U.S. Department of the Interior 

U.S. Geological Survey

Assessment of Regional Earthquake Hazards and 

Risk Along the Wasatch Front, Utah
FRONT COVER. View to the southeast of downtown Salt Lake City, Utah, circa 

1980. Wasatch Range with prominent Lake Bonneville shorelines in the 

background.
Assessment of Regional Earthquake Hazards and 

Risk Along the Wasatch Front, Utah

PAULA L. GORI and WALTER W. HAYS, Editors

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-K-R

Contributions from 

Utah Geological Survey, 

University of Utah, and 

Utah State University

Chapters K R are issued as a single volume 

and are not available separately. 

Chapter titles are listed in the volume 

table of contents.
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, 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.

Author affiliations given in this volume were correct at the time 

the report was approved for publication.

Published in the Eastern Region, Reston, Va. 

Manuscript approved for publication October 5,1995.

Library of Congress Cataloging in Publication Data

Assessing regional earthquake hazards and risk along the Wasatch Front, Utah / edited by Paula L.

Gori and Walter W. Hays.

p. cm. (U.S. Geological Survey professional paper : 1500K-R) 

Includes bibliographical references. 

Supt.ofDocs.no.: 119.16: 1500KZ. 

1. Earthquakes Wasatch Range (Utah and Idaho) 2. Earthquakes Utah. I. Gori, Paula. II.

Hays, Walter W. III. Series: U.S. Geological Survey professional paper; 1500. 

QE535.2.U6A84 2000

551.2'2'097922 dc20 92-33618

CIP

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225
CONTENTS

[Letters designate the chapters]

Introduction, by Paula L. Gori and Walter W. Hays

(K) Site Amplification in the Salt Lake City-Ogden-Provo Urban Corridor and the Implications for EarthquakeResistant Design, by Walter W. Hays

(L) Predicting Strong Ground Motion in Utah, by Kenneth W. Campbell

(M) Probabilistic Analysis 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

(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

(O) In Situ Poisson's Ratio Measurements Near Prove, Utah, by Richard D. Miller, Don W. Steeples, Kenneth 

W. King, and Ralph W Knapp

(P) Earthquake Losses in Central Utah, by S.T. Algermissen, E.P. Arnold, K.V. Steinbrugge, Margaret G. 

Hopper, and PS. Powers

(Q) Isoseismals of Some Historical Earthquakes Affecting the Wasatch Front Area, Utah, by Margaret G. 

Hopper

(R) Seismic-Risk Methods and Estimates for Utility Systems and State-Owned Buildings Along the Wasatch 

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

III

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 earthquake'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 of 

magnitude1 greater than or equal to 6 have occurred in 

Utah. The two largest were the magnitude (M L 7.0) 6.6 

Hansel Valley earthquake of 1934 (fig. 2) and the M L 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.

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) surfacewave magnitude (Ms), and (3) body-wave magnitude (ms). 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).
REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

113° IDAHO 112

41

h- /'

0 50 100 KILOMETERS 

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

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 

(Ms) 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, earthquakeinduced 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 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 earthquake 

potential. The second volume covers ground-shaking
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.

hazards and aspects of loss estimation. 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 composing this section are 

described in Chapters A through J:

Chapter 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. 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. 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
INTRODUCTION

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

Valley, Utah, earthquake.

an average return period of 24±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 Quaternary 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: 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. 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 

is being 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 composing this section are 

described in Chapters K through R:

Chapter K: Site Amplification in the Salt Lake CityOgden-Provo Urban 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 CityOgden-Provo 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.
REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

 x- 

- « -. 

:J< i'Zi* fi. ' 

- Svt.Jf'sF --13$*-- 

' - 1

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 Analysis of Earthquake GroundShaking 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.
INTRODUCTION

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 

potential for the Wasatch Front urban corridor, a 

better understanding of how shallow underlying 

geology affects ground shaking was developed.

Chapter O: 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 S.T. 

Algermissen, E.P. Arnold, K.V. Steinbrugge, Margaret 

G. Hopper, and P.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 ML 5.5 shock on the 

Provo segment to $5.5 billion for a M s 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 MMIVII.

Chapter R: Seismic-Risk Methods and Estimates for 

Utility Systems and State-Owned Buildings Along the 

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

Ward. Results of seismic risk projects on Utah 

facilities reaffirm the iterative nature of these projects.

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).

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, 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 are included in Professional Paper 1519 

(Gori, 1993), 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, O.K., 1883, Earthquakes: Salt Lake Daily Tribune, Sept. 

16,1883, p. 4.

Gori, P.L., ed., 1993, Applications of research from the U.S. Geological Survey program, assessment of regional earthquake 

hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1519,167 p.

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, PL, 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.

Site Amplification in the Salt Lake City-Ogden-Provo 

Urban Corridor and the Implications for 

Earthquake-Resistant Design

By WALTER W. HAYS

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-K

CONTENTS

Page

Abstract....................................................................................................... Kl

Introduction................................................................................................. 2

Background................................................................................................. 2

Evaluation of the Bedrock Ground-Shaking Hazard in Utah........... 5

Acknowledgments............................................................................ 6

Methodology for Evaluating Site Amplification ........................................ 6

Soils Data......................................................................................... 7

Site-Amplification Data.................................................................... 9

Maps of the Site-Amplification Factors for Salt Lake City, Ogden,

and Provo..................................................................................... 10

SaltLake City Region ...................................................................... 11

Ogden Region................................................................................... 11

Provo Region.................................................................................... 12

Possible Uses of Site-Amplification Data and Maps.................................. 12

Uniform Building Code.................................................................... 12

Applications in Utah......................................................................... 14

Arguments to Minimize or Eliminate Controversy in the

Application of These Data.................................................. 14

Technical Aspects of Site Amplification.................................................... K15

Site and Building Periods................................................................. 16

Level of Dynamic Shear Strain........................................................ 17

Thickness of the Soil Column.......................................................... 18

Near Field......................................................................................... 18

Bedrock Motions.............................................................................. 18

Variability in Site-Amplification Effects ......................................... 18

Worldwide Data............................................................................... 18

1967 Caracas, Venezuela, Earthquake.................................... 18

1970 Gediz, Turkey, Earthquake ............................................ 19

1976 Friuli, Italy, Earthquake................................................. 19

1985 Mexico Earthquake........................................................ 19

Conclusions, Unresolved Technical Issues, and

Recommendations....................................................................... 19

Technical Issues Needing Resolution.............................................. 20

Recommendations............................................................................ 20

References Cited......................................................................................... 20

ILLUSTRATIONS

Page

FIGURE 1. Map showing the Wasatch fault zone................................................................................................................................................................... K2

2. Seismogram showing example of site amplification in Salt Lake City................................................................................................................ 4

3. Schematic representation of the soil-rock column, the bedrock and surface acceleration time histories, the corresponding

response spectra derived from them, and the transfer function............................................................................................................................ 5

4. Graph showing the effect of different types of soil columns on the spectral composition of ground motion...................................................... 5

5, 6. Maps showing:

5. Probabilistic bedrock peak horizontal ground acceleration in Utah............................................................................................................. 6

6. Probabilistic bedrock peak horizontal ground velocity in Utah.................................................................................................................... 6

7. Graph showing the range of shear-wave velocities of surficial soils in the San Francisco Bay region........................................................ 7

8. Schematic representation of the correlation of the response spectrum derived from a strong-motion

accelerogram with a series of damped single-degree-of-freedom harmonic oscillators............................................................................... 9

9-13. Maps and graphs showing range of transfer functions showing horizontal ground response of sites relative 

to a rock site on the Wasatch Front, Salt Lake City area:

9. Underlain by thin deposits of sands and gravels (stations 6, 8, 10, 11, 12).................................................................................................. 10

10. Underlain by thick, soft deposits of silts and clays (stations 17-19, 24, 25)................................................................................................ 10

11. Underlain by rock (stations 5, 9, 13)............................................................................................................................................................. 11

12. Underlain by thick, soft deposits of silts and clays (stations 2, 6)................................................................................................................ 11

13. Underlain by thick, soft deposits of silts and clays (stations 3, 4)................................................................................................................ 12

14. Maps showing the spatial variation of site-amplification factors for the period band 0.2 to 0.7 s for the Salt Lake City,

Ogden, and Provo areas........................................................................................................................................................................................ 13

15, 16. Graphs showing:

15. Estimated range of values of horizontal spectral velocity that might be expected in Salt Lake City, Ogden, or Provo

within about 35 km of a large earthquake on the Wasatch fault zone........................................................................................................... 15

16. Estimate of the level of dynamic shear strain as a function of epicentral distance and magnitude.............................................................. 16

17. Diagram comparing strong-motion accelerograms in a low-strain environment recorded on firm soil and in soft soil in the

September 19, 1985, Mexico City earthquake..................................................................................................................................................... 19

18. Graph showing the shape of normalized response spectra for three different soil columns proposed in 1985 by the

Seismology Committee of the Structural Engineers Association of California................................................................................................... 20

III
IV CONTENTS

TABLES

TABLE 1. Site-amplification effects in various urban areas ............................................................................................................................................................ K3

2. Locations of portable broadband seismograph stations in Utah that recorded nuclear explosions................................................................................. 8
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK 

ALONG THE WASATCH FRONT, UTAH

SITE AMPLIFICATION IN THE SALT LAKE CITY-OGDEN-PROVO

URBAN CORRIDOR AND THE IMPLICATIONS FOR

EARTHQUAKE-RESISTANT DESIGN

By WALTER W. HAYS

ABSTRACT

Like many other urban areas in the United States, Utah's Salt Lake 

City-Ogden-Provo urban corridor is underlain by soil deposits that will 

amplify earthquake ground motions in narrow period bands under certain conditions. When soil amplification occurs, the demand on a structure can exceed its capacity to resist the enhanced ground shaking in the 

narrow period band. In that case, the structure may experience severe 

damage or collapse.

Estimating the level of surface ground shaking in the Salt Lake 

City-Ogden-Provo urban corridor is complex because the cities are 

founded on soils of differing thicknesses, softnesses, and spatial extents. 

Also, no empirical data from past earthquakes in Utah have been 

recorded on these soil deposits to demonstrate the effects that they have 

on the amplitude and spectral composition of the surface ground 

motion. This gap in knowledge makes it difficult to estimate the level 

and variability of ground motion expected from future moderate- to 

large-magnitude earthquakes occurring along the Wasatch Front.

In terms of bedrock motions, a 1982 analysis by S.T. Algermissen 

and others indicates that earthquakes along the Wasatch Front could, in 

a 50-year exposure time, generate bedrock ground motions having peak 

amplitudes of horizontal acceleration and velocity that reach 0.28 g and 

16 cm/s, respectively. Ordinates of spectral velocity for 5-percent damping are expected to reach 100 cm/s at sites underlain by bedrock and 

even higher values at sites underlain by soil.

The amplitude and spectral characteristics of the surface ground 

motions at sites in the urban corridor underlain by soil are difficult to 

quantify precisely, because one must consider (1) the magnitude and 

epicentral distance of the earthquake, (2) the propagation paths of the 

body and surface seismic waves, and (3) variations in the thicknesses, 

softnesses, and geometries of the near-surface soil deposits. To aid the 

analysis, small-amplitude nuclear-explosion ground-motion data, representative of the low-strain environment, were recorded at 71 sites in the 

urban corridor underlain by soil and rock. These data have provided 

new insight, especially in the period band 0.2 to 0.7 s, which corresponds 

to the fundamental mode of response of two- to seven-story buildings, a 

dominant category in the Salt Lake City-Ogden-Provo urban corridor. 

These data, combined with observations of site amplification throughout the world, indicate that the ground-shaking hazard in the Salt Lake 

City-Ogden-Provo area is worst for sites in the centers of the valleys 

underlain by thick, soft silts and clays deposited in connection with the 

filling of the Great Salt Lake in the Pleistocene Epoch. These soft soil

deposits cause frequency-dependent amplification of ground motion relative to a bedrock site on the Wasatch Front, where soil-to-rock amplification factors can reach 10 (1,000 percent) in the period band 0.2 to 0.7 s 

under conditions of low (0.0001-0.1 percent) levels of shear strain. The 

soil-to-rock amplification factors in this period band are about 2 (200 

percent) when the soil deposits are thin gravels and sands near the 

Wasatch Front. Because the thickness of silts and clays increases 

toward the valley centers, the soil-to-rock amplification factors also 

increase as distance from the Wasatch fault zone increases out to a distance of about 35 km, the valley centers.

Evaluation of site-amplification effects in Utah, in the context of 

results obtained from worldwide postearthquake investigations and 

other regional studies, indicates unequivocally that the soil deposits in 

the Salt Lake City-Ogden-Provo urban corridor should be expected to 

amplify earthquake ground motions, especially where the level of shear 

strain induced in the soil column is less than about 0.5 percent This 

level of strain would be expected to occur either in large-magnitude 

earthquakes located tens of kilometers or more from the site or in smallto moderate-magnitude earthquakes having epicenters located as close 

as a few kilometers from the site. ALso, such an evaluation indicates 

that a high potential exists for damaging resonance in some two- to 

seven-story buildings at some locations in the Salt Lake City-OgdenProvo urban corridor during future earthquakes. Data from the September 19, 1985, Mexico earthquake are the best example of what might 

happen in Utah; they showed that "surprising" damage can occur at 

great epicentral distances (for example, 400 km) when the frequency 

response of a soft soil column and the building are the same or almost 

the same.

The technical concern along the Wasatch Front is how to account 

for the nonlinear soil response in future moderate- to large-magnitude 

earthquakes that might nucleate directly under a city. Accurate depiction of the nonlinear response of soil to ground shaking in the nearsource area is still a controversial issue that will remain unresolved in 

Utah until adequate near-source ground-motion data can be acquired to 

define the bounds on the effect.

The potential for site amplification of ground motion and the 

matched responses of some soil columns and buildings (resonance) are 

the most important future implications for earthquake-resistant design 

of buildings along the Wasatch Front. The seismic design provisions of 

the Uniform Building Code need to be evaluated to determine if the level 

of ground motion (that is, peak ground acceleration, peak ground velocity, spectral velocity) expected in a given exposure time and the soil-toKl
K2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

rock amplification factors defined by the soil-structure interaction factor in the code are adequate for designing buildings and other facilities 

in the Salt Lake City-Ogden-Provo urban corridor.

INTRODUCTION

BACKGROUND

This paper presents information on site amplification 

in the Utah's Salt Lake City-Ogden-Provo urban corridor 

located adjacent to the Wasatch fault zone (fig. 1) and 

suggests ways to incorporate this information in earthquake-resistant design and land use practices. The 

Wasatch fault zone is 370-km-long zone of active, normal 

faulting within the Intermountain Seismic Belt. The largest population growth in Utah is occurring in this urban 

corridor, where approximately 1,000,000 people (almost 

80 percent of the State's population) now live. The building stock in this urban corridor is quite varied and 

includes low-, medium-, and high-rise buildings, schools, 

hospitals, the capital, government command centers, and 

lifeline systems (energy, water, communication, and 

transportation systems). The Wasatch fault zone has the 

potential for generating moderate- to large-magnitude 

earthquakes (Schwartz and Coppersmith, 1984). Ground 

shaking, surface fault rupture, and earthquake-induced 

landslides and liquefaction generated in such earthquakes 

could cause serious social and economic disruption 

throughout the State unless realistic mitigation measures 

are devised and implemented before these earthquakes 

occur.

The Salt Lake City-Ogden-Provo urban corridor, like 

many other urban areas in the United States (table 1), is 

underlain by soil deposits that have the potential for 

amplifying earthquake ground motions in narrow period 

bands. Before elaborating further, this paper will review 

some of the terminology used in ground motion literature.

Several terms (for example, site response, soil 

response, ground response, ground-motion response) are 

used interchangeably with site amplification in the literature. All of these terms generally carry the same meaning 

as the term "site amplification," which is used in this 

report. Amplification is always defined relative to a rock 

site (figs. 2, 3). Site amplification refers to the frequencyand strain-dependent physical processes that occur when 

an overlying soil deposit responds to bedrock ground 

shaking. These physical processes can modify the peak 

amplitudes of acceleration, velocity, and displacement 

recorded at the ground surface, but these effects are 

small and relatively insignificant in comparison with the 

effects that occur in a discrete period band of the 

response spectrum. The result is that the spectral composition of the ground motions recorded at the surface of a 

soil deposit will differ from that of the subsurface bedrock ground motions or the ground motion recorded on

44°00' 114°00

42°00

40°00' -

38°00' -

150 MILES

I ' ' I

0 150 KILOMETERS

FIGURE 1. The Wasatch fault zone, a major zone of active, normal 

faulting. The largest urban centers of Utah are located along the fault 

zone. Geologic and geomoiphic evidence acquired primarily from 

paleoseismicity studies show that earthquakes having magnitudes of 

7 or greater have occurred on the fault in the past 8,000 years.

surface outcrops of bedrock. Whether the peak amplitude 

of surface acceleration is increased or decreased by the 

soil column and by how much are the most controversial 

aspects of site amplification and the subject of continuing 

debate.

The soil-to-rock amplification factor is a multiplicative factor ranging from a fraction (deamplification) to 

values of about 10 (1,000 percent). The multiplicative factor is related to a specific part (period band) of the spectrum. A typical description is that the amplification is a 

factor of 5 (500 percent) for the period band 0.2 to 0.7 s. 

Unless noted, the soil-to-rock amplification describes a 

frequency-domain effect for the horizontal component of
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN 

TABLE 1. Site-amplification effects in various urban areas

K3

Location

Period band of dominant response

Short Intermediate Long 

(0.05-0.5 s) (0.5-3.0 s) (3.0-10 s)

References

California

San Francisco region 

Los Angeles region....

Las Vegas region 

Tonopah region.. 

Beatty region......

XXX Wood (1908), Reid (1910), Steinbrugge (1968), Idriss and Seed (1968),

Borcherdt (1975), Borcherdt and others (1975,1978); Joyner and

others (1981). 

XXX Murphy and others (1971a), Boore (1973), Davis and West (1973), Hanks

(1976), Hays (1977), Hays and others (1979), Rogers and others (1979,

1985).

Nevada

XXX Murphy and others (1971b), Bennett (1974), Murphy and Hewlett (1975).

X Murphy and others (1971b), Murphy and Hewlett (1975).

X Murphy and others (1971b), Murphy and West (1974).

Washington State

Seattle region Ihmen and Hadley (1987).

Illinois

Chicago region. Peak and Reed (1954).

Tennessee

Memphis region X X Sharma and Kovacs (1980).

Massachusetts

Boston region. X Whitman (1983).

South Carolina

Charleston region. X Elton and Martin (1986).

Puerto Rico

San Juan region. Molinelli (1985).

ground motion rather than a time-domain effect. The factor describes empirically what happened at the site underlain by soil relative to the reference site underlain by rock 

and is described in terms of a specific period, or period 

band, or the period where the maximum or dominant 

response occurred (fig. 4). A factor of 5 in the frequency 

domain does not correspond to a factor of 5 in the time 

domain. The extent to which the peak amplitude of 

ground acceleration is increased or decreased depends on

what happens physically at zero period (that is, the very 

high frequencies).

The concept of a transfer function comes from the 

theory of linear operators (that is, Fourier and Laplace 

transforms). In this paper, the transfer function is used to 

describe site amplification in the frequency domain, and, 

unless noted, the soil-to-rock transfer function is always 

derived. For a site underlain by a soil deposit, the procedure for estimating the transfer function is to divide the
K4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

16 96 112 

SECONDS

160 176 192

FIGURE 2. Example of site amplification in Salt Lake City. The velocity time histories depict low-amplitude ground 

motions representing a low-strain environment, which causes essentially elastic response. The potential for site 

amplification in the Salt Lake City-Ogden-Provo urban corridor was recognized in the 1960's when ground-motion 

records at sites underlain by deep, soft soil exhibited higher levels of ground motion than records at nearby sites 

underlain by rocks or thinner soil deposits.

response spectrum by the corresponding response spectrum for a nearby site that is underlain by rock. The distance between the two sites must be small in comparison 

with the epicentral distance to each site. The soil-to-rock 

amplification factor for a discrete period band is determined from the transfer function. In the Fourier analysis 

of ground-motion data, the typical procedure until 

recently was to use the amplitude spectrum (which is a 

display of the amplitude of the sinusoids composing the 

time history as a function of frequency) and to neglect the
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN K5

SURFACE

SOIL 

COLUMN

SURFACE

Output

Rock 

Acceleration

ROCK

Input Exciting 

Motion

FIGURE 3. (from left to right) The soil-rock column, the 

bedrock and surface acceleration time histories, the 

corresponding response spectra derived from them, and 

the transfer function. The peak amplitude of acceleration

IIo £;

0.0

a> <D

<D Q. 

GCC/>

SURFACE

Period o

Occ

oo . w 1

ROCK Period

Period

is read from the acceleration time histories. The soil-torock amplification factor for a discrete period band is read 

from the transfer function. A is peak acceleration.

OLU

 10.

UJl- =

0-Z 

C/)UJ

o

ioo.o r

10.0

1.0

Stiff . Medium . Soft

BUILDING NATURAL PERIODS 

1-2 stories | stl^s I >7 stories

0.1 

0.01 0.1 1.0 10.0 

PERIOD, IN SECONDS

100.0

FIGURE 4. Effect of different types of soil columns on the spectral 

composition of ground motion. The general correlation with the 

height of the building is also shown for comparison. The soil column 

in the lakebed zone of Mexico City caused a dominant response 

centered at 2 s. The period band 0.2 to 0.7 s is emphasized in this 

paper because two- to seven-story buildings are a dominant category 

in the Salt Lake City-Ogden-Provo urban corridor.

phase spectrum (which is a display of the phase lead or 

lag of the sinusoids making up the time history as a function of frequency). One advantage of using the transfer 

function is that the source terms (for example, the 

nuclear-explosion forcing function) can be eliminated in 

analyzing site-amplification effects. This process makes 

the result essentially independent of the energy source 

(Rogers and Hays, 1978). Thus, nuclear-explosion sources 

can be used to add to the ground-motion data sample in 

an urban area if proper judgment is used.

EVALUATION OF THE BEDROCK GROUND-SHAKING 

HAZARD IN UTAH

The bedrock ground-shaking hazard for a community 

is usually presented in map format, and the soil effects 

are considered in separate analyses. Such a map displays 

the spatial variation and relative severity of a physical 

parameter (such as peak bedrock ground acceleration or 

bedrock spectral velocity) and provides a basis for dividing a region into geographic zones, each region having a 

similar severity throughout its extent to earthquakegenerated ground acceleration (or some other groundmotion parameters such as ground velocity or spectral 

acceleration). Once the level and extent of ground shaking have been defined for all zones in a region, public policy can be devised to mitigate the potential effects 

through implementation of appropriate actions such as 

avoidance, land use planning, engineering design, and distribution of losses through insurance (Hays, 1981). Each 

mitigation strategy requires some sort of map to zone the 

ground-shaking hazard. The most familiar zoning map is 

contained in the Uniform Building Code (UBC). Soil 

amplification is not a part of the UBC zone map but is 

considered separately in the analysis.

Algermissen and Perkins (1976) and Algermissen and 

others (1982) constructed the first probabilistic maps that 

could be used to define the bedrock ground-shaking hazard in Utah. Later, Youngs and others (this volume) constructed a probabilistic ground-shaking hazard map for 

Utah that included consideration of the surface soil type 

through the attenuation relation. These maps were based 

on an integrated analysis of the following types of data:

1. Seismicity.

2. Earthquake source (seismogenic) zones.
K6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

114°00 42°00'

37 °00'

100 KILOMETERS

FIGURE 5. Probabilistic bedrock peak horizontal ground acceleration 

in Utah. The map is a portion of the national map (scale 1:7,500,000). 

It represents a 90-percent probability of nonexceedance in a 50-year 

exposure time (from Algermissen and others, 1982). The contours 

are expressed in terms of a percentage of the force of gravity. The 

effects of soil amplification are considered separately.

3. Attenuation of peak acceleration (or another groundmotion parameter), holding bedrock or a soil type constant and allowing magnitude to vary.

The maps prepared by Algermissen and others (1982) 

were national maps; the portion for Utah is shown in figures 5 and 6. These maps represent a 90-percent probability of nonexceedance in an exposure time of 50 years. 

They display bedrock ground shaking at levels as high as 

0.28 g (fig. 5) and 16 cm/s (fig. 6).

ACKNOWLEDGMENTS

The technical contributions of Kenneth King and 

Robert Miller (U.S. Geological Survey) are gratefully 

acknowledged. Both individuals contributed significantly 

to the acquisition of ground-motion data and surficial 

geology data along the Wasatch Front and assisted in the 

analysis of the integrated data.

100 MILES 

I

I 1

0 100 KILOMETERS

FIGURE 6. Probabilistic bedrock peak horizontal 

ground velocity in Utah. The map is a portion of the 

national map (scale 1:7,500,000). The map represents a 90-percent probability of nonexceedance in a 

50-year exposure time (from Algermissen and others, 1982). The contours are expressed in centimeters per second. The effects of soil amplification are 

considered separately.

METHODOLOGY FOR EVALUATING SITE 

AMPLIFICATION

Evaluation of the ground-shaking hazard, including 

soil effects, in the Salt Lake City-Ogden-Provo urban corridor has proceeded slowly. Scientific and geotechnical 

data must be acquired along the Wasatch fault zone, analyzed, and integrated with other relevant worldwide data, 

and a number of technical problems must be resolved 

(Hays and Gori, 1984).

Evidence from seismic reflection lines shows that 

individual faults in the Wasatch fault zone dip at low 

angles and that they probably become listric at a depth of 

about 8 km, the depth above which earthquakes having 

magnitudes less than about 5.5 occur. Moderate- to largemagnitude (that is, magnitudes (Ms) of about 6.5-7.5) 

earthquakes, although completely absent in the record of 

historical seismicity dating from 1847, appear to nucleate
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN K7

at depths of 10 to 15 km in or near the zone of brittleductile transition (Smith and others, 1985). When 

frequency-magnitude plots of the recent instrumental 

seismicity data in Utah are extrapolated linearly, the 

frequency of small earthquakes is underestimated, 

and the frequency of large earthquakes is overestimated 

(Schwartz and Coppersmith, 1984). Geologic data from 

trenching (that is, fault-offset distances) show that the 

Wasatch fault is segmented and that repeated large-magnitude earthquakes have occurred in the past 8,000 years 

at average recurrence intervals of several hundred years 

(Schwartz and Coppersmith, 1984). The geologic and seismologic data also suggest that the Wasatch fault zone may 

generate characteristic earthquakes of magnitudes (M §) 

7.0 to 7.5 and that the standard log N=a-bM relation does 

not apply in the strict sense (Schwartz and Coppersmith, 

1984).

The estimated regional attenuation relation must be 

based on and constrained by California strong-groundmotion data, not Utah data No acceleration-attenuation 

relation exists for Utah earthquakes because strongground-motion data have been recorded from only one 

earthquake in Utah, the 1962 magnitude 5.7 Cache Valley 

earthquake.

Empirical strong-motion data from past Utah earthquakes showing the frequency-dependent characteristics 

of ground response under low- to high-strain ground shaking are completely lacking along the Wasatch Front. Data 

on building response in Utah earthquakes are also 

lacking.

Special studies performed to overcome these limitations and to evaluate the potential for site amplification 

are described below.

SOILS DATA

A special effort was made in the 1970's (Miller, 1980) 

to quantify the physical properties of unconsolidated 

materials along the Wasatch Front so that the response of 

the various soil deposits to earthquake ground shaking 

could be determined. Miller's work yielded the following 

information. Salt Lake City, Ogden, and Provo are 

founded on several different types of soil deposits. These 

deposits are from several lakes that filled the Great Salt 

Lake Basin during the Pleistocene Epoch and can be 

grouped into three depositional types: onshore, nearshore, and offshore. Onshore deposits are primarily fluvial gravels and coarse sands that form a parallel band 

along the major mountain fronts. In contrast, nearshore 

and offshore deposits are lacustrine. Nearshore sediments were deposited on beaches, long shore spits and 

bars, deltas, and marshes. Offshore deposits consist of 

very fine sands, silts, and clays that accumulated in quiet, 

relatively deep water and probably represent the finer

RELATIVE SOFTNESS OF SOIL DEPOSITS

Very soft to soft clay; silty clay 

I Medium stiff clay; silty clay 

Loose to dense sand 

Very stiff to hard silty clay 

Sandy clay; silty loam 

I Interbedded sandy gravels; silty clay 

^^| Dense to very dense near-surface sand, 0-12 m thick

Dense to very dense sand, 12-30 m thick 

Near-surface gravel 

_____ Deep gravel 

SOIL *-U ROCK 

500 1000 1500 

SHEAR-WAVE VELOCITY, IN METERS PER SECOND

2000

FIGURE 7. Range of shear-wave velocities of surficial soils in the San 

Francisco Bay region. The relative softness increases from right to 

left. For comparison, the thick, soft clay and silt deposits in the Salt 

Lake City-Ogden-Provo urban corridor have an average shear-wave 

velocity of 200 m/s. (After Gibbs and others, 1976.)

portion of debris being carried into the lake from the 

mountains.

The soil columns in the Salt Lake City area range in 

thickness from about 100 m near the Wasatch Front to 900 

m in the deepest part of the Jordan River valley, where 

surficial materials have shear-wave velocities averaging 

about 200 m/s and a natural moisture content by weight of 

about 43 percent (Miller, 1980). The available information 

on Ogden and Provo, although less than that on Salt Lake 

City, suggests that the physical properties of the soil column underlying Ogden and Provo are similar to those 

underlying the Salt Lake City area. A borehole in the 

Provo area indicates a shear-wave velocity of about 155 

m/s for the surficial materials (Miller, 1980). To provide a 

comparison with another urban area having soils that 

vary in thickness and softness, the range of values of 

shear-wave velocities determined for soils in the San 

Francisco Bay region are given below (Gibbs and others, 

1976) and are shown graphically in figure 7:

55 to 115 m/s Very soft to soft clay and silty clay 

160 to 190 m/s Medium stiff clay to silty clay 

150 to 250 m/s Loose to dense sand 

170 to 305 m/s Very stiff to hard clay and silty clay 

210 to 310 m/s Sandy clay and silty loam 

240 to 270 m/s Interbedded sandy gravels and silty clay 

250 to 400 m/s Dense to very dense near-surface sand 0 

to 12 m thick
K8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 2. Locations of portable broadband seismograph stations in 

Utah that recorded nuclear eocplosions

Station Location Lat, °N. Long, °W.

Logan area

1..... 

3..... 

4..... 

5.....

Logan Airport 

Young Ward 

90 S. Main, Providence 

Utah State University 

Logan Canyon

41.846 

41.708 

41.707 

41.745 

41.741

111.894 

111.909 

111.816 

111.813 

111.786

Ogden area

1..... 

2..... 

3..... 

4..... 

5..... 

6..... 

7..... 

8..... 

11..... 

12..... 

13..... 

16..... 

20.....

2095 N 100 E 

1049 Kiesel 

4376 W 2350 N 

Rainbow Bowling Lanes 

Rogers Poultry Farm 

Weber Main Library 

4463 Forrest Green Drive 

Syracuse Fire Department 

4725 S 300 W 

2550 S 4476 W, Taylor 

Hooper Post Office 

6675 900 S, West Warren 

100 N 844 W, Farmington

41.288 

41.246 

41.303 

41.236 

41.253 

41.221 

41.180 

41.092 

41.180 

41.164 

41.164 

41.266 

39.989

111.983 

111.974 

112.086 

111.928 

111.969 

111.964 

111.942 

112.055 

111.970 

112.123 

112.123 

112.152 

111.912

Salt Lake City area

1..... 

2.....

4.....

6......

8......

10...... 

11...... 

12 ...... 

US ...... 

14...... 

15 ...... 

16...... 

17...... 

18...... 

19......

20...... 

21......

West Bank of Jordan River 

1663 Mt. View 

Rotary Park 

Liberty Park 

Georges Hollow, Fort Douglas 

Utah Geological and Mineral Survey 

3100 Kennedy Drive 

Southern side of Parleys Creek 

Mill Creek Canyon 

4500 South Street 

Castro Springs 

North bank of Dry Creek 

Little Cottonwood Canyon 

1932 Parkridge 

Sandy City Fire Station 

Murray City 

Salt Lake City International Airport 

Saltair 

Salt Lake City International Airport 

fire station 

Hidden Valley G.C. 

12000 S 1700 E

40.652 

40.639 

40.826 

40.749 

40.768 

40.755 

40.757 

40.708 

40.692 

40.679 

40.673 

40.569 

40.573 

40.616 

40.580 

40.639 

40.785 

40.769 

40.783

40.556 

40.536

111.929 

111.844 

111.801 

111.870 

111.809 

111.827 

111.791 

111.809 

111.770 

111.801 

111.809 

111.821 

111.763 

111.839 

111.872 

111.892 

111.955 

112.101 

111.979

111.839 

111.830

TABLE 2. Locations of portable broadband seismograph stations in 

Utah that recorded nuclear explosions Continued.

Station Location Lat, °N. Long, °W.

Salt Lake City area Continued

22 

23 

24 

25 

26 

27. 

28 

29 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 

41

...... Bateman Dairy Farm

...... 10875 S 12 W

......North Jordan Park

......Salt Lake City Fire Station #7

......Capitol Building 

...... University of Utah 

...... Taylorsville 

......4060 S 725 W

......Sugar House Library 

...... 3702 S 645 E

...... 2838 Imperial

......6100 S 700 W

......4130 S 1221 E

......2270 E. Evergreen Street

...... 1560 Industrial Road

......4036 South Golden Circle

......3915 Brooklane Drive

...... 1759 Mt. View

...... 1964 East Evergreen Street

40.612 

40.554 

40.752 

40.776 

40.777 

40.776 

40.663 

40.683 

40.721 

40.691 

40.709 

40.641 

40.682 

40.699 

40.773 

40.684 

40.687 

40.691 

40.697

111.927 

111.922 

111.918 

111.920 

111.887 

111.847 

111.949 

111.910 

111.866 

111.872 

111.843 

111.917 

111.856 

111.832 

112.011 

111.852 

111.848 

111.841 

111.832

Provo area

1

3 

4. 

5 

6 

7 

8. 

9 

10 

11.

...... Olmstead Office Building

...... Olmstead

...... Or em Fire Station

...... Orem city offices 

...... Cherry Hill Farm

..... .Provo Airport

......Boat house, Utah Lake

......Provo city offices 

...... K-96 Radio Station

......Youd Farm

...... Springville Fire Station

40.315 

40.326 

40.314 

40.299 

40.264 

40.221 

40.236 

40.238 

40.213 

40.133 

40.167

111.655 

111.639 

111.695 

111.693 

111.717 

111.718 

111.736 

111.671 

111.671 

111.730 

111.610

Cedar City area

1

3 

4 

5. 

6. 

7. 

8. 

9. 

10.

......Three Peaks

...... Coal Canyon 

...... Rush Lake

......Sevensville

......College of Southern Utah 

...... SUSC football field 

......Highway 14 trailer park 

......Cemetery 

...... Settling pond 

......Imperial 400 Motel

37.717 

37.679 

37.864 

37.786 

37.675 

37.676 

37.678 

37.689 

37.656 

37.673

113.225 

113.050 

113.064 

113.092 

113.067 

113.077 

113.055 

113.063 

113.068 

113.061

ISites underlain by rock; all other sites underlain by unconsolidated sediments.
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN K9

SCHEMATIC ENSEMBLE OF SINGLEDEGREE-OF-FREEDOM SYSTEMS

M

0.05 0,1 0.5 1.0 3.0 6.0 I

PERIOD, IN SECONDS

[ 1-2 stories [3-7 stories)____>7 stories 

BUILDING NATURAL PERIOD RANGE

100

10

Q

I

Ul 

CO

<r 

ff

CO<r

LU

P

I

Z

n 1

CO

0.1

I IMINI 

5 PERCENT ^ 

DAMPED

0.01 0.1 1 

PERIOD, IN SECONDS

FIGURE 8. Correlation of the response spectrum derived from a strongmotion accelerogram with a series of damped single-degree-offreedom harmonic oscillators. The peak amplitude of acceleration is 

the zero-period asymptote of the spectrum. The amplitude of peak 

displacement (D) is the infinite-period asymptote. The site and 

building response are analyzed through the response spectrum. M is 

mass, A is peak ground acceleration, and Vis peak ground velocity.

250 to 570 m/s Dense to very dense sand 12 to 30 m

thick

400 to 55 m/s Near-surface gravel 

420 to 750 m/s Deep gravel

SITE-AMPLIFICATION DATA

Two strategies were used to offset the limitations 

posed by the lack of ground-motion data from past Utah 

earthquakes (Hays and others, 1978; Hays, 1980): (1) 

nuclear-explosion ground-motion data recorded simultaneously at selected sites (table 2) underlain by soil and 

rock in each city were used as a basis for deriving soil 

transfer functions to give a bound on site amplification 

under low-strain conditions and (2) soil transfer functions 

were related to the depositional environment and physical properties (for example, shear-wave velocity, bulk

density, lithology, and water content) of the 71 recording 

sites in the Salt Lake City-Ogden-Provo urban corridor. 

Ground-motion measurements from a dozen nuclear 

explosions at the Nevada Test Site were made simultaneously at sites underlain by soil and rock in each city as 

well as sites in Logan and Cedar City to provide some 

comparison of effects. Recording sites underlain by rock 

on the Wasatch Front encompassed limestone, quartz 

monozite, shale, and sandstone. Portable broadband 

velocity seismographs were deployed at each location. 

Acceleration and displacement time histories were 

derived from the velocity records along with 5-percent 

damped response spectra. These data provide complete 

information on the amplitude and spectral response of the 

ground motion recorded ar each site, but they are 

representative only of the small-amplitude, low-strain 

environment.

A brief discussion of the response spectrum is provided below to facilitate understanding. The response 

spectrum is a common representation of the spectral 

composition of earthquake and ground motion. The 

response spectrum (fig. 8) depicts graphically the peak 

amplitude of each time history, representing the individual response of each one of a series of damped, simple 

harmonic oscillators to the acceleration time history 

input at the base of the oscillator. The structural engineer 

uses the response spectrum having 2 to 5 percent damping as a tool to correlate characteristics of ground motion 

with the elastic response of buildings of various heights 

and types. The concepts of the response spectrum are 

also incorporated in the UBC and other building codes.

Soil transfer functions were derived in each city from 

the response spectra (figs. 9-13). Many investigators (for 

example, Borcherdt, 1975; Rogers and Hays, 1978; Hays 

and others, 1978, 1979; Hays, 1980) have shown that the 

transfer function, defined as the average ration of the 5- 

percent damped, horizontal velocity response spectra for 

a pair of soil-rock sites, is a sensitive function of the properties of the soil columns underlying the sites. Rogers and 

Hays (1978) showed that a transfer function can be determined reasonably well from either earthquake or nuclearexplosion ground-motion data because the source effects 

are essentially eliminated when taking the ratios, 

although there may be a significant difference in the levels of peak ground accelerations and dynamic shear strain 

(defined approximately by the ratio of the peak particle 

velocity to the shear-wave velocity of the soil). Hays and 

others (1978) concluded from the lack of dependence of 

site amplification on amplitude that the response of some 

soil deposits in Nevada is probably linear up to about 0.5- 

percent strain. However, this conclusion cannot be verified for the Salt Lake City-Qgden-Provo urban corridor 

until earthquake data have been recorded.
K10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

FIGURE 9. Range of transfer functions showing horizontal 

ground response of sites underlain by thin (150 m or less) 

deposits of sands and gravels (stations 6, 8, 10, 11, 12) 

relative to a rock site (station 7) on the Wasatch Front in

10

SPECTRA y 

Fo 

O 1

LJJ

LOCI

O

1-

cc

0.1

THIN, SEMISATURATED, 

UNCONSOLIDATED MATERIAL

I V \ 

Upper ^ / \ i^ 

envelope j ^ .** i I 

^ -" ~~ ..*../* \ ' V^

Mean *'*" /^ / ^ \\."*" . ^^\

Lower envelope ~^ . \ 

\ 

\

-

I i ili i J

0.1 1 10

PERIODS, IN

the Salt Lake City area Ground response is shown in 

terms of a mean transfer function. These data represent a 

low-strain environment The dashed line shows the Salt 

Lake City limits.

10

u

O 1

0.1

Upper 

envelope

Mean*'*'"

Lower envelope

THICK, SATURATED, \ 

UNCONSOLIDATED MATERIAL

I i

0.1 10

PERIODS, IN SECONDS

FIGURE 10. Range of transfer functions showing horizontal 

ground response of sites underlain by thick (450-750 m), 

soft deposits of silts and clays (stations 17-19, 24, 25) 

relative to a rock site (station 7) on the Wasatch Front in

the Salt Lake City area. Ground response is shown in 

terms of a mean transfer function. These data represent a 

low-strain environment. The dashed line shows the Salt 

Lake City limits.

MAPS OF THE SITE-AMPLIFICATION FACTORS 

FOR SALT LAKE CITY, OGDEN, AND PROVO

Maps showing the spatial variation of the soil-to-rock 

amplification factors were prepared for Salt Lake City, 

Ogden, and Provo (fig. 14). These maps were based on the 

mean value of the soil transfer function in a specific

period band 0.2 to 0.7 s derived from small-amplitude 

nuclear-explosion data (Hays and King, 1982, 1984). This 

period band was selected because the signal-to-noise 

ratio is best for this band and also because it correlates 

roughly with the fundamental response mode of two- to 

seven-story buildings, a dominant group in this urban 

corridor.
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN Kll

SALT LAKE CITY REGION

Soil-to-rock or site-amplification factors in the Salt 

Lake City region for the period band 0.2 to 0.7 s are shown 

in figure 14A Data for this period band indicate the following:

1. Site amplification factors vary from 1 to 8.5 within the 

corporate limits of the city.

2. Site amplification factors (relative to a site underlain 

by rock on the Wasatch Front) increase to the west 

with increasing distance from the Wasatch fault zone. 

This effect, which correlates with the transition from 

stiff soils near the Wasatch Front to thick, soft soils in

10.0 rr

0.01 0.1 1.0 

PERIOD, IN SECONDS

10 .0

FIGURE 11. Range of transfer functions showing horizontal ground 

response of sites underlain by rock (stations 5, 9, 13). The reference 

bedrock station is station 7 on the Wasatch Front in the Salt Lake City

the valley center, offsets to some degree the normal 

decay of the peak amplitudes and spectral ordinates of 

ground motion with distance from the fault zone. The 

effect would be most important when the hypocenters 

are 20 to 50 km from Salt Lake City. 

3. Site-amplification factors are as much as 10 (1,000 percent) at sites underlain by the thick (500-900 m), soft 

deposits of clays and silts in the center of the valley, 

whereas they are about 2 (200 percent) at sites underlain by the relatively thinner (150 m) deposits of sand 

and gravels near the Wasatch Front. These data suggest 

a high damage potential for two- to seven story buildings in parts of Salt Lake City when the response of the 

soil column and the response of the building are 

closely matched.

OGDEN REGION

The site-amplification factors in the Ogden region for 

the period band 0.2 to 0.7 s are shown in figure 145. Data 

for this period band show the following:

1. The spatial variation of the site-amplification factors is 

rapid but not as rapid as that in the Salt Lake City (and 

Provo) area.

2. The site-amplification factors (relative to a rock site on 

the Wasatch Front) are as great as 5 (500 percent) and 

are greatest for the thick, soft deposits of clays and 

silts located in the valley center some distance from 

the Wasatch Front. Except in one zone, the factors also 

increase with increasing distance from the surface 

trace of the fault zone. The explanation for the zone of

<

cc 

o

LU 

Q_ 

CO

>

o 

o

f\l

10

0.1

j :

Upper ^"..// 

envelope jus'\ (

. */ ^ 

Mean , /^ J

Lower envelope

THICK, SATURATED, 

UNCONSOLIDATED MATERIAL

0.1 10

PERIOD, IN SECONDS

FIGURE 12. Range of transfer functions showing horizontal ground response of sites underlain by thick, soft silts and clays 

(stations 2, 6) relative to a rock site (station 4) on the Wasatch Front in the Ogden area. These data represent a lowstrain environment. The dashed line shows the Ogden city limits.
K12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

o 

o

10

0.1

Upper 

envelope j ...... */'

THICK, SATURATED, 

UNCONSOLIDATED MATERIAL

0.1 1

PERIOD, IN SECONDS

10

FIGURE 13. Range of transfer functions showing horizontal ground response of sites underlain by thick, soft deposits of silts 

and clays (stations 3, 4) relative to a rock site (station 2) on the Wasatch Front in the Provo area. Stations 3 and 4 are 

located in the Orem area. These data represent a low-strain environment. The dashed lines show municipal boundaries.

low site amplification is unknown at present, but a 

local soil deposit having a value of shear-wave velocity 

higher than normal is suspected as the cause.

PROVO REGION

The site-amplification factors (relative to a rock site 

on the Wasatch Front) in the Provo area are shown in figure 14(7 for the period band 0.2 to 0.7 s. These data lead to 

the following conclusions:

1. The site-amplification factors vary from 1 to 7.5 over a 

distance of only 10 km and increase with increasing 

distance westward from the surface trace of the 

Wasatch fault zone, as they do in Salt Lake City and 

Ogden.

2. The greatest site-amplification factors (fig. 14) correlate with thick, soft deposits of clays and silts located 

some distance from the surface trace of the fault zone, 

and the smallest site-amplification factors correlate 

with relatively thin deposits of gravel and sand near the 

Wasatch Front.

POSSIBLE USES OF SITE-AMPLIFICATION DATA 

AND MAPS

UNIFORM BUILDING CODE

In Utah, considerable new knowledge is becoming 

available on the earthquake potential of specific faults, 

the bedrock ground-shaking hazard, soil amplification, 

earthquake-induced ground failure, and other earthquakerelated phenomena. Also, recent damaging earthquakes 

(for example, the September 19, 1985, Mexico earthquake) have provided engineers with important information on earthquake-resistant design. This new knowledge 

has not yet been fully incorporated in the UBC or in the 

design of structures in Utah.

The UBC (Uniform Building Code) (International 

Conference of Building Officials, 1988), the most widely 

used standard for earthquake-resistant design, contains 

the following commentary about seismic design:

Basically, the problem is that the entire phenomenon, from the 

earthquake ground motion to the realistic response of structures 

to this ground motion, is very complex. Codes, of necessity, are 

generalized simplifications. Complex mathematical analyses 

have been made on simple and idealized structures subjected to 

past earthquake ground motions. These have been helpful in 

improving our understanding of the phenomenon. However, for 

purposes of design of the vast majority of structures, it is necessary to reduce this complex, dynamic problem to one of equivalent static lateral forces. These can be related to the dynamic 

characteristics of the structure. They provide the basic code criteria, applied with stresses within the elastic limit. However, in 

applying these simplified concepts, the structural engineer must 

do this with sound judgment that can only be developed with 

experience, observation, and study of the earthquake phenomenon. He must be especially aware of the nature of the response 

of the particular structure under design and he must evaluate the 

capabilities of the structure to perform satisfactorily beyond the 

elastic-code-stipulated stresses.

The UBC is a minimum standard to assure public 

safety. Its requirements are intended to safeguard against 

major failures and loss of Me. The aim of the code is to 

provide structures that will:
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN K13

40°45' \

Map is preliminary and should 

not be used for planning 

decisions without considerations of high-strain effects, 

surface faulting, and the 

seismic radiation pattern.

Ground response is 

relative to station 7

 Seismograph station 

 Salt Lake City limit

Rock

5 MILES 

J

5 KILOMETERS

Map is preliminary and should 

not be used for planning 

decisions without considerations of high-strain effects, 

surface faulting, and the 

seismic radiation pattern.

Ground response is 

relative to station 7

 Seismograph station

- Ogden limit 

W//, Rock

Ground response is 

relative to station 7

 Seismograph station 

 Provo limit 

Rock

Map is preliminary and should 

not be used for planning 

decisions without considerations of high-strain effects, 

surface faulting, and the 

seismic radiation pattern.

FIGURE 14. Spatial variation of site-amplification factors for the period 

band 0.2 to 0.7 s. Values on contours indicate the ratios of velocity 

response spectra that would be expected at sites underlain by soil 

relative to that same ratio experienced at sites underlain by rock on 

the Wasatch Front These values provide an estimate of the spatial 

variation of ground motion in the period band for which two- to 

seven-story buildings have their fundamental natural period of 

vibration (from Hays and King, 1982). These data represent a lowstrain environment. Repeatability would be least certain when a 

moderate- to large-magnitude earthquake nucleates underneath the 

city. A, Salt Lake City area; B, Ogden area; C, Provo area.

1. Resist minor earthquakes without damage.

2. Resist moderate earthquakes without structural damage but with some nonstructural damage.

3. Resist major earthquakes without collapse but with 

some structural and nonstructural damage. 

The basic formula used in the UBC is

V = ZIKCSW (1)

where:

 V is the base shear or total lateral force at the base of 

the structure.

 Z is the zone factor, the numerical coefficient that 

depends on the seismic risk zone. California is in
K14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

zones 3 and 4; the Salt Lake City-Ogden-Provo urban 

corridor is in zone 2.

 / is the occupancy importance factor for the building.

 K is an arbitrary horizontal force factor that, in 

effect, is a safety factor adjustment. Based on an 

arbitrary classification of the type of construction, it 

recognizes that different degrees of risk for collapse 

are inherent in different types of construction.

 C is a numerical coefficient based on the fundamental period of the building. A plot of C against period 

represents a response spectrum and roughly parallels the spectrum derived from the accelerogram of 

the Imperial Valley, Calif., earthquake recorded ar El 

Centro in 1940.

 S is the soil-structure interaction factor that incorporates the potential for site-structure resonance. The 

values of S range from 1 to 1.5, the highest value 

occurring when the fundamental periods of vibration 

of the building and the soil column are equal.

 W is the total dead load of the building. 

A strategy for incorporating new information on 

earthquake hazards along the Wasatch Front should 

include the following activities:

1. Updating the seismic risk zone map that gives the Z 

factor to reflect the latest information on the groundshaking hazard, as depicted probabilistically. For 

example, the map of Utah can be used to give the peak 

horizontal bedrock acceleration for a 50-year exposure 

time (the useful life of ordinary buildings) and a 90-percent probability of nonexceedance.

2. Reviewing the soil-structure interaction factor that is

prescribed in the building code to determine if Utah

soils are represented adequately, especially the thick,

soft clays and silts in the valley centers.

Since the UBC undergoes vigorous evaluation and

recommendations for change every 3 years, a mechanism

already exists to implement changes that are relevant for

Utah.

APPLICATIONS IN UTAH

Maps of the bedrock ground-shaking hazard (for 

example, those produced by Algermissen and others, 

1982) and soil-to-rock amplification factors can be 

applied in building codes, land use guidelines, construction practices, and criteria for repairing, strengthening, 

and retrofitting existing buildings. Implementation of 

these kinds of loss-reduction applications in Utah would 

involve the following technical tasks:

Evaluation of the Uniform Building Code. The 

need to modify the current edition of the UBC to provide 

design criteria that are realistic in terms of the current 

knowledge of both the earthquake potential and the 

ground-shaking hazard in the Salt Lake City-Ogden-Provo

urban corridor should be evaluated carefully. In addition, 

the best available data on soil-structure interaction 

should be incorporated into recommended code changes 

to ensure that either poor site-structure combinations are 

avoided or appropriate engineering is performed to 

develop adequate structural capacity.

Evaluation of regional and urban land-use 

practices. Alternatives to current practices should be 

identified to reduce the probability of constructing at 

sites having poor site-structure combinations (that is, siting of buildings on soil-rock columns where the natural 

period of the soil column, Ts, is the same as the natural 

period, Tb, of the building). Where there is no viable alternative, appropriate engineering can reduce the chance of 

severe damage.

Evaluation of construction practices for new 

buildings. Alternatives to current practices that will 

ensure a higher level of quality of materials, workmanship, and inspection should be specified.

Evaluation of current practices to repair and 

strengthen existing buildings. Alternatives to current 

practices should be identified to increase the seismic 

resistance of buildings designed and built to an earlier 

standard of earthquake resistance.

ARGUMENTS TO MINIMIZE OR ELIMINATE CONTROVERSY 

IN THE APPLICATION OF THESE DATA

The most important requirement for maps of the 

earthquake ground-shaking hazard (including siteamplification effects) is that they be useful for guiding 

engineering, social, and political decisions. From a scientific viewpoint, such a map must encompass all possible 

earthquake source-path-site combinations for a given 

exposure time, or the scale must be such that variations 

in source and path effects cause negligible changes in the 

expected values. The repeatability of site amplification 

must be reasonably consistent from earthquake to earthquake, especially for the larger magnitude earthquakes 

used to set design and construction practices and to guide 

response planning.

The results in the Salt Lake City-Ogden-Provo urban 

corridor are unequivocal, except for considerations 

involving near-source locations. At these locations, the 

source effects dominate the site effects. The trends of site 

amplification in the Salt Lake City, Ogden, and Provo 

areas shown in figure 14 can be expected to recur in 

small- to moderate-magnitude earthquakes occurring 

along the Wasatch fault zone and producing smallamplitude, low-strain ground motion causing essentially 

elastic response. The most significant implication of these 

results is that ground motion may be amplified by as 

much as a factor of 10 (1,000 percent) at some sites in the 

0.2- to 0.7-s period band corresponding with the natural
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN K15

100.0

10.0 -

Upper envelope of 

spectra for sites 

/ underlain by 

unconsolidated 

materials \ X

0.01 o.i 1.0

PERIOD, IN SECONDS

10.0

FIGURE 15. Estimated range of values of horizontal spectral velocity 

that might be expected in Salt Lake City, Ogden, or Provo within 

about 35 km of a large (Mg 7.5) earthquake on the Wasatch fault zone. 

Such an earthquake is currently estimated to produce a peak bedrock 

velocity of 16 cm/s (Algermissen and others, 1982). The greatest 

controversy is over the question of strain dependency and whether 

the peak bedrock acceleration (zero-period asymptote of the 

response spectrum) would be increased or decreased at sites 

underlain by soil. The period band 0.2 to 0.7 s emphasized in this 

paper is shown on the graph.

periods of vibration of two- to seven-story buildings. This 

type of elastic or essentially elastic response could also 

be realized in a large, distant earthquake (occurring as far 

away as 100 km), which could cause unusual damage to 

some buildings (for example, in a manner similar to the 

September 19,1985, Mexico earthquake).

