslip direction on the Fort Canyon fault 

section has been estimated by using the results of the 

paleostress tensor analysis on mesoscopic faults within 

the cataclastic carapace on the southern margin of the 

Little Cottonwood stock (Fort Canyon domain, table 2). 

The slip direction on the fault zone was assumed to 

coincide with the trace of the maximum resolved shear 

stress vector in the plane of the fault zone (Angelier, 

1979). This trace is the projection of the total stress 

vector (8) onto the fault plane, and the orientation of this 

trace is uniquely determined by the orientation of the 

fault, the directions of the principal stresses, and the 

principal stress parameter (<f>) (Angelier, 1979). Knowledge of the actual magnitudes of the principal stresses is 

not required. The range of possible paleoslip vectors in 

the plane of the fault zone was determined graphically on 

a stereonet by using a construction proposed by Angelier 

(1979). If <f>=O, then 82=83, and the slip direction is 

defined by the projected trace of 8 1 on the fault plane. 

Conversely, if <P= 1, then 81 =82 , and the predicted slip 

direction coincides with the trace of 83 on the fault plane. 
H12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

,,'~ 

Cottonwood c,, \ stock 

,-, /)qm~\ ...... -. ,,., ..... ----J' ,' , ,-' ~,. I ('.,,-' 

,---- r ~ ," ,.,""" ,'' -- " 

Little 

~ qm1 ,""\I \ ,'' .. ---- I ('1 }! -Draper '-,._.."" '' \ '-----~...... qm, \ : ', qm, -'l Corner c. \ ..... · ': c'' I I ·~ \, · ...... -. ···-.. ·-···-. q ,I ,.••'' : _,I I ;• --........;I '-·'1''-. ··...t:ltofJ Ls qm2 .. /::···· .. --~~\ /' ' ti -···-... .,... '· "··.-... - ... -··t· .. l·-... -·,, .. / qm2 ,/qm,: \ ,' '' 

' --- --, ,, ,' ,.,' qm2 ;,~ qm3 :: • ---·• '- qm -, ' , ... ,~ ~ '' .. , •'\ .... ' ......... y:. .. - . . .. 2-·· q~3, I, _,c;.~,.\A;'t-T-, v ,' J ~ .. ,. .... , . -. ,..::-.:_ ... ~.. v .. ,___ 's \..adue ' : ,'~ .. ,..... "' q , ---------------- Jacob \ qm ... -, , qm1 .... _ /'-----, ____ .... ' ~7 1--;, .. .,..,. ', .. v qm2 

1 .-·-· ,._ .... ~ - --, --\ ~--T--- phv .... J - EXPLANATION ,_, .. 

( 

I 

/ 

) 

I 

.,_ 

J 

Cataclasite 

8 

Moderately deformed 

quartz monzonite 

,,---... I I 

1.,._,'*"1 

Outcrop 

Contact ----- 0 

I 

0 

1 MILE 

I 

KILOMETER 

........ __ ~/pine 

......... ~ 

8 

Slightly deformed 

quartz monzonite 

-r---

Fault-Ball and bar on hanging wall, dashed 

where buried or inferred 

N 

t 8 /"•'-. ...-. 

Unimproved road 

Phyllonite -~··-~~ ......... 

Stream drainage 

Andesite 

Limestone 

FIGURE 6. -Geology of the Fort Canyon fault zone in the Fort Canyon structural domain shown in figure 1. 

Slip directions for O<cj>< 1lie in the plane of the fault zone 

between these two slip directions (fig. 7). The slip 

direction limits estimated for the Fort Canyon fault zone 

are 24 o /238° for c1> = 0 to 18°/252° for c1> = 1. The paleostress 

tensor inversion estimated that cp=0.2, which defines the 

preferred estimate for the slip direction as 22°/242°. This 

slip direction requires dextral-normal movement on the 

Fort Canyon fault zone. 

Structural fabrics of relic phyllonite preserved in the 

fault zone are also consistent with a component of dextral 

slip. These fabrics predate the cataclastic deformation 

(Parry and Bruhn, 1986) that resulted in development of 

the mesoscopic fault populations used in the stress tensor 

inversion analysis. The oldest fault rock preserved in the 

faulted carapace in Fort Canyon consists of isolated 

outcrops of phyllonite, which contain a penetrative foliation and, in some locales, an extension lineation. 

Locally, these rocks also contain a mesoscopic shear band 

fabric (S-C bands of Simpson and Schmidt (1983)) in 

which the primary foliation is deflected into planar 

millimeter-wide zones of intensive shearing. The sense of 

slip on these S-C bands as measured in outcrop is 

consistent with dextral normal movement on the fault 

zone and slip directions plunging from 20° to 30° to 210° to 

230° (Parry and Bruhn, 1986). Slip directions determined 

from S-C bands and asymmetric augen in oriented thin 

sections from the phyllonite also indicate dextral to 

dextral-normal movement in the fault zone (Houghton, 

1985). 

The southernmost section of the Salt Lake segment of 

the Wasatch fault zone-the Draper section-is marked 

by a discontinuous series of scarps in Quaternary deposits that strike 035° for a distance of about 7 km along the 

western edge of Little Cottonwood stock. The fault zone 

is not exposed in bedrock in this area, although numerous 

discrete normal faults occur in the margins of the stock 

and presumably represent subsidiary normal faulting 

associated with the Wasatch fault zone. We have 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H13 

N junction with the slip direction across the Fort Canyon 

fault zone, is sufficient to constrain a maximum probable 

dip on the Draper section of the Wasatch fault zone. 

The primary faults in the Traverse Mountain barrier 

form a tripartite fault array. We do not have complete 

information concerning the magnitudes of the three slip 

vectors, but we do know the slip direction on the Fort 

Canyon section and require that the slip direction across 

the northern Traverse Mountain fault zone have a normal 

component of movement. This latter criterion constrains 

w E the slip direction on the Draper fault section in that the 

resultant slip vector between the Fort Canyon and the 

Draper fault sections cannot cause convergence across 

the Traverse Mountain fault. Consequently, we seek 

acceptable solutions for slip directions on the Draper 

fault section by assuming various dip angles and applying 

the paleoprincipal stress orientations and a value of 

<!>=0.4 determined from stress tensor inversion of the 

mesoscopic fault populations at the southernmost end of 

the Draper section. 

s 

EXPLANATION 

CD Draper fault section 

0 Fort Canyon fault 

~ Preferred slip lines 

FIGURE 7. -Orientation of the Fort Canyon fault, the preferred 

orientation of the Draper fault section at the southern end of the Salt 

Lake segment, and slip directions for various <1> values of the 

paleostress field. The preferred slip direction on the Draper fault 

section is 240° on a fault plane dipping about 65° NW. Lower 

hemisphere, equal-area projection. See figure 5 for a geometric 

model of these faults in the Traverse Mountain barrier. 

attempted to constrain possible dip angles for this section 

of the fault zone on the basis of the geometry of faulting 

within the Traverse Mountain barrier. The approach is 

based on finding possible slip directions on the Draper 

section that are consistent with slip directions on the 

other two primary fault zones in the barrier-namely, 

the Fort Canyon fault zone and the north-dipping fault 

zone that extends along the northwestern edge of the 

Traverse Mountains. The paleoslip direction on the Fort 

Canyon fault zone has been estimated as about 22°/242°. 

We have no data bearing on the slip direction on the fault 

bounding the northern edge of the Traverse Mountains 

other than the strike of the fault zone, which is approximately N. 60° E. (240°), and drill data and gravity 

modeling indicating that the fault zone dips steeply 

northward with a minimum throw of about 1 km (Cook 

and Berg, 1961; Zoback, 1983). This latter evidence 

indicates that the fault zone has a normal component of 

slip, which requires an unknown component of extension 

perpendicular to strike. This observation, taken in conThis approach is shown graphically in figure 7. The 

maximum dip angle (&) on the Draper fault section that 

meets the criterion for extension across the fault zone 

along the northern edge of the Traverse Mountains is 65° 

(fig. 5). In this case, the difference vector between the 

Fort Canyon and the Draper fault sections is parallel to 

the trend of the fault zone along the northern edge of 

Traverse Mountain. Lesser dip angles on the Draper 

section are also satisfactory in that the difference vector 

across the Traverse Mountain fault zone requires extension. Steeper dips on the Draper fault section require 

convergence across the northern edge of the Traverse 

Mountains and are therefore unacceptable solutions to 

the slip direction analysis. Further refinement of the dip 

estimate of the Draper fault section would require more 

information on the dip angle and slip vector in the fault 

zone along the northern edge of the Traverse Mountains. 

The structural model for the Traverse Mountains 

barrier consists of a triple junction formed by a tripartite 

array of primary fault zones (fig. 5). The northern arm, 

consisting of the Draper fault section, dips approximately 65° or less toward the west-northwest; the eastern arm, comprised of the Fort Canyon fault section (or 

western end of the Deer Creek fault), dips approximately 

35° S.; the western arm dips steeply northward. The 

difference in slip between the Draper and the Fort 

Canyon fault sections is presumably accommodated by 

sinistral-normal faulting on this third arm of the triple 

junction. The barrier must plunge toward the southwest 

into the crust, presumably paralleling the intersection 

line between the Draper and the Fort Canyon fault 

sections. This line plunges approximately 28° to 230°, 

subparallel to the strike (240°) of the third arm of the 

triple junction. This structure is clearly a nonconserva-
H14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

tive barrier and represents a likely site for the arrest and 

(or) initiation of seismic ruptures within the Wasatch 

normal fault zone. 

0 

0 5 MILES 

0 5 KILOMETERS 

\ 

\ 

\ 

-'\ 

\ 

G))Q (~ v 

0 / 

FIGURE 8. -Surface fault traces in the Salt Lake segment and preferred dip angles based on geometric modeling. Numbers of barriers 

are the same as those in figure 2 and table 1. Solid lines indicate 

normal faults (ball and bar on hanging wall, dashed where inferred or 

buried). Large arrows indicate inferred directions of extension. See 

text for discussion. 

SOUTHERN FAULT SECTIONS 

The surface trace of the Wasatch fault zone consists of 

three rectilinear sections between the Traverse Mountain barrier (no. 1) and the bifurcation of the fault trace 

at branch point 4 (fig. 2). These rectilinear sections are 

separated by two bends (branch points 2 and 3, fig. 2) 

that mark significant changes in the strike of the fault 

zone. These bends are classified as conservative barriers 

(table 2) because there is no apparent development of a 

major third direction of faulting in either the hanging 

walls or the footwalls of the fault zone in the regions of 

the bends. This absence implies that the slip direction on 

the Draper fault section parallels the slip directions on 

the two sections to the north and that the fault zone in 

this area can be modeled as a cylindrical surface. Consequently, one can use the dip estimate and slip direction 

(approximately 240° trend) on the Draper fault section to 

model the dip of these two adjacent sections. The intersection lines between the fault sections must parallel the 

plunge and trends of the slip vector in the Draper fault 

section. This problem can be solved on a stereonet by 

using a geometric construction, which results in estimated dip angles of 55° for the fault section between 

barriers 2 and 3 and 45° for the section between barriers 

3 and 4 (fig. 8). These solutions are directly dependent on 

our estimate of the dip of the Draper fault section. If the 

65° dip estimate in this section is too great, then the 

remaining sections would have more gentle dips; conversely, if the estimate for the Draper section is too low, 

the other sections would dip more steeply. A similar type 

of error would be introduced by the estimated slip 

direction in the Draper fault section, in that too shallow 

a plunge would result in underestimating the dip angle of 

the adjacent fault sections and too steep a plunge would 

cause an overestimation of the fault dip angles. 

An unreversed seismic refraction-reflection profile 

across the fault zone in the section between barriers 2 

and 3 by Bashore and others (1981) provides an independent check on our fault zone modeling in this area. 

The fault zone was modeled by using two-dimensional 

synthetic seismograms and gravity data as dipping about 

60° W. near the surface and decreasing in dip to between 

40° and 50° at a depth of 4 to 5 km. Their result is 

generally consistent with the 55° westward dip in our 

model, which is based on a totally different approach and 

a different set of data. 

NORTHERN FAULT SECTIONS 

The geometry of the northern half of the Salt Lake 

fault segment is different than that of the southern part, 

where the fault zone is delineated by a narrow zone of 

surface scarps along the Wasatch mountain front. The 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H15 

northern part of the segment, in contrast to the southern 

part, is characterized by a major bifurcation of the 

Quaternary fault trace into a zone about 12 km long by 6 

km wide (fig. 2) (Marsell, 1969; Van Horn, 1972) beginning at branch point 4 . This zone extends northward and 

terminates in the Salt Lake salient, a bedrock ridge that 

separates the proposed Ogden rupture segment to the 

north from the Salt Lake segment to the south (Schwartz 

and Coppersmith, 1984). Two zones of surface scarps 

delineate an eastern branch extending along the mountain front for 10 km north of the branch point and a 

western branch that extends about 15 km northward 

until it curves abruptly eastward and intersects the 

mountain front at the southern end of the Salt Lake 

salient (bend 8). Each of these branches is composed of 

discontinuous, mostly linear zones of surface scarps 

separated by bends in the fault-zone trace (figs. 2, 8). 

According to Van Horn (1972), surface scarps in the 

eastern branch are older than most of those in the 

western branch. The eastern scarps presumably formed 

between 5,000 years and 3 m.y. ago, whereas most 

scarps in the western branch formed during the last 

5,000 years. Greater activity on the western branch of 

the fault is also indicated by the distribution of Quaternary sediments, which thicken abruptly westward across 

the western branch of the fault zone, whereas the area 

in between the two branches is underlain by only a thin 

veneer of Quaternary deposits overlying bedrock 

(Marsell, 1969; Crone and Harding, 1985). 

The zone of bifurcation in the fault zone between 

branch point 4 and bends 5a and 5b apparently represents a nonconservative barrier. There are two lines of 

evidence for this conclusion. First, this region lies along 

a major east-northeast-trending basement ridge beneath 

Jordan Valley (Cook and Berg, 1961; Zoback, 1983). This 

ridge separates the Jordan graben into a deeper southern 

part and a shallower northern half, according to the 

gravity modeling of Zoback (1983). Presumably, this 

ridge reflects structural accommodation of the hanging 

wall to changes in slip vector direction and (or) magnitude in the vicinity of branch point 4. Second, the fault 

section between bends 5a and 5b dips steeply northward 

and trends between 240° and 245°, subparallel to the 240° 

average slip vector for the Salt Lake segment as a whole. 

A north-dipping fault section having this strike cannot 

link with the fault section immediately to the south and 

conserve a slip vector orientation of 240° around bend 5a. 

Consequently, bend 5a must be a nonconservative barrier. 

The dips of the fault sections between branch point 4 

and bend 9 along the western branch of surface faulting 

have been estimated by assuming that each of these 

bends represents a conservative barrier. The only support for this assumption is that no major structures have 

N 

w ~s, E 

s 

FIGURE 9. -North-striking faults having dips (8) between 20° and 80° 

W. and slip lines for the paleostress field in the Red Butte structural 

domain (fig. 1, table 2). Slip directions for <1>=0.0, 0.5, and 1.0 are 

defined by intersections of great circles defining fault planes having 

lines of constant <j>. The best estimate for slip in a 240° direction is a 

fault plane dipping 50°. See text for discussion. 

been inferred from the gravity data in Jordan Valley 

between the basement ridge near branch point 4 and the 

Salt Lake salient barrier (Cook and Berg, 1961; Zoback, 

1983). The preferred dip of the north-trending section 

between bends 7 and 8 was first found by assuming a slip 

vector trending 240° and using the paleostress data from 

the Red Butte domain, where <t>=0.54. The solution for 

the preferred fault plane dip is shown in the stereoplot of 

figure 9, where the orientation of the maximum shear 

stress vector for this paleostress system is plotted as a 

series of three curves for <t>=O, 0.5, and 1.0 on faults 

dipping between 20° and 80° W. The 240° slip line does 

not intersect the <t>=0.5 shear stress vector curve but lies 

a few degrees to the north of it at the closest point of 

approach. A fault plane dipping 50° W. lies closest to this 

maximum resolved shear stress vector; therefore, 8=50° 

is the preferred dip estimate for the north-dipping fault 

section. The dip angles of the remaining fault sections in 

the western branch of surface rupturing were then found 

by assuming a cylindrical fault-zone surface. These 

angles vary between 45° and 72°, as figure 8 shows. 

SALT LAKE SALIENT 

The Salt Lake salient is a ridge of Paleozoic and 

Tertiary bedrock that extends west of the Wasatch 
H16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Front at the northern end of the Salt Lake rupture 

segment (Gilbert, 1928; Schwartz and Coppersmith, 

1984). A large west-dipping normal fault having a total 

throw of about 1.5 km forms the eastern boundary of the 

salient, where Tertiary strata are faulted against Precambrian metamorphic and Paleozoic sedimentary rock 

in the footwall (fig. 2) (Gilbert, 1928). The western 

boundary of the salient is formed by the Warm Springs 

fault, a north-trending normal fault that dips approximately 70° W. (Gilbert, 1928). Quaternary strata in the 

hanging wall are faulted directly against Paleozoic rocks 

in the footwall of the normal fault. We have not mapped 

the internal structure of the salient in this study, but 

1:24,000-scale mapping by Van Horn (1981) shows a 

number of north-northwest- and northeast-trending normal faults that postdate Oligocene deposits and were 

probably formed during the late Tertiary. 

The geometry of linkage between the main rupture 

zones in the Salt Lake segment and faults in the interior 

of the Salt Lake salient is not clear. Surface scarps at the 

southern margin of the salient are discontinuous but 

apparently extend into the large normal fault along the 

eastern boundary of the segment. There is no reported 

evidence for Quaternary movement on this fault in the 

interior of the salient, so, presumably, the Quaternary 

ruptures have not reactivated most of this fault. The 

Virginia Street fault trends westward along the southern 

margin of the salient and conceivably represents a bifurcation of the rupture zone, in which displacement is 

transferred onto the Warm Springs fault at the western 

boundary of the salient (Pavlis and Smith, 1980; Zoback, 

1983). 

Gravity and drill data from the valley west of the Salt 

Lake salient indicate that the salient forms part of a 

major, west-trending basement ridge and fault zone that 

extends across the entire width of the valley (Cook and 

Berg, 1961; Zoback, 1983). This basement ridge is 

reflected in an elongate gravity high at the northern end 

of Jordan Valley and by a significant change in the 

thickness of Tertiary valley fill indicated by drill data and 

gravity modeling. The fill in the northern end of Jordan 

Valley is only several hundred meters thick, whereas the 

fill is in excess of 1 km immediately north of the 

basement ridge, according to Zoback (1983). 

The structure of the Salt Lake salient indicates that it 

marks the site of a large nonconservative barrier in the 

Wasatch fault zone. The salient is marked by three 

primary fault systems-the Salt Lake rupture segment 

on the south, the Ogden rupture segment to the north, 

and inferred west-trending fault zone associated with the 

gravity saddle and modeled basement ridge at the northern end of Jordan Valley. The structure of the Ogden 

fault segment has not been mapped in this study, but the 

ohzervation that the Tertiary valley fill is thicker immediately north of the salient relative to the area to the 

south indicates that throw across the Ogden segment is 

greater than that in the northern part of the Salt Lake 

segment. This difference could be owing to several 

factors, the most likely being (1) that the dip on the 

Ogden fault segment is steeper than that on the adjacent 

Salt Lake segment or (2) that extension is greater across 

the Ogden segment. The resolution of this problem 

requires further mapping and detailed gravity studies in 

the Salt Lake salient and the southern part of the Ogden 

fault segment. 

DISCUSSION 

GEOMETRY OF FAULT SECTIONS 

Our modeling of dips for fault sections in the Salt Lake 

segment is based on the premise that traces of surface 

ruptures forming linear to slightly curvilinear patterns 3 

km or more in length reflect the strike of fault sections in 

the subsurface. This premise is supported by (1) the 

physiography of the Wasatch Front and (2) the overall 

shape of gravity contours in the l:!,$tnging-wall basin, both 

of which partly mimic the geometric pattern of the fault 

zone (Cook and Berg, 1961; Zoback, 1983). 

The estimated dip angles are steeper than those 

reported by Gilbert (1928), according to measurements 

of bedrock fault exposures on the southwestern edge of 

the Little Cottonwood stock at the southern end of our 

Draper structural domain (fig. 1) and at Mill Creek 

Canyon in the Olympus Cove domain. Gilbert estimated 

the dip of the fault zone as 35° in the Draper domain, and 

we agree that the cataclastic carapace in the fault zone 

dips 35° to 45° SW. in the same area. However, because 

this area is located in the curvilinear zone of intersection 

between the Fort Canyon and the Draper fault sections, 

it is not representative of the main part of the northeasttrending Draper fault section. Our work predicts a 20° to 

30° SW. dip at this locale because of the intersection of 

the Fort Canyon fault, which dips 35° S., and the Draper 

section, which dips approximately 65° NW. or less (fig. 

5). 

We have not estimated the dip of the eastern rupture 

zone in the Olympus Cove domain because of the nonconservative barrier at bends 5a and 5b. We prefer a dip 

of about 45° to 50° (fig. 8) for the fault section about 6 km 

west of the mountain front. This dip is approximately 10° 

greater than the dip measured in the bedrock by Gilbert 

(1928). 

Fault scarps in Quaternary deposits, which define 

almost all of the rupture traces in the Salt Lake segment, 

dip steeply between 60° and 70° (Gilbert, 1928). Our 

model of the fault zone predicts that these dips do not 

extend into bedrock at depth in some of the fault sections 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H17 

but are probably confined to the surficial deposits. This 

conclusion is similar to the conclusion reached by 

Bashore and others (1981) on the basis of their seismic 

reflection-refraction experiment in the southern part of 

the fault segment. We do not have any evidence as to 

whether the fault zone is listric or remains planar at 

depth. 

DIRECTIONAL CHARACTERISTICS OF RUPTURE 

PROPAGATION 

Predicting the direction of rupture propagation during 

a large earthquake in the Salt Lake segment is an 

important goal in earthquake hazards mitigation. If the 

position of rupture nucleation within the fault segment 

can be predicted, this region should be selectively monitored for precursory phenomena. Also, the distribution 

of strong ground motion during a large earthquake 

depends on the direction of rupture propagation, which is 

in turn related to the position of rupture nucleation in the 

fault segment. 

Three types of evidence need to be considered in 

discussing the possible directions of large-earthquake 

rupture propagation within the Salt Lake segmentlong-term deformation rates, the dimensions of nonconservative barriers, and the overall geometry of Quaternary rupture traces. Here we focus our attention only on 

large earthquakes of approximately M 7.0 to 7.5, the 

"characteristic earthquakes" defined by Schwartz and 

Coppersmith (1984), which would presumably rupture 

most, if not all, of the segment during a single event. We 

do not consider the effects of large-earthquake ruptures 

that could originate in adjacent segments and propagate 

into the Salt Lake segment. These types of events are 

beyond the scope of this study. 

Large kilometer-scale variations in total displacement 

along the strike of a normal fault zone provide evidence 

that average rates of deformation have varied systematically along the length of the zone over extended periods 

of time. The rationale for relating long-term deformation 

rates to rupture nucleation is as follows. An increase in 

deformation rate within a fault zone raises ambient shear 

stress levels and enhances the transition from quasiplastic to cataclastic (frictional) deformation. This latter 

effect may increase the permeability of the fault zone and 

lead to greater fluid access. Increased shear stress levels 

and the chemical and physical effects of fluids enhance 

the nucleation and growth of quasi-static ruptures, which 

may in turn become unstable and trigger earthquakes 

(Das and Scholz, 1981). Consequently, we suggest that 

large variations in displacement along the strike of a 

rupture segment are one criterion for identifying potential sites of rupture nucleation. This criterion is not 

necessarily valid for predicting the site of rupture nucleation for each large earthquake but must be used in a 

long-term statistical sense. That is, we can assume that 

the greatest frequency of nucleation events, when considered over a large number of M 7.0 to 7.5 earthquakes, 

is likely to occur in the area where long-term rates of 

deformation are greatest. 

Differential displacement data from the footwall of the 

Salt Lake segment indicate that the greatest long-term 

deformation rate has occurred at the southern end of the 

segment. The amount of net slip across the Salt Lake 

segment is difficult to quantify because Paleozoic and 

Mesozoic rocks in the hanging wall are buried beneath 

upper Cenozoic sedimentary deposits in Jordan Valley 

(Cook and Berg, 1961; Mattick, 1970; Zoback, 1983). 