Significant differences in the trends of site amplification shown by these data should be expected in a moderate- to large-magnitude earthquake occurring at a depth of 

10 to 15 km directly under one of the cities and causing a 

peak bedrock acceleration of 0.2 g or greater. Such a scenario would be the worst case of the near-source problem 

for a listric fault. The variability of ground motion can be 

expected to be greatest at sites closest to and aligned with 

the fault rupture. In such a case, the effects of the source 

and the path are both important; they are also difficult to 

separate. The amplification of ground motion expected to 

occur in the period band 0.2 to 0.7 s at sites underlain by 

thick, soft clays and silts located at distances ranging 

from the epicenter to 35 km from the zone of earthquake 

nucleation would tend to offset the normal attenuation of

ground motion with increasing distance from the hypocenter. The most likely outcome in this case is that locations in a broad area in each city within about 35 km of 

the hypocenter would experience about the same level of 

ground shaking, as represented by peak ground acceleration or peak ground velocity, but exhibit considerable 

variability, as represented by the response spectrum. The 

range, represented on the smooth response spectrum, is 

shown in figure 15. The effect of strain dependence on 

site amplification in the near-source area is the primary 

unknown (fig. 16). This question will not be completely 

resolved for Utah until ground motions near the fault are 

recorded in large-magnitude earthquakes either in Utah or 

in other parts of the world having a similar seismotectonic setting.

TECHNICAL ASPECTS OF SITE AMPLIFICATION

Because site amplification of earthquake ground 

motion is a controversial subject and the limited earthquake data in Utah require utilization of small-amplitude 

nuclear-explosion data, a brief review of worldwide siteamplification data will be given in the next section to provide a broader framework of understanding. Such understanding is needed for interpreting and using the siteamplification data obtained in Utah until a more complete 

data set is available.

Scientists and engineers throughout the world have 

recognized and documented since the 1800's that site 

amplification commonly occurs in earthquakes (MacMurdo, 1824; Idriss and Seed, 1968; Seed and Idriss, 1969; 

Seed and others, 1972; Tezcan and others, 1977; Joyner 

and others, 1981; Rosenblueth, 1986; Savy and others, 

1986; State of California, 1990). Many earthquakes have 

shown scientists and engineers that two frequencydependent phenomena site response and structural 

response must be considered explicitly in earthquakeresistant design. A number of factual statements can now 

be made about various aspects of these phenomena:

1. During an earthquake, the characteristics of the earthquake ground motion in any city will vary widely, 

depending on the thickness, softness, and geometry of 

the local soil-rock columns and on the source phenomena causing the directivity and focusing of energy 

(Earthquake Engineering Research Institute, 1986). 

This observation means that there probably will be 

some "surprises" in the Salt Lake City-Ogden-Provo 

urban corridor in that some ground-motion phenomena in future earthquakes will not be able to be 

explained except in terms of site-amplification 

phenomena.

2. Damage to a structure at a particular site in an earthquake is complexly related to the dynamic frequencydependent properties of the earthquake source, the
K16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

0.65

0.325

0.065

0.006

0.001

DEEP, FIRM SOIL

Magnitude

X ^ X? N x

< \ \ \ \

\ \ 

'^

\ \ 

\ \

1 , , \ , , I \ I

3 10 30 100 200 

DISTANCE, IN KILOMETERS

FIGURE 16. Estimate of the level of dynamic shear strain as a function 

of epicentral distance and magnitude. A deep soil column having a 

near-surface shear-wave velocity of about 200 m/s is modeled.

low-pass filtering characteristics of the wave propagation path, and the band-pass filtering characteristics of 

both the structure and the soil-rock column underlying 

the structure. The physical parameters of the earthquake and the earth that cause the soil-rock column 

and the structure to vibrate at the same period increase 

the potential for damage (Yamahara, 1970). These 

parameters are not completely known yet in the Salt 

Lake City-Ogden-Provo urban corridor. Additional geologic investigations needed to improve basic understanding are underway.

3. The level of dynamic shear strain and its effects on soil 

properties are the most controversial aspects of site 

amplification. The level of shear strain induced in the 

soil column by the input bedrock motion increases as 

the magnitude of the earthquake increases and 

decreases as the distance from the center of energy 

release increases (see fig. 16).

4. The response of the site to ground shaking depends 

strongly on the strain-dependent properties of the soilrock column underlying the site. The level of dynamic 

shear strain and the contrast in physical properties of 

the soil and rock determine whether the soil acts to

transmit the energy or to dissipate it. As an energy 

transmitter, the soil column acts as a band-pass filter 

by modifying the amplitude and phase spectra of the 

incident body and surface seismic waves in selected 

periods (Murphy and others, 1971b) and increasing the 

duration of shaking (Hays, 1975). As an energy dissipator, the soil column can damp the earthquake ground 

motion, thereby reducing the level of peak acceleration. As a soil-structure system, part of the vibrational 

energy of both the soil column and the structure can be 

transmitted back into the Earth; this transmittal permits vertical movement, rocking, and side-to-side 

movement of the structure on its base (Wolf, 1985).

5. Site amplification (the frequency- and strain-dependent 

response of the soil-rock column to body and surface 

seismic waves) increases the amplitude of the surface 

ground motion in a narrow period band that can be 

related to the thickness, softness, shear-wave velocity, 

bulk density, and geometry of the soil column (Seed 

and others, 1976). The transfer function is one way to 

categorize the dominant spectral response in terms of 

the period band where it occurs. Although the 

response can be related to many discrete period bands, 

three are typically used to categorize the effect: short 

period (0.05-0.5 s), intermediate period (0.5-3 s), and 

long period (3-10 s). Each period band correlates 

approximately with the fundamental mode of response 

of buildings of increasing heights and decreasing 

stiffness

6. The structure also acts as a band-pass filter as it 

responds to the ground shaking as its base. The spectral response of the structure can be increased or 

decreased in selected period bands that depend on the 

type of structure, the construction materials used in 

the lateral-force-resisting system, the lateral and vertical dimensions, the physical properties of the soil-rock 

column, and the wavelengths and amplitudes of the 

incident seismic waves. The demand on the structure is 

greatest when the dominant period of the bedrock 

motion, the fundamental natural period of vibration of 

the structure, and the natural period of the soil column 

are the same, a case that creates a condition for sitestructure resonance (figs. 3, 4).

SITE AND BUILDING PERIODS

Earthquake-resistant design must take into account 

the conditions that cause site amplification of ground 

motion and damaging soil-structure resonance. In an 

urban area, careful evaluation is required to identify the 

wide range of soil-rock columns and their physical properties, the various types of buildings, and the physical 

conditions that cause the soil and building responses to 

occur at or close to the same period. A soil column, like a
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN K17

building or structure (see fig. 13), has a natural period of 

vibration, Ts, given by the relation

(2)

where H is the thickness of the soil column and Vs is the 

shear-wave velocity measured at low levels (0.0001 percent) of strain. Soils, depending on their physical properties, typically have shear-wave velocities ranging from 50 

to 600 m/s, whereas rocklike material and rock have 

shear-wave velocities greater than about 750 m/s.

One characteristic of site amplification in the Salt 

Lake City-Ogden-Provo urban corridor is that the 

response is not dominated by a specific period or period 

band. The entire spectrum at sites underlain by soil 

exceeds that at sites underlain by rock.

Soils clearly exhibit strain-dependent properties in 

laboratory tests (Seed and Idriss, 1969). Tests on "undisturbed" samples have shown that, as the level of dynamic 

shear strain increases, the material damping increases 

and the shear modulus decreases. The result is that Ts 

increases as the level of shear strain increases. The basic 

empirical relation has been given by Seed (1975) as

T. = 4H/(RV.) (3)

where R is an empirical factor having the following values: (1) 0.9 for a magnitude 6 earthquake producing a 

peak effective acceleration of 0.1 g, (2) 0.8 for a magnitude 6 earthquake producing a peak effective acceleration 

of 0.2 g, and (3) 0.67 for a magnitude 7 earthquake producing a peak effective acceleration of 0.3 to 0.4 g. The fundamental natural period of vibration Tb of a building is given 

approximately by the relation

rb = AVIO (4)

where N is the number of stories. However, the stiffness 

and flexibility of a building (design parameters controlled 

by the structural engineer and architect) can make its 

actual fundamental natural period shorter or longer. 

Observations from postearthquake investigations have 

shown that Tb lengthens as the thresholds of various 

states of damage are reached. In an earthquake, the 

"worst" case for damage is when the value of Ts coincides 

with that of Tb . This situation causes resonance of the 

building and can result in severe damage or collapse 

unless the building has been designed to withstand the 

forces generated by this phenomenon.

The type of structure being sited controls the scope 

of the evaluation. For example, amplification of surface 

waves is not typically considered in siting a nuclear powerplant, which is more sensitive to short-period vibrations 

of body waves than to long-period vibrations of surface

waves. On the other hand, potential site amplification of 

long-period Rayleigh waves is an important consideration 

when siting a high-rise building, especially if the building 

is founded on a deep, soft soil deposit like those in the 

Salt Lake City-Ogden-Provo urban corridor.

LEVEL OF DYNAMIC SHEAR STRAIN

Assessing the level of dynamic shear strain and its 

effects on the physical properties of the soil column 

requires laboratory tests and good technical judgment. 

Laboratory measurements demonstrating that soils have 

shear moduli and damping characteristics that depend on 

the level of strain suggest that, under certain conditions, 

nonlinearities and inelasticities in the soil will attenuate 

rather than amplify the peak amplitudes of surface 

ground motion at sites underlain by soil. Current understanding is limited because the high levels of strain produced in the laboratory have not been duplicated in the in 

situ environment by actual strong-motion records of past 

earthquakes. For example, the greatest value of peak 

ground velocity ever recorded (in the 1971 San Francisco 

and 1979 Imperial Valley, Calif., earthquakes) is 110 cm/s. 

The empirical rule that

strain= peak velocity recorded at the site____ 

shear-wave velocity of the soil column at the site (5)

leads to the conclusion that the greatest level of strain 

induced in situ in soil columns in past earthquakes has 

reached only about 0.5 percent, much less than the level 

of strain developed in the laboratory environment.

Some researchers (for example, Hays, 1978; Hays 

and others, 1979; Hays and King, 1982) have shown that 

site response is essentially linear up to in situ strain levels 

of about 0.5 percent for some soil-rock columns. The epicentral distance to the strain level of 0.5 percent is only a 

few kilometers for soil columns like those in Salt Lake 

City having a shear-wave velocity of 200 m/s. The epicentral distance to 0.5 percent decreases as the shear-wave 

velocity of the soil approaches the shear-wave velocity of 

the bedrock.

The complexity in the specification of the dynamic 

properties of the soil is directly related to the differences 

in the laboratory and in situ environments and the problem of obtaining an "undisturbed" sample. The complexity increases below depths of 30 m because of the 

difficulty and expense of sampling.

THICKNESS OF THE SOIL COLUMN

Two different points of view have been used to define 

the critical thickness of the soil column, the physical 

parameter that strongly affects the period of the dominant
K18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

site amplification. One view (Seed, 1975) considers that 

the soil column includes only near-surface material having a shear-wave velocity less than 765 m/s. The other 

view (Kobayashi and Nagahashi, 1982) considers that the 

soil column extends to bedrock having a compressionalwave velocity of at least 3,600 m/s. In the first case, surface ground motions are assumed to be affected mainly 

by a short soil column, typically about 30 m thick, 

whereas, in the'second case, surface ground motions are 

assumed to be affected by a much thicker soil column. 

The short soil column will mainly affect the short periods 

and the peak amplitudes of acceleration, whereas the 

thick soil column will affect mainly the intermediate- and 

long-period bands and the peak amplitudes of velocity 

and displacement. In the Salt Lake City-Ogden-Provo 

urban corridor, the effects of both short and deep soil columns are present because the thickness of the soil deposits adjacent to the Wasatch Front varies from about 150 to 

900m.

NEAR FIELD

The near field (or near source) of an earthquake is 

the most complex area to evaluate for potential site 

amplification. The site is defined as being in the far field 

(or "far-source" region) of an earthquake when it is 

located far enough from the physical extent of the 

earthquake source that the solid angle subtended by the 

source and the site is very small. Estimating the characteristics of ground motion is much easier in the far fields 

than it is in the near field, where seismic energy arrives 

nearly simultaneously from many different directions. 

The difference is essentially one of using a point-source 

(the far field) or a line-source (the near field) approximation. The near-field problem in Utah is complicated 

because of the listric nature of the Wasatch fault zone, 

which makes it plausible for earthquakes to nucleate 

directly under any of the three cities. Analyses of strongground-motion data close to the causative fault have been 

made by a number of investigators (for example, Idriss, 

1978; Hays, 1980; Singh, 985). For the near field, these 

analyses indicate the following:

1. Analytical separation of the frequency-dependent 

effects of the source from the effects of the soil-rock 

column is very difficult. Near the fault, the source 

tends to dominate the path and site effects. The directivity of the source can cause considerable variability 

in the values of the peak amplitudes of ground acceleration, velocity and displacement, and the ordinates of 

spectral velocity (Singh, 1985).

2. A "killer pulse," a pulse of approximately 1-s duration 

that typically does not have the greatest peak amplitude of acceleration but does have the greatest kinetic 

energy, is generated in some cases in the near field (for

example, in the 1971 San Fernando earthquake) as a 

consequence of the "fling" of the fault (Bertero and 

others, 1978). Breakout and stopping phases of the 

fault rupture also can affect the near-field ground 

motion and complicate the analysis.

BEDROCK MOTIONS

Estimating bedrock ground motions is also a 

difficult task in evaluating site-amplification effects. The 

frequency-dependent characteristics of the bedrock 

motion that is put into the soil column depend on the 

details of the geology of the propagation path. These 

details are usually imprecise. Therefore, analytical calculations must be bounded; a suite of strong-motion accelerograms acquired in past earthquakes at sites underlain 

by rock can be used to constrain the analyses. The ideal 

data are from sites underlain by the same type of rock, 

located at about the same distance from the zone of 

energy release, and having the same geology underlying 

the propagation path as the site being evaluated. An adequate analog having the appropriate source-path-site 

characteristics does not yet exist for the Salt Lake CityOgden-Provo urban corridor.

VARIABILITY IN SITE-AMPLIFICATION EFFECTS

Several investigators (for example, Murphy and others, 1971b; Hays, 1980) have shown that site-amplification 

effects for some soils are fairly repeatable if the soil is 

soft relative to the bedrock, the level of strain is low (for 

example, 0.0001 to 0.1 percent), and the site is not near 

the fault zone. The degree of repeatability of the site 

transfer function is currently unknown for sites near the 

fault when the shear strain approaches and exceeds 0.5 

percent.

WORLDWIDE DATA

Four examples of site amplification of ground 

motions representing a low-strain environment are 

described below because of their relevance to the Salt 

Lake City-Ogden-Provo urban corridor.

1967 CARACAS, VENEZUELA, EARTHQUAKE

Soil-structure resonance occurred in Caracas, 56 km 

from the epicenter of this moderate (Mg 6.4) earthquake. 

Severe damage was restricted mainly to tall buildings 

(higher than 12 stories) sited on thick, soft soil columns 

that were at least 160 m thick. The dominant site response 

occurred in the intermediate-period band, centered 

around 1.4 s (Seed and others, 1972).
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN K19

E-W DIRECTION Peak=17 percent 9

10 20 30 40 50

TIME, IN SECONDS 

STATION SCT (MEXICO CITY LAKEBED)

60

Peak =3.5 percent g

-20

00 UJ -40 

<Z 0 40 60 80 100 

TIME, IN SECONDS

120 140

STATION TACUBAYA (MEXICO CITY ROCK SITE)

FIGURE 17. Comparison of strong-motion accelerograms in a low-strain 

environment recorded on firm soil and in the soft soil (of the lakebed 

zone) in the September 19, 1985, Mexico earthquake. The soft soil in 

the lakebed zone of Mexico City located 400 km from the epicenter 

amplified the ground motion in a period band centered around 2 s. 

The records were provided by the Universidad Nacional Autonoma de 

Mexico.

1970 GEDIZ, TURKEY, EARTHQUAKE

Soil-structure resonance under small-amplitude 

ground motions caused the collapse of a one-story garage 

and paint workshop (a part of the Tofias automobile factory) located 225 km from the epicenter of this large (M s 

7.0) earthquake. The cause was the similarity of the predominant periods of the bedrock motions, the response of 

the 120- to 135-m column of alluvium, and the response of 

the building. The resonance occurred in the intermediateperiod band and was centered around 1.2 s (Tezcan and 

others, 1977).

1976 FRIULI, ITALY, EARTHQUAKE

Site amplification of a factor of 4 (400 percent) 

occurred in the short- to intermediate-period band (0.2- 

0.7 s) for a site underlain by 15 m of alluvium located 25 

km from the epicenter. The peak amplitude of bedrock 

accelerations ranged from 0.10 to 0.53 g (Savy and others, 

1986). The magnitude (Ms) of the main shock was 6.9.

1985 MEXICO EARTHQUAKE

The great (Ms 8.1) earthquake produced two 

surprises: the low value of the peak amplitude of 

acceleration (0.17 g) in the epicentral region and the high 

(3.5 g) value of the peak amplitude of acceleration in certain parts of Mexico City located 400 km from the epicenter (fig. 17). Extensive damage occurred in 5- to 20-story 

buildings sited in the lakebed zone of Mexico City underlain by soft lakebed deposits (Rosenblueth, 1986). These

deposits had been recognized in the 1960's (Zeevaert, 

1964) as a 2-s band-pass filter. The largest ground motions 

in Mexico City occurred at sites underlain by 35 to 50 m of 

soft lakebed deposits having a shear-wave velocity of 

about 100 m/s. The dominant period of the accelerogram 

recorded in the lakebed zone was 2 s. The response of the 

lakebed deposits was essentially elastic. They amplified 

the peak amplitude of acceleration (caused by Rayleigh 

waves) by about a factor of 5 (500 percent) relative to the 

level of peak acceleration observed at nearby sites underlain by stiffer, rocklike material (Rosenblueth, 1986). The 

ordinate of 5-percent damped spectral acceleration at 2 s 

reached 1 g. The similarity of the soil response and the 

building response at many locations in the lakebed zone 

caused resonance that was a major cause of the severe 

damage to and collapse of about 400 engineered buildings. These buildings generally had fundamental natural 

periods ranging from 0.5 to about 2 s. The number of 

severely damaged and collapsed buildings represented 

less than 1 percent of the total number of engineered 

buildings in Mexico City, an indication that buildings constructed according to the Mexican building code performed well everywhere but in the lakebed zone.

CONCLUSIONS, UNRESOLVED TECHNICAL 

ISSUES, AND RECOMMENDATIONS

Research on the ground-shaking hazard in the Salt 

Lake City-Ogden-Provo urban corridor has provided technical information that can be used to improve building 

codes, land use regulations, and construction practices in 

Utah. The soil columns underlying this urban corridor 

cause distinctive characteristics of site amplification 

depending on their softnesses, thicknesses, and locations 

relative to the zone of energy release. Observations at 71 

sites in Utah, augmented by observations in other parts of 

the world, suggest that two- to seven-story buildings sited 

on thick, soft deposits of clays and silts in the Salt Lake 

City-Ogden-Provo urban corridor may face a demand that 

exceeds their capacity in moderate- to large-magnitude 

earthquakes. The hazard is far less for structures sited on 

thin deposits of gravel and sand near the Wasatch Front. 

In the case of a large earthquake (for example, the characteristic earthquake of magnitude MS 7.0 to 7.5) nucleating 

at a depth of 10 to 15 km on the listric Wasatch fault zone 

directly under one of the cities, a large portion on the city 

within about 35 km of the surface trace of the fault zone 

could possibly experience severe ground shaking, and 

buildings could suffer extensive damage or possible collapse. In this case, the characteristics of the ground 

motion will probably be controlled mainly by parameters 

of the earthquake source, and directivity effects could be 

large, causing considerable variability in the distribution 

of ground motion and damage patterns. The trends indi-
K20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

Soft to medium 

clays and sands

Deep cohesionless or 

' stiff clay soils

0.5 1.0 1.5 2.0 

PERIOD, IN SECONDS

2.5

FIGURE 18. Shape of normalized response spectra for three different 

soil columns proposed by the Seismology Committee of the Structural 

Engineers Association of California (1985). These curves are based 

mainly on ground-motion data recorded in California. A relevant 

question is, "To what degree do they represent soil columns in the 

Salt Lake City-Ogden-Provo urban corridor?" This paper asserts that 

the amplifying effects for soft to medium clays may be 

underestimated except for the near-source high-strain environment, 

the rarest case, which cannot be quantified empirically at present.

cated by the maps in this report would not be expected to 

be closely replicated because of lower soil-to-rock amplification factors and a shift to longer periods of dominant 

response, the primary results of high-strain effects. The 

trends shown by the maps would be replicated best when 

the hypocenter of a magnitude 7.0 to 7.5 earthquake is 50 

km or more from any one of the principal cities, the result 

being a low-strain environment.

TECHNICAL ISSUES NEEDING RESOLUTION

Additional data and research on site amplification 

will aid in understanding and quantifying the following 

technical issues in Utah:

1. Do the characteristics of site amplification in the Salt 

Lake City-Ogden-Provo urban corridor derived from

' distant nuclear-explosion ground-motion data adequately represent what will happen on future earthquakes on the Wasatch Front?

2. How different will the characteristics of site amplification be in earthquakes having magnitudes of 7.0 to 7.5 

for a site close to the source? For the same site far 

from the source?

3. What levels of dynamic shear strain will be generated 

in the in situ environment at sites near the source in 

moderate- to large-magnitude earthquakes? How will

these levels of strain compare with strain produced in 

the laboratory environment?

4. Are the soil-to-rock amplification factors in current 

building codes and practices used in California adequate for earthquake-resistant design in the Salt Lake 

City-Ogden-Provo urban corridor (fig. 18)?

RECOMMENDATIONS

More detailed geologic investigations are needed to 

determine the controlling factors at each site, especially 

in the valley centers. Also, integration of the empirical 

ground-motion data with the results of analytical models 

is needed. Additional ground-motion data from earthquakes in the Inter-mountain Seismic Belt or in other parts 

of the world having analogous seismotectonic settings 

and soil-rock columns must be acquired and analyzed to 

quantify the ground-shaking hazard for the near-source 

and far-source environments of the Salt Lake City-OgdenProvo urban corridor.

Although current research on site amplification in 

Utah is incomplete and gaps in our knowledge exist, the 

results of this research can be implemented as preliminary design criteria that will probably have a useful life of 

5 to 10 years because of the slowness of data acquisition 

to address the issues described above. Appropriate uses 

include:

1. Risk maps (zoning maps) for inclusion in the 1991 and 

subsequent editions of the UBC and other model 

codes.

2. Maps of site-amplification factors for discrete period 

bands that correspond to the full range of new and 

existing buildings in Utah.

3. Preliminary seismic microzonation studies and products that identify the areas of the Salt Lake City-OgdenProvo urban corridor that are best suited for certain 

types of construction and land use.

4. Studies of the potential vulnerability of existing two- to 

seven-story buildings as well as other categories of 

existing buildings, lifelines, and other facilities.

REFERENCES CITED

Algermissen, S.T., and Perlrins, D.M., 1976, A probabilistic estimate of maximum acceleration in rock in the contiguous 

United States: U.S. Geological Survey Open-File Report 76- 

416, 45 p.

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, 

99 p.

Bennett, T.J., 1974, Amplitude and propagation characteristics of 

short-period seismic surface waves in the Las Vegas Valley:
SITE AMPLIFICATION AND EARTHQUAKE-RESISTANT DESIGN K21

Report NVO-1163-242, Environmental Research Corporation, 75 p.

Bertero, V.V., Mahin, S.A., and Herrera, R.A., 1978, A seismic 

design implications of near-field San Fernando records: 

Journal of Earthquake Engineering and Structural Dynamics, v. 6, p. 31-42.

Boore, D.M., 1973, The effect of simple topography on seismic 

waves: Implications for the acceleration recorded at 

Pacoima Dam, San Fernando Valley, California: Bulletin of 

the Seismological Society of America, v. 63, p. 1603-1610.

Borcherdt, R.D., ed., 1975, Studies of seismic zonation of the San 

Francisco Bay region: U.S. Geological Survey Professional 

Paper 941-A, 102 p.

Borcherdt, R.D., Gibbs, J.R., and Fumal, T.E., 1978, Progress on 

ground motion predictions for the San Francisco Bay 

region: International Conference on Microzonation for 

Safer Construction Research and Applications, 2d, San 

Francisco, Calif., January 1, 1978, Proceedings, v. 1, p. 241- 

254.

Borcherdt, R.D., Joyner, W.D., Warrick, R.E., and Gibbs, J.F., 

1975, Response of local geologic units to ground shaking, in 

Borcherdt, R.D., ed., Studies for seismic zonation of the San 

Francisco Bay region: U.S. Geological Survey Professional 

Paper 941-A, p. A52-A67.

Davis, L.L. and West, L.R., 1973, Observed effects of topography 

on ground motion: Bulletin of the Seismological Society of 

America, v. 63, p. 283-298.

Earthquake Engineering Research Institute, 1986, Reducing 

earthquake hazards: Lessons learned from earthquakes: El 

Cerrito, Calif., Publication 86-02,108 p.

Elton, D.J., and Martin, J.R., 1986, Site period study for Charleston, S.C.: U.S. Conference on Earthquake Engineering, 3d, 

Charleston, S.C., 1986, Proceedings, v. 1, p. 497-504.

Gibbs, J.F., Fumal. T.E., and Borcherdt, R.D., 1976, In situ measurements of seismic velocities in San Francisco Bay 

region: U.S. Geological Survey Open-File Report 76-731, 

145 p.

Hanks, T.C., 1976, Observations and estimation of long period 

strong ground motion in the Los Angeles Basin: International Journal of Earthquake Engineering and Structural 

Dynamics, v. 4, p. 473-488.

Hays, W.W., 1975, A note on the duration of earthquake and 

nuclear explosion ground motions: Bulletin of the Seismological Society of America Bulletin, v. 65, p. 875-844.

 1977, Evaluation of the seismic in the Sylmar-San 

Fernando area, California, from the 1971 San Francisco 

earthquake: American Society of Civil Engineers, Mechanics Division Specialty Conference of Dynamic Response of 

Structures, Los Angeles, 1977, Proceedings, p. 502-511.

 1978, Ground response maps for Tonopah, Nevada: Bulletin of the Seismological Society of America, v. 68, p. 451- 

469.

 1980, Procedures for estimating earthquake ground 

motions: U.S. Geological Survey Professional Paper 1114, 

77 p.

 ed., 1981, Facing geologic and hydrologic hazards: Earth 

science considerations: U.S. Geological Survey Professional Paper 1240-B, 90 p.

Hays, W.W., and Gori, PL., eds., 1984, Evaluation of regional and 

urban earthquake hazards and risk in Utah: U.S. Geological 

Survey Open-File Report 84-763, 687 p.

Hays, W.W., and King, K.W., 1982, Zoning of the earthquake 

ground-shaking hazard along the Wasatch fault zone, Utah: 

International Conference on Microzonation, 3d, Seattle, 

Wash., 1982, Proceedings, v. 3, p. 1307-1317.

 1984, Seismic microzonation along the Wasatch fault 

zone, Utah: World Conference on Earthquake Engineering, 

8th, San Francisco, Calif., July 21-28, 1984, Proceedings, 

v. 1, p. 1-12.

Hays, W.W., Algermissen, S.T., Miller, R.D., and King, K.W., 1978, 

Preliminary ground response maps for the Salt Lake City 

area: International Conference on Microzonation, 2d, San 

Francisco, 1978, Proceedings, v. 2, p. 487-508.

Hays, WW, Rogers, A.M., and King, K.W., 1979, Empirical data 

about local ground response: Earthquake Engineering 

Research Institute, National Conference on Earthquake 

Engineering, 2d, Stanford, Calif., 1979, Proceedings, p. 223- 

232.

Idriss, I.M., 1978, Characteristics of earthquake ground motions: 

American Society of Civil Engineers Earthquake Engineering and Soil Dynamics Specialty Conference, Proceedings, 

v. 3, p. 1151-1267.

Idriss, I.M., and Seed, H.S., 1968, Analysis of ground motions 

during the 1957 San Francisco earthquake: Bulletin of the 

Seismological Society of America, v. 58, p. 2013-2032.

Ihmen, Steven, and Hadley, D.M., 1987, Seismic hazard maps for 

Puget Sound, Washington: Bulletin of the Seismological 

Society of America, v. 77, p. 1091-1109.

International Conference of Building Officials, 1988, Uniform 

Building Code: Whittier, Calif., International Conference of 

Building Officials.

Joyner, W.B., Warrick, R.E., and Fumal, T.E., 1981, The effects of 

Quaternary alluvium on strong ground motion in the Coyote 

Lake, California earthquake of 1979: Bulletin of the Seismological Society of America, v. 71, p. 1333-1349.

Kobayashi, Hiroyoshi, and Nagahashi, Sumio, 1982, Response 

spectra on seismic bedrock during earthquakes, in Engineering seismology: Tokyo Institute of Technology, p. 22-27.

MacMurdo, J., 1824, Papers relating to the earthquake which 

occurred in India in 1819: Philadelphia Magazine, v. 63, 

p. 105-177.

Miller, R.D., 1980, Surficial geologic map along part of the 

Wasatch Front, Salt Lake Valley, Utah: U.S. Geological Survey Miscellaneous Field Studies Map MF-1198, scale 

1:100,000.

Molinelli, Jose, 1985, Earthquake vulnerability study for the metropolitan area of San Juan, Puerto Rico, in Hays, WW, and 

Gori, PL., eds., A workshop on reducing potential losses 

from earthquakes in Puerto Rico, Proceedings of Conference XXX: U.S. Geological Survey Open-File Report 85-731, 

p. 211-277.

Murphy, J.R., and Hewlett, R.A., 1975, Analysis of seismic 

response in the city of Las Vegas, Nevada: A preliminary 

microzonation: Bulletin of the Seismological Society of 

America, v. 65, p. 1575-1598.

Murphy, J.R., and West, L.R., 1974, An analysis of surface and 

subsurface seismic measurements demonstrating the ampli-
K22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

fication effect of near-surface geology: Report NVO-1163- 

TM41 (to Atomic Energy Commission), Environmental 

Research Corporation, 21 p.

Murphy, J.R., Lynch, R.D., and O'Brien, L.J., 1971a, Predicted San 

Fernando earthquake spectra: Report NVO-1163-TM30 (to 

Atomic Energy Commission), Environmental Research Corporation, 38 p.

Murphy, J.R., Weaver, N.L., and Davis, A.H., 1971b, Amplification 

of seismic body waves by low-velocity layers: Bulletin of 

the Seismological Society of America, v. 61, p. 109-146.

Peak, R.B., and Reed, W.C., 1954, Engineering properties of Chicago subsoils: University of Illinois Engineering Experiment Station Bulletin 423, 43 p.

Reid, H.F., 1910, The California earthquake of April 18, 1906, in 

The mechanics of the earthquake: Report of the State Earthquake Investigation Commission: Carnegie Institute of 

Washington Publication 87, v. 2. 500 p.

Rogers, A.M., and Hays, W.W., 1978, Preliminary evaluation of 

site transfer functions developed from nuclear explosions 

and earthquakes: International Conference on Microzonation, 2d, San Francisco, 1978, Proceedings, v. 2, p. 753-764.

Rogers, A.M., Tinsley, J.C., Hays, W.W., and King, K.W., 1979, 

Evaluation of the relation between near-surface geological 

units and ground response in the vicinity of Long Beach, 

California: Bulletin of the Seismological Society of America, 

v. 69, p. 1603-1622.

Rosenblueth, Emilio, 1986, The Mexican earthquake: A firsthand report: Civil Engineering, January, p. 38-40.

Savy, Jean, Bernreuter, Don, and Chen, J.C., 1986, Site effects: A 

generic method for modeling site effects in seismic hazard 

analyses, in Hays, W.W, ed., Proceedings of Conference 

XXXIV: U.S. Geological Survey Open-File Report 86-185, 

p. 249-284.

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

characteristic earthquakes: Examples from the Wasatch 

and San Andreas faults: Journal of Geophysical Research, v. 

89, no. B7, p. 5681-5698.

Seed, H.B., 1975, Design provisions for assessing the effects of 

local geology and soil conditions on ground and building 

response during earthquakes, in New earthquake design 

provisions: Seminar sponsored by Professional Development Committee of Structural Engineers Association of 

Northern California and San Francisco Section of American 

Society of Civil Engineers, Proceedings, p. 38-63.

Seed, H.B., and Idriss, I.M., 1969, Influence of soil conditions on 

ground motions during earthquakes: Proceedings of the 

American Society of Civil Engineers, Journal of the Soil 

Mechanics and Foundations Division, v. 95, p. 1199-1218.

Seed, H.B., Murarka, R., Lysmer, John, and Idriss, I.M., 1976, 

Relationships of maximum acceleration, maximum velocity, 

distance from source, and local site conditions for moderately strong earthquakes: Bulletin of the Seismological Society of America, v. 66, p. 1323-1342.

Seed, H.B., Whitman, R.V., Dezfulian, H., Dobry, R., and Idriss, 

I.M., 1972, 1972, Soil conditions and building damage in the 

1967 Caracas earthquake: Proceedings of the American 

Society of Civil Engineers, Journal of the Soil Mechanics 

and Foundations Division, v. 98, p. 787-806.

Singh, J.P., 1985, Earthquake ground motions: Implications for 

designing structures and reconciling structural damage: 

Earthquake Spectra, v. 1, p. 239-270.

Sharma, S., and Kovacs, W.D., 1980, The microzonation of the 

Memphis, Tennessee, area: U.S. Geological Survey OpenFile Report 80-914,129 p.

Smith, R.B., Richins, W.D., and Doser, D.I., 1985, The 1983 Borah 

Peak, Idaho, earthquake: Regional seismicity, kinematics of 

faulting, and tectonic mechanics, in Stein, R.S., and Bucknam, R.C., eds., On the Borah Peak, Idaho, earthquake: U.S. 

Geological Survey Open-File Report 85-290, p. 236-263.

State of California, 1990, Competing against time: Report to Governor George Deukmejian from the Governor's Board of 

Inquiry on the 1989 Loma Prieta Earthquake, 264 p.

Steinbrugge, K.V. chairman, 1968, The safety of fills: San Francisco, Report prepared for the San Francisco Bay Conservation and Development Commission, 33 p.

Structural Engineers Association of California, 1985, Tentative 

lateral force requirements: Sacramento, Calif., Report of 

Seismology Committee, 76 p.

Tezcan, S.S., Seed, H.B., Whitman, R.v, Serff, N., Christian, J.T., 

Durgunoglu, H.T., and Yegian, M., 1977, Resonant period 

effects in the Gediz, Turkey, earthquake of 1970: Earthquake 

Engineering and Structural Dynamics, v. 5, p. 157-179.

Whitman, R.V., 1983, An engineer's perspective on the Cape 

Anne, Massachusetts earthquake of 1755 [abs.]: Earthquake 

Notes, v. 54, no. 1, p. 8.

Wolf, J.P., 1985, Dynamic soil-structure interaction: Englewood 

Cliffs, N.J., Prentice-Hall, 466 p.

Wood, H.O., 1908, Distribution of apparent intensity in San Francisco, in The California earthquake of April 18, 1906: Washington, D.C., Report of the State Earthquake Investigation 

Commission, Carnegie Institute of Washington, p. 220-245.

Yamahara, H., 1970, The interrelation between frequency characteristics of ground motion and earthquake damage to structure: Soils and Foundations, v. 10, p. 57-74.

Zeevaert, L, 1964, Strong ground motion recorded during earthquakes of May 11 and 19th, 1962 in Mexico City: Bulletin of 

the Seismological Society of America, v. 54, p. 209-232.
Predicting Strong Ground Motion in Utah

By KENNETH W. CAMPBELL

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-L

CONTENTS

Page

Abstract............................................................................................................................................................. LI

Introduction...................................................................................................................................................... 1

Ground-Motion Model..................................................................................................................................... 1

Strong-Motion Parameters..................................................................................................................... 2

Magnitude.............................................................................................................................................. 2

Source-to-Site Distance.......................................................................................................................... 2

Data Base.......................................................................................................................................................... 3

Regression Analysis.......................................................................................................................................... 4

Far-Field Constraints.............................................................................................................................. 12

Analysis of Residuals ....................................................................................................................................... 14

Ground-Motion Estimates for North-Central Utah.......................................................................................... 16

Stress Regime......................................................................................................................................... 17

Fault Type............................................................................................................................................... 21

Anelastic Attenuation............................................................................................................................. 22

Site Effects............................................................................................................................................. 23

Source Directivity.................................................................................................................................. 25

Ground-Motion Estimates...................................................................................................................... 25

Summary........................................................................................................................................................... 26

References Cited............................................................................................................................................... 29

ILLUSTRATIONS

FIGURES 1-13. Graphs showing:

1. Distribution of strong-motion recordings with respect to magnitude and distance ...................................................................................... L10

2. Attenuation relationships of peak horizontal acceleration for various magnitudes ...................................................................................... 12

3. Predicted magnitude scaling of peak horizontal acceleration for various distances..................................................................................... 13

4. Attenuation relationships of peak horizontal velocity for various magnitudes............................................................................................. 14

5. Predicted magnitude scaling of peak horizontal velocity for various distances............................................................................................ 15

6. Dependance of peak horizontal acceleration on building embedment.......................................................................................................... 16

7. Dependence of peak horizontal velocity on sediment depth......................................................................................................................... 17

8. Normalized weighted residuals plotted as a function of distance................................................................................................................. 18

9. Normalized weighted residuals plotted as a function of magnitude............................................................................................................. 19

10. Normalized weighted residuals plotted as a function of predicted value...................................................................................................... 20

11. Estimates of peak horizontal acceleration for north-central Utah................................................................................................................. 26

12. Estimates of peak horizontal velocity for north-central Utah....................................................................................................................... 27

13. Median estimate of a 5-percent-damped pseudo-relative velocity spectrum for a site in north-central Utah located

	approximately 5 km from the zone of seismogenic rupture of aM§=7.5 earthquake................................................................................... 28

TABLES

TABLE 1. Earthquake data............................................................................................................................................................................................................... L4

2. Strong-motion data.......................................................................................................................................................................................................... 5

3. Station data..................................................................................................................................._^ 7

4. Distribution of recordings by distance ............................................................................................................................................................................ 11

5. Definitions ofK^ftei, Kj, R), and^, Kt, D). ................................................................................................................................................................ 11

6. Summary of regression coefficients..................................................................................................................._^ 11

	III
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK 

ALONG THE WASATCH FRONT, UTAH 

PREDICfiNG STRONG GROUND MOTION IN UTAH 

By KENNETH w. CAMPBELL 

ABSfRACI' 

Near-source attenuation relationships for predicting peak horizontal acceleration and velocity in terms of earthquake magnitude, sourceto-site distance, and several source and site parameters are used to estimate strong ground motion in north-central Utah. The range of estimates 

provided reflects uncertainty in the state of knowledge regarding the 

effects of stress regime, fault type, anelastic attenuation, and local site 

conditions. Regional data were used to quantify these effects when possible. For a site located directly above the rupture zone of an hypothesized Ms=7.5 earthquake on the Wasatch fault (a distance of 

approximately 5 km), median estimates of peak horizontal acceleration 

and velocity were found to range from 0.55 to 1.1 g and 45 to 120 cm/s, 

respectively. These ground-motion estimates, when translated into 

median estimates of 5-percent-darnped response spectral values using 

the Newmark-Hall procedure, result in estimates of pseudo-absolute 

acceleration for the 0.1- to 0.5-s period band that range from 1.05 to 2.1 g 

and estimates of pseudo-relative velocity for the 0.5- to 3.0-s period band 

that range from 70 to 200 cm/s. 

INTRODUCTION 

Near-source attenuation relationships for predicting 

peak horizontal acceleration and peak horizontal velocity 

from earthquake magnitude, source-to-site distance, and 

various source and site parameters are developed from 

strong-motion recordings of worldwide earthquakes having magnitudes ranging from 5.0 to 7. 7. This report revises 

several previous studies (Campbell, 1981a, b, 1982a, b, 

1983, 1984a). Major revisions include changes in the definition of parameters representing source-to-site distance, 

site geology, and building size and embedment 

Two types of attenuation relationships are developed. The first, a near-source relationship, is empirically 

derived from a set of near-source strong-motion recordings having no constraint on its far-field behavior. It is 

similar to the unconstrained model of Campbell (1981a). 

The second relationship includes an anelastic attenuation 

term as a means of extrapolating the model beyond the 

near-source region. This latter relationship, similar to the 

constrained model of Campbell (1981a), provides a means 

of incorporating regional differences in anelastic attenuation when predicting strong ground motion. 

GROUND-MariON MODEL 

The equation used to model the attenuation and magnitude-scaling characteristics of peak horizontal acceleration and velocity is given by the expression (Campbell, 

1984a) 

where Y is the ground-motion parameter being predicted, 

M is earthquake magnitude, R is source-to-site distance, 

Ki are variables representing source and site effects, y is 

the coefficient of anelastic attenuation, and E is a random 

error term having zero mean and standard deviation equal 

to the standard error of regression. The coefficients a 

through e are parameters to be determined from the data. 

All logarithms in equation 1 are given in terms of the natural logarithm. The nonlinear form of equation 1 was 

selected to accommodate any saturation of peak horizontal acceleration and velocity with magnitude and distance 

if supported by the data. The far-field properties of the 

model are characterized by the magnitude scaling coefficient b, the coefficient of geometrical attenuation d, and 

the coefficient of anelastic attenuation y. The near-source 

properties of the model are characterized by the coefficients c1 and c2. The form of the term in brackets was first 

proposed by Esteva ( 1970) and later used by Hadley and 

others (1982) to model the near-source attenuation of simulated ground motions. For c1>0, the strong-motion 

parameter saturates with decreasing distance, approaching a fmite value at R=O. F~r c2>0, the strong-motion 

parameter saturates with increasing magnitude at short 

L1 
L2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

distances until C2=-b/d, when it becomes totally independent of magnitude at R=0.

The parameters Y, M, R, and KI were defined according to guidelines set forth by Campbell (1985a). Brief definitions of these parameters follow.

STRONG-MOTION PARAMETERS

Peak horizontal acceleration and velocity were 

defined as the arithmetic mean of the peak horizontal values of acceleration and velocity, respectively. This mean, 

referred to as the mean peak horizontal component, was 

chosen because (1) it tends to minimize the variability 

associated with the near-random azimuthal orientation of 

the two orthogonal horizontal components and, thus, is 

statistically superior to the largest horizontal component 

(Campbell, 1982a) and (2) it avoids the error associated 

with using both peak horizontal components as independent observations (Campbell, 1985a).

The latter observation is particularly important 

because past investigators have commonly used both 

peak horizontal components as independent observations. This practice artificially increases the number of 

degrees of freedom and thereby invalidates the statistical 

interpretation of the results. This flaw is also true of analyses that use one of .the horizontal components to predict 

the other. The only correct use of both horizontal components as individual variables is by means of multivariate 

multiple regression (Johnson and Wichern, 1982) a technique that properly accounts for statistical correlation 

between dependent variables.

The mean peak horizontal component of acceleration 

has been found to be approximately 12 percent less and 

the mean peak horizontal component of velocity approximately 17 percent less than the largest peak horizontal 

component (Campbell, 1981a; Joyner and Fumal, 1985). 

On the other hand, median predictions of ground-motion 

parameters based on the mean peak horizontal component have been found to be virtually identical to those 

based on the use of both peak horizontal components 

(Campbell, 1982a). The above factors can be used to compare median predictions of ground-motion parameters in 

the current study with those in other studies available 

from the literature.

Peak accelerations were scaled from either 

uncorrected1 time series or copies of the original accelerograms to avoid the reduction in amplitude caused by 

decimation and low-pass filtering of the records during 

routine processing. Peak velocities were scaled directly 

from the processed velocity time series.

First stage of routine processing in which the accelerogram is digitized and the baseline corrected. Unequal time intervals are used to preserve the peak amplitudes of the original accelerogram.

MAGNITUDE

Magnitude was defined as surface-wave magnitude 

(Mg) when both local magnitude (ML) and Mg were 

greater than or equal to 6.0 or as ML when both Mg and ML 

were less than 6.0. Using the Mg scale for the larger earthquakes serves as a uniform basis for characterizing the 

size of worldwide events and thus avoids saturation of the 

nib and ML scales (Campbell, 1985a, fig. 2).

Bakun (1984) and Hanks and Boore (1984) have 

found ML and moment magnitude, the magnitude measure 

recommended by Joyner and Boore (1981), to be roughly 

equivalent for 5.0<ML^6.0, and Hanks and Kanamori 

(1979) cited a similar equivalence between moment magnitude and Ms for 5.0<MS<7.5. Furthermore, Nuttli (1979) 

has interpreted the commonly cited Richter magnitude 

scale (Richter, 1958), used in many previous groundmotion attenuation relationships, to represent ML for 

earthquakes less than about magnitude 6.0 and Mg for 

larger earthquakes. Thus, the magnitude scale used in the 

current study is considered to be consistent with both the 

moment magnitude and the Richter magnitude scales for 

the range of magnitudes used in this study and thus does 

not constitute a significant departure from the magnitude 

scales used in previous ground-motion studies.

SOURCE-TO-SITE DISTANCE

Source-to-site distance was defined as the shortest 

distance between the recording station and the zone of 

seismogenic rupture, which was identified, when possible, from aftershocks of the earthquake. When the aftershock distribution was too diffuse, too inaccurate, or too 

limited to reliably define this zone, then modeling studies, 

past seismicity, geodetic data, refraction experiments, 

and geologic information particularly the depth to basement-rock contact on the fault zone were used to infer 

its spatial extent. Adopting this distance measure implicitly assumes that dynamic stresses within the upper sediments are too small to be seismogenic, even if significant 

displacement occurs within these sediments.

The current definition of distance is a revision of that 

used in the author's previous studies. The former distance 

measure defined as the shortest distance between the 

site and the rupture zone was found to be difficult to 

determine for many moderate-sized earthquakes. For 

large earthquakes (M>6.5), where surface rupture was 

clearly identified, the former distance was measured from 

the surface trace of the fault. For small to moderate earthquakes (M<5.5). where it could be reasonably assumed 

that rupture was confined to basement rock beneath the 

sediments, the former distance was measured from the 

inferred fault rupture at depth. However, for moderatesized earthquakes (M=5.5-6 5), where the surface expres-
PREDICTING STRONG GROUND MOTION IN UTAH L3

sion of rupture was identified only by a zone of small, discontinuous ground cracks in the vicinity of the fault trace, 

it was not clear whether rupture was confined to basement rock, terminated somewhere within the sediments, 

or propagated to the surface of the sediments. The 

revised definition of distance avoids these ambiguities 

and, in the opinion of this author, represents a more physically sound basis for modelling wave-propagation characteristics of strong ground motion.

The strongest phase of shaking recorded at a site 

may not always come from the closest part of the seismogenic rupture zone. In fact, Shakal and Bernreuter 

(1981, p. 19) suggested that ground-motion predictions 

made by using attenuation relationships based on closestdistance measures will "at best be accurate and at worst 

may significantly underpredict ground-motion levels." 

This assertion would be true only if the relationships were 

developed in terms of closest distance to seismogenic 

rupture and then applied in terms of distance to the actual 

source of the strongest phase of shaking the distance 

measure preferred by Shakal and Bernreuter. Because it 

is not known in advance where the true source of the 

strongest ground motion will be, it is not feasible in practice to make predictions in terms of the latter distance 

measure. The distance measure used in the current study 

can be reliably estimated for both actual and hypothesized earthquakes and, if consistently applied, will appropriately account for uncertainty in the location of the 

actual source of the strongest recorded ground motion by 

including it as inherent variability.

DATA BASE

The data base consists of peak horizontal acceleration and velocity parameters obtained from accelerograms recorded in the near-source regions of selected 

worldwide earthquakes. The study was not restricted to 

earthquakes occurring only in western North America, as 

has been common practice in the past. Several factors 

have minimized the potential bias of the foreign data used 

in the analyses. The restriction to the near-source regime 

has made any regional differences in anelastic attenuation 

negligible in comparison with the inherent scatter from 

other factors. In addition, the foreign data used in this 

investigation come from events occurring within active 

tectonic provinces generally similar to the seismotectonic 

regime of western North America. Deep subduction 

events were excluded from the analyses because of 

potential significant differences in travel paths and tectonic stress conditions of such earthquakes in comparison with those of the shallow events used in the current 

study. To avoid a potential instrument bias, all the foreign 

data were recorded on instruments having dynamic characteristics similar to those commonly used in the United 

States.

The data base was assembled by using criteria 

designed to select only consistent and reliable data over a 

range of magnitudes and distances of greatest interest to 

engineers and engineering seismologists. Data were 

selected if (1) the largest horizontal component of peak 

acceleration was at least 0.02 g; (2) the accelerograph triggered early enough to record the strong phase of shaking; 

(3) the magnitude of the earthquake was 5.0 or larger; (4) 

the closest distance to seismogenic rupture was less than 

30 or 50 km, depending on whether the magnitude of the 

earthquake was less than or greater than 6.25; (5) the shallowest extent of seismogenic rupture was no deeper than 

25 km; and (6) the recording site was located on unconsolidated deposits.

Rock sites sites located on sedimentary or crystalline rock were excluded from the analyses because of 

their potentially significant and extremely variable site 

effects (Campbell, 1983; Tucker and others, 1984; Cranswick and others, 1985). Such sites are typically located in 

areas of significant topographic relief, such as near the 

abutments of dams, or founded on rock having a large 

degree of weathering. In addition, the limited number of 

near-source recordings on rock makes it virtually impossible to empirically quantify any significant site effects for 

such sites.

Ideally, it would be desirable to restrict the analyses 

to strong-motion records obtained in the free field. However, as Campbell (1986) pointed out, there are very few 

true free-field recordings. Therefore, it was necessary to 

include records obtained at the foundation level of buildings in order to have a sufficient number of recordings for 

analysis. In accordance with the recommendations of 

Campbell (1984b, 1985a, 1986), the analyses were carefully evaluated for potential soil-structure interaction 

effects, which were modeled when found to be significant. Accelerograms recorded on instruments located on 

the abutments and toes of dams were excluded because 

of the potential effects of the dam

The earthquakes selected for analysis, together with 

their magnitudes and types of faulting, are listed in 

table 1; the peak accelerations, peak velocities, and 

source-to-site distances for each recording site are given 

in table 2; recording sites are described in table 3. Figure 1 

shows the distribution of recordings with respect to magnitude and distance. This figure indicates that there is little correlation between magnitude and distance for either 

peak horizontal acceleration or peak horizontal velocity, a 

positive attribute in regression analyses that treat these 

parameters as independent variables. The final data base 

contains 134 acceleration values, 107 velocity values, 130 

recording stations, and 21 earthquakes that meet the criteria presented above.
L4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 1. Earthquake data 

[All earthquakes occurred in California, unless otherwise noted]

Earthquake

Long Beach

Imperial Valley

Kern County

Daly City

Parkfield

Borrego Mountain

Lytle Creek

San Fernando

Managua, Nicaragua

Point Mugu

Hollister

Oroville

Kalapana, Hawaii

Gazli, U.S.S.R

Santa Barbara

Tabas, Iran

Bishop

Malibu

St. Elias, Alaska

Coyote Lake

Imperial Valley

Date 

(year-month-date)

33-03-11

40-05-19

52-07-21

57-03-22

66-06-28

68-04-09

70-09-12

71-02-09

72-12-23

73-02-21

74-11-28

75-08-01

75-11-21

76-05-17

78-08-13

78-09-16

78-10-04

79-01-01

79-02-28

79-08-06

79-10-15

Magnitude Fault type

6.2

7.1

7.7

5.3

6.0

6.7

5.4

6.6

6.2

5.9

5.1

5.7

7.1

7.0

5.7

7.4

5.8

5.0

7.2

5.9

6.9

Strike slip

Strike slip

Reverse

Strike slip

Strike slip

Strike slip

Reverse

Reverse

Strike slip

Reverse

Strike slip

Normal

Thrust

Reverse

Reverse

Reverse

Strike slip

Reverse

Thrust

Strike slip

Strike slip

No. of recordings

Acceleration Velocity

2

1

1

4

5

1

5

53

1

1

3

1

1

1

5

1

2

4

1

6

35

2

1

1

4

4

1

4

44

1

0

3

0

0

1

3

1

0

0

1

5

31

REGRESSION ANALYSIS

The coefficients a through e in equation 1 were estimated by using a nonlinear least-squares regression routine developed by More and others (1980). The method 

was modified to incorporate weights used to control the 

influence of those earthquakes having a large number of 

recordings. The weighting scheme is identical to that 

described by Campbell (1981a, 1982a). Because it is 

designed to give each earthquake an equal weight in the 

analysis at each of nine distance intervals, it thereby balances the information on magnitude scaling contributed 

by earthquakes whose recordings are poorly distributed 

with respect to distance with the information on distance 

scaling contributed by earthquakes whose recordings are 

well distributed with respect to distance. The distance 

intervals were selected to be roughly equal in logarithmic 

increments between 0 and 56.6 km.

The weight of each recording was computed from the 

expression

(2)

where n is the total number of recordings used in the 

analysis and n^ is the number of recordings associated 

with the specific earthquake and distance interval containing the ith recording. The quantity n^ is not unique to 

a single recording; rather, it is the same for all recordings 

of the same earthquake whose distance falls within the 

specified distance interval. The above expression assures 

that the sum of the weights equals n, thus preserving the 

correct number of degrees of freedom. Table 4 gives the 

distribution of recordings with respect to the nine distance intervals used in the analyses.

Other investigators have proposed alternate methods 

of compensating for the bias associated with the erratic 

distribution of distances and numbers of recordings 

between earthquakes. The two most notable are the twostep regression procedure suggested by Joyner and Boore 

(1981) and the random-effects procedure suggested by 

Brillinger and Preisler (1984). Although the later technique may be more mathematically rigorous, all three 

techniques are fundamentally similar in their results. 

Therefore, the weighted regression procedure has been 

retained for the current study to be consistent with the 

author's previous analyses.
PREDICTING STRONG GROUND MOTION IN UTAH L5

TABLE 2. Strong-motion data 

[ , not available]

Earthquake (year)

Long Beach (1933) ..........

Imperial Valley (1940) .....

Kern County (1952) .........

Daly City (1957) ...............

Parkfield (1966) ...............

Borrego Mountain (1968) 

Lytle Creek (1970) ...........

San Fernando (1971).......

Station 

no.