However, there is evidence that the footwall has been 

warped on a regional scale-a minimum of 5 km and 

perhaps as much as about 9 km of uplift of the southern 

end of the footwall with respect to the northern end. The 

evidence is as follows. The unconformity at the base of 

the Paleocene-Eocene Wasatch Formation in the northern part of the segment provides a geologic datum; this 

unconformity marked the surface during Paleocene to 

Eocene time. This formation is in turn overlain by the 

Norwood Tuff, Oligocene volcaniclastic deposits that 

were coeval with emplacement of the Little Cottonwood 

and Alta stocks at the southern end of the Salt Lake 

segment (Hintze, 1973). Our studies of fluid inclusions 

(Parry and Bruhn, 1986) and geobarometry on Oligocene 

skarn deposits on the margins of the Alta stock (Parry 

and Bruhn, 1987) in the southern part of the footwall 

indicate that fault rock now exposed at an elevation of 

about 1,515 m originated at depths of approximately 11 

km, whereas the skarns at an elevation of 3 km formed at 

depths of approximately 5 km. The fact that the skarn 

deposits are now at the same elevation as the projected 

base of the Oligocene tuffaceous deposits (Norwood Tuff) 

at the northern end of the segment indicates differential 

warping of the footwall, the southern end being uplifted 

a minimum of 5 km with respect to the northern end. The 

maximum value of approximately 9 km is obtained by 

subtracting the elevation of the Eocene-Oligocene unconformity (approximately 3 km) from the estimated depth 

of origin of the fault rock (approximately 11 km) now at 

an elevation of 1,515 m. 

Fission-track and K-Ar dating of the uplift history and 

time of origin of fault rock at the southern end of the 

segment indicate that faulting had begun by about 17 

m.y. ago. The differential uplift rate between the southern and the northern ends of the footwall is approximately 0.3 to 0.5 mm/yr averaged over this time interval. 

In fact, the fission-track dating of Evans and others 

(1985) suggests that the greatest rates of footwall uplift 

occurred in the last 10 m.y. Notably, the long-term 

average differential uplift or "north-south warping" rate 
H18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

of the footwall is a significant fraction of estimated net 

slip rates across the fault zone. Trenching studies done 

by Swan and others (1980) indicate that these latter rates 

were between 0.6 and 1.4 mm/yr during the last 19,000 

years. Clearly, the southern end of the segment meets 

our criterion as a probable site for repetitive rupture 

nucleation on the basis of its long-term differential 

displacement rates. 

The dimensions of nonconservative barriers within the 

fault segment may be an additional factor in controlling 

the direction of rupture propagation during large earthquakes (M approximately 7.0-7.5 ). King and Yielding 

(1984) suggested that fracture toughness is a geometric 

property of the fault zone and can be crudely estimated 

by considering the barrier as a zone of plastic deformation. If several nonconservative barriers are present in a 

fault zone, rupture nucleation may be favored in the 

barrier having the least fracture toughness. 

The length of a plastic deformation zone can be estimated for an ideal elastic-plastic material from simple 

fracture mechanics considerations. The length (Db) of the 

plastic deformation zone at the tip of a mode III (antiplane shear) crack is given by the Bilby-Cottrell-Swinden 

model 

(1) 

where I(' is fracture toughness and Sy is plastic yield 

stress (Knott, 1973, p. 70-71). The fracture toughness 

(K') is a stress intensity factor reflecting the stress 

concentration at the tip of a mode III rupture owing to 

(1) the regional stress field and (2) crack size. I(' is that 

stress intensity factor at which the crack can undergo 

self-extension in the zone of plastic deformation at its tip. 

K increases with increased plastic zone length (Db). The 

plastic deformation zone effectively "blunts" the tip of 

the propagating crack and thus reduces the stress concentration below the concentration for an identical crack 

having no plastic tip zone. 

King and Yielding (1984) suggested that nonconservative barriers act as regions of plastic deformation in fault 

zones and applied a formula like equation 1 to a study of 

rupture propagation in the Al Asnam fault zone of 

Algeria. Here plastic deformation is used in only an 

approximate sense and implies distortion of a volume of 

material by contemporaneous movement on a population 

of broadly distributed fault planes that mutually interfere and interlock. In this sense, a barrier is considered 

mechanically equivalent to a plastic yield zone in an ideal 

elastic-plastic material. 

The magnitude Sy of the yield stress in equation 1 is 

unknown. King and Yielding (1984) used a maximum 

value of 200 MPa, a typical shear strength of intact rock, 

and a minimum of 20 MPa, a nominal stress drop for large 

earthquakes in their study of the Al Asnam earthquake 

fault zone. Two large earthquakes within the Rocky 

Mountain region have recently occurred in fault segments exhibiting geometries and dimensions similar to 

those of the Salt Lake segment. The Ms 7.5 Hebgen 

Lake, Mont., earthquake had a stress drop of approximately 11 MPa, whereas the the stress drop estimated 

for the Borah Peak, Idaho, earthquake (Ms 7.3) was 

approximately 1 MPa (Doser, 1985). These stress drops 

are significantly lower than the nominal values used by 

King and Yielding (1984). 

Here we tentatively evade the problem of the magnitude of Sy by comparing the relative fracture toughnesses of any two nonconservative barriers. The relationship, derived directly from equation 1, is simply 

(2) 

where the numerical subscripts refer to barriers 1 and 2. 

Using equation 2 implies that the yield stress (Sy) is the 

same in all nonconservative barriers within the fault 

zone. This assumption is partly supported by rock friction experiments, where the static coefficient of sliding 

friction is essentially constant over a large range of 

temperatures, strain rates, and effective confining pressures for rocks of many different lithologies (Byerlee, 

1967). The situation is similar in a large barrier, where 

highly fractured rocks of different lithologies extend to 

depth within the fault zone. However, other factors, such 

as the geometry of the fault population and fluid pressures and chemistry, may differ significantly between 

barriers and strongly influence relative yield stresses. At 

present, we do not have enough data to determine the 

differences between the overall fault population geometries of the various barriers or whether major differences 

in fluid pressure or chemistry may occur. These features 

of barriers are important parameters for further 

research. 

Three large nonconservative barriers have been identified within the Salt Lake segment (1,9, and the composite barrier at 4). The lengths of these barriers can be 

determined from geologic mapping (fig. 2). Db for the 

Traverse Mountain barrier is approximately 8 km, 

whereas Db for the Salt Lake salient barrier is approximately 10 km (fig. 10). The difference between the 

relative fracture toughness of the Salt Lake barrier and 

that of the Traverse Mountain barrier based on equation 

2 is only 10 percent, and, in our opinion, it is not at all 

certain that such a small difference is meaningful, given 

the assumptions required in using equation 2. 

The structural characteristics of the northern half of 

the Salt Lake segment, between the Salt Lake salient 

and branch point 4, can be interpreted in two fundamentally different ways. The zone of branching between 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H19 

0 

I 

0 

N 

t 

I 

5 

5 

I 

' 

I 

10 

• • 

• • 

• • 

. • 

Db=4km 

10 MILES 

I 

I 

15 KILOMETERS 

FIGURE 10.-Trace of the Salt Lake fault segment and estimated 

dimensions Db of barriers modeled as equivalent "plastic" deformation zones. Solid lines indicate normal faults (ball and bar on hanging 

wall, dashed where inferred or buried). 

branch point 4 and bends 6 on the west and 5b on the east 

may represent a separate nonconservative barrier. The 

size of this barrier is estimated by assuming that the 

barrier can be approximated as a right circular cylinder 

having an axis dipping 45° W. The estimated diameter of 

this cylinder is 4 km and is taken to be the effective 

length (Db) of the nonconservative barrier. The relative 

fracture toughness of this barrier is approximately 60 

percent less than that of the Salt Lake salient and 

approximately 40 percent less than that of the Traverse 

Mountain barrier. Consequently, in this interpretation, 

the barrier in the center of the segment has the least 

fracture toughness, and, according to the model of King 

and Yielding (1984), a large-earthquake rupture is most 

likely to nucleate from the central part of the segment. 

The alternative interpretation of the mechanical characteristics is that the entire northern half of the segment 

(north of branch point 4) may act as a large nonconservative barrier having length Db of approximately 20 to 24 

km (fig. 10). This interpretation is based on speculation 

about the geometric and mechanical characteristics of the 

region between the two primary fault traces north of the 

branch point (4). Mapping in the footwall of the fault zone 

shows that the footwall is extensively faulted for a 

distance of about 1 to 2 km from the trace of the primary 

fault zone (fig. 11). These faults form a conjugate system 

of east- and west-dipping normal to oblique-slip faults. 

We suspect that the region between the two primary 

fault zones in the northern half of the segment is pervaded by conjugate fault systems like those exhumed in 

the footwall. This region may then approximate a large 

"plastic" deformation zone comprised of numerous interlocking and mutually interfering conjugate faults. Two 

schematic cross sections of this zone are shown in figure 

12. In one, the two primary fault traces are shown as 

parallel fault planes, whereas, in the other, the two 

primary fault planes intersect at depth. At present, 

there is insufficient evidence to choose one section over 

the other. In either case, the zone of highly faulted rock 

between the two primary faults presumably forms an 

elongate plastic deformation zone. When this zone is 

combined with the Salt Lake salient immediately to the 

north, it has a length Db of approximately 24 km, much 

larger than the Db length of 8 km estimated for the 

Traverse Mountain barrier. The large length of the 

former barrier invalidates the small tip zone approximation inherent in the derivation of equation 1, and, therefore, equation 2 cannot be used to estimate the relative 

fracture toughnesses of this barrier and others. Clearly, 

the fracture toughness of the northern half of the fault 

segment is greater than that of the Traverse Mountain 

barrier. This interpretation of the mechanical characteristics of the Salt Lake segment implies that large-earth-
H20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

;t(54 

0 

EXPLANATION 

Normal fault-Bar and ball on downthrown side; 

dashed where concealed 

Strike and dip of bedding 

N 

t 

1 MILE 

0 1 KILOMETER 

CONTOUR INTERVAL=200 FEET 

A 

I 

I 

I 

I 

I 0 I 2000 FEET 

\ I 

I 0 250 500 METERS 

'\ NO VERTICAL EXAGGERATION 

, 

B 

FIGURE 11. -A, Normal faults in the northern half of the Red Butte structural domain (fig. 1). B, Cross section B-B' illustrating 

the conjugate geometry of faulting in the footwall. Dashed and queried lines indicate projected positions in the subsurface. 

quake ruptures are most likely to nucleate at the southern end of the segment and propagate northward. 

The geometries of two other seismogenic fault zones in 

the Basin and Range-Rocky Mountain transition zone 

support the contention that geologic structure may provide information about directions of rupture propagation 

during large earthquakes. Ruptures in the 1959 Hebgen 

Lake (M8 7.5) and the 1983 Borah Peak (M8 7.3) earthquakes propagated unilaterally in both events, rupturing 

toward points of structural bifurcation in the fault zone 

(Smith and others, 1984; Doser, 1985a, b) (fig. 13). At 

Borah Peak, the bifurcation zone was marked by a major 

bedrock ridge that extends northwest of the main mountain front (Crone and Machette, 1984), whereas the fault 

zone at Hebgen Lake bifurcated into two normal fault 

zones, one extending well into the main mountain range 

and the other paralleling and locally cutting into the main 

mountain front along the edge of the hanging-wall valley 

(Myers and Hamilton, 1964). The structural geometries 

of both fault zones clearly developed over an extended 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H21 

A 

en 

a: 

w 

1-

10 

0 

~ 5 

0 

..J 

Q 

10 

8 

WEST 

0 

0 

WEST 

FIGURE 12.-Possible structure of the branched fault zone in the 

northern half of the Salt Lake segment. Conjugate faults like 

those in figure 11 are interpreted to pervade the region between 

the eastern and western primary fault planes. A, The two 

EAST 

5 MILES 

5 KILOMETERS 

EAST 

primary fault branches are assumed to dip at the same angle. B, 

The two primary fault branches are assumed to intersect at a 

depth of several kilometers. Faults labeled Rand R' are Riedel 

and anti-Riedel shears, respectively. 
H22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

HEBGEN LAKE, 

MONTANA 

BORAH PEAK, 

IDAHO 

SALT LAKE, 

UTAH 

--==- ,, ,, 

' ' I 

\ 

N 

1 ""--....... ~ 

-\ I 

\ 

\ 

\ I 

'\\:-.. \ I \ I 

'( 

\ 

'\ 

I 

~i fl \ \ 

' 

0 

0 

' '\ 

FIGURE 13. -Rupture traces of the Borah Peak, Idaho, and Hebgen 

Lake, Mont., earthquake rupture zones in comparison with the 

Quaternary trace of the Salt Lake rupture segment. Solid lines 

indicate normal faults (ball and bar on hanging wall, dashed where 

inferred or buried). The large arrows indicate the directions of 

? 

11' 

5 MILES 

5 KILOMETERS 

I 

I 

\ 

\ 

\ 

\ 

\ 

) 

, ... ... -

\ 

\ 

\ 

' \ 

\ 

\ 

unilateral rupture propagation for the Borah Peak and Hebgen Lake 

events. The question mark by the arrow on the Salt Lake rupture 

zone indicates the inferred rupture direction during most earthquakes. The Borah Peak rupture trace is after Crone and Machette 

(1984); the Hebgen Lake trace is after Myers and Hamilton (1964). 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H23 

period of time and presumably reflect the summation of 

numerous large earthquakes. 

The branchlike structures of the Borah Peak and 

Hebgen Lake earthquake fault zones are characteristic 

of bifurcated cracks in experiments on crack branching. 

That is, the direction of propagation in experiments is 

toward the direction of bifurcation (Broek, 1983), implying that the geometric characteristics of an earthquake 

fault zone developed as the result of numerous rupturing 

events and may reflect a consistent directionality of 

rupture propagation. This rationale is particularly applicable to normal fault zones, where the hanging walls and 

footwalls remain essentially stationary along the strike of 

the fault zone during numerous rupturing events. In 

strike-slip zones, on the other hand, such evidence is 

likely to be destroyed as the opposite walls of the fault 

zone are translated into adjacent regions of changing 

rupture characteristics. 

The overall geologic structure of the Salt Lake segment is comparable to the structures of the Borah Peak 

and Hebgen Lake earthquake fault zones in that it also 

contains a bifurcation point, where the fault zone divides 

into two primary branches. However, caution must be 

taken in making direct comparisons, because there currently is no reported evidence for simultaneous rupturing of both fault branches in the northern half of the Salt 

Lake segment (Van Horn, 1975). That is, there is a 

possibility that the eastern branch, which exhibits scarps 

between 5,000 years and 3 m.y. in age, may have become 

inactive, and earthquake rupturing may now be confined 

to the western branch, which displays the youngest 

scarps. Alternatively, slip may have occurred at depth on 

the eastern fault branch during the youngest earth-

. quakes without propagation of the rupture to the surface. 

Two fundamentally different, but equally important, 

views of the structural comparison between the Borah 

Peak, Hebgen Lake, and Salt Lake rupture segments 

arise from the previous discussion. On one hand, the 

bifurcated geometry of the Salt Lake segment may still 

control seismic ruptures, even though the youngest 

scarps are located on only one branch. In this case, the 

geometries of the Borah Peak and Hebgen Lake earthquake scarp systems may provide direct analogies for the 

expected directionality of rupturing within the Salt Lake 

segment. Notably, the direction of inferred rupture 

would agree with the direction implied by arguments 

concerning (1) long-term displacements and (2) the effective fracture toughness model, in which the bifurcated 

northern half of the fault segment is considered as a large 

plastic deformation zone. On the other hand, the fact that 

the youngest surface scarps apparently occur only on the 

western fault branch of the Salt Lake segment may 

indicate that the bifurcated fault geometry has been 

bypassed during the Holocene, as the fault zone continued to evolve structurally. In this case, comparative 

studies of the three earthquake fault zones may further 

our knowledge of barrier evolution within seismically 

active normal fault zones, an important topic in itself. 

CONCLUSIONS 

The Salt Lake rupture segment (Schwartz and Coppersmith, 1984) is composed of several linear to slightly 

curvilinear fault sections that intersect in geometric 

barriers defined by bends and branch ·points in the trace 

of the fault zone. The dips of these fault sections vary 

with the strike of the fault zone, estimated dips ranging 

between 45° and 90°. 

Extension across the normal fault zone was to the 

southwest, averaging approximately 240°. This extension direction superimposed a component of sinistral slip 

on individual sections of the rupture segment; this slip 

varied as a function of the strike and dip of each fault 

section. The stress parameter ( <1>) varied from a low of 

approximately 0.2 in the Fort Canyon fault section at the 

Traverse Mountain barrier to approximately 0. 5 in the 

remainder of the rupture segment. 

The Salt Lake segment is bounded on its northern and 

southern ends by two large nonconservative barriers. A 

third nonconservative barrier occurs in the central part 

of the segment, located at the point of bifurcation in the 

trace of surface ruptures. The mechanical characteristics 

of this third barrier may be interpreted in two ways. The 

barrier can be considered as the smallest nonconservative barrier in the segment, possessing the corresponding least effective fracture toughness (K). Alternatively, the barrier can be viewed as part of a much larger 

elongate zone of effective "plastic" deformation comprising the entire northern bifurcated half of the rupture 

segment. In this case, the Traverse Mountain barrier, at 

the southern end of the segment, would be the nonconservative barrier having the least K'. 

Long-term deformation rates have been highest at the 

southern end of the Salt Lake segment and thus provide 

a rationale for selecting this part of the rupture segment 

as the most likely site for repetitive initiation of largeearthquake (M approximately 7.0 to 7.5) ruptures. This 

hypothesis is consistent with the second interpretation of 

fault zone fracture toughness and, possibly, with 

observed directions of rupture propagation in the Borah 

Peak and Hebgen Lake earthquakes. These earthquakes 

occurred in fault zones displaying overall bifurcated 

geometries similar to the geometry of the Salt Lake 

segment, although the latest earthquakes in the Salt 

Lake segment apparently did not produce surface scarps 

on both primary fault branches, as those at Borah Peak 
H24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

and Hebgen Lake did. Whether this absence represents 

a fundamental difference in the mechanical characteristics of the Salt Lake rupture segment relative to those of 

the Borah Peak and Hebgen Lake ruptures remains an 

important and fundamental problem. 

REFERENCES CITED 

Aki, K., 1979, Characteristics of barriers on earthquake faults: Journal 

of Geophysical Research, v. 84, no. B11, p. 6140-6148. 

Angelier, J., 1979, Determination of the mean principal directions of 

stresses for a given fault population: Tectonophysics, v. 56, p. 

T17-T26. 

Angelier, J., Tarantola, A., Valette, B., and Manoussis, S., 1982, 

Inversion of field data in fault tectonics to obtain the regional 

stress, pt. I, Single phase fault populations: A new method of 

computing the stress tensor: Geophysical Journal of the Royal 

Astronomical Society, v. 69, p. 607-621. 

Arabasz, W.J., 1985, Earthquake behavior in the Wasatch front area: 

Association wi~h geologic structure, space-time occurrence and 

stress state, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, p. 310-339. 

Arabasz, W.J., and Julander, D.R., 1986, Geometry of seismically 

active faults and crustal deformation within the Basin and RangeColorado Plateau transition in Utah: Geological Society of America 

Special Paper 208, p. 43-7 4. 

Arabasz, W.J., Smith, R.B., and Richins, W.D., 1980, Earthquake 

studies along the Wasatch front, Utah: Network monitoring, 

seismicity, and seismic hazards: Bulletin of the Seismological 

Society of America, v. 70, p. 1479-1500. 

Baker, A.A., and Crittenden, M.D., Jr., 1961, Geology of the Timpanogos Cave quadrangle, Utah: U.S. Geological Survey Geologic 

Quadrangle Map GQ-132, scale 1:24,000. 

Bashore, W.M., Smith, R.B., Zandt, G., and Ansorge, J., 1981, Upper 

crustal structure of the Salt Lake Valley and the Wasatch Front 

from seismic modelling: EEBS, Transactions of the American Geophysical Union, v. 62, p. 961. 

Broek, D., 1983, Elementary engineering fracture mechanics: Boston, 

Martinus Nijhoff, 469 p. 

Bullock, R.L., 1954, The geology of Lehi quadrangle, Utah: Provo, 

Utah, Brigham Young University, unpublished M.Sci. thesis, 57 p. 

Byerlee, J.D., 1968, Brittle-ductile transition in rocks: Journal of 

Geophysical Research, v. 73, no. B14, p. 4741-4750. 

Cook, K.L., and Berg, J.W., Jr., 1961, Regional gravity survey along 

the central and southern Wasatch Front, Utah: U.S. Geological 

Survey Professional Paper 316-E, p. E75-E89. 

Crittenden, M.D., Jr., 1976, Stratigraphic and structural setting of the 

Cottonwood area, Utah, in Hill, J.G., ed., Geology of the Cordilleran hingeline: Denver, Colo., Rocky Mountain Association of 

Geologists, p. 281-317. 

Crittenden, M.D., Jr., Stuckless, J.S., Kistler, R.W., and Stern, T.W., 

1973, Radiometric dating of intrusive rocks in the Cottonwood 

area, Utah: U.S. Geological Survey Journal of Research, v. 1, p. 

173-178. 

Crone, A.J., and Harding, S.T., 1985, Near-surface faulting associated 

with Holocene fault scarps, Wasatch fault zone, Utah: A preliminary report, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, p. 241-268. 

Crone, A.J., and Machette, M.N., 1984, Surface faulting accompanying 

the Borah Peak earthquake, central Idaho: Geology, v. 12, p. 

664-667. 

Das, S., and Scholz, C.H., 1981, Theory of time-dependent rupture in 

the earth: Journal of Geophysical Research, v. 86, no. B7, p. 

6039-6051. 

Davis, F.D., 1983a, Geologic map of the central Wasatch front, Utah: 

Utah Geological and Mineralogical Survey Map 55-A, scale 

1:100,000. 

---1983b, Geologic map of the southern Wasatch front, Utah: Utah 

Geological and Mineralogical Survey Map 55-A, scale 1:100,000. 

Doser, D.l., 1985a, The 1983 Borah Peak, Idaho, and 1959 Hebgen 

Lake, Montana, earthquakes: Models for normal fault earthquakes 

in the Intermountain Seismic Belt, in Stein, R. S., and Bucknam, 

R. C., convenors, Proceedings of Workshop XXVIII on the Borah 

Peak, Idaho, earthquake: U.S. Geological Survey Open-File 

Report 85-290, p. 368-384. 

---1985b, Source parameters and faulting processes of the 1959 

Hebgen Lake, Montana, earthquake sequence: Journal of Geophysical Research, v. 90, no. B6, p. 4537-4556. 

Evans, S.H., Parry, W.T., and Bruhn, R.L., 1985, Thermal, mechanical and chemical history of Wasatch fault cataclasite and phyllonite, Traverse Mountains area, Salt Lake City, Utah: Age and 

uplift rates from K/Ar and fission track measurements: U.S. 

Geological Survey Open-File Report 85-22, p. 422-429. 

Gibler, P.R., 1985, Bedrock deformation along the Salt Lake segment 

of the Wasatch fault: Implications for stress magnitudes, stress 

directions and seismicity: Salt Lake City, University of Utah, 

unpublished M.Sci. thesis, 60 p. 

Gilbert, G.K., 1928, Studies of basin-range structure: U.S. Geological 

Survey Professional Paper 153, 92 p. 

Hintze, L.F., 1973, Geologic history of Utah: Brigham Young University Geology Studies, v. 20, p. 1-181. 

Houghton, W., 1986, Structural geology of the Deer Creek and 

Wasatch faults: Salt Lake City, University of Utah, unpublished 

M.Sci. thesis, 64 p. 

King, G., 1983, The accommodation of large strains in the upper 

lithosphere of the earth and other solids by self-similar fault 

systems: The geometrical origin of b-value: Pure and Applied 

Geophysics, v. 121, p. 761-814. 

King, G., and Nabelek, J., 1985, Role of fault bends in the initiation and 

termination of earthquake rupture: Science, v. 228, p. 987. 

King, G., and Yielding, G., 1984, The evolution of a thrust fault system: 

Processes of rupture initiation, propagation and termination in the 

1980 El Asnam (Algeria) earthquake: Geophysical Journal of the 

Royal Astronomical Society, v. 7, p. 915-933. 

Knott, J.F., 1973, Fundamentals of fracture mechanics: London, 

Butterworth, 273 p. 

Lawson, D.L., and Hanson, R.J., 1974, Solving least squares problems: Englewood Cliffs, N.J., Prentice-Hall, 340 p. 