.... 131

288 

.... 117

.... 1095

.... 1080

1065 

1078 

1049

.... 1013

1014 

1015 

1438 

1016

.... 117 

.... 290

116 

112 

274 

113

.... 241

458 

267 

128 

461

253 

466 

127 

264 

232

235 

238 

446

475 

133

135 

262 

145 

148 

142

199 

208 

211 

223 

431 

452 

449

Source- Peak 

to-site horizontal 

distance acceleration 

(km) (<jr)

6.4 

22.0

8.3 

48.4

12.9 

14.8 

14.8 

24.6

3.0 

6.3 

10.1 

11.0 

15.2

45.0

12.4 

17.9 

20.0 

28.3 

28.8

13.8 

16.0 

18.1 

18.7 

20.5

21.6 

21.6 

22.6 

25.0 

25.4

25.4 

25.5 

25.5 

25.7 

26.7

26.7 

27.6 

28.3 

28.3 

28.6

28.7 

28.7 

28.7 

28.7 

28.7 

28.7 

28.8

0.180 

.140

.292 

.186

.083 

.053 

.047 

.038

.400 

.435 

.275 

.350 

.066

.102

.175 

.175 

.068 

.090 

.043

.205 

.115 

.195 

.325 

.140

.225 

.185 

.155 

.195 

.165

.115 

.105 

.145 

.105 

.130

.205 

.120 

.135 

.185 

.090

.180 

.135 

.155 

.105 

.105 

.075 

.100

Peak 

horizontal 

velocity 

(cm/s)

22.6 

22.9

34.4 

16.7

4.6 

2.2 

3.8 

1.6

61.2 

24.0 

11.3

7.5 

20.3 

8.8

3.5 

4.3 

2.2

26.8 

30.1 

11.6

19.2

21.6 

25.6

13.1 

18.8

17.4 

13.0 

12.1 

11.1 

18.2

18.8 

11.8

17.4

20.4 

19.4 

21.2

14.1

TABLE 2. Strong-motion data Continued

Earthquake (year)

San Fernando (1971) 

Continued ....................

Managua (1972) ...............

Point Mugu (1973) ..........

Hollister (1974) ...............

Oroville (1975) ................

Kalapana (1975) ..............

Gazli(1976)......................

Santa Barbara (1978) ......

Tabas (1978) ....................

Bishop (1978) ..................

Malibu (1979) ..................

Station 

no.

482

160 

455 

163 

166

125 

416 

437 

469 

428

443 

140 

175 

413 

425

181 

184 

187 

121 

205

288 

269 

229 

244 

247 

1052

...... 3501

...... 272

...... 1377

1028 

1250

...... 1291

...... 2808

...... 9110

...... 5093

885 

283 

5137 

9022

...... 9124

...... 1008

1490 

...... 508"!

5079

Sourceto-site 

distance 

(km)

29.0 

29.1 

29.1 

29.3 

29.3

29.6 

29.9 

30.0 

30.0 

30.1

30.1 

31.0 

31.0 

31.0 

31.1

31.3 

31.3 

31.3 

32.1 

33.1

34.8 

35.5 

42.2 

42.2 

42.2 

48.2

5.4 

24.0

8.9 

10.8 

10.8

30.0 

37.9 

3.5

12.7 

12.9 

14.6 

14.6 

16.3

5.4

30.7 

30.8

18.1 

20.2

Peak 

horizontal 

acceleration 

07)

.120 

.220 

.185 

.105 

.235

.145 

.140 

.105 

.205 

.084

.130 

.095 

.135 

.115 

.090

.140 

.090 

.145 

.160 

.070

.100 

.125 

.060 

.040 

.030 

.090

.365 

.105

.085 

.135 

.120

.065 

.165 

.666

.353 

.331 

.154 

.176 

.267

.755

.045 

.060

.080 

.065

Peak 

horizontal 

velocity 

(cm/s)

13.7 

18.6 

15.7 

16.4 

20.2

16.7 

17.9 

19.1 

19.2 

26.8

22.1 

8.4 

20.8 

14.6 

13.5

16.9 

13.0 

13.2

17.6 

16.3

14.0 

10.5 

12.1 

7.3

32.7

5.0 

8.6 

3.7

74.5

32.9 

10.9 

13.8

98.2
L6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 2. Strong-motion data Continued

Earthquake (year)

Malibu (1979) 

Continued .....................

St. Elias (1979)

Coyote Lake (1979)

Imperial Valley (1979)

Station 

no.

657

757

2734

1411

1410

1409

1377

1492

1422

6618

6616

6617

5028

5155

942

952

958

5054

955

6621

5060

5165

9034

117

5055

5154

6619

412

5057

5053

5115

5058

6622

5051

5056

5169

931

6605

5059

5061

5062

6610

5052

724

Sourceto-site 

distance

(km)

20.7

23.7

36.4

5.4

6.6

8.4

14.4

16.2

28.0

5.0

5.2

5.2

5.5

5.5

5.6

5.6

6.7

6.8

6.9

7.3

7.5

7.5

7.5

8.3

9.3

9.4

9.8

10.1

10.9

11.9

12.3

13.8

14.6

15.0

16.0

16.0

18.8

21.7

22.7

23.7

29.5

30.8

32.5

36.4

Peak

horizontal 

acceleration

(0)

.040

0.45

.136

.243

.256

.225

.109

.106

0.45

.254

.278

.310

.440

.307

.585

.480

.570

.735

.495

.265

.195

.440

.352

.335

.240

.240

.385

.215

.245

.250

.380

.380

.169

.155

.150

.094

.130

.292

.135

.110

.060

.142

.060

.087

Peak

horizontal 

velocity

(cnVs)

27.3

28.7

23.2

21.1

6.3

6.5

31.7

34.6

76.4

81.3

85.9

65.3

50.6

43.9

57.4

27.4

36.2

55.2

36.4

46.6

48.7

43.3

41.6

17.8

28.9

37.1

12.4

15.9

12.9

20.5

18.5

27.0

14.4

13.7

7.0

4.5

10.1

For peak horizontal acceleration and velocity, the 

weighted nonlinear regression analysis resulted in the following unconstrained relationships:

In PHA=-2.817+0.702M-1.20 ln[#+0.0921 exp(0.584M)]

 In PHV=-0.798+1.02M-1.26 ln[fl+0.0150 exp(0.812M)]

(3)

(4)

where peak horizontal acceleration (PHA), peak horizontal velocity (PHV), and distance (R) have units of g, 

centimeters per second, and kilometers, respectively. 

Magnitude and distance are as defined previously, and D 

is the depth (in kilometers) of sediments beneath the 

recording site. Parameters Kiy functions /(e^, Kiy R) and 

f(et, KI, D) and coefficients e^ are summarized in tables 5 

and 6; the attenuation relationships are plotted in figures 

2 through 5 for J^=0. The standard errors of regression 

are 0.30 and 0.26 for equations 3 and 4, respectively.

The definitions and functional relationships for KI 

were determined from an analysis of residuals described 

later in the text. Because of the nonlinear form of equation 1, rigorous statistical tests to determine the significance of coefficients a through e require Monte Carlo 

simulation (Gallant, 1975), which is beyond the scope of 

this study. Therefore, the statistical significance of these 

coefficients is not precisely known. A visual inspection of 

the residuals did indicate a very strong need for all coefficients, except possibly 02, an indication that these coefficients are statistically significant at least at the 90-percent 

confidence level. Coefficient c% is controlled by too few 

recordings to be judged solely on visual inspection. It 

should be noted, however, that the regression analyses for 

both strong-motion parameters resulted in a value for 02 

at its practical upper limit, C2=-b/d. This value represents 

complete saturation of peak horizontal acceleration and 

velocity at the source of an earthquake, in accordance 

with some geophysicists' interpretations of earthquake 

rupture mechanics (Campbell, 1985a). Confirmation of 

the statistical significance of c% will have to await the outcome of Monte Carlo simulations.

The results presented in table 6 indicate that peak 

horizontal acceleration is systematically high when 

recorded on ground-level instruments located on shallow 

soils and that it is systematically low when recorded in 

basements of buildings more than three stories high. Shallow soils are defined as unconsolidated deposits less than 

about 10 m deep that overlie sedimentary or crystalline 

rock (Campbell, 1981a). The reduction in peak horizontal 

acceleration due to embedment was found to be dependent on building size and distance, buildings 10 stories 

and higher having lower accelerations than 3- to 9-story 

buildings and buildings further from the source having
PREDICTING STRONG GROUND MOTION IN UTAH

TABLE 3. Station data 

[All stations in California, unless otherwise noted. , not available]

L7

Station 

no.

112

113

116

117

121

125

127

128

131

133

135

140

142

145

148

160

163

166

175

181

184

187

199

205

208

211

223

229

232

235

238

241

244

247

253

262

264

267

269

272

274

283

288

290

412

413

416

425

428

431

Description

Cedar Springs, Pump House

Colton, SCE Substation

Devil Canyon, Filter Plant

El Centre Array Station 9

Fairmont Reservoir

Lake Hughes Array Station 1

Lake Hughes Array Station 9

Lake Hughes Array Station 12

Long Beach, Public Utilities Building

Los Angeles, Hollywood Storage Building

Los Angeles, Hollywood Storage PE Lot

Los Angeles, UCLA Math Science Building

Los Angeles, 120 N. Robertson

Los Angeles, 222 Figueroa

Los Angeles, 234 Figueroa

Los Angeles, 533 S. Fremont

Los Angeles, 611 W. Sixth Street

Los Angeles, 646 S. Olive

Los Angeles, 808 S. Olive

Los Angeles, 1640 Marengo

Los Angeles, 1900 Avenue of the Stars

Los Angeles, 1901 Avenue of the Stars

Los Angeles, 3407 W. Sixth Street

Los Angeles, USC Phillips Hall

Los Angeles, 3470 Wilshire

Los Angeles, 3550 Wilshire

Los Angeles, 4680 Wilshire

Los Angeles, 5250 Century

Los Angeles, 6430 Sunset

Los Angeles, 6464 Sunset

Los Angeles, 7080 Hollywood

Los Angeles, 8244 Orion

Los Angeles, 8639 Lincoln

Los Angeles, 9841 Airport

Los Angeles, 14724 Ventura

Pahndale, Fire Station

Pasadena, CIT Millikan Library

Pasadena, JPL Building 180

Pearblossom, Pumping Plant

Port Hueneme, Naval Lab

San Bernardino, Hall of Records

Santa Barbara, Courthouse

Vernon, CMD Terminal Building

Wrightwood, 6074 Park Drive

El Centre Array Station 10

Los Angeles, 1177 S. Beverly Drive

Beverly Hills, 9100 Wilshire

Los Angeles, 1800 Century Park East

Los Angeles, 5900 Wilshire

Los Angeles, 616 S. Normandie

Housing

Building

do

do

do

do

do

do

do

do

do

Shelter

Building

do

do

do

do

do

do

do

do

Building

do

do

do

do

do

do

do

do

do

do

do

do

do

do

do

do

do

Shelter

Building

do

Building

do

do

do

do

do

do

do

do

No. of 

stories

1

1

1

2

1

1

1

1

4

14

7

9

19

19

10

42

7

8

7

27

20

8

12

12

20

6

7

15

11

11

7

12

14

14

1

9

9

1

6

2

6

2

1

7

10

15

31

16

Level

Ground

do

do

Basement

Ground

do

do

do

Basement

do

Ground

Basement

do

Ground

do

Basement

do

Ground

do

do

Basement

do

Ground

Basement

do

do

do

Ground

do

Basement

do

Ground

Basement

do

Ground

do

Basement

do

Ground

do

Basement

Basement

do

Ground

do

Basement

do

do

do

do

Sediment 

depth (km)

0.09

.18

5.0

3.0

2.6

3.7

3.7

.91

3.4

3.0

3.0

3.0

2.9

3.4

3.7

4.6

2.9

2.9

3.0

2.4

2.9

2.9

2.9

4.2

2.4

2.4

1.8

.20

.01

.027

1.1

8.5

6.7

.068

4.5

4.1

3.7

3.7

3.5

Geology1

A

A

E

A

E

A

E

E

B

A

A

B

A

A

A

B

A

A

A

B

B

B

A

A

B

A

B

B

A

A

A

A

B

B

A

A

B

B

E

A

A

B

A

B

A

B

A

B

B

A
L8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

TABLE 3. Station data Continued

Station 

no.

437

443

446

449

452

455

458

461

466

469

475

482

657

724

757

885

931

942

952

955

958

1008

1013

1014

1015

1016

1028

1049

1052

1065

1078

1080

1095

1250

1291

1377

1409

1410

1411

1422

1438

1490

1492

2734

2808

3501

5028

5051

5052

5053

Description

Los Angeles, 1150 S. Hill

Los Angeles, 6200 Wilshire

Los Angeles, 1760 N. Orchid

Los Angeles, 2500 Wilshire

Beverly Hills, 435 N. Oakhurst

Beverly Hills, 450 N. Roxbury

Los Angeles, 15107 Van Owen

Los Angeles, 15910 Ventura

Los Angeles, 15250 Ventura

Los Angeles, 1625 W Olympic

Pasadena, CIT Athenaeum

Alhambra, 900 S. Fremont

Santa Monica, 201 Ocean

Niland, 8071 Luxor

Sepulveda Canyon Control Facility

Santa Barbara, UCSB Physical Plant

El Centre Array Station 12

El Centre Array Station 6

El Centre Array Station 5

El Centre Array Station 4

El Centre Array Station 8

Bishop, Los Angeles Water Department

Cholame-Shandon Array Station 2

Cholame-Shandon Array Station 5

Cholame-Shandon Array Station 8

Cholame-Shandon Array Station 12

Hollister, City Hall

Oakland, City Hall

Oso Pumping Plant

San Francisco, Alexander Building

San Francisco, South Pacific Building

San Francisco, State Building

Taft, Lincoln School

Gilroy, Gavilan College

Marysville, CDOT Maintenance Building

San Juan Bautista, 24 Polk

Gilroy Array Station 2

Gilroy Array Station 3

Gilroy Array Station 4

Halls Valley, Grant Ranch

Cholame-Shandon Array, Temblor

Mammoth Lakes, High School

San Juan Bautista, 101/156 Overpass

Icy Bay, Gulf Timber Company

Hilo, UH Cloud Physics Lab

Nicaragua, Managua ESSO Refinery

El Centre Array Station 7

El Centre, Parachute Test Site

Plaster City

Calexico, Fire Station

Housing

do

do

do

do

do

do

do

do

do

do

do

do

Building

do

do

do

Shelter

do

do

do

do

do

do

do

Building

Shelter

Building

do

Shelter

Building

do

Building

do

do

do

do

Shelter

do

Building

Shelter

do

Building

Shelter

Building

do

do

do

do

do

do

No. of 

stories

10

17

23

13

10

10

7

16

12

10

2

12

18

1

9

1

1

1

15

15

12

6

1

1

1

1

1

1

1

1

1

1

1

1

Level

do

Ground

do

Basement

do

Ground

Basement

do

do

Ground

Basement

do

Basement

Ground

Basement

Ground

do

do

do

do

do

do

do

do

do

do

Basement

do

Ground

Basement

do

Basement

Tunnel

Ground

do

do

do

do

do

do

do

do

do

do

do

do

do

do

do

do

Sediment 

depth (km)

2.6

3.5

2.9

3.8

3.7

2.9

1.8

1.8

2.9

.91

1.2

4.1

8.4

4.4

5.3

5.5

5.7

5.1

1.7

1.7

1.7

1.7

1.4

.3

3.0

.043

.087

.062

6.4

.020

1.2

4.0

4.0

4.0

.015

6.1

2.0

5.2

3.4

2.0

4.3

Geology^

A

A

A

A

A

A

A

A

A

B

B

B

B

A

B

A

A

A

A

A

A

A

A

A

B

A

A

B

A

A

F

A

A

B

A

A

A

A

A

A

E

B

B

F

E

A

A

A

A

A
PREDICTING STRONG GROUND MOTION IN UTAH L9

TABLE 3. Station data Continued

Station 

no.

5053

5055

5056

5057

5058

5059

5060

5061

5062

5079

5081

5093

5115

5137

5154

5155

5165

5169

6605

6610

6616

6617

6618

6619

6621

6622

9022

9034

9110

9124

Description

Bonds Corner

Holtville, Post Office

El Centro Array Station 1

El Centro Array Station 3

El Centro Array Station 11

El Centro Array Station 13

Brawlee, Airport

Calipatria, Fire Station

Salton Sea Wildlife Refuge

Malibu, Kilpatrick Boy's School

Topanga, Fire Station

Santa Barbara, UCSB North Hall

El Centro Array Station 2

Santa Barbara, Freitas Building

El Centro, Imperial County Center

El Centro, Meloland Overpass

El Centro, Differential Array SMA-1

Westmorland, Fire Station

Mexico, Delta

Mexico, Victoria

Mexico, Aeropuerto

Mexico, Cucapah

Mexico, Agrarias

Mexico, Mexicali, SAHOP

Mexico, Chihuahua

Mexico, Compuertas

Goleta, SCE Substation

El Centro, Differential Array

U.S.S.R., Karakyr

Iran, Tabas

Housing

do

Building

do

do

do

Shelter

Building

do

do

do

do

do

Shelter

Building

Shelter

Vault

Shelter

Building

Shelter

do

do

Shelter

do

Building

Shelter

do

do

Vault

Building

do

No. of 

stories

1

1

1

1

1

do

2

1

1

1

3

4

2

do

1

1

Level

do

Ground

do

do

do

do

1

do

do

do

do

do

do

Basement

Ground

do

do

do

do

do

do

Ground

do

do

do

do

do

do

do

Sediment 

depth (km)

5.7

5.6

5.1

5.7

4.5

3.8

5.2

5.0

5.5

8.5

4.8

5.1

5.0

5.1

7.5

8.0

7.0

5.5

7.0

5.5

5.0

1.4

2.0

Geology

A

A

A

A

A

A

A

A

A

E

E

E

A

B

A

A

A

A

A

A

A

A

A

A

A

A

E

A

A

A

1 Geology code adopted from Campbell (1981a): A, Holocene deposits, rock >10 m deep; B, Pleistocene deposits, rock >10 m deep; E, Holocene or Pleistocene 

deposits <10 m deep overlying rock; F, extremely soft or loose Holocene deposits such as beach sand or recent flood-plain, lake, swamp, estuarine, and delta 

deposits.

smaller reductions than those closer in. This distance 

dependence was adequately modeled by a hyperbolic tangent function, where distance is expressed in kilometers. 

This function is plotted in figure 6.

The results given in table 6 indicate that peak horizontal velocity is dependent on sediment depth (that is, 

depth to crystalline basement rock), increasing rapidly 

with depth at relatively shallow depths and becoming virtually independent of depth at depths of 5 to 6 km. This 

dependence was found to be dependent on building size, 

buildings five stories and higher having peak velocities 

that increase faster with depth than those of smaller 

buildings and instrument shelters. For depths greater 

than about 5 km, at which point the effect of sediment 

depth remains relatively constant, the effect of building 

size is less than about 10 percent. The depth dependence 

was adequately modeled by a hyperbolic tangent

function, where sediment depth is expressed in kilometers. This function is plotted in figure 7. No amplification 

due to shallow soils nor any reduction as a result of building embedment was observed for peak velocity.

Both peak horizontal acceleration and peak horizontal velocity were found to be influenced by fault type, 

being higher for reverse and thrust faults than for strikeslip faults. The effect for velocity is somewhat larger than 

that for acceleration, but the difference may not be statistically significant.

During the analysis of residuals for both peak horizontal acceleration and velocity, three recordings were 

found to have such large positive residuals that they 

appeared to be outliers. All three recordings had weighted 

residuals exceeding four standard errors. Further investigation revealed that all three recordings had three factors 

in common: (1) they were associated with earthquakes
L10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

8.0

7.5 -

7.0

6.5

6.0

5.5

5.0

4.5

 

 

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

B

i 

10 15 20 25 30 35 40 

DISTANCE TO SEISMOGENIC RUPTURE, IN KILOMETERS

45 50

FIGURE 1. Distribution of strong-motion recordings with respect to magnitude and distance. A, Peak horizontal acceleration; B, peak

horizontal velocity.
PREDICTING STRONG GROUND MOTION IN UTAH Lll

TABLE 4. Distribution of recordings by distance

Distance

range (km)

0-2.4

2.5-4.9

5.0-7.4

7.5-9.9

10.0-14.0

14.1-19.9

20.0-28.2

28.3-40.0

28.3-40.0

40.1-56.6

Earthquake (year)

Parkfield (1966) 

Gazli (1976)

Long Beach (1933)

Parkfield (1966)

Managua (1972)

Tabas (1978)

Coyote Lake (1979)

Imperial Valley (1979)

Imperial Valley (1940)

Hollister (1974)

Santa Barbara (1978)

Coyote Lake (1979)

Imperial Valley (1979)

Daly City (1957)

Parkfield (1966)

Lytle Creek (1970)

San Fernando (1971)

Hollister (1974)

Imperial Valley (1979)

Daly City (1957)

Parkfield (1966)

Lytle Creek (1970)

San Fernando (1971)

Santa Barbara (1978)

Malibu (1979)

Coyote Lake (1979)

Imperial Valley (1979)

Long Beach (1933)

Daly City (1957)

Lytle Creek (1957) 

San Fernando (1971)

Point Mugu (1973)

Malibu (1979)

Coyote Lake (1979)

Imperial Valley (1979)

Lytle Creek (1970)

San Fernando (1971)

Oroville (1975)

Kalapana (1975)

Bishop (1978)

St. Elias (1979)

Imperial Valley (1979)

Kern County (1952)

Borrego Mountain (1968)

San Fernando (1971)

No. of recordings

Acceleration

0

1 

1

1

1

1

1

2

11

1

1

2

1

7

1

2

1

1

2

5

2

1

1

3

3

1

2

5

1

1

1 

13

1

3

1

3

2

32

1

1

2

1

4

1

1

4

/ciocitiy

0

1 

1

1

1

1

1

2

10

1

1

1

1

5

1

1

1

1

2

5

2

1

0

2

2

0

2

5

1

1

1 

12

0

0

0

3

2

25

0

0

0

1

3

1

1

4

TABLE 5. Definitions 0/Kj, fife^, K$, R), and ffe^K^, D)

[f(&i, KI, K)=e\K\+t 2K2+eaK3+e4KA+e^s+eR(I£A+Kci)tai\h(e'7R) for peak

horizontal acceleration; f(e^, KI, D^eiKi+ez^+esKs tanh(e4fl)+e5 [1-

.KsJtamXegD) for peak horizontal velocity]

Parameter

Fault type

Source directivity

Shallow soil

Building size

Embedment

Definition

Peak horizontal Peak horizontal 

acceleration velocity

#i=0 Strike slip #i=0 Strike slip

1 Reverse and 1 Reverse and

thrust. thrust.

#2=0 All other #2=0 Allother

rupture rupture

geometries. geometries.

1 Rupture 1 Rupture 

toward toward

site. site.

#3=0 Soils > 10m 

deep.

1 Soil<10m

deep.

 #3=0 Buildings >5

stories.

1 Shelters and

buildings

<5 stories.

#4=0 All other 

recordings.

1 Basements of 

3- to 9-story

buildings.

#5=0 All other

recordings.

1 Basements of

>10-story

buildings.

TABLE 6. Summary of regression coefficients, ej

[flei, KI, R)=eiKi+e fa+esKa+e^+e^s+e^Ki+KsytonhfaR) for peak

horizontal acceleration; f[_eit Kit Z))=eiXi+e2-K2+e3^3tann(e4£))+e5 UXa]tanh(egD) for peak horizontal velocity]

Coefficient

.

Peak horizontal velocity acceleration

Equations Equation 4 Equations Equation 6

e\

&i 

PI K3 

64,

65

66

67

0.32 0.34 0.47 0.49

52 53 95 .99 

.41 .41 .63 .53

-.85 -.86 .39 .41

-1.14 -1.12 .72 .60

87 .89 .75 .88

.068 .065 .0 .0
L12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

! I I

0.01

10 20 40 100 

DISTANCE TO SEISMOGENIC RUPTURE, IN KILOMETERS

200 400

FIGURE 2. Attenuation relationships of peak horizontal acceleration for magnitudes of 5.0, 6.0, 7.0, and 8.0. Solid line indicates unconstrained model 

(eq. 3); dashed line, constrained model, where y=0.0059 (eq. 5). Relationships are plotted for Ki=Q.

exhibiting unilateral rupture towards the site, (2) their 

azimuths with respect to the source were within 5° to 10° 

of the direction of rupture, and (3) they were sited on sediments at least 5 km deep. These results appeared to indicate a directivity effect, possibly enhanced by the 

thickness of the sediments. The effect was so strong, 

especially for peak velocity, that it was decided to include 

it as a parameter, even though it was represented by only 

three recordings. Other recordings in the data base may 

be affected to some degree by directivity, but only these 

three seem to represent a combination of factors that lead 

to a near-maximum effect of this phenomenon. A more 

thorough investigation of directivity effects is beyond the 

scope of the current study.

FAR-FIELD CONSTRAINTS

Because this study was not directly concerned 

with predicting far-field ground motion, accelerograms 

recorded farther than 30 to 50 km from seismogenic rupture were not included in the analyses. In order to 

develop attenuation relationships that could be extrapolated to larger distances, a second analysis was performed where y, the coefficient of anelastic attenuation in 

equation 1, was constrained to values appropriate for the 

majority of data making up the data base.

Because of the similarity between the regional 

makeup of the current data base and that of Joyner and 

Boore (1981), we adopted their statistically determined
PREDICTING STRONG GROUND MOTION IN UTAH L13

1.0

0.4

0.2

0.1

0.04 -

0.02 -

0.01

4.5 5.0 5.5 6.0 6.5 

MAGNITUDE

7.0 7.5 8.0

FIGURE 3. Predicted magnitude scaling of peak horizontal acceleration for distances (/?) of 2,5,10,20, 50, and 100 km. Solid line indicates unconstrained 

model (eq. 3); dashed line, constrained model, where y=0.0059 (eq. 5). Relationships are plotted for K^=0.

values of y for our constrained analyses. Repeating the 

weighted nonlinear regression analyses using y=0.0059 

resulted in the following constrained relationships 2 :

In PHA=-3.303+0.850M-1.251n[£+0.0872 exp(0.678M)] 

-jR+fte^K^K)

In PHV=-1.584+1.18M-1.24 ln[£+0.00907exp(0.951M)]

(5)

(6)

where all parameters are defined as in equations 3 and 4. 

The coefficients e^, parameters K^, and functions f(e^ K^,

n

''The analyses of Joyner and Boore (1981) resulted in the same 

value of Y for both peak acceleration and peak velocity, suggesting that 

anelastic attenuation may be nearly independent of frequency in 

California.

R) and/^, KI, D) are summarized in tables 5 and 6; the 

relationships are plotted in figures 2 through 5. The standard errors of regression for equations 5 and 6 are 0.30 

and 0.27, respectively.

Equations 5 and 6 were compared with the attenuation relationships of Joyner and Boore (1981) to verify 

that the attenuation characteristics were similar beyond 

distances of 30 to 50 km as originally intended The comparison was made for M=6.5 and R>3Q km after modification to account for differences between the definitions of 

parameters in the two studies. The magnitude was chosen 

to be near the median magnitude of both studies. Comparisons were restricted to R>30 km to minimize the impact 

of differences in the near-source behavior of the two functional forms. Close agreement in the attenuation rates of 

both studies suggested that the extrapolation of equations
L14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

100

40

o 

o

LU 

C/3

cc

LU 

O.

OC 20 LU ^u 

LU

LU 

<J

1 10

4 -

2 -

i i I i i i

4 10 20 40 100 

DISTANCE TO SEISMOGENIC RUPTURE, IN KILOMETERS

200 400

FIGURE 4. Attenuation relationships of peak horizontal velocity for magnitudes of 5.0, 6.0, 7.0, and 8.0. Solid line indicates unconstrained model 

(eq. 4); dashed line, constrained model, where Y=0.0059 (eq. 6). Relationships are plotted for K^=Q.

5 and 6 beyond 30 to 50 km using y=0.0059 was valid to 

distances of at least several hundred kilometers.

Coefficients derived for the constrained relationships 

are generally consistent with those derived for the unconstrained relationships, an indication that only relatively 

small adjustments were required to accommodate the 

constraint on anelastic attenuation imposed during the 

second analysis. It should be noted, however, that the 

coefficients presented in table 6 are constrained only by 

near-source recordings and may not be valid for distances 

greater than 30 to 50 km. This fact is especially true for 

the shallow-soil and building-embedment parameters, 

which may become less important as distance (and, 

hence, wave period) increases.

ANALYSIS OF RESIDUALS

An analysis of residuals was used to (1) select and 

define the parameter used in the attenuation relationships 

and (2) check the adequacy of the final relationships. For 

convenience, the analysis used the normalized weighted 

residual,

(7)

where NWR^ is the normalized weighted residual, i is the 

index of the recording, wi is the weight assigned to the 

recording, yi is the observed value of In Y, yi is the predicted value of In Y, o is the standard error of the regres-
PREDICTING STRONG GROUND MOTION IN UTAH L15

100

40

20

1 10

4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0

MAGNITUDE

FIGURE 5. Predicted magnitude scaling of peak horizontal velocity for distances (R) of 2, 5, 10, 20, 50, and 100 km. Solid line indicates 

unconstrained model (eq. 4); dashed line, constrained model, where y=0.0059 (eq. 6). Relationships are plotted for ^=0.

sion, and MWR is the mean weighted residual, defined by 

the expression

(8)

The definitions and functional forms of the parameters listed in table 5 were developed by an iterative procedure. First, a list of candidate parameters was selected 

from theoretical considerations of source, site, and soilstructure interaction effects available from the literature. 

Second, regression analyses were used to establish attenuation relationships in terms of M and R by setting Ki=0 

in equation 1. In the case of continuous parameters such 

as sediment depth, the normalized weighted residuals 

were then plotted against the candidate parameters. In

the case of classification or so-called "dummy" parameters such as fault type, the normalized weighted residuals 

associated with each class were plotted against M and R. 

When visually perceptible trends were identified from the 

residual plots, those parameters were added to the model. 

The process was repeated until all significant parameters 

and their proper functional relationships were identified 

and included in the model. From a purely scientific point 

of view, the above procedure might be criticized for its 

strong empirical basis, although, from a statistical standpoint, such procedures are accepted as standard modelbuilding techniques (Draper and Smith, 1971; Tukey, 

1977).

The adequacy of the final attenuation relationships 

was assessed by plotting the normalized weighted residuals against each dependent and independent variable
L16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

1.1

1.0

0.9

g 0.8

0.7

Basements of >10-story buildings

w 0.6

6

z

Q

0.4

0.3

0.2

Basements of 3- to 9-story buildings

/

10 15 20 25 30 35 

DISTANCE TO SEISMOGENIC RUPTURE, IN KILOMETERS

40 45 50

FIGURE 6. Dependence of peak horizontal acceleration on building embedment. Solid line indicates unconstrained model (eq. 3); dashed line, constrained 

model (eq. 5). The embedment factor is a multiplicative factor given by the expression expje^^f e^K^-e^K^- ^tanh^y/?)], where ei and KI are defined 

in tables 5 and 6.

included in the models. The residual plots against distance, magnitude, and predicted value are given in figures 

8 through 10 for the unconstrained relationships (eqs. 3, 

4). A lack of any significant trend indicated that the models were statistically adequate. Similar results were found 

for equations 5 and 6 as well as for other independent 

variables included in the relationships.

GROUND-MOTION ESTIMATES 

FOR NORTH-CENTRAL UTAH

The region of north-central Utah contains the major 

population centers of Ogden, Provo, and Salt Lake City. 

The dominant feature of this region is the Wasatch Front, 

a massive 370-km-long scarp extending along the western 

margin of the Wasatch Mountains from Gunnison, Utah, 

to Malad City, Idaho. The scarp is a result of normal faulting along the Wasatch fault, which marks the boundary 

between the Basin and Range and Middle Rocky Mountain physiographic provinces (Fenneman, 1946). This 

boundary marks the transition from a northwest-southeast extensional stress regime on the west to a northwestsoutheast compressional stress regime on the east 

(Zoback and Zoback, 1981).

The Wasatch fault poses the greatest seismic threat 

of major earthquakes in this region. Geologic evidence 

indicates that surface faulting associated with earthquakes of magnitude 6.5 to 7.5 has occurred repeatedly 

along several segments of this fault at an average rate of 

one event every 444 years (Swan and others, 1980). In 

contrast, the greatest seismic threat in the next 100 years, 

assuming a Poisson model of earthquake occurrences, 

comes from small to moderate earthquakes located in the 

historically more active regions of the State (Youngs and 

others, this volume).
PREDICTING STRONG GROUND MOTION IN UTAH L17

Shelters and buildings <5 stories

456 

SEDIMENT DEPTH, IN KILOMETERS

FIGURE 7. Dependence of peak horizontal velocity on sediment depth. Solid line indicates unconstrained model (eq. 4); dashed line, constrained model (eq. 6). 

The sediment-depth factor is a multiplicative factor given by the expression exptestfstanh^DJ+^l-^tanhtee^O], where ei and KI are defined in tables 5 

and 6.

There are several important issues that need to be 

addressed before attempting to estimate strong-motion 

parameters in north-central Utah. These issues are discussed below.

STRESS REGIME

McGarr (1984) presented theoretical and empirical 

results indicating that peak acceleration and peak velocity are strongly dependent on stress state and focal depth. 

He contended that, for a given seismic moment, both 

parameters are smaller for earthquakes occurring in an 

extensional stress regime than for earthquakes occurring 

in a compressional stress regime and that both parameters increase with focal depth. Differences due to stress 

regime are quite large, being as great as a factor of 3 for 

peak acceleration and a factor of 2 for peak velocity at a 

given depth.

Taken at face value, McGarr's results would have a 

considerable impact on the estimation of ground motion 

in the region of Utah west of the Wasatch Front. This 

region, including the Wasatch fault, is characterized by

Basin and Range extensional tectonics. In contrast, the 

majority of the strong-motion data used to develop the 

attenuation relationships in this study come from coastal 

California, a region undergoing predominantly northsouth compression (Zoback and Zoback, 1981). Thus, if 

McGarr were correct, one would expect substantially 

smaller ground motion in western Utah than equations 3 

through 6 would predict. However, McGarr's results are 

not consistent with other empirical observations.

McGarr's results appear to be contradicted by this 

author's study of peak acceleration data from near-source 

recordings in California (Campbell, 1985b, 1989). In an 

attempt to identify regional differences in peak acceleration scaling relations, Campbell analyzed strong-motion 

data recorded within 20 km of ML=2.5 to 5.0 earthquakes 

in Oroville, Calif., Imperial Valley, Calif., and coastal California The Oroville region falls along the western margin 

or the Sierra Nevada physiographic province (Fenneman, 

1946), a region characterized by east-to-northeast extension (Zoback and Zoback, 1981). Focal mechanisms in 

this area are predominantly normal. The other two areas 

are within the San Andreas physiographic province
L18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

I 

Q

-2

-3

-4

; . 'i

t 

1 '

O

E 0

Q

-1

-2

-3

-4

B

10 15 20 25 30 35 40 

DISTANCE TO SEISMOGENIC RUPTURE, IN KILOMETERS

45 50

FIGURE 8. Normalized weighted residuals plotted as a function of distance. A, Peak horizontal acceleration; B, peak horizontal velocity. 

Plots are shown for the unconstrained models (eqs. 3,4). Similar distributions were found for the constrained models.
PREDICTING STRONG GROUND MOTION IN UTAH L19

3 -

I

(A

Q 

I

I o

-2

-3

-4

B

3 -

1 2

o 

55

UJ

« 1 

o

I o

-2

-3

-4

4.5 5.0 5.5 6.0 6.5 

MAGNITUDE

7.0 7.5 8.0

FIGURE 9. Normalized weighted residuals plotted as a function of magnitude. A, Peak horizontal acceleration; B, peak horizontal 

velocity. Plots are shown for the unconstrained models (eqs. 5, 4). Similar distributions were found for the unconstrained models.
L20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

1 '

i ,

o

o

-2

-3

..? t ' .. .

 o %

J_____I___I I I I I

0.01 0.02 0.04 0.1 0.2

PREDICTED VALUE, IN g

0.4 1.0

r

I '

O

8-t

-2 -

-3 -

B

 

' *' *' '

J_____L i i i i i

4 10 20

PREDICTED VALUE, IN CENTIMETERS PER SECOND

40 100

FIGURE 10. Normalized weighted residuals plotted as a function of predicted value. A, Peak horizontal acceleration; B, peak horizontal 

velocity. Plots are shown for unconstrained models (eqs. 3,4). Similar distributions were found for the constrained models.
PREDICTING STRONG GROUND MOTION IN UTAH L21

(Fenneman, 1946), a region characterized by north-south 

compression (Zoback and Zoback, 1981). Focal mechanisms in this region are predominantly strike slip on 

northwest-trending faults and reverse or reverse oblique 

on west-trending faults.

Campbell's regression analyses indicated only minor 

regional differences in magnitude and distance scaling. 

For magnitudes ranging from 3.5 to 4.5 and epicentral distances ranging from 5 to 10 km (about one to two focal 

depths), the scaling relations predict peak accelerations 

that vary by less than 10 to 15 percent between regions. 

Furthermore, the study found no significant dependence 

of peak acceleration on focal depth for depths in the 

range 4- to 10-km range. These depths represent the major 

seismogenic portion of the crust in California Similar 

results were found whether hypocentral distance or epicentral distance was used in the analyses.

The reasons for the discrepancy between the above 

results and those of McGarr are largely speculative. One 

major difference between the two studies that may help 

to explain this discrepancy is a difference in magnitude 

scaling. McGarr assumed that, at relatively short 

distances, peak acceleration should be relatively independent of seismic moment. On the other hand, the nearsource regression analyses of Campbell (1985b, 1989) 

indicate a strong correlation between peak acceleration 

and ML and, hence, seismic moment, hi PHA being proportional to 0.5 to 0.7 ML- The latter dependence is generally consistent with magnitude scaling of peak acceleration from ML>5.0 earthquakes (Campbell, 1981a, 1984a; 

Joyner and Boore, 1981). In fact, Campbell (1985b) found 

that, when an estimate of closest distance to fault rupture 

was used as the distance metric in his regression analyses, the near-source magnitude and distance scaling properties of peak acceleration for M^<5.0 earthquakes were 

remarkably consistent with the extrapolation of the nearsource attenuation relationships for ML>S.O earthquakes 

developed by Campbell (1984a). Thus, either McGarr's 

assumption of magnitude independence of peak horizontal acceleration for hypocentral distances on the order of 

10 km is wrong, or else ML scales with seismic moment, 

state of stress, and focal depth in a manner similar to his 

proposed scaling relationship for peak horizontal acceleration.

The hypothesis that ML and peak horizontal acceleration might have similar dependencies on stress state and 

focal depth is important because, if correct, it would 

imply that scaling relations between the two parameters 

are relatively independent of stress state and, thus, of tectonic regime. Some empirical evidence suggesting that 

this hypothesis may be correct stems from an observation 

by Boore (1980) that Western U.S. strong-motion records 

show a direct relationship between peak horizontal velocity and Wood-Anderson amplitude and, hence, ML- Combining Boore's findings with McGarr's observation that 

both peak horizontal acceleration and peak horizontal 

velocity are dependent on stress state and focal depth 

suggests that ML is also dependent on stress state and 

focal depth.

A further implication of the above hypothesis is that 

the relationship between seismic moment and ML should 

be dependent on stress state. Such a relationship has 

been proposed by Thatcher and Hanks (1973) from 

source-parameter relations developed for Southern California Their proposed relationship,

logM0=2.0ML+14.2-log ACT (9)

suggests that the relationship between seismic moment 

and ML is dependent on stress drop, Aa. The hypothesis is 

then validated if Aa can be considered dependent on 

stress state. McGarr used just such a proposed dependence to give theoretical credibility to his observations by 

inferring direct relationships between localized stress 

drop and crustal strength and between crustal strength 

and stress state. From considerations of tectonic stress, 

he suggested that crustal strength (and, thus, stress drop) 

is nearly a factor of 4 greater in compressional tectonic 

regimes than it is in extensional tectonic regimes and that 

stress drop is expected to increase linearly with depth. It 

is interesting to note that regional variations in stress 

drop and moment-magnitude relationships in Southern 

California and northern Baja California have also been 

attributed to regional differences in tectonic stresses by 

Thatcher (1972) and Thatcher and Hanks (1973).

All things considered, it would seem that McGarr's 

results may be irrelevant with respect to the estimation of 

strong-motion amplitudes, at least when magnitude is 

used as the measure of source strength. If his hypothesis 

is wrong, as suggested by the analyses of Campbell 

(1985b, ,1989), then there is no need to consider stress 

regime as being important. If his hypothesis is correct, 

then there is evidence to suggest that scaling relations 

between ground-motion parameters and magnitude are 

relatively independent of stress state.

FAULT TYPE

In this study, as well as in previous studies (Campbell, 1983,1984a), fault type has been found to be a significant factor in predicting strong ground motion. Its 

importance has been confirmed theoretically by studies 

done by Anderson and Luco (1983) and Boore and Boatwright (1984). Boore and Boatwright computed average 

radiation coefficients for three types of faulting: a vertical 

strike-slip fault, a 30°-dipping dip-slip fault, and a 45°-dipping oblique-slip fault.-They found that the average radiation coefficients for dipping faults were 14 to 28 percent
L22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

higher than those for vertical strike-slip faults for distances within about a focal depth of the source. At 

regional distances (tens to hundreds of kilometers), the 

opposite was found, dipping faults having average radiation coefficients 14 to 20 percent smaller than those of 

vertical strike-slip faults. These results suggest that, on 

average, near-source amplitudes of ground motion from 

dipping faults will start off higher but attenuate more rapidly than those from near-vertical faults, the transition 

occurring at a distance of approximately one focal depth. 

Indeed, such a result was observed in the analyses of the 

Oroville and Imperial Valley strong-motion recordings 

described in the preceding section.

Boore and Boatwright's results would seem to indicate that the effects of fault type should be negligible, on 

average, over the range of distances used in this study if 

differences in attenuation rate between different fault 

types are neglected. Yet, the current study found significantly higher ground motions from earthquakes on dipping faults. The most plausible explanation for these 

results is offered by the ground-motion simulation studies 

of Anderson and Luco (1983), who found that sites 

located directly above the rupture surface of dipping 

faults had the highest amplitudes of any other rupture 

configuration. Because the data in the present study 

include several such sites (for example, the 1976 Gazli 

and 1978 Tabas recordings), it is possible that these sites 

are largely responsible for the observed results. These 

results can have serious implications for cities like 

Ogden, Provo, and Salt Lake City that are located directly 

above the Wasatch fault. For this reason, the author has 

chosen to incorporate the uncertainty associated with the 

potential effect of fault type by estimating ground 

motions for a range of fault types (^=0 and K^=\ from 

table 5).

ANELASHC ATTENUATION

There have been a number of attenuation studies 

conducted in the Basin and Range in the past with widely 

varying results. The closest study geographically was that 

conducted by King and Hays (1977). They measured 

ground motions from four aftershocks of the March 27, 

1975, Pocatello Valley, Idaho, earthquake (M 3=6.0) along a 

150-km linear array extending from southern Idaho to Salt 

Lake City. They concluded that this region exhibits 

higher attenuation of 1- to 10-Hz pseudo-relative velocity 

response spectra than either California or Southern 

Nevada. This conclusion, however, is in direct contradiction to the results of Singh and Herrmann (1983).

Singh and Herrmann quantified the regional attenuation of Lg coda waves throughout the United States 

through the relationship

(10)

where Q is a measure of anelastic attenuation referred to 

as the quality factor, Q0 is a measure of Q at 1 Hz, and/is 

frequency. Their maps indicate that attenuation in northcentral Utah is represented by Q0=400 and n=0.2, substantially higher than attenuation in coastal California, 

where Q0<200 and n=0.5.

There is similar controversy regarding regional attenuation elsewhere in the Basin and Range. For example, in 

the southern Great Basin, Rogers and others (1987) found 

Q0=600 and n=0.1 from vertical peak-amplitude earthquake data, Q0=143 and n=0.84 from vertical coda-amplitude earthquake data, and Q0=188 and n=1.05 from 

vertical coda-amplitude nuclear explosion data. In this 

same region, Singh and Herrmann (1983) found Q0=200 

and n=0.4, and King and Hays (1977) presented attenuation rates for pseudo-relative velocity spectra from 

nuclear explosions that are similar to earthquake attenuation rates in California over a broad range of frequencies. 

In the northern Great Basin, Chavez and Priestley (1985) 

found Q0=125 to 280 from synthetic Wood-Anderson 

earthquake amplitudes, Chavez and Priestley (1986) 

found Q0=130 and n=0.85 from nuclear explosion Lg 

amplitudes, and Singh and Herrmann (1983) found 

Q0=200 to 300 and n=0.4 from Lg-coda amplitudes.

The above results seem to imply that there are large 

regional variations of attenuation characteristics throughout the Basin and Range and that the computed attenuation rates for the region are highly dependent on method 

and data. Large regional variations in attenuation are supported by Taylor and Patton (1986), who found that structural inversions of regionalized Rayleigh-wave dispersion 

curves indicate significant lateral variations in the crust 

and upper mantle structure of the Basin and Range and 

transition areas. The uncertainty in computed attenuation 

rates is supported by the extreme variability in attenuation characteristics derived for the same area by different 

investigators and different techniques.

Singh and Herrmann's maps also suggest that there is 

significant azimuthal variation in attenuation in northcentral Utah, although this variation is no larger than the 

variability associated with estimates of Q in the Basin and 

Range provided by various investigators. Until a definitive 

study of attenuation in north-central Utah becomes available, it is necessary to estimate ground motion by using a 

range of attenuation rates that can be assumed to reasonably bracket the true attenuation rate of the region.

In order to incorporate anelastic attenuation in the 

prediction of ground-motion amplitudes in north-central 

Utah, the predictions were derived from equations 5 and 

6, where y is estimated from the relationship

(ID
PREDICTING STRONG GROUND MOTION IN UTAH L23

.where Q0 and n are as defined in equation 10 and U is the 

group velocity of the particular phase amplitude used to 

derive Q in kilometers per second. According to Singh 

and Herrmann (1983), the group velocity of Lg waves is 

approximately equal to 3.5 km/s.

For n^l, Y will be different for peak horizontal acceleration and velocity, because these parameters sample 

different parts of the frequency spectrum. For the present 

study, peak horizontal acceleration and velocity are 

assumed to have fundamental frequencies on the order of 

5 and 1 Hz, respectively (Nuttli, 1979). In actuality, these 

strong-motion parameters should be dependent on magnitude and frequency (Seed and others, 1969), but such a 

refinement seems unwarranted at this time in light of the 

large variability associated with Q0 and n. The various 

studies of attenuation in the Basin and Range cited above 

were used to bracket estimates of y- The largest values 

were derived from the assumption of California-type 

attenuation, for which y=0.0059 and Q=152 for both peak 

horizontal acceleration and peak horizontal velocity. This 

value of Y was used to quantify anelastic attenuation for 

the purposes of deriving equations 5 and 6. The smallest 

values of y were obtained from the study of Rogers and 

others (1987) for the southern Great Basin. Their nuclear 

explosion, vertical coda-amplitude study resulted in estimates of Q=1020 and y=0.0044 for 5-Hz waves, and their 

vertical peak-amplitude earthquake study resulted in estimates of Q=600 and y=0.0015 for 1-Hz waves. Therefore, 

for the purposes of the present study, peak horizontal 

acceleration was evaluated by using y=0.0044 to 0.0059, 

and peak horizontal velocity was evaluated by using 

y=0.0015 to 0.0059.

SITE EFFECTS

Recordings of nuclear explosions in Ogden, Provo, 

and Salt Lake City (Hays and others, 1978; Hays and King, 

1982, 1984; Hays, this volume; King and others, this volume) show significant amplification of pseudo-relative 

velocity spectra on valley sediments in comparison with 

those on rock over a wide range of frequencies. In Salt 

Lake City, average amplifications as high as a factor of 10 

were observed in the 5- to 10-Hz and the 1.4- to 5-Hz frequency bands, even higher amplifications being observed 

at specific frequencies. Area! contours of amplification 

show a rapid increase along profiles extending west from 

the range front out into the valley, maximum amplification being obtained within distances of as little as 5 to 10 

km. The 5- to 10-Hz contours appear to correlate with the 

depth of unconsolidated deposits, maximum amplification being attained for depths exceeding approximately 

300 m (Hays and others, 1973). The 1.4- to 5-Hz contours 

appear to correlate more with the depth of low-density

sedimentary rock, which attains depths as great as 4 km 

along the range front valleys (Mabey, 1991).

The correlation of site amplification with subsurface 

geology, especially sediment depth, is well documented in 

the literature (Seed and others, 1976; Trifunac and Lee, 

1978, 1979; Campbell, 1984a, this report; Joyner and 

Fumal, 1984, 1985; Rogers and others, 1984, 1985). However, simple relationships between ground-motion parameters and sediment depth appear to significantly 

underestimate the observed amplification of ground 

motion in Salt Lake valley. For example, the maximum 

amplification observed in Salt Lake City in the 1.4- to 5-Hz 

frequency band is larger by a factor of 2 or more than that 

estimated from the relationships of Trifunac and Lee 

(1978) and Rogers and others (1985) using reasonable 

estimates of sediment depth in Salt Lake valley (Mabey, 

1991). For low to intermediate frequencies, this discrepancy may be caused by the rather large depth-to-width 

ratio of the range front valleys, which have the potential 

to amplify these ground motions by two to three times 

over valleys having relatively small aspect ratios (Bard 

and Bouchon, 1985). For intermediate to high frequencies, this discrepancy may be the result of seismic resonance within the unconsolidated deposits.

The phenomenon of seismic resonance is well documented in the literature (Campbell, 1981a, 1983, 1984a; 

Chiaruttini and Siro, 1981; Faccioli, 1981; Mueller and others, 1982; Prince, 1984; Rogers and others, 1984, 1985; 

Anderson and others, 1986). Resonance occurs when 

upward-propagating waves are trapped in a surface layer 

of relatively low velocity. For vertically incident seismic 

waves, the maximum amplification of ground motion at 

the surface (ignoring free-surface effects) is proportional 

to pVrfpVi, which is reached when the frequency of the 

incidence waves coincides with the natural frequency of 

the surface layer, Vi/4H (Vi and V2 are the propagation 

velocities of the surface layer and underlying layer, 

respectively, and H is the thickness of the surface layer). 

pVz/pVi is referred to as the seismic impedance ratio. 

Theoretically, resonance can occur within a deposit of 

any thickness, as long as the impedance is large enough to 

overcome the effects of attenuation within the deposit. 

This condition becomes more difficult to meet as the 

deposits become thicker. The phenomenon is most commonly observed in relatively shallow deposits, where seismic impedances are more likely to be relatively large, as 

they are, for example, in the saturated unconsolidated 

deposits of the range front valleys of north-central Utah.

A recent example of the potentially devastating 

effects of seismic resonance is the destruction that beset 

Mexico City on September 19, 1985, as a result of the 

Ms=8.1 Michoacan, Mexico, earthquake (Anderson and 

others, 1986), which occurred off the western coast of 

Mexico some 350 km from Mexico City. The central por-
L24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

tion of the city lies on lakebed deposits as much as 500 m 

thick. These deposits, composed of soft saturated clays, 

were known to resonate at a natural frequency of about 

0.5 Hz from ground motions recorded during previous 

earthquakes. During the September 19 earthquake, 5-percent-damped absolute acceleration response spectra 

recorded on the lakebed deposits were again observed to 

peak near 0.5 Hz, amplitudes being as much as a factor of 

10 larger than those recorded on a nearby rock site south 

of the central portion of the city. In contrast, peak horizontal accelerations and spectral accelerations at nonresonant frequencies showed amplifications of a factor of 3 

or less. The amplification due to resonance at the surface 

of the lakebed deposits was large enough to destroy hundreds of buildings whose natural frequencies coincided 

with the natural frequency of these deposits. Similar phenomena have been observed in Caracas, Venezuela (Seed 

and Idriss, 1982), and in Japan (Kanai, 1983).

The Mexico City experience is extremely relevant 

to north-central Utah. The average amplification of 

5-percent-damped pseudo-relative velocity spectra from 

nuclear explosions recorded in Salt Lake valley reaches 

values at least as large as those observed in Mexico City 

for frequencies of 1.4 to 10 Hz (Hays and others, 1978; 

Hays and King, 1982, 1984; Hays, this volume; King and 

others, this volume). Thus, buildings having natural frequencies in this range are potentially vulnerable to damage from ground motions of even moderate strength, 

depending on their location within the valley. High-frequency resonance is also possible along the margins of 

the range front valleys, where shallow unconsolidated 

deposits and high impedance ratios are likely to be found. 

Large amplifications are also possible at frequencies 

greater than 1.4 Hz in the deepest parts of the valleys.

It must be emphasized that the observed amplification of pseudo-relative velocity spectra in Ogden, Provo, 

and Salt Lake City are for low-amplitude, low-strain 

ground motions. It is possible that high-amplitude ground 

motions from a large nearby earthquake on the Wasatch 

fault would induce strains large enough to significantly 

reduce the amount of amplification (Prakash, 1981), 

although this effect has been documented only from laboratory tests on disturbed soil samples. In any event, this 

effect would be restricted to only those sites located very 

near the source of the seismic waves. The only exception 

would be those sites susceptible to strong nonlinear 

behavior, such as loose, saturated sands and soft, sensitive clays where large strains could be induced by relatively small amplitudes

The spectral amplification data for Utah are not 

directly applicable to the estimation of peak horizontal 

acceleration and velocity. However, these data may be 

used to develop relative spectral amplification factors for 

Utah and California, which can then be used to infer relative ratios for the peak ground-motion parameters of the 

two regions. Because the attenuation relationships developed in this study are based on California strong-motion 

recordings and thus represent California site conditions, 

the relative ratios of the peak ground-motion parameters 

of the two regions can be used to approximately adjust 

these relationships for Utah site conditions. The technique is admittedly crude, but it will have to suffice until 

attenuation relationships for response spectra are 

developed.

Spectral amplification factors for thick (450-750 m), 

saturated, unconsolidated sediments in Salt Lake valley 

(Hays and King, 1982) were used to establish site 

response for Utah. These factors represent an upper 

bound to those found for shallower deposits in Salt Lake 

valley, as well as for those found in the Provo and Ogden 

areas. Spectral amplification factors developed for the 

Los Angeles region by Rogers and others (1985) were 

used to establish site response for California This latter 

study developed average spectral amplification factors 

for three frequency bands: 0.1 to 0.3, 0.3 to 1.0. and 1.0 to 

5.0 Hz. The data consisted of recordings of nuclear explosions and accelerograms recorded during the 1971 San 

Fernando earthquake. Spectral amplification factors were 

computed in terms of Fourier spectra ratios, a crystalline 

rock site in Pasadena, Calif., being used as the reference 

site. For the current study, only the intermediate- and 

high-frequency bands are of interest, since these roughly 

coincide with the assumed predominant frequencies of 

peak velocity and peak acceleration, respectively.