Marsell, R.E., 1969, The Wasatch fault zone in north central Utah, in 

Jensen, M.L., ed., Guidebook of northern Utah: Utah Geological 

and Mineralogical Survey Bulletin, v. 82, p. 125-139. 

Mattick, R.E., 1970, Thickness of unconsolidated to semiconsolidated 

sediments in Jordan Valley, Utah: U.S. Geological Survey Professional Paper 700-C, p. C119-C124. 

Michael, A.J., 1984, Determination of stress from slip-data; faults and 

folds: Journal of Geophysical Research, v. 89, no. B13, p. 

11,517-11,526. 

Myers, W.B., and Hamilton, W., 1964, Deformation accompanying the 

Hebgen Lake earthquake of August 17, 1959: U.S. Geological 

Survey Professional Paper 435, p. 55-98. 

Parry, W.T., and Bruhn, R.L., 1984, Pore fluid and seismogenic 

characteristics of fault rock at depth on the Wasatch fault, Utah: 
STRUCTURE OF THE SALT LAKE SEGMENT, WASATCH NORMAL FAULT ZONE H25 

EEBS, Transactions of the American Geophysical Union, v. 65, p. 

11-14. 

---1986, Pore fluid and seismogenic characteristics of fault rock at 

depth on the Wasatch fault, Utah: Journal of Geophysical 

Research, v. 91, no. B1, p. 730-744. 

---1987, Fluid inclusion evidence for minimum 11 km vertical 

offset on the Wasatch fault, Utah: Geology, v. 15, p. 67-70. 

Pavlis, T.L., and Smith, R.B., 1980, Slip vectors from faults near Salt 

Lake City from Quaternary displacement and seismicity, in Arabasz, W.J., Smith, R.B., and Richins, W.D., eds., Earthquake 

studies in Utah 1850 to 1978: Salt Lake City, University of Utah 

Seismograph Stations, p. 378-382. 

Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquake: Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

Schwartz, D.P., Hanson, K.L., and Swan, F.H., III, 1983, Paleoseismic investigations along the Wasatch fault zone: An update, in 

Geologic excursions in neotectonics and engineering geology in 

Utah, Guidebook, pt. IV: Salt Lake City, Utah, Geological Society 

of America, p. 45-48. 

Simpson, C., and Schmid, S.M., 1983, An evaluation of criteria to 

deduce the sense of movement in sheared rocks: Geological Society 

of America Bulletin, v. 94, p. 1281-1288. 

Smith, R.B., and Bruhn, R.L., 1984, Intra-plate extensional tectonics 

of the eastern Basin-Range; inferences of structural style from 

seismic reflection data, regional tectonics and thermal-mechanical 

models of brittle-ductile deformation: Journal of Geophysical 

Research, v. 89, no. B7, p. 5733-5762. 

Smith, R.B., Richins, W.D., Doser, D.I., Pechmann, J.C., and 

Langer, C., 1984, The 1983 M 7.3 Borah Peak, Idaho earthquake: 

A model for active crustal extension: Geological Society of America 

Abstracts with Programs, v. 16, p. 16. 

Swan, F.H., III, Schwartz, D.P., and Cluff, L.S., 1980, Recurrence of 

moderate-to-large magnitude earthquakes produced by surface 

faulting on the Wasatch fault zone: Bulletin of the Seismological 

Society of America, v. 70, p. 1431-1462. 

Van Horn, R., 1972, Map showing relative ages of faults in the Sugar 

House quadrangle, Salt Lake County, Utah: U.S. Geological 

Survey Miscellaneous Investigations Map 1-766-B, scale 1:24,000. 

---1981, Geologic map of the pre-Quaternary rocks of the Salt 

Lake City North quadrangle, Davis and Salt Lake Counties, Utah: 

U.S. Geological Survey Miscellaneous Investigations Map 1-1330, 

scale 1:24,000. 

Zoback, M. L., 1983, Structure and Cenozoic tectonism along the 

Wasatch fault zone, Utah: Geological Society of America Memoir 

157, p. 3-27. 

---1985, Constraints on in-situ stress field along the Wasatch 

Front, in Hays, W.W., and Gori, P.L., eds., Proceedings of 

Conference XXVI; a workshop on Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, p. 286-309. 

Late Quaternary Displacement on the 

Morgan Fault, A Back Valley Fault in the 

Wasatch Range of Northeastern Utah 

By J. TIMOTHY SULLIVAN and ALAN R. NELSON, U.S. BuREAu oF 

RECLAMATION 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-1 

CONTENTS 

Abstract .......................................................................... . 

Introduction ..................................................................... . 

Acknowledgments ....................................................... . 

Back Valleys of the Wasatch Range .................................... .. 

Morgan Valley ................................................................. . 

Morgan Fault ................................................................... . 

Age of Quaternary Deposits Adjacent to the Central Section 

of the Morgan Fault ................................................. . 

Trench Investigations of Quaternary Faulting on the Central 

Section of the Morgan Fault .................................. .. 

Stratigraphy in Trench M4 ........................................... . 

Page 

I1 

1 

1 

1 

2 

2 

5 

12 

13 

Page 

Trench Investigations of Quaternary Faulting on the Central 

Section of the Morgan Fault-Continued 

Ages of Faulted Deposits............................................... 115 

Events in Trench M4.................................................... 15 

Other Trenches........................................................... 16 

Slip Rates on the Morgan Fault........................................... 16 

Quaternary Slip Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Cenozoic Slip Rates...................................................... 16 

Conclusions....................................................................... 17 

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 17 

ILLUSTRATIONS 

Page 

FIGURES 1, 2. Maps showing: 

1. Location of the Morgan fault, the Wasatch fault, and other middle or late Quaternary faults in northeastern 

Utah............................................................................................................................................ I3 

2. Cenozoic geology of Morgan Valley......................................................................................................... 4 

3. Generalized topographic profiles and geologic section parallel to the Morgan fault drawn along the crest of the footwall 

escarpment and along the hanging wall at the base of the escarpment . .. .. . .. .. .. .. .. .. .. .. .. .. . .. .. .. .. .. . .. .. .. .. .. . .. . .. . .. .. .. . .. 6 

4, 5. Maps showing surficial geology of: 

4. Central section of the Morgan fault . .. .. .. .. .. .. . .. .. .. .. .. . .. .. .. . .. .. .. .. . .. . .. . .. . .. . .. . .. .. .. .. . .. . .. . .. . .. .. . .. .. .. . .. . .. .. .. .. .. .. .. . 7 

5. Southern end of the central section of the Morgan fault, showing locations of trenches at the Robeson Springs 

trench site and sampling sites.......................................................................................................... 8 

6. Log of trench M4 at the Robeson Springs trench site, showing faulted middle and late Quaternary colluvial deposits 

at the southern end of the central section of the Morgan fault........................................................................... 14 

TABLES 

Page 

TABLE 1. Selected properties of soils on alluvial, colluvial, and lacustrine sediments along the Morgan fault, Morgan Valley, 

north-central Utah . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . .. . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . .. . . . . .. . . . . . . . .. . .. . . . . . . . . . I10 

2. D-alloisoleucine/L-isoleucine ratios in the total amino acid fraction and calculated ages for Oreohelix cf. strigosa from 

alluvial and colluvial sediments along the Morgan fault, Morgan Valley, north-central Utah....................................... 12 

III 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, A 

BACK VALLEY FAULT IN THE WASATCH RANGE OF 

NORTHEASTERN UTAH 

By J. TIMOTHY SuLLIVAN1 and ALAN R. NELSON2 

ABSTRACT 

The Morgan fault is a 16-km-long north-trending normal fault bounding the eastern side of Morgan Valley, a back valley of the Wasatch 

Range. A history of late Cenozoic surface displacements on the Morgan 

fault is suggested by triangular facets preserved on the footwall 

escarpment of the fault, back-tilted erosion surfaces adjacent to the 

fault, and the preservation of a 1,000-m-thick fanglomerate in the 

hanging wall of the fault. Trenches across the Morgan fault show that 

middle Pleistocene colluvial deposits are displaced about 4 m and 

Holocene deposits are displaced about 1 m. Amino acid ratio minimum 

age estimates on shells, radiocarbon dating of peat deposits, soil 

development indices, displacement data, and stratigraphic correlations 

of late Quaternary deposits among the trenches suggest a minimum 

average middle to late Pleistocene slip rate of about 0.01 to 0.02 mm/yr 

for the Morgan fault. Stratigraphic relations in one of the trenches 

suggest that repeated surface displacements on the fault have been 

about 1 m or less, an indication that paleoearthquakes of magnitude 61/2 

to 7 have occurred on the fault. 

INTRODUCTION 

Most neotectonic studies in Utah have emphasized the 

hazard posed by the occurrence of large-magnitude 

earthquakes on the Wasatch fault (Swan and others, 

1980; Schwartz and Coppersmith, 1984), but fault maps 

(Anderson and Miller, 1979; Nakata and others, 1982) 

and other studies show late Quaternary (less than 125 ka) 

faults east of the Wasatch fault, including those in Cache 

Valley (Swan and others, 1983; Nelson and Sullivan, this 

volume), Bear Lake Valley (Williams and others, 1962), 

the Bear River fault zone (West, 1986), and Strawberry 

Manuscript approved for publication November 20, 1990. 1

Now at Yucca Mountain Project Office, U.S. Department of Energy, P.O. Box 

539, Las Vegas, NV 89109. 2

Now at U.S. Geological Survey, Mail Stop 966, Federal Center, Denver, CO 

80225. 

Valley (southeast of Kamas Valley, fig. 1) (Nelson and 

Martin, 1982; Nelson and VanArsdale, 1986). A diffuse 

band of contemporary seismicity including earthquakes 

as large as magnitude 5. 7 and extending through this 

area about 20 km east of the Wasatch fault (Arabasz and 

others, 1980, this volume) suggests that additional late 

Quaternary faults may be present. 

ACKNOWLEDGMENTS 

This study was part of a regional seismotectonic study 

for the assessment of seismic hazards to U.S. Bureau of 

Reclamation dams in the Wasatch Range in central Utah 

(Sullivan and others, 1986). The staff of the Bonneville 

Construction Office of the U.S. Bureau of Reclamation in 

Provo, Utah, provided the logistical support essential to 

the completion of this work. We especially thank both 

Leyland Kippen and the Union Pacific Railroad for 

permission to excavate on private properties. Edward 

Baltzer and Carol Krinsky (geologists formerly with the 

U.S. Bureau of Reclamation) assisted in the trench 

logging. Dean Ostenaa (U.S. Bureau of Reclamation, 

Denver, Colo.) reviewed early drafts of this manuscript. 

Soils analyses by Rolf Kihl (Institute for Arctic and 

Alpine Research (INSTAAR), University of Colorado) 

are appreciated. Nelson determined amino acid ratios in 

the INSTAAR Amino Acid Geochronology Laboratory, 

with the help of Dan Gater and under the direction of 

Gifford Miller. 

BACK VALLEYS OF THE WASATCH RANGE 

The back valleys of the northern Wasatch Range are in 

the transition zone between the eastern Basin and Range 

11 
I2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

province and the Middle Rocky Mountains and Colorado 

Plateaus provinces. This zone is coincident with late 

Cretaceous and early Paleogene thrust faults of the 

Sevier thrust belt (Armstrong, 1968). Seismic reflection 

data generated for oil and gas exploration, locally 

detailed surface mapping, and seismologic investigations 

in the transition zone indicate a complex interaction 

between these stacked thrust faults and younger normal 

faults (Royse and others, 1975; Lamerson, 1982; McKee 

and Arabasz, 1982; Smith and Bruhn, 1984). The back 

valleys are structural and physiographic basins exhibiting less late Cenozoic structural relief than morphologically similar basins in the Basin and Range (Zoback, 

1983; Sullivan and others, 1986). Gilbert (1928) suggested recent displacement on some faults in Cache 

Valley, Ogden Valley, Morgan Valley, and Kamas Valley, structural basins in the Wasatch Range that he 

termed "back valleys" (fig. 1). Late Quaternary displacement has been inferred on faults in Ogden Valley, near 

Mantua, and on the East Canyon fault (Sullivan and 

others, 1986). Other unstudied late Cenozoic normal 

faults that may have Quaternary displacement are the 

Crawford Mountains fault and faults in the Bear River 

Range (Sullivan and others, 1986). 

MORGAN VALLEY 

Morgan Valley is a 25-km-long, 10- to 15-km-wide, 

northwest-trending back valley, located 20 km east of the 

Wasatch fault, that was described by Gilbert (1928) as a 

structural trough similar to Ogden and Cache Valleys to 

the north. Eardley (1944) later described Morgan Valley 

as a syncline but subsequently recognized the role of 

normal faulting in the development of the valley (Eardley, 1955). Most recently, Morgan Valley has been interpreted as an asymmetric graben bounded on the east by 

the Morgan fault (Hopkins, 1982; Hopkins and Bruhn, 

1983). 

Cenozoic deposits of three ages, separated by angular 

unconformities, are present in Morgan Valley and suggest a deformational history dating from the Eocene. The 

conglomerates of the Eocene Wasatch Formation, dipping 40° to 65° to the east and northeast, are exposed on 

the western margin of the valley unconformably overlying Paleozoic and Mesozoic sedimentary rocks (Mullens 

and Laraway, 1973; Bryant, 1984). The overlying tuffaceous sandstones and conglomerates of the Norwood 

Tuff, described and dated as late Eocene to early Oligocene by Eardley (1944), are exposed throughout the 

valley dipping 10° to 40° to the northeast. Schick (1955) 

and Coody (1957) mapped a fanglomerate unconformably 

overlying the Norwood Tuff on the eastern side of the 

valley (Thv, fig . 2) that they informally correlated with 

the Huntsville fanglomerate (informal name), which is 

mapped in Ogden Valley and near the East Canyon fault 

(fig 1). 

MORGAN FAULT 

The 16-km-long Morgan fault is mapped on the eastern 

side of Morgan Valley at the base of an escarpment 

formed in Paleozoic and early Tertiary rocks (Mullens 

and Laraway, 1973). The escarpment consists of three 

linear 5- to 8-km-long sections that correspond to en 

echelon steps in the trace of the fault and to differences 

in escarpment height and elevation of hanging-wall 

deposits (figs. 2, 3). There are no fault scarps in unconsolidated deposits along the three sections of the fault. 

However, triangular facets along the base of the escarpment that are 100 to 250 m high and slope 20° to 25° 

suggest that Quaternary displacements have occurred on 

all sections of the fault. 

Evidence suggests that Cenozoic displacement may be 

greatest on the northern section of the fault. On the basis 

of (1) the correlation of thrust faults mapped on Durst 

Mountain in the footwall of the Morgan fault with those 

exposed on the western side of the valley and (2) the 

thickness of Cenozoic deposits in the valley, Hopkins 

(1982) concluded that 6,800 m of Cenozoic normal displacement has occurred on the northern section of the 

fault. Along this section of the fault, tilted late Cenozoic 

erosion surfaces are cut on the Huntsville fanglomerate 

(fig. 2), which overlies the Norwood Tuff in the hanging 

wall of the fault. Schick (1955) concluded that this 

fanglomerate is of Pliocene(?) age and suggested that it 

was displaced by the Morgan fault. Recent mapping 

(Mullens and Laraway, 1973; Hopkins, 1982) indicates 

that this fanglomerate dips 5° to 38° into the fault and 

also suggests that the fanglomerate is faulted. Although 

triangular facets are also present on the southern section 

of the fault, Cenozoic displacement appears to be less 

there than it is on the northern section (Hopkins, 1982, p. 

35). 

Our investigation of the late Quaternary displacement 

history of the Morgan fault focused on the central section 

of the fault, where Quaternary alluvial fans (Qaof) set 

into the Huntsville fanglomerate suggest displacement 

by the Morgan fault (figs. 2, 3 , 4). We infer this 

topographic low on the hanging wall of the fault to be a 

0.5- to 1-km-wide graben filled with alluvial fan deposits 

that is bounded on the east by the main scarp of the 

Morgan fault and on the west by an inferred east-facing 

antithetic fault (fig. 4). Dissected alluvial fan deposits 

(Qaof) slope 6° to 9° to the southwest within this inferred 

graben. The lower 50 m of some of the facets slope 1 o to 

3° more steeply than their upper portions. Thus, we infer 

the surface trace of the fault to be at the 50- to 100-m-
LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, NORTHEASTERN UTAH 

IDAHO 

UTAH 

I ,(_r • •• ....., 

\ 

I 

I 

t 

' Kamas 

\Valley 

' 0 

0 

Q Evanston 

(

(BEAR RIVER 

FAULT ZONE 

\ WYOMING L.-------1~ 

UTAH 

25 MILES 

25 KILOMETERS 

13 

FIGURE !.-Location of the Morgan fault, the Wasatch fault, and other middle or late Quaternary faults (ball and bar on downthrown side) 

in northeastern Utah. Outlines of the back valleys of the northern Wasatch Mountains are indicated by dashed lines. The area of figure 

2 is shown. 
14 

Tn 

Tn 

0 

I 

0 

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

I 

2 

N 

I 

I 

2 MILES 

I 

3 KILOMETERS 

Tn 

\ 

\ 

\ 

\ 

\ 

DURST 

MOUNTAIN 

., 

Tw 

Pzpf 

Area of figure 4 

Pzpf 

Tw 
LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, NORTHEASTERN UTAH I5 

EXPLANATION 

UNITS 

Oay Younger alluvium 

~ Oao Older alluvium 

~ Oaof Older alluvial fan deposits 

~ Oaf Alluvial fan deposits 

~ Ot Fluvial terrace deposits 

""' Otp Terrace graded to Provo shoreline § Ob Lake Bonneville deposit 

01 Landslide 

6;: Thv Huntsville fanglomerate 

!-

§ Tn Norwood Tuff (Eocene and Oligo-

: cene) 

Tw Wasatch Formation (Eocene) 

Pzpf Paleozoic and Precambrian rocks 

SYMBOLS 

• Erosion surface 

--- Contact-Dashed where inferred or gradational 

......!..___-- Normal fault-Dashed where inferred: bar and 

ball on downthrown side 

------- Bonneville shoreline 

WR-96 Soil description locality 

WR-16• Paleomagnetic sampling locality 

FIGURE 2. -Explanation. 

wide break in slope between the facets and the fans. 

Although a few small, discontinuous Holocene fans along 

the break in slope in the larger drainages are undissected, channels (some up to 20 m deep) are incised into 

the surfaces of all the larger alluvial fans along the 

central section (fig. 4). Longitudinal profiles down these 

channels show that they are graded to the Bonneville 

shoreline along all but their lowest reaches. Knickpoints 

in the profiles caused by the fall of the lake (dated at 15 

ka by Scott and others (1983)) do not appear to have 

migrated more than 100 m up the fan channels. Holocene 

alluvial fans have been deposited where the channels 

reach the flood plain of the Weber River. Thus, we 

conclude that most of the sediment derived from the 

footwall of the fault is being carried beyond the hangingwall fan surfaces. Distinguishing tectonic responses from 

climatic or changing base-level responses in alluvial fan 

systems is difficult at best (for example, Funk, 1976; 

Bull, 1977), but the dissection of older fans along the 

<111111 FIGURE 2. -Cenozoic geology of Morgan Valley modified from 

Mullens and Laraway (1973). Reversely magnetized samples of a 

silty clay from the B horizon of a soil on an erosion surface west 

of the Weber River (site WR-16) indicate that this soil, the 

underlying deposits, and higher erosion surfaces in the valley are 

older than 730 ka (Sullivan and others, 1988). Most younger 

alluvium postdates the fall of Lake Bonneville from the Provo 

shoreline (14 ka). Fluvial terraces are all of Pleistocene age, but 

several ages are represented, including a terrace graded to the 

Provo shoreline (Qtp). The area of figure 4 is shown. 

central section suggests that fault slip rates are lower 

than stream-downcutting rates. 

AGE OF QUATERNARY DEPOSITS ADJACENT 

TO THE CENTRAL SECTION OF THE MORGAN 

FAULT 

The Quaternary deposits along the central section of 

the Morgan fault consist of Holocene alluvium in the 

major drainages, deposits of Lake Bonneville, and older 

colluvial and alluvial deposits at elevations above the 

Bonneville shoreline (fig. 4). Soils developed in the 

alluvium in the larger drainages (Qay, figs. 4, 5) and in 

the fans (Qayf, figs. 4, 5) deposited where the drainages 

reach the floor of Morgan Valley are weakly developed 

and contain no argillic B horizons. Comparison of these 

soils with the Holocene soils described by Shroba (1982) 

indicates that the alluvium is of Holocene age. Soils 

began forming on the buff-colored sands and silts 

between the present flood plain and the highest stand of 

Lake Bonneville in the valley following the fall of the lake 

about 14 to 15 ka. A soil developed on these deposits near 

Robeson Springs (M-2) (table 1, fig. 5) lacks an argillic 

horizon but contains substantial amounts of pedogenic 

carbonate. A sequence of older fan deposits (Qaof, figs. 

3, 4, 5) derived from the mountains to the east is exposed 

above the Bonneville shoreline. These deposits are overlain by thin (1- to 3-m-thick) hillslope colluvium that 

thickens to 7 m adjacent to the Morgan fault near 

Mahogany Canyon. Exposures and test pits (all sampling 

localities, fig. 4) show thick calcium carbonate soil horizons (stages II and III of Gile and others (1966)) on both 

the colluvial and alluvial deposits; a 1-m-thick petrocalcic 

horizon (stage IV) in fan deposits was exposed beneath 

about 3 m of colluvium near Mahogany Canyon (locality 

1, fig. 4). Thus, comparisons with similar soils developed 

in similar deposits elsewhere in the region (Machette, 

1985a) indicate that many of these alluvial and colluvial 

sediments are probably of middle Quaternary age (125-

730 ka). However, the differing degree of carbonate 

development in units of similar lithology, unconformities 

between most alluvial and overlying colluvial units, and 

uncertainty in correlating individual alluvial units in 

isolated exposures suggest that units deposited during a 

number of episodes during the Pleistocene may be 

present. 

We used three relative dating methods in an attempt 

to estimate more accurately the ages of the older colluvial and alluvial sediments along the central section of 

the Morgan fault-two measures of the degree of soil 

development and amino acid ratios measured on fossil 

gastropod shells in the deposits. 

Soil development indices (Harden and Taylor, 1983) 

provide an objective way of comparing the degree of soil 
16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

N s 

-e------ NORTHERN SECTION -------.-

3000 

2500 

(/) 

a: 

w 

1-

w 

2 

~ 

z 

0 2000 i= 

< > 

w 

_J 

w 

Bonneville 

shoreline 

..-cENTRAL SECTION-

~ SOUTHERN SECTION.__.. 

Tw 

c: 

0 

> 

c: 

Cl) 

u 

"0 

0 

0 

~ 0 

z 

1500 -----.IY!.odern profile of W_e_~~er_R_iv_e_r --- Tn 

Salt Lake 

Valley 

0 2 4 6 8 10 12 14 16 18 

DISTANCE, IN KILOMETERS 

FIGURE 3. -Generalized topographic profiles (solid lines) and geologic 

section parallel to the Morgan fault drawn along the crest of the 

footwall escarpment and along the hanging wall at the base of the 

escarpment. Profiles are from a 1:62,500-scale map having a vertical 

exaggeration of approximately 6:1. The approximate position of the 

contact between the Huntsville fanglomerate (Thv) and the Norwood 

development on deposits of unknown age with that on 

deposits of similar lithology in areas where numerical 

ages are available (Birkeland, 1984). Several calibrated 

(independently dated) soil profiles available from Morgan 

Valley (discussed by Sullivan and others (1988)) include 

soil M-6, which overlies a peat dated at 832±100 ka 

(Beta-9244) in a trench near Robeson Springs (table 1, 

fig. 5); soils (M-2, table 1; WR-9, WR-18, fig. 2) on both 

fine and coarse materials younger than the Bonneville 

shoreline (14-15 ka); and profile WR-16, which has a 

reversely magnetized B horizon that shows it to be older 

than 730 ka (fig. 2). The variable but high carbonate 

content of the soils described on alluvial and colluvial 

deposits along the central section of the Morgan fault 

(table 1, fig. 4) makes the indices of Harden and Taylor 

(1983) (table 1) less useful in estimating the age of these 

soils than in estimating the ages of many of the soils 

elsewhere in the Wasatch Range (Sullivan and others, 

1988). 

Measuring the rates of total pedogenic carbonate accumulation in soils is another method that has proven 

Tuff (Tn) is shown by a dashed line north of Mahogany Canyon. Thv is 

overlain by older alluvial fan deposits south of the antithetic fault at 

Mahogany Canyon. The modern profile of the Weber River and the 

locations of erosion surfaces (hachured lines) are also shown. Geologic 

units are the same as those used in figure 2. 

useful for estimating the age of soils in a number of areas 

in the arid and semi-arid Western United States 

(Machette, 1985a). Age estimates based on total carbonate values cannot be relied on for soils significantly 

younger than the last interglacial (125 ka) (unless many 

independent age estimates for similar soils in the region 

are available) because of probable major changes in 

carbonate accumulation rates::over this period. However, 

over longer time spans, inultip~e cycles of climate change 

tend to attenuate accumulation rate changes; as a result, 

age estimates for older soils are relatively more accurate 

FIGURE 4.-Surficial geology of the central section of the Morgan ~ 

fault, modified from Mullens and Laraway (1973). See figure 2 

for location. Older alluvial fan deposits have an estimated age of 

more than 400 ka, and colluvial deposits (not shown here) are 

inferred to range in age from Holocene to older than 400 ka. The 

upslope inflections in contour lines on unit Qaof mark channels 

incised into the older alluvial fan deposits. Younger alluvium 

and alluvial fan deposits postdate Lake Bonneville deposits 

(16-14 ka) in the valley. Sampling localities and the area of 

figure 5 at southern end of the central section are shown. 
LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, NORTHEASTERN UTAH 17 

Base is Morgan. Utah. 

7 Y2' quadrangle 0 

0 

1 MILE 

1 KILOMETER 

EXPLANATION 

QUATERNARY 

Qay Younger alluvium 

Oayf Younger alluvial fan deposits 

Qb Deposits of Lake Bonneville 

Oaof Older alluvial fan deposits 

Qao Older alluvium 

TERTIARY 

Thv 

in 

Tw 

Fanglomerate-:--Subrounded cobble and 

boulders in red-bro\m sandy silt 

Norwood Tuff-Light-9ray tuffaceous 

sandstone 

Wasatch Formation-Red sandstone and 

conglomerate 

PALEOZOIC 

Pzu Paleozoic sedimentary rocks, undivided 

Geologic contact-Dashed where 

approximately located 

Morgan fault (soltd) and inferred 

antithetic faUlt {dashed), bar and 

ball on downthrown side 

Highest Bonneville shoreline 

Amino acid sampling locality 

Soil test pit locality 
I8 

Qb 

0 

I 

0 

~ 

~ 

~ :::J 

0 

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

500 FEET 

I 

I 

100 METERS 

~ Younger alluvium 

~ Colluvium 

~ Deposits of Lake 

Bonneville 

I Qaof I Older alluvial 

fan deposits 

G Paleogene Wasatch 

Formation 

~ Paleozoic sedimentary rocks, undivided 

North Morgan Springs 

Pzu 

Robeson Springs trench site 

0 

I 

0 

25 

l 

I 

50 FEET 

I 

7-........,;-.... ______________ 10 METERS ..... 

/ '\ 

// Oao \/ 

/ / / --\ Pzu 

I // \ 

, _ _. \ 

-- EXPLANATION 

I Oayf I Younger alluvial 

fan deposits 

~ Older alluvium 

./ 

r- _..,... _, ' ....... 

Oay 

---- Geologic contact-Dashed where 

approximately located 

---- Morgan fault-Dashed where 

approximately located, short 

dashed where inferred, queried where uncertain, dotted 

where concealed 

~ Trench locations 4. Amino acid sampling locality 

M-2.& Soil description locality 

-o Spring 

Spring sapping scarp 

)( Highest Bonneville shoreline 

( River or stream 

FIGURE 5.-Surficial geology of the southern end of the central section of the Morgan fault, showing locations of trenches at the Robeson Springs 

trench site and sampling sites. 
LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, NORTHEASTERN UTAH 19 

age estimates for older soils (Machette, 1985b; Colman 

and others, 1988). 

Machette's (1985a) methods were used to calculate 

amounts of carbonate, in grams per square centimeter, in 

the total profile for the Morgan soils and to compare 

them with values for carbonate-rich soils in the northern 

Wasatch Range having some independent age control. 

Sullivan and others (1988) calculated latest PleistoceneHolocene (0-15 ka) carbonate accumulation rates of as 

much as 1 g/cm2 per thousand years from two soils (M-2 

and M-6, table 1) in Morgan Valley and one soil in Heber 

Valley (about 65 km to the south). However, ground 

water may have added carbonate to the soil in profile 

M-2, and primary carbonate values are difficult to estimate for all soils. The longer term (0-150 ka) rate of 

Sullivan and others (1988)-0.5 g/cm2 per thousand years 

(again based on only three soils along the Weber River, 

fig. 1)-is roughly half the latest Pleistocene-Holocene 

rate. Because the back valley soils are located east of the 

crest of the Wasatch Range, it is unlikely that latest 

Pleistocene-Holocene carbonate accumulation rates are 

higher than the 0.5 g/cm2 per thousand years calculated 

by Scott and others (1982) for soils of similar age near 

Salt Lake City. Rates of about 0.15 g/cm2 per thousand 

years for Fisher Valley (Colman and others, 1988), 0.14 

g/cm2 per thousand years for the Beaver area (Machette, 

1985b), and maximum rates of 0.14 to 0.26 g/cm2 per 

thousand years for Spanish Valley (Harden and others, 

1985) elsewhere in Utah indicate that middle and late 

Quaternary rates in the eastern Wasatch Range may 

well have been less than 0.2 g/cm2 per thousand years. 

We use 0.5 g/cm2 per thousand years as a maximum rate 

to estimate minimum ages of 70 to 100 ka for the soils in 

profiles M-3, M-4, and M-5 (table 1). These calculations 

suggest that soils on the alluvial fans (unit Qaof, fig. 4) 

are certainly older than 50 ka and probably date to before 

the last interglacial age (125 ka). Because the fans are 

being eroded, even finite soil ages would be minimum 

ages for the fan sediments. 

Amino acid ratios derived from analysis of the organic 

matrix within carbonate fossils have proven useful in the 

relative dating and correlation of a variety of Quaternary 

stratigraphic units worldwide (Wehmiller, 1982). This 

methodology, termed aminostratigraphy by Miller and 

Hare (1980), is valid within a region as long as all samples 

have had very similar temperature histories and if the 

amino acids in the species analyzed racemize at about the 

same rate (Williams and Smith, 1977). Although most 

studies have used marine mollusks (Wehmiller, 1982), 

recent work indicates that amino acid ratios from nonmarine gastropods are useful for relative dating (Miller 

and others, 1982; Scott and others, 1983; Nelson and 

others, 1984; Barnes and others, 1986). The Paleozoic 

carbonates and alluvial fan surfaces having carbonaterich soils along the central section of the Morgan fault 

provide a favorable environment for lime-loving terrestrial gastropods. The large terrestrial gastropod Oreohelix cf. strigosa is abundant in the surface litter along the 

trace of the fault, and fragments and whole shells of this 

species were found in the older alluvial fan sediments 

(Qaof) and overlying colluvium (Qc) at four sites (fig. 4, 

table 2). Ratios of D-alloisoleucine to L-isoleucine in 

these samples in the alluvium are greater than 0.4, 

whereas those in the colluvial units range from 0.20 to 

0.57; these ratios suggest that the shells are of several 

ages. 

Accurate numerical age estimates are difficult to 

obtain from amino acid ratios. Such estimates require 

accurate kinetic models of amino acid racemization in the 

mollusk genera of interest, along with estimates of the 

temperature histories of the fossil samples. A ± 1 o uncertainty in the effective diagenetic temperature (EDT) 

(integrated chemical effect of the sample's temperature 

history) results in a 20 percent uncertainty in the age 

estimate, and we have no way of estimating the uncertainty in our EDT's. Although the diagenetic models that 

we used were developed by using different genera (table 

2), the models for many genera differ little, and there is 

no reason to think that these models do not apply to 

Oreohelix shells in Morgan Valley. 

We used two kinetic models to calculate (minimum) 

ages for our samples (table 2). The linear model assumes 

a constant rate for the isoleucine racemization reaction in 

shells but probably applies only to our younger samples, 

because the reaction rate has been shown to decrease 

markedly in older samples (Wehmiller, 1982). One of 

several possible nonlinear models suggests much greater 

ages for the older samples. A few of the samples that 

were collected less than 2 m below the surface may have 

been affected by seasonal temperature changes (which 

would increase their apparent age), but, because the 

alluvial fan sediments are being eroded, most samples 

must have been buried more deeply during most of their 

burial history. If the shallower samples were less than 2 

m deep for their entire burial history, the maximum 

possible surface heating effect would reduce our calculated ages by about half (for example, Wehmiller, 1977). 

Because of large uncertainties in sample temperature 

histories and appropriate kinetic models, only the linearmodel age estimates (table 2) are used-and they are 

only minimum age estimates. However, enough is known 

about EDT's in the region and the reaction rate in 

gastropods to suggest that the samples from the colluvium exhibiting ratios of 0.20 and 0.27 are less than 200 

to 400 ka and less than 300 to 500 ka, respectively. 

Thus, on the basis of six samples (table 2), the older 

alluvial fan deposits are thought to be older than 400 ka. 

The colluvial deposits overlie these older fan deposits at 
TABLE 1.-Selected properties of soils on alluvial, colluvial, and lacustrine sediments along the Morgan fault, Morgan Valley, north-central Utah ~ 

...... [-,not detectable] 0 

Average Estimated percentage by volume Percentage by weight2 Organic Soil Total 

Horizon1 depth Parent Munsell matter3 Carbonate4 

profile pedogenic 

material color Pebbles Cobbles Boulders Sand Silt Clay (percent) carbonate6 

(em) (0.2-8 em) (8-25 em) (>25 em) (2--0.5 mm) (50-2J.Lm) (<2J.Lm) (percent) index5 

(g/cm2

) 

Profile M-1 

A ....... 0-10 Slopewash colluvium 10YR 6/2 5 0 0 27 56 18 1.7 16.5 15 63 

Bwk ..... 10-23 ............ do ............ 10YR 5/3 5 0 0 28 52 21 1.2 24.9 ~ 

Bkm ..... 23-50 ............ do ............ 10YR 8/0n 5 0 0 28 58 15 .5 43.7 t_:tj 

0 

Bk1 ..... 50-84 ............ do ............ 7.5YR 8/2 5 0 0 18 75 7 .4 48.9 ~ 

0 

Bk2 ..... 84-161 ............ do ............ 7.5YR 7/3 2 0 0 14 79 7 .3 35.6 z 

> 

c ....... 161-228+ ............ do ............ 7.5YR 7/3 2 0 0 28 62 11 3.3 - t-t 

t_:tj 

Profile M-2 > 

~ 

~ 

Ap ...... 0-9 Loess and lake 7.5YR 5/3 0 0 0 21 73 6 1.5 - 15 19 :::0 

10 

sediments. c:: 

A ....... 9--20 ............ do ............ 7.5YR 5/3 0 0 0 22 70 9 1.1 > 

~ 

Bw ...... 20-42 Lake sediments 7.5YR 6/4 0 0 0 44 43 13 .3 - t_:tj 

Cox1 .... 42-50 ............ do ............ 7.5YR 7/4 0 0 0 50 40 11 .1 5.5 :::0 

> 

Bk1 ..... 50-61 ............ do ............ 7.5YR 7/4 0 0 0 44 45 11 .2 13.9 N 

> Bk2 ..... 61-71 . ........... do ............ 7.5YR 8/3 0 0 0 39 47 15 .4 35.5 ~ 

Bk3 ..... 71-107 ............ do ............ 7.5YR 8/3 0 0 0 6 81 14 .1 21.7 t::l 

w Cox2 .... 107-155+ ............ do ............ 7.5YR 7/3 0 0 0 6 82 12 .1 13.0 > z 

Profile M-3 t::l 

~ ~ 

Ap ...... 0-10 Loess and alluvium 7.5YR 5/3 10 10 2 28 57 15 6.5 19 35 w - ~ 

Bt ....... 10-32 ............ do ............ 7.5YR 5/3 10 10 2 20 52 28 3.4 14.3 > 

2Bkm .... 32-35 Alluvium 7.5YR 8/0 20 5 2 17 66 17 1.2 50.4 t-t 

0 

2Bk ..... 35-73 ............ do ............ 7.5YR 8/3 20 5 2 20 66 14 .4 56.0 z 

0 

2Bt ...... 73-128 ............ do ............ 7.5YR 6/3 5 0 0 19 73 7 .2 21.6 ~ 

3Ckm .... 128-135+ ............ do ............ 7.5YR 8/1 20 10 2 27 41 32 .3 51.0 :::0 

t_:tj 

Profile M-4 ~ 

> w 

A1 ...... 0-13 Loess and colluvium 7.5YR 4/3 10 5 1 22 57 20 4.5 21.6 12 49 > 

~ 

A2 ...... 13-37 ............ do ............ 7.5YR 4/3 10 5 1 22 62 16 3.2 28.0 0 

:::0 

2BA ..... 37-50 Alluvium 7.5YR 7/3 30 1 0 36 50 15 1.3 53.8 l'l:j 

2Bk ..... 50-71 ............ do ............ 7.5YR 7/4 30 1 0 31 43 26 .7 54.6 ~ 

0 

3Ck ..... 71-120 ............ do ............ 7.5YR 8/3 10 5 2 17 73 10 .4 47.6 z 

3C ...... 120-140+ ............ do ............ 7.5YR 7/3 10 5 2 19 53 29 .2 39.8 ~ 
TABLE I.-Selected properties of soils on alluvial, colluvial, and lacustrine sediments along the Morgan fault, Morgan Valley, north-central Utah-Continued 

Average Estimated percentage by volume Percentage by weighe Organic Soil 

Horizon1 depth Parent Munsell matter3 Carbonate4 profile material color Pebbles Cobbles Boulders Sand Silt Clay (percent) (em) (0.2-8 em) (8-25 em) (>25 em) (2--0.5 mm) (50-2~-tm) (<2~-tm) (percent) index5 

Profile M-5 

Ap ...... 0--9 Loess and colluvium 7.5YR 5/3 15 10 1 24 58 18 4.7 - 14 

A ....... 9--30 ............ do ............ 7.5YR 5/4 15 10 1 21 53 26 2.7 12.8 

BA ...... 30--44 ............ do ............ 7.5YR 7/4 15 10 1 29 48 23 1.4 46.1 

2Bkm .... 44-52 Alluvium 7.5YR 8/2 20 10 0 32 48 20 .4 47.9 

2Bk ..... 52--88 ............ do ............ 7.5YR 8/3 20 10 0 38 43 19 .4 46.6 

3CB ..... 88-138 ............ do ............ 7.5YR 7/4 40 1 0 31 48 21 .2 41.6 

4C ...... 138-149+ ............ do ............ 7.5YR 7/4 5 0 0 34 48 18 .2 46.0 

Profile M-6 

Ap ...... 0--16 Slopewash colluvium 7.5YR 4/2 5 0 0 31 56 13 9.8 10.7 16 

A ....... 16-37 Colluvium 7.5YR 4/3 5 0 0 28 55 17 3.4 19.8 

Bt ....... 37-69 ............ do ............ 7.5YR 5/3 5 0 0 30 55 14 2.2 25.0 

Ck ...... 69-98 ............ do ............ 7.5YR 7/3 5 0 0 33 57 10 1.4 12.4 

2C1 ..... 98-135 ............ do ............ 10YR 7/3 10 0 0 28 49 23 .9 36.4 

2C2 ..... 135-175+ ............ do ............ 10YR 7/3 5 2 0 23 50 27 1.0 11.6 

1

Horizon nomenclature of Guthrie and Witty (1982) and Birkeland (1984), except that the master K 3

Method of Walkley and Black (1934). 

horizon is not used. 4

Method of Allison and Moodie (1965, p. 1387). 2

Particle size distribution of <2-mm fraction obtained by using sieve-pipette methods (for example, 5

Nonarid index of Harden and Taylor (1983). 

Carver, 1971) and a Sedigraph for some silt-clay fractions from which carbonates and organic matter 6

Method of Machette (1985a). 

have been removed by Jackson's (1956) method. 

Total 

pedogenic 

carbonate6 

(g/cm2

) 

40 

7 

t"" 

~ trj 

.0 

c::: 

~ trj 

~ 

z 

> 

~ 

I<! 

t:::; 

-

r:n 

t-0 

t"" 

> 0 

trj 

~ trj 

z 

t-3 

0 

z 

t-3 

::I: 

trj 

~ 

0 

~ 

~ z 

toxj 

> 

c::: 

t"" 

~t-3 

z 

0 

~ 

t-3 

::I: 

trj 

> r:n 

t-3 

trj 

~ 

z 

c::: 

~ ::I: 

~ 

1--' 

1--' 
112 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

TABLE 2. -D-alloisoleucine/L-isoleucine ratios in the total (free +peptide-bound) amino acid fraction and calculated ages for Oreohelix cf. 

strigosa from alluvial and colluvial sediments along the Morgan fault, Morgan Valley, north-central Utah 

[-, age not calculated] 

Minimum age estimate (ka) Location5 

INSTAAR Depth below Number of Mean total 

lab no. 1 surface (m) sample preparations allellle ratio2 Linear kinetic Nonlinear 

model3 model4 Site Unit 

Modern surface float (mean annual temperature 7.2 °C-7. 7 °C) 

DAN-194 ............ 0 8 0.025±.005 1 

North Morgan Springs 

DAN-190 ............ >4 3 0.61±.05 709-777 1,511-1,660 4 Qaof 

Gravel pit 

DAN-207 ............ 2.3 3 0.27±.06 317-289 2 Qc 

DAN-209 ............ .7 3 .35±.06 422-386 2 Qc 

DAN-189 ............ 3 2 .41±.06 456-503 2 Qaof 

DAN-208 ............ 5.5 3 .43±.04 485-530 3 Qaof 

Mahogany Canyon roadcut 

DAN-198 ............ >1.6 3 0.30±.04 337-369 1 Qc 

DAN-193, 195, 196, 

197, 199 ............ >2 8 .49±.06 582-637 861-942 1 Qaof 

Trench at site M6, Mahogany Canyon6 

DAN-206 ............ 1.7 3 0.20±.01 214-234 1 Qc 

DAN-201 ............ 5.0 3 

DAN-200 ............ 5.1 3 

DAN-203 ............ 6.0 3 

DAN-204 ............ 4.8 3 

DAN-205 ............ 5.8 3 

1

lnstitute for Arctic and Alpine Research, University of Colorado, Boulder, 

Colo. 2

alle/Ile ratio (peak area) measured by using methods of Miller and Hare 

(1980). Mean ratios include one standard deviation. Extraneous values rejected 

by using methods of Dixon (1965). 3 Age calculated by using a linear kinetic model of isoleucine racemization 

(equation 18 of Williams and Smith (1977)), where k=O. 77; a modern ratio of 0.025 

for Oreohelix cf. strigosa (A.R. Nelson, unpub. data, 1984); Arrhenius parameters determined for Vallonia by Nelson and others (1984); values of constants in 

the Arrhenius equation (equation 9 of Williams and Smith (1977)); and an effective 

diagenetic temperature for the late Quaternary in this region of 8 oc less than the 

all the sampling sites. The lowest ratio obtained from 

colluvium, in the trench near Mahogany Canyon, suggests an age of more than 200 ka. Higher ratios from 

other colluvial units indicate either that these units are 

older or that shells in the slope colluvium are reworked 

from the older fan deposits. The minimum age estimates 

from the soil carbonate data are consistent with these 

age estimates from amino acid ratios. 

TRENCH INVESTIGATIONS OF QUATERNARY 

FAULTING ON THE CENTRAL SECTION OF 

THE MORGAN FAULT 

The Morgan fault is exposed at the base of the escarpment in 2-m-high exposures at Robeson Springs (fig. 5) 

.35±.05 400-438 1 Qc 

.57±.03 691-757 1,350-1,478 Qc 

.40±.05 463-508 1 Qc 

.44±.03 515-564 1 Qaof 

.46±.03 542-593 677-740 1 Qaof 

present mean annual temperature (Nelson and others, 1984) (for example, 

Wehmiller and Belknap, 1982). Age range calculated by using ±0.25 oc range in 

estimated effective diagenetic temperature. 4

Age calculated by using a nonlinear model, a Quaternary effective diagenetic 

temperature (see footnote 3) (for example, Wehmiller and Belknap, 1982), and the 

same relationships used in the linear model, except that the reaction rate is 

assumed to decrease to one-fifth the initial rate for samples having alle/Ile ratios 

greater than 0.4. 5 As seen in figure 4. Qc not mapped. 6

Trench discussed in detail by Sullivan and others (1988). 

as a planar, N. 7° W.-striking, 65° W.-dipping sheared 

contact between Paleozoic carbonates and light-brown 

silty colluvium. Sheared and altered dolomite is exposed 

for a distance of about 15 m to the east in the footwall of 

the fault, but no other shears are evident in colluvial 

deposits in the exposure, which extends about 25 m west 

of the fault. This exposure and additional exposures at 

North Morgan Springs (fig. 5) showed that unconsolidated deposits, exposed above the Bonneville shoreline, 

were displaced by the Morgan fault in a narrow zone at 

the base of the triangular facets on the escarpment. 

The Robeson Springs trench site is located at the 

southern end of the central section of the Morgan fault 

(fig. 5). The linear trace of the footwall escarpment ends 
LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, NORTHEASTERN UTAH 113 

at a small, east-trending ephemeral drainage south of the 

trench site. South of this drainage, red sandstone and 

conglomerate of the Wasatch Formation are exposed 

dipping 40° to the west and extending across the projection of the central section of the fault. Projecting the 

southern section of the fault north across the Weber 

River indicates that the Morgan fault steps westward 

about 200 m between the southern and central sections 

(figs. 2, 5). 

At the Robeson Springs site, five trenches were 

excavated at or near the break in slope at the base of the 

footwall escarpment of the Morgan fault (fig. 5). Two of 

these trenches (M2U and M4) exposed the main trace of 

the Morgan fault. Another normal fault trending northwest between the central and southern sections of the 

Morgan fault is exposed in trenches M2U, M1, and M3. 

The Cambrian and Devonian dolomite that forms the 

escarpment is exposed in all the trenches. To the east, it 

is overlain by Devonian and Mississippian sedimentary 

rocks that generally dip to the west but that have been 

complexly folded and faulted (Mullens and Laraway, 

1973). In trenches M2U and M4, colluvial deposits overlie 

the dolomite in the hanging wall of the fault, but, in the 

exposures at North Morgan Springs, the colluvial deposits overlie older alluvial fan deposits that are correlated 

with similar deposits exposed at four locations north of 

the Robeson Springs site (figs. 4, 5). 

STRATIGRAPHY IN TRENCH M4 

The Morgan fault is clearly expressed in trench M4 as 

a zone of sheared dolomite and fault gouge from within 1 

m of the ground surface to the base of the trench (fig. 6). 

The fault zone juxtaposes colluvial deposits and bedrock 

along aN. 7° W.-striking, 50° W.-dipping planar contact. 

The eastern boundary of the fault zone is an abrupt 

planar shear separating fractured bedrock (unit 1) from a 

fine-grained fault breccia (unit 1b) derived from the 

bedrock. Near the base of the trench, a plastic clay gouge 

(unit 1c) forms part of the fault zone. 

On the western margin of the fault zone, a 4-m-thick 

sequence of colluvial deposits has been displaced by the 

fault (units 3, 6, 7a, 7b, fig. 6). The colluvial deposits are 

all of similar lithology-clayey silts containing variable 

but small (less than 15 percent) amounts of dispersed 

angular pebbles of dolomite. Slight differences in color, 

clay content, carbonate content, and induration have 

been used to map three main colluvial units (units 3, 6, 7) 

and to identify separate facies (identified by lower case 

letters) within two of the units. Unit 3 is massive and 

moderately indurated and contains upper and lower 

zones of pedogenic carbonate (stage II). Unit 6 is siltier, 

lacks carbonate, and is loose and unconsolidated. Modern 

soil is developed in unit 7, including a cambic B horizon 

containing weak stage II carbonate in some parts (unit 

7a). 

Discrete downslope-thinning colluvial wedges that are 

derived from erosion of the free face of a fault scarp are 

typically found adjacent to faults displaying scarps more 

than 1 m high in unconsolidated deposits. The stratigraphy and thickness of these colluvial wedges have been 

used to estimate the size of the individual surface displacements on faults (for example, Schwartz and Coppersmith, 1984). Near the floor of trench M4, two 

0.1-m-thick, 0.5- to 0.8-m-long fingers of fault breccia 

(unit 1b) are interbedded with colluvial unit 3c (fig. 6). 