For the deeper parts of the Los Angeles basin, Rogers 

and others (1985) found average spectral amplification 

factors ranging from 3.6 to 4.5 for the intermediate-frequency band and 3.6 to 6.5 for the high-frequency band. 

Hays and King (1982) found spectral amplification factors 

of roughly 9.0 and 7.0 for these same frequency bands for 

thick, unconsolidated sediments in Salt Lake valley. Thus, 

site amplification in Salt Lake valley appears to be 

roughly a factor of 2 times higher at intermediate frequencies and roughly a factor of 1.5 times higher at high frequencies in comparison with site amplification in the Los 

Angeles basin. The larger amplification in Salt Lake valley 

may reflect the two-dimensional resonance of the valley, 

as Bard and Bouchon (1985) suggested, or it may simply 

be that geotechnical factors such as soft soils, a shallow 

water table, and large velocity gradients lead to larger 

amplifications.

On the basis of the preceding discussion, equation 5 

with K$=Q was used to establish a lower bound median 

estimate of peak horizontal acceleration with respect to 

site effects for north-central Utah. This estimate is 

believed to be appropriate for sites located in Provo and 

Ogden and for those sites located on relatively shallow 

(<150 m), unconsolidated sediments in Salt Lake valley.
PREDICTING STRONG GROUND MOTION IN UTAH L25

This estimate is probably also appropriate for rock, at 

least for sources within about 50 km of the site, although 

the actual values could be somewhat smaller or larger 

than this estimate, depending on factors such as the type 

of rock, the degree of weathering, and local topography 

(Campbell, 1983; Tucker and others, 1984). For very shallow, unconsolidated deposits near the range front, the 

shallow soil factor in table 5 (-£3=!) was used to establish 

an upper bound median estimate for peak horizontal 

acceleration. This multiplicative factor of 1.5 was also 

used to account for the additional amplification expected 

at high frequencies at sites underlain by thick, unconsolidated deposits in Salt Lake valley.

Equation 6 with K^=l and Z)=0 was used to establish 

a lower bound median estimate for peak horizontal velocity with respect to site effects. This estimate is appropriate for sites located on hard rock. The soil-depth factor in 

table 5 (.£3=1, Z^00), augmented with an additional factor of 2 to represent the additional amplification at intermediate frequencies expected at sites underlain by thick, 

unconsolidated sediments, was used to establish an upper 

bound median estimate for peak horizontal velocity.

SOURCE DIRECTIVITY

Source directivity is the focusing or defocusing of 

seismic energy as rupture propagates toward or away 

from a site. The current study has shown that rupture 

toward a site can lead to substantial amplification of 

ground motion in some cases (see table 6). Similar effects 

have been observed by Boatwright and Boore (1982) and 

Singh (1985). The cities of Ogden, Provo, and Salt Lake 

City are particularly vulnerable to this phenomenon 

because they lie directly above the Wasatch fault. 

Because of their locations, they lie at relatively small azimuths with respect to the fault plane and would probably 

be subject to some kind of focusing for almost any rupture configuration. The only configuration that would 

tend to defocus ground motions would be unilateral rupture away from one of these cities.

Although potentially important, directivity was not 

taken into account in the prediction of ground motion in 

north-central Utah for several reasons. First, there is no 

definitive empirical study on which to base the selection 

of appropriate amplification factors. Second, directivity 

effects have never been known to result in peak horizontal accelerations and velocities in excess of about 1.3 g 

and 100 cm/s, respectively, these values being within the 

upper bounds estimated for a Mg=7.5 earthquake in northcentral Utah. If one is concerned about the potential 

effects of directivity, it is recommended that estimates of 

ground motion presented in the next section be multiplied 

by an additional factor to account for these effects. However, one should be careful not to exceed reasonable

upper bound values for peak horizontal acceleration and 

velocity. What constitutes reasonable upper bound limits 

of these parameters is a subject of some controversy. 

Practically speaking, median estimates that approach the 

maximum near-source estimates of peak horizontal acceleration and velocity for the Ms=7.5 earthquake presented 

below might be considered reasonable upper limits. A 

rough estimate of what may be considered near-maximum 

directivity effects can be obtained from the factors in 

tables 5 and 6, which suggest that amplification due to 

directivity may be as large as a factor of 1.7 for peak horizontal acceleration and 2.7 for peak horizontal velocity.

GROUND-MOTION ESTIMATES

Ground-motion attenuation curves for north-central 

Utah are presented in figures 11 and 12. The range of values reflects variability associated with fault type, site 

amplification, and anelastic attenuation, as discussed in 

the preceding sections. This variability can be reduced 

only with regional studies designed to better define and 

quantity these effects. Until such studies become available, it is believed that a range of estimates is the only reasonable means of quantifying this variability.

The development of attenuation relationships for 

pseudo-relative velocity response spectra is currently in 

progress. Until these relationships become available, it is 

recommended that response spectra be estimated from 

estimates of peak acceleration and velocity according to 

the procedures proposed by Newmark and Hall (1982). 

This method is preferred over the use of attenuation relationships available in the literature (for example, Joyner 

and Fumal, 1985), because regional uncertainties in fault 

type, anelastic attenuation, and site effects included in 

estimates of peak horizontal acceleration and velocity are 

readily incorporated in the spectra.

As an example of the procedure, figure 13 displays a 

median estimate of a 5-percent-damped pseudo-relative 

velocity spectrum for a site located approximately 5 km 

from the zone of seismogenic rupture of a Mg=7.5 earthquake. This scenario probably represents the largest 

ground motion that can be expected anywhere in Utah. It 

is roughly equivalent to the level of ground motion that 

might be expected in Ogden, Provo, or Salt Lake City as a 

result of the largest earthquake that has been geologically 

inferred on the Wasatch fault (Swan and others, 1980).

Figures 11 through 13 present median estimates of 

ground motion. If some other percentile is desired, then it 

should be estimated from the expression (Draper and 

Smith, 1981)

il/2 (12)
L26 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

2.0

1.0

0.4

0.2

0.1

0.04

0.02

0.01

\ I I TT

I I I I I I I I I I I

10 20 40 100 200 

DISTANCE TO SEISMOGENIC RUPTURE, IN KILOMETERS

400 1000

FIGURE 11. Estimates of peak horizontal acceleration for north-central Utah. The shaded zone represents the range in median estimates of acceleration due to

variability in fault type, site amplification, and anelastic attenuation (see text).

where y represents hi PHA or hi PHV, ya is the cc-percentile value ofy,y is the median or predicted value of y, t(^v 

is the t statistic associated with a cumulative probability a 

and v=n-p-l degrees of freedom, n is the number of 

observations used in the analysis, p is the number of coefficients in the model, n0 is the number of future observations (usually 1), a is the standard error of regression, and 

a0 is the standard deviation of y.

For large v, ta>v can be replaced by za, the standard 

normal variable, and because one is usually interested in 

the prediction of a single future observation, n0=l. Thus, 

the only quantity that is not known is a£, which, because 

of the nonlinear form of equations 5 and 6, can only be 

estimated by Monte Carlo simulation. Such simulation is 

beyond the scope of this study. For estimates near the 

centroid of the data (that is, near the middle of the scattergrams in fig. 1), c$ is very small and can be neglected. 

In this case, equation 12 becomes

(13)

For estimates involving an extrapolation of the model, 

a0 cannot be neglected, and equation 13 will substantially underestimate the value of ya for ct>0.5.

SUMMARY

In this study, empirical attenuation relationships for 

horizontal components of peak acceleration and peak 

velocity have been developed from as many as 134 nearsource strong-motion recordings. These records were
PREDICTING STRONG GROUND MOTION IN UTAH L27

200

100

O

O 

o ui

CO

V)

oc

I

F

o

UJ

40

20

10

I I 4

1 I I I I | 1 I r T T rr

i i i i

10 20 40 100 200 

DISTANCE TO SEISMOGENIC RUPTURE, IN KILOMETERS

400 1000

FIGURE 12. Estimates of peak horizontal velocity fornorth-central Utah. The shaded zone represents the range in median estimates of velocity due to variability

in fault type, site amplification, and anelastic attenuation (see text).

obtained within 30 to 50 km of 21 earthquakes worldwide 

whose magnitudes range from 5.0 to 7.7. Because all 

recordings come from sites located on relatively low 

velocity, unconsolidated deposits, limitations associated 

with the small number of available rock recordings are 

avoided. These attenuation relationships provide models 

for predicting peak acceleration and peak velocity from a 

suite of parameters characterizing source, propagation, 

and site effects, the most important being earthquake 

magnitude and distance to the zone of seismogenic rupture. Empirical analyses indicated that factors such as 

fault type, building size, building embedment, and shallow 

soil influence the prediction of peak acceleration, 

whereas factors such as fault type, building size, and 

depth of sediments influence the prediction of peak 

velocity.

Two types of attenuation relationships were developed in the study: an unconstrained relationship, whose 

coefficients were all derived directly from regression 

analyses, and a constrained relationship, whose anelastic 

attenuation term was constrained to provide a relationship that could be reliably extrapolated beyond the nearsource limit of the strong-motion data. Both relationships 

provide virtually identical estimates of ground motion 

within about 30 to 50 km of the source, but only the constrained relationship provides reliable estimates of 

ground motion to distances of about 100 km. A significant 

advantage of the constrained relationship is its ability to 

provide estimates of ground motion in regions whose 

attenuation characteristics are different from California's.

A striking feature of all four attenuation relationships 

developed in this study is the empirical constraint that
L28 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

1000

o

UJ

tr

6 d

z>

LLJ 

C/3

a.

400 -

200 -

100 

0.01 0.02 0.04 0.1 0.2 0.4 1 

UNDAMPED NATURAL PERIOD, IN SECONDS

FIGURE 13. Estimate of the 5-percent-damped pseudo-relative velocity spectrum for a site in north-central Utah located approximately 5 km from the zone of 

seismogenic rupture of a MS =7.5 earthquake. The shaded zone represents the range in the median estimate of pseudo-relative velocity due to variability in fault 

type and site amplification (see text). RD, relative displacement; PSAA, pseudo-absolute acceleration.

peak ground-motion amplitudes be independent of magnitude at the earthquake source. This characteristic, 

referred to as ground-motion saturation, has been proposed by many geophysicists as a fundamental property 

of the earthquake source. Its empirical basis, however, 

has been the subject of great controversy in the past 

(Campbell, 1985a).

The constrained attenuation relationships developed 

in this study were used to provide median estimates of 

strong ground motion for north-central Utah. A range of 

estimates was provided to reflect uncertainties in the 

state of knowledge and regional variability associated 

with the potential effects of fault type, anelastic attenuation, and site effects. A range of values for anelastic atten-
PREDICTING STRONG GROUND MOTION IN UTAH L29

uation believed to be appropriate for the Basin and Range 

was taken from the literature. Regional studies of site 

effects were used to modify the estimates for local site 

conditions. The range of estimates for a given value of 

magnitude and distance was found to be quite large, an 

indication that regional studies of ground motion should 

be undertaken to better define the effects of fault type 

and anelastic attenuation.

A method developed by Newmark and Hall (1982) for 

estimating response spectra from estimates of peak 

ground-motion values was used to provide a median 5- 

percent-damped pseudo-relative velocity spectrum for a 

Ms=7.5 earthquake hypothesized to occur on the Wasatch 

fault approximately 5 km from a site located within one of 

the range front valleys. This scenario leads to what the 

author considers to be the largest ground motion that can 

be expected to occur in the cities of Ogden, Provo, and 

Salt Lake City.

REFERENCES CITED

Anderson, J.G., Bodin, P., Brune, J.N., Prince, J., Singh. S.K., 

Quaas, R., and Onate, M., 1986, Strong ground motion from 

the Michoacan, Mexico, earthquake: Science, v. 233, p. 

1043-1049.

Anderson, J.G., and Luco, J.E., 1983, Parametric study of nearfield ground motions for oblique-slip and dip-slip dislocation models: Bulletin of the Seismological Society of America, v. 73, p. 45-57.

Bakun, W.H., 1984, Seismic moments, local magnitudes, and 

coda-duration magnitudes for earthquakes in central California: Bulletin of the Seismological Society of America, v. 

74, p. 439-458.

Bard, P.-Y., and Bouchon, M., 1985, The two-dimensional resonance of sediment-filled valleys: Bulletin of the Seismological Society of America, v. 75, p. 519-541.

Boatwright, J.A, and Boore, D.M., 1982, Analysis of the ground 

accelerations radiated by the 1980 Livermore Valley earthquakes for directivity and dynamic source characteristics: 

Bulletin of the Seismological Society of America, v. 72, p. 

1843-1865.

Boore, D.M., 1980, On the attenuation of peak velocity: World 

Conference on Earthquake Engineering, 7th, Istanbul 1980, 

Proceedings, v. II, p. 577-584.

Boore, D.M., and Boatwright, J.A, 1984, Average body-wave 

radiation coefficients: Bulletin of the Seismological Society 

of America, v. 74, p. 1615-1621.

Brillinger, D.R., and Preisler. H.K., 1984, An exploratory analysis 

of the Joyner-Boore attenuation data: Bulletin of the Seismological Society of America, v. 74, p. 1441-1450.

Campbell, K.W., 1981a, Near-source attenuation of peak horizontal acceleration: Bulletin of the Seismological Society of 

America, v. 71, p. 2039-2070.

 1981b, A ground motion model for the Central United 

States based on nearsource acceleration data: Conference 

on Earthquakes and Earthquake Engineering: The Eastern 

U.S., Knoxville, Term., 1981, Proceedings, v. 1, p. 213-232.

 1982a, Near-source scaling characteristics of peak horizontal acceleration for moderate-to-large earthquakes, in 

Boatwright, J., ed., Proceedings of Workshop XVI; The 

dynamic characteristics of faulting inferred from recordings 

of strong ground motion: U.S. Geological Survey Open-File 

Report 82-591, p. 120-184.

 1982b, A preliminary methodology for the regional zonation of peak acceleration: Third International Conference 

on Earthquake Microzonation, 3d, Seattle, Wash., 1982, Proceedings, v. I, p. 365-376.

 1983, The effects of site characteristics of near-source 

recordings of strong ground motion, in Hays, WW, ed., A 

workshop on "Site-specific effects of soil and rock on 

ground motion and the implications for earthquake-resistant design": U.S. Geological Survey Open-File Report 83- 

845, p. 280-309.

 1984a, Near-source attenuation of strong ground motion 

for moderate to large earthquakes; an update and suggested 

application to the Wasatch fault zone of north-central Utah, 

in Hays, WW, and Gori, PL., 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. 483-499.

 1984b, Observed structural modification of recorded 

strong ground motion, in Critical aspects of earthquake 

ground motion potential: Palo Alto, Calif., Report ATC-10- 

1, Applied Technology Council, p. 43-52.

 1985a, Strong motion attenuation relations: A ten-year 

perspective: Earthquake Spectra, v. 1, p. 759-804.

 1985b, Near-source estimation of strong ground motion 

for the eastern United States: Washington, D.C., Secondquarter progress report to the U.S. Nuclear Regulatory 

Commission, 5 p.

 1986, Empirical prediction of free-field ground motion 

using statistical regression models, in Workshop on soilstructure interaction, Bethesda, Md., 1986: Upton, N.Y., 

Report NUREG/CP-0054, Brookhaven National Laboratory, 

p. 12-41. 

-1989, The dependence of peak horizontal acceleration on

magnitude, distance, and site effects for small-magnitude 

earthquakes in California and Eastern North America: Bulletin of the Seismological Society of America, v. 79, p. 1311- 

1346.

Chavez, D.E., and Priestley, K.F., 1985, M L observations in the 

Great Basin and M0 versus ML relationships for the 1980 

Mammoth Lakes, California, earthquake sequence: Bulletin 

of the Seismological Society of America, v. 75, p. 1583-1598.

 1986, Attenuation of regional crustal phases in the Great 

Basin [abs.]: Earthquake Notes, v. 57, no. 1, p. 9.

Chiaruttini, C., and Siro, L., 1981, The correlation of peak ground 

horizontal acceleration with magnitude, distance, and seismic intensity for Friuli and Ancona, Italy, and the Alpine 

Belt: Bulletin of the Seismological Society of America, v. 71, 

p. 1993-2009.

Cranswick. E., Wetmiller, R., and Boatwright. J., 1985. High-frequency observations and source parameters of 

microearthquakes recorded at hard-rock sites: Bulletin of 

the Seismological Society of America, v. 75, p. 1535-1567.
L30 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

Draper, N.R., and Smith, H., 1981, Applied regression analysis 

(2d ed.): New York, John Wiley.

Esteva, L., 1970, Seismic risk and seismic design decisions, in 

Hanson, R.J., ed., Seismic design for nuclear power plants: 

Cambridge, Mass., M.I.T. Press, p. 142-182.

Faccioli, E., 1981. The use of RMS acceleration as a measure of 

soil amplification in strong earthquakes, in Erguray, O., and 

Erdik, M., eds., State-of-the-art in earthquake engineering, 1981: Ankara, Turkey, Turkish National Committee on 

Earthquake Engineering, p. 98-105.

Fenneman, N.M., 1946, Physical divisions of the United States: 

U.S. Geological Survey Map, scale 1:7,000,000.

Gallant, A.R., 1975, Nonlinear regression: The American Statistician, v. 29, p. 73-81.

Hadley, D.M., Helmberger, D.V., and Orcutt, J.A., 1982, Peak 

acceleration scaling studies: Bulletin of the Seismological 

Society of America, v. 72, p. 959-979.

Hanks, T.C., and Boore. D.M., 1984, Moment-magnitude relations 

in theory and practice: Journal of Geophysical Research, v. 

89, no. B7, p. 6229-6235.

Hanks, T.C., and Kanamori. H., 1979. A moment magnitude scale: 

Journal of Geophysical Research, v. 84, p. 2348-2350.

Hays, W.W., and King, K.W., 1982, Zoning of the earthquake 

ground-shaking hazard along the Wasatch fault zone, Utah: 

International Earthquake Microzonation Conference, 3d, 

Seattle, Wash., 1982, Proceedings, v. Ill, p. 1307-1318.

 1984, The ground-shaking hazard along the Wasatch fault 

zone, Utah, in Hays, W.W, and Gori, PL., 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. 133-147.

Hays, W.W., Algermissen, S.T., Miller, R.D., and King. K.W., 1978, 

Preliminary ground response maps for the Salt Lake City 

area: International Earthquake Microzonation Conference, 

2d, San Francisco, Calif., 1978, Proceedings, v. 2. p. 497-508.

Johnson, R.A., and Wichern, D.W, 1982, Applied multivariate statistical analysis: Englewood Cliffs, N.J., Prentice-Hall, 594 p.

Joyner, W.B., and Boore, D.M., 1981, Peak horizontal acceleration and velocity from strong-motion records including 

records from the 1979- Imperial Valley, California, earthquake: Bulletin of the Seismological Society of America, v. 

71, p. 2011-2038.

Joyner, W.B., and Fumal, T.E., 1984, Use of measured shear-wave 

velocity for predicting geologic site effects on strong 

ground motion: World Conference on Earthquake Engineering, 8th, San Francisco, Calif., 1984, Proceedings, v. II, p. 

777-784.

 1985, Predictive mapping of earthquake ground motion, 

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

Angeles region An earth-science perspective: U.S. Geological Survey Professional Paper 1360, p. 203-220.

Kanai, K, 1983, Engineering seismology: Tokyo, University of 

Tokyo Press, 251 p.

King, K.W., and Hays, W.W, 1977, Comparison of seismic attenuation in northern Utah with attenuation in four other 

regions of the western United States: Bulletin of the Seismological Society of America, v. 67, p. 781-792.

Mabey, D.R., 1992, Subsurface geology along the Wasatch Front, 

in Gori, PL., and Hays, W.W, eds., Assessment of regional 

earthquake hazards and risk along the Wasatch Front, Utah: 

U.S. Geological Survey Professional Paper 1500-C, p. ClC16.

McGarr, A., 1984, Scaling of ground motion parameters, state of 

stress, and focal depth: Journal of Geophysical Research, v. 

89, no. B8, p. 6969-6979.

More, J.J., Garbow, B.S., and Hillstrom, K.E., 1980, User guide 

for MINPACK-1: Argonne, 111., Report ANL-80-74, Argonne 

National Laboratory, 260 p.

MueUer, C.S., Boore, D.M., and PorceUa, R.L., 1982, Detailed 

study of site amplification at El Centro strong-motion array 

station no. 6: International Earthquake Microzonation Conference, 3d, Seattle, Wash., 1982, Proceedings, v. I, p. 413- 

424.

Newmark, N.M., and Hall, W.J., 1982, Earthquake spectra and 

design: Berkeley, Calif., Earthquake Engineering Research 

Institute Monograph, 103 p.

Nuttli, O.W, 1979, The relation of sustained maximum ground 

acceleration and velocity to earthquake intensity and magnitude, in State-of-the art for assessing earthquake hazards 

in the United States: U.S. Army Engineer Waterways Experiment Station Miscellaneous Paper S-73-1, Report 16.

Prakash, S., 1981, Soil dynamics: New York, McGraw-Hill, 426 p.

Prince, J., 1984, Influence of the number of response maxima on 

the observed seismic behavior of structures, in Critical 

aspects of earthquake ground motion and building damage 

potential: Palo Alto, Calif., Report ATC-10-1, Applied Technology Council, p. 57-66.

Richter, C.F., 1958, Elementary seismology: San Francisco, 

Calif., WH. Freeman, 768 p.

Rogers, A.M., Tinsley, J.C., and Borcherdt, R.D., 1984, Geographic variation in ground shaking as a function of 

changes in near-surface properties and geologic structure 

near Los Angeles, California: World Conference on Earthquake Engineering, 8th, San Francisco, Calif., 1984, Proceedings, v. II, p. 737-744.

 1985, Predicting relative ground response, in Ziony, J.I., 

ed., Evaluating earthquake hazards in the Los Angeles 

region An earth-science perspective: U.S. Geological Survey Professional Paper 1360, p. 221-247.

Rogers, A.M., Harmsen, S.C., Herrmann, R.B., and Meremonte, 

M.E., 1987, A study of ground motion attenuation in the 

southern Great Basin, Nevada-California, using several 

techniques for estimates of Qs, log A0, and coda Q: Journal 

of Geophysical Research, v. 92, no. B5, p. 3527-3540.

Seed, H.B., and Idriss, I.M., 1982, Ground motions and soil liquefaction during earthquakes: Berkeley. Calif., Earthquake 

Engineering Research Institute Monograph, 134 p.

Seed, H.B., Idriss, I.M., and Keifer, F.W, 1969, Characteristics of 

rock motions during earthquakes: Proceedings of the American Society of Civil Engineers, Journal of the Soil Mechanics and Foundations Division, v. SM95, p. 1199-1218.

Seed, H.B., Ugas, C., and Lysmer, J., 1976, Site-dependent spectra for earthquake-resistant design: Bulletin of the Seismological Society of America, v. 66, p. 221-243.
PREDICTING STRONG GROUND MOTION IN UTAH L31

Shakal, A.F., and Bernreuter, D.L., 1981, Empirical analysis of 

near-source ground motion: Washington, D.C., Report 

NUREG/CR-2095, U.S. Nuclear Regulatory Commission, 

65 p.

Singh, J.P., 1985, Earthquake ground motions: Implications for 

designing structures and reconciling structural damage: 

Earthquake Spectra, v. 1, p. 239-270.

Singh, S., and Herrmann, R.B., 1983, Regionalization of crustal Q 

in the continental United States: Journal of Geophysical 

Research, v. 88, no. Bl, p. 527-538.

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 Bulletin, v. 70, p. 1431- 

1478.

Taylor, S.R., and Patton, H.J., 1986, Shear-wave velocity structure from regionalized surface-wave dispersion in the Basin 

and Range: Geophysical Research Letters, v. 13, p, 30-33.

Thatcher, W., 1972, Regional variations of seismic source parameters in the northern Baja, California, area: Journal of Geophysical Research, v. 77, no. B8, p. 1549-1565.

Thatcher, W., and Hanks, T.C., 1973, Source parameters of Southern California earthquakes: Journal of Geophysical 

Research, v. 78, no. B35, p. 8547-8576.

Trifunac, M.D., and Lee, V.W., 1978, Dependence of the Fourier 

amplitude spectra of strong motion acceleration on the 

depth of sedimentary deposits: Los Angeles, Calif., Report 

CE78-14, University of Southern California Department of 

Civil Engineering, 39 p.

 1979, Dependence of pseudo-relative velocity spectra of 

strong motion acceleration on the depth of sedimentary 

deposits: Los Angeles, Calif., Report CE79-02, University of 

Southern California Department of Civil Engineering.

Tucker, B.E., King, J.L., Hatzfeld, D., and Nersesov, I.L., 1984, 

Observations of hard rock site effects: Bulletin of the Seismological Society of America, v. 74, p. 121-136.

Tukey, J.W., 1977, Exploratory data analysis: Reading, Mass., 

Addison-Wesley.

Zoback, M.L., and Zoback, M.D., 1981, State of stress in the conterminous United States: Journal of Geophysical Research, 

v. 86, no. B6, p. 4981-4998.

Probabilistic Analysis of Earthquake Ground-Shaking 

Hazards Along the Wasatch Front, Utah

.R. YOUNGS, EH. SWAN, and M.S. POWER, GeoMatrix Consultants, D.P. 

SCHWARTZ, U.S. Geological Survey, andR.K. GREEN, Woodward- Clyde Consultants

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-M

CONTENTS

Abstract....................................................................................................... Ml

Introduction................................................................................................. 1

Acknowledgments............................................................................ 3

Approach..................................................................................................... 3

Regional Seismicity.................................................................................... 5

Earthquake Catalog.......................................................................... 5

Identification of Dependent Events.................................................. 8

Catalog Completeness...................................................................... 9

Seismic Source Characterization................................................................ 9

Area Source Model for Background Seismicity............................... 12

Quaternary Faulting.......................................................................... 12

Earthquake Magnitude Distribution and Recurrence

Assessment............................................................... 12

Unsegmented Versus Segmented Fault-Rupture

Model....................................................................... 16

Fault Dip and Depth................................................................ 17

Maximum Earthquake Magnitude Assessment

Techniques............................................................... 17

Wasatch Fault Zone................................................................. 18

Segmentation................................................................. 18

Maximum Earthquake Magnitude................................ 23

Earthquake Recurrence Rate......................................... 23

Oquirrh Mountain Fault Zone................................................. 29

Segmentation................................................................. 29

Activity......................................................................... 30

Total Length.................................................................. 30

Maximum Earthquake Magnitude................................ 30

Earthquake Recurrence Rate......................................... 30

East Cache Fault..................................................................... 30

Segmentation................................................................. 31

Maximum Earthquake Magnitude................................ 31

Earthquake Recurrence Rate......................................... 31

West Valley Fault Zone........................................................... 31

Activity......................................................................... 31

Seismic Source Characterization Continued

Maximum Earthquake Magnitude................................ M32

Earthquake Recurrence Rate........................................ 32

Hansel Valley Fault Zone..................................................... 32

Segmentation................................................................ 32

Activity......................................................................... 32

Maximum Earthquake Magnitude................................ 32

Earthquake Recurrence Rate........................................ 32

East Bear Lake Fault............................................................... 33

Segmentation................................................................ 33

Maximum Earthquake Magnitude................................ 33

Earthquake Recurrence Rate........................................ 33

Comparison of Predicted and Observed Earthquake

Frequency........................................................................... 34

Attenuation Relationships.......................................................................... 34

Earthquake Source Characteristics................................................... 34

Source-to-Site Attenuation Characteristics...................................... 34

Local Site Effects............................................................................. 35

Selected Attenuation Relationships for Peak Ground

Acceleration........................................................................ 36

Selected Attenuation Relationships for Response Spectral

Values................................................................................. 39

Analysis Results and Sensitivity ................................................................ 40

Computed Hazard............................................................................ 40

Contributions to Uncertainty............................................................ 44

Additional Sensitivity Analyses....................................................... 48

Peak Acceleration Hazard Maps...................................................... 55

Response Spectra............................................................................. 58

Summary..................................................................................................... 62

Postscript.................................................................................................... 62

References Cited......................................................................................... 63

Appendix: Comparison of Strong-Motion Data from the Study

Region with Selected Attenuation Relationships........................ 69

ILLUSTRATIONS

Page 

FIGURE 1. Map showing location of the study area for ground-shaking hazard mapping and the Quaternary faults modeled

in the seismic-hazard analysis.............................................................................................................................................................................. M2

2. Diagram of hazard model logic tree used for evaluating seismic hazards of soil sites in this study.................................................................... 4

3,4. Maps showing:

3. Catalog of independent events of magnitude ML>2 occurring prior to July 1, 1962.................................................................................... 6

4. Instrumental catalog of independent events of magnitude ML>2 occurring between July 1, 1962, and April 1,1986................................ 7

5-11. Graphs showing:

5. Criteria used for time and distance windows in identifying independent events.......................................................................................... 8

6. Estimates of catalog completeness and regional recurrence for north-central Utah..................................................................................... 10

7. Cross sections through six segments of the Wasatch fault showing instrumental Seismicity recorded between

October 1974 and April 1986....................................................................................................................................................................... 11

8. Recurrence estimates for background seismicity source delineated in figures 3 and 4................................................................................ 13

III
IV CONTENTS

9. Comparison of recurrence models used in the analysis for equal recurrence rate of maximum events and equal rate Paee 

of seismic moment release............................................................................................................................................................................ M14

10. Wasatch fault-specific b values..................................................................................................................................................................... 15

11. Relationship between seismic moment and earthquake magnitude.............................................................................................................. 16

12. Diagram showing weighting factors for maximum magnitude assessment techniques for two cases................................................................. 18

13. Map showing segmentation models for fault-specific sources............................................................................................................................. 24

14. Diagram showing maximum magnitude logic tree for the Wasatch fault zone.................................................................................................... 26

15-23. Graphs showing:

15. Comparison of predicted recurrence rates for seismic sources and observed frequency of earthquakes...................................................... 29

16. Peak horizontal acceleration attenuation relationships used in seismic hazard analysis.............................................................................. 38

17. Peak horizontal acceleration relationships for a magnitude 6.5 event modified for a slower rate of attenuation......................................... 39

18. Typical response spectral acceleration attenuation relationships for soil-site conditions used in seismic-hazard

analysis............................................................................................................................................................................................^ 40

19. Computed distributions for frequency of exceeding various levels of peak ground acceleration on soil at three

locations ..............................................................................................................................................................................^ 41

20. Peak ground acceleration hazard curves on soil for three locations.............................................................................................................. 42

21. Contributions of various sources to the computed mean hazard for six locations........................................................................................ 43

22. Variation in the computed mean hazard along the length of the Wasatch fault zone from Nephi to Collinston .......................................... 44

23. Contributions of different magnitude events to total frequency of exceeding various levels of peak ground

acceleration at three locations....................................................................................................................................................................... 45

24-32. Graphs showing contribution to uncertainty in exceedance frequency from uncertainty in:

24. Selecting the appropriate soil-site attenuation relationship.......................................................................................................................... 46

25. Recurrence model.......................................................................................................................................................................................... 47

26. Source segmentation ..................................................................................................................................................................................... 48

27. Segmentation model at six locations along the Wasatch fault...................................................................................................................... 49

28. Source activity. .........................................................................................................................................................................^ 50

29. Fault dip......................................................................................................................................................................................................... 51

30. Use of either recurrence data from the segment only or an average value from the central segments of the fault in

modeling earthquake recurrence for the fault segments................................................................................................................................ 52

31. Maximum magnitude ................................................................................................................................................................................... 52

32. Recurrence rate.............................................................................................................................................................................................. 53

33. Graph showing comparison of mean hazard curves for soil and rock sites at three locations............................................................................. 53

34-36. Graphs showing effect on computed hazard of:

34. Postulated lower Utah attenuation rate in Utah in comparison with California rates at three locations....................................................... 54

35. Choice of lower bound magnitude cutoff at three locations.......................................................................................................................... 55

36. Using realtime estimates of earthquake occurrence probabilities at two locations along the Wasatch fault zone........................................ 56

37. Maps showing contours of peak ground acceleration on soil sites having 10-percent probability of being exceeded in

10, 50, and 250 years............................................................................................................................................................................................. 57

38-40. Graphs showing equal hazard response spectra at three locations for:

38. Soil and rock sites computed by using two attenuation relationships........................................................................................................... 58

39. Soil sites ........................................................................................................................................................................................................ 59

40. Rock sites ...................................................................................................................................................................................................... 60

41-43. Graphs showing:

41. Proposed spectral shapes for developing equal-hazard response spectra...................................................................................................... 61

42. Example of the development of equal-hazard response spectra for a 10-percent probability of exceedance in 50 years

for soil and rock sites near Magna................................................................................................................................................................ 62

43. Preliminary estimates of ranges in response spectral shapes for lake deposits............................................................................................. 63

A1-A4. Graphs comparing:

Al. Peak ground accelerations recorded at Logan during the 1962 Cache Valley, Utah, earthquake and median attenuation

relationships.................................................................................................................................................................................................. 70

A2. Response spectra of ground motions recorded during the 1962 Cache Valley, Utah, earthquake and median and

84th-percentile spectra from attenuation relationships................................................................................................................................. 71

A3. Peak ground accelerations recorded during the 1983 Borah Peak, Idaho, earthquake and median attenuation

relationships.................................................................................................................................................................................................. 72

A4. Peak ground accelerations recorded during the 1983 Borah Peak, Idaho, aftershocks and median attenuation

relationships.................................................................................................................................................................................................. 73
CONTENTS V

TABLES

TABLE 1. Fault characterization parameters and associated weights for the seismic-hazard model.............................................................................................. M19

2. Parameters and relationships used to estimate maximum magnitude and associated weights ....................................................................................... 22

3. Geologic parameters used to characterize earthquake recurrence rate for the Wasatch fault zone................................................................................. 27

4. Values of coefficients for the attenuation relationships of Sadigh and others (1986) for horizontal peak ground acceleration and

pseudo-absolute spectral accelerations........................................................................................................................................................................... 37

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK 

ALONG THE WASATCH FRONT, UTAH

PROBABILISTIC ANALYSIS 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

ABSTRACT

This paper presents a probabilistic assessment of the ground-shaking hazard along the Wasatch Front urban corridor, utilizing the extensive geological and seismological data base that has been developed for 

the region. Using a logic tree methodology, the approach formally incorporates uncertainty in earthquake source geometry and fault segmentation, maximum earthquake magnitude, earthquake recurrence rate, 

ground-motion attenuation, and other parameters and model components. The analysis results are presented on maps showing contours of 

peak ground acceleration on soil-site conditions for probabilities of 

exceedance of 10 percent in 10 years, 10 percent in 50 years, and 10 percent in 250 years. The major contributions to uncertainty in the hazard 

are uncertainties in selecting the appropriate models for earthquake 

recurrence on the faults and ground-motion attenuation. Repeating the 

analyses for rock-site attenuation relationships indicates that rock-site 

peak accelerations can be obtained by multiplying the soil-site accelerations by a factor of 1.1 for a probability of exceedance of 10 percent in 

10 years and by a factor of 1.2 for probabilities of exceedance of 10 percent in 50 years and 10 percent in 250 years.

Response spectral shapes for soil- and rock-site conditions are 

presented for the probability of exceedance levels used in mapping peak 

ground acceleration. Absolute response spectra can be obtained by multiplying these spectral shapes by the corresponding values of peak 

ground acceleration. A preliminary estimate is presented for the 

additional spectral amplification that might occur in areas of soft lake 

deposits.

INTRODUCTION

The urban corridor along the Wasatch Front, which 

includes more than 85 percent of Utah's population, is 

considered one of the highest risk areas within the conterminous United States with respect to earthquake hazards. 

The seismic hazards of this region are related primarily to 

the presence of the Wasatch fault zone, which extends for 

more than 370 km along the western front of the Wasatch 

Range. Paleoseismicity studies (Swan and others, 1980; 

Hanson and others, 198 la, b; Schwartz and others, 1984;

Schwartz and Coppersmith, 1984) have demonstrated that 

the Wasatch fault zone has repeatedly produced largemagnitude earthquakes throughout the Holocene and that 

it is likely to be the source of future damaging earthquakes. In addition, the region may be affected by earthquakes originating on other faults exhibiting Quaternary 

movement, including the Oquirrh Mountains fault zone, 

the East Cache fault, the West Valley fault, the Hansel Valley fault, and the East Bear Lake fault. As figure 1 indicates, Logan, Brigham City, Ogden, Salt Lake City, and 

Provo, the largest cities in Utah, all lie along active faults 

capable of producing damaging earthquakes.

During the past two decades, geological studies have 

been conducted for the specific purpose of identifying 

and evaluating the potential earthquake hazards along the 

Wasatch Front (Utah Geological Association, 1972; Evernden, 1980; Hays and Gori, 1984). Extensive telemetered 

seismic arrays have been operating to assess the earthquake hazard along the Intermountain Seismic Belt since 

1974. Prior to that, instrumental monitoring of the 

Wasatch Front dates back only to 1962 (Arabasz and 

others, 1980).

This study makes a quantitative assessment of the 

ground-shaking hazard along the Wasatch Front urban 

corridor (fig. 1), utilizing the extensive geological and 

seismological data base that has been assembled for the 

region. The ground-shaking hazard is related to (1) the 

location and geometry of the earthquake sources relative 

to urban areas, (2) the recurrence of earthquakes of various magnitudes on these sources, and (3) the attenuation 

of ground motions from the sources to various locations 

within the study area. A probabilistic approach is used to 

formally incorporate the uncertainty in modeling the 

regional seismicity and attenuation characteristics into 

the hazard assessment. In this approach, distributions of

Ml
M2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

113° 112° 111°

EXPLANATION

Quaternary fault

Salt Lake City

> 

Little Cottonwood Canyon

40°

25 50 75 MILES

0

I 

25

r

50 75

I 

100 KILOMETERS

FIGURE 1. Location of the study area for ground-shaking hazard mapping (shaded regions) and the Quaternary faults

modeled in the seismic-hazard analysis.
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M3

values for source geometry, maximum earthquake magnitude, earthquake recurrence rate, characteristics of 

ground-motion attenuation, and other parameters that are 

not known with certainty are selected to reflect the best 

understanding of the available data. The seismic-hazard 

analysis integrates these data on source characteristics 

and ground motion attenuation to evaluate the probability 

of exceeding specified levels of ground motion within 

designated time intervals. The ground-motion parameters 

addressed include both horizontal peak acceleration and 

horizontal spectral acceleration on soil- and rock-site conditions. A principal product of the study consists of maps 

showing the spatial variation of peak horizontal acceleration levels for three selected levels of probability of 

exceedance. The uncertainty in the estimated hazard is 

analyzed and discussed with respect to the uncertainty in 

the various input parameters and sensitivity to models 

incorporated in the analysis. These analyses provide a 

means of evaluating the impact of new information on the 

results presented in this paper.

ACKNOWLEDGMENTS

The work presented in this paper was carried out 

under U.S. Geological Survey (USGS) contracts 14-08- 

0001-2194 and 14-08-0001-G1189. The authors wish to 

thank Walter Arabasz and James Pechmann of the University of Utah, Bernice Bender and Kathie Fraser of the 

USGS, Ken Campbell of Dames and Moore (Evergreen, 

Colo.), and Peter Knuepfer of the State University of New 

York for their helpful review comments. We also appreciate the help of Ethan Brown of the University of Utah in 

providing the earthquake catalog used in this study and 

Tony Crone and Michael Machette of the USGS for providing unpublished data for the East Bear Lake fault zone.

APPROACH

In probabilistic terms, seismic hazard is defined as 

the likelihood that various levels of ground motion will be 

exceeded at a site during a specified time period. It is 

commonly assumed -that the occurrence of individual 

main shocks can be represented as a Poisson process. 

Following the approach developed by Cornell (1968), the 

probability that, at a given site, a ground motion parameter Z will exceed a specified level z during a specified 

time period t is given by the expression

P(Z>z (1)

where v(z~) is the average frequency during time period t 

at which the level of ground-motion parameter Z exceeds 

z at the site resulting from earthquakes on all sources in 

the region. The inequality at the right of equation 1 is valid 

regardless of the appropriate probability model for earth-

)= V a (m0 )/" f

° =

quake occurrence, and v(z~)»t provides an accurate and 

slightly conservative estimate of the hazard for probabilities of 0.1 or less, provided that v(z~) is the appropriate 

value for the time period of interest.

The frequency of exceedance, v(z), is a function of 

the uncertainty in the time, size and location of future 

earthquakes and uncertainty in the level of ground 

motions that they may produce at the site. It is computed 

by the expression

f(m)*f(r)«P(Z>z|m, r) dr dm (2)

where a^m) is the frequency of earthquakes on source 

n above a minimum magnitude of engineering significance, m°; f(m) is the probability density function for 

event size between m° and a maximum event size for the 

source, m"; f(r) is the probability density function for distance to the earthquake rupture; and P(Z>z m, r) is the 

probability that, given a magnitude m earthquake at a distance r from the site, the ground motion exceeds level z.

The probability functions contained in equations 1 

and 2 represent the uncertainties inherent in the natural 

phenomena of earthquake generation and seismic-wave 

propagation, hi the north-central Utah region (as is usually the case in any region), selecting the appropriate 

models and model parameters required to apply equation 

2 involves considerable uncertainty because of limited 

data and (or) alternative interpretations of the available 

data The approach used in this study explicitly incorporates these additional uncertainties into an analysis to 

assess their impact on the estimate of the expected level 

of seismic hazard as well as the uncertainty in that estimate.

The uncertainty in modeling natural phenomena is 

incorporated into the hazard analysis through the use of 

logic trees. The logic tree formulation for seismic-hazard 

analysis (Power and others, 1981; Kulkarni and others, 

1984; Youngs and others, 1985; Coppersmith and Youngs, 

1986; Electric Power Research Institute, 1987) involves 

specifying discrete alternatives for states of nature or 

parameter values and specifying the relative likelihood 

that each discrete alternative is the correct value or state 

of the input parameter. The relative likelihoods of the different parameter values are typically based on subjective 

judgment because the available data are too limited to 

allow an objective statistical analysis.

Figure 2 displays the logic tree representing the seismic-hazard model developed for this study. The logic tree 

is laid out to provide a logical progression from general 

aspects or hypotheses regarding the characteristics of 

seismicity and seismic-wave propagation in the region to 

specific input parameters for individual faults and fault 

segments. The rationale for developing the various levels
Attenuation 

Relationship

Fault 

Recurrence 

Model

Sources Segmentation

Segmentation 

Model

Segments

State of 

Activity

Total 

Length

Dip

Maximum 

Magnitude

Oquirrh 

Mountains

Unsegmented 

(0~2J

Unnamed

Collinston

Ogden

(0.5)

Campbell, 

this volume

(0.333)

Sadigh 

and others (1986)

(0.334)

Exponential

' (0.3)

/ Model A

Wasatch / / (0.4)

East Cache \ /

y Segmented /

West Valley (0.8) T

Hansel \ 

Valley \

\ Model B

Bear Lake (0.6)

Salt Lake City Active 37 km 7 / (0.15)

(1.0) * (1.0) \ / 

Provo \ 45 / 7V4

(0.5) ^ (0.7) 

\

1 (0.15) 

6

/ (6.3)

Background, N/A _ N/A Active N/A N/A / 6'/4

/ 

[ 

\ 

(1.0) (0.5)

L Characteristic 

(0.7)

(0.2)

Joynerand

Boore (1982)

(0.333)

FIGURE 2. Hazard model logic tree used for evaluating seismic hazards at soil sites in this study. Parameter values are shown above the branches, and assessed pro

below the branches. Not all branches are shown for all sources.
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M5

of the logic tree is discussed below. The bases for selecting the parameter values and assigning relative weights 

are presented in the following sections of this paper.

The first node of the logic tree represents the uncertainty in selecting the appropriate strong-ground-motion 

attenuation relationship. Attenuation was placed first in 

the tree because it is felt that a single relationship (whichever relationship may be "correct") is applicable to all 

earthquake sources in the region. Alternative, recently 

developed attenuation relationships are used in the analysis. As figure 2 indicates, three attenuation relationships 

developed for soil-site conditions were used. Four attenuation relationships were used to evaluate peak accelerations for rock-site conditions. Response spectral values 

were evaluated by using two published relationships for 

soil sites and two for rock sites.

The second node of the logic tree represents the 

uncertainty in selecting the appropriate form of the recurrence relationship for modeling the distribution of earthquake size for events occurring on faults. Although the 

exponential model has been commonly used in hazard 

analyses, it was derived on the basis of large regional seismicity catalogs (Gutenberg and Richter, 1954). Recent 

studies (for example, Schwartz and Coppersmith, 1984) 

suggest that the size distribution for individual faults and 

fault segments may be better represented by a "characteristic" model that implies more frequent large events than 

intermediate-sized events. In modeling the magnitudefrequency distribution for the regional background 

seismicity, only the exponential model is assumed to be 

applicable.

At this point, the logic tree is expanded into subtrees, 

one for each of the seismic sources included in the analysis, to model the additive hazard from multiple sources. 

Seven seismic sources were included in the analysis. Six 

sources consist of mapped fault zones having Quaternary 

displacement that have the potential of generating largemagnitude events (fig. 1). In addition, a regional background zone was included in the analysis to model possible (random) locations for small- to moderate-magnitude 

events, a reflection of the diffuse zone of seismic activity 

observed in the region (figs. 3, 4). The historical seismicity and its implications for modeling earthquake occurrence are discussed in following sections. To the right of 

this node of the logic tree, each source is considered to be 

acting independently, and the distribution in the total 

computed hazard is obtained by convolving the independent distributions obtained for each seismic source.

The first two nodes of the logic tree for each fault 

represent the uncertainty in whether a fault zone is segmented and, if segmented, in choosing the appropriate 

segmentation model. Segmentation affects both the 

assessment of maximum earthquake magnitude and the 

distribution of seismicity along the fault zone. If the fault

is segmented, the logic tree is again expanded into subtrees, one for each segment identified for a particular segmentation model. As was the case for the expansion at 

the seismic source level, each segment is considered to be 

acting independently, and the distribution in the computed hazard for the fault as a whole is obtained by convolving the independent distributions obtained for each 

segment.

The remaining nodes of the logic tree for faults 

address specific parameters used in the hazard computation: whether the fault or fault segment is currently active, 

affecting whether it contributes to the hazard; uncertainty 

in the fault geometry in terms of total fault length and 

fault plane dip, affecting the distribution of distance to 

fault rupture, the assessment of maximum magnitude, 

and earthquake recurrence rate based on seismic moment 

rate; uncertainty in maximum magnitude; and uncertainty 

in estimating earthquake recurrence. The development of 

the conditional maximum magnitude distribution incorporates additional variables that are not shown in the 

overall logic tree of figure 2. These variables are discussed further in "Seismic Source Characterization" 

(p. M17).

The logic tree branches shown in figure 2 are representative of the type of input parameters developed at 

each level for the seven seismic sources. As shown, many 

of the levels of the logic tree are not applicable to the 

background zone, and only uncertainty in maximum magnitude and recurrence rate (shown in fig. 2 as the return 

period for a magnitude 5 or greater event within the zone) 

is incorporated.

REGIONAL SEISMICITY

EARTHQUAKE CATALOG

A comprehensive earthquake catalog for the State of 

Utah has been developed by researchers at the University 

of Utah Seismograph Stations. Their published catalog 

(Richins, 1979) together with recent updates through 

March 1986 (E. Brown, personal commun., 1986) were utilized in this study. The catalog can be divided into two primary periods of reporting, pre-instrumental and 

instrumental. The pre-instrumental catalog covers the 

time period from 1850, when the first newspaper was published in Utah, to July 1962, when the first Statewide seismograph network was established. The assigned location 

for most events occurring before 1950 is the population 

center where the event was most strongly felt, and the 

location error is estimated to be 25 to 50 km (Richins, 

1979). Nearly all of the magnitudes reported in the catalog 

for events occurring before July 1962 were obtained from 

epicentral intensity by using the relationship ML=%/O+! 

developed by Gutenberg and Richter (1956). Earthquake
M6

42°

41°

40°

39°

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

113° 112° 111°

EAST HEAR LAKE FAULT

0

WYOMING

EXPLANATION

MAGNITUDE

O 5

O 4

O 3

O 2

0

T~

25

25 

_L

50 75 MILES 

J

50 75 100 KILOMETERS

FIGURE 3. Catalog of independent events of magnitude M^2 occurring prior to July 1, 1962. Magnitudes represent either instrumentally 

determined values or values obtained from epicentral intensity by using the relationship ML=2^/o+l- Dashed boundary defines limits of 

background seismic source.
40'

PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS

113° 112°

M7

EXPLANATION

MAGNITUDE

o

o

O *

O 4

O 3

O 2

39° -

3

1

0 25

25 

1

1

50

50 

1

1

75

75 MILE 

1

1

100 KILOMETERS

FIGURE 4. : Instrumental catalog of independent events of Mi>2 occurring etween July 1, 1962, and April 1, 1986. Dashed boundary define

limits of background seismic source.
M8

2000

1000

500

200

> 100

50

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

2000

I I I I I

EXPLANATION

 Gardner and Knopoff (1974)

 Uhrhammer (1966)

 Arabasz and Robinson (1976)

1000

500

£ 200 

ai

O 100 _j

- 50 

I

3 4 

MAGNITUDE

20

10

5

I I I I 

EXPLANATION

I

B

 Gardner and Knopoff (1974)

 Uhrhammer (1966)

- Wyss(1979)

3 4 

MAGNITUDE

FIGURE 5. Criteria used for time (A) and distance (B) windows in identifying independent events.

recurrence estimates for the study region were found to 

be relatively insensitive to the use of other intensity-magnitude conversions. Figure 3 displays the locations of 

events in the pre-instrumental catalog for the region long 

110.5° to 113.5°W., lat 39°to 42.5° N.

The instrumental catalog used extends from July 1, 

1962 to April 1, 1986. Events occurring in the period July 

1962 to October 1974 were located with the focal depth 

fixed at 7.0 km, and the size of the horizontal error ellipsoid is 10 km or greater for most of the events (Richins, 

1979). From 1974 to 1978, a Statewide telemetered network was deployed, resulting in improved seismic monitoring. The size of the horizontal error ellipsoid for events 

occurring after 1974 is generally less than 5 km (Richins, 

1979).

The focal depths are less well determined owing to 

the regional station spacing of the network (Arabasz, 

1984). Magnitudes are reported in terms of local magnitude, ML- Figure 4 displays the locations of earthquakes 

contained in the instrumental catalog for the region.

IDENTIFICATION OF DEPENDENT EVENTS

The mathematical formulation used in this (and 

most) hazard analysis is based on an assessment of the 

frequency of occurrence of independent earthquakes. It

has been shown that the inclusion of dependent events 

(for example, foreshocks and aftershocks) in the analysis 

results in only a minor increase in the computed hazard 

(Mertz and Cornell, 1973; Veneziano and Van Dyck, 1985). 

For this study, dependent events in the earthquake catalog were identified by the following procedure. The largest event in the catalog was identified, and a spatial and 

temporal window was specified on the basis of empirical 

criteria for the size of foreshock-aftershock sequences as 

a function of earthquake magnitude. All events falling 

within the window were flagged as dependent events. The 

next largest unflagged event was then selected, and the 

process was repeated down to the smallest-magnitude 

event.

Three sets of empirical criteria for foreshockaftershock sequence size were used: Gardner and Knopoff s (1974) criteria for southern California, Uhrhammer's 

(1986) criteria for California, and the time window developed by Arabasz and Robinson (1976) from California and 

New Zealand data combined with a multiple of earthquake rupture size evaluated from data presented by Wyss 

(1979). These three sets of criteria for window size are 

shown graphically in figure 5. The time window for aftershocks was used to identify foreshocks. The three criteria 

were judged to perform equally well in identifying after-
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M9

shocks because they all (1) identified as dependent events 

nearly all of the earthquakes labeled as aftershocks by 

Arabasz and McKee (1979) in the pre-instrumental 

catalog, (2) yielded instrumental catalogs of independent 

events that pass the Kolmogorov-Smirnov goodness-of-fit 

test for a Poisson process at similar levels of significance, 

and (3) yielded identical catalogs of events of magnitude 

Mu>4. The number of independent events in the magnitude range of 2 to 4 varies by about 25 percent, depending 

on which set of window criteria is used. As discussed 

below, this variation results in differences in the b value 

computed for the region. The uncertainty in identifying a 

catalog of independent events was incorporated in the 

earthquake recurrence estimates used in the hazard analysis. The catalog data shown in figures 3 and 4 are for 

independent earthquakes of magnitude ML>2.

CATALOG COMPLETENESS

To estimate earthquake recurrence frequencies, one 

must estimate the time periods over which independent 

events of various magnitudes can be considered completely reported in the catalog. These time periods were 

estimated by plotting the observed frequency of occurrence of independent events in different magnitude intervals as a function of time before April 1, 1986, the 

observed frequency being equal to the number of events 

observed in the last T years divided by T. If it is assuming 

that the earthquake occurrence rate on a regional scale is 

stationary in time, then the time when the observed frequency begins to steadily decrease with increasing time 

represents the date before which the catalog cannot be 

considered complete. This approach to evaluating catalog 

completeness is similar to the method proposed by Stepp 

(1972).

Figure 6A presents a plot of the frequency of earthquakes as a function of time for earthquakes within the 

region outlined in figures 3 and 4. This region was defined 

to encompass the seismic activity along the Intermountain Seismic Belt from lat 39° to 42.5° N. The region 

around Price, Utah (area of concentrated seismicity in the 

southeastern corner of fig. 4), was excluded because it 

contains a significant level of mining-induced seismicity 

(Arabasz, 1984). The events are sorted into magnitude 

intervals two-thirds of a magnitude unit wide centered on 

the magnitudes obtained from the Gutenberg and Richter 

(1956) relationship given above by using unit intensities. 

The periods of catalog completeness for the study region 

estimated from the results shown in figure 6A are:

Magnitude range 

0.7-1.3 

1.3-4.0 

4.0-5.3

Period of completeness 

Incomplete

October 1974-March 1986 

January 1938-March 1986

The numerical results presented in figure QA do not lend 

themselves to precise estimates of periods of completeness. The dates tabulated above were selected to correspond with known major changes in seismicity reporting 

in the Utah region: the initial operation of a telemetered 

network in October 1974 and the development of a cooperative agreement between the University of Utah and the 

U.S. Coast and Geodetic Survey (now the National Ocean 

Survey) for routine transmission of seismographic information in 1938 (Arabasz, 1979). For earthquakes larger 

than magnitude ML 5.3 (epicentral intensity VII or 

greater), there are too few events to make an estimate of 

completeness by using the above method, so the completeness periods estimated by Arabasz (1979) for events 

of intensity VII (1875) and for events of intensity VIII 

(1850) were used in the analysis. These dates correspond 

to the start of the Townley and Alien (1939) catalog for 

Utah in 1875 and the first routinely published newspaper 

in 1850.