This inter bedding of fault breccia and colluvium appears 

to have resulted from the erosion of fault breccia from 

the free face of a scarp that was formed during two 

separate surface displacements on the fault. The colluvium between these fingers of fault breccia is 0.4 m thick, 

and 0.5 m of colluvium is preserved between the lower 

finger and the underlying bedrock. These thicknesses 

provide minimum estimates of the height of the scarp and 

of the vertical displacement that produced it. 

The lack of discrete horizons within unit 3 suggests 

that it did not accumulate as a succession of scarpderived colluvial wedges. Unit 3 consists of 2 m of 

massive pebbly, clayey silt deposited by surface wash 

and creep from above the trench site. The uniform 

thickness of fault breccia preserved adjacent to this unit 

along a planar 50°-W.-dipping contact indicates that unit 

3 has been faulted into its present position. We interpret 

this unit to consist of multiple indistinguishable colluvial 

units that have been downdropped along the fault during 

successive small surface displacements and subsequently 

buried by continuing deposition from the escarpment 

above the fault. These colluvial units inferred to comprise unit 3 are lithologically identical; unconformities 

between them or any differences in soil development 

from one unit to another apparently have been masked 

by carbonate accumulation. On the basis of the depth of 

carbonate in the modern soil (unit 7a), it is thought that 

the pedogenic carbonate zones of unit 3 probably developed 1 to 2 m below the ground surface. The lack of 

interbedding within unit 3 (other than at the floor of the 

trench, where fingers of fault breccia divide portions of 

unit 3c) indicates that the displacements did not expose 

the fault breccia in the scarp free face. Thus, the 

individual surface displacements were not significantly 

greater than the present thickness of slope colluvium 

(unit 7)-about 0.5 m-on the footwall of the fault. Thus, 

the displacements were probably about the size of the 

minimum displacements inferred from the thickness of 

colluvium preserved below the fingers of fault breccia 

near the floor of the trench. 
114 

11 

10 c: ,.... Q) 

E :t: Q) c: (.) :::l 

CQ -

9 a. 0 rn a. :c .8 cti E 

8 ·f ~ Q) 

(j) > 0:: 

LU 7 ILU 

~ 

~ 6 LU• 

u 

z 

<( 5 I-

(j) 

Ci 

4 

3 

2 

0 

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

M4-South face 

DISTANCE, IN METERS 

DESCRIPTION OF UNITS 

EXPLANATION 

---- Contact-Dashed where indistinct 

Fault 

Fault zone 

7 Colluv~um (HOLOCENE)-Dark brown, loose, pebbly, silt with organic material, A horizon 

7b Colluv1um (HOLOCENE)-Dark brown, loose, pebbly silt 

7a Colluv~um (HOLOCENE)-Light brown, moderately indurated, pebbly silt with cambic 8 and BCa horizon 

6 Colluv1um (HOLOCENE)-Brown, loose, clayey silt 

3 Colluvium (MIDDLE AND LATE OUATERNARY)-Light brown, moderately indurated, pebbly, clayey silt. 

Uppe_r and lower zones of carbonate accumulation stippled 

3c Colluv~um (MIDDLE OUATERNARY)-Yellowish-brown, well-indurated, pebbly 

2 Colluvium (MIDDLE OUATERNARY)-Yellow, well-indurated, sandy silt 

1 Bedrock (PALEOZOIC)-Dark gray, moderately weathered dolomite with no recognizable bedding, DCd of 

Mullens and Laraway (1973) 

1 b Fault breccia-Dark grey, uncemented silt-sized dolomite fragments 

1c Fault gouge-Yellow plastic clay 

FIGURE 6.-Log of trench M4 at the Robeson Springs trench site, showing faulted middle and late Quaternary colluvial deposits at the southern 

end of the central section of the Morgan fault. Sloping lines at the left are projections of displaced surfaces (the amount of the displacement 

is indicated). 

The total displacement since the deposition of unit 3c 

has been estimated by projecting the base of unit 3 to the 

footwall of the fault at the same slope as the ground 

surface (fig. 6). The measured vertical displacement 

between the present bedrock surface and the top of unit 

3 is about 4.0 m. This estimate, however, is a minimum, 

because this bedrock surface in the footwall must have 

been eroded as unit 3 was deposited. 

In the upper portion of trench M4, the apparent 

displacement of the base of unit 6 records the most recent 

displacement event on the fault at this locality. Unit 6 is 

a key stratigraphic unit that is present in all of the 

trenches at the site and that overlies a dated peat deposit 

in trenches M2L and M3 (discussed below). The amount 

of this displacement has been estimated by projecting the 

base of unit 6 to the footwall of the fault and using the 

slope of the present ground surface (fig. 6). The vertical 

displacement measured between the bedrock surface and 

the base of unit 6 is about 1.0 m. The fault plane on which 

this displacement occurred is preserved in unit 2, a 

lighter colored, well-indurated pocket of colluvium, as a 

carbonate-impregnated plane dipping 50° to the west. 

We assume that this plane approximately represents the 

actual vertical displacement, not a minimum value, 

because little erosion of the bedrock surface is presumed 

to have occurred since the most recent Holocene event. 
LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, NORTHEASTERN UTAH I15 

Estimated displacements of 0. 5 to 1 m per event are 

small in comparison with displacements estimated for 

other Quaternary faults in Utah (for example, Schwartz 

and Coppersmith, 1984; Nelson and Van Arsdale, 1986; 

Nelson and Sullivan, this volume). These small displacements and empirical relationships (for example, Bonilla 

and others, 1984) between earthquake magnitude and 

the amount of surface displacement suggest paleoearthquake magnitudes of 61/2 to 7 for the 16-km-long Morgan 

fault, as do similar relationships between earthquake 

magnitude and fault rupture length. 

The preservation of unit 2 directly above the main fault 

appears to be related to an earlier displacement. A 

near-vertical carbonate-filled shear extends from the 

base of unit 2 to the floor of the trench in the bedrock east 

of the main fault and strikes N. go E. across the trench. 

Unit 7a and its more pebbly upslope facies, unit 7b, are 

interpreted to be parts of a horizon of slope colluvium 

downdropped to its present position by the most recent 

displacement event on the main fault zone. Unit 6 thins 

upslope to its contact with unit 2 in the hanging wall of 

the fault, and it is not clear if it ever extended across the 

fault. The predominance of silt and the lack of induration 

in unit 6 suggest that it consists principally of loess (or 

loess redeposited by surface wash) rather than slope 

colluvium eroded from the escarpment. Because it is the 

only colluvial unit in this trench and in other trenches 

that thickens downslope, unit 6 may have been deposited 

on only the lower parts of the slope and may not have 

extended across the fault zone. 

AGES OF FAULTED DEPOSITS 

Estimates of the ages of displaced colluvial deposits at 

the Robeson Springs trench site are based on radiocarbon dates for peat interbedded with colluvial deposits in 

trenches M2L and M3 and on amino acid ratios from 

snails in colluvial and alluvial fan deposits exposed in four 

other locations along the central segment of the Morgan 

fault. A small (3 x 6 m) bog deposit of peat that underlies 

colluvial unit 6 and overlies bedrock in the hanging wall 

of the Morgan fault is exposed in trenches M2L and M3. 

Unit 6 is correlated between trench M4 and the other 

trenches 32m to the south (fig. 5) on the basis of similar 

lithologies and stratigraphic and geomorphic positions 

(Sullivan and others, 1988). Radiocarbon dates of 

8,320±100 ka on peat (Beta-9244) and 9,105±270 ka on 

wood (GX-9968) from this deposit indicate an early 

Holocene age for the bog and an early or middle Holocene 

age for overlying colluvial unit 6. 

A post-early Holocene displacement event of 1 m 

might be expected to have produced scarps along the 

central section of the fault that would still be visible 

today. Scarps 1 m high on gently sloping alluvial fans 

probably would be preserved for tens of thousands of 

years (Hanks and others, 1984). However, the Morgan 

fault is mapped at the base of a 20° to 25° sloping 

escarpment, and figure 6 shows that the 1-m-high scarp 

has been completely covered since the deposition and 

faulting of unit 6. Erosion and deposition of younger 

alluvial deposits apparently obliterated the scarp in 

drainages crossing the escarpment. 

We infer a middle to late Pleistocene age for the older 

colluvial deposits (unit 3) in trench M4 by using their 

similar lithologies, degrees of carbonate development, 

stratigraphic positions, depths, and positions adjacent to 

the fault to correlate them with the sequence at Mahogany Creek for which we have amino acid age estimates 

(table 2). At Mahogany Creek, about 2.5 km north of the 

Robeson Springs trench site (fig. 5), a similar sequence of 

colluvial deposits varies in thickness from 7 m in a trench 

at the mountain front to about 1 m in a roadcut 180 m to 

the west. In the trench at Mahogany Canyon, this 

colluvium overlies older alluvial fan deposits in which a 

petrocalcic horizon containing stage IV carbonate is 

developed. This degree of pedogenic carbonate development suggests that the fan deposits are at least 200 to 

300 ka and probably older than 500 ka. Snails from this 

colluvium at depths of 1 to 5 m and from other sites at 

similar depths have estimated ages of 200 to 400 ka or 

more (discussed above, table 2). However, the upper 

parts of unit 3 may date from the late Pleistocene (10-125 

ka) because individual units cannot be correlated from 

Mahogany Creek to Robeson Springs. 

EVENTS IN TRENCH M4 

On the basis of the stratigraphic relations in trench M4 

and our estimates of the age of the colluvial deposits, we 

have interpreted the following sequence of events: 

1. During the middle and late Pleistocene, units 3 and 3c 

were deposited as a series of 0.5- to LO-rn-thick 

horizons of slopewash that were downdropped and 

preserved by a series of small-displacement events 

comprising a minimum of 3 m of displacement on the 

Morgan fault. Before or during this period, unit 2 was 

deposited as slopewash across the fault zone and 

displaced about 1 m on a near-vertical shear; as a 

result, it was preserved from erosion upslope from the 

main fault zone. 

2. Following an interval of nondeposition of unknown 

duration, a depression in the hanging wall of the fault 

exposed in trench M2L (fig. 5) was filled with bog 

deposits (radiocarbon dates of 8.3 and 9.1 ka). 

3. Unit 6 was deposited above unit 3 and the bog 

deposits principally as loess that may have been 

derived from exposed Lake Bonneville sediments in 

the valley. 
116 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

4. Units 7a and 7b were deposited by surface wash 

across the fault zone. 

5. Units 2, 7a, and 7b were displaced about 1.0 m along 

the main fault zone. 

6. Modern surface wash continued to deposit colluvium 

(unit 7) on the slope across the fault zone. 

OTHER TRENCHES 

In trench M2U, about 30 m south of M4 (fig. 5) at the 

southern end of the central section of the fault, vertical 

relief on the bedrock surface across the projection of the 

fault exposed in M4 is about 3.5 m, although the fault 

plane is not distinct. A 1.5-m-high, near-vertical displacement of the bedrock surface striking N. 7° W. marks 

the Morgan fault in this trench. This step is overlain by 

about 1. 0 m of undisplaced colluvium having the same 

lithology and in the same stratigraphic position as units 7 

and 7a in trench M4 (Sullivan and others, 1986). If this 

correlation is correct, it suggests that the most recent 

surface displacement recorded in trench M4 did not 

extend south to M2U. However, below this horizon, 

massive colluvium about 1.5 m thick, equivalent to unit 3 

in trench M4, is interpreted to have been displaced 

during small (less than 1 m) faulting events (Sullivan and 

others, 1988). 

About 10m south and west of trench M2U, a normal 

fault striking N. 65° W. and dipping about 50° to the 

southwest that is exposed in trenches M1, M3, and M2L 

(fig. 5) displaces fine-grained colluvial deposits equivalent to unit 3, but overlying colluvial units are not 

displaced. Trench M1 crosses the projection of the N. 7° 

W.-striking fault exposed in trenches M2U and M4 and 

shows that the main trace of the Morgan fault has been 

terminated by this northwest-striking fault, as the bedrock relationships at the trench site (discussed earlier) 

suggest. Because the base of the colluvium was not 

exposed in the hanging wall of the fault in trench M1, an 

estimate of the amount of displacement since the deposition of the colluvial deposits is precluded. 

SLIP RATES ON THE MORGAN FAULT 

QUATERNARY SLIP RATES 

Mapping and trench investigations on the central 

section of the Morgan fault provided evidence that 

middle to late Pleistocene and Holocene displacements 

have occurred on this back valley fault. At the Robeson 

Springs trench site, two faults were identified: (1) the 

north-trending central section of the main fault that 

displaces middle to late Pleistocene and Holocene colluvial deposits a minimum of 4 m and (2) a northwesttrending trace of a subsidiary fault that displaces similar 

middle to late Pleistocene colluvial deposits. Radiocarbon dates of 8.3 and 9.1 ka provide a maximum age for 

the younger colluvium that has been displaced about 1 m 

by the main fault in trench M4. If it is assumed that the 

lower part of the underlying sequence of colluvial deposits in trench M4 is 200 to 400 ka, a minimum of 4 m of 

displacement yields a minimum middle to late Pleistocene slip rate of 0.01 to 0.02 mm/yr. Because only one 

event is recorded, a Holocene slip rate cannot be calculated. These Pleistocene slip rates are an order of 

magnitude lower than long-term Pleistocene rates on the 

Wasatch fault (Machette and others, this volume), but 

some other faults in the region may have rates nearly as 

low (Nelson and Van Arsdale, 1986; McCalpin and others, this volume; Nelson and Sullivan, this volume). 

We interpret the colluvial stratigraphy in trench M4 as 

suggesting recurrent Quaternary surface displacements 

of 0.5 to 1.0 m. If 0.5 m most nearly represents the 

average size of the surface-displacement events that are 

represented by the 4 m of displacement in at least the 

last 200 to 400 ka (as the lack of discrete scarp-derived 

colluvial wedges in the trench suggests), about eight 

individual events have occurred. These estimates yield a 

maximum average middle to late Quaternary event 

recurrence interval of 25 to 50 k. y. If only four events of 

1m displacement have occurred, the maximum average 

recurrence interval would be 50 to 100 k.y. 

CENOZOIC SLIP RATES 

The effect of the late Cenozoic uplift of the Wasatch 

Range on the evolution of landforms in Morgan Valley 

depends partly on the distribution of late Cenozoic faults. 

Naeser and others (1983) estimated uplift rates of 0.8 

mm/yr over the last 5 Ma and 0.4 mm/yr over the last 10 

Ma for the north-central Wasatch Range west of Morgan 

Valley. As a section constructed across the northern 

portion of Morgan Valley by Hopkins (1982, fig. 5) 

shows, the faults bounding the western side of Morgan 

Valley as mapped by Bryant (1984) have limited displacement, and the structural relief in the valley is primarily 

caused by Cenozoic displacement on the Morgan fault. If 

the late Cenozoic slip rate on the Wasatch fault is much 

greater than that on the Morgan fault, the effect of this 

relative uplift of the Wasatch Range would be to accelerate late Cenozoic erosion in Morgan Valley. The fact 

that the Weber River has incised the Norwood Tuff more 

than 300 m in Morgan Valley (fig. 3) indicates that such 

erosion has occurred and suggests that the average late 

Cenozoic slip rate on the Morgan fault is much lower than 

that on the Wasatch fault. 

The accumulated thickness of the Huntsville fanglomerate preserved adjacent to the northern section of the 

Morgan fault provides a crude estimate of the average 
LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, NORTHEASTERN UTAH I17 

Cenozoic slip rate on the fault. In Hopkins' (1982) cross 

section, the Huntsville fanglomerate overlies the late 

Eocene-early Oligocene Norwood Tuff and dips 13° E. 

into the fault. It has an estimated maximum thickness of 

1,000 m, which provides a minimum estimate of displacement since the beginning of deposition of the fanglomerate. The age of this deposit is poorly constrained; previous workers have suggested that it is of Pliocene age 

(Eardley, 1944; Coody, 1957). However, similar unconsolidated gravel deposits of Oligocene age have been 

mapped further south in the Wasatch Range overlying 

and interbedded with the Oligocene Keetley Volcanics 

(Bromfield and Crittenden, 1971; Sullivan and others, 

1988). Using an age range of 5 to 35 Ma for the Huntsville 

fanglomerate and a minimum displacement estimate of 

1,000 m yields an estimated average middle and late 

Cenozoic slip rate of 0.03 to 0.2 mm/yr. 

The tilting of late Cenozoic erosion surfaces adjacent to 

the northern section of the fault also provides a crude 

estimate of the average late Cenozoic slip rate on the 

fault. In this area, locally well-preserved, gently 

(0.5°-1. 7°) east-dipping erosion surfaces, cut on the 

Huntsville fanglomerate, slope up to 1. 7o northeast, into 

the fault (fig. 2). Our observations in less dissected 

terrains of the Weber River drainage suggest that these 

surfaces probably once sloped at least 3° toward the 

center of the valley; therefore, we interpret the back 

tilting of these surfaces to have resulted from displacement on the Morgan fault. If the most steeply tilted 

surface, south of Bohman Hollow and 1. 7 km west of the 

main trace of the fault, has been uniformly rotated from 

a 3° westerly dip to its present 1. 7o easterly dip, then 

projection of this surface to the fault indicates about 150 

m of displacement. If the erosion surface has been 

downdropped relative to the footwall as well as rotated, 

total displacement would be greater. These erosion surfaces are probably significantly older than a much lower 

erosion surface on the western side of the valley (fig. 2) 

paleomagnetically dated at more than 730 ka. Assuming 

an age of 1 to 5 Ma for this surface yields an average 

latest Cenozoic slip rate of 0.03 to 0.15 mm/yr for this 

section of the fault, which is consistent with the rate 

estimated from the thickness and age of the Huntsville 

fanglomerate. 

These slip-rate values are poorly constrained and do 

not take into account probable significant variations in 

slip rates during the late Cenozoic. Estimates of 0.01 to 

0.02 mm/yr determined at the Robeson Springs trench 

site for the middle to late Quaternary are at the low end 

of the ranges oflate Cenozoic slip rates. Because we infer 

only one Holocene event and because our estimates of 

average minimum recurrence intervals range from 25 to 

100 ka, we have no way of judging whether the Holocene 

slip rate or recurrence interval differs from our estimates 

for the middle to late Quaternary, as Wallace (1984) has 

suggested for the Great Basin and as Machette and 

others (1987, this volume) have suggested for the 

Wasatch fault. Our dating control is too imprecise to 

determine whether there has been a significant change in 

slip rates during the late Cenozoic. If there is a difference 

in rates, rates on the Morgan fault for the middle to late 

Quaternary are probably lower than those for earlier 

periods. 

CONCLUSIONS 

Quaternary surface displacements have occurred on 

the Morgan fault, a north-trending, range-bounding fault 

in the back valleys of the Wasatch Mountains. Colluvial 

deposits having an age of 200 to 400 ka, estimated from 

amino acid ratios on gastropods found in correlative 

deposits, are displaced a minimum of 4 m at the Robeson 

Springs trench site on the central section of the fault. 

These data yield a minimum average middle to late 

Pleistocene slip rate of 0.01 to 0.02 mm/yr. The tilting of 

late Cenozoic erosion surfaces and estimates of the 

displacement of a Cenozoic fanglomerate along the northern section of the fault suggest that these rates are at the 

low end of a range of estimates of the average late 

Cenozoic slip rate on the Morgan fault. 

Radiocarbon dates of 8.3 and 9.1 ka on bog deposits 

underlying displaced alluvial units show that the most 

recent surface-displacement event (approximately 1 m) 

occurred during the Holocene. The colluvial stratigraphy 

in trench M4 is interpreted as showing that earlier 

individual surface displacements on the fault at this 

location have also been small (0.5-1.0 m). Because this 

trench site is near the end of the central segment of the 

fault, this estimate of individual surface displacements 

may be somewhat smaller than the maximum surface 

displacement, which is commonly used to estimate 

paleoearthquake magnitudes. Empirical relationships 

between surface displacement and rupture length, however, suggest results that are consistent with the trench 

data. Maximum surface displacements of 0.5 to 1.0 mare 

estimated for rupture of the entire 16-km-long fault from 

the empirical relationships of Bonilla and others (1984). 

These data suggest that paleoearthquakes in the magnitude range 61/2 to 7 have occurred on the Morgan fault. 

REFERENCES CITED 

Allison, L.E., and Moodie, C.D., 1965 Carbonate, in Black, C.A., 

Methods of soil analysis, pt. 2, Chemical and microbiological 

properties: Madison, Wis., American Society of Agronomy, p. 

1379-1396. 
118 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Anderson, L.W., and Miller, D.G., 1979, Quaternary fault map of 

Utah: Long Beach, Calif., Fugro, Inc., 39 p., scale 1:1,000,000. 

Arabasz, W.J., Smith, R.B., and Richins, W.D., 1980, Earthquake 

studies along the Wasatch Front, Utah-Network monitoring, 

seismicity, and seismic hazards: Bulletin of the Seismological 

Society of America, v. 70, p. 1479-1500. 

Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and Utah: 

Geological Society of America Bulletin, v. 79, p. 429-459. 

Barnes, D.A., Clark, P.U., and Nelson, A.R., 1986, Preliminary 

aminostratigraphy of Mississippi Valley loess: American Quaternary Association Biennial Meeting, 9th, Champaign-Urbana, Ill., 

1986, Programs and Abstracts, p. 117. 

Birkeland, P.W., 1984, Soils and geomorphology: New York, Oxford 

University Press, 372 p. 

Bonilla, M.G., Mark, R.K., and Lienkaemper, J.J., 1984, Statistical 

relations among earthquake magnitude, surface rupture length, 

and surface fault displacement: Bulletin of the Seismological Society of America, v. 74, p. 2379-2411. 

Bromfield, C.S., and Crittenden, M.D., Jr., 1971, Geologic map of the 

Park City East Quadrangle, Summit and Wasatch Counties, Utah: 

U.S. Geological Survey Geologic Quadrangle Map GQ-852, scale 

1:24,000. 

Bryant, B., 1984, Reconnaissance geologic map of the Precambrian 

Farmington Canyon Complex and surrounding rocks in the 

Wasatch Mountains between Ogden and Bountiful, Utah: U.S. 

Geological Survey Miscellaneous Investigations Map I-1447, scale 

1:50,000. 

Bull, W.B., 1977, The alluvial-fan environment: Progress in Physical 

Geography, v. 1, p. 222-270. 

Carver, R.E., ed., 1971, Procedures in sedimentary petrology: New 

York, Wiley-Interscience, 653 p. 

Colman, S.M., Choquette, A.F., and Hawkins, F.F., 1988, Physical, 

soil, and paleomagnetic stratigraphy of the upper Cenozoic sediments in Fisher Valley, southeastern Utah: U.S. Geological Survey Bulletin 1686, 33 p. 

Coody, G.L., 1957, Geology of the Durst Mountain-Huntsville area: 

Salt Lake City, University of Utah, unpublished M.Sci. thesis, 63 

p. 

Dixon, W .J., 1965, Extraneous values, in Black, C.A., Methods of soil 

analysis, pt. 2, Chemical and microbiological properties: Madison, 

Wise., American Society of Agronomy, p. 43-49. 

Eardley, A.J., 1944, Geology of the north-central Wasatch Mountains, 

Utah: Geological Society of America Bulletin, v. 55, p. 819-894. 

---1955, Tertiary history of north central Utah, in Eardley, A.J., 

ed., Tertiary and Quaternary geology of the eastern Bonneville 

Basin: Utah Geological Society Guidebook to the Geology of Utah, 

no. 10, p. 37-44. 

Funk, J.W., 1976, Climatic and tectonic effects on alluvial fan systems, 

Birch Creek Valley, east central Idaho: Lawrence, University of 

Kansas, unpublished Ph.D. thesis, 246 p. 

Gilbert, G.K., 1928, Studies of Basin and Range structure: U.S. 

Geological Survey Professional Paper 153, 89 p. 

Gile, L.H., Peterson, F .F., and Grossman, R.B., 1966, Morphological 

and genetic sequences of carbonate accumulation in desert soils: 

Soil Science, v. 101, p. 347-360. 

Guthrie, R.L., and Witty, J.E., 1982, New designations for soil 

horizons and layers and the new Soil Survey Manual: Soil Science 

Society of America Journal, v. 46, p. 443-444. 

Hanks, T.C., Bucknam, R.C., Lajoie, K.R., and Wallace, R.E., 1984, 

Modification of wave-cut and faulting-controlled landforms: Journal of Geophysical Research, v. 89, no. B7, p. 5771-5790. 