Figure 6J3 shows cumulative annual frequencies 

based on the data in figure 6A for periods of complete 

reporting. The error bars represent 16th- to 84th-percentile cumulative rates computed by using the formulation 

given by Weichert (1980). The data indicate that there 

may not be complete reporting at present for events 

smaller than magnitude 2.0. The regional b value for independent events computed by using the Weichert (1980) 

maximum likelihood technique ranges from 0.75(±0.03) to 

0.83(±0.03), depending on the choice of aftershock window criteria. Within the zone outlined in figure 4, the frequency of events of magnitude ML 4 or larger is one event 

every 1.3 years.

SEISMIC SOURCE CHARACTERIZATION

Active Quaternary faults along the Wasatch fault and 

in the adjacent areas have long been recognized as potential earthquakes sources (for example, Gilbert, 1890). 

Numerous studies have been made in recent years to 

identify the locations of and assess the seismic potential 

of faults having Quaternary displacement in the study 

area. At a regional scale, the concentration of earthquakes 

in the Intermountain Seismic Belt appears to align with 

the Quaternary active faults (fig. 4), but, when examined 

in detail, many of the small- to moderate-magnitude 

events cannot be directly associated with mapped Quaternary faults (Arabasz and others, 1980; Arabasz, 1984). It 

should be noted that the apparent concentration of historical seismicity along the Wasatch fault indicated in figure 

3 reflects the fact that noninstmmental epicenters were 

assigned to the location reporting the strongest felt 

effects and that the established communities are concentrated along the Wasatch Front (Arabasz and others, 

1980).
M10 

200 

100 

50

20

10

5

UJ

8 2 UJ

oc

z

0.5

0.2

0.1

0.05

0.02

0.01

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

200 

100

X

M 0. 

M 1

M 0.67-1.33 

.33-2

2-2.67

M 4.67-5.33

s s o> o> sCO

U

UJ

D 

O

UJ

oc

D 

Z 

Z

<

UJ

P

50

20

10

«

1 5 10 20 50 100 200 

TIME, IN YEARS

O 0.2

0.1 

0.05

0.02

0.01

B

MAGNITUDE

FIGURE 6. Estimates of catalog completeness and regional recurrence for north-central Utah. A, Curves showing the frequency of occurrence for events in different magnitude intervals as a function of time prior to April 1, 1986. B, Cumulative event frequencies obtained for the periods of completeness indicated in A. 

The vertical line denotes the 68-percent confidence interval (16th-84th percentile) on the cumulative frequency.

The lack of signature of the Quaternary faults in the 

instrumental seismicity is illustrated in figure 7. Shown 

are approximately east-west cross sections of the instrumental seismicity recorded in the period October 1974 to 

April 1986. In each cross section, the events occurring 

along the length of one of the segments of the Wasatch 

fault identified by Schwartz and Coppersmith (1984) are 

projected onto a plane normal to the fault trace. The locations and estimated range of dips of the identified Quaternary faults are shown, the Wasatch fault being located at 

distance zero in the plots. As can be seen, there is a poor 

correlation of instrumental seismicity with the identified 

faults.

Because of the poor correlation of smaller magnitude 

seismicity with mapped Quaternary surface faulting, two

types of earthquake sources are included in the seismichazard model: fault-specific sources representing the 

mapped active faults that may be the source of moderateto large-magnitude earthquakes (ML>6) and an area 

source that models the background seismicity of smaller 

magnitude earthquakes that may be occurring on 

unknown faults or faults that are not mapped as active in 

the Quaternary. On the fault-specific sources, only the 

occurrence of events of magnitude 6 or larger was modeled. Although smaller magnitude events may also occur 

on the mapped active faults, they do not appear to occur 

with a frequency discernible from the background rate. 

Therefore, the background source was used to model the 

occurrence of all smaller magnitude events.
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS Mil

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1ft

3 "§ 

«?&

§ i

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II

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M12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

AREA SOURCE MODEL FOR BACKGROUND SEISMICITY

Conceivably, there could be many faults within the 

region capable of generating small to moderate earthquakes that do not rupture to the surface. For this analysis, the spatial distribution of future small- to moderatemagnitude seismicity is assumed to be uniform within the 

zone outlined in figures 3 and 4. Seismicity occurring in 

areas to the east and west of the source zone shown in figures 3 and 4 was not modeled in the analysis, because 

small to moderate earthquakes occurring in these areas 

would be too far from the study area defined in figure 1 to 

contribute significantly to the ground-shaking hazard.

The maximum magnitude for events in the background zone is constrained by the threshold for surfacefaulting events, which has typically been considered to be 

about magnitude 6 to QVz. Surface faulting was reported 

for the 1934 ML 6.6 Hansel Valley earthquake, whereas no 

surface faulting was found for the 1962 M L 5.7 Cache Valley and 1975 ML 6.0 Pocatello Valley earthquakes. It is 

very unlikely that there are faults along the Wasatch Front 

long enough to generate earthquakes larger than about 

magnitude 6^ that have not been mapped. Accordingly, 

the maximum magnitude for events in the background 

source was judged to lie in the range of 6 to 6V£. The maximum magnitude and assigned weights are magnitude 6 

(0.3), magnitude 6V4 (0.5), and magnitude 6Yz (0.2).

Earthquake recurrence for the background zone was 

based on the recorded seismicity located within the zone 

shown in figures 3 and 4. As discussed previously, the 

three different criteria used to identify aftershocks in the 

catalog resulted in a variation of about 25 percent in the 

number of events of magnitude 2 to 4. Figure 8A shows 

the cumulative frequency plot for the three catalogs of 

independent events and the associated recurrence relationships obtained by using Weichert's (1980) maximum 

likelihood technique. The three recurrence curves shown 

in figure 8A were used in the hazard analysis with equal 

weight. The mean annual frequency of events of magnitude Mi>5 ranges from 0.10 to 0.12 and has associated b 

values of 0.75 to 0.83. The uncertainty in the regional 

recurrence estimates was incorporated into the analysis 

by constructing a joint distribution for cumulative earthquake frequency and b value using relative likelihoods for 

various parameter combinations (for example, Bender, 

1983). Figure 8B shows the resulting 90-percent confidence interval estimates of earthquake recurrence utilizing the joint distributions of recurrence relationship 

parameters.

QUATERNARY FAULTING

The major faults having Quaternary displacement in 

the region that are close enough to the study area to be

significant to the seismic-hazard evaluation of the study 

area are shown in figure 1. These faults include the 

Wasatch fault zone; a discontinuous zone of faults along 

the western side of the East Tintic and Oquirrh Mountains, referred to here as the Oquirrh Mountains fault 

zone; the East Cache fault; the West Valley fault zone; the 

Hansel Valley fault; and the East Bear Lake fault. The Quaternary active back-valley faults east of the Wasatch, such 

as the Strawberry Valley fault, are not specifically 

included in the seismic source model because of their distances from urban areas and their low recurrence rates 

(Nelson and Van Arsdale, 1986).

Fault-specific parameters that can significantly affect 

seismic hazard are shown in the overall logic tree for the 

study in figure 2. Certain elements of the seismic-hazard 

logic tree are common to all the faults, either because all 

the faults are believed to have certain characteristics that 

are shared in common or because, for some characteristics (for example, dip and maximum depth of faulting), 

there are insufficient data to discriminate among the different faults. Elements that are common for all the faults 

include the form of earthquake magnitude distribution 

and the methods used to assess recurrence; whether a 

fault consists of separate segments; fault dip and maximum depth of rupture; and the techniques used to assess 

maximum earthquake magnitude. These factors are discussed below.

EARTHQUAKE MAGNITUDE DISTRIBUTION AND 

RECURRENCE ASSESSMENT

Although the exponential magnitude distribution 

generally fits regional seismicity data, as the data in figure 

QB show, recent studies (for example, Schwartz and Coppersmith, 1984) have suggested that the magnitude distribution for individual faults and fault segments may be 

better represented by a "characteristic" size implying that 

large events are relatively more frequent than intermediate-sized events. The characteristic model of Schwartz 

and Coppersmith (1984) was developed on the basis of 

data from the Wasatch fault zone and the San Andreas 

fault in California. For this study, both the exponential 

magnitude distribution and the characteristic magnitude 

distribution the latter as developed by Youngs and Coppersmith (1985a, b) were used to model earthquake 

recurrence for the faults. The characteristic model was 

favored over the exponential model (probability 0.7 versus 0.3) because comparisons of the geologic and seismicity data from the Wasatch fault zone originally formed the 

basis for the model.

Recurrence estimates for the fault-specific sources 

were based on two approaches. Where displacement data 

for individual past events were available, they were used 

to estimate the recurrence rate for the characteristic or
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M13

100

50

20

10

D 

O

UJ

£ 2

\ '

<

lil

> 0.5

D 0.2 

u

0.1 

0.05

0.02

0.01

EXPLANATION 

O Aftershock window 1 

D Aftershock window 2 

A Aftershock window 3

\ \ 

v \ 

\ \

\ \ 

\ \ 

\ \

\ V

1 6 71 2 

MAGNITUDE

B q 

EXPLANATION

Observed frequency

and 1-sigma error

bars 

90-percent confidence

interval on cumulative

annual frequency

l\

\ V

\\

\ V

\\ \\\

\ \ \

\ \

\ \ \

V V

V \

\ \

V V

\1

I_k

6

FIGURE 8. Recurrence estimates for background seismicity source delineated in figures 3 and 4. A, Recurrence relationships resulting from use of the 

three aftershock window criteria shown in figure 5. B, 90-percent confidence interval, including uncertainty in cumulative frequency and b value.

maximum event, and the form of the characteristic or 

exponential magnitude distribution was used to define 

the frequency of events down to magnitude 6.0. If only 

slip rate data were available for a fault or fault segment, 

then a moment rate approach was used, the seismicity 

rate being defined by the relationship given by Anderson 

(1979) for the exponential model and by the relationship 

given by Youngs and Coppersmith (1985a, b) for the characteristic model. In these approaches, the recurrence estimates for individual faults are anchored to the mean 

repeat times of the largest events estimated from geological evidence rather than from an extrapolation of the 

observed frequency of small events. The geologically 

based estimates of recurrence of large-magnitude events 

are considered more reliable than the extrapolation of 

small-magnitude seismicity rates because of the difficulty 

in attributing seismicity to specific faults.

In applying Youngs and Coppersmith's (1985a, b) 

characteristic magnitude distribution, the maximum magnitude assessed for the fault, ramax, is taken to be the 

expected magnitude for the characteristic size event, individual events being uniformly distributed in the range of 

mmax -^ magnitude units. The cumulative frequency for 

earthquakes of magnitude mmax -1A is then set equal to 

the annual frequency of maximum events assessed for the 

fault, and the upper bound magnitude, mu, in equation 2 is 

equal to mm^ +1/4. To provide a consistent interpretation 

for the exponential model, the standard truncated exponential distribution (Cornell and Van Marke, 1969) was 

modified to treat the upper bound magnitude in the density function as uniformly distributed over the range of 

mmax ±1A magnitude units. The effect is to smear out the 

upper boundary of the magnitude distribution without 

altering the general shape of the recurrence relationship.
M14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

aUJ 

DC

I

o

10

10

10

10

A :

EXPLANATION

 Exponential 

 Modified exponential

--- Characteristic

B

EXPLANATION

 Exponential

 Modified exponential

 -- Characteristic

8 4 

MAGNITUDE

FIGURE 9. Comparison of recurrence models used in the analysis for (A) equal recurrence rate of maximum events (M>7) and (B) equal rate of seismic moment

release. Curves are constructed by using a b value of 0.8.

The cumulative frequency for earthquakes of magnitude 

mmax ~1/4 is again set equal to the annual frequency of 

maximum events assessed for the fault and the upper 

bound magnitude, mu, is equal to mmax +Vi In this modified form, the distribution of events in the range mmax ±1A 

remains nearly exponential.

Figure 9 compares the shape of the exponential, 

modified exponential, and characteristic magnitude distributions. Figure QA shows the three distributions developed for an assessed fault mmax of 7.25, the frequency of 

events larger than magnitude 7 being held constant in all 

three models. Figure 9B shows the magnitude distributions developed on the basis of equal rate of seismic 

moment release. As can be seen, the modified exponential 

distribution is essentially equal to the exponential distribution except at the upper bound. The characteristic magnitude distribution results in about a factor of 10 

reduction in the frequency of small-magnitude events in 

comparison with the exponential model when the absolute level of the distribution is fixed by either the frequency of the largest events or by the rate of moment 

release.

Application of the two models requires specification 

of a b value for the fault in order to be able to define the 

frequency of smaller magnitude events. Figure 10 presents frequency data for independent events from a 15- 

km-wide corridor representing the surface projection of 

the west-dipping Wasatch fault between lat 40° and 41.5° 

N., which encompasses the more active central portion of 

the fault zone. The earthquake frequencies for the smallmagnitude events are based on instrumental seismicity, 

and the frequency of large-magnitude events is based on 

the paleoseismic data described later in this paper. In figure WA, characteristic and exponential magnitude distributions are anchored to the frequency of large-magnitude 

events and then extended to smaller magnitudes by using 

the b value of 0.8 obtained for the regional seismicity. As 

can be seen, the frequency of smaller magnitude events
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M15

UJ

O

0.5

0.2

<

z 0.1

<

> 0.05

O

0.02

0.01

0.005

0.002

0.001

EXPLANATION

- Characteristic 6=0.8

 Exponential 6=0.8

B

EXPLANATION

 Characteristic b=Q.7

 Exponential b= 0.5

7 8 1

MAGNITUDE

FIGURE 10. Wasatch fault-specific b values. A, Recurrence curves using regional b values. B, Recurrence curves using selected fault-specific b values. In 

modeling the fault-specific sources for the hazard analysis, only events of magnitude greater than 6 are modeled; smaller events occur in the background 

source. Solid circles show the earthquake frequencies and their I-CT error bars.

that may be occurring on the fault is overestimated, especially by the exponential model. To provide more reasonable estimates of the frequency of smaller magnitude 

events occurring on the faults, fault-specific b values were 

developed. Figure IQB shows that the recurrence curves 

obtained by using a b value of 0.7 with the characteristic 

model and a b value of 0.5 with the exponential model 

provide a much better fit to the observed frequencies of 

small magnitude events. These two fault-specific b values 

were used to model earthquake recurrence for all of the 

fault-specific sources. As discussed above, only the faultspecific recurrence estimates for events of magnitude 6 or 

greater are included in the analysis.

Use of the moment rate approach requires a relationship between earthquake magnitude and seismic moment. 

Doser and Smith (1982) analyzed the relationship 

between seismic moment and magnitude for the Utah 

region and concluded that the relationship was significantly different from that developed for California 

earthquakes. Doser and Smith's preferred relationship 

estimates seismic moment, M0, as a function of ML magnitude and was developed on the basis of events of M L magnitude 3.7 to 6.0. This study requires a seismic momentmagnitude relationship applicable to events as large as 

Ms 7?4 and consistent with the attenuation relationships 

used in the hazard analysis. These relationships were 

derived largely on the basis of strong-motion data from 

California, where the moment magnitude scale as defined 

by Hanks and Kanamori (1979) is equivalent to ML in the 

range of 3 to 7 and equivalent to M s in the range of 5 to 

7.5. To apply these relationships in Utah, we have made 

the assumption that ground motions are more directly 

correlated with magnitude than with seismic moment. 

This assumption is supported by the theoretical model of 

ground motion developed by Hanks and McGuire (1981) 

in which ground-motion levels are fixed by both seismic
M16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

moment and stress drop. Boore (1983) has shown that 

this model can be used to simulate the characteristics of 

both strong-motion attenuation and measurement of M L 

in California. Regional differences in the seismic moment 

and stress drop thus should have similar effects on strong 

ground motion and on measured magnitude. We have 

made the assumption that events having similar measured 

magnitudes in Utah and California will have similar levels 

of strong ground motion, although the total seismic 

moment release may be different in the two regions.

For this study, we have developed a modified form of 

the Doser and Smith relationship by including data for 

large Ms magnitude events reported by Doser and Smith 

(1982) and by Doser (1984). These events include the 1959 

Hebgen Lake and 1983 Borah Peak earthquakes. The 

resulting data set is plotted in figure 11. A least-squares fit 

to the data yields the relationship log(M0)=1.04M+18.8, 

which is only slightly different from Doser and Smith's 

(1982) preferred relationship. This modified Doser-Smith 

relationship was used for all moment-rate-constrained 

recurrence estimates.

Also shown in figure 11 for comparison is the relationship proposed by Hanks and Kanamori (1979), which 

clearly does not fit the trend of the data. Because the 

Hanks and Kanamori relationship predicts larger seismic 

moments for earthquakes of magnitude 6 or greater, its 

use would result in somewhat lower earthquake frequencies estimated on the basis of the moment rate approach.

Application of the moment rate approach requires an 

estimate of the average slip rate for the fault or fault segment. Slip rates estimated from data for a single location 

or only a few locations may not reflect the average slip 

rate along the entire fault because the locations selected 

for detailed study are usually where the fault is well 

expressed topographically; hence, these locations may 

represent points where the slip rate is somewhat greater 

than the average over the whole fault or segment length. 

Accordingly, the distributions of possible values of the 

average slip rate assessed for the faults reflect the uncertainty in the point estimates of slip rate together with a 

possible bias in the point estimate. The amount of possible bias was judged to range from 1.0 (no bias) to 0.5 

(point estimate twice the average rate), the expected 

amount of bias in the point estimates being set to 0.8.

UNSEGMENTED VERSUS SEGMENTED 

FAULT-RUPTURE MODEL

The formulation used in this study (and in most other 

seismic hazard analyses) treats the location of future 

earthquake rupture on a seismic source as a random process, usually a uniform distribution. For fault zones modeled as a single fault-specific source, this formulation 

would result in earthquake ruptures uniformly distributed

1027

10"

LU

F

O 10"

LU

I

LU

O 1024

o

1023

10"

EXPLANATION

Doser and Smith (1982) primary data

Doser and Smith (1982) secondary data

Doser (1984)

Doser and Smith (1982)

Hanks and Kanamori (1979)

This study

5 6 

MAGNITUDE

FIGURE 11. Relationship between seismic moment and earthquake magnitude. "Primary data" are data used by Doser and Smith (1982) to develop 

their preferred relationship, shown in the figure.

along the length of the fault. However, on the basis of 

the results of paleoseismic investigations, Schwartz and 

Coppersmith (1984) proposed that faults may consist of 

separate segments that appear to rupture completely and 

independently during successive earthquakes of characteristic sizes, implying that the individual segments of a 

fault should be modeled as separate sources in the hazard 

analysis. The segmentation model not only affects the distribution of ruptures along the fault, it also provides constraints on the maximum rupture length and, therefore, 

on maximum earthquake magnitude and may also affect 

recurrence estimates.

Convincing arguments supporting fault segmentation 

have been presented by Schwartz and Coppersmith (1984, 

1986), Aki (1979, 1984) and Slemmons and Depolo (1986). 

Studies by several investigators (Schwartz and Coppersmith, 1984; Mayer and MacLean, 1986; Machette and others, 1986; Personius, 1986) report evidence for distinct
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M17

segments along the Wasatch fault zone. However, detailed 

analysis of some historical surface-faulting earthquakes 

suggests that faulting can rupture across and (or) skip 

over barriers between adjacent segments. For example, 

the October 10, 1980, El Asnam, Algeria, earthquake 

appears to have been produced by rupture of two or three 

adjacent segments (Deschamps and others, 1982; King 

and Yielding, 1984). Similarly, surface faulting during the 

October 28,1983, Borah Peak, Idaho, earthquake ruptured 

across the bedrock hills east of Willow Creek Summit, 

which may represent a major segment boundary along the 

Lost River fault separating the Warm Springs Valley segment from the Thousand Springs Valley segment (Crone 

and Machette, 1984).

To account for the uncertainty in fault-rupture behavior, both an unsegmented model and a segmented model 

were included for each fault. The unsegmented model 

allows for rupture across the segment boundaries during 

events and for the simultaneous rupture of two or more 

adjacent segments. Rupture during successive earthquakes is assumed to be uniformly distributed along the 

entire length of the fault zone, and the maximum length of 

rupture is not necessarily constrained by boundaries 

between segments. The segmented model assumes that 

rupture will not occur across the segment boundaries and 

that the segments behave independently, the maximum 

length of rupture on a given segment being equal to the 

segment length. The results of paleoseismic investigations 

indicate that the segmented fault model is more representative of the fault-rupture behavior of normal-slip Basin 

and Range faults. Accordingly, the unsegmented and segmented conditions for all the faults are assigned weights 

of 0.2 and 0.8, respectively. For the Wasatch fault zone, 

two segmented fault models are included to account for 

uncertainty in the actual number of segments, as discussed later.

FAULT DIP AND DEPTH

The downdip geometry of the fault plane affects the 

seismic-hazard computation in several ways. For nearby 

faults, it can affect distribution for distance to earthquake 

rupture. It is a factor in determining fault-rupture area 

used in the assessment of maximum earthquake magnitudes and earthquake recurrence estimates based on seismic moment rate.

All the faults included in the seismic source model 

are high-angle normal faults. There are insufficient data 

on the downdip geometry of these faults to discriminate 

among them. Therefore, the downdip geometry is treated 

the same way for all the faults.

Nearly all the microseismicity along the Wasatch 

Front occurs at depths of about 15 km or less (fig. 7). On

the basis of microseismicity and aftershock data, the July 

6, 1954, Fallon Stillwater, Nev, earthquake (M S 7.1), the 

August 18, 1959, Hebgen Lake, Mont, earthquake (Ms 

7.5), and the October 28, 1983, Borah Peak, Idaho, earthquake (Mg 7.3) are all interpreted to have occurred on 45 °- 

to 65°-dipping normal faults that nucleated at depths of 

about 15 km (Smith and others, 1985). Accordingly, all 

faults are assumed to have dips ranging between 45 ° and 

65°, a maximum depth being 15 km. In the analysis, dips 

of 45° and 65° were used with equal likelihood.

MAXIMUM EARTHQUAKE MAGNITUDE ASSESSMENT 

TECHNIQUES

Maximum earthquake magnitudes for the fault-specific sources are assessed by estimating the physical 

parameters of length, area, and displacement for the maximum-size event and then relating these parameters to 

earthquake magnitude by using published correlations for 

magnitude as a function of rupture length RL [Slemmons 

(1982), Ms=0.819+1.341 logio (RL in meters); Bonilla and 

others (1984), Ms=5.17 1.237 logio (RL in kilometers)], 

rupture area RA, [Wyss (1979), Ms=4.15+log10 (RA in 

square kilometers)], and seismic moment. The relationship used to estimate magnitude from seismic moment, 

M0, was developed by utilizing the data presented by 

Doser and Smith (1982) and Doser (1984) and shown in 

figure 11. A least-squares fit to the data shown in figure 11 

yielded the relationship M=-16.22+0.885 Iog 10 (M0 in 

dyne-cm) where M is assumed to be equivalent to M L in 

the range of 3 to 7 and Ms in the range of 5 to 7.5. The distribution of possible values for the physical parameters of 

maximum rupture length, maximum rupture area, and 

average displacement in a maximum event are developed 

by extending the logic tree formulation shown in figure 2.

Completion of the assessment of maximum magnitude requires selection of the appropriate relationship 

between rupture parameters for a maximum event and 

earthquake magnitude. The relative weights assigned to 

the various techniques used to estimate the maximum 

earthquake magnitude for each fault and fault segment 

are illustrated in figure 12. Where three techniques (that 

is, length versus magnitude, area versus magnitude, and 

moment versus magnitude) could be used, the greatest 

weight, 0.5, is assigned to magnitudes based on seismic 

moment because this relationship incorporates essentially all the physical factors controlling earthquake size 

(fault rupture area, amount of slip, and rigidity of the 

rock). The lowest weight, 0.2, is assigned to the lengthversus-magnitude relationship because it incorporates the 

least number of these controlling factors. The balance, 

0.3, is assigned to the area-versus-magnitude relationship.

The Wasatch and East Cache faults are the only faults 

having sufficient data to estimate the average fault dis-
M18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

Case A

Case B

(1.0)

(1.0)

FIGURE 12. Weighting factors for maximum magnitude assessment techniques for case A, in which data on displacement per event are available, 

and case B, in which data on displacement per event are not available. L is 

the log of the rupture length, A is the log of the rupture area, M0 is seismic 

moment, MS is surface-wave magnitude, and Mw is moment magnitude.

placement. Therefore, the moment-versus-magnitude 

technique could not be used for the other faults. Where 

only length-versus-magnitude and area-versus-magnitude 

relations were used, the techniques are assigned relative

weights of 0.4 and 0.6, respectively. Given the lengthversus-magnitude relationship, the relationships of Slemmons (1982) and Bonilla and others (1984) are assigned 

equal probability of reflecting the correct value. After 

maximum magnitude distributions are obtained from the 

logic trees such as those illustrated in figure 12, these distributions are discretized into quarter-magnitude intervals 

for use in the analysis.

The remaining parameters incorporated in the analysis (fig. 2) include different segmentation models, 

whether a fault is active (that is, capable of independently 

producing earthquakes), total fault length, maximum 

earthquake magnitude, and earthquake recurrence rate. 

The assessed fault-specific parameter values and associated weights are summarized in table 1 and are discussed 

below for each source. Table 2 lists the fault parameters 

and probability weights used for assessing maximum 

magnitude for each of the faults.

WASATCH FAULT ZONE

The Wasatch fault zone is an active westward-dipping 

normal fault (figs. 1, 13). Paleoseismic studies demonstrate that repeated large-magnitude earthquakes have 

occurred on most segments of this fault zone throughout 

the Holocene (Swan and others, 1980, 1981a, b, c, 1982; 

Hanson and others, 1981a, b; Schwartz and Coppersmith, 

1984). These data are incorporated in the seismic-hazard 

model in the following manner.

Segmentation

In addition to the question of whether the fault-rupture process is characterized by unsegmented or segmented behavior, discussed earlier, there is uncertainty 

regarding the actual number of fault segments along the 

Wasatch fault zone. Schwartz and Coppersmith (1984) 

proposed that the fault zone south of Collinston, Utah, 

consists of six separate segments. They did not discuss 

the fault zone north of Collinston, which may include a 

seventh unnamed segment (fig. ISA). On the basis of morphological discriminant analysis of selected geomorphic 

parameters (drainage basin shape, mountain front sinuosity, steepness of trunk streams, locations of steep sections 

of stream channels and valley shape), Mayer and Maclean 

(1986) proposed that the zone may consist of nine separate segments. Machette and others (1986, 1991) suggest 

that the zone consists of at least 10 segments.

At present, there are insufficient data to conclusively 

define the actual number of segments (Wheeler, 1984). 

Therefore, two segmentation models are included in the 

hazard analysis (fig. 13, table 1). The minimum number of 

segments proposed along the Wasatch fault zone is 7
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M19

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TABLE 1. Summary of fault characterization parameters and associated weights (in parentheses) for the seismic-hazard model 

Fault and State of , , Dip 

. .^ length . . ^ 

segment activity ° (deg) 

(km)

,, . . Recurrence ,. , , ^i Recurrence 

Maximum magnitude , . i Slip rate (mm/yr) 

basis J 

Wasatch fault Continued

Nephi 

Levan

Fayette

Active (1.0) 40(1.0) 45(0.5) 

65 (0.5) 

Active (1.0) 35(1.0) 45(0.5)

65. (0.5) 

Active (1.0) 14(1.0) 45(0.5)

7 (0.45); 7.25 (0.55) 

7 (0.45); 7.25 (0.55) 

7 (0.55); 7.25 (0.45)

7 (0.85); 7.25 (0.15) 

6.25 (0.1); 6.5 (0.4); 

7(0.15)

Segment (0.5) Not used 

Zone (0.5) Not used 

Segment (1.0) Not used

6.75 (0.35); Segment (1 .0) Not used

2000 (0.28); 167

3570 (0.2); 192

9090. (0.04); 833

6670(0.16) 

6250 (0.12); 58

10,000 (1.0)

Oquirrh Mountains fault

Unsegmented Active (1.0) 180 (0.35) 45 (0.5) 

model (0.2)

65 (0.5)

250 (0.05) 45 (0.5)

65 (0.5)

Segmented model (0.8) 

Segments: 

EastTintic Active (1.0) 35(1.0) 45(0.5)

65 (0.5)

Oquirrh Active (1.0) 50(1.0) 45(0.5) 

marginal

65 (0.5)

Northern Active (1.0) 35(1.0) 45(0.5) 

Oquirrh Mountain

65 (0.5) 

Great Salt Lake Active (1.0) 60(1.0) 45(0.5)

7 (0.13); 7.25 (0.72); 7.5 (0.14); Zone (1.0)

7.75 (0.01) 

7 (0.37); 7.25 (0.51); 7.5 (0.11);

7.75 (0.01) 

7 (0.13); 7.25 (0.72); 7.5 (0.14); Zone (1.0)

7.75 (0.01)

7 (0.37); 7.25 (0.51); 7.5 (0.11); 

7.75 (0.01)

0.08 (0.03); 0.1 (0.23); 0.15 (0.41); 0.2 Not used 

(0.27); 0.3 (0.06)

0.08 (0.03); 0.1 (0.23); 0.15 (0.41); 0.2 

(0.27); 0.3 (0.06)

Not used

7(1.0)

7(1.0)

7 (0.2); 7.25 (0.8)

7 (0.8); 7.25 (0.2) 

7(1.0)

7(1.0) 

7.25 (1.0)

65 (0.5) 7.25 (1.0)

Promontory Active (0.5) 70(1.0) 45(0.50) 7.25 (0.8); 7.5 (0.2) 

Mountain

Inactive (0.5) 65.5 (0.5) 7.25 (0.8); 7.5 (0.2)

Zone (1.0) 

Zone (1.0) 

Zone (1.0)

Zone (1.0) 

Zone (1.0)

0.08 (0.03); 0.1 (0.23); 0.15 (0.41); 0.2 Not used 

(0.27); 0.3 (0.06)

0.08 (0.03); 0.1 (0.23); 0.15 (0.41); 0.2 Not used 

(0.27); 0.3 (0.06)

0.08 (0.03); 0.1 (0.23); 0.15 (0.41); 0.2 Not used 

(0.27); 0.3 (0.06)

0.08 (0.03); 0.1 (0.23); 0.15 (0.41); 0.2 Not used 

(0.27); 0.3 (0.06)

0.08 (0.03); 0.1 (0.23); 0.15 (0.41); 0.2 Not used 

(0.27); 0.3 (0.06)
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M21

maximum event

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M22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 2. Summary of parameters and relationships used to estimate maximum magnitude and associated weights

Rupture length 

(km)

Dip 

(deg)

Average displacement 

(m)

Estimation 

technique*

Estimation 

relationship**

Wasatch fault

Unsegmented model (0.2) 

Segmented model (0.8)

35 (0.5); 45 (0.4); 

65 (0.5); 100 

(0.05) 

Segment lengths 

(1.0)

45 (0.5); 65 (0.5) 

45 (0.5); 65 (0.5)

1 (0.1); 1.5 (0.6); 

2 (0.3)

1(0.1); 1.5 (0.6); 

2 (0.3)

L versus M (0.2); 

A versus M (0.3); 

M0 versus M (0.5) 

L versus M (0.2); 

A versus M (0.3); 

M0 versus M (0.5)

1 (0.5); 2 (0.5); 

3(1.0); 

4(1.0) 

1(0.5); 2 (0.5); 

3(1.0); 

4(1.0)

Oquirrh Mountains fault

Unsegmented model (0.2) 

Fault length 250 km (0.5) ...

Fault length 180 km (0.5) ... 

Segmented model (0.8)

... 35 (0.05); 45 (0.4); 

65 (0.5); 100 

(0.05) 

,... 35 (0.5); 65 (0.5)

Segment lengths 

(1.0)

45 (0.5); 65 (0.5)

45 (0.5); 65 (0.5) 

45 (0.5); 65 (0.5)

Not used

Not used 

Not used

L versus M (0.4); 

A versus M (0.6)

L versus M (0.4); 

A versus M (0.6) 

L versus M (0.4); 

A versus M (0.6)

1(0.5); 2 (0.5); 

3(1.0)

1(0.5); 2 (0.5); 

3(1.0) 

1 (0.5); 2(0.5); 

3(1.0)

East Cache fault

Unsegmented model (0.2) 

Segmented model (0.8)

36 (0.3); 47 (0.7)

Segment lengths 

(1.0)

45 (0.5; 65 (0.5) 

45 (0.5; 65 (0.5)

1 (0.1); 1.5 (0.6); 

2 (0.3)

1(0.1); 1.5 (0.6); 

2 (0.3)

L versus M (Q.2); 

A versus M (0.3); 

M0 versus M (0.5) 

L versus M (Q.2); 

A versus M (0.3); 

M0 versus M (0.5)

1(0.5); 2 (0.5); 

3(1-0); 

4(1.0) 

1(0.5); 2 (0.5); 

3(1-0); 

4(1.0)

West Valley fault

Unsegmented model (0.8) 16 (1.0) 45 (0.5); 65 (0.5) Not used L versus M (0.4); 

A versus M (0.6)

1 (0.5); 2 (0.5); 

3(1.0)

Hansel Valley fault

Unsegmented model (0.6) 

Segmented model (0.4)

25 (1.0)

Segment lengths 

(1.0)

45 (0.5); 65 (0.5) 

45 (0.5); 65 (0.5)

Not used 

Not used

L versus M (0.4); 

A versus M (0.6) 

L versus M (0.4); 

A versus M (0.6)

1 (0.5); 2 (0.5); 

3(1.0) 

1(0.5); 2 (0.5); 

3(1.0)

East Bear Lake fault

Unsegmented model (0.2) 

Segmented model (0.8)

50 (0.4); 27 (0.6)

Segment lengths 

(1.0)

45 (0.5); 65 (0.5) 

45 (0.5); 65 (0.5)

Not used 

Not used

L versus M (0.4); 

A versus M (0.6) 

L versus M (0.4); 

A versus M (0.6)

1(0.5); 2 (0.5); 

3(1.0) 

1(0.5); 2 (0.5); 

3(1.0)

L, length; M, magnitude; A, area; MO, seismic moment. 

*1, Slemmons (1982) (L versus M); 2, Bonilla and others (1984) (L versus M); 3, Wyss (1979) (A versus M); 4, this study (M=16.22+0.885 log M0).
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M23

(segmentation model A, fig. 13A); the maximum number 

proposed is 10 (segmentation model B, fig. 135). The 

actual number of segments probably lies within this 

range. Model A and model B were assigned relative 

weights of 0.4 and 0.6, respectively, to reflect our judgment that model B is slightly more likely. Both models are 

conditional on the fault's being segmented (fig. 2).

Maximum Earthquake Magnitude

Figure 14 presents the logic tree used to assess maximum magnitude for the Wasatch fault zone. Given an 

unsegmented fault model for the Wasatch fault, the rupture length likely to be associated with the maximum 

earthquake may be significantly longer than the length of 

individual mapped segments. Bonilla and others (1984) 

have developed a relationship relating displacement to 

rupture length for normal faults. The maximum net vertical displacement reported from paleoseismic investigations along the Wasatch fault zone is 4.75 m at Dry Creek 

(Lund and Schwartz, 1987). Using Bonilla and others' 

(1984) relationship relating rupture length to maximum 

displacement for normal faults and displacements of 4.75 

m yields an expected rupture length (mean value) of 53 

km. The longest historical surface fault rupture reported 

from the Basin and Range is 61 km associated with the 

October 3, 1915, Pleasant Valley, Nev, earthquake. However, if two or three of the identified fault segments were 

to rupture simultaneously (for example, the Ogden and 

Salt Lake City segments or the Provo, Spanish Fork, and 

Nephi segments), a maximum rupture length of about 100 

km might be expected. This case, however, is considered 

to be very unlikely. To account for uncertainty in the maximum rupture length, values ranging from 35 to 100 km 

are considered in the hazard analysis. Table 2 summarizes 

the assessed maximum rupture lengths and the relative 

weights assigned to each value for the unsegmented fault 

model. Given the segmented fault models, the maximum 

rupture lengths are assumed to be the lengths of the individual segments as shown in table 1 and figure 14.

Schwartz and others (1983) and Schwartz and Coppersmith (1984, 1986) summarized the available displacement data from five locations along the Wasatch fault 

zone on the basis of paleoseismic investigations. The values, which have been corrected for distortions due to 

effects of graben formation and backtilting, represent net 

vertical tectonic slip at specific locations along the fault 

zone. The values range from 1.6 to 2.7 m per event and 

average about 2 m per event. It is not known where these 

measurements are located with respect to the maximum 

displacement during the individual rupture events or how 

they relate to the average displacement along these surface ruptures.

Detailed measurements of recent surface-faulting 

events show that there is a great deal of variability in the 

displacement along the length of the surface rupture. For 

example, the maximum displacement of 2.7 m during the 

October 28, 1983, Borah Peak earthquake occurred along 

less than 1 km of the 34-km-long rupture. Only 49 percent 

of the rupture was characterized by displacements equal 

to or greater than the average displacement of 0.8 m 

(Crone and Machette, 1984). Therefore, it is unlikely that 

the measured displacements along the Wasatch fault represent the maximum displacement along the surface rupture. If the locations where measurements were made had 

been randomly selected, it would be reasonable to 

assume that the average of all the displacements, which is 

about 2 m, would approximate the average displacement 

per event for past surface-faulting events. However, 

because the locations were selected where the fault is 

well expressed topographically, the results may be biased 

toward values that are too large.

To account for uncertainties in the displacement 

data, a range of values for the average displacement associated with the maximum earthquake is included in the 

probabilistic assessment of the maximum earthquake 

magnitude. Values of 1,1.5, and 2 m are assigned probabilities of 0.1, 0.6 and 0.3, respectively (fig. 14, table 2). Given 

the unsegmented model, the expected maximum earthquake on the Wasatch fault zone is about MS 71/4. If the 

fault is segmented, the expected maximum magnitudes 

for the segments range from MS 6V& to 1% (table 1).

Earthquake Recurrence Rate

The slip rate and displacement per event data presented in table 3 are used to assess the earthquake recurrence rate of major surface-faulting events on the various 

segments of the Wasatch fault zone and for the zone as 

whole. In addition to incorporating uncertainly in the correct recurrence model, described previously, the seismichazard model also includes uncertainty in the primary 

data namely, uncertainties in the measured cumulative 

displacement, in the age of the displaced strata, and in 

the average displacement per event. Quantifying the 

uncertainty in each of these parameters allows the calculation of the probability distributions for the slip rate. The 

resulting probability distributions are presented in 

table 1.

Given the unsegmented fault model, the earthquake 

recurrence rate was calculated on the basis of the seismic 

moment rate obtained from the estimated average slip 

rate. To arrive at an average rate, the slip-rate data in table 

3 were weighted to reflect the segment lengths for model 

A, which was used because there are no independent sliprate data for the American Fork and Provo segments in
M24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

EXPLANATION

Segment boundary

. Promontory 

' Mountains 

\ segment

Inferred Quaternary 

fault

Inferred 

reajfSilt Lake 

segment

Northern

Oquirrh Mountains 

fault

\

Oquirrh

marginal

fault

East Tintic 

fault

40° -

100 KILOMETERS

FIGURE 13. Segmentation models for fault-specific sources. A, Wasatch fault model A and all other faults; B, Wasatch fault

model B.
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M25

113° 112° 111°

42°

41 C

40°

EAST BEAR LAKE FAULT-I

l IDAHO I

Bear Lake

HANSEL VALLEY 

FAULT

EXPLANATION

Segment boundary 

Quaternary fault

Inferred Quaternary 

fault

WYOMING

Salt Lake City

American Fork

Provo

Spanish Fork

B 25 

I

50 

I

75 MILES 

I

r

25

I

50

I

75 100 KILOMETERS

FIGURE 13. Segmentation models for fault-specific sources. A, Wasatch fault model A and all other faults; B, Wasatch fault

model B Continued.
Segmentation

Model 1 Segment name

Rupture 

length, in 

kilometers

Width dip/ 

depth, in 

kilometers

Average 

displacement

WASATCH

FAULT ZONE

Unnamed

Collinston

Ogden

Salt Lake City .

Provo

Nephi t

Levan

30(1.0)

78(1.0)

45 (1.0)

70(1.0)

40(1.0)

49(1.0) _

Collinston

Brigham City

Weber

Salt Lake City

American Fork

Provo

Spanish Fork

Nephi

Levan

Fayette

60 (1.0)

40 (1.0)

57 (1.0)

45 (1.0)

18 (1.0)

19 (1.0)

32 (1.0)

40 (1.0)

35 (1.0)

' 14 (1.0)

 *c

48(1.0)

EXP

 R

1 Slemm

2 Bonilla 

3 Wyss 

4 This s

+0.8

FIGURE 14. Maximum magnitude logic tree for the Wasatch fault zone. L is the log of the rupture length, A is the log of the rupture area, M0 is sei

magnitude, and M is moment magnitude.
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS

TABLE 3. Summary of geologic parameters used to characterize earthquake recurrence rate for the Wasatch fault zone

[N/A, not applicable]

M27

Recurrence 

basis

Segment

Model A Model B

Site

Cumulative 

vertical 

displacement 

(m)

Point Age 

, , , , estimate of (thousands of ,. D _.. slip rate 

years B.P.) . , . } (mm/yr)

Vertical

displacement per 

event 

(m)

Mean

recurrence 

interval for 

maximum 

earthquake

(yr)

Comments and 

sources of data

Segmented fault-rupture model

Zone

Zone

Segment

Segment

Segment

Segment

Segment

"Zone"

Unnamed Collinston

Collrnston Brigham

City

Ogden Weber

Salt Lake Salt Lake

City. City.

American

Fork.

Provo

Provo Spanish

Fork.

Nephi Nephi

Levan Levan

Fayette

Central part of zone

(Brigham City to

Nephi).

N/A

Wellsville

Mountains

near

Honeyville.

Kaysville

Little

Cottonwood

Canyon.

N/A

N/A

Hobble Creek

North Creek

Deep Creek

N/A

N/A

N/A

26

10 (0.5)

11 (0.5)

11.5(0.2)

14.5 (0.7)

24.5 (0.1)

N/A

N/A

11.5(0.5)

13.5 (0.5)

6.5 (0.2)

7.0 (0.6)

7.5 (9.2)

2.5(1)

N/A

N/A

N/A

150

6 (0.1)

8 (0.7)

9 (0.2)

17 (0.2)

19 (9.6)

21 (0.2)

N/A

N/A

11(0.2)

13.5 (0.7)

25. (0.2)

5.1 (0.5)

5.5 (0.5)

6.3 (0.2)

7.3 (0.6)

8.3 (0.2)

N/A

N/A

0.15

0.17

1.3

0.76

N/A

N/A

0.93

1.3

0.35

0.20

1.1

N/A

N/A

1.6

[2]

2.0

[2]

N/A

N/A

1.9

[6-7]

2.3

[3]

2.5

[1]

N/A

2.1

8,800

7,250

1,350

2,550

1,900

1,900

2,050

1,750

7,100

10,000

1,900

per segment

Slip rate assumed to be

comparable to or less

than that on East Cache

fault (Swan and others,

1983). Slip rate assumed

to be accurate to within

about 30 percent.

Recurrence estimate

based on seismic

moment rate (Scott,

1980). Slip rate assumed

to be accurate to within

30 percent.

Swan and others (1980,

Schwartz and others

(1984).

Swan and others (1981b),

Schwartz and Coppersmith (1984).

"Zone" recurrence basis

only.

"Zone" recurrence basis

only.

Swan and others (1980,

1981c).

Hanson and others(1981a),

Hanson and Schwartz

(1982), Schwartz and

Coppersmith (1984).

Hanson and others (1981b),

Schwartz and Coppersmith (1984).

This report. Slip rate

assumed to be accurate

to within about

50 percent.

Average of data from

Kaysville, Little Cottonwood Canyon, Hobble

Creek, North Creek, and

Deep Creek sites.

Unsegmented model

N/A N/A N/A N/A N/A N/A 0.80 N/A 280

Combined models

N/A N/A N/A N/A N/A N/A N/A N/A 330

Standard deviation of mean assumed to be 0.15. Number of events averaged is shown in brackets.
M28 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

model B. The overall average slip rate (weighted by segment length) is 0.8 mm/yr. The slip rate was distributed 

along the fault so as to produce a slip rate of 1.1 mm/yr in 

the central part of the fault consistent with the observed 

variation in rate.

Given the segmented fault model, the earthquake 

recurrence rate for the segments north of Brigham City 

and south of Nephi were estimated by using the seismic 

moment rate approach because there are no data on the 

slip per event or on the timing of individual events for 

these segments.

For the central part of the fault zone, where there are 

slip-rate and displacement-per-event data from several 

localities, the recurrence rate for maximum events is estimated by dividing the slip rate by the average displacement per event. Uncertainty in these parameters was 

incorporated in the analysis as indicated by the distribution for recurrence rate shown in table 1. Two bases were 

used to estimate the average slip rate and displacement 

per event. In one case (the "zone" recurrence basis in 

table 1 and fig. 2), the segments along the central part of 

the fault between Brigham City and Nephi are considered 

to have the same rate of activity, and the recurrence basis 

for these segments was calculated on the basis of the 

average of the slip-rate and displacement-per-event data 

from the five sites located along this stretch of the fault 

(table 1). The slip rate for the central part of the zone, 

based on data summarized by Schwartz and others 

(1984), is 1.1 (±0.15) mm/yr. The average displacement 

per event reported for paleoseismic events identified at 

the five sites along the central segments of the Wasatch 

fault zone is 2.1 (±0.1) m. These values were used to calculate the distribution for the recurrence interval for the 

maximum event shown in table 1 for the "zone" recurrence basis. In the second case (the "segment" recurrence 

basis in table 1), the slip rate and displacement data from 

each site shown in table 3 were used to characterize the 

earthquake recurrence for the specific segments. In some 

cases (for example, the American Fork and Provo segments of segmentation model B), only the "zone" recurrence basis could be used. Where both the "zone" 

recurrence basis and the "segment" recurrence basis were 

used, they were given equal weight.

The activity on the fault north of about Brigham City 

is significantly less than that on the central part of the 

zone (Personius, 1986). On the basis of the subdued geomorphic expression of the northernmost part of the zone 

(that is, the unnamed segment in fig. ISA), it is assumed 

that the long-term slip rate on this part of the fault is comparable to or less than the rate on the East Cache fault 

(see discussion below), and a nominal slip rate of 0.15 

mm/yr was assigned to this part of the fault zone.

Immediately north of Honeyville, Utah (lat 41 ° 33' N) 

(that is, the Collinston segment on fig. 13-A and the

Brigham City segment on fig. 135), Scott (1980) reported 

that an alluvial fan, which he estimated to be about 

150,000 years old, is displaced 26 m. This displacement 

suggests an average slip rate of 0.17 mm/yr, comparable to 

the rate inferred for the northernmost part of the zone. 

There is insufficient information to directly assess the 

accuracy of these slip rates. In estimating the uncertainty 

in the earthquake recurrence rate on these segments of 

the fault, it is assumed that these slip rate values are accurate only to within about 30 percent.

The basis for slip-rate and displacement-per-event 

data at the Kaysville, Little Cottonwood Canyon, Hobble 

Creek, North Creek, and Deep Creek sites (fig. 1, table 3) 

are well documented in several papers and U.S. Geological Survey (USGS) Open-File Reports by Swan and others 

(1980, 1981a, b, c) and have been summarized by 

Schwartz and others (1984) and Schwartz and Coppersmith (1984, 1986). To quantify the uncertainty in the 

earthquake recurrence rates, distributions have been 

assigned to displacement and age data to reflect the 

uncertainty in these primary data. For example, at the 

Kaysville site, the cumulative vertical displacement is 

reported as 10 to 11 m since deposition of an alluvial fan 

estimated to be 8,000 (+1,000, -2,000) years old. The 

actual displacement is equally likely to be anywhere 

within the 10- to 11-m range, so these values are assigned 

equal weight (table 3). The preferred age is assigned the 

highest probability weight (0.7). Ages of 6,000 and 9,000 

years were assigned weights of 0.1 and 0.2, respectively, 

to reflect the judgement that it is unlikely that the fan is as 

young as 6,000 years and that it could be as old as 9,000 

years (Schwartz and Coppersmith, 1984). Where the age 

of the displaced unit is based on radiocarbon dates (for 

example, the age of 7,300 (±1,000) 14C years B.P. for the 

age of the displaced alluvium at the Deep Creek site), the 

reported error for this date is assumed to be within about 

one standard deviation of the actual age.

The available data indicate that the activity on the 

Wasatch fault zone dies out south of Levan. There are no 

reports on the rate of slip on the Fayette segment (model 

B), which was assigned a nominal slip rate of 0.2 mm/yr 

on the assumption of an event about every 10,000 years.

Table 3 presents the mean recurrence times for major 

surface-faulting events on each of the fault segments. 

These times are interpreted to represent the average 

repeat time for the maximum earthquake. Table 1 shows 

that the distribution in the earthquake recurrence rates 

reflects the uncertainty in the slip rate and recurrence 

data. Figure 15 presents the predicted recurrence rate for 

the entire zone. On the basis of the hazard model, the estimated return period for events of magnitude 7 or greater 

is 330 (±90) years for the entire zone. This value compares 

favorably with an earlier estimate of 400 to 666 years 

based on the number of paleoseismic events inferred to
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M29

10

o

UJ

2 10 >

<r

10

10

10

EXPLANATION

 Total

 Background 

' Wasatch

 East Cache

Oquirrh Mountains 

West Valley 

Hansel Valley 

East Bear Lake 

Historical catalog

B

8 3 

MAGNITUDE

FIGURE 15. Comparison of predicted recurrence rates for seismic sources and observed frequency of earthquakes. A, Mean recurrence estimates for individual 

sources and total of all sources. B, Mean (solid) fractiles of predicted distributions of earthquake recurrence in comparison with one standard deviation error 

bars for regional recurrence rate based on observed seismicity.

have occurred on the central segments (Brigham City to 

Nephi) during approximately the past 8,000 years 

(Schwartz and Coppersmith, 1984). The shorter return 

period predicted by the model developed here is due to 

the inclusion of additional segments within the central 

part of the fault zone in the alternative segmentation 

model and contributions from the northern and southern 

segments of the fault zone that were not included in the 

Schwartz and Coppersmith estimate.

OQUIRRH MOUNTAINS FAULT ZONE

The Oquirrh Mountains fault zone lies 25 to 50 km 

west of the Wasatch Front (figs. 1,13). The fault zone consists of an alignment of late Pleistocene fault scarps at or 

near the boundary between the alluvial basin fill and the 

bedrock along the western side of the East Tintic and

Oquirrh Mountains (Andersen and Miller, 1979) that indicates the presence of active westward-dipping normal 

faults along this trend. This fault zone is discontinuous 

and is shown as two faults by Everitt and Kaliser (1980) 

the Oquirrh marginal fault along the eastern margin of 

Tooele Valley and the Oquirrh-Boulter-Tintic fault zone 

along the eastern margin of Rush Valley. The mapped 

onland faults extend northward along the western flank 

of Antelope Island beneath the Great Salt Lake (fig. 13 A). 

The northern limit of Quaternary faulting is uncertain, 

and similar faults may exist along the western flank of the 

Promontory Mountains.

Segmentation

Both unsegmented and segmented fault models are 

used in the seismic-hazard model to characterize the
M30 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

fault-rupture behavior of the Oquirrh Mountains fault 

zone. Figure ISA shows the locations of the five segments 

considered if the zone is segmented. Additional studies 

would undoubtedly result in a refinement of these somewhat arbitrarily defined segments, but comparison of the 

results from the alternative segmentation models used to 

characterize the Wasatch fault zone suggests that further 

refinement would not significantly affect the overall 

results of the hazard analysis.

Activity

Quaternary faulting has been recognized along all 

segments of the Oquirrh Mountains fault zone except the 

Promontory Mountains segment. Given the segmented 

fault model, a probability of 0.5 that the fault is an active 

seismogenic fault was assigned to the Promontory Mountains segment to account for the uncertainty regarding the 

existence of Quaternary faulting along this part of the 

zone.

Total Length

Uncertainty in the total length of an unsegmented 

Oquirrh Mountains fault zone was modeled to account for 

the uncertainty concerning the existence of active faulting north of the Great Salt Lake. Values of 180 and 250 km 

are considered; 180 km is the combined lengths of the 

mapped traces along the four southern segments, and 250 

km is the combined length if the zone extends to the 

northern end of the Promontory Mountains (fig. 13 A). The 

two values were assigned equal weight.

Maximum Earthquake Magnitude

Because there are insufficient paleoseismic data on 

the amount of displacement during past surface-faulting 

events on this fault, the assessment of earthquake magnitude was based on rupture length and rupture area only 

(fig. 12, case B; table 2). If the fault zone is unsegmented 

and 250 km long, the rupture-length distribution was 

assumed to be the same as the rupture-length distribution 

for the unsegmented Wasatch fault (table 2). If the fault 

zone length is 180 km, a 100-km rupture length is not considered credible, and weights were assessed for the 

remaining rupture lengths (table 2). Given the segmented 

model, the maximum rupture lengths are assumed to be 

equal to the segment lengths (table 1).

Table 1 presents the results of the maximum earthquake magnitude assessment for the Oquirrh Mountains 

fault zone. If the fault zone is unsegmented, the most 

likely maximum earthquake is about magnitude 11A. If the 

fault zone is segmented, the most likely maximum earthquakes on the individual segments range from about magnitude 7 to 71A,

Earthquake Recurrence Rate

The available data do not tightly constrain the time 

since the most recent surface-faulting event or the recurrence interval between events on the Oquirrh Mountain 

fault zone. Along the northern part of the Oquirrh marginal fault (fig. 13.A), a series of recessional shorelines 

below the Provo level (younger than about 13,500 yr B.P.) 

appears to be displaced across a prominent curvilinear 

fault scarp. The fault scarp just north of Bates Canyon is 

about 9 m high. It consists of an upper slope 4.6 to 6 m 

high that has been modified by shoreline or sublacustrine 

wave erosion and a lower, steeper slope 3.6 to 4.6 m high 

that appears unaffected by lake processes and therefore 

represents a post-Provo surface-faulting earthquake 

(Everitt and Kaliser, 1979; Woodward-Clyde Consultants, 

1982). On the basis of the geomorphic expression of the 

fault and the absence of faulting in younger alluvial fans, 

Swan and Schwartz estimated that this event occurred 

between about 8,000 and 10,000 years ago and that the 

recession of the lake from the Provo level occurred about 

13,500 years ago (Woodward-Clyde Consultants, 1982).

Most of the faulting along the East Untie fault occurs 

within pre-Bonneville alluvial fan deposits (older than 

about 15,000-14,000 yr B.P.) that lie above the Bonneville 

shoreline and therefore provides little information on the 

recency of faulting. Locally, Everitt and Kaliser (1979) 

have mapped 0.6- to 3-m-high scarps in strandline and 

alluvial-fan deposits just below the Bonneville shoreline. 

Although the field relationships are not unequivocal, 

there seems to have been at least one major post-Bonneville surface-faulting earthquake along the East Tintic fault 

(Woodward-Clyde Consultants, 1982).

The slip rate on the Oquirrh Mountain fault zone is 

very poorly constrained. The scarp heights reported 

above suggest a post-Provo slip rate of about 0.3 mm/yr 

on the Oquirrh marginal fault and a post-Bonneville slip 

rate of about 0.04 to 0.2 mm/yr on the East Tintic fault. 

The assessed distribution for average slip rate shown in 

table 1 was based on the uncertainty in these point estimates of slip rate together with possible biases (described 

previously). Because of the limited data, no attempt was 

made to discriminate between the unsegmented and segmented models, and all segments are assumed to have 

slip rates within this range. On the basis of the fault characterization parameters presented in table 1, the estimated return period for earthquakes of magnitude 7 or 

greater on the Oquirrh Mountains fault zone is 2,200 

(±800) years.

EAST CACHE FAULT

The East Cache fault lies on the eastern side of Cache 

Valley along the western base of the Bear River Range 

approximately 20 to 40 km east of the Wasatch Front
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M31

(figs. 1, 13). It extends as a zone of westward-dipping 

Quaternary normal faults in a north-south direction for a 

distance of about 72 km from just north of James Peak in 

Utah to Preston, Idaho (Cluff and others, 1974). Late Quaternary fault scarps along the East Cache fault are discontinuous and in many locations are subdued owing to 

erosion or are buried by Holocene alluvial fans. Paleoseismic investigations (Swan and others, 1983; McCalpin, 

1987) show that there have been repeated late Pleistocene 

surface-faulting events and at least one Holocene event 

on the East Cache fault near Logan, Utah.

Segmentation

Both unsegmented and segmented fault models are 

used to characterize the rupture behavior of the East 

Cache fault. In this analysis, the fault was divided into two 

segments the Logan segment, which is 47 km long, and 

the Richmond segment, which is 25 km long (fig. 13A). A 

salient in the range front immediately north of Smithfield 

Canyon is interpreted to reflect a structural barrier 

between these segments. McCalpin (1987) proposed a 

three-segment model suggesting the possibility of shorter 

segments. This model, however, has not been explicitly 

incorporated into the analysis.

Maximum Earthquake Magnitude

The maximum magnitude logic tree for the East 

Cache fault is similar to that for the Wasatch fault shown 

in figure 14. If the fault is assumed to be unsegmented, 

two rupture lengths are considered (table 2) 36 km, 

which is 50 percent of the total fault length, and 47 km, 

which is equivalent to the longest segment included in the 

seismic source model. There are insufficient paleoseismic 

data along the East Cache fault to assess the average displacement per event. Swan and others (1983) and 

McCalpin (1987) reported displacements for individual 

events on the fault near the mouth of Logan Canyon of 1.4 

m to possibly as much 2.4 m. These values are comparable to the fault displacements observed along the Wasatch 

fault zone (table 3). Therefore, the range of values and 

assigned probabilities used to characterize the average 

displacement per event during a maximum earthquake on 

the Wasatch fault zone were also assumed to be representative of the East Cache fault. If the East Cache fault is 

unsegmented, the most likely maximum earthquake is 

about magnitude 71A If the fault is segmented, the most 

likely maximum earthquakes on the Logan and Richmond 

segments are about magnitude 71A and 7, respectively 

(table 1).

Earthquake Recurrence Rate

The slip rate and recurrence data for the East Cache 

fault are not well constrained. Two surface-faulting events

that postdate the recession of Lake Bonneville from the 

Bonneville shoreline 14,000 to 15,000 years ago have been 

identified near the mouth of Logan Canyon (Swan and 

others, 1983). These events suggest an average recurrence 

rate of about 7,250 (±250) years for maximum events. The 

second-most-recent event occurred after 15,000 yr B.P. 

and before formation of the Provo delta about 13,500 

years ago. The age of the most recent post Provo displacement is not well constrained, so the actual interval 

between these events is not known.

At the mouth of Logan Canyon, the cumulative displacement of the Bonneville shoreline deposits is about 

2.8 m, and the displacement of the Provo delta is 1.4 m, 

suggesting average slip rates of about 0.2 and 0.1 mm/yr, 

respectively. These rates are about 4 to 13 time slower 

that the rates reported for the Wasatch fault zone between 

Brigham City and Nephi (table 3).

Slip rates ranging from 0.05 to 0.2 mm/yr were 

included in the source model for the East Cache fault 

(table 1); these rates reflect possible bias in the single 

point estimate as well as the facts that the slip rates are 

calculated on the basis of only two slip events and that 

the timing of the next event could have a significant 

impact on the calculated slip rates. Most of the weight is 

assigned to values close to the rates observed near Logan. 

The average slip rate is assumed to be the same on both 

segments of the East Cache fault. On the basis of the seismic-hazard analysis, the average recurrence interval for 

earthquakes of magnitude 7 or greater on the whole East 

Cache fault zone is 7,500 (±2,700) years.

WEST VALLEY FAULT ZONE

The West Valley fault zone, originally called the Jordan Valley fault zone and subsequently renamed by 

Keaton and others (1986), consists of a series of mostly 

east-dipping normal faults that displace late Quaternary 

lake deposits in Salt Lake Valley. West-facing scarps form 

graben locally. As mapped by Keaton and others (1986), 

the total length of the zone is about 18 km. The fault zone 

is within about 5 km of downtown Salt Lake City (figs. 1, 

13).

Activity

The relation of the West Valley fault zone to the 

Wasatch fault zone is not clear. The short length relative 

to the height of the fault scarps, the discontinuous nature 

of the fault, and its close proximity to the Wasatch fault 

indicate that the West Valley fault zone may represent 

sympathetic faulting caused by large earthquakes on the 

Wasatch fault zone. Keaton and others (1986) argued that 

the size of the fault scarps, the location of earthquake epi-
M32 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

centers in the central part of Salt Lake Valley, and the evidence for repeated Holocene displacements strongly 

suggest that the West Valley fault zone is a seismogenic 

structure that operates independently of the Wasatch 

fault zone. Both of these interpretations are included in 

the analysis with equal probability.

Maximum Earthquake Magnitude

Given the fact that the West Valley is an independently active source, it is modeled as a single fault segment because its overall length is so short. The maximum 

rupture length is assumed to be equal to the total length of 

the zone (18km). The probability distribution for the 

maximum earthquake magnitude on the West Valley fault 

was calculated on the basis of a rupture length of 18 km. 

The range of downdip geometries considered included 

the effect of truncation of the fault by the west-dipping 

Wasatch fault. The maximum earthquake magnitude probability distribution is presented in table 1. The available 

data indicate that the most likely maximum magnitude on 

the West Valley fault zone is about §Vz if the fault is an 

independent seismogenic structure.

Earthquake Recurrence Rate

The earthquake recurrence on the West Valley fault 

zone is estimated on the basis of Holocene slip-rate data 

from Keaton and others (1986). The ranges of values that 

they calculated for different post-Lake Bonneville marker 

horizons (12,000 years old and younger) indicate that the 

Holocene slip rate is likely to be in the range of 0.3 to 0.5 

mm/yr. The actual slip rate is judged equally likely to be 

anywhere within this range; therefore, both values are 

assigned equal weights (table 1). If it is assumed that the 

West Valley fault zone is independently generating earthquakes, these values yield a repeat time for earthquakes 

of magnitude 6M: or greater of about 8,000 years. This 

value is within the range of Keaton and others' preliminary estimates for the recurrence of surface-faulting 

events based on their displacement data.

HANSEL VALLEY FAULT ZONE

Hansel Valley was the source of the March 12, 1934, 

Hansel Valley earthquake (M L 6.6). Surface-fault rupture 

occurring along the western side of Hansel Valley for a 

distance of about 11 km produced a series of scarps up to 

half a meter high in an area of older Holocene fault scarps 

(Anderson and Miller, 1979). Slemmons (1977) reported a 

maximum displacement of 1.2 m for this event. The zone 

of Quaternary faulting is inferred to extend northward 

along the western flank of the Hansel Mountains (Anderson and Miller, 1979), this inference suggesting a total

fault length of about 25 km. The activity of the northern 

part of the zone is not known.

Segmentation

Despite the short length of the fault, both an unsegmented and a segmented model are included in the source 

model for the Hansel Valley fault because of the apparent 

differences in the Quaternary displacements along the 

fault. Nonetheless, because of the short overall length of 

the fault, it seems less likely that the segmented model 

would define the maximum earthquake. Accordingly, the 

unsegmented model is assigned a slightly higher weight 

(0.6) than the segmented model (0.4) (table 1).

Activity

If the fault is unsegmented, then the entire length of 

the fault is assumed to be active. In the segmented model, 

the fault is divided into two segments. The southern segment coincides with the 1934 surface rupture, and the 

northern segment consists of the 14-km-long inferred 

extension to the north. The segment boundary coincides 

with a marked change in the character of the range and a 

large saddle along the crest of the range. The northern 

segment is assumed to have a 50-50 chance of being active 

(table 1).

Maximum Earthquake Magnitude

Given the unsegmented model, it is assumed that the 

entire 25-km-long fault could rupture during the maximum earthquake; given the segmented model, the maximum rupture lengths are assumed to be equal to the 

segment lengths (table 1). Table 1 shows the discretized 

distribution resulting from the maximum earthquake magnitude logic tree. Given the unsegmented model, the 

expected maximum magnitude is about 6%. Given the 

segmented model, the expected value is about magnitude 

&/z for both segments. In this case, the 1934 Hansel Valley 

earthquake would represent a maximum event

Earthquake Recurrence Rate

For the unsegmented fault model and for the segmented model for the North Hansel Mountains segment of 

the zone, the rate of earthquake recurrence is estimated 

on the basis of slip rate. The rate on the South Hansel 

Mountains segment also incorporates the results of 

McCalpin's (1985) study of the Quaternary history of faulting in the Hansel Valley area.

Slip-rate data on the Hansel Valley fault are not well 

constrained. McCalpin (1985) reported that the probable 

tectonic throw during the past 26,000 years ranges from
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M33

1.2 to 2.5 m. This suggests a slip rate of from 0.05 mm/yr 

to 0.1 mm/yr. These values were used to characterize the 

average slip rate on the Hansel Mountains segment and 

for the entire fault for the unsegmented model (table 1). 

The values were assigned equal weight.

McCalpin inferred that there have been three surfacefaulting events in Hansel Valley during about the past 

26,000 years. This estimate suggests an average recurrence interval of about 8,700 years. He reported that the 

most recent event prior to the 1934 earthquake occurred 

about 12,000 years ago, indicating an interval of about 

12,000 years between events. These two estimates of the 

recurrence rate were assigned equal weight (table 1).

Given the segmented model, there are no available 

data to constrain the timing of past events or the slip rate 

on the North Hansel Mountains segment. The lack of evidence for late Quaternary faulting and the subdued geomorphic expression of the fault indicate that the slip rate 

on the North Hansel Mountains segment is comparable to 

or less than the slip rate on the South Hansel Mountains 

segment. Slip rates ranging from 0.01 to 0.1 mm/yr were 

assigned for the North Hansel Mountains segment. The 

recurrence interval for earthquakes of magnitude &/2 or 

greater on the Hansel Valley fault zone is 11,000 (±5,000) 

years.

EAST BEAR LAKE FAULT

The East Bear Lake fault is approximately 65 to 75 

km east of the Wasatch Front (figs. 1, 13). It is an approximately 50-km-long westward-dipping normal fault that 

lies along the eastern side of Bear Lake and Bear Lake Valley, separating the valley to the west from the Bear Lake 

Plateau and the southern part of the Aspen Range to the 

east.

Segmentation

Both unsegmented and segmented fault-rupture models are included in the source model for the East Bear 

Lake fault. Preliminary reconnaissance of the East Bear 

Lake fault by the USGS suggested that the late Quaternary 

history of faulting is different for the northern and southern parts of the fault (A.J. Crone and M.N. Machette, 1986, 

personal commun., 1986). North of Bear Lake, the fault 

appears to have undergone very recent displacement. 

Where the fault crosses the alluvial terraces along Bear 

River, the fault scarps appear to displace all but the 

youngest meander scars, suggesting late Holocene faulting. To the south, no evidence of Holocene faulting has 

been observed. Wave-cut scarps associated with higher 

middle Holocene(?) lake levels postdate the most recent 

faulting. The youngest deposit, displaced by the fault

where it crosses North Eden Canyon and South Eden 

Canyon, is estimated by A.J. Crone (personal commun., 

1986) to be late Pleistocene in age (approximately 15,000 

years old). It is possible that the recent faulting observed 

to the north also occurs to the south on a fault trace that 

might be beneath the lake west of the mapped trace of the 

fault. Accordingly, two models are considered: an unsegmented model that treats the fault as a single 50-km-long 

fault and a segmented model that treats the 23-km-long 

Montpelier segment to the north and the 27-km-long Bear 

Lake segment to the south as independent sources characterized by different fault behaviors. The available data 

suggest that the fault is segmented, so the unsegmented 

model is assigned a weight of only 0.2 (tables 2, 3).

Maximum Earthquake Magnitude

Except for the lack of data on the amount of displacement per event, the approach used to estimate the 

probability distribution for the maximum earthquake 

magnitude is the same as that used for the Wasatch fault 

zone. If the East Bear Lake fault is unsegmented, two 

maximum rupture lengths are considered. Because there 

have been historical surface-fault ruptures in the Basin 

and Range as long as 61 km, 100 percent rupture of the 50- 

km-long East Bear Lake fault is considered as a possible 

maximum rupture length. Twenty-seven kilometers, 

which is the length of the longest segment and is approximately equivalent to 50 percent of the mapped length of 

the fault, is considered to be a slightly more likely maximum rupture length (table 2). Regardless of the segmentation model, the most likely maximum earthquake on the 

East Bear Lake fault is 6?4 to 7 (table 1).

Earthquake Recurrence Rate

The available data indicate a long interval between 

surface-faulting events on the East Bear Lake fault. The 

rate of activity is probably less than the rate of activity on 

the East Cache fault. On the basis of their limited reconnaissance investigations of the fault, M.N. Machette and 

A.J. Crone (personal commun., 1987) estimated that there 

has been only one surface-faulting event on the Montpelier segment during approximately the past 8,000 years. 

The freshness of the fault scarp indicates that this event, 

which had a displacement of about 2 m, probably 

occurred fairly recently. There do not appear to have been 

any faulting events on the Bear Lake segment for about 

the past 15,000 years. Despite the poor constraints on the 

recurrence interval for the East Bear Lake fault, the available evidence clearly indicates that the fault is significantly less active than the Wasatch fault. Intervals of 8,000 

and 15,000 years are assumed to characterize the recur-
M34 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

rence of surface-faulting events on the Montpelier and 

Bear Lake segments, respectively, and the slip rate is 

assumed to be on the order of 0.05 to 0.1 mm/yr (table 1). 

The seismic-hazard model gives a return period for maximum events of 7,200 (±2,000) years for the fault zone.

COMPARISON OF PREDICTED AND OBSERVED 

EARTHQUAKE FREQUENCY

The hazard model developed above can be used to 

predict regional earthquake recurrence rates. Figure 15 

compares predicted earthquake cumulative frequencies 

with the observed seismicity rate. Figure 15A shows the 

mean recurrence estimates for individual sources and the 

sum of the recurrence for all sources. The recurrence estimates for the West Valley fault and the Oquirrh Mountains 

fault zone have been weighted by the probabilities of 

activity assigned to the various segments. Figure 155 

compares the fractiles of the distribution for earthquake 

recurrence obtained from the distribution in source 

parameters with the one-standard-deviation error bars on 

the regional seismicity rate. As can be seen, the sum of 

the predicted rates for all sources yields an estimated rate 

consistent with the historical record. The wider distribution in the estimated frequency of events near magnitude 

6 results from uncertainty in the appropriate form of the 

magnitude distribution. The Wasatch fault zone represents the dominate source of potential future large-magnitude events in the region.

ATTENUATION RELATIONSHIPS

Attenuation relationships define the values of a 

ground-motion parameter, such as peak ground acceleration, velocity, displacement, or response spectral values, 

as a function of earthquake magnitude and distance. For 

this study, attenuation relationships for peak ground 

acceleration and response spectral accelerations are 

required as inputs to the probabilistic ground-motion 

analysis.

Attenuation relationships in the United States that 

are well constrained by strong-motion data have been 

developed on the basis of statistical analyses of data that 

are largely from California. To date, only one strongmotion recording has been obtained in Utah in Logan at 

a distance of about 30 km from the 1962 M L 5.7 Cache Valley earthquake. During the 1983 M s 7.3 Borah Peak, Idaho, 

earthquake, several recordings were obtained in Idaho at 

distances of about 90 to 100 km. Aftershocks of the Borah 

Peak earthquake produced recordings at closer distances. 

The appendix to this paper compares ground-motion data 

recorded during these earthquakes with recently developed attenuation relationships that are based largely on 

data from California. From the comparisons presented in

the appendix, it is concluded that the Cache Valley and 

the Borah Peak data exhibit the degree of scatter relative 

to the attenuation curves that is typically observed in 

strong-motion data. The data are too limited to either 

strongly support or refute the applicability of California 

attenuation relationships to Utah.

In general, the earthquake ground motion recorded 

at a site can be viewed as the product of the earthquake 

source characteristics, the attenuation characteristics 

along the travel path between the earthquake source and 

the site, and the local site effects. Each of these factors 

may influence the attenuation relationships appropriate 

for a given region. Evidence regarding similarities or differences between these factors for Utah and California 

are briefly reviewed below.

EARTHQUAKE SOURCE CHARACTERISTICS

McGarr (1982, 1984) has suggested that normal faulting, the type of faulting expected for major earthquakes 

along the Wasatch front, produces weaker ground motion 

than strike-slip faulting or reverse faulting for a given size 

event. Because California-type attenuation relationships 

are based on strong-motion data predominantly from 

strike-slip and reverse faulting earthquakes, McGarr's 

studies suggest that the normal faulting environment 

might result in lower ground motion in Utah than in California. On the other hand, studies reported by Campbell 

(this volume) indicated only minor differences in groundmotion attenuation between extensional and compressive 

stress environments within California. Campbell (this volume) also found significantly stronger ground motion for 

dip-slip faulting than for vertical strike-slip faulting. However, his results for dip-slip faulting are based on data 

taken almost entirely from reverse- and thrust-faulting 

earthquakes. The most comprehensive study to date of 

ground motion recorded during normal-faulting earthquakes was done by Westaway and Smith (1989). They 

evaluated peak ground accelerations recorded during 

normal-faulting earthquakes in the Western United States, 

Italy, Greece, Turkey, and New Zealand and concluded 

that such earthquakes result in peak ground accelerations 

equivalent to those seen in reverse-faulting and strike-slip 

earthquakes. Their studies found no evidence that weaker 

ground motions result from normal-faulting earthquakes.

SOURCE-TO-SITE ATTENUATION CHARACTERISTICS

Several studies have addressed regional attenuation 

characteristics in Utah and in adjacent and broader 

regions of the Western United States. King and Hays 

(1977) analyzed velocity seismograph data in northern 

Utah from four aftershocks (ML 2.9-4.1) of the 1975 Pocatello Valley, Idaho, earthquake. They concluded that
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M35

higher frequency (greater than 1 Hz) response spectral 

values in northern Utah attenuated at a faster rate than 

those in California.

Griscom and Arabasz (1979) examined regional 

attenuation in Utah from a study of Wood-Anderson 

seismograph data. They concluded that the rate of attenuation in Utah might be greater than the rate of attenuation 

in southern California. Griscom (1980) analyzed WoodAnderson seismograph data for Utah and also analyzed 

the attenuation of Modified Mercalli intensities for five 

Utah earthquakes. On the basis of both these analyses, 

she concluded that attenuation in Utah may be slightly 

greater than that in coastal California.

McGuire (1983) analyzed the attenuation of Modified 

Mercalli intensities for five northern Utah earthquakes 

and four other earthquakes from the Rocky Mountain 

region. He combined results of his analyses of intensity 

attenuation with ground motion versus intensity relationships (the latter relationships based largely on California 

strong-motion data) to derive ground motion attenuation 

relationships for northern Utah. By comparing his derived 

expression for velocity attenuation with the attenuation 

relationships for Wood-Anderson seismograph standard 

amplitudes for California, McGuire concluded that the 

rate of velocity attenuation in Utah could be slightly lower 

than that in California, but that, given the uncertainties in 

the analysis, the differences were not resolvable.

Several studies have examined Modified Mercalli 

intensity attenuation in broad regions of the United 

States. As they pertain to evaluating attenuation along the 

Wasatch Front in northern Utah, all of these studies have 

the limitation that the regions considered are much larger 

than the specific region of the Wasatch Front. The studies 

by Evernden (1975), Howell and Schultz (1975), Chandra 

(1979), and Brazee (1980) all indicate that the rate of 

intensity attenuation in the western mountain States is 

somewhat lower than that in California.

Recent studies have also been conducted of the 

intrinsic attenuation properties of the crust in different 

regions of the United States. Singh and Herrmann's (1983) 

regionalization of Q throughout the United States indicates that the rate of attenuation in Utah is lower than 

that in California. Chavez and Priestly (1985) found that 

the rate of attenuation at the Wood-Anderson instrument 

frequency (approximately 1.25 Hz) in Nevada was greater 

than that in southern California and equal to or slightly 

less than that in central California. On the other hand, 

Rogers and others (1987) concluded that the rate of attenuation in the southern Great Basin of Nevada and California was lower than that in central and southern 

California. Although these studies address intrinsic 

crustal attenuation over broad regions of the country, 

including Utah and other parts of the Great Basin, they

have the limitation of not specifically addressing attenuation in Utah.

The available evidence summarized above is equivocal. The studies that have focused specifically on Utah 

(King and Hays, 1977; Griscom and Arabasz, 1979; 

Griscom, 1980; McGuire, 1983) suggest that the rate of 

attenuation in Utah is comparable to or perhaps greater 

than the rate of attenuation in California. On the other 

hand, some broad regional studies suggest that the rate of 

attenuation in Utah may be less than that in California

LOCAL SITE EFFECTS

Local site conditions can also influence attenuation 

relationships. With the exception of areas underlain by 

lake deposits, the subsurface soil and rock conditions 

along the Wasatch Front are considered to be comparable 

to the range of conditions in California where ground 

motions have been recorded. Therefore, except for areas 

underlain by lake deposits, site effects are not expected 

to lead to significant differences between attenuation 

relationships for Utah and those for California.

Shear-wave velocities of sediments for many locations in Salt Lake Valley along the Wasatch Front have 

been reported by Tinsley and others (1991). At several of 

these locations, the lake deposits exhibit shear-wave 

velocities that are lower than those of typical alluvial sites 

in California where ground motions have been recorded. 

At such locations on the lake deposits, it may be anticipated that low-amplitude peak ground accelerations will 

be amplified relative to peak accelerations on firm soils 

and rock; however, high-amplitude peak accelerations 

may experience little or no amplification or might even 

experience deamplification owing to nonlinear soil 

response (Seed and others, 1976a; Idriss, 1991). Studies 

by Idriss (1990) suggest that the demarcation between 

amplification and deamplification of peak ground acceleration may be defined by a peak rock acceleration of about 

0.4 g for earthquakes of magnitude approximately equal 

to 7.

Hays and King (1982, 1984a, b), King and others 

(1983), and Rogers and others (1984) have investigated 

site response effects on response spectral values along 

the Wasatch Front during very small amplitude motions 

produced mainly by distant underground nuclear blasts at 

the Nevada Test Site. Their studies indicate large spectral 

values for ground motions on the lake deposits relative to 

rock or to shallower, coarser grained soils along the 

Wasatch Front. Also, comparison of their results with similar data for younger soil deposits in the Los Angeles 

region of California (Rogers and others, 1985) indicates 

that soil-to-rock spectral ratios for the lake deposits are 

significantly larger than those for the California soil 

deposits. The results of studies by Hays and King (1982,
M36 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

1984a, b), King and others (1983), and Rogers and others 

(1984) provide an excellent guide to the relative soil 

amplification hazard of different geologic conditions 

along the Wasatch Front during future earthquakes. At 

present, it is difficult to use these results to quantify absolute levels of response spectral attenuation relationships 

for the lake deposits for strong ground-shaking conditions 

because of potentially significant differences in the values 

of the spectral ratios for strong earthquake shaking as 

compared with those for very small amplitude shaking. 

The potential differences arise from nonlinear soil 

response effects and possibly from differences in the 

duration and frequency content of motions from local 

earthquakes and distant nuclear blasts.

Another factor that could influence ground motion 

along the Wasatch Front is the valley subsurface geometry (two- and three-dimensional wave-propagation 

effects). Benz and Smith (1988) conducted two-dimensional simulations of wave propagation and site amplification due to normal-faulting earthquakes in the Salt Lake 

Valley. Their studies indicate that the basin geometry 

could significantly influence ground motions. However, 

basin effects on attenuation relationships have not been 

quantified to date.

On the basis of presently available data and the evaluations discussed earlier, it is reasonable to assume that 

California attenuation relationships are applicable to 

areas along the Wasatch Front underlain by rock and 

stiffer, denser soils such as predominantly sandy and 

gravelly fluvial and deltaic deposits. These relationships 

are probably not applicable to areas of soft lake clays and 

silts, but alternative relationships are not available at 

present. The possible influence of soft soil conditions on 

response spectra obtained from the analysis is discussed 

later.

The attenuation relationships selected for the basic 

analyses are summarized below. Also summarized is a 

modified peak acceleration attenuation relationship that 

exhibits a lower rate of attenuation than the relationships 

used in the basic analyses. This relationship was developed and used to test the sensitivity of the results to a rate 

of crustal intrinsic attenuation that may possibly be lower 

in Utah than it is in California.

SELECTED ATTENUATION RELATIONSHIPS 

FOR PEAK GROUND ACCELERATION

Two sets of peak acceleration attenuation relationships have been used in the basic analyses: one set for 

firm soil-site conditions and the other for rock-site conditions. The soil-site set of relationships includes the relationships of Joyner and Boore (1982), Sadigh and others 

(1986), and Campbell (this volume). The modified relationship, which is consistent in this respect with the relationships of Sadigh and others (1986) and Campbell (this 

volume), was used. Sadigh and others' (1986) relationship 

is summarized in table 4. Campbell's (this volume) relationship incorporates an anelastic attenuation term; his 

value of anelastic attenuation for California was used. 

Sadigh and others (1986) and Campbell (this volume) distinguished the effect of the type of faulting (whether 

strike slip or reverse) in their attenuation relationships. 

Their relationships for strike-slip faulting, which give 

lower peak accelerations than those for reverse faulting, 

were used.

McGuire's (1983) attenuation relationships were not 

used in the analysis because of the large uncertainties 

involved in deriving peak acceleration attenuation relationships from intensity data McGuire's relationships 

show about the same rate of attenuation as those selected 

for use and differ significantly in amplitude (give higher 

amplitudes) only for events of magnitude less than about 

ML 6.

The median attenuation curves for the selected relationships are compared in figure 16 for earthquake magnitudes of 5.5, 6.5, and 7.5. The relationships were 

developed by using magnitude definitions that are equivalent to ML magnitude in the range of 3 to 7 and M g magnitude in the range of 5 to 7.5. As discussed earlier, it is 

assumed that the relationship between strong ground 

motion and ML and Mg magnitudes in Utah is similar to 

what has been observed in California.

Distance definitions for the attenuation relationships 

differ. For purposes of the comparisons shown in figure 

16, distance is as defined by each investigator (closest distance to fault-rupture surface for Sadigh and others 

(1986), closest distance to fault-rupture surface in the 

seismogenic zone for Campbell (this volume), and closest 

horizontal distance to the vertical projection of fault rupture to the ground surface for Joyner and Boore (1982). In 

using Campbell's (this volume) relationship, a depth to 

the seismogenic zone of 2 km has been assumed. Therefore, in figure 16, acceleration values are not plotted for 

distances less than 2 km for Campbell's relationship. In 

the probabilistic seismic-hazard analysis, distances used 

in each case are consistent with the definitions for the 

respective relationships.

The rock-site set of relationships selected for the 

analysis includes two of the above-mentioned relationships Joyner and Boore (1982) and Campbell (this volume) and two additional relationships those of Seed 

and Schnabel (1980) and Sadigh and others (1986) (the 

latter relationship is summarized in table 4). The relationships of Seed and Schnabel (1980) and Sadigh and others 

(1986) shown in figure 16 were specifically developed for 

rock-site conditions. The relationship of Joyner and 

Boore (1982) does not distinguish between peak ground
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M37

TABLE 4. Values of coefficients for the attenuation relationships ofSadigh and others (1986) for horizontal peak ground acceleration and

pseudo-absolute spectral accelerations

[Coefficients &i through 67 are for use in the relationship ln?/+&i+&2M=&3(8.5-M) 4+65 hi(E+&6e7^0i where Mis moment magnitude. Gy is the standard error of estimate of the relationship. The relationships are those derived for strike-slip-faulting earthquakes]

Ground motion parameter y Period (s)

61

Coefficient

&2 &3 64 &5 &6 &7 by

Soil-site conditions

Peak ground acceleration

Pseudo-absolute spectral acceleration (5-percent damping)

-0.03

.1

.2

.3

.5

1.0

2.0

4.0

-2.611

-2.024

-1.696

-1.638

-1.659

-1.975

-2.414

-3.068

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

0

.007

0

-.008

-.025

-.060

-.105

-.160

2.5

2.5

2.5

2.5

2.5

2.5

2.5

2.5

-1.75

-1.75

-1.75

-1.75

-1.75

-1.75

-1.75

-1.75

0.8217 1

.3152

.8217 1

.3152

.82171

.31572

.82171

.31572

.8217 1

.31572

.82171

.31572

.82171

.31572

.82171

.31572

0.4814 1

.6282

.4814 1

.62862

.4814 1

.62862

.48141

.62862

.48141

.62862

.48141

.62862

.4814 x

.62862

.4814 1

.62862

1.26 0-14M1

.352

1.332 -1.48M1

.372

1.453-.162M1

.402

1.486-.164M1

.422

1.584-.176M1

.442

1.62-.18M1

.452

1.62-.18M1

.452

1.62-.18M1

.452

Rock-site conditions

Peak ground acceleration

Pseudo-absolute spectral acceleration (5-percent damping)

-0.03

.1

.2

.3

.5

1.0

2.0

4.0

-1.406

-.688

-.479

-.543

-.793

-1.376

-2.142

-3.177

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

0

.007

-.008

-.018

-.036

-.065

-.100

-.150

2.5

2.5

2.5

2.5

2.5

2.5

2.5

2.5

-2.05

-2.05

-2.05

-2.05

-2.05

-2.05

-2.05

-2.05

1.353 1

.5792

1.353 x

.5792

1.353 l

.5792

1.353 1

.5792

1.353 1

.5792

1.353 l

.5792

1.353 1

.5792

1.353 x

.5792

0.406 x

.5372

.406 1

 5372

.4061

.5372 .

.406 1

.5372

.4061

.5372

.4061

.5372

.406 1

.5372

.406 1

.5372

1.26-0. UM1

.352

1.332-.148M1

.372

1.453-162M1

402

1.486-.164M1

>422

1.584-.176M1

.442

1.62-.18M1

.452

1.62-.18M1

.452

1.62-.18M1

.452

^Magnitude <6.5. 

2Magnitude >6.5.
M38 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

0.5

Ok z" 

o

0.2

0.1

0.05

0.02

0.01

EXPLANATION

 - Sadigh and others (1986)

 Campbell (this volume)

 - Joyner and Boore (1982)

B

EXPLANATION 

Sadigh and others (1986) 

Seed and Schnabel (1980)

10 20 50 100 1 2

DISTANCE, IN KILOMETERS

10 20 50 100

FIGURE 16. Peak horizontal acceleration attenuation relationships used in seismic hazard analysis. A, Soil. B, Rock.

accelerations on soil and those on rock. The relationship 

of Campbell (this volume) uses data from soil sites only; 

however, Campbell (this volume) indicated that the 

ground-motion estimates obtained therefrom are probably also appropriate for rock, at least for source-to-site 

distances of about 50 km or less. K.W. Campbell (personal 

commun., 1987) indicated that predicting peak accelerations on rock and on soil may be little different at small 

distances but that, at large distances, predictions on rock 

may be substantially lower than those on soil, an 

indication that using his relationship for rock-site conditions may be conservative. Overall, the four rock-site relationships used in the analysis describe median peak 

accelerations averaging about 15 percent higher than 

those of the three soil-site relationships within a distance 

of 50 km, decreasing to about 5 percent higher at a distance of 200 km.

The standard deviations in the predicted accelerations recommended by the developers of each attenuation 

relationship are incorporated into the seismic-hazard 

analysis through the evaluation of the term P(Z>z\m, r) in 

equation 2. The standard deviations differ significantly 

among the three relationships, being lowest for the relationship of Campbell (this volume) and highest for that of 

Joyner and Boore (1982). Seed and Schnabel (1980) did 

not report a standard deviation for their attenuation 

curves, so the standard deviations defined by Sadigh and 

others (1986) were used for their curves.

Separate sets of probabilistic ground-motion analyses were made by using the soil-site set and the rock-site 

set of attenuation relationships described above. In each 

set of analyses, the different attenuation relationships 

were given equal weight in the logic tree formulation (fig. 

2).
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M39

0.5

0.2

cc

3 0.1

0.05

0.02

0.01

EXPLANATION

Sadigh and others (1986) 

Sadigh and others (1986) 

modified for lower 

rate of attenuation

5 10 20 

DISTANCE, IN KILOMETERS

50 100

FIGURE 17. Peak horizontal acceleration relationships for a magnitude 6.5 

event modified for slower rate of attenuation.

For the sensitivity analyses conducted for a lesser 

rate of attenuation, the following equation was derived to 

modify the attenuation relationships used in the basic 

analyses:

(3)

where as is the peak acceleration adjusted for the lesser 

degree of attenuation used in the sensitivity analysis, a is 

the peak acceleration defined by each attenuation relationship used in the basic analyses, and R is source-to-site 

distance in kilometers. The effect of this modifying relationship on attenuation is illustrated in figure 17 for an 

attenuation curve for magnitude 6.5 constructed from the 

relationship of Sadigh and others (1986).

Equation 3 was derived in the following manner. 

Owing to differences in anelastic attenuation between 

regions, the ratio of peak acceleration in a region having a 

slower rate of attenuation to peak acceleration in a region 

having a faster rate of attenuation can be expressed as

(4)

where y/ and ys are the coefficients of anelastic attenuation for the regions of faster and slower attenuation, 

respectively. Campbell (1982) evaluated the values of the 

coefficient of anelastic attenuation for California and 

Utah on the basis of the regionalization of crustal Q in the 

United States by Singh (1981) (see also Singh and Herrmann, 1983) and the recommendations for predominant 

periods of ground motion developed by Seed and others 

(1969). The values of y derived by Campbell (1982) for 

California are higher than those for Utah. Equation 3 is a 

close approximation to equation 4 if Campbell's (1982) 

results over the magnitude range 5.5 to 7.5 and the distance range 0 to 100 km are used.

Rogers and others (1986) estimated Q in the southern 

Great Basin to be nearly frequency independent and in the 

range of about 700 to 900. The corresponding value of y 

can be evaluated from the expression y=nf/vQ, where/is 

frequency and v is the seismic-wave velocity. Taking v=3.5 

km/s, as Rogers and others (1986) assumed, and Q=800 

leads to the following approximate expression for y:

y =0.0011/ (5)

If it is assumed that Rogers and others' (1987) results are 

applicable to Utah, then the ratio of acceleration between 

Utah and California can be obtained by evaluating y for 

Utah as a function of frequency by using equation 5, taking y for California equal to 0.0059 (incorporated in Campbell's (1982) acceleration attenuation relationship), and 

applying equation 4. The results of this analysis are that, 

for frequencies equal to or higher than about 5 Hz, y 

Utah>y California. For frequencies in the range of about 1 

Hz to 5 Hz, y Utah<y California, but the effect of a lower y 

for Utah would result in modifications to the basic attenuation relationships that are less than what is defined by 

equation 3. Within source-to-site distances of 50 km, it is 

likely that the frequencies associated with peak accelerations exceed 1 Hz, an indication that equation 3 would 

provide a conservative assessment of possible modifications to the basic attenuation relationships for peak acceleration. However, for ground motions occurring at low 

frequencies (such as response spectral values at frequencies less than 1 Hz), Rogers and others' (1987) results, if 

applicable to Utah, would indicate an effect on the attenuation rate greater than that defined by equation 3.

SELECTED ATTENUATION RELATIONSHIPS 

FOR RESPONSE SPECTRAL VALUES

Sadigh and others (1986) and Joyner and Boore 

(1982) have developed attenuation relationships for 

response spectral values at 5-percent damping for a num-
M40 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

0.5

0.2

0.1

0.05

0.02

0.01

0.005

EXPLANATION 

7=0.2 s

 Sadigh and others (1986)

 Joyner and Boore (1982)

EXPLANATION 

7=2s

 Sadigh and others (1986)

 Joyner and Boore (1982)

10 20 50 100 1 2

DISTANCE, IN KILOMETERS

10 20 50 100

FIGURE 18. Typical response spectral acceleration attenuation relationships (5-percent damping) for soil-site conditions used in seismic-hazard analysis.

Periods of vibration (7) are 0.2 and 2 s.

ber of periods of vibration, for both soil- and rock-site 

conditions. Sadigh and others' (1986) relationships are 

summarized in table 4. Joyner and Boore's (1982) relationships have also been summarized by Joyner and Fumal 

(1985). The relationships of Sadigh and others (1986) and 

Joyner and Boore (1982), equally weighted, were used in 

separate sets of analyses for soil- and rock-site conditions 

in the study. Examples of the attenuation curves defined 

by both relationships for soil-site conditions are illustrated in figure 18 for periods of vibration of 0.2 and 2 s.

ANALYSIS RESULTS AND SENSITIVITY

COMPUTED HAZARD

The seismic hazard was mapped by performing hazard computations at 80 points located on an irregular grid 

throughout the study region defined in figure 1. For hazard computations, the individual faults were modeled as 

segmented planar surfaces, and the background zone was 

modeled as a set of closely spaced parallel fault planes 

occupying the background source region outlined in figures 3 and 4. The probability density function for distance 

to earthquake rupture for each source was computed on 

the assumption that earthquake ruptures were uniformly 

distributed horizontally along the fault and that the focal 

depths were distributed according to the observed depth 

distribution for the region independent of magnitude (see 

fig. 7). The distance density functions were computed 

consistent with the distance measure used in each of the 

attenuation relationships. The rupture size of an event 

was specified by the relationship ln(rupture area) 

=2.146M-8.139, developed from data presented by Wyss 

(1979). This relationship gives the mean rupture area for a 

specific magnitude rather than the median (mean log) 

rupture area Studies by Bender (1984) have shown that 

using mean estimates of rupture size in the computation 

of hazard yields results nearly equal to those obtained 

when the statistical uncertainty in the size of individual 

ruptures is incorporated in the analysis.

The hazard was computed by considering the contributions of earthquakes of magnitude M L 5 and larger. 

Engineering experience has shown that, even though relatively high peak accelerations have been recorded for
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS 

Salt Lake City Brigham City Logan

M41

0.050 

- (lOOyrRP)

0.050 

- (lOOyrRP)

0.2

3 0.1

< 

m

0.2

0.1

0.70 

- (2500 yrRP)

0.50 

- (2500yrRP)

0.40 

- (2500yrRP)

10 10 10 10 2 10 MO 5 10* 10 3 10 2 10 1 10 5 

EXCEEDANCE FREQUENCY

10

FIGURE 19. Computed distributions for frequency of exceeding various levels of peak ground acceleration on soil at three locations. The top row of plots are for 

ground-motion levels having a mean frequency of exceedance corresponding to a return period (RP) of approximately 100 years, the middle row to a return 

period of 500 years, and the bottom row to a return period of 2,500 years.

earthquakes smaller than magnitude 5, events of this size 

generally have not caused structural damage to engineered buildings. Certain facilities may be sensitive to the 

effects of high-frequency motions from smaller magnitude 

events, so sensitivity analyses were made in which the 

contributions to hazard from earthquakes in the magnitude 4 to 5 range were included.

At each point in the study region, distributions for 

the annual frequency of exceeding various levels of peak 

ground acceleration were developed by performing hazard computations using equation 2, the input parameters 

being defined by each end branch of the logic tree shown 

in figure 2. At each acceleration level, the complete set of 

computed results forms a discrete distribution for frequency of exceedance. Figure 19 shows the computed distributions of frequency of exceedance at three locations 

for a range of peak accelerations on soil sites. The 

selected acceleration levels correspond to the hazard levels used for the regional hazard mapping discussed below, 

and the identified return periods represent the mean frequency of exceedance for the distributions shown. The 

results shown are representative of the range in the computed hazard obtained for the study area At all locations, 

the computed distributions span approximately one order 

of magnitude in frequency of exceedance. The factors 

contributing to the uncertainty in the computed frequencies are discussed later in this section.

The computed distributions were used to obtain the 

mean frequency of exceeding various levels of peak 

ground acceleration (mean hazard curve) as well as hazard curves representing various percentiles. Figure 20 

shows the resulting hazard curves for points at Salt Lake 

City, Brigham City, and Logan. These results are for peak 

horizontal acceleration on soil sites. As indicated in the 

figure, the distributions in frequency of exceedance are 

somewhat skewed about the mean value, the mean hazard curve being located at about the 60th percentile of the 

distributions. The 90-percent confidence interval (5th to 

95th percentile levels) in peak acceleration at a given frequency of exceedance typically represent a variation of
M42 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

10

10

10

V

V^xNc*%

B^A^^^X^X J

X^ XV >^X1 Logan Vv > vX85%\ ;*.>.

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6

PEAK ACCELERATION, g

0.8 0 0.2 0.4 0.6 0.8

FIGURE 20. Peak ground acceleration hazard curves on soil for three locations. Shown are the mean hazard curves and hazard curves corresponding to the 5th, 

15th, 50th, 85th, and 95th percentiles of the distributions for frequency of exceeding various levels of peak acceleration obtained at each site.

±50 percent about the level given by the mean hazard 

curve.

The computed hazard distributions represent our 

best judgment as to the uncertainty in defining the input 

parameters and thus represent our confidence in the estimated hazard. As tables 1 through 3 indicate, extensive 

distributions are used to model the input parameters in 

the hazard model logic tree, the result being several thousand end branches. Limited sensitivity analyses conducted to investigate the effect of including more detailed 

modeling of the individual parameter distributions 

showed no significant change in the computed fractiles. 

However, the more extreme fractiles should be considered to be less precisely defined than the central 

estimates.

The results shown in figure 20 yield similar levels of 

peak acceleration at the three sites for return periods on 

the order of 100 years but substantially different peak 

accelerations for longer return periods. The increased 

variation in acceleration level at longer return periods 

results from changes in the relative contributions of various earthquake sources to the hazard as a function of 

acceleration level and return period. Figure 21 shows the 

contributions of various sources to the hazard at six sites. 

The top row of plots presents the source contributions for 

the three sites illustrated in figure 20. At each of these

sites, the hazard at low acceleration levels is dominated 

by the background seismicity. Because the rate of occurrence of moderate-magnitude events is assumed to be 

uniform throughout the background zone of seismicity, 

similar hazard levels are obtained at all three sites. For 

longer return periods, the hazard is dominated by contributions from the identified faults, and their relative rates 

of seismic activity result in differences in the computed 

hazard, depending on proximity to the fault. This pattern 

was generally observed throughout the study area, as 

indicated by the results for additional sites shown in the 

bottom row of plots in figure 21.

The differences between the computed hazard at Salt 

Lake City and that at Brigham City reflect the modeled 

variations in the rate of activity along the length of the 

Wasatch fault zone. Figure 22 illustrates the variation in 

the computed hazard along the zone from Nephi to Collinston. As indicated in the figure, nearly identical results are 

obtained for sites along the central, active portion of the 

fault (lat 40°-41.5° N.). The hazard decreases slightly to 

the south as Nephi is approached and decreases significantly to the north beyond Brigham City, where geologic 

evidence indicates a substantial decrease in the rate of 

fault movement. The spatial variation of seismic hazard 

throughout the study region is mapped in terms of contours of peak ground acceleration on soil sites for three
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M43

10

10

10-"

10- 

10- 1

^ i ' r

I Salt Lake City

10

10- 3

10

^ r

I Magna

Brigham City

i\ . \ .

^ T 

'. Tooele

.

I Logan

% \ \\ 

'.. I \ \ *>

^ I 

I Collinston

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

PEAK ACCELERATION, g

EXPLANATION

 Total East Bear Lake

---- Wasatch East Cache

 Background West Valley

 Oquirrh Mountains

FIGURE 21. Contributions of various sources to the computed mean hazard for six locations. Shown for each location are the mean hazard curves for various

sources and the total mean hazard curve.

return periods. These hazard maps are presented later in 

figure 37.

Figure 23 illustrates the relative contributions of 

events in different magnitude and distance intervals to the

computed mean hazard. Each plot in the figure presents a 

histogram showing the percentage of contributions of 

events in 0.25-magnitude-wide intervals. The histograms 

are in turn divided into the contributions from distance
M44 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

10

UJ

O

I 10

10 '

^ I ' I

EXPLANATION

 -. -. Nephi

 Provo

 Salt Lake City

 Ogden

 Brigham City 

Collinston

0.2 0.4 0.6 

PEAK ACCELERATION, g

0.8

FIGURE 22. Variation in the computed mean hazard along the length of the 

Wasatch fault zone from Nephi to Collinston.

ranges of 0 to 10 km, 10 to 25 km, and greater than 25 km. 

As indicated by figure 23, at higher probabilities of 

exceedance (short return periods), the contributions to 

hazard come from events in a wide range of magnitudes 

and distances. As the probability of exceedance 

decreases (return period increases), the hazard becomes 

increasingly dominated by the larger and closer events. 

The magnitude and distance range of contributions also 

narrows as the more active portion of the Wasatch fault is 

approached, indicative of the increasing dominance of the 

fault contributions to the hazard as illustrated in figure 21.

CONTRIBUTIONS TO UNCERTAINTY

The distributions in the computed hazard shown in 

figures 19 and 20 represent the cumulative effect of all 

levels of parameter uncertainty included in the hazard 

model (fig. 2). The relative contributions of various components of the model to the overall uncertainty can be

identified readily from the logic tree formulation by computing the hazard and holding individual parameters fixed 

at specific values. For example, the contribution of uncertainty to the soil-site attenuation relationship can be 

obtained by computing the mean hazard under the 

assumption that each of the three attenuation relationships is, in turn, the "correct" relationship, one having a 

weight of 1.0 and the other two having zero weight. The 

resulting hazard curves are shown in figure 24 for three 

sites. In each plot, the solid curve corresponds to the 

mean hazard, and the heavy dashed curves correspond to 

the 5th and 95th percentiles of the distribution in exceedance frequency shown in figure 20. The three labeled 

curves are the resulting mean hazard for each of the soilsite attenuation relationships, and the differences 

between them represent the uncertainty in the computed 

hazard due to uncertainty in selecting the appropriate 

attenuation relationship.

The results shown in figure 24 indicate that uncertainty in selecting the appropriate attenuation relationship generally has less significance at sites having higher 

hazard near the more active portion of the Wasatch fault 

than it does at sites having lower hazard away from the 

fault The reason for the variability in the importance of 

the choice of attenuation relationship can be found by 

examining the contributions of events from various magnitude and distance ranges and the comparisons of the 

median attenuation relationships used in the analysis. At 

sites of lower hazard, such as Logan, there is a substantial 

contribution from events in the magnitude range of 5 to 6 

(fig. 23). The differences in the hazard computed by using 

the three attenuation relationships are greatest for this 

range of magnitudes and result from greater variability in 

the predicted median accelerations (fig. 16) enhanced by 

differences in the standard deviation about the median. 

For events at distances beyond 10 km, which have a large 

contribution to the hazard at Logan, Campbell's (this volume) relationship has the lowest median values and the 

lowest standard deviation (0.30), whereas Joyner and 

Boore's (1982) relationship has the highest median values 

and the highest standard deviation (0.62). The smallest 

variability between the hazard curves is obtained for a 

return period of 500 years at Salt Lake City (similar 

results were observed at other sites along the more active 

part of the Wasatch fault). The contributions to hazard are 

largely from events of magnitude 6 to 7 occurring at distances of 10 km or less. The best agreement between the 

median attenuation curves shown in figure 15 is for 

events of magnitude 6.5, and the differences in the median 

curves at distances of less than 10 km are partly compensated for by the differences in the standard deviation. In 

this magnitude and distance range, Campbell's relationship has the highest median values but the lowest standard deviation, whereas Joyner and Boore's relationship
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M45

Salt Lake City Brig ham City Logan

8 5

0-10 km

6 7 

MAGNITUDE

EXPLANATION 

10-25 km

8 5

>25km

FIGURE 23. Contributions of different magnitude events to total frequency of exceeding various levels of peak ground acceleration at three locations. The top 

row of plots corresponds to a return period (RP) of approximately 100 years, the middle row to 500 years, and the bottom row to 2,500 years.

has the lowest median and the highest standard deviation. 

In general, as the range of magnitudes that contribute to 

the hazard increases, more variability between the 

median estimates of the three relationships is introduced 

into the analysis, and the variability in computed hazard 

increases.

The results shown in figure 24 also indicate that, in 

areas of lower hazard, such as Logan, the uncertainty in 

hazard due to uncertainty in selecting the appropriate 

attenuation relationship becomes a major part of the total 

uncertainty in the hazard. At such sites, the computed 

hazard is largely dominated by the background seismicity 

(fig. 21), which has been modeled with a relatively smaller 

amount of uncertainty in the source parameters in

comparison with that used in the fault-specific sources 

(fig. 2). Thus, at such sites, the choice of attenuation relationship provides a relatively large proportion of the total 

uncertainty in the computed hazard.

The impact of uncertainty in recurrence model on the 

overall uncertainty in the frequency of exceedance is 

shown in figure 25. Here the effect is opposite that shown 

in figure 24, greater variation being obtained at locations 

of higher hazard. At sites where the hazard is largely due 

to events in the background source, such as at Logan, 

there is little uncertainty in the hazard because there is little uncertainty modeled for the earthquake recurrence 

rate in the background source. The variability increases 

as the dominance of the hazard by contributions from the
M46 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

z

LU

D 

O

1 10-

10

Salt Lake City

__ __I

0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

PEAK ACCELERATION, fif

Campbell (this

volume) 

Mean hazard

EXPLANATION 

Sadigh and others (1986)

5th and 95th percentiles 

from figure 20

 Joyner and Boore (1982)

FIGURE 24. Contribution to uncertainty in exceedance frequency from uncertainty in selecting the appropriate soil-site attenuation relationship. Conditional 

mean hazard curves obtained by using each of the three attenuation relationships used in the analysis are labeled.

faults increases (fig. 21) and reflects the large differences 

in the rate of magnitude 6 to 6.5 events predicted by the 

two recurrence models used for the faults (fig. 10). The 

percentage variability from the mean curve is somewhat 

lower at the highest acceleration levels because of the 

increasing contribution by the largest magnitude events, 

whose predicted rate is insensitive to the choice of recurrence model. The fact that the overall mean hazard is 

skewed toward the characteristic hazard curve reflects 

the greater weight assigned to this model in the hazard 

logic tree (fig. 2).

Figure 26 presents the influence of uncertainty in 

whether the faults are segmented. As indicated, the contribution to the total uncertainty is relatively small, being 

greatest where the contribution to hazard from the fault 

sources is greatest (and where the differences in recurrence estimates between the segmented and unsegmented models is the greatest). The opposite effects 

obtained at Salt Lake City and Brigham City reflect the 

difference between the recurrence estimate for individual 

segments and the average recurrence rate assigned to the 

unsegmented model. Examination of the recurrence data

presented in table 1 indicates that the Wasatch fault segments near Brigham City (Ogden segment for segmentation model A, Weber segment for segmentation model B) 

have the highest recurrence rate relative to the average 

for the central active portion of the Wasatch, whereas the 

Salt Lake City segment has the lowest recurrence rate relative to the average.

For the Wasatch fault, two models of segmentation 

were considered, given that the fault is segmented. Figure 

27 shows the effect of uncertainty in segmentation model 

at six locations along the fault. The two segmentation 

models produce significantly different results only in the 

vicinity of Brigham City. These differences arise owing to 

differences between the northern extent of the most 

active portion of the Wasatch in the two models (Brigham 

City in model A and approximately 15 km south of 

Brigham City in model B).

The variation in hazard due to uncertainty in fault 

activity is shown in figure 28. Figure 28A shows the effect 

of uncertainty in the existence of a tectonically active 

extension of the Oquirrh Mountain fault zone to the north 

of the Great Salt Lake. This uncertainty was modeled in
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M47

10

> 10

z

iu

a

HI

cc. u.

10

10

Salt Lake City

I . I

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

PEAK ACCELERATION, g

EXPLANATION

 Characteristic --- Exponential

 Mean hazard 5th and 95th percentiles

from figure 20

FIGURE 25. Contribution to uncertainty in exceedance frequency from uncertainty in recurrence model. Conditional mean hazard curves obtained by using the

two recurrence models are labeled.

terms of uncertainty in the total length conditional on the 

Oquirrh Mountain fault zone's being unsegmented or in 

terms of the state of activity of the northernmost segment 

(fig. 2, table 1). The combined effect of these two models 

is shown in the figure. The curve labeled "active northern 

extension" corresponds to an active Promontory segment 

for a segmented fault zone or a total length of 250 km for 

an unsegmented fault zone. The curve labeled "inactive 

northern extension" corresponds to an inactive Promontory segment for a segmented fault zone or a total length 

of 180 km for an unsegmented fault zone. As can be seen, 

only at locations near the postulated northern extension 

of the fault zone, such as Promontory Point, is there any 

noticeable effect on the hazard, and, even at this location, 

the effect is small owing to the dominance of the hazard 

by the background source and the more active Wasatch 

fault.