Harden, D.R., Biggar, N.E., and Gillam, M.L., 1985, Quaternary 

deposits and soils in and around Spanish Valley, Utah, in Weide, 

D.L., ed., Soils and Quaternary geology of the southwestern 

United States: Geological Society of America Special Paper 203, p. 

43-64. 

Harden, J.W., and Taylor, E.M., 1983, A quantitative comparison of 

soil development in four climatic regimes: Quaternary Research, v. 

20, no. 3, p. 342-359. 

Hopkins, D.L., 1982, A structural study of Durst Mountain and the 

north central Wasatch Mountains, Utah: Salt Lake City, University of Utah, unpublished M.Sci. thesis, 50 p. 

Hopkins, D.L., and Bruhn, R.L., 1983, Extensional faulting in the 

Wasatch Mountains, Utah [abs.]: Geological Society of America 

Abstracts with Programs, v. 15, no. 5, p. 402. 

Jackson, M. L., 1956, Soil chemical analysis, advanced course: Madison, 

University of Wisconsin, Department of Soil Science, unpublished 

laboratory manual, 656 p. 

Lamerson, P.R., 1982, Fossil Basin area and its relation to the 

Absaroka thrust system, in Powers, R.B., ed., Geological studies 

of the Cordilleran thrustbelt: Denver, Colo., Rocky Mountain 

Association of Geologists, p. 279-340. 

Machette, M.N., 1985a, Calcic soils of the southwestern United States, 

in Weide, D.L., ed., Soils and Quaternary geology of the southwestern United States: Geological Society of America Special 

Paper 203, p. 1-23. 

---1985b, Late Cenozoic geology of the Beaver Basin, southwestern Utah: Brigham Young University Studies in Geology, v. 32, 

pt. 1, p. 19-37. 

McKee, M.E., and Arabasz, W.J., 1982, Microearthquake studies 

across the Basin and Range-Colorado Plateau transition in central 

Utah, in Nielson, D.L., ed., Overthrust Belt of Utah: Utah 

Geological Association Publication 10, p. 137-149. 

Miller, G.H., and Hare, P.E., 1980, Amino acid geochronology: Integrity of the carbonate matrix and potential of molluscan fossils, in 

Hare, P.E., Hoering, T.C., and King, K., Jr., eds., Biogeochemistry of amino acids: New York, John Wiley, p. 415-443. 

Miller, G.H., McCoy, W.D., and Nelson, A.R., 1982, Environmental 

controls in the amino acid racemization rate from continentalinterior deposits [abs. ]: Geological Society of America Abstracts 

with Programs, v. 14, no. 7, p. 565. 

Mullens, T.E., and Laraway, W.H., 1973, Geologic map of the Morgan 

7.5-minute quadrangle, Morgan County, Utah: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-318, scale 1:24,000. 

Nakata, J.K., Wentworth, C.M., and Machette, M.N., 1982, Quaternary fault map of the Basin and Range and Rio Grande Rift 

provinces, Western United States: U.S. Geological Survey OpenFile Report 82-579, scale 1:2,500,000. 

Naeser, C.W., Bryant, B., Crittenden, M.D., and Sorenson, M.L., 

1983, Fission-track ages of apatite in the Wasatch Mountains, 

Utah-An uplift study, in Miller, D.M., Todd, V.R., and Howard, 

K.A., eds., Tectonic and stratigraphic studies in the eastern Great 

Basin: Geological Society of America Memoir 157, p. 29-36. 

Nelson, A.R., and Martin, R.A., 1982, Seismotectonic study for Soldier 

Creek Dam, Central Utah Project: Denver, Colo., Seismotectonic 

Report 82-1, Engineering and Research Center, U.S. Bureau of 

Reclamation, 115 p. 

Nelson, A.R., and Van Arsdale, R.B., 1986, Recurrent late Quaternary movement on the Strawberry normal fault, Basin and RangeColorado Plateau transition zone, Utah: Neotectonics, v. 1, p. 

1-30. 

Nelson, A.R., Madole, R.F., Evanoff, E., Piety, L.A., and Scott, 

G.R., 1984, Quaternary paleotemperature estimates using aminoacid ratios measured on terrestrial gastropods from fluvial 

sequences in Colorado: American Quaternary Association Biennial 

Meeting, 8th, Boulder, Colo., 1984, Program and Abstracts, p. 92. 

Royse, F., Jr., Warner, M.A., and Reese, D.L., 1975, Thrust belt 

structural geometry and related stratigraphic problems, 
LATE QUATERNARY DISPLACEMENT ON THE MORGAN FAULT, NORTHEASTERN UTAH 119 

Wyoming-Idaho-northern Utah, in Bolyard, D.W., ed., Symposium on deep drilling frontiers of the central Rocky Mountains: 

Denver, Colo., Rocky Mountain Association of Geologists, p. 

41-54. 

Schick, R. B., 1955, Geology of the Morgan-Heneter area, Morgan and 

Summit Counties, Utah: Salt Lake City, University of Utah, 

unpublished M.Sci. thesis, 54 p. 

Scott, W.E., Shroba, R.R., and McCoy, W.D., 1982, Guidebook for the 

1982 Friends of the Pleistocene, Rocky Mountain Cell, field trip to 

Little Valley and Jordan Valley, Utah: U.S. Geological Survey 

Open-File Report 82-845, 58 p. 

Scott, W.E., McCoy, W.D., Shroba, R.R., and Rubin, M., 1983, 

Reinterpretation of the exposed record of the last two cycles of 

Lake Bonneville, western United States: Quaternary Research, v. 

20, p. 241-265. 

Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquakes-Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

Shroba, R.R., 1982, Soil B-horizon properties as age indicators for late 

Quaternary deposits along the Wasatch Front, north-central Utah 

[abs.]: Geological Society of America Abstracts with Programs, v. 

14, no. 4, p. 233. 

Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional tectonics of 

the eastern Basin-Range-Inferences on structural style from 

seismic reflection data, regional tectonics, and thermal-mechanical 

models of brittle-ductile deformation: Journal of Geophysical 

Research, v. 89, no. B7, p. 5733-5762. 

Sullivan, J.T., Nelson, A.R., LaForge, R.C., Wood, C.K., and 

Hansen, R.A., 1988, Central Utah regional seismotectonic study 

for USBR dams in the Wasatch Mountains: Denver, Colo., Seismotectonic Report 88-5, U.S. Bureau of Reclamation, 269 p. 

Swan, F.H., III, Schwartz, D.P., and Cluff, L.S., 1980, Recurrence of 

moderate- to large-magnitude earthquakes produced by surface 

faulting on the Wasatch fault zone, Utah: Bulletin of the Seismological Society of America, v. 70, p. 1431-1462. 

Swan, F.H., III, Hanson, K.L., Schwartz, D.P., andBlack,J.H., 1983, 

Study of earthquake recurrence intervals on the Wasatch fault, 

Utah: San Francisco, Calif., Woodward-Clyde Consultants, 8th 

semi-annual technical report to the U.S. Geological Survey under 

contract 14-08-0001-19842, 36 p. 

Walkley, A., and Black, I.A., 1934, An examination of the Degtjareff 

method for determining soil organic matter, and a proposed 

modification of the chromic acid titration method: Soil Science, v. 

34, p. 29-38. 

Wallace, R.E., 1984, Patterns and timing of late Quaternary faulting in 

the Great Basin province and relation to some regional tectonic 

features: Journal of Geophysical Research, v. 89, no. B7, p. 

5763-5769. 

W ehmiller, J. F., 1977, Amino acid studies of the Del Mar, California, 

midden site: Apparent rate constants, ground temperature models, and chronological implications: Earth and Planetary Science 

Letters, v. 37, p. 184-196. 

---1982, A review of amino acid racemization studies in Quaternary mollusks-Stratigraphic and chronologie applications in 

coastal and interglacial sites, Pacific and Atlantic coasts, United 

States, United Kingdom, Baffin Island, and tropical islands: Quaternary Science Reviews, v. 1, p. 82-120. 

Wehmiller, J.F., and Belknap, D.F., 1982, Amino acid age estimates, 

Quaternary Atlantic coastal plain-Comparison with U-series 

dates, biostratigraphy, and paleomagnetic control: Quaternary 

Research, v. 18, no. 3, p. 311-336. 

West, M.W., 1986, Quaternary extensional reactivation of the Darby 

and Absaroka thrusts, southwestern Wyoming and north-central 

Utah [abs.]: Geological Society of America Abstracts with Programs, v. 18, no. 5, p. 422. 

Williams, J.S., Willard, A.D., and Parker, V., 1962, Recent history of 

Bear Lake Valley, Utah-Idaho: American Journal of Science, v. 

26G, p. 24-36. 

Williams, K.M., and Smith, G.G., 1977, A critical evaluation of the 

application of amino acid racemization to geochronology and geothermometry: Origins of Life, v. 8, p. 91-144. 

Zoback, M.L., 1983, Structure and Cenozoic tectonism along the 

Wasatch fault zone, Utah, in Miller, D.M., Todd, V.R., and 

Howard, K.A., eds., Tectonic and stratigraphic studies of the 

eastern Great Basin: Geological Society of America Memoir 157, p. 

3-27. 

CONTENTS 

Abstract .......................................................................... . 

Introduction ..................................................................... . 

Acknowledgments ....................................................... . 

Geologic Setting ............................................................... . 

Structure .................................................................. . 

Quaternary Deposits ................................................... . 

Scarp and Mountain-Front Morphology .......................... . 

Page 

J1 

1 

1 

2 

2 

4 

4 

Age of Faulted Deposits .................................................... . 

Fault-Displacement History ................................................ . 

Slip Rates and Recurrence ................................................. . 

Relationship to the East Cache Fault ................................... . 

Conclusions ...................................................................... . 

References Cited .............................................................. . 

ILLUSTRATIONS 

FIGURES 1, 2. Maps showing: 

Page 

J4 

6 

10 

10 

12 

12 

Page 

1. Location of the southern Cache Valley region.......................................................................................... J2 

2. Generalized Quaternary geology of the area north of James Peak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. 3 

3. Plot of rubification versus nonarid soil development indices for soils near the trench site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

4. Log of the eastern wall of the trench across the scarp of the James Peak fault.......................................................... 8 

TABLE 

Page 

TABLE 1. Selected properties of soils on the northern flank of James Peak, north-central Utah........................................................ J7 

III 

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH 

LATE QUATERNARY HISTORY OF THE JAMES PEAK FAULT, 

SOUTHERNMOST CACHE VALLEY, NORTH-CENTRAL UTAH 

By ALAN R. NELSON 1 and J. TIMOTHY SuLLIVAN2 

ABSTRACT 

The James Peak normal fault is marked by a northeasterly trending 

7-m-high scarp that cuts outwash fans of Bull Lake age on the northern 

flank of James Peak. The occurrence of two surface-faulting events, 

each having about 2m of displacement, on the fault in the last 140 ka 

indicates a late Quaternary slip rate of about 0.03 mm/yr. The degree 

of soil development on the outwash fans and on fault-related colluvial 

wedges provides only limited control on the timing of events. Average 

event recurrence intervals are at least 50,000 years, but recurrence 

may be nonuniform, and the most recent event may be as young as 30 

ka. The amount of displacement during each event suggests that 

surface breaks may have extended north and ruptured the southern 

portion of the East Cache fault. If so, the James Peak fault may be a 

westerly splay of the East Cache fault rather than a separate valleybounding fault. 

INTRODUCTION 

Comparing Quaternary fault-slip rates is a widely used 

means of assessing the earthquake potential of geologically young faults. The Wasatch fault poses the greatest 

hazard to population centers in north-central Utah 

because its slip rate may be several times greater and its 

recurrence of surface faulting 3 to 10 times greater than 

those of other faults in the Wasatch Mountain region 

(Schwartz and Coppersmith, 1984). However, other 

faults in the region may still pose a significant earthquake hazard, although their histories and slip rates have 

been documented in only a few cases (Nelson and 

VanArsdale, 1986; Sullivan and Nelson, this volume; 

McCalpin and others, this volume). Anderson and Miller 

(1979) and Nakata and others (1982) mapped a number of 

Manuscript approved for publication November 20, 1990. 1

Now at U.S. Geological Survey, Mail Stop 966, Federal Center, Denver, CO 

80225. 2

Now at Yucca Mountain Project Office, Department of Energy, P.O. Box 539, 

Las Vegas, NV 89109. 

faults on which significant Quaternary slip had occurred, 

but other faults in the eastern Wasatch Mountainssome displaying late Quaternary (younger than 125 ka) 

slip-have only recently been identified (Sullivan and 

Nelson, 1983; Sullivan and others, 1988). One such fault, 

the James Peak normal fault, is marked by scarps on 

bedrock, outwash fans, and other alluvium on the northern flank of James Peak and extends west-southwest 

from the southern tip of the East Cache fault in southern 

Cache Valley almost to Ogden Valley (fig. 1). We will 

summarize trench data used to estimate a slip rate on the 

James Peak fault and suggest how surface breaks on the 

fault may relate to ruptures on the adjacent East Cache 

fault, which is recognized as posing a significant earthquake hazard (Swan and others, 1983). 

ACKNOWLEDGMENTS 

This study was part of a regional seismotectonic study 

for the assessment of seismic hazards to large dams in 

the Wasatch Mountains and was supported by the U.S. 

Bureau of Reclamation (Sullivan and others, 1988). The 

staff of the Bonneville Construction Office of the U.S. 

Bureau of Reclamation in Provo, Utah, provided considerable logistical support. We especially thank Karl A. 

Jensen of Tremonton, Utah, for permission to excavate 

on his property. Edward Baltzer (U.S. Bureau of Reclamation, Denver) assisted in mapping and logging the 

trench. Soil analyses by Rolf Kihl (Institute for Arctic 

and Alpine Research, University of Colorado) are appreciated. Dean Ostenaa (U.S. Bureau of Reclamation, 

Denver) reviewed earlier drafts of the manuscript, and 

Anthony Crone (U.S. Bureau of Reclamation, Denver) 

and James McCalpin (Utah State University) substantially improved this draft. 

Jl 
J2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

112°00' 111°45' 

\ Providence Canyon 

CACHE OJ 

m VALLEY 'P 

:D 

~ 

~ '\ Canyon 

'i " :D 

Paradise~ < m 

~ 

0 

't,1~ I// 

/,_,I " , 

Index map showing 

location of study area 

0 

I 

0 

/ 

James Peak 

5 

I 1 I 

5 10 

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Area of 

figure 2 

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10 MILES I 

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FIGURE 1.-Location of the southern Cache Valley region. Selected 

late Cenozoic normal faults (modified from Crittenden and Sorensen, 

1985a, b) are indicated by heavy lines (bar and ball on downthrown 

side), dashed where approximately located. The edge of the Willard 

thrust fault is indicated by sawteeth on the overriding plate. 

GEOLOGIC SETTING 

Our review of 1:58,000-scale color infrared and 

1:15,000- to 1:40,000-scale black-and-white aerial photographs shows that the scarp of the James Peak normal 

fault is the only previously unreported fault scarp on 

unconsolidated deposits in the eastern Wasatch Mountains. The scarp marks the southern edge of an unnamed 

structural and topographic basin between James Peak 

and Middle Mountain (fig. 2) at the southern end of Cache 

Valley and 5 km north of Ogden Valley (fig. 1). The 

basin, about 3 km wide and 9 km long, contains Tertiary 

Salt Lake Formation covered by Quaternary colluvium 

and alluvium (Blau, 1975; Hintze, 1980). The basin is 

bounded on the north by highly dissected Paleozoic 

limestones, shales, dolomites, and quartzites and on the 

south by Proterozoic and Paleozoic orthoquartzites, 

argillites, and volcanic rocks that make up the highlands 

around James Peak (King, 1965; Blau, 1975; Davis, 1983). 

STRUCTURE 

Cenozoic structural basins termed "back valleys" occur 

throughout the Wasatch Mountains east of the Wasatch 

fault (Gilbert, 1928). Cache Valley is a late Cenozoic 

structural and topographic basin, bounded by normal 

faults, that developed in the Cache allocthon, the upper 

plate of the Willard (fig. 1) and Woodruff thrusts (Bjorkland and McGreevy, 1971; Crittenden, 1972; Hintze, 

1980). The East Cache fault, the principal fault in the 

valley, is marked by triangular facets cut on the eastdipping Proterozoic and Paleozoic rocks at the western 

base of the Bear River Range. Geophysical data (Stanley, 1972; Peterson, 1974) compiled by Zoback (1983) 

suggest that the upper Cenozoic basin fill in the valley is 

1. 7 to 2.1 km thick. A seismic reflection profile across the 

East Cache fault shows that Cache Valley is an asymmetric basin, deepest on the eastern margin adjacent to 

the East Cache fault (Smith and Bruhn, 1984). Consistent east dips of reflectors in the late Tertiary basin fill 

suggest a listric geometry for the East Cache fault, 

although the fault plane cannot be resolved from the 

available data. 

The James Peak normal fault, at the base of the 

escarpment in Proterozoic and Paleozoic rocks on the 

northern side of James Peak (fig. 2), marks the southern 

boundary of the east-west-trending basin at the southern 

end of Cache Valley. Isolated outcrops of Paleozoic 

sedimentary rocks suggest that the Salt Lake Formation 

may be less than 100 m thick in the basin, an indication 

FIGURE 2.-Generalized Quat~rnary,geology of the area north of ~ 

James Peak. The James Peak fault scarp (hachures point down 

the slope of the scarp) trends northeast-southwest in the middle 

of the figure. An f following the unit symbol indicates that the 

surficial unit has alluvial fan morphology. A slash within a unit 

symbol shows a deposit of multiple genesis (for example, Qa/c 

indicates Quaternary alluvium and colluvium). Qay deposits are 

primarily of Holocene and latest Pinedale age (younger than 15 

ka). Most Qao (Qaof) units are pre-Pinedale (older than 30 ka). 

Qa and Qc units are undifferentiated on the basis of age. Qcffsl 

marks thin deposits of colluvium and alluvium over Tertiary 

Salt Lake Formation. Most faults in bedrock south of the James 

Peak fault scarp are from Blau (1975). Many units (such as 

Qablf) contain smaller areas of younger and older deposits. 
LATE QUATERNARY HISTORY OF THE JAMES PEAK FAULT, NORTH-CENTRAL UTAH 

... 1 

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R 

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N 

1 

R 

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... r--r!---- ,./ 

\ Tsl ;··."" ) ··., 

(}--RSJ / b _,/ I,./ 

\ ..,, / 

L.:- .,, / R 

,-.J' \ / I -\I 

I ._ I . 

I 

I 

I 

/ R 

I 

I 

I 

I 

\ Oc 

\ 

R I 

Tsl 

R 

Qa 

~ Qay a: 

<{ Qablf z 

a: 

w 

1- Qaof <{ 

:::> Qc 0 Qg 

Tsl 

R 

0 

I 

0 

z 

~ z 

:::> 

0 

~ 

_, 

w 

0 

0 

~ 

R 

*James Peak 

EXPLANATION 

UNITS 

Alluvium ____.,-. ... _ Younger alluvium 

Alluvium deposited during ..J..--

Bull Lake glaciation 

Older alluvium ...u....a...L.. 

Colluvium 

Glacial deposits .....____ Salt Lake Formation (Miocene) JP-4 .& 

Bedrock (Paleozoic and JP-1! Proterozoic undifferentiated) 

MILE 

KILOMETER 

/ 

R 

Qc / Tsl / Oaf 

/ 

I 

/ 

I / Oaf 

/ --...... 4 ' 

~/I ?Oaf.., 

/ ... .....---

... / 

... / 

~-· 

_.,... ./"··~:) : 

SYMBOLS 

Contact-Dashed where inferred or 

gradational 

Normal fault-Dashed where inferred, 

bar and ball on downthrown side 

Normal fault scarp of probable late 

Quaternary age 

Moraine crest 

Soil description locality 

Trench site 

J3 

R 
J4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

that cumulative subsidence is much less here than it is in 

Cache Valley. North of the basin, north-dipping lower 

Paleozoic rocks are cut by north-trending normal faults, 

including the Middle Mountain fault. Near Broadmouth 

Canyon on the western side of James Peak, Blau (1975) 

mapped north-trending normal faults having estimated 

displacements of about 1,500 m that are about on trend 

with the Middle Mountain fault. Bedrock escarpments 

mark these north-trending faults, but north-trending 

scarps are not developed on the sediments in the basin. 

No east-west-trending faults are mapped along the 

northern edge of the basin, where fluvial dissection has 

produced extensive outcrops of the Salt Lake Formation. 

Thus, the basin appears to be a shallow half-graben 

connecting the larger and deeper grabens at the southern 

end of Cache Valley and the northwestern end of Ogden 

Valley (Zoback, 1983; Sullivan and others, 1988). 

QUATERNARY DEPOSITS 

Quaternary deposits in the basin consist of locally 

derived, bouldery colluvial deposits (unit Qc, fig. 2) in 

the central and northern portions that thicken and are 

interbedded with fan alluvium to the south. Alluvial fan 

deposits (Qaf), including two large, bouldery outwash 

fans (Qaof and Qablf) derived from the quartzites 

exposed on James Peak, occur along the southern edge of 

the basin. These fans become larger and appear to 

thicken from west to east; if so, this thickening may 

indicate downfaulting, tilting, or deeper erosion of the 

eastern part of the basin near Wells ville Creek and the 

East Cache fault. Sandy till (Qg) makes up the high, 

steep moraines built by the glaciers that deposited the 

outwash fans in two of the unnamed drainages on the 

northern flank of James Peak. 

SCARP AND MOUNTAIN-FRONT MORPHOLOGY 

Mountain-front slopes developed on bedrock along the 

James Peak fault are steep, especially on James Peak 

(26°), despite the fact that this side of the peak has been 

extensively eroded by glaciers. On the lower half of the 

mountain, eroded spurs (ridge crests) are faceted. Three 

breaks in slope on the spurs, as well as the north-facing 

faceted spur (25° slope) just west of Wellsville Creek at 

the eastern end of the basin (fig. 2), suggest a history of 

recurrent Quaternary displacements on the fault (for 

example, Gilbert, 1928; Hamblin, 1976). To the east, the 

mountain front is lower and more dissected and has 

gentler slopes, although here, as elsewhere, the steepest 

facets are at the base of the slopes along the fault. 

However, by comparison, the facets along the fault are 

smaller, less continuous, and less steep than those along 

the East Cache and Wasatch faults. The short (less than 

7 km) mountain front along the James Peak fault makes 

quantitative comparisons with other mountain fronts (for 

example, Bull and MacFadden, 1977) of little value. 

Overall, the mountain-front morphology of the northern 

flank of James Peak suggests that Quaternary slip rates 

are much lower here than they are on major nearby 

faults such as the East Cache and Wasatch faults. 

The James Peak fault scarp is expressed primarily as a 

scarp on bedrock over much of its length, but, along the 

central part of the fault, the scarp offsets alluvial and 

colluvial deposits and the large alluvial fans (units Qablf 

and Qaof, fig. 2) issuing from the glaciated drainages on 

the northern flank of the mountain. The scarp is generally higher (10-30 m) and steeper (20°-35° maximum 

slope angle) on bedrock (or thin colluvium over bedrock) 

than it is on fan sediments (1-4 m, 15°-30°). Where the 

scarp is on alluvium undissected by small drainages, it is 

uniformly 3 to 4 m high. 

An en echelon offset at the eastern end of the scarp 

forms a junction with the prominent scarp at the southern end of the East Cache fault; the scarps join at about 

a 110° angle. The eastern half of the scarp is subdued 

(less than 2 m high, 20° maximum slope angle) where it 

crosses the apex of the easternmost alluvial fan (Qaof, 

fig. 2), but the scarp is steeper (20°-25°) in the Cambrian 

quartzite on the en echelon segment (500 m west of 

Wells ville Creek). The lower slopes of the Paleozoic 

limestone and sandstone bedrock facets, 2. 7 km to the 

north on the East Cache fault, are steeper still (25°-30°). 

Just west of the large alluvial fans, the western half of 

the scarp is steep (22°-30° maximum slope angle) near 

the base of dissected facets in quartzite, but, farther 

west, the scarp is indistinct because the bedrock hills 

west of James Peak are lower and more rounded than the 

faceted mountain flanks to the east. 

Just south of the western end of the basin, the 

Broadmouth Canyon normal faults trend about N. 30° E. 

through low, dissected bedrock hills (Blau, 1975). A 

major topographic boundary between the western flank 

of James Peak and the lower hills to the west marks the 

largest normal fault, which exhibits 1,500 m of throw, 

that extends into Broadmouth Canyon. One large topographic step (120 m difference in elevation) and several 

smaller ones (most down to the northwest) (fig. 2) that 

parallel the largest fault are interpreted as related 

subsidiary faults. 

AGE OF FAULTED DEPOSITS 

Our chronology of Quaternary deposits and the scarps 

bounding or cutting them near James Peak is based on 

the morphology and relative position of fans, moraines, 

and scarps and on the data from four soils developed on 
LATE QUATERNARY HISTORY OF THE JAMES PEAK FAULT, NORTH-CENTRAL UTAH J5 

deposits in the largest glaciated drainage. Most mapped 

Quaternary deposits are undifferentiated as to age (fig. 

2), but even the oldest are probably no more than a few 

hundred thousand years old. 

The height of topographic scarps along the Broadmouth Canyon faults suggests Pliocene to Quaternarybut not necessarily late Quaternary -displacement. 

Most of the faults cannot be traced into the Salt Lake 

Formation in the basin to the north; thus, they appear to 

be buried by Tertiary sediments. However, in two areas 

where the Broadmouth Canyon faults meet the James 

Peak fault, faint lineaments extend into dissected Salt 

Lake Formation sediments; exposures are not sufficient 

to resolve the relationships between the lineaments and 

the faults. Although the Broadmouth Canyon faults may 

join the James Peak fault, the young scarp on the James 

Peak fault indicates that the Broadmouth Canyon faults 

have been displaced by it. 

The steep, continuous scarp on the alluvial fans along 

the central part of the James Peak fault suggests late 

Quaternary displacement on the fault. However, except 

for the smallest, youngest fans, which lack scarps where 

the fault crosses them, fans of significantly different ages 

(on which to compare scarp heights) were not identified. 

Topographic profiling across the scarp was difficult 

because of the thick alder-aspen forest and the soft, 

loose, organic-rich soil on the surface of the scarp. For 

this reason, profiling to obtain estimates of displacement 

and relative age (for example, Bucknam and Anderson, 

1979) was not attempted. 

Most Quaternary chronology studies in the Rocky 

Mountain region have divided deposits of the most recent 

major glaciations between the Bull Lake glaciation and 

the Pinedale glaciation; older deposits have been designated as pre-Bull Lake (Madole, 1976; Pierce, 1979). 

These groupings are usually based on relative age data 

such as relative position in the landform sequence, 

landform morphology, degree of soil development, and 

surface-weathering characteristics (Birkeland and others, 1979). Because the data used to subdivide these 

deposits into relative age groups are usually compared 

only for sites within a single mountain range or drainage 

basin, deposits in different ranges or basins elsewhere in 

the region that have been assigned to the same relative 

age group may not be the same age (Pierce, 1979; Porter 

and others, 1983). Colman and Pierce (1981) have identified moraines in Idaho probably dating from about 60 to 

70 ka, intermediate in age between the accepted ages of 

15 and 25 ka for most Pinedale deposits and 130 and 150 

ka for Bull Lake deposits. Thus, deposits placed in the 

Bull Lake relative age group in some drainages might be 

as young as 60 to 70 ka. Following Pierce (1979) and 

Porter and others (1983), we use "Pinedale" and "Bull 

Lake" to name the more recent major Quaternary glaciations in the Rocky Mountains and assume ages for the 

deposits of each glaciation by comparing our soils data 

with similar data from numerically dated glacial deposits 

(Pierce and others, 1976; Porter and others, 1983; Colman and Pierce, 1986). 

Soil development indices are an objective way of 

comparing the degrees of development of soils (Birkeland, 1984). The soil properties that vary the most 

systematically with time in the Rocky Mountain region 

include horizon thickness, color, texture, and calcium 

carbonate accumulation (Shroba and Birkeland, 1983). 

Ways to express changes in these properties include 

simple, widely used soil development indices (for example, depth to base of observable oxidized parent material 

and increase of maximum percentage of clay), profile 

summations (Machette, 1985), and the indices of Harden 

(1982) and Harden and Taylor (1983). Harden's indices 

refine those of Bilzi and Ciolkosz (1977) and incorporate, 

in a quantitative way, most of the concepts represented 

by development indices used previously in the Western 

United States. The profile development index (Harden, 

1982) (fig. 3) is particularly useful because the degree of 

development of all selected properties can be objectively 

summarized by a single value. 

We followed Harden and Taylor (1983) in using X-Y 

plots to compare indices for the soils that we described 

and sampled near the trench site (table 1, fig. 2) with 

those for soils of known age elsewhere in the Wasatch 

Mountains (fig. 3). Soils on a lateral moraine (JP-4) and 

on the outwash fan in front of it (JP-2) were described to 

determine whether the deposits were Bull Lake or 

Pinedale. For comparison, a Holocene soil (JP-3) was 

described on very coarse, bouldery alluvium in a narrow 

channel where the moraines are narrowly breached. The 

thick soil developed on the colluvial wedges in the trench 

(JP-1) was also described to help estimate the age of the 

wedges. Soils JP-1 and JP-2 are developed in deposits of 

more than one age. To help estimate the time interval 

represented by the degree of soil development in each of 

the younger deposits as well as by the degree of development of the whole soil, we calculated soil indices for 

each depositional unit of these two soils (table 1, fig. 3). 

Soil development indices (fig. 3) calculated by using 

field and laboratory data show that the outwash gravels 

(unit Qablf, fig. 2) in the lower parts of soils JP-1 and 

JP-2 are in relative age group (RAG) 2 and are probably 

chronocorrelative with outwash from the Bull Lake 

glaciation. Although it is possible that this outwash 

significantly postdates the last interglacial (about 125 ka) 

(discussed above), the reddish clay-rich argillic horizons 

of these soils suggest that they began forming about 140 

ka. 

Soil JP-4 on the steep, high, left-lateral moraine (fig. 2) 

is poorly developed, and its soil indices are lower than 
J6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

100 

X 80 

w 

Cl 

~ 

z 60 

0 

i= 

<{ 

u 40 u::: 

ill 

:J 

a: 20 

0 

0 

• 

.,, .,d RAG 1 

' (pre-Bull Lake) 

\~ . . ' 

RAG 2 ~, (Bull Lake) JP-~ •\.) . .l ' . '· ... ~JP-2 

RAG 4 e RAG 3 ' 

• 

/ • (Pinedale) \

1 

JP-2a '~ JP-3 1\; 

JP-4 • 'JP-la 

20 40 60 80 

NONARID INDEX 

100 

FIGURE 3.-Rubification versus nonarid soil development indices (solid 

squares) (after Harden, 1982; Harden and Taylor, 1983) for soils near 

the trench site (fig. 2, table 1). Point JP-1a is a plot of indices for the 

upper 83 em of soil JP-1 (unit 4, fig. 4), JP-1b is for the upper 205 em 

of the same soil (units 3 and 4), JP-2a is for the upper 53 em of soil 

JP-2, and JP-1, JP-2, JP-3, and JP-4 are indices for the entire 

profiles. Dashed lines divide the diagram into four relative age 

groups (RAG's) defined by indices from soils of known age (marked 

by solid circles) elsewhere in the eastern Wasatch Mountains (Nelson 

and Krinsky, 1982; Sullivan and others, 1988, p. 54). Question marks 

indicate that we do not know if the dashed lines forming approximate 

boundaries between RAG's can be extended, because we have no 

development indices for soils in the areas beyond the dashed lines. 

Comparing the degree of development of soils in these RAG's with 

that of soils on glacial deposits elsewhere in the Rocky Mountain 

region (Madole, 1976; Pierce, 1979; Shroba and Birkeland, 1983; 

Colman and Pierce, 1986) suggests that soils in RAG 4 are of 

Holocene age or a little older (younger than 12 ka), those in RAG 3 

correlate with glacial deposits of Pinedale age (15-25 ka), those in 

RAG 2 may be of Bull Lake age (130-150 ka), and those in RAG 1 are 

older than Bull Lake. 

the indices for most soils on tills and outwash of the 

Pinedale glaciation (fig. 3). However, the till on which 

the soil is developed is derived entirely from quartzites, 

which do not weather easily. In addition, the steep slopes 

and narrow crests of the moraines indicate that this soil 

may have been partially stripped by erosion on the 

moraine crest. Thus, both because soil development 

proceeds so slowly in quartzite-derived tills and because 

of erosion, the moraine is probably much older than the 

soil indices suggest. 

The bouldery surfaces, narrow crests, and steep slopes 

of the moraines on the northern flank suggest that they 

were built during the Pinedale glaciation, but the indices 

for soils developed on the outwash apron in front of the 

moraines suggest an older age, probably before the last 

interglacial (more than 125 ka). The most probable 

interpretation of the age of the moraines is that the 

Pinedale and Bull Lake glaciers extended the same 

distance downvalley but that, during the Pinedale glaciation, high Bull Lake lateral moraines protected all but 

the center of the fan surfaces from burial by younger 

outwash. Thus, the outwash fans that are displaced by 

the James Peak fault (Qablf and Qaof, fig. 2) are 

probably primarily of Bull Lake age (130-150 ka). 

Soil JP-3 and the uppermost parts of JP-1 (JP-1a) and 

JP-2 (JP-2a) are only weakly developed and fall in RAG 

4 (fig. 3). Cambic and weak argillic B horizons have 

developed in the mixed fine-grained sheetwash and loess 

deposits in the upper parts of soils JP-1 and JP-2. Soil 

JP-3 has a weak argillic horizon but no increase in hue or 

chroma to bright reddish colors (table 1). Soil indices 

suggest a Holocene or very latest Pleistocene age for this 

soil, and its position near the main stream channel of the 

largest drainage basin on the northern flank also suggests that it is of Holocene (probably late Holocene) age. 

Argillic horizon development in deposits this young is 

probably caused by a locally high rate of eolian dust 

influx (fine silt as well as clay) from the large areas of 

exposed Bonneville lake sediments in southern Cache 

Valley. Shroba (1980) has also described argillic horizons 

from deposits of early to middle Holocene age along the 

Wasatch fault. 

FAULT-DISPLACEMENT HISTORY 

To estimate the sizes, number, and ages of the more 

recent surface-displacement events on the James Peak 

fault, a single trench was excavated across the 7-m-high 

scarp on the outwash fan 200m west of the left-lateral 

moraine (fig. 2). Dense aspen forest and soft soils prevented access to all other promising trench sites. The 

trench exposed white, coarse, sandy, quartzite-derived 

outwash (unit 1, fig. 4) on which was developed a reddish 

argillic horizon (unit 1bB, fig. 4) overlain by bouldery, 

silty colluvial wedges (unit 2, fig. 4). The wedges were 

overlain by silty colluvial units (units 3 and 4, fig. 4) 

marked by thick cambic and argillic B horizons. Despite 

the slumping and raveling of the very loose, unconsolidated outwash in the lower trench walls, we were able to 

expose the fault zone and former free face of the scarp at 

the southern end of the trench. A topographic profile 

across the scarp indicates that the fan has been displaced 

about 4.2 +0.6/-0.2 m at this site. The stratigraphic 

relations, unit contacts, lithologies of the outwash and 

the colluvial wedges, and soils developed on them do not 

clearly show whether one event of about 4-m displacement or two events of about 2-m displacement occurred 

on the same fault in the trench. As we will discuss later, 

we favor a two-event interpretation. 

On the basis of regional correlation of map and trench 

units as suggested by the soil development indices of 

figure 3, we infer the following sequence of events: 
TABLE 1.-Selected properties of soils on the northern flank of James Peak, north-central Utah 

Estimated percentage by volume Percentage by weight2 

Average 

Trench depth Parent Munsell Pebbles Cobbles Boulders Sand Silt 

Profile log unit Horizon1 (em) material dry color (0.2-8 em) (8-25 em) (>25 em) (2--{).5mm) (50-211-m) 

JP-1 ............... 4b Al 0-10 Colluvium-loess 7.5YR 2/2 10 2 0 20 57 

4b A2 10-46 ......... do .......... 7.5YR 3/2 10 2 0 22 54 

4a Bw 46--83 ......... do .......... 7.5YR 5/3 10 5 0 28 51 

3bB 2Btl 83-100 ......... do .......... 7.5YR 5/4 10 5 0 26 53 

3Ba 2Bt2 100-205 ......... do .......... 7.5YR 6/4 25 15 0 28 48 

2 3Cox/3Bt 205-248 Scarp colluvium 7.5YR 7/4 20 20 2 33 45 

lbB 4Bt 248-278 Outwash 5YR 4/6 30 30 10 54 18 

1 4Cox 248-390+ ......... do .......... 7.5YR 7/3 30 30 10 90 8 

Extra samples4 ••••• 2b 3C 260 Scarp colluvium 7.5YR 7/4 10 15 25 36 45 

2a 3C 355 ......... do .......... 7.5YR 6/4 10 20 20 21 39 

JP-2 ..................... Ap 0-10 Sheetwash-loess lOYR 6/3 20 25 0 29 52 

A 10-20 ......... do .......... 7.5YR 6/4 20 25 0 28 52 

Bt 20-53 ......... do .......... 7.5YR 6/4 20 25 0 37 37 

2Bt 53-85 Outwash 2.5YR 4/6 50 25 0 72 17 

2BC 85-114 ......... do .......... 5YR 7/4 50 25 0 88 7 

2Cox 114-136+ ......... do .......... 5YR 7/4 40 35 1 84 11 

JP-3 ..................... Al 0-16 Loess-alluvium 7.5YR 4/3 15 20 25 35 53 

A2 16-41 ......... do .......... 7.5YR 5/3 15 20 25 41 41 

Bt 41-85 ......... do .......... 7.5YR 5/4 15 20 25 63 27 

2C 85-132+ Alluvium 7.5YR 7/3 25 20 30 82 13 

JP-4 ..................... Al 0-13 Sheetwash-till 7.5YR 3/3 20 5 10 62 31 

A2 13-48 ......... do .......... 7.5YR 4/3 10 20 10 66 27 

2Bt 48-72 Till 7.5YR 8/1 20 10 10 72 20 

2C 72-105+ ......... do .......... 7.5YR 8/0 20 15 10 81 15 

3Determined by Walkley and Black's (1934) method. 1

Horizon nomenclature of Guthrie and Witty (1982) and Birkeland (1984). 4

From center of units 2a and 2b at distance m (fig. 4). 2Particle size distribution of <2-mm fraction obtained by using sieve-pipette methods (for example, 

Carver, 1971) and a Sedigraph for some silt-clay fractions from which organic matter has been removed 

by Jackson's (1956) method. No samples contained carbonate. 

Clay 

(<2~J.m) 

23 

24 

21 

21 

24 

23 

28 

3 

19 

40 

19 

20 

26 

11 

5 

5 

12 

19 

10 

5 

8 

6 

9 

4 

Organic 

matter3 

(percent) 

9.4 

8.8 

3.2 

2 

.7 

.6 

.3 

.2 

.9 

.4 

5.2 

3.7 

1.4 

.6 

.2 

.3 

13.4 

5.2 

1.2 

.4 

10.4 

3.6 

.4 

.4 

t""4 

> 1-3 

M 

.0 

c:: 

> 1-3 

M 

~ 

z 

> 

~ 

~ 

::t: 

-

00 

1-3 

0 

~ 

~ 

0 

~ 

1-3 

::t: 

M 

~ 

> 

~ 

M 

00 

~ 

M 

> p::: 

~ 

> c:: 

t"'i 

~1-3 

z 

0 

~ 

1-3 

::t: 

0 

M 

z 

1-3 

~ 

> t""4 

c:: 

1-3 

> ::t: 

t!....j 

-:::J 
J8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

EXPLANATION 

Argillic 8 horizon 

, Cobbles and boulders 

Lithologic and soil horizon contact-Dashed 

where gradational 

Fault-Arrow indicates relative sense of 

displacement 

Soil horizon contacts not coincident with 

lithologic unit contacts 

Surface of erosional scarp 

UNIT DESCRIPTIONS 

COLLUVIUM OUTWASH 

2 Scarp-derived proximal facies 

2a Gravelly clay 

1 Clean, loose, coarse sandy gravel 

1a Zone of some infiltrated silt and clay 

2b Loose, gravelly, sandy silt 

38 Slopewash and loess facies(argillic 8 horizon) 

38a Gravelly clayey silt 

1 8b Zone of patchy argillic horizon development and iron staining 

38b Pebbly silt loam 

4 Slopewash and loess facies 

4a Pebbly silt loam (cambic 8 horizon) 

4b Pebbly silt loam(A horizon) 

0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 

DISTANCE, IN METERS 

FIGURE 4.-Log of the eastern wall of the trench across the scarp of 

the James Peak fault (fig. 2). The segmented line at the top of the log 

is the central portion of topographic profile JP-1 measured across the 

site by using the methods of Bucknam and Anderson (1979). To 

trench as high as possible on the scarp, the trench was dug in two 

parts; the first part (0-12 m in distance) was backfilled before the 

1. Moraines and an extensive outwash fan were deposited on the northern flank of James Peak, probably 

during the Bull Lake glaciation (about 130-150 ka). 

Soils developed on stable areas of moraines and 

outwash, but gradual erosion prevented welldeveloped soils from forming on moraine crests. The 

length of time required to develop the reddish argillic 

B horizons on the outwash is difficult to estimate, 

partly because the soil north of the trench site (JP-2) 

appears to have been stripped. The fact that the 

argillic horizon and the fine-grained units overlying it 

are thin suggests that soil JP-2 has been eroded, 

perhaps by meltwater in braided outwash channels 

(which did not reach the trench site) during the later 

Pinedale glaciation. These horizons may have developed rapidly because of high rates of dust influx owing 

to the silt and clay from lake sediments exposed in 

Cache Valley to the north. Because the B horizons on 

second part (12-19 min distance) was dug. Lowercase letters in unit 

symbols indicate sublithofacies, and B marks argillic B horizons. The 

lithology of the proximal colluvial wedges-units 2a and 2b-suggests that the scarp was formed by two surface-faulting events (see 

text). 

these deposits are much more strongly developed 

than the B horizons on late Pinedale or Holocene 

deposits, these horizons must have taken at least a 

few tens of thousands of years to develop, perhaps as 

long as 70,000 years. 

2. Surface faulting vertically displaced the outwash (unit 

1, fig. 4) about 1.6 to 2.2 m down to the north. A 

proximal colluvial debris wedge (unit 2a, fig. 4), 

formed by spall, slump, and slopewash, rapidly buried 

the argillic horizon developed on the outwash (unit 

1bB, fig. 4) and the lower part of the free face of the 

scarp. The source of the bouldery debris wedge, 

which has a matrix of silty clay (table 1), must have 

been the surface colluvium of similar lithology overlying the outwash on top of the scarp (not exposed in 

our trench). The uppermost part of unit 1bB near the 

fault is mapped as part of the soil developed on unit 1. 

But the thickening of the reddish clayey silt near the 
LATE QUATERNARY HISTORY OF THE JAMES PEAK FAULT, NORTH-CENTRAL UTAH J9 

top of unit 1bB (note 1, fig. 4) and the inclusion of 

reddish clayey peds in the matrix of less reddish 

clayey silt near the base of unit 2a suggest that 

material along the contact between units 1bB and 2a 

may be fragments of an argillic horizon eroded from 

the crest of the scarp immediately after the event. We 

infer that none of the loose, sandy outwash (unit 1, fig. 

4) was exposed in the free face by this event; if it had 

been, the free face would have slumped almost immediately after faulting and contributed much coarse 

sand to the debris wedge. Equilibrium scarp degradation models at gently sloping sites suggest that the 

maximum fault displacement is about two times the 

maximum colluvial wedge thickness (for example, 

Nash, 1981). However, the steep slope above the site 

and the inferred active surface transport processes 

(discussed below) suggest that the amount of vertical 

displacement during this event was probably not 

much greater than the wedge thickness (1. 6 m). 

3. After the argillic horizons had formed and probably 

after the first fault event, soil development on the 

lower part of the outwash fan continued. However, 

the steep (20° -25°) slope of the fan above the trench 

site, the lack of well-developed B horizons on the 

proximal wedges, and the high clay content of surface 

colluvium above the scarp suggest that creep and 

solifluction may have been the dominant slope processes during periods of cold, moist climate (onset of 

Pinedale glaciation, about 75 ka?) during or after the 

deposition of the first proximal wedge. These processes may have eroded existing soils or prevented 

well-developed soils from forming at and above the 

site. 

4. A second displacement event on the same fault moved 

unit 2a down against the outwash. The loose, sandy 

lithology of the proximal debris wedge (unit 2b, fig. 4) 

that was produced following this event indicates that 

some outwash was exposed in the free face in addition 

to the bouldery, clayey silt (not exposed in the trench) 

overlying the outwash. Erosion of the scarp following 

the first event also probably reduced the proportion of 

bouldery silt to outwash exposed in the free face 

during the second event and resulted in a sandy 

wedge. Slumping of the outwash in the lower part of 

the free face helped to spread loose debris downslope 

and buried the debris wedge from the first event. The 

loose debris produced a thin wedge (unit 2b, fig. 4) 

that extended farther downslope than the first wedge 

(unit 2a, fig. 4); this geometry is typical of debris 

wedges produced by the second and third events on 

multiple-event scarps (Ostenaa, 1984). Displacement 

during the second event is more difficult to estimate 

from the thickness of the debris wedge; it must be 

close to the difference between the total topographic 

displacement across the scarp and the displacement of 

the first event (4.2 m-(1.6-2.2 m)=2.0-2.6 m). We 

could not measure total throw because the top of the 

outwash unit in the footwall was not exposed in the 

trench. The lack of any evidence of soil development 

on the debris wedge resulting from the first event 

suggests that the two events may have occurred less 

than a few tens of thousands of years apart. However, 

the active surface processes on the steep fan (discussed earlier) could also have eroded a moderately 

developed soil or prevented one from forming. It is 

also possible that weak soil development near the top 

of wedge 2a is masked by later development of more 

strongly developed soils on units 2b and 3Ba (fig. 4). 

An alternate interpretation that must be considered is 

that both units 2a and 2b are proximal debris wedges 

produced following a single displacement event of 

about 4 m. This interpretation would explain the 

absence of (1) an abrupt contact between units 2a and 

2b, (2) any evidence of soil development on unit 2a, 

and (3) deformation of the first debris wedge (unit 2a) 

during the second event. The apparent overlap of the 

time estimates for pre-first event and post-second 

event soil-forming intervals (discussed later) would 

also be explained by a single event. Clear evidence of 

shearing along the contact between units 2a and 1a 

was not observed, but three cobbles were imbricated 

parallel with the contact near the base of unit 1a, and 

the outwash is so unconsolidated that evidence of 

shearing may not have been preserved. Possibly, unit 

1a is a fissure that filled with outwash that slumped 

from the free face after a second event. In any case, 

the stratigraphic relations near the fault show that 

most of unit 2a must have been deposited immediately 

after faulting, and its lithology shows that this unit 

was derived from units that contained little sand. 

Because we find it difficult to explain how a thick 

proximal debris wedge could be deposited adjacent to 

an almost vertical free face of loose, sandy outwash 

several meters high in such a way that the outwash 

contributed no sediment to the wedge, we favor a 

two-event fault history. In addition, 2-m displacement events are much more typical of faults in the 

region, even faults that are much longer than the 

James Peak fault (for example, Schwartz and Coppersmith, 1984). 

5. A silty colluvial wedge (unit 3B, fig. 4) was deposited 

over the proximal debris wedges by wash erosion of 

the scarp crest and by rill and wash transport of fines 

from above the site. The thickness and high silt 

content of the wedge (table 1) suggest that it has a 

strong eolian component, probably owing to exposed 

lake sediments in Cache Valley. As the deposition 
J10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

rate of the colluvial wedge decreased, B horizon 

development in the upper part of the wedge became 

more distinct and culminated in a 122-cm-thick argillic 

horizon (table 1) on the wedge. Because of its position 

on the scarp slope, this soil is not directly comparable 

to soils in more stable landscape positions whose age 

is better known. We interpret this soil as forming in a 

climate not greatly different from the present; thus, it 

must either predate the main Pinedale glaciation 

(18-25 ka) or postdate the fall of Lake Bonneville from 

its high stand (15 ka) (Scott and others, 1983). The 

total amount of clay in this soil (9 g/cm2

) in comparison 

with regional rates of clay accumulation (for example, 

Colman and others, 1988) and most other soil indices 

(fig. 3) for this part of the soil on unit 3 suggest an age 

as great as 100 to 150 ka because these horizons are so 

thick. However, the limited soil reddening and weak 

B horizon structure, the probable high rate of dust 

influx during part of the history of the site, and the 

likelihood that some of unit 3 is derived from clay-rich 

soils upslope all suggest a much younger age. Thus, 

we estimate that this soil, which postdates both fault 

events, developed over a period of 30,000 to 70,000 

years. 