The hazard model also included uncertainty in 

whether the West Valley fault is an independent active 

source. As shown in figure 28B, the effect on the computed hazard in the vicinity of Salt Lake City was very 

small because the West Valley fault makes only a small 

contribution to the total hazard (see fig. 21) owing to its

limited extent, both in terms of length and downdip 

width.

The impact of uncertainty in fault dip is illustrated in 

figure 29. Uncertainty in fault dip is greatest at locations, 

such as Magna, that lie approximately 10 to 25 km west of 

the surface trace of the Wasatch fault zone. At these locations, variations in fault dip produce the maximum relative change in the distribution for distance to earthquake 

rupture. However, as shown, even at these locations, the 

effect of uncertainty in fault dip is small.

Figure 30 shows the effect of choosing to combine 

data from all segments or using only the data from an individual segment for modeling earthquake recurrence for 

the segments in the central portion of the Wasatch fault 

zone. As can be seen, the effect is variable along the fault, 

primarily reflecting differences between the data for an 

individual segment and the average behavior of the central segments of the fault zone.

For the remaining two parameters in the hazard 

model maximum magnitude and recurrence rate the 

contributions to total uncertainty were evaluated by computing the 5th- and 95th-percentile hazard curves, considering only the uncertainty in the individual parameter. 

Figures 31 and 32 show the influence of uncertainty in
M48 

10

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

> 10 -

z

o

Ui

cc

10

Salt Lake City

I . I

Brigham City

I

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

PEAK ACCELERATION, g

EXPLANATION

 Unseg merited - Segmented

- Mean hazard ~~ 5th and 95th percentiles

from figure 20

FIGURE 26. Contribution to uncertainty in exceedance frequency from uncertainty in source segmentation. Conditional mean hazard curves obtained for

unsegmented and segmented faults are labeled.

maximum magnitude and recurrence rate, respectively. 

The variability in the computed hazard due to uncertainty 

in these two parameters is generally similar throughout 

the study region, tending to be smaller at locations where 

the background source has a greater contribution to the 

computed hazard.

In summary, for sites near the Wasatch fault zone 

south of Collinston, the uncertainty in selecting the 

appropriate recurrence model is the largest contributor to 

uncertainty in the hazard, followed by uncertainty in 

selecting the appropriate attenuation relationship. For 

sites in areas of lower hazard, the uncertainty in selecting 

the appropriate attenuation relationship is the major contributor to uncertainty in the hazard. The uncertainties in 

the remaining parameters in general have relatively small 

contributions to the uncertainty in the hazard, although 

they may have more impact locally.

ADDITIONAL SENSITIVITY ANALYSES

Additional analyses were performed to evaluate the 

sensitivity of the results to local site conditions, possible 

lower rates of ground-motion attenuation, alternative

lower magnitude cutoff, and use of realtime estimates of 

earthquake occurrence.

The hazard results presented earlier are applicable to 

locations underlain by firm soil deposits. As discussed 

previously, three published attenuation relationships 

were used for estimating peak ground accelerations on 

soil sites, and four published relationships were used for 

rock sites. Differences between ground motions on soil 

and rock sites were evaluated by repeating hazard computations for the four rock-site attenuation relationships, 

equally weighted. Figure 33 compares the resulting rocksite hazard curves with the soil-site hazard curves for 

three locations. Similar comparisons made throughout 

the study region indicate that peak accelerations on rock 

should be about 10 percent greater than those on soil for 

return periods of about 100 years and about 20 percent 

greater for return periods of 500 years or longer. These 

factors can be used to obtain accelerations on rock from 

accelerations on soil.

Sensitivity analyses were performed to evaluate the 

effect of a possible lower rate of ground-motion attenuation in Utah in comparison with that observed in California. The modeled lower attenuation rate is described in
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M49

10

10

10

o

UJ

2 10" 

* 10 -

Nephi

I . I

10

10

10

Ogden

I I I I I I \

Brigham City

. v .x Collinston

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0

PEAK ACCELERATION, g

0.2 0.4 0.6 0.8

EXPLANATION

 Model A Model B

 Mean hazard 5th and 95th percentiles

from figure 20

FIGURE 27. Contribution to uncertainty in exceedance frequency from uncertainty in segmentation model at six locations along the Wasatch fault. Conditional

mean hazard curves obtained by using segmentation models A and B are labeled.

the previous section and shown in figure 17. Figure 34 

compares the mean hazard curves obtained by using the 

modified attenuation relationships with those used in the 

basic analysis. As can be seen, there is very little effect on 

the hazard, primarily because the hazard is dominated by

contributions from nearby events, either occurring on 

adjacent faults or within the background seismicity 

source. The largest percentage differences occur at lower 

acceleration levels, where the contributions from more 

distant events are the greatest (see fig. 23).
M50 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

10

o

z

LLJ

D 

O 111

DC

10- 3

10

A

EXPLANATION

 Active northern extension 

 » » Inactive northern extension

Mean hazard

5th and 95th percentiles 

from figure 20

Promontory 

Point

0.2 0.4 0.6 0.8

I ' I ' I

EXPLANATION 

 West Valley included 

l\ ........ West Valley not included

i^^ Mean hazard

5th and 95th percentiles 

from figure 20

B '

\

\

Salt Lake City

0.2 0.4 0.6 0.8

PEAK ACCELERATION, g

FIGURE 28- Contribution to uncertainty in exceedance frequency from 

uncertainty in source activity. A, Effect of uncertainty as to the existence of 

a northern extension of the Oquirrh Mountains fault zone. The curve labeled 

"active northern extension" corresponds to an active Promontory segment 

for a segmented fault zone or a total length of 250 km for an unsegmented

fault zone. The curve labeled "inactive northern extension" corresponds to 

an inactive Promontory segment for a segmented fault zone or a total length 

of 180 km. B, Effect of whether the West Valley fault zone is included as an 

independent earthquake source.

Uncertainty in the rate of attenuation could have 

been directly included in the primary analysis as additional branches in the logic tree. If it had been, it would 

have been necessary to incorporate an alternative for 

both a higher and a lower rate of attenuation. The effect 

of a greater rate of attenuation in Utah in comparison 

with the rate in California is expected to be a slight reduction in the computed hazard similar in magnitude to the 

increase shown in figure 34. The small effect on the 

results from using a lower rate of attenuation in the sensitivity study (fig. 34) indicates that the inclusion of this 

source of uncertainty has a negligible effect on the mean 

hazard. Its inclusion would slightly increase the dispersion about the mean hazard shown in figure 20.

The results in figure 34 can be viewed as showing the 

sensitivity to any factor that would result in an effect on 

the rate of attenuation of the order of magnitude given by 

equation 3 and shown in figure 17. Although the effect 

was specifically derived to model a crustal anelastic 

attenuation that is possibly lower in Utah than it is in California, it could also be considered to model the possible 

influence of soft lake deposits on attenuation. As 

described in the previous section, the influence of 

decreasing soil stiffness may be to increase peak accelerations at low amplitudes and to decrease them at high 

amplitudes. The tendency toward increasing accelerations at low amplitudes results in an effect on attenuation 

similar to that shown in figure 17. However, the offsetting
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M51

10

10

a

10

10 <

I r I '

EXPLANATION

Mean hazard

5th and 95th percent!les 

from figure 20

Salt Lake City 

i i

f I ' 

EXPLANATION 

Mean hazard

5th and 95th percentiles 

from figure 20

Magna

0.2 0.4 0.6 0.8 0 v 0.2 

PEAK ACCELERATION, g

0.4 0.6 0.8

FIGURE 29. Contribution to uncertainty in exceedance frequency from uncertainty in fault dip. Conditional mean hazard curves for each dip are labeled.

tendency toward decreasing accelerations at high amplitudes is not modeled by the relationship shown in 

figure 17.

The hazard curves were computed by considering the 

contributions to hazard from events of magnitude 5 or 

greater because it is judged that the ground shaking from 

smaller magnitude events poses little risk to engineered 

structures. The effect of including the contributions from 

events in the magnitude range 4 to 5 on the computed hazard is shown in figure 35. As can be seen, there is a small 

increase in the frequency of exceedance only at low acceleration levels, for which smaller magnitude events contribute relatively more to the hazard. The increase in the 

computed hazard at Logan is greater than that at Salt Lake 

City for the same reason. When longer period ground 

motions are considered, the effect of including smaller

magnitude events will be even less than the effect shown 

in figure 35.

The hazard analyses were conducted by using a 

memoryless, or Poisson, model for earthquake recurrence; that is, the frequency of events in the near future 

(for example, the expected number of events in the next 

50 years) is a function of the long-term earthquake recurrence rate, no consideration being given to the time 

elapsed since the most recent event. Although the Poisson model is almost exclusively used in seismic-hazard 

analyses, it is at odds with the generally accepted physical 

model in which earthquake occurrence on a fault is a process of the gradual accumulation of strain, followed by 

sudden release. The physical model implies that, for individual faults undergoing relatively constant loading, the 

repeat time for those events that release the built-up
M52 

10 'r

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

D 

O

111

oc

z 

< 10

10

Prove Salt Lake City

I . I

Ogden

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

PEAK ACCELERATION, g

EXPLANATION 

Segment 

Mean hazard 

 Fault

5th and 95th percentiles 

from figure 20

FIGURE 30. Contribution to uncertainty in exceedance frequency along the Wasatch fault from use of either recurrence data from the segment only, denoted 

"segment," or an average value for the central segments of the fault zone, denoted "fault," in modeling earthquake recurrence for the fault segments.

10

10 o 

ui

O ui

BE

z 

< 10

10

Salt Lake City

I

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0

PEAK ACCELERATION, g

0.2 0.4 0.6 0.8

FIGURE 31. Contribution to uncertainty in exceedance frequency from uncertainty in maximum magnitude. The long-dashed curves are the 5th and 95th percentiles for uncertainty in all parameters (see fig. 20), and the short-dashed curves are the 5th and 95th percentiles for uncertainty in maximum magnitude.
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M53

10

10-

UJ

O

UJ 

QC

10

10

Salt Lake City

I . I

v Logan \ \ N,

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

PEAK ACCELERATION, g

FIGURE 32. Contribution to uncertainty in exceedance frequency from uncertainty in recurrence rate. The long-dashed curves are the 5th and 95th percentiles for 

uncertainty in all parameters, and the short-dashed curves are the 5th and 95th percentiles for uncertainty in recurrence rate only.

10

O

ui 10 2

O

10

10

Salt Lake City

1 . I

Brigham City

I . I

Logan

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 

PEAK ACCELERATION, g

EXPLANATION 

 Soil ---- Rock

FIGURE 33. Comparison of mean hazard curves for soil and rock sites at three locations.
M54 

10

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10 -

10 -'

10

Salt Lake City Brigham City

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6

PEAK ACCELERATION, g

0.8 0 0.2 0.4 0.6 0.8

EXPLANATION 

 California rate of attenuation Lower rate of attenuation

FIGURE 34. Effect on computed hazard of postulated lower Utah attenuation rates in comparison with California rates at three locations.

stress should be more or less cyclic in nature. However, 

on a regional basis, the Poisson model for earthquake 

occurrence has been shown to be appropriate both on the 

basis of observation (for example, Gardner and Knopoff, 

1974) and on the basis of theory as the limiting result of a 

number of independent or nearly independent processes 

of whatever type on individual features (for example, 

Brillinger, 1982). In addition, Cornell and Winterstein 

(1988) have shown that the Poisson model is conservative 

as long as the time elapsed since the most recent renewing event is less than the mean repeat time.

The renewal model provides the simplest approach 

for making realtime estimates of earthquake occurrence. 

The renewal model specifies that, if the occurrence of 

multiple events is neglected, the probability of a renewing 

event in the next t years, given £Q years since the last 

renewing event, is given by the expression

P(event in t t0~) =

F(*+«b)-F(«o)

(6)

where F( ) is the cumulative density function for interarrival time of events. Investigators have employed different models for the distribution of interarrival times, such 

as the normal, lognormal, gamma, and Weibull distributions. Cornell and Winterstein (1988) indicated that the

Weibull distribution provides the desired characteristics 

of a simple analytical form and a hazard function that 

increases monotonically with increasing tQ . (The hazard 

function is a measure of the likelihood of an event in the 

next increment of time, dt, given no events in time £0 0 

They gave a simple approximation for the Weibull distribution as

in which

_ i-o.5V[r](i-vm)

(7)

(8)

V[T\ and ~E[T\ being the coefficient of variation and the 

mean of interarrival times, respectively. Equations 7 and 8 

are good approximations for 0<V[T]<1 and have the 

advantage of degenerating to the Poisson model when 

V[7>1 (Cornell and Winterstein, 1988).

Equations 6 through 8 were used to obtain realtime 

estimates of the frequency of future large events at the six 

locations along the Wasatch fault zone for which data are 

available on the time since the most recent event 

(Schwartz and others, 1984). The coefficient of variation 

of interarrival time, V[T], was assumed to be 0.7, reflect-
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M55

10

o

yj 10 

O

LU 

DC

10

10

Salt Lake City Brigham City 

I

Logan

0.2 0.4 0.6 0.8 0 0.2 0.4 0.6

PEAK ACCELERATION, g

EXPLANATION

0.8 0 0.2 0.4 0.6 0.8

FIGURE 35. Effect of choice of lower bound magnitude cutoff on computed hazard at three locations.

ing the large variations observed in the limited data for 

interarrival times for large earthquakes on the Wasatch 

fault zone. The ratios of the estimated number of renewing events in the next 50 years to the number given by the 

Poisson model at the six locations are as follows:

Location

Kaysville

Little Cottonwood Canyon

Hobble Creek

North Creek

Deep Creek

Mean

repeat 

time

(years)

1,300

2,600

2,100

1,800

7,000

Elapsed

time

since

most

recent

event

(years)

<500

^lOOO

>1000

300-500

<1750

Ratio of renewal

to Poisson

model

of expected

number

of events

in 50 years

<0.83

>.82

>.90

.67

<.68

Assumed to be similar to the Hobble Creek site.

The above ratios are all less than 1.0, reflecting the 

fact that the estimated elapsed times are less than the 

estimated mean repeat times at all locations. The above 

ratios are assumed to apply only to the largest or characteristic events on the appropriate fault segments. Smaller

events are not likely to result in substantial stress release 

and are considered to occur randomly (Poisson) in time. 

The effect of the realtime estimates of the near-future rate 

of large events on the computed hazard along the Wasatch 

fault zone is shown in figure 36. The effect is a minor 

decrease in hazard, the largest decrease occurring at long 

return periods where the hazard is dominated by contributions from the largest events (fig. 23).

PEAK ACCELERATION HAZARD MAPS

The variation in seismic hazard throughout the study 

region is shown in figure 37 in terms of contours of peak 

ground-acceleration level corresponding to a 10-percent 

probability of exceedance in time periods of 10, 50, and 

250 years. These probability levels span the range of probabilities typically considered in evaluating seismic hazard 

at commercial and industrial sites and also correspond to 

the levels used by the USGS in its national maps of 

seismic hazard (Algermissen and others, 1982). If it is 

assumed that earthquake occurrence is a Poisson process, the three probability levels can be related to the 

mean frequencies of exceedance (and to their inverse, 

return period) by applying equation 1:
M56 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

o

HI

cc

10

10

10

Nephi 

i I

Salt Lake City

0.2 0.4 0.6 0.8 0 0.2 

PEAK ACCELERATION, g

0.4 0.6 0.8

EXPLANATION 

Poisson Real time

FIGURE 36. Effect on computed hazard of using realtime estimates of earthquake occurrence probabilities at two locations along the Wasatch

fault zone.

10 percent probability 

of exceedance (years)

10

50

250

Equivalent exceedance 

frequency 

(events/year)

0.0105

.0021

.00042

Return period 

(years)

95

475

2,373

Hazard computations were made for 80 points 

located on a grid in the study region. Peak accelerations 

corresponding to the above frequencies of exceedance 

were interpolated from the computed mean hazard 

curves, and the resulting values were contoured to produce the maps shown in figure 37. As indicated by the

exceedance frequency fractiles shown in figure 20, the 

mean hazard curves are somewhat higher than the 

median (50th percentile) hazard curves and thus represent a more conservative estimate of the hazard. Peak 

accelerations obtained from the median hazard curves are 

about 10 percent less than those presented on the hazard 

maps. The computed 90-percent confidence intervals for 

peak ground acceleration at a given return period typically range from minus 40 percent to plus 60 percent 

about the values shown in figure 37.

The hazard maps further illustrate the spatial trends 

shown in figure 21. The spatial variation in hazard 

increases with decreasing probability of exceedance 

(increasing return period), reflecting the increased domi-
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS

112° ... 112° _ 112

M57

41 s

40°

\\ / S CA6HE

^ ^ v ^ \

V ^ \ m \

Brigham^ \ 5; \ /

o ' o s, -iS7 ?» ^

^/-'

^^ North Crefek

\( S v i / v

u-A ^-1 /

V N^dX^o*

t_\ i_______i___,x___

;Salt Lake City

ittle\CottonwoC|d 

Canyon

\

'l m \ I 

\ \ephi

i \

25 

I

50 MILES

25 50 75 KILOMETERS

FIGURE 37. Contours of peak ground acceleration on soil sites having 10-percent probability of being exceeded in (A) 10 years, (fi) 50 years, and (Q 250 

years. Peak accelerations on rock sites are expected to be approximately 10 percent higher than the values shown on the map for 10-percent probability 

of exceedance in 10 years and approximately 20 percent higher than the values shown on the maps for 50 and 250 years.

nance of the hazard by contributions from the individual 

faults as the probability of exceedance decreases. The 

increased dependence of the hazard on proximity to the 

mapped faults is particularly true for the central portion 

of the Wasatch fault zone, which has by far the highest 

estimated rate of activity for large events.

The hazard maps shown in figure 37 are for peak 

accelerations on soil-site conditions. Using rock-site 

attenuation relationships to repeat the analysis for all 

locations indicates that rock-site acceleration levels can 

be obtained by increasing the soil-site acceleration levels 

shown in figure 37 by about 10 percent for 10-percent
M58 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

Salt Lake City Brigham City Logan

1 "l 

- m 2500 yr RP - - 2500 yrRP X***"*""*

0.04 0.1 0.2 0.5 1 24 0.04 0.1 0.2 0.5 1 24 0.04 0.1 0.2 0.5 1 24

PERIOD, IN SECONDS

EXPLANATION 

 Sadigh and others (1986) -- « Joyner and Boore (1982)

FIGURE 38. Equal-hazard response spectra (5-percent damping) for soil and rock sites at three locations computed by using two attenuation relationships. Each

plot shows the spectra for three return periods (RP).

probability of exceedance in 10 years and by about 20 percent for 10-percent probability of exceedance in 50 and 

250 years.

In those areas underlain by soil, the peak accelerations in figure 37 apply directly. Soil sites are considered 

to be sites where the depth to bedrock is 20 m or greater. 

In areas of shallow soil or areas underlain directly by 

rock, it is recommended that the peak accelerations 

shown in figure 37 be adjusted upward, as described in 

the preceding paragraph.

RESPONSE SPECTRA

Response spectral accelerations appropriate for use 

with the mapped peak acceleration levels were evaluated 

by developing equal-hazard response spectra for a number of locations in the study region. The equal-hazard 

response spectra were developed by conducting hazard 

analyses using response spectral attenuation relationships described previously in this paper for periods of 

vibration between 0.04 and 4 s. The levels of spectral
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M59

10 PERCENT IN 10 YEARS 10 PERCENT IN 50 YEARS 10 PERCENT IN 250 YEARS

Joyner and 

Boore (1962)

. . ...I . .... ...I

Joyner and 

Boore (1962) 

. ...I

Sadigh and 

others (19196) 

i . ...I

Sadigh and 

others (19196)

. . ...I

0.04 0.1 0.2 0.5 1 0.04

Salt Lake City

0.1 0.2 0.5 1 2 

PERIOD, IN SECONDS

EXPLANATION 

 Brigham City

4 0.04 0.1 0.2

---- Logan

0.5 2 4

FIGURE 39. Equal-hazard response spectra (5-percent damping) completed for soil sites at three locations. The upper row of plots shows the spectral 

amplifications computed by using the Joyner and Boore (1982) attenuation relationships, and the lower row of plots shows the spectral amplification computed 

by using the Sadigh and others (1986) attenuation relationships.

acceleration corresponding to the three probability levels 

used in the hazard maps were then interpolated from the 

hazard curves for each period of vibration, and smooth 

response spectra were drawn through the computed 

points to construct equal-hazard spectra.

Figure 38 shows the resulting soil and rock equalhazard spectra for three geographic locations. The plotted 

points indicate the computed spectral acceleration levels 

for the three probability levels. Using the Sadigh and others (1986) and Joyner and Boore (1982) response spectra

attenuation relationships yields similar results except for 

low probability levels on soil sites, where the Joyner and 

Boore (1982) relationship gives spectral accelerations 

that are about Wz to 2 times higher than those obtained by 

using the Sadigh and others (1986) relationships.

To directly compare the relative frequency content of 

the various equal-hazard spectra, the absolute spectra 

were normalized by their corresponding peak groundacceleration levels. The resulting spectral shapes are 

shown in figures 39 and 40 for soil and rock sites, respec-
M60 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10 PERCENT IN 10 YEARS 10 PERCENT IN 50 YEARS 10 PERCENT IN 250 YEARS

Joyner and 

- Boore(1982)

Sadigh and 

- others (1986)

Joyner and 

Boore (1982) 

. ...I

Joyner and 

Boore (1982)

0.04 0.1 0.2 0.5 1 2 4 0.04 0.1 0.2 0.5 1 24 0.04 0.1 0.2 0.5 1 

PERIOD, IN SECONDS

Salt Lake City

EXPLANATION 

 Brigham pity - - - - Logan

FIGURE 40. Equal-hazard response spectral shapes (5-percent damping) computed for rock sites at three locations. The upper row of plots shows the spectral 

amplifications computed by using the Joyner and Boore (1982) attenuation relationships, and the lower row of plots shows the spectral amplifications 

computed by using the Sadigh and others (1986) attenuation relationships.

tively. In general, the computed spectral shape for a 

particular site classification, probability level, and attenuation relationship is similar at all three geographic locations. Average spectral shapes for each probability level 

and site classification were obtained by averaging the six 

shapes computed from the Sadigh and others (1986) and 

Joyner and Boore (1982) relationships at the three locations. These spectral shapes are shown in figure 41. The 

average spectral shapes differ by a maximum of 20 percent from the spectral shapes obtained at any given location. Consequently, we propose that the spectral shapes 

shown in figure 41 be used for the respective probability

level and site classifications, regardless of location within 

the study area.

It can be seen in figures 39 through 41 that the 

response spectral shapes broaden significantly with 

decreasing probability level. This broadening reflects the 

increasing contribution of large-magnitude events as the 

probability level decreases. It is therefore not appropriate 

to use a single spectral shape for all probability levels.

Absolute acceleration equal-hazard spectra for soil 

sites can be obtained by multiplying the soil spectral 

shapes shown in figure 41 by the appropriate peak ground 

acceleration obtained from the hazard maps for the sam-
10 PERCENT IN 10 YEARS 

3i nun r

2.5

I «

u

a. 1.5

'

UJ 

Q. to 0.5

PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M61 

10 PERCENT IN 50 YEARS 10 PERCENT IN 250 YEARS

0.04 0.1 0.2 0.5 1 24 0.04 0.1 0.2 0.5 1 24 0.04 0.1 0.2 0.5 1 24

PERIOD, IN SECONDS

EXPLANATION 

 Soil - - Rock

FIGURE 41. Proposed spectral shapes (5-percent damping) for developing 

equal-hazard response spectra. For soil sites, equal-hazard spectra are developed by multiplying the soil spectral amplifications shown in the figure by 

the appropriate peak ground accelerations obtained from the soil-hazard 

maps shown in figure 37. For rock sites, the appropriate peak acceleration to

use with the rock spectral amplifications shown in the figure is obtained by 

multiplying the peak ground accelerations shown on the soil-hazard maps 

shown in figure 37 by a factor of 1.1 for a 10-percent probability of exceedance in 10 years and by a factor of 1.2 for 10-percent probability of exceedance in 50 and 250 years.

pie probability level (fig. 37). Absolute acceleration equalhazard spectra for rock sites can be obtained by first estimating the peak acceleration on rock by multiplying the 

soil acceleration levels by a factor of 1.1 for 10-percent 

probability of exceedance in 10 years or a factor of 1.2 for 

lower probability levels and then multiplying by the 

appropriate rock spectral shapes shown in figure 41.

As an example application, figure 42 presents the 

equal-hazard response spectra that would be developed 

for rock and soil sites in the vicinity of Magna corresponding to a 10-percent probability of exceedance in 50 years. 

The hazard map shown in figure 37 indicates that the 

appropriate peak ground acceleration on soil sites is 0.25 

g. If a factor of 1.2 is used, the corresponding peak acceleration on rock would be 0.30 g. The absolute acceleration spectra shown in figure 42 were obtained by 

multiplying the appropriate spectral shapes in figure 41 by 

these two acceleration levels.

The soil-site spectra shown in figure 41 are considered appropriate for firm soil conditions. Firm soils are 

considered to consist of medium-dense to dense sands 

and gravels and stiff silts and clays. As discussed in the 

section of the paper describing attenuation relationships, 

there is uncertainty as to the relative amplitudes of

ground motions on softer lake deposits (soft clays and 

silts). A preliminary indication of the possible additional 

spectral amplification in areas of soft lake deposits can be 

obtained from the response spectral shapes developed by 

Seed and others (1976b) for different generalized categories of subsurface conditions. Seed and others (1976b) 

found that response spectral amplifications for "soft to 

medium clay and sand" exceeded the amplifications for 

"deep cohesionless" and "stiff soils for periods greater 

than about 0.4 s and were less than the amplifications for 

"deep cohesionless" and "stiff soils at shorter periods. 

The average increase in amplification in the period range 

of about 0.75 to 1.5 s corresponded to a factor of about 

1.6. Using this factor with the response spectral curve for 

soil shown in figure 41 and neglecting any possible 

decrease in amplification at periods less than 0.4 s, we 

obtained the ranges in response spectral shapes shown in 

figure 43. The lower curves correspond to the shapes for 

soil shown in figure 41 and are for firm soil conditions. 

The upper curves are preliminary estimates of response 

spectral shapes applicable to soft lake deposits. These 

curves can be multiplied by the appropriate peak acceleration values in figure 37 to obtain preliminary estimates of 

absolute response spectra on soft lake deposits.
M62 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

O

I

a

8

0.8

0.6

0.4

0.2

I 111^

« « « Rock

0.04 0.1 0.2 0.5 1 

PERIOD, IN SECONDS

FIGURE 42. Example of the development of equal-hazard response spectra (5- 

percent damping) for a 10-percent probability of exceedance in 50 years 

for soil and rock sites near Magna, Utah.

SUMMARY

A quantitative assessment has been made of the 

ground-shaking hazard along the Wasatch Front urban 

corridor by utilizing the extensive geological and seismological data base that has been developed for the region. 

A probabilistic approach was used to express levels of 

ground motion as a function of their probability of 

exceedance. Using a logic tree methodology, the 

approach formally incorporates variations in values for 

earthquake source geometry and fault segmentation, 

maximum earthquake magnitude, earthquake recurrence 

rate, ground-motion attenuation, and other parameters 

and model components that are not known with certainty 

(table 1).

On the basis of probabilistic analyses conducted at 

80 points located on a grid in the study area, maps were 

prepared showing contours of peak ground acceleration 

for probabilities of exceedance of 10 percent in 10 years, 

10 percent in 50 years, and 10 percent in 250 years (fig. 

37). These maps are for peak acceleration on soil-site conditions, considered to be sites where depth to bedrock

was 20 m or more. By repeating the analyses using rocksite attenuation relationships, we concluded that rocksite peak accelerations can be obtained by multiplying the 

soil-site accelerations by a factor of 1.1 for a probability 

of exceedance of 10 percent in 10 years and by a factor of 

1.2 for probabilities of exceedance of 10 percent in 

50 years and 10 percent in 250 years.

Response spectra were also developed from the 

probabilistic analyses. Response spectral shapes (for a 

damping ratio of 0.05) for firm soil conditions and for 

rock-site conditions were developed for the same probability of exceedance levels used in mapping peak ground 

acceleration (fig. 41). Absolute response spectra can be 

obtained by multiplying these spectral shapes by the corresponding values of peak ground acceleration. In areas 

underlain by soft lake silts and clays, considerable uncertainty exists regarding the spectral shapes. A preliminary 

estimate is presented for the additional spectral amplification that might occur in areas of soft lake deposits (fig. 

43).

Extensive analyses were conducted to examine the 

sensitivity and uncertainty of the results of this study with 

respect to variations in the various input parameters and 

models. The computed 90-percent confidence intervals 

for peak ground acceleration represent a range of minus 

40 percent to plus 60 percent about the values shown on 

the hazard maps (fig. 37). The two biggest contributors to 

uncertainty in the results were uncertainties in selecting 

the appropriate models for earthquake recurrence on the 

faults and ground-motion attenuation.

POSTSCRIPT

The state of knowledge about earthquake hazards 

along the Wasatch Front has continued to evolve since the 

completion of this study (1986). Continued paleoseismic 

studies have led to a consensus model for the segmentation of the Wasatch fault zone and improved data on the 

timing of late Holocene surface rupturing events 

(Machette and others, 1991). Geophysical data obtained 

from the Great Salt Lake has confirmed the existence of a 

Great Salt Lake fault and indicated a somewhat greater 

slip rate than that used for the Oquirrh Mountains fault 

zone in this study (Pechmann and others, 1987). Continued examination of the relationship between local magnitude (ML) and seismic moment (M0) has led to the 

conclusion that the moment magnitude relationship 

developed for California earthquakes is appropriate for 

the Utah region (Shemeta and Pechmann, 1989). Studies 

of strong-motion data have suggested values for the standard error in peak ground acceleration (Youngs and others, 1990; Campbell, 1991) that are somewhat higher than 

those used in this study. In addition, Brockman and Bollinger (1992) have reported results of analyses that fur-
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M63

10 PERCENT IN 10 YEARS 10 PERCENT IN 50 YEARS 10 PERCENT IN 250 YEARS

0.1 0.2 0.5 1 24 0.04 0.1 0.2 0.5 1 24 0.04 0.1 0.2 0.5 1 24

PERIOD, IN SECONDS

EXPLANATION 

 Firm soil - Soft soil

FIGURE 43. Preliminary estimates of ranges in response spectral shapes (5-percent damping) for lake deposits. Lower curves are soil curves shown in figure 41 

and are intended for firm soil conditions. Upper curves hi the ranges are preliminary estimates for soft lake silts and clays.

ther confirm the assumption used in this study that the 

rate of attenuation of seismic waves in Utah is similar to 

that in California,

Many of these results would suggest modifications of 

the inputs to the seismic-hazard analysis. We have performed a limited number of calculations to evaluate the 

impact of updated input parameters on the results presented in the regional hazard maps (fig. 37). The impact 

varies with location and sometimes results in higher hazard and sometimes in lower hazard. Generally, the 

updated input parameters result in differences of 15 percent or less in peak ground acceleration for the three 

selected probability levels. These differences are well 

within the uncertainty bounds presented in our study. 

Therefore, we believe that figure 37 still presents a valid 

representation of the seismic hazard along the Wasatch 

Front.

REFERENCES CITED

Aki, K, 1979, Characterization of barriers on an earthquake

fault: Journal of Geophysical Research, v. 84, no. Bll, p.

6140-5148. 

 1984, Asperities, barriers, and characteristic earthquakes:

Journal of Geophysical Research, v. 89, no. B7, p. 5867-

5872.

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, 

99 p.

Anderson, J.G., 1979, Estimating the seismicity from geological 

structure for seismic risk studies: Bulletin of the Seismological Society of America, v. 69, no. 1, p. 135-158.

Anderson, L.W., and Miller, D.G., 1979, Quaternary fault map of 

Utah: Long Beach, Calif., FUGRO, Inc., 35 p.

Arabasz, W.J., 1979, Historical review of earthquake-related 

studies and seismographic recording in Utah, 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 Special Publication, p. 33-56.

 1984, Earthquake behavior in the Wasatch front area: 

Association with geologic structure, space-time occurrence, and stress state, in Hays, W.W., and Gori, PL., eds., 

Proceedings of Conference XXVT 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 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 Special 

Publication, p. 119-121.
M64 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

Arabasz, W.J., and Robinson, R., 1976, Microseismicity and geologic structure in the northern South Island, New Zealand: 

New Zealand Journal of Geology and Geophysics, v. 19, no. 

2, p. 561-601.

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, no. 5, p. 1479-1499.

Bender, B., 1983, Maximum likelihood estimation of b values for 

magnitude grouped data: Bulletin of the Seismological Society of America, v. 73, no. 3, p. 831-851.

 1984, Seismic hazard estimation using a finite fault rupture model: Bulletin of the Seismological Society of America, v. 74, no. 5, p. 1899-1923.

Benz, H.M., and Smith, R.B., 1988, Elastic-wave propagation and 

site amplification in the Salt Lake Valley, Utah, from simulated normal faulting earthquakes: Bulletin of the Seismological Society of America, v. 78, no. 6, p. 1851-1874.

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, no. 6, p. 2379-2411.

Boore, D.M., 1983, Stochastic simulations of high-frequency 

ground motions based on Seismological models of the radiated spectra: Bulletin of the Seismological Society of America, v. 73, no. 5, p. 1865-1894.

Brazee, R.J., 1980, An analysis of earthquake intensities with 

respect to attenuation, magnitude, and rate of recurrence 

(revised ed.): Washington, D.C., U.S. Nuclear Regulatory 

Commission Report NUREG/CR-1805, 35 p.

Brillinger, D.R., 1982, Some bounds for seismic risk: Bulletin of 

the Seismological Society of America, v. 72, no. 4, p. 1403- 

1410.

Brockman S.R, and Bollinger, G.A., 1992, Q estimates along the 

Wasatch front in Utah derived from Sg and Lg wave amplitudes: Bulletin of the Seismological Society of America, v. 

82, no. 1, p 135-147.

Campbell, K.W., 1982, A preliminary methodology for the 

regional zonation of peak ground acceleration: International Earthquake Microzonation Conference, 3d, Seattle, 

Wash., 1982, Proceedings, v. I, p. 365-374.

 1987, A random-effects analysis of near-source ground 

motion for the Diablo Canyon Power Plant site, San Luis 

Obispo County, California: Bethesda, Md., report prepared 

for Lawrence Livermore National Laboratory, Dames and 

Moore.

Chandra, U., 1979, Attenuation of intensities in the United 

States: Bulletin of the Seismological Society of America, v. 

69, no. 6, p. 2003-2024.

Chavez, D.E., and Priestly, K.F., 1985, ML observations in the 

Great Basin and M0 versus ML relationships for the 1980 

Mammoth Lakes, California earthquake sequence: Bulletin 

of the Seismological Society of America, v. 75, no. 6, p. 

1583-1598.

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 safety: Oakland, Calif.,

report prepared for Utah Geological and Mineralogical Survey, Woodward-Lundgren and Associates, 146 p.

Coppersmith, K.C., and Youngs, R.R., 1986, Capturing uncertainty in probabilistic seismic hazard assessments within 

intraplate environments: National Conference on Earthquake Engineering, 3d, Charleston, S.C., 1986, Proceedings, 

v. I, p. 301-312.

Cornell, C.A., 1968, Engineering seismic risk analysis: Bulletin of 

the Seismological Society of America, v. 58, no. 5, p. 1583- 

1606.

Cornell, C.A., and Van Marke, E.H., 1969, The major influences 

on seismic risk: World Conference on Earthquake Engineering, 3d, Santiago, Chile, Proceedings, v. A-l, p. 69-93.

Cornell, C.A. and S.R. Winterstein, 1988, Temporal and magnitude dependence in earthquake recurrence models: Bulletin 

of the Seismological Society of America, v. 78, no. 4, p. 

1522-1537.

Crone, A.J., and Machette, M.N., 1984, Surface faulting accompanying the Borah Peak earthquake, central Idaho: Geology, v. 

12, p. 664-667.

Deschamps, A., Guademer, Y., and Cisternas, A,, 1982, The El 

Asnam, Algeria earthquake of 10 October 1980 Multiplesource mechanism determined from long-period records: 

Bulletin of the Seismological Society of America, v. 72, no. 

4, p. 1111-1128.

Doser, D.I., 1984, The 1983 Borah Peak, Idaho and 1959 Hebgen 

Lake, Montana earthquakes; models for normal fault earthquakes in the Intel-mountain 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-A, p. 368-384.

Doser, D.I. and Smith, R.B., 1982, Seismic moment rates in the 

Utah region: Bulletin of the Seismological Society of America, v. 72, no. 2, p. 525-551.

Electric Power Research Institute, 1987, Seismic hazard methodology for the central and eastern United States, v. 1, Methodology (revised ed.): EPRI Report NP-4726, prepared for 

Seismicity Owners Group and Electric Power Research 

Institute under research projects P101-38, -45, -46, 2256- 

14.

Everitt, B.L., and Kaliser, B.N., 1980, Geology for assessment of 

seismic risk in the Tooele and Rush valleys, Tooele County, 

Utah: Salt Lake City, Utah Geological and Mineral Survey 

Special Studies 51, 33 p.

Evernden, J.F., 1975, Seismic intensities, "size" of earthquakes, 

and related parameters: Bulletin of the Seismological Society of America, v. 65, no. 5, p. 1287-1313.

 compiler, 1980, Proceedings of Conference X: Earthquake 

hazards along the Wasatch and Sierra-Nevada frontal fault 

zones: U.S.Geological Survey Open-File Report 80-801, 

688 p.

Gardner, J.K., and Knopoff, L., 1974, Is the sequence of earthquakes in Southern California, with aftershocks removed, 

Poissonian?: Bulletin of the Seismological Society of America, v. 64, no. 5, p. 1363-1367.

Gilbert, G.K., 1890, Lake Bonneville: U.S. Geological Survey 

Monographs 1, 438 p.

Griscom, M., 1980, Space-time seismicity patterns in the Utah 

region and an evaluation of local magnitude as the basis of a
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M65

uniform earthquake catalog: Salt Lake City, University of 

Utah, unpublished M.Sci. thesis, 95 p.

Griscom, M., and Arabasz, W.J., 1979, Local magnitude (M L) in 

the Wasatch front and Utah region: Wood-Anderson calibration, coda-duration estimates of ML, and ML versus mg, 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 Special Publication, p. 

433-443.

Gutenberg, B. and Richter, C.F., 1954, Seismicity of the Earth 

and associated phenomena, (2d ed.): Princeton, N.J., Princeton University Press, 310 p.

 1956, Earthquake magnitude, intensity, energy, and acceleration: Bulletin of the Seismological Society of America, v. 

46, no. 3, p. 105-145.

Hanks, T.C., and Kanamori, H., 1979, A moment magnitude scale: 

Journal of Geophysical Research, v. 84, no. B5, p. 2348- 

2350.

Hanks, T.C., and McGuire, R.K., 1981, The character of high frequency ground motion: Bulletin of the Seismological Society of America, v. 71, no. 6, p. 2071-2095.

Hanson, K.L. and Schwartz, D.P., 1982, Guidebook to late Pleistocene and Holocene faulting and the vicinity Little Cottonwood Canyon to Scipio, Utah: American Geophysical 

Union Chapman Conference on Fault Behavior and the 

Earthquake Process, Snowbird, Utah, 1982, 40 p.

Hanson, K.L., Swan, F.H., and Schwartz, D.P., 1981a, Study of 

earthquake recurrence intervals on the Wasatch fault, Utah 

(North Creek site): Sixth semiannual technical report prepared for the U.S. Geological Survey (Menlo Park, Calif.) 

under contract no. 14-07-0001-19842, 22 p.

 1981b, Study of earthquake recurrence intervals on the 

Wasatch fault, Utah: Fifth semiannual technical report prepared for the U.S. Geological Survey (Menlo Park, Calif.) 

under contract no. 14-08-0001-19115,15 p.

Hays, W.W., and Gori, PL., eds., 1984, 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, 692 p.

Hays, W.W., and King, K.W, 1982, Zoning of the earthquake 

ground-shaking hazard along the Wasatch fault zone, Utah: 

International Earthquake Microzonation Conference, 3d, 

Seattle, 1982, Proceedings, v. Ill, p. 1307-1318.

 1984a, The ground-shaking hazard along the Wasatch 

fault zone, Utah: World Conference on Earthquake Engineering, 8th, San Francisco, 1984, Proceedings, v. 1, p. 7-14. 

-1984b, The ground-shaking hazard along the Wasatch

fault zone, Utah, in Hays, W.W., and Gori, PL., 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. 143- 

147.

Howell, B.F., Jr., and Schultz, T.R., 1975, Attenuation of modified 

Mercalli intensity with distance from the epicenter: Bulletin 

of the Seismological Society of America, v. 65, no. 1, p. 641- 

665.

Idriss, I.M., 1990, Response of soft soil sites during earthquakes, 

in Duncan, J.M., ed., H. Bolton Seed Memorial Symposium, 

Berkeley, Calif., 1990: Proceedings, v. 2, p. 273-289.

Jackson, S.M., and J. Boatwright, 1987, Strong ground motion in 

the Borah Peak earthquake and its aftershocks: Bulletin of 

the Seismological Society of America, v. 77, no. 3, p. 724- 

738.

Joyner, W.B., and Boore, D.M., 1982, Prediction of earthquake 

response spectra: U.S. Geological Survey Open-File Report 

82-977,17 p.

Keaton, J., Curry, D.R., and Olig, S.J., 1986, Paleoseismicity and 

earthquake hazards evaluation of the West Valley fault zone, 

Salt Lake City urban area: Report prepared for U.S. Geological Survey (Menlo Park, Calif.) under contract no. 14-08- 

0001-22048, 18 p.

King, K.W., and Hays, W.W., 1977, Comparison of seismic attenuation in Northern Utah with attenuation in four other 

regions of the western United States: Bulletin of the Seismological Society of America, v. 67, no. 3, p. 781-792.

King, K.W., Hays, W.W., and McDermott, P.J., 1983, Wasatch front 

urban area seismic response data report: U.S. Geological 

Survey Open-File Report 83-452, 68 p.

King, G., and Yielding, G., 1984, The evolution of a thrust fault 

system Process of rupture initiation, propagation and termination in the El Asnam (Algeria) earthquake: Geophysical Journal of the Royal Astronomical Society, v. 77, p. 913- 

933.

Kulkarni, R.B., Youngs, R.R., and Coppersmith, K.J., 1984, 

Assessment of confidence intervals for results of seismic 

hazard analysis, in World Conference on Earthquake Engineering, 8th, San Francisco, 1984, Proceedings, v. 1, p. 263- 

270.

Lund, W.R., and Schwartz, D.P, 1986, Fault behavior and earthquake recurrence at the Dry Creek site, Salt Lake segment, 

Wasatch Fault Zone, Utah: Eos, Transactions of the American Geophysical Union, v. 67, no. 44, p. 1107.

Machette, M.N., Personius, S.F., Scott, WE., and Nelson, A.R., 

1986, Quaternary geology for ten fault segments and largescale changes in slip rate along the Wasatch fault zone: 

Paper presented at workshop on earthquake hazards along 

the Wasatch Front, Utah, Salt Lake City, Utah, 1986.

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, no. 2, p. 137-149.

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.

McCalpin, J., 1985, Quaternary fault history and earthquake 

potential of the Hansel Valley area, north central Utah, in 

National Earthquake Hazards Reduction Program; summaries of technical reports, v. XX: U.S. Geological Survey 

Open-File Report 85-464, p. 105-108.

 1987, Quaternary deformation along the East Cache fault, 

north-central Utah [abs.]: Geological Society of America 

Abstracts with Programs, v. 19, no. 5, p. 320.

McGarr, A., 1982, Upper bounds on near-source peak ground 

motion based on a model of inhomogeneous faulting: Bulletin of the Seismological Society of America, v. 72, no. 6, 

p. 1825-1841.
M66 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

 1984, Scaling of ground motion parameters, state of 

stress, and focal depth: Journal of Geophysical Research, v. 

89, no. B8, p. 6969-6979.

McGuire, R.K., 1983, Estimation of seismic ground motion in 

northern Utah: Report submitted to U.S. Geological Survey 

(Menlo Park, Calif.) by Dames and Moore under contract 

14-08-0001-19825, 63 p.

Mertz, H.A., and Cornell, C.A., 1973, Aftershocks in engineering 

seismic risk analysis: Cambridge, Mass., Research Report 

R73-25, Massachusetts Institute of Technology, Department 

of Civil Engineering, 23 p.

Nelson, A.R., and Van Arsdale, 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.

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.]: Eos, Transactions of the American 

Geophysical Union, v. 68, p. 1369.

Personius, S.F., 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.

Power, M.S., Coppersmith, K.J., Youngs, R.R., Schwartz, D.P., 

and Swan, F.H., III, 1981, Seismic exposure analysis for the 

WNP-2 and WNP-1/4 site: Appendix 2.5K to Amendment 

no. 18: Richland Wash., Final Safety Analysis Report WNP-2 

for Washington Public Power Supply System, 58 p.

Richins, W.J., 1979, Earthquake data for the Utah region, in Arabasz,WJ., Smith, R.B., and Richins W.D., eds., Earthquake 

studies in Utah 1850 to 1978: Salt Lake City, University of 

Utah Seismograph Stations Special Publication, p. 57-251.

Rogers, A.M., Carver, D.L, Hays, WW, King, K.W., and Miller, 

R.D., 1984, Preliminary estimates of geographic variation in 

relative ground shaking in the Wasatch front urban corridor, 

in Hays, WW, and Gori, PL., 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. 547-556.

Rogers, A. M., Harmsen, S.C., Herrmann, R.B., and Meremonte, 

M.E., 1986, A study of ground motion attenuation in the 

southern Great Basin, Nevada-California, using several 

techniques for estimates of Qs, log AQ, and coda Q\: Journal 

of Geophysical Research, v. 92, no.B5, p. 3527-3540.

Rogers, A.M., J.C. Tinsley, and R.D. Borcherdt, 1985, Predicting 

relative ground response, in Ziony, J.I., ed., Evaluating 

earthquake hazards in the Los Angeles region An earthscience perspective: U.S. Geological Survey Professional 

Paper 1360, p. 221-247.

Sadigh, K., Egan, J.A., and Youngs, R.R., 1986, Specification of 

ground motion for seismic design of long period structures: 

Earthquake Notes, v. 57, no. 1, p. 13.

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

characteristic earthquakes from the Wasatch and San 

Andreas faults: Journal of Geophysical Research, v. 89, no. 

B7, p. 5681-5698.

 1986, Seismic hazards: New trends in analysis using geologic data, in Wallace, R.E., chairperson, Active tectonics: 

Washington D.C., National Academy Press, p. 215-230.

Schwartz, D.P., Hanson, K.L., and Swan, F.H., 1983, Paleoseismic 

investigations along the Wasatch fault zone: An update, in 

Crone, A.J. (ed.), Paleoseismicity along the Wasatch Front 

and adjacent areas, central Utah: Geological Society of 

America Rocky Mountain and Cordilleran Sections Meeting, 

Guidebook pt. 2: Utah Geological and Mineral Survey Special Studies 62, p. 45-40.

Schwartz, D.R, Swan, F.H., and Cluff, L.S., 1984, Fault behavior 

and earthquake recurrence along the Wasatch fault zone, in 

Hays, WW, and Gori, PL, 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.

Scott, WE., 1980, Quaternary stratigraphy of the Wasatch Front, 

in Evernden, J.F., compiler, Summaries of technical reports, 

v. IX, National Earthquake Hazards Reduction Program: 

U.S. Geological Survey Open-File Report 80-6, p. 62-64.

Seed, H.B., and Idriss, I.M., 1982, Ground motions and soil liquefaction during earthquakes, in Engineering monographs on 

earthquake criteria, structural design, and strong motion 

records: Berkeley, Calif., Earthquake Engineering Research 

Institute, v. 5, p. 134.

Seed, H.B., Idriss, I.M., and Keifer, F. W, 1969, Characteristics of 

rock motions during earthquakes: Proceedings of the American Society of Civil Engineers, Journal of the Soil Mechanics and Foundation Division, v. 95, no. SM5, p. 1199-1218.

Seed, H.B., Murarka, R., Lysmer, J., and Idriss, I.M., 1976a, Relationships of maximum accelerations, maximum velocity, 

distance a from source and local site conditions for moderately strong earthquakes: Bulletin of the Seismological Society of America, v. 66, no. 4, p. 1323-1342.

Seed, H.B., C. Ugas, and J. Lysmer, 1976b, Site-dependent spectra for earthquake-resistant design: Bulletin of the Seismological Society of America, v. 66, n. 1, p. 221-244.

Shemeta, J.E., and Pechmann, J.C., 1989, Source parameters of 

aftershocks of the 1983 Borah Peak, Idaho, earthquake 

[abs.]: Eos, Transactions of the American Geophysical 

Union, v. 70, p. 1188.

Singh, S., 1981, Regionalization of crustal Q in the continental 

United States: St. Louis, Mo., St. Louis University, unpublished Ph.D dissertation, 103 p.

Singh, S., and Herrmann, R.B., 1983, Regionalization of crustal 

coda Q in the continental United States: Journal of Geophysical Research, v. 88, no. Bl, p. 527-538.

Slemmons, D.B., 1977, State-of-the-art for assessing earthquake 

hazards in the United States, Report 6, Faults and earthquake magnitude: U.S. Army Corps of Engineers, Waterways Experiment Station Miscellaneous Paper S-73,129 p.

 1982, Determination of design earthquake magnitudes for 

microzonation, in International Earthquake Microzonation 

Conference, 3d, Seattle, Wash., 1982, Proceedings v. 1., p. 

119-130.

Slemmons, D.B., and Depolo, G.M., 1986, Evaluation of active 

faulting and associated hazards, in Studies in geophysics 

Active tectonics: Washington, D.C., National Academy 

Press, p. 45-62.

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.C., and Buck-
PROBABILISTIC ANALYSIS OF EARTHQUAKE GROUND-SHAKING HAZARDS M67

nam, R.C., convenors and eds., Proceedings of Workshop 

XXVIII on the Borah Peak, Idaho, earthquake: U.S. Geological Survey Open-File Report 85-290-A, p. 236-263.

Stepp, J.C., 1972, Analysis of completeness of the earthquake 

sample in the Puget Sound area and its effect on statistical 

estimates of earthquake hazard, in International Conference on Microzonation, Seattle, Wash., 1972, Proceedings v. 

2, p. 897-909.

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, Utah: Bulletin of the 

Seismological Society of America, v. 70, no. 5, p. 1431-1462.

Swan, F.H., III, Schwartz, D.P., Hanson, K.L., Knuepfer, PL., and 

Cluff, L.S., 1981a, Study of earthquake recurrence intervals 

on the Wasatch fault at the Kaysville site, Utah: U.S. Geological Survey Open-File Report 81-0228, 52 p.

Swan, F.H., III, Hanson, K.L., Schwartz, D.P, and Knuepfer, PL, 

1981b, Study of earthquake recurrence intervals on the 

Wasatch fault at the Little Cottonwood Canyon site, Utah: 

U.S. Geological Survey Open-File Report 81^50, 30 p.

Swan, F.H., III, Schwartz, D.R, Cluff, L.S., Hanson, K.L., and 

Knuepfer, PL., 1981c, Study of earthquake recurrence intervals on the Wasatch fault at the Hobble Creek site, Utah: 

U.S. Geological Survey Open-File Report 81-229, 58 p.

Swan, F.H., III, Hanson, K.L., and Schwartz, D.P, 1983, Study of 

earthquake recurrence intervals on the Wasatch fault, Utah: 

Eighth semiannual technical report prepared for the U.S. 

Geological Survey (Menlo Park, Calif.) under contract 14- 

07-0001-19842, 40 p.

Tinsley, J.C., King., K.W., Trumm, D.A., Carver, D.L., and Williams, R., 1991, Geological aspects of shear-wave velocity 

and relative ground response in Salt Lake Valley, Utah, in 

McCalpin, J.P, ed., Proceedings of the Symposium on Engineering Geology and Geotechnical Engineering, 27th, Logan 

Utah, 1991, p. 25-1-25-9.

Townley, S.D. and Alien, M.W, 1939, Descriptive catalog of 

earthquakes of the Pacific coast of the United States 1769 

to 1928: Bulletin of the Seismological Society of America, v. 

29, no. 1, p. 1-297.

Uhrhammer, R.A., 1986, Characteristics of northern and central 

California seismicity [abs.]: Earthquake Notes, v. 57, no. 1, 

p. 21.

Utah Geological Association, 1972, Environmental geology of 

the Wasatch Front, 1971: Utah Geological Association Publication 1, 336 p.

Veneziano, D., and Van Dyck, J., 1985, Analysis of earthquake 

catalogs for incompleteness and recurrence rates, in Seismic hazard methodology for nuclear facilities in the Eastern 

United States [Electric Power Research Institute Research 

Project P101-29]: Palo Alto, Calif., EPRI/SOG Draft 85-1, v. 

2, app. A-6.

Weichert, D.H., 1980, Estimation of the earthquake recurrence 

parameters for unequal observation periods for different 

magnitudes: Bulletin of the Seismological Society of America, v. 70, no. 4, p. 1337-1346.

Westaway, R., and Smith, R.B., 1989, Strong ground motion in 

normal-faulting earthquakes: Geophysical Journal, v. 96, p. 

529-559.

Wheeler, R., 1984, A plan for evaluating hypothesized segmentation of the Wasatch fault: in Hays, WW, and Gori, PL., 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.

Woodward-Clyde Consultants (San Francisco), 1982, Preliminary assessment of the seismic potential of the Oquirrh 

fault zone, Utah: Report prepared by Woodward-Clyde for 

Davy McKee Corporation (San Mateo, Calif.), 16 p.

Wyss, M., 1979, Estimating maximum expectable magnitude of 

earthquakes from fault dimensions: Geology, v. 7, no. 7, p. 

336-340.

Youngs, R.R. and Coppersmith, K.J., 1985a, Development of a 

fault-specific recurrence model: Earthquake Notes [abs.], v. 

56, no. 1, p. 16.

 1985b, Implications of fault slip rates and earthquake 

recurrence models to probabilistic seismic hazard estimates: Bulletin of the Seismological Society of America, v. 

75, no. 4, p. 939-964.

Youngs, R.R., Coppersmith, K.J., Power, M.S., and Swan, F.H., III, 

1985, Seismic hazard assessment of the Hanford region, 

eastern Washington State, in Department of Energy Natural 

Phenomena Hazards Mitigation Conference, Las Vegas, 

1985, Proceedings, p. 169-176.

Youngs, R.R., Makdisi, R, Sadigh, K, and Abrahamson, N.A., 

1990, The case for magnitude dependent dispersion in peak 

ground acceleration [abs.]: Seismological Research Letters, 

v. 61, no. 1, p. 30.