6. Finally, an episode of rapid slopewash deposition, 

probably marked by some eolian influx, produced a 

silty colluvial unit (unit 4, fig. 4) that is very similar to 

unit 3. A weak cambic B horizon (unit 4a) and a thick, 

silty A horizon (unit 4b, fig. 4) on this unit indicate a 

Holocene age (soil JP-1a, fig. 3). Deposition during 

the warmer and probably drier Altithermal of the 

middle Holocene (Baker, 1983) is likely. 

SLIP RATES AND RECURRENCE 

The topographic profile across the scarp at the trench 

site (fig. 4) shows that the outwash, most likely about 140 

ka, is displaced about 4.2 m for an average late Quaternary vertical slip rate of 0.03 mm/yr on the James Peak 

fault. 

Because the soils developed on the outwash and on the 

colluvium overlying the faulting-related wedges provide 

only maximum and minimum age estimates for the 

wedges, the true intervals of time between surfacefaulting events are difficult to estimate. Although displacement data for each of the two fault events are 

uncertain, scarp erosion models that consider the position of the scarp at the foot of a steep slope, the thickness 

of the first colluvial wedge, and the volumes and lithologies of both wedges suggest that the second event may 

have been larger than the first. We estimate average 

displacements of about 1.8+0.4/-0.2 m for the first 

event and 2.4+0.8/-0.6 m for the second event. The 

reddish argillic horizon on the outwash beneath the 

colluvial wedges probably required at least several tens 

of thousands of years (we assume more than 30,000 but 

perhaps even 70,000 years) to develop before burial. This 

well-developed soil adjacent to the fault shows that no 

significant displacement occurred on the fault for a long 

period of time after outwash deposition ceased. Thus, the 

first event could have occurred as early as 110 ka (140-30 

ka) or as late as 70 ka (140-70 ka). The thick argillic 

horizon on unit 3B (fig. 4), which postdates the second 

displacement event, also suggests a soil-forming interval 

of at least 30,000 and perhaps 70,000 years. Given these 

broad constraints, a time interval of 80,000 years (110-30 

ka) is available in which the two events could have 

occurred. If we take the mean of possible times at which 

each event occurred (first event 90 ka, second event 50 

ka), the events are separated by 40,000 years (90-50 ka). 

Because the maximum estimates of the length of the 

soil-forming intervals following each event overlap, no 

minimum estimate of the length of this period can be 

made. If the two events were separated by 40,000 years, 

the recurrence interval between these events and earlier 

and future events would be more than 50,000 years 

(50,000+40,000+50,000= 140,000 years). Thus, the average recurrence interval for the two events since 140 ka is 

at least 50,000 years, but the lack of a soil between the 

two debris wedges suggests nonuniform recurrence. 

However, if there was only one event (discussed earlier), 

it probably occurred about 70 ka. 

RELATIONSHIP TO THE EAST CACHE FAULT 

The location of the James Peak fault at the southern 

end of the East Cache fault suggests that. their surfacerupture histories may be related. Blau's (1975) mapping 

showed that the East Cache fault is abruptly terminated 

at its high-angle intersection with the James Peak fault 

(fig. 2), because no north-trending fault extends up 

Wellsville Creek. The James Peak fault is also oriented 

at near-right angles to the contemporary regional extensional stress field (Zoback and Zoback, 1980). However, 

portions of the Wasatch fault that intersect at high 

angles have ruptured concurrently during the Holocene 

(Nelson and Personius, 1990), and similar fault patterns 

are found along late Quaternary fault zones elsewhere in 

the Basin and Range (for example, Wallace, 1979). The 

morphology of the bedrock scarps on the Broadmouth 

Canyon faults (fig. 2) does not suggest recurrent late 

Quaternary displacement but also does not preclude 

some Quaternary slip. These scarps trend southwesterly, nearly parallel to the southern part of the East 

Cache fault. We speculate that the Broadmouth Canyon 

faults may be the southernmost part of a Quaternary 

rupture zone encompassing all three faults. 
LATE QUATERNARY HISTORY OF THE JAMES PEAK FAULT, NORTH-CENTRAL UTAH J11 

Some of the larger surface-displacement events on the 

James Peak fault may have ruptured part of the East 

Cache fault. Empirical relations between the amount of 

surface displacement and fault rupture length (Bonilla 

and others, 1984) suggest that more than 7 km of surface 

faulting is associated with events that produce the 2 m of 

displacement that we infer from our trench stratigraphy. 

On the Wasatch fault, surface displacements of 1.6 to 2.6 

m are thought to be associated with ruptures on fault 

segments 30 to 60 km long (Schwartz and Coppersmith, 

1984). The 1983 Borah Peak, Idaho, earthquake had a 

maximum displacement of about 2. 5 m and a total 

rupture length of 36±3 km (Crone and Machette, 1984). 

Thus, even if small, unrecognized ruptures occurred on 

the Broadmouth Canyon faults during the events 

recorded at James Peak, at least 15 to 20 km of the 

southern part of the East Cache fault may also have 

ruptured. Near the southern end of the East Cache fault, 

Swan and others (1983) could not determine the age of 

most recent displacement because the main trace of the 

fault is well above the Bonneville shoreline at the range 

front and because scarps along the fault are almost 

completely masked by younger (post-Provo shoreline, 

younger than 14 ka) (Scott and others, 1983) alluvial fans. 

Mullens and Izett (1964) also did not find evidence of 

surface faulting in the Paradise area (fig. 1). Thus, any 

scarps along the East Cache fault related to the James 

Peak events are probably covered by younger deposits. 

Farther north along the East Cache fault near Logan, 

Utah (fig. 1), Swan and others (1983) concluded that two 

1- to 2-m post-Bonneville surface-displacement events 

have occurred on this part of the East Cache fault for a 

slip rate of 0.10 to 0.2 mm/yr for the last 15,000 years. 

The first event occurred between the high stand of Lake 

Bonneville (15-16 ka) (Scott and others, 1983) and its 

catastrophic fall to the Provo shoreline (14-15 ka). The 

second event postdates the Provo level of the lake and 

predates alluvial fans thought to be about 6 to 10 ka. 

Excavations at several places along the East Cache fault 

have also revealed post-Bonneville displacements (B.N. 

Kaliser, oral commun., 1981), but these investigations 

are undocumented. Thus, the average recurrence interval on the section of the East Cache fault marked by the 

post-Provo scarps near Logan since the high stand of 

Lake Bonneville appears to be about 7,000 years, but the 

actual interval between events could be as short as 2, 000 

to 3,000 years (Swan and others, 1983). 

Estimates of the thickness of basin fill in southern 

Cache Valley (Zoback, 1983) and topographic relief on 

the bedrock escarpment of the East Cache fault, both of 

which decrease south of Logan, suggest that total displacement on the fault also diminishes to the south. Late 

Pleistocene scarps are mapped along the fault at least as 

far south as Blacksmith Fork Canyon (Cluff and others, 

1974; Swan and others, 1983), but Swan and others (1983) 

speculated that the rupture near Logan may die out 

south of Providence Canyon. 

Site-specific fault slip data indicate differing histories 

for the East Cache and James Peak faults, but little 

reliable information on the southern half of the East 

Cache fault is available to distinguish among several 

possible relations. One possibility is that displacements 

on the James Peak fault occur near the end of surface 

ruptures produced by very large but infrequent (average 

recurrence interval of more than 50,000 years) earthquakes that break a large part (more than 50 km) of the 

East Cache fault. The length of these ruptures and the 

fact that the amount of displacement near the center is 

likely to be greater than that near the end of the rupture 

segment indicate that these earthquakes would be as 

large as or larger than those inferred on the central 

segments of the Wasatch fault (Bonilla and others, 1984; 

Schwartz and Coppersmith, 1984), a supposition that 

seems very unlikely. Alternatively, the East Cache fault 

may consist of unrecognized segments, the recurrence 

interval on the southern part of the fault (and the James 

Peak fault) being an order of magnitude longer than that 

on the East Cache fault near Logan. The southern part of 

the East Cache fault could also have a slip rate and a 

recurrence interval similar to those of the central part of 

the fault; if displacements were occurring near the base 

of the facets along the range front, on faults buried by 

the post-Provo alluvial fans along the range front, or on 

faults basinward of the alluvial fans, scarps on these 

unrecognized faults would be either buried or quickly 

eroded. 

Although the apparent slip rates on the East Cache 

and James Peak faults differ by a factor of 3 to 6, it 

should be emphasized that the ages of the datums used to 

estimate those rates differ by almost an order of magnitude. A similar situation is reported by Machette and 

others (this volume), whose mapping suggests that the 

high slip rates and short recurrence intervals on the 

central segments of the Wasatch fault zone may apply 

only to the latest Pleistocene and Holocene; longer term, 

late Quaternary rates appear to be much lower, closer to 

those estimated for other faults in the region (for example, Nelson and VanArsdale, 1986; Sullivan and Nelson, 

this volume). The rapid changes in subsurface pore 

pressures and the isostatic loading and unloading accompanying the rise and fall of Lake Bonneville about 15 ka 

may have triggered many earthquakes on the Wasatch 

and East Cache faults and resulted in a latest Pleistocene 

and Holocene recurrence rate several times larger than 

the late Quaternary rate (Swan and others, 1983; 

Machette and others, this volume). Thus, we speculate 

that late Quaternary slip rates on the East Cache fault 
J12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

and the James Peak fault, and perhaps recurrence intervals as well, do not differ nearly as much as the apparent 

slip rates suggest. 

CONCLUSIONS 

Two surface-displacement events of about 2 m each 

have occurred on the James Peak fault in the last 140 ka, 

indicative of a late Quaternary slip rate of about 0. 03 

mm/yr. Surface-faulting recurrence intervals are difficult to estimate because of poor age control, but the 

average interval between events is at least 50,000 years. 

The fault may be a westerly splay of the East Cache fault 

rather than a separate valley-bounding fault like the ones 

to the south. The lack of detailed slip and recurrence data 

from the southern half of the East Cache fault precludes 

determining whether the central part of the East Cache 

fault behaves independent of the southernmost part of 

the fault and of the James Peak fault or whether all parts 

of both faults have had a similar late Quaternary history. 

REFERENCES CITED 

Anderson, L.W., and Miller, D.G., 1979, Quaternary fault map of 

Utah: Long Beach, Calif., Fugro, Inc., 39 p. 

Baker, R.G., 1983, Holocene vegetational history of the western 

United States, in Wright, H.E., Jr., ed., The Holocene, in 

Wright, H.E., Jr., ed., Late Quaternary environments of the 

United States: Minneapolis, University of Minnesota Press, v. 2, 

p. 109-127. 

Bilzi, A.F., and Ciolkosz, E.J., 1977, Time as a factor in the genesis of 

four soils developed in recent alluvium in Pennsylvania: Soil 

Science Society of America Journal, v. 41, p. 122-127. 

Birkeland, P.W., 1984, Soils and geomorphology: New York, Oxford 

University Press, 372 p. 

Birkeland, P.W., Colman, S.M., Burke, R.M., Shroba, R.R., and 

Meierding, T.C., 1979, Nomenclature of alpine glacial depositsOr what's in a name?: Geology, v. 7, p. 532-536. 

Bjorkland, C.J., and McGreevy, C.J., 1971, Geologic map and section 

of Cache Valley, Utah and Idaho: Utah Department of Natural 

Resources Publication 36, 1 pl., scale 1:180,000. 

Blau, J.C., 1975, Geology of the southern part of the James Peak 

quadrangle: Logan, Utah State University, unpublished M.Sci. 

thesis, 55 p. 

Bonilla, M.G., Mark, R.K., and Lienkaemper, J.J., 1984, Statistical 

relations among earthquake magnitude, surface rupture length, 

and surface fault displacement: Bulletin of the Seismological Society of America, v. 74, p. 2379-2411. 

Bucknam, R.C., and Anderson, R.E., 1979, Estimation of fault-scarp 

ages from a scarp-height-slope-angle relationship: Geology, v. 7, 

p. 11-14. 

Bull, W.B., and McFadden, L.D., 1977, Tectonic geomorphology north 

and south of the Garlock fault, California, in Doehring, D.O., ed., 

Geomorphology in arid regions: Annual Geomorphology Symposium, 8th, Binghamton, N.Y., 1977, Proceedings, p. 115-138. 

Carver, R.E., ed., 1971, Procedures in sedimentary petrology: New 

York, Wiley-Interscience, 653 p. 

Cluff, L.S., Glass, C.E., and Brogan, G.E., 1974, Investigation and 

evaluation of the Wasatch fault north of Brigham City and Cache 

Valley faults, Utah and Idaho; A guide to land use planning with 

recommendations for seismic safety: Oakland, Calif., WoodwardLundgren and Associates, report to U.S. Geological Survey under 

contract 14-08-001-13665, 146 p. 

Colman, S.M., and Pierce, K.L., 1981, Weathering rinds on andesitic 

and basaltic stones as a Quaternary age indicator, Western United 

States: U.S. Geological Survey Professional Paper 1210, 56 p. 

---1986, Glacial sequence near McCall, Idaho: Weathering rinds, 

soil development, morphology, and other relative-age criteria: 

Quaternary Research, v. 25, p. 25-42. 

Colman, S.M., Choquette, A.F., and Hawkins, F.F., 1988, Physical, 

soil, and paleomagnetic stratigraphy of the upper Cenozoic sediments in Fisher Valley, southeastern Utah: U.S. Geological Survey Bulletin 1686, 33 p. 

Crittenden, M.D., Jr., 1972, Willard thrust and Cache allochthon, 

Utah: Geological Society of America Bulletin, v. 83, p. 2871-2880. 

Crittenden, M.D., Jr., and Sorensen, M.L., 1985a, Geologic map of the 

North Ogden quadrangle and part of the Ogden and Plain City 

quadrangles, Box Elder and Weber Counties, Utah: U.S. Geological Survey Miscellaneous Investigations Map I-1606, 1 pl., scale 

1:24,000. 

---1985b, Geologic map of the Mantua quadrangle and part of the 

Willard quadrangle, Box Elder, Weber, and Cache Counties, 

Utah: U.S. Geological Survey Miscellaneous Investigations Map 

I-1605, 1 pl., scale 1:24,000. 

Crone, A.J., and Machette, M.N., 1984, Surface faulting accompanying 

the Borah Peak earthquake, central Idaho: Geology, v. 12, p. 

664-667. 

Davis, F.D., 1983, Geologic map of the central Wasatch Front, Utah: 

Utah Geological and Mineral Survey Map 54-A, 2 pls., scale 

1:100,000. 

Gilbert, G.K., 1928, Studies of Basin and Range structure: U.S. 

Geological Survey Professional Paper 153, 89 p. 

Guthrie, R.L., and Witty, J.E., 1982, New designations for soil 

horizons and layers and the new Soil Survey Manual: Soil Science 

Society of America Journal, v. 46, p. 443-444. 

Hamblin, W.K., 1976, Patterns of displacement along the Wasatch 

fault: Geology, v. 4, p. 619-622. 

Harden, J.W., 1982, A quantitative index of soil development from 

field descriptions-Examples from a chronosequence in central 

California: Geoderma, v. 28, p. 1-28. 

Harden, J.W., and Taylor, E.M., 1983, A quantitative comparison of 

soil development in four climatic regimes: Quaternary Research, v. 

20, p. 342-359. 

Hintze, L.F., 1980, Geologic map of Utah: Utah Geological and Mineral 

Survey, 2 pls., scale 1:500,000. 

Jackson, M.L., 1956, Soil chemical analysis, advanced course: Madison, 

University of Wisconsin, Department of Soil Science, unpublished 

laboratory manual, 656 p. 

King, H.D., 1965, Paleozoic stratigraphy of the James Peak quadrangle, Utah: Logan, Utah State University, unpublished M.Sci. 

thesis, 47 p. 

Machette, M.N., 1985, Calcic soils of the southwestern United States, 

in Weide, D.L., ed., Soils and Quaternary geology of the southwestern United States: Geological Society of America Special 

Paper 203, p. 1-21. 

Madole, R.F., 1976, Glacial geology of the Front Range, Colorado, in 

Mahaney, W.C., ed., Quaternary stratigraphy of North America: 

Stroudsburg, Pa., Dowden, Hutchinson, and Ross, p. 297-318. 

Mullens, T.E., and Izett, G.A., 1964, Geology of the Paradise quadrangle, Utah: U.S. Geological Survey Geologic Quadrangle Map 

GQ-185, 1 pl., scale 1:24,000. 

Nakata, J.K., Wentworth, C.M., and Machette, M.N., 1982, Quaternary fault map of the Basin and Range and Rio Grande Rift 

provinces, Western United States: U.S. Geological Survey OpenFile Report 82-579, 2 pls., scale 1:2,500,000. 
LATE QUATERNARY HISTORY OF THE JAMES PEAK FAULT, NORTH-CENTRAL UTAH J13 

Nash, D.B., 1981, Fault-A Fortran program for modeling the degradation of active normal fault scarps: Computers and Geosciences, 

v. 7, p. 249-266. 

Nelson, A.R., and Krinsky, C.K., 1982, Late Cenozoic history of the 

upper Weber and Provo Rivers, NE. Utah [abs.]: Geological 

Society of America Abstracts with Programs, v. 14, no. 3, p. 344. 

Nelson, A.R., and Personius, S.F., 1990, Preliminary surficial geologic 

map of the Weber segment, Wasatch fault zone, Weber and Davis 

Counties, Utah: U.S. Geological Survey Miscellaneous Field Studies Map MF-2132, scale 1:50,000. 

Nelson, A.R., and VanArsdale, R.B., 1986, Recurrent late Quaternary 

movement on the Strawberry normal fault, Basin and RangeColorado Plateau transition zone, Utah: Neotectonics, v. 1, p. 

1-30. 

Ostenaa, D.A., 1984, Relationships affecting estimates of surface fault 

displacements based on scarp-derived colluvial deposits [abs.]: 

Geological Society of America Abstracts with Programs, v. 16, no. 

5, p. 327. 

Peterson, D.L., 1974, Bouger gravity map of part of the northern Lake 

Bonneville Basin, Utah and Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-627, 1 pl., scale 1:250,000. 

Pierce, K.L., 1979, History and dynamics of glaciation in the northern 

Yellowstone National Park area: U.S. Geological Survey Professional Paper 729-F, 90 p. 

Pierce, K.L., Obradovich, J.D., and Friedman, I., 1976, Obsidian 

hydration dating and correlation of Bull Lake and Pinedale glaciations near West Yellowstone, Montana: Geological Society of 

America Bulletin, v. 87, p. 703-710. 

Porter, S.C., Pierce, K.L., and Hamilton, T.D., 1983, Late Wisconsin 

mountain glaciation in the western United States, in Porter, S.C., 

ed., The Late Pleistocene, in Wright, H.E., Jr., ed., Late Quaternary environments of the United States: Minneapolis, University of Minnesota Press, v. 1, p. 71-111. 

Scott, W.E., McCoy, W.D., Shroba, R.R., and Rubin, M., 1983, 

Reinterpretation of the exposed record of the last two cycles of 

Lake Bonneville, western United States: Quaternary Research, v. 

20, p. 241-265. 

Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and 

characteristic earthquakes-Examples from the Wasatch and San 

Andreas fault zones: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698. 

Shroba, R.R., 1980, Influence of parent material, climate, and time on 

soils formed in Bonneville-shoreline and younger deposits near 

Salt Lake City and Ogden, Utah [abs.]: Geological Society of 

America Abstracts with Programs, v. 12, no. 6, p. 304. 

Shroba, R.R., and Birkeland, P.W., 1983, Trends in late Quaternary 

soil development in the Rocky Mountains and Sierra Nevada of the 

western United States, in Porter, S.C., ed., The Late Pleistocene, 

in Wright, H.E., Jr., ed., Late Quaternary environments of the 

United States: Minneapolis, University of Minnesota Press, v. 1, 

p. 145-156. 

Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional tectonics of 

the eastern Basin and Range-Inferences on structural style from 

seismic reflection data, regional tectonics, and thermal-mechanical 

models of brittle-ductile deformation: Journal of Geophysical 

Research, v. 89, no. B7, p. 5733-5762. 

Stanley, W.D., 1972, Geophysical study of unconsolidated sediments 

and basin structure in Cache Valley, Utah and Idaho: Geological 

Society of America Bulletin, v. 83, p. 1817-1830. 

Sullivan, J. T., and Nelson, A. R., 1983, Late Cenozoic faulting in Heber 

and Keetley valleys, northeastern Utah, in Gurgel, K.D., ed., 

Guidebook, pt. IV, in Crone, A.J., ed., Field trip 5; Paleoseismicity along the Wasatch front and adjacent areas, central Utah: 

Geologic excursions in neotectonics and engineering geology in 

Utah: Utah Geological and Mineralogical Survey Special Studies 

62, p. 55-61. 

Sullivan, J.T., Nelson, A.R., LaForge, R.C., Wood, C.K., and 

Hansen, R.A., 1988, Central Utah regional seismotectonic study 

for USBR dams in the Wasatch Mountains: U.S. Bureau of 

Reclamation Seismotectonic Report 88-5, 338 p. 

Swan, F.H., III, Hanson, K.L., Schwartz, D.P., and Black, J.H., 1983, 

Study of earthquake recurrence intervals on the Wasatch fault, 

Utah: San Francisco, Woodward-Clyde Consultants, 8th semiannual technical report to U.S. Geological Survey under contract 

14-08-0001-19842, 36 p. 

Walkley, A., and Black, LA., 1934, An examination of the Degtjareff 

method for determining soil organic matter, and a proposed 

modification of the chromic acid titration method: Soil Science, v. 

34, p. 29-38. 

Wallace, R.E., 1979, Map of young fault scarps related to earthquakes 

in north-central Nevada: U.S. Geological Survey Open-File Report 

79-1554, 2 pls., scale 1:125,000. 

Zoback, M.L., 1983, Structure and Cenozoic tectonism along the 

Wasatch fault zone, Utah, in Miller, D.M., Todd, V.R., and 

Howard, K.A., eds., Tectonic and stratigraphic studies in the 

eastern Great Basin: Geological Society of America Memoir 157, p. 

3-29. 

Zoback, M.L., and Zoback, M.D., 1980, State of stress in the conterminous United States: Journal of Geophysical Research, v. 85, no. 

B11, p. 6113-6156. 


"' • l ~ • -~-"'" ' ' 
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radioactivity, which reflect subsurface structures that are of economic 

or geologic significance. Many maps include correlations with the 

geology. 

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also 

includes 7.5-minute quadrangle photogeologic maps on planimetric 

bases that show geology as interpreted from aerial photographs. Series 

also includes maps of Mars and the Moon. 

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial 

geology, stratigraphy, and structural relations in certain coal-resource 

areas. 

Oil and Gas Investigations Charts show stratigraphic 

information for certain oil and gas fields and other areas having 

petroleum potential. 

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases for quadrangle or 

irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations. 

Hydrologic Investigations Atlases are multicolored or blackand-white maps on topographic or planimetric bases presenting a wide 

range of geohydrologic data of both regular and irregular areas; 

principal scale is 1:24,000, and regional studies are at 1:250,000 scale 

or smaller. 

Catalogs 

Permanent catalogs, as well as some others, giving 

comprehensive listings of U.S. Geological Survey publications are 

available under the conditions indicated below from the U.S. 

Geological Survey, Book and Open-File Report Sales, Federal Center, 

Box 25425, Denver, CO 80225. (See latest Price and Availability List.) 

"Publications of the Geological Survey, 1879-1961" may be 

purchased by mail and over the counter in paperback book form and as 

a set of microfiche. 

"Publications of the Geological Survey, 1962-1970" may be 

purchased by mail and over the counter in paperback book form and as 

a set of microfiche. 

"Publications of the U.S. Geological Survey, 1971-1981" may 

be purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche. 

Supplements for 1982, 1983, 1984, 1985, 1986, and for 

subsequent years since the last permanent catalog may be purchased by 

mail and over the counter in paperback book form. 

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by 

mail and over the counter in paperback booklet form only. 

''Price and Availability List of U.S. Geological Survey 

Publications," issued annually, is available free of charge in paperback 

booklet form only. 

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" are available free of charge by mail or may 

be obtained over the counter in paperback booklet form only. Those 

wishing a free subscription to the monthly catalog "New Publications of 

the U.S. Geological Survey" should write to the U.S. Geological 

Survey, 582 National Center, Reston, VA 22092. 

Note.-Prices of Government publications listed in older 

catalogs, announcements, and publications may be incorrect. 

Therefore, the prices charged may differ from the prices in catalogs, 

announcements, and publications.