APPENDIX
M70 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

APPENDIX: COMPARISON OF STRONGMOTION DATA FROM THE STUDY REGION 

WITH SELECTED ATTENUATION 

RELATIONSHIPS

This appendix compares strong-ground-motion data 

from recordings of earthquakes in the study region with 

recently developed attenuation relationships based primarily on data from California. The regional data include 

one recording in Logan, Utah, at a distance of about 

30 km from the 1962 ML 5.7 Cache Valley earthquake; several recordings at distances of about 90 to 100 km from 

the 1983 MS 7.3 Borah Peak, Idaho, earthquake; and many 

recordings in the distance range of about 2 to 50 km during aftershocks of the 1983 Borah Peak earthquake. For 

Borah Peak earthquake aftershocks, only data for earthquakes of ML>4 are included. The ground-motion data 

from the study region are compared with the relationships of Joyner and Boore (1982), Sadigh and others 

(1986), and Campbell (this volume).

Figure Al compares the peak ground accelerations 

of the only strong-motion recording to date in Utah (the 

Logan recording obtained on a soil site during the 1962 M L 

5.7 Cache Valley earthquake) with the median attenuation 

curves from the three attenuation relationships. The data 

are higher than the curves. However, they are within the 

range of variation about the median curves typically 

found in strong-motion data, the average of the two data 

points being approximately 0.8, 1.3, and 2.6 standard deviations above the median relationships of Joyner and 

Boore (1982), Sadigh and others (1986), and Campbell 

(this volume), respectively (using the standard deviations 

presented by the authors for their respective relationships).

Figure A2 compares response spectra of the Logan 

recording with median and 84th-percentile spectra predicted by the relationships of Sadigh and others (1986) 

and Joyner and Boore (1982). The spectra of the recorded 

motions range from the median to well above the 84th 

percentile (median plus one standard deviation) of the 

two relationships.

Peak acceleration data recorded during the MS 7.3 

Borah Peak earthquake are compared with the respective 

attenuation relationships in figure A3. Because most of 

the data are from soil sites, Sadigh and others' (1986) relationship for soil sites rather than that for rock sites was 

used. The recorded data, particularly those recorded in 

the free field rather than in basements, as indicated in the 

figure, tend to be somewhat higher overall than two of the 

attenuation curves and about equal to or lower than the 

third curve. A moment magnitude of 7.1 was calculated 

for this earthquake by using the moment versus magnitude relationship adopted for this study (as described in 

the main text of this paper) and the seismic moment

0.3

0.1

0.03

0.01

0.003

0.001

EXPLANATION 

RECORDED DATA 

 1962 Cache Valley earthquake, 

ML 5.7

ATTENUATION RELATIONSHIPS

 Sadigh and others (1986)

 Joyner and Boore (1982)

_______ Campbell (this volume)

10 30 

DISTANCE, IN KILOMETERS

100

FIGURE A-l. Comparison of peak ground accelerations recorded at Logan 

during the 1962 Cache Valley, Utah, earthquake and median attenuation 

relationships.
APPENDIX M71

z 

o

0.5

0.2

0.1

0.05

0.02

0.01

0.005

0.002

0.001

84th percentile

EXPLANATION

 Cache Valley (north) 

 Cache Valley (east)

 Sadigh and others (1986)

i i i i i

0.04 0.1 0.2

_ 84th percentile

EXPLANATION

- Cache Valley (north) 

 Cache Valley (east)

 Joyner and Boore (1982)

. . . ,l

0.5 2 4 0.04 0.1 

PERIOD, IN SECONDS

0.2 0.5

FIGURE A-2. Comparison of response spectra (5-percent damping) of ground motions recorded during the 1962 Cache Valley, Utah, earthquake and median and

84th-percentile spectra from attenuation relationships.

reported by Jackson and'Boatwright (1985). The attenuation curves for magnitude 7.1 would be slightly lower that 

those for MS 7.3 shown in figure A3, which would emphasize the overall trend for the recorded data for this earthquake to be slightly above the California attenuation 

curves.

Figure A4 compares peak accelerations from the 

1983 Borah Peak earthquake aftershocks (magnitudes 

greater then 4) with the respective attenuation curves. As 

is the case for the main shock, most of the aftershock 

data were recorded on soil sites, so Sadigh and others' 

(1986) soil-site relationship was used for the comparisons. The data are compared with the Joyner and Boore 

(1982) attenuation curves separate from the Sadigh and 

others (1986) and the Campbell (this volume) curves

because of significant differences in hypocentral and epicentral distances for some of the records. Hypocentral 

distance is most consistent with the distance definitions 

used by Sadigh and others and by Campbell, and epicentral distance is most consistent with the distance definition used by Joyner and Boore (see main text for these 

distance definitions). However, for magnitudes less than 

5, only comparisons with the Sadigh and others and the 

Campbell curves are presented, because Joyner and 

Bo ore's relationships are restricted to magnitudes equal 

to or greater than 5. (In fact, Campbell's relationships 

were also developed for magnitudes equal to or greater 

than 5 but can be reasonably be used for magnitudes in 

the range of 4 to 5 (K.W. Campbell, personal commun., 

1986).) For some of the aftershocks, significant differ-
M72 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

0.3

0.1

O)

z 

o

0.03

0.01

0.003

0.001

EXPLANATION 

RECORDED DATA 

1983 Bo rah Peak earthquake, Ms 7.3 

 Free field 

O Basement 

ATTENUATION RELATIONSHIPS

 Sadigh and others (1986)

 Joyner and Boore (1982)

 Campbell (this volume)

FIGURE A-3. Comparison of peak ground accelerations recorded during 

the 1983 Borah Peak, Idaho, earthquake and median attenuation 

relationships.

10 30 100 

DISTANCE, IN KILOMETERS

300

ences exist between the ML magnitudes reported by the 

University of Utah Seismograph Stations and the moment 

magnitudes obtained by Jackson and Boatwright (1987). 

Therefore, the data are compared with the curves for both 

sets of magnitudes. Regardless of whether the data are 

compared with the curves for ML or for moment magnitude, it can be concluded from figure A4 that the data 

tend to lie below the attenuation curves, substantially so 

for some of the aftershocks.

For the Borah Peak main shock and aftershock 

ground-motion data, observations similar to those stated 

above have been made by Jackson and Boatwright (1985).

Our overall conclusion from these comparisons is 

that the data are too few to either strongly support or 

refute the applicability of California attenuation relationships to Utah. The data show no consistent trend to lie 

above or below the values predicted by the attenuation 

relationships.
APPENDIX M73

0-5 | , , , ,

0.2

0.1

0.05

0.02

0.01

0.005

at

2 0.002 

O

gj 0.001 

8 0.5

0.2

0.1

0.05

0.02

0.01

0.005

0.002

0.001

i i i r

M L=5.8

=4.7

EXPLANATION 

Event 2329, M L=5.8, Mw=4.7 

O Recorded data

 Sadigh and others (1986)

- - - Campbell (1987)

. . I______ . . .

=5.4 -

=4.3 -

EXPLANATION 

Event 2339, M L=5.4, Mw=4.3 

O Recorded data

 Sadigh and others (1986)

- - - - Campbell (this volume)

* *

6

O

O * ,MW=4.7

EXPLANATION 

Event 2329, ML =5.8, Mw=4.7 

O Recorded data 

 * Joyner and Boore (1982)

i . i i I______i___i .

* *

6 \

vx \

O

o

EXPLANATION 

Event 2339, M L =5.4, Mw=4.3

O Recorded data 

 * Joyner and Boore (1982)

* M L=5.4 -

* Mw=4.3

O

5 10 20 50 100 5 10 20 50 100 

HYPOCENTRAL DISTANCE, IN KILOMETERS EPICENTRAL DISTANCE, IN KILOMETERS

FIGURE A-4. Comparison of peak ground accelerations recorded during 1983 Borah Peak, Idaho, aftershocks and median attenuation

relationships.
M74 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

0.1. , , , f.

0.05

0.02

0.01

0.005

0.002

0.001

Z 0.0005 

O

I

ui

iu 0.0002

^ 0.1

=4.8

EXPLANATION 

Event 0124, ML =4.8, Mw=3.8 

O Recorded data

 Sadigh and others (1986)

- - - - Campbell (this volume)

w

0.05

0.02

0.01

0.005

0.002

0.001

0.0005

0.0002

=3.5 :

=4.2 -

EXPLANATION O 

Event 2343, ML =4.2, /WU,=3.5 

O Recorded data 

 Sadigh and others (1986)

- - - Campbell (this volume)

. i i I______.

M, =4.7

=3.6

EXPLANATION 

Event 0159, ML =4.7, Mw=3.6 

O Recorded data

 Sadigh and others (1986)

- - - - Campbell (this volume)

, , , I______. . . .

iiiir_

=4.6 -

=3.5 -

EXPLANATION 

Event 2104, ML =4.6, Mw=3.5 

O Recorded data 

 Sadigh and others (1986)

- - - - Campbell (this volume)

i i . . I______.

10 20 50 100 5 10 20 

HYPOCENTRAL DISTANCE, IN KILOMETERS

50 100

FIGURE A-4. Comparison of peak ground accelerations recorded during 1983 Borah Peak, Idaho, aftershocks and median attenuation

relationships Continued.
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

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-N

A study of ground response in the Salt Lake City and 

Springville, Utah, areas and correlations of response data with 

reflection data

CONTENTS

	Page 

Abstract............................................................................................................................................................. Nl

Introduction...................................................................................................................................................... 1

Previous Studies................................................................................................................................................ 1

Geology............................................................................................................................................................. 4

Data Acquisition............................................................................................................................................... 4

Strong-Motion Network.......................................................................................................................... 4

Site Response......................................................................................................................................... 5

Shallow Reflection................................................................................................................................. 5

Data Reduction and Analysis ........................................................................................................................... 5

Site Response......................................................................................................................................... 5

Seismic-Reflection Data Processing...................................................................................................... 9

Discussion......................................................................................................................................................... 11

Conclusions...................................................................................................................................................... 21

References Cited............................................................................................................................................... 23

ILLUSTRATIONS

Page

FIGURES 1,2. Maps showing:

1. Locations of sites in Salt Lake City at which ground-motion recordings of nuclear explosions and shallow seismicreflection profiles were acquired.................................................................................................................................................................. N2

2. Location of sites in the Springville-Spanish Fork area at which ground-motion recordings of nuclear explosions and

shallow seismic-reflection profiles were acquired........................................................................................................................................ 3

3. Source input shooting pattern, station interval spacing, and source-to-geophone distances used on each reflection

profile.................................................................................................................................................................................^ 6

4, 5. Time histories showing vertical, horizontal north-south, and horizontal east-west components of ground motion from 

4. Nuclear explosion event B recorded simultaneously at nine sites in Salt Lake City.................................................................................... 7

5. Nuclear explosion event A recorded simultaneously at five sites in the Springville-Spanish Fork area...................................................... 8

6-11. Graphs showing:

6. Amplitude spectral comparison for three sites computed from recordings of vertical and horizontal ground motion

generated by nuclear explosion event B ....................................................................................................................................................... 9

7. Amplitude spectral comparison for three sites computed from recordings of vertical and horizontal ground motion

generated by nuclear explosion event A....................................................................................................................................................... 10

8. Comparison of signal and noise amplitude spectra for three sites in Salt Lake City computed from ground motion

generated by nuclear explosion event B ....................................................................................................................................................... 11

9. Comparison of signal and noise amplitude spectra for three sites in the Springville-Spanish Fork area computed from

ground motion generated by nuclear explosion event A .............................................................................................................................. 12

10. Spectral ratios of vertical and horizontal components of ground motion recorded on sites having geologic conditions

different from those at the crystalline rock site in Salt Lake City................................................................................................................ 14

11. Spectral ratios of vertical and horizontal components of ground motion recorded on sites having geologic conditions

different from those at the crystalline rock site in the Springville-Spanish Fork area................................................................................. 16

12, 13. Maps comparing horizontal component average spectral ratios from Hays and King (1982) and from this study for 

12. Salt Lake City area........................................................................................................................................................................................ 17

13. Springville-Spanish Fork area...................................................................................................................................................................... 18

14-18. Seismic reflection profiles from 

14. SLC site 5 (Bonneville Golf Course)............................................................................................................................................................ 20

15. SLC site 12 (Liberty Park)................................................................................._^ 20

16. SLC site 8 (Forest Dale Golf Course)........................................................................................................................................................... 21

17. SSF site 3 (Hobble Creek)............................................................................................................................................................................ 22

18. SSF site 5 (Spanish Fork Airport)................................................................................................................................................................ 22

III
IV CONTENTS

TABLE

Page

TABLE 1. Seismic-reflection and ground-response data ................................................................................................................................................................. N23
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK 

ALONG THE WASATCH FRONT, UTAH

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

ABSTRACT

Earthquake site-response estimates were computed for 14 sites in 

Salt Lake City and the Springville-Spanish Fork areas of Utah. Lowstrain ground motions, generated by nuclear tests located in Nevada, 

were recorded on a wide range of surficial geologic materials by threecomponent digital seismographs. Site response is determined by the 

spectral-ratio method in which the Fourier spectrum of a site under 

investigation is divided by the spectrum of a reference site located on 

crystalline bedrock. We assume that the relative response of higher 

strain ground motions produced during a local earthquake at the sites in 

question can be estimated by low-strain ground-motion response. We do 

not consider local nonlinear soil response or directivity effects that 

could occur during a large local earthquake. High-resolution seismicreflection profiles were also acquired at some of the sites to examine the 

feasibility of correlating site response with the impedance structure 

inferred from the seismic-reflection data.

The spectral ratio values are described in terms of amplification 

factors relative to the crystalline bedrock site. Results indicate that an 

increase in ground motion can be expected westward from the Wasatch 

Front toward the center of the valley. In the period band 0.2 to 3.33 s, 

amplification factors of 1.5 to 4.5 are common near the mountain front, 

whereas factors of 7 to 12 characterize sites closer to the center of the 

Salt Lake valley. At similar distances from the Wasatch Front in the 

Springville-Spanish Fork area, the amplification factors of 5 to 15 are 

higher than those in the Salt Lake valley. Correlations between seismic 

reflections and site response suggest that part of the cause for higher 

amplification factors is due to near-surface high-impedance contrast 

amplification of seismic waves.

INTRODUCTION

The principal population centers and urban areas of 

Utah are located near the Wasatch fault zone. Although 

the Wasatch fault and associated faults have not produced 

an earthquake larger than magnitude 6 since 1850 (Arabasz and others, 1979), the potential for a damaging and 

(or) life-threatening earthquake to occur within the next 

100 years is very high (Swan and others, 1980). Structural 

damage and life risks from ground motions induced by

earthquakes could be lessened if the potential groundshaking intensity for urban areas could be accurately predicted. This study is part of a U.S. Geological Survey-Utah 

Geological and Mineral Survey program to assess 

earthquake ground-shaking hazards in Utah's Wasatch 

Front urban areas.

The general objective of this study is to improve the 

understanding of how shallow underlying geology affects 

ground shaking. The study (1) compares site-response 

amplification factors derived from Fourier amplitudes 

with those derived from pseudo-relative velocity algorithms (Hays and King, 1982); (2) develops a detailed 

ground-response map of the Salt Lake City urban area in 

the low- to medium-response areas of Hays and King 

(1982); and (3) examines the feasibility of correlating site 

response with geologic data that have been derived from 

high-resolution shallow seismic-reflection profiles.

The study included analysis of ground-shaking data 

induced by distant nuclear explosions at the Nevada Test 

Site recorded at 14 sites in the Salt Lake City and 

Springville-Spanish Fork areas (figs. 1, 2). The methodology was similar to that used by Borcherdt (1975), Rogers 

and Hays (1978), Hays and King (1982), and Rogers and 

others (1985). High-resolution shallow-reflection profiles 

were acquired at five of the sites. In addition, a semipermanent network of strong-motion seismic instruments 

was deployed in the Salt Lake City-Ogden urban areas to 

record higher intensity ground shaking if local earthquakes should occur during these urban hazards studies.

PREVIOUS STUDIES

The variability of damage to structures caused by 

induced vibrations has been documented since the early 

20th century. Milne (1908) and Reid (1910) observed that

Nl
N2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112°00' 57'30" 55' 52°30" 50' 111 047'30"

40°47'30"

45'

42'30"

40°40'

EXPLANATION

0 Dr-200site

 Reflection Hne

 Strong-motion 

seismoscope

^ Strong-motion 

accelerograph

3500 SOUTH STREET

J* 8400 SOUTH STREET

2 3 MILES 

J_____I

I 1 f 1 I 

01234 KILOMETERS

FIGURE 1. Locations of sites in Salt Lake City at which ground-motion recordings of nuclear explosions and shallow seismic-reflection profiles were 

acquired. The estimated depths to basement for the Salt Lake City (SLC) sites shown on figure 4 are, from top to bottom, 0,30, 80,140,140,180, 

190,270, and 440 m (R. Van Horn, unpublished data, 1986).

structures underlain by loose soil, marshy deposits or 

filled land incurred more damage during ground shaking 

induced by earthquakes than similar structures underlain 

by bedrock. Advances in seismic instrument technology 

permitted Kanai (1952) and Gutenberg (1957) to make

quantitative studies of ground shaking induced by earthquakes at sites underlain by different materials. Their 

studies indicated that sites underlain by alluvium or filled 

land may experience up to 10 times more earthquake 

ground shaking than sites underlain by bedrock.
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N3

111'J42'30" 40' 37'30" 35' 1ir32'30"

40°10'lEXPLANATION

 Reflection line 

O Borehole site 

Dr-200site

40"5

4 KILOMETERS

FIGURE 2. Location of sites in the Springville-Spanish Fork area at which ground-motion recordings of nuclear explosions and shallow s eismic

reflection profiles were acquired.

The testing of nuclear devices at the Nevada Test Site 

and the advancement of computer analysis have 

increased the sophistication of quantitative, detailed, and 

controlled empirical investigations into the variability of 

the frequency and amplitude of ground shaking at 

selected sites. Studies by Borcherdt (1970), Murphy and 

others (1971), Rogers and Hays (1978), and Hays and others (1978) have shown that realistic ground-shaking 

estimates for moderate-sized earthquakes can be made by 

using ground-motion scaling laws derived from the 

analysis of low-strain nuclear-explosion-induced ground 

motions. Studies by Borcherdt and Gibbs (1976), Hays 

and King (1982), and Rogers and others (1985) have 

shown that local site conditions have a greater influence 

on ground shaking than source azimuth, wave type, or

angles of incidence. A ground-shaking study of the San 

Francisco Bay area (Borcherdt and Gibbs, 1976) showed 

an average spectral amplification for horizontal ground 

motions on bay mud sites to range between 3.7 and 11.3, 

relative to sites located on bedrock. A similar study of the 

Wasatch Front area (Hays and King, 1982) indicated that 

the greatest potential hazard from earthquake ground 

shaking would occur at sites underlain by thick, finegrained, water-saturated, nearshore and offshore lacustrine deposits of silts and clays. A ground-response study 

of the Los Angeles area (Rogers and others, 1985) showed 

a ground-motion amplification factor of 4 to 6 (at certain 

frequencies) at sites underlain by Holocene and Pleistocene sediments relative to sites underlain by crystalline 

rock. The Los Angeles area study also indicated a correla-
N4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

tion of ground-motion amplification with the mean void 

ratio of near-surface materials when the ground-shaking 

period was 0.2 to 0.5 s.

GEOLOGY

The Salt Lake City and Springville-Spanish Fork 

areas investigated in this study are located in similar geologic settings. The Wasatch Front borders both study 

areas on the east, where Jurassic to Precambrian limestone, dolomite, shale, sandstone, and quartzite are 

exposed (McGregor and others, 1974). These cratonderived rocks are thought to underlie the alluvial sediments upon which most of the Wasatch Front urban areas 

are located. These sediments are unconsolidated fluvial 

and lacustrine materials deposited in and around Pleistocene lakes and alluvial fans of Pleistocene age or 

younger (Van Horn, 1972; Swan and others, 1980). McGregor and others (1974) categorized the alluvial sequence 

beneath the Salt Lake City area as Quaternary alluvium at 

the surface underlain by Tertiary alluvial deposits, which 

in turn are underlain by Jurassic to Precambrian bedrock. 

The cumulative thickness of the alluvial stratum (Quaternary and Tertiary) increases westward as the distance 

from the Wasatch Front increases. The estimated range of 

total alluvial thickness underlying the Salt Lake City study 

area is from 0 to approximately 450 m (McGregor and others, 1974). Tertiary alluvium, varying in thickness from 

0 to 200 m, rests on Jurassic and other bedrock in the Salt 

Lake City study area. The Tertiary deposits are generally 

semiconsolidated layers whose composition ranges from 

boulders to clay interbedded with some well-cemented 

layers.

The unconsolidated sediment classification used in 

the study areas of Hays and King (1982) was adapted from 

Miller's (1980) study of the Salt Lake valley surficial 

deposits. The Miller study divided the surficial deposits of 

the Salt Lake valley into three categories according to the 

environment of deposition: onshore, nearshore, and offshore. The study indicated that the deposits become more 

fine grained with increasing distance from the Wasatch 

Front. The onshore deposits are mostly fluvial in origin 

and generally form a narrow band along the base of the 

Wasatch Front and into the Salt Lake valley as terraces 

along the edges of the streams draining the Wasatch 

Range. The onshore deposits also are found as extensive 

flood-plain deposits overlying thick sequences of finegrained lacustrine deposits. The onshore deposits consist 

mainly of sand, gravel, cobbles, and boulders and grade 

into the nearshore deposits, which can be coarse grained 

and poorly sorted. The nearshore sediments were deposited chiefly in shallow water along the lakeshores and 

formed beaches, longshore spits and bars, deltas, and 

marsh deposits. The offshore deposits represent a quietwater depositional environment and consist mainly of 

interbedded very fine sand, silt, and clay.

In the Salt Lake City area, the recording sites 

described in this report are located on the full range of 

surficial geological materials as defined by Miller (1980). 

Site 1 is located on limestone of Jurassic age, sites 2, 3, 9, 

and 10 are underlain by onshore deposits, sites 4 and 6 are 

underlain by nearshore deposits, and sites 5 and 8 are 

underlain by offshore deposits (fig. 1).

The geology of the Springville-Spanish Fork area is 

generally similar to that of the Salt Lake City area The 

thickness of alluvial deposits is less well known owing to 

sparse subsurface geotechnical information in the area. 

Site 1 in the Springville area is underlain by quartzite of 

Pennsylvanian-Permian age. Geologic descriptions of 

surficial deposits near sites 2 and 3 (fig. 2) (Swan and others, 1980) and a description of the subsurface material 

from a nearby borehole (J.C. Tinsley, written commun., 

1986) indicate that the materials at this location are similar to the Salt Lake City onshore deposits. A description 

of the material from a borehole at the Spanish Fork Airport (site 4) and at the Youd farm (site 5) is similar to the 

descriptions of the Salt Lake City offshore deposits (J.C. 

Tinsley, unpublished data, 1986). The subsurface geology 

at these and other sites is being investigated.

DATA ACQUISITION

STRONG-MOTION NETWORK

Nine strong-motion accelerographs and seven strongmotion seismoscopes are deployed in the Salt Lake City 

and Ogden areas of Utah. Three strong-motion accelerographs (SMA-1) are permanently located in the Salt Lake 

City area in standby operation mode. Six additional SMA1 accelerographs have been added to the three-station 

network for the duration of the urban hazards investigation. These instruments were installed to provide 

expanded recording of ground shaking in the Salt Lake 

City area if a medium to large earthquake should occur in 

or near the study area The sites selected for the deployment of the six additional accelerographs followed criteria similar to those used for selecting site-response 

station locations. One accelerograph is located on a hardrock site near Salt Lake City (site 1) to establish a base 

against which to compare the other seismic data The 

remaining five instruments were deployed at sites underlain by a variety of depths and types of alluvium. Two are 

located at sites occupied by Hays and King (1982), and 

two are located at sites investigated in this report (fig. 1).

Induced vibrations from large buildings during an 

earthquake could compromise the ground-shaking data 

recorded by the strong-motion instrument if the instrument is located near or in the structure. Therefore, the 

strong-motion instruments are located at a distance of
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N5

more than twice the height of the nearest large building. 

The strong-motion instruments begin recording when 

ground shaking exceeds 0.1 g acceleration in the 1.0- to 

40-Hz bandwidth.

Seven seismoscopes were installed in the Salt Lake 

City area to supplement the strong-motion network. The 

seismoscopes will record the amplitudes of ground shaking at the instrument's natural period (0.75 s) (Cloud and 

Hudson, 1961). The sites chosen for the seismoscopes 

represent different thicknesses of alluvium overlying the 

basement bedrock of the valley. The site locations (fig. 1) 

were selected from the Salt Lake valley gravity map data 

of Adhidjaja and others (1981). No earthquake large 

enough to trigger or record on the accelerographs or seismoscopes has occurred since the instruments were 

installed.

SITE RESPONSE

The site-response ground-shaking data presented in 

this report were recorded by portable digital seismographs. The seismometers are three-component velocitysensing transducers that have a natural period of 0.62 s 

and are damped at 60 percent of critical. The data are digitally recorded on 1/4-inch magnetic tape at 50 samples 

per second per channel. Carver and others (1986) have 

described the seismographs and their calibrations.

The sites at which response studies were conducted 

(1) represent sites from the low-, medium-, and highresponse areas described by Hays and King (1982); (2) are 

in the Salt Lake City area; (3) are at sites where additional 

geotechnical experiments could be done to reveal details 

of the underlying geology; (4) are in areas where major 

differences of surficial geology occur as described by 

Miller (1980); and (5) are in locations where there are 

enough open areas to conduct high-resolution shallowreflection studies, drill a borehole, and conduct future 

experiments. The study did not include Salt Lake City 

locations in high-response areas described by Hays and 

King (1982), because most of those sites have been inundated by the rise of the Great Salt Lake. Site-response 

sites selected in the Springville-Spanish Fork area (fig. 2) 

included two locations in the deeper parts of the basin 

that are believed to be similar to the high-response sites 

near Salt Lake City as reported by Hays and King (1982).

An underground nuclear test at the Nevada Test Site 

provided the seismic energy. Previous reports by 

Borcherdt (1970), Hays and King (1982), and Rogers and 

others (1985) have demonstrated the validity of using the 

ground motions from a Nevada Test Site nuclear test for a 

site-response study. A minimum of 2 min of pre-event seismic data were recorded for the tests. The pre-event data 

at each site were used to establish the level of ambient 

seismic background noise at that station before the 

recording of ground shaking from the nuclear event.

SHALLOW REFLECTION

High-resolution, shallow seismic-reflection profiles 

were collected at five sites in the Salt Lake City and 

Springville-Spanish Fork areas that coincided with the 

locations used to study relative ground shaking and site 

response (figs. 1, 2). The data for the reflection profiles 

were collected from a linear array that ranged from 

approximately 40 to 160 m in length. Station separations 

of 0.5 to 1.5 m became the positions of the geophones and 

the reference locations for the input sources. The data 

were amplified, filtered, summed, digitally sampled at 0.5- 

ms intervals, and stored on V^-inch magnetic tape by a 

high-resolution 24-channel reflection system.

Single 100-Hz (resonant frequency) compressionalwave geophones with 15-cm ground coupling spikes were 

firmly planted in the ground surface at 0.5- to 1.5-m intervals to detect P-wave reflections. The -6 dB/octave rolloff 

of the 100-Hz geophones and -18 dB/octave rolloff of the 

80-Hz low-cut filter in the recording system help discriminate against the surface waves generated by the energy 

source. The seismic energy source was a 12-gauge shotgun that fired a 28-g lead slug vertically into the ground. 

The slug dissipated its energy quickly and usually penetrated less than 0.2 m into the ground. The data from three 

shots at each shotpoint are summed and recorded. Summing of data from several shots at a single shotpoint 

improves the signal-to-noise ratio owing to enhancement 

of the coherent reflective seismic signal and by canceling 

some of the less coherent ambient seismic noise.

The shotpoints were located in the middle of the geophone array (split-spread). The minimum and maximum 

source-receiver remained constant (constant offsets) as 

the source locations were moved along the array in increments equal to the station (geophone) spacing distance. 

Figure 3 shows the source input shooting pattern, the station interval spacing, and the source-to-geophone distances used on each profile. The half-integer source input 

configuration was used to achieve -3 dB ambient seismic 

noise attenuation during the data processing (Knapp, 

1985). The array also allows common-depth-point (CDP) 

processing of the data (Mayne, 1962). The array pattern 

and CDP processing achieve a summing of 12 different 

source-to-geophone raypath combinations for each subsurface point along the length of the profile (twelvefold 

coverage).

DATA REDUCTION AND ANALYSIS

SITE RESPONSE

Ground motion due to a nuclear test in Nevada was 

digitally recorded at nine sites in the Salt Lake City area 

and at five sites in the Springville-Spanish Fork area by 

wideband digital-recording seismograph systems using
N6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

101 105 110 125 130

GEOPHONES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

EXPLANATION

SLC site 5 (Bonnevitle Golf Course): A, 9.1 m; B, 33.5 m; C, 3.0 m 

SLC site 8 (Forest Dale Golf Course): A, 5.5 m; B, 13.4 m; C, 1.2 m 

SLC site 12 (Liberty Park): A, 5.5 m; B, 13.4 m; C, 1.2 m 

SSF site 3 (Hobble Creek (Orchard)): A, 5.5 m; B, 13.4 m; C, 1.2 m 

SSF site 5 (Spanish Fork Airport): A, 9.0 m; B, 33.0 m; C, 3.0 m

FIGURE 3. Source input shooting pattern, station interval spacing, and source-to-geophone distances used on each reflection profile. S hotpoint 

locations are halfway between stations. Station designations used in the reflection profiling are not related to site response s tation numbers.

three-component velocity-sensing seismometers (Carver 

and others, 1986). Approximately 2 min of pre-event and 6 

min of postevent seismic data from each site were preserved as time histories in digital form on VAX 11/750 

files. These data were reduced to amplitude-normalized 

time histories (figs. 4, 5), spectra (figs. 6, 7), spectral signal to spectral noise comparisons (figs. 8, 9), and spectral 

ratios (figs. 10, 11). The data-reduction methods were 

similar to those used by Hays and King (1982) and Rogers 

and others (1985). Spectral ratios (site-transfer functions 

of sites on rock to sites underlain by alluvium) derived 

from nuclear-test ground motions show reasonable agreement with site-transfer functions similarly derived from 

earthquake-induced ground motions (Rogers and Hays, 

1978). The results presented here are a reasonable estimate of the site effects of a damaging earthquake.

The time-history data were low pass filtered (10-Hz 

corner, -18 dB/octave) to help eliminate local ambient 

seismic noise. Rogers and others (1985) found that most 

of the seismic energy from nuclear tests above 5 Hz is 

attenuated before reaching recording sites that are more 

than approximately 400 km from the source. Therefore, 

the filtering was acceptable for this study because all the 

recording stations are more than 500 km from the source.

Approximately 200 s of seismic signal were selected 

(windowed) to the nearest power of 2 of the number of 

samples and tapered with a whole-cosine-bell (Harming 

window) before being Fourier transformed by a Fast 

Fourier Transform (FFT) algorithm (A.C. Tarr and E.

Cranswick, unpublished data, 1986). A similar procedure 

was used on the pre-event time-history data to derive the 

spectra for ambient seismic noise at each site. None of 

the spectral amplitudes have been normalized by the window length because the spectra used for site-to-site comparisons are derived from time series of identical 

duration. The methods used by Rogers and others (1980) 

to estimate the reliability of the spectral estimates are not 

directly applicable to this study owing to the absence of 

multiple observations (recordings) from each site. The 

horizontal spectra were used to derive site-transfer functions from the sites located on rock relative to the sites 

underlain by unconsolidated sediments. The site-transfer 

function was calculated by the general formula STF= Sa/Sr 

where STF is the site-transfer function, Sa is the derived 

spectrum of a site underlain by alluvium, Sr is the derived 

spectrum of a site underlain by rock, and all spectra are in 

the frequency band between 0.3 and 5.0 Hz.

Rogers and others (1979) discussed the method of 

deriving the site-transfer function in terms of spectral 

ratios. The site-transfer functions of this study were calculated directly from the unsmoothed, independent spectral amplitudes. Similarly, the frequency band of the 

spectral ratios in this report represents the averages of 

the unsmoothed spectral ratios between the defined frequency limits (A.C. Tarr and E. Cranswick, unpublished 

data, 1986). Only spectra with a signal-to-noise separation 

greater than 6 dB were used for deriving the spectral 

ratios (figs. 8, 9). The absence of data or "holes" in some
STATIONS

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VERTICAL COMPONENT HORIZONTAL N-S COMPONENT HORIZONTA

3

40 80 120 0 40 80 

TIME, IN SECONDS

120 

FIGURE 5. Vertical, horizontal north-south, and horizontal east-west components of ground motion from nuclear explosion event A recorded si

Springville-Spanish Fork area. Recording amplitudes normalized to station 1 (hard rock). 
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N9

VERTICAL COMPONENT

1 5 

HORIZONTAL N-S COMPONENT

1 5 

HORIZONTAL E-W COMPONENT

FREQUENCY, IN HERTZ

EXPLANATION 

Station 1 - Station 2 Station 9

FIGUEE 6. Amplitude spectral comparison for three sites computed 

from recordings of vertical and horizontal (north-south and eastwest) ground motion generated by nuclear explosion event B. The 

spectra are identified by station number location (see figs. 1, 2), 

arranged by decreasing alluvial thickness (top to bottom), and 

plotted at frequencies for which the signal-to-noise ratio is greater 

than 2.

of the spectral ratios shown on figures 10 and 11 are due 

to the elimination of low signal-to-noise data. The frequency bands used to derive the average ratios in this 

report are the same as those used in the Hays and King 

(1982) report to enable a direct comparison of the spectral ratio numbers. The spectra and spectal ratios shown 

on the figures presented in this report are smoothed by 

convolving the derived spectra with a whole-cosine-bell 

16 samples wide. The graphic smoothing is intended to 

emphasize what are judged to be the significant trends in 

the data; the reliability of these trends can be estimated 

by inspecting the composite signal and noise plots shown 

in figures 8 and 9.

SEISMIC-REFLECTION DATA PROCESSING

Five high-resolution shallow seismic-reflection profiles were completed for this report. The sites were chosen to coincide with site-response locations, for variety of 

subsurface geology, and for the availability of a sufficient 

area in which an unobstructed reflection line could be 

obtained. Level sites were selected to eliminate the static 

corrections needed in the data processing. Correction for 

topography variations was needed, however, for Salt Lake 

City site 8. The field data were digitally recorded on magnetic tape and processed by software on a VAX 11/780 

computer. The data were first edited, sorted into CDP 

gathers, and then plotted in preliminary form. The CDP 

gathers supplied the two-way traveltime data to be used 

for further analysis. All seismic data for which two-way 

traveltime was greater than 100 ms were eliminated. The 

data truncation (or data elimination) was necessary 

because the arrival of surface-wave seismic signals interfered with reflection data after 100 ms. The stacking 

velocities of the truncated CDP data were determined by 

using a constant-velocity panel search. The correct stacking velocity adjusts traveltime differences in the CDP data 

that were due to different source-to-receiver distances.

The velocity analysis procedure needed the input of a 

series of best-estimate traveltime velocities ( V) likely to 

be encountered in the area. The following equation:
N10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

VERTICAL COMPONENT

</)

DC 

LUQ- in-2

1 5 

HORIZONTAL N-S COMPONENT *: 10

HORIZONTAL E-W COMPONENT

FREQUENCY, IN HERTZ 

EXPLANATION

Station 1 ----- Station 2 Station 5

FIGURE 7. Amplitude spectral comparison for Springville-Spanish 

Fork stations 1,2, and 5 computed from recordings of vertical and 

horizontal (north-south and east-west) ground motion generated 

by nuclear explosion event A. The spectra are identified by station 

number location (see fig. 3), arranged in order of decreasing 

alluvial thickness (top to bottom), and plotted at frequencies for 

which the signal-to-noise ratio is greater than 2.

where Tx is the actual two-way traveltime, X is the offset 

distance from the analyzed trace to the source position, 

and TO is the zero-offset two-way traveltime for the given 

velocity, was used by the software program to automatically calculate TQ on a sample-by-sample basis on each 

CDP trace for each velocity. A sequence of traveltime 

velocities was selected that would give CDP traces having 

the least amount of normal-moveout curvature.

These selected traveltime velocities also produced 

the maximum trace amplitude at the two-way traveltime 

of the reflection pulse when the CDP's were summed. The 

time-corrected data were again edited to remove any 

interfering electrical noise, spikes, or pre-first reflective 

pulse seismic signals that may have been moved into the 

desired data from the velocity-time corrections. The data 

were then filtered to help attenuate the seismic energy 

below 100 Hz. The final data were displayed in stacked 

CDP sections that showed seismic-reflection amplitudes 

in a two-way traveltime versus horizontal distance format. The processing parameters are listed below:

1. Reformatting of field data

2. Editing of dead or bad traces

3. Definition of reflection line geometry

4. CDP sorting of common-shot gathers

5. Band-pass filter analysis of CDP gathers

6. Seismic velocity analysis of CDP gathers

7. Muting analysis of normal move-out corrected CDP 

gathers

8. Preliminary stack

9. Reexamination of chosen velocities, filters, and 

mutes

10. Amplitude scaling of final stack before plotting

11. Final stack

The average vertical resolution for the 100- to 150-Hz 

stacked seismic data was about 1 to 2 m, which was calculated by using the V4-wavelength criteria on velocities 

derived from the reflection data (Widess, 1973). All processed data were truncated to give a minimum of 800 percent (eightfold) coverage or better on the resultant 

profiles.
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH Nil

VERTICAL COMPONENT

STATION 1 

HORIZONTAL N-S COMPONENT HORIZONTAL E-W COMPONENT

Signal

STATION 2

STATION 9

0.3 1 5 

FREQUENCY, IN HERTZ

FIGURE 8. Comparison of signal and noise amplitude spectra for three sites in Salt Lake City computed from ground motion generated by nuclear 

explosion event B. Noise spectra calculated from analysis of a 50-s segment before the arrival of event B.

DISCUSSION

The time histories of the site-response data for the 

Salt Lake City and Springville-Spanish Fork areas are 

shown in figures 4 and 5. The data have been amplitude 

normalized to the sites underlain by rock and arranged 

from top to bottom on the illustration, according to a general increase in the thickness of underlying alluvium and 

an increased distance west from the Wasatch Front 

toward the center of the valley. The sites in figures 4 and 5 

are ordered according to thickness of underlying alluvium, this order owing to the limited amount of geologic 

information available at this time. The estimated depths

to basement for the Salt Lake City (SLC) sites shown on 

figure 4 are, from top to bottom: 0, 30, 80, 140, 140, 180, 

190, 270, and 440 m (R. Van Horn, unpublished data, 

1986). The depths of alluvium at the Springville-Spanish 

Fork (SSF) sites are less well known. The projection of 

the dip of an exposed outcropping of bedrock, approximately 50 m from SSF site 2, to a point at depth beneath 

SSF site 2 indicates that the underlying thickness of alluvium at that station probably does not exceed 30 m. Borehole tests near SSF site 3 and SSF site 4 did not penetrate 

bedrock at a total depth of 150 m, and borehole tests near 

SSF site 5 did not penetrate bedrock at a total depth of 

300 m (R. Van Horn, unpublished data, 1986). Figures 4
N12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

VERTICAL COMPONENT

STATION 3 

HORIZONTAL N-S COMPONENT HORIZONTAL E-W COMPONENT

STATION 4

STATION 5

0.3 1

FREQUENCY, IN HERTZ

0.3

FIGURE 9. Comparison of signal and noise amplitude spectra for three sites in the Springville-Spanish Fork area computed from ground m otion 

generated by nuclear explosion event A. Noise spectra calculated from analysis of a 50-s time segment before the arrival of even t A.

and 5 show an increase in signal amplitude in both P-wave 

and surface-wave portions of the seismograms and an 

increase in the wavetrain duration with increasing alluvium depth and distance from the Wasatch Front (toward 

the center of the valley). Figure 6 shows the derived spectra from the data recorded at SLC site 1 (rock), SLC site 2 

(30 m of alluvium), and SLC site 9 (440 m of alluvium). 

Figure 7 shows the derived spectra from the data 

recorded at SSF site 1 (rock), SSF site 2 (approximately 

30 m of alluvium), and SSF site 5 (approximately 300 m of 

alluvium). The spectra show that an increased spectral 

amplification and a slight spectral peak shift toward the 

lower frequencies may correlate with increased thickness 

of alluvial deposits.

Transfer functions, or averaged spectral ratios, were 

derived from unsmoothed spectra for bandwidths similar 

to those used in the Hays and King (1982) report. Spectra 

from the stations underlain by alluvium were divided by 

the spectra of the stations underlain by rock. The spectral 

ratios show that amplification occurs over most of the frequency band for which a good signal-to-noise ratio exists 

(figs. 10,11). The average spectral amplification over four 

period bands 0.2 to 0.7, 0.7 to 1.0, 1.0 to 2.0, and 2.0 to 

3.33 s are shown for Salt Lake City in figure 12 and for 

Springville-Spanish Fork in figure 13. The amplification 

factors range from approximately 1.5 to 4.5 for sites near 

the Wasatch Front, underlain by thinner sections of alluvium, to factors of 7 to 12 for sites in the Springville-Spanish Fork area that are located near the center of the valley
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N13

and are underlain by more than 300 m of alluvium. Sites 

near the middle of Salt Lake valley, where Hays and King 

(1982) had obtained relatively high values of spectral ratio 

response numbers, could not be reoccupied for direct 

comparison owing to flooding caused by the rise of the 

Great Salt Lake. However, data from sites in midvalley 

locations in the general Springville-Spanish Fork area, 

similar to the midvalley Salt Lake City locations, show 

site response in the 0.2- to 0.7-s period bandwidth to be 

higher than that for the rock site by a factor of 7 to 12, 

which compares well with Salt Lake City data given by 

Hays and King (1982).

Figures 4 through 13 show a general trend in 

increased response as the depth of alluvium or distance 

from the Wasatch Front increases, but anomalies in this 

trend are observed. For example, SLC site 5 clearly shows 

more time-history amplitude spectral energy and higher 

average spectral ratio factors in the 0.2- to 0.7- and the 0.7- 

to 1.0-s period bandwidths than SLC site 4. Yet SLC site 5 

is located closer to the Wasatch Front and is underlain by 

a thinner alluvial sequence than SLC site 4. Hays and King 

(1982) postulated the existence of areas that would show 

high ground-shaking amplification in close proximity to 

sites displaying average ground response. An explanation 

for the anomalously high amplification observed at SLC 

site 5 can be found in the high-resolution shallowreflection profile made at that site (fig. 14). The analysis 

of the data shown on the reflection profile yielded seismic 

P-wave velocities of the near-surface layers that are much 

slower (550-600 m/s) than those of the deeper alluvium 

and underlying bedrock (4.5 km/s) (Arnow and Mattick, 

1968). Because the difference in impedance of vertically 

adjacent layered media amplifies seismic waves in proportion to the contrast in impedances of these layers 

(Medvedev, 1962), the slow near-surface velocities calculated for this site, in contrast to the deeper rocks having 

higher velocities, may be producing the impedance difference needed to generate the higher ground response. An 

aquifer at a 50-m depth is encountered in a well located 

270 m northeast of the recording site, but the aquifer was 

not detected on the reflection profile. Either the aquifer 

reflector is very weak, or, more reasonably, the water 

table is deeper than the reflection methods used can 

resolve (deeper than 75 m). Therefore, a shallow water 

table, which Evernden and Thomson (1985) suggested to 

explain the increased ground motion, apparently does not 

exist at this site.

High-resolution shallow-reflection profiles were also 

run in Salt Lake City at SLC sites 8 and 12. The near-surface P-wave velocities measured at SLC site 12 (915-1650 

m/s) were the highest observed among the three sites. 

The P-wave velocities at SLC site 12 were calculated from 

the reflection two-way traveltimes of the three two-cycle 

reflections (F, G, and H, fig. 15) shown in the stacked

record section. A shallow depth to the water table (1-3 m) 

(Mower and Van Horn, 1973) indicates that the sediments 

are saturated, a condition that probably contributed to 

the increased seismic velocities calculated for this site. 

Additionally, impedance contrast amplification across the 

horizons indicated by the strong reflections at SLC site 12 

may also be a significant factor here in amplifying the 

upward-propagating waves from the nuclear test.

The P-wave velocities measured at SLC site 8 

(approximately 1,000 m/s) were intermediate to velocities 

at SLC sites 5 and 12. The high-resolution reflection profile at this site crosses the East Bench fault (A, fig. 16) and 

shows a reflection at 20 to 40 ms (10-20 m depth) downdropped to the west approximately 5 ms (2.5 m) at position A. West of position A, other reflection discontinuities 

that suggest faulting are indicated at positions B, C, D, 

and E. Except for position E, the reflection offsets are 

down to the west. There is a slight indication that beds 

between positions B and C may be back rotated toward 

the fault. The fault probably penetrates deeper into the 

section than figure 16 shows; however, the surface wave 

and the shallower reflectors obscure later reflective arrivals from deeper structures. Ground response was also 

intermediate, average spectral ratio factors (ASRF) being 

4.2 to 6.6 at this site, in comparison with SLC site 5 (2^4.5) 

or SSF site 5 (7.7-10.1).

The seismic-reflection profile from SSF site 3 is 

underlain by approximately 100 m of alluvium and shows 

shallow reflections in the 20- to 50-ms two-way traveltime 

range (6-14 m depth range) (J, K, and L, fig. 17). The origin of these undulating structures is unknown, but a possibility is that they are due to the fluvial processes of 

nearby Hobble Creek. The seismic P-wave velocities computed for the labeled reflections in figure 17 are relatively 

slow 550 m/s. Again, the slow seismic-reflection velocities, calculated at this site for near-surface deposits, may 

be an indicator of an acoustic impedance contrast that 

would amplify upward-traveling seismic waves into the 

intermediate ground-response range (ASRF between 4.0 

and 8.0).

The reflection profile from SSF site 4, a high-groundresponse site (ASRF between 8 and 12), shows a moderately deep (approximately 55 m), flat-lying two-cycle 

reflection at about 80-ms two-way traveltime (M, fig. 18). 

A shallow water-table depth (1.5 m), measured in the 

borehole at this site, probably improved the source coupling to the ground and therefore allowed deeper penetration of the seismic signal. Resolution of any horizons 

above this event was unsuccessful because ground roll 

and air blast dominate this part of the record. Also, the 

absence of acoustic contrasts large enough to generate an 

observable reflection earlier in the record may have contributed to the lack of reflections. Still, the reflection 

recorded in this profile is the latest event (in two-way
N14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

VERTICAL COMPONENT

STATION 2/STATION 1 

HORIZONTAL N-S COMPONENT HORIZONTAL E-W COMPONENT

10

0.1

10

0.1

10

0.1

STATION 5/STATION 1

STATION 4/STATION 1

0.3 1 5 

FREQUENCY, IN HERTZ

STATION 6/STATION 1

10

VERTICAL DATA LOST 1

0.3 1 5 0.3 1 5 

FREQUENCY, IN HERTZ FREQUENCY, IN HERTZ

FIGURE 10. Spectral ratios of vertical and horizontal (north-south and east-west) components of ground motion recorded on 

sites having geologic conditions different from those at the crystalline rock site (station 1) in Salt Lake City. Spectral 

ratios are not plotted where the signal-to-noise ratio is less than 2.
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N15

VERTICAL COMPONENT

10

0.1

STATION 10/STATION 1 

HORIZONTAL N-S COMPONENT HORIZONTAL E-W COMPONENT

10

0.1

STATION 3/STATION 1

10

0.1

STATION 8/STATION 1

10

0.1

0.3

STATION 9/STATION 1

5 0.3 1 5 0.3 

FREQUENCY, IN HERTZ

FIGUEE 10. Continued.
N16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

10

0.1

VERTICAL COMPONENT

STATION 10/STATION 1 

HORIZONTAL N-S COMPONENT HORIZONTAL E-W COMPONENT

STATION 3/STATION 1

g

1

10

0.1

STATION 4/STATION 1

10

0.1

STATION 5/STATION 1

0.3 0.3 1 5 

FREQUENCY, IN HERTZ

0.3

FIGURE 11. Spectral ratios of vertical and horizontal (north-south and east-west) components of ground motion recorded on 

sites having geologic conditions different from those at the crystalline rock site in the Springville-Spanish Fork area. 

Spectral ratios not plotted where the signal-to-noise ratio is less than 2.
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N17

Salt Lake City, Utah B

40°45' -

40°42'30" -

40°45' -

40°42'30" -

Salt Lake City, Utah

111 0 50'

40°45' -

40°42'30" -

111 0 50'

0 1 2 3 KILOMETERS

EXPLANATION

* Dr-200 site * Pseudo-relative velocity response 

D Shallow reflection site (Hays and King, 1982)

40°45' -

40°42'30" -

* Dr-200 site

O Shallow reflection site

3 KILOMETERS

EXPLANATION

* Pseudo-relative velocity response 

(Hays and King, 1982)

FIGURE 12. Horizontal component average spectral ratios from Hays and King (1982) and from this study for the Salt Lake City area A, Top 

shows ratios for the period band 0.2 to 0.7 s; bottom shows ratios for the period band 0.7 to 1.0 s. B, Top shows ratios for the period 

band 1.0 to 2.0 s; bottom shows ratios for the period band 2.0 to 3.33 s.
N18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

Springvillc-Spanish Fork area

11T40' 111 a 37'30"

40°10' -

40°07'30" -

FIGURE 13. Horizontal component average spectral ratios 

from Hays and King (1982) and 

from this study for the Springville-Spanish Fork area A, Top 

shows ratios for the period 

band 0.2 to 0.7 s; bottom shows 

ratios for the period band 0.7 to 

1.0 s. B, Top shows ratios for 

the period band 1.0 to 2.0 s; bottom shows ratios for the period 

band 2.0 to 3.33 s.

111 °40'

40°10' -

40°07'30" -

I I I T

01234 KILOMETERS

Dr-200 site

Shallow reflection site

EXPLANATION

D Pseudo-relative velocity response

(Hays and King, 1982)
B

RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N19

FIGURE 13. Continued. 

Springville-Spanish Fork area

10' 111°37'30" ____

r

40° 10' -

40C07'30" -

40°10' -

40°07'30" -

1H 040' 111°37'30"

* Dr-200site

% Shallow reflection site

EXPLANATION

Q Pseudo-relative velocity response

(Hays and King, 1982)
N20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

FIGURE 14. Seismic reflection profile from SLC site 5 (Bonneville Golf Course) showing reflection labeled I.

FIGURE 15. Seismic reflection profile from SLC site 12 (Liberty Park) showing reflections labeled F, G, and H. Ground roll

dominates in the zone labeled N.
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N21

S-^

i8-

8 8

^^ -Fk'P

~ Jig?

8-

SM

8-

iSo

NI

'31AII113AVU1 

AVM-OM1

o

00

 a 

|

1

O

ft§

traveltime) seen on any of the profiles. The seismic Pwave velocity determined for the reflection labeled M (fig. 

18) is 1,370 m/s, which is significantly higher than the 

velocity determined for the reflections at SSF site 3.

The data in table 1 suggest a weak correlation 

between ground response and the results of the seismicreflection work. Two sites, SLC 12 and SSF 4, have high 

ground response (ASRF 8-12.1) the only two sites 

where seismic-reflection data were collected in which a 

reflector was detected in the 70- to 80-ms time range (55- 

to 60-m depth range). Two other sites, SLC 8 and SSF 3, 

have intermediate ground response (ASRF 4.5-8.5) and a 

reflection arrival in the 30- to 45-ms range. The remaining 

reflection profile, SLC 5, does not fit into this pattern of 

ground response versus depth and arrival time of reflections; it has low response (ASRF 1.8-4.5) and a moderately deep reflection (60 ms). It is possible that the 

calculated velocities for SLC 5 are in error or that different field methods would resolve more reflections that 

might fit the above pattern. However, the reflection depth 

versus ground response database is too small to be considered statistically significant at this time. Additional 

reflection profiles have been acquired and are being 

added to the data set.

CONCLUSIONS

The results obtained by Hays and King (1982), who 

developed a contoured site-response map of the Salt Lake 

City area based on transfer functions derived from 

pseudo-velocity spectra ratios, and those in this report, 

which used transfer functions derived from Fourier transformed spectra ratios, show good agreement (figs. 12,13). 

Both data sets show an increase in response westward 

from the Wasatch Front toward the center of the valley. 

Also, both data sets show that the average spectral ratio 

factors for Salt Lake City, at similar distances from the 

Wasatch Front, differ from those for Springville-Spanish 

Fork; for sites near the Wasatch Front in Salt Lake City, 

the ASRF vary from 1.5 to 4, whereas, for sites at similar 

distances from the Wasatch Front in the Springville-Spanish Fork area, the factors are 5 to 15. The good agreement 

between the results of this study and those of Hays and 

King (1982) demonstrates the repeatability of generating 

site-response values and gives a sense of reliability to the 

overall procedure used by Borcherdt and Gibbs (1976), 

Hays and King (1982), and Rogers and others (1985) for 

mapping ground response and developing groundresponse predictions. The site-response estimates in this 

study are made without consideration of directivity 

effects, source characteristics, or high-strain effects of 

nearby earthquakes.

The analysis of the high-resolution shallow-reflection 

profiles shows that the technique is a viable means of
N22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

FIGUEE 17. Seismic reflection profile from SSF site 3 (Hobble Creek) showing reflections labeled J, K, and L.

FIGUEE 18. Seismic reflection profile from SSF site 5 (Spanish Fork Airport) showing reflection labeled M.

mapping shallow structures in urban areas underlain by 

unconsolidated sedimentary sections. The profiles suggest promising correlations between the later arriving 

reflections (70-80 ms) and higher ground response and 

between intermediate site response and earlier arriving 

reflections (30-45 ms). However, the high-resolution shallow-reflection method will need a larger database to show

if a statistically significant correlation exists. Additional 

site-specific geologic studies and high-resolution shallowreflection studies are being conducted at the groundresponse sites used in this study and in the Hays and King 

(1982) report. The resultant geotechnical analysis and 

correlations among geologic factors and the site-response 

factors are in progress.
RELATIVE GROUND RESPONSE IN SALT LAKE CITY AND AREAS OF SPRINGVILLE-SPANISH FORK, UTAH N23

TABLE 1. Seismic-reflection and ground-response data 

[ , not observed at this site]

Average spectral ratio factor: 

0.2 -0.7 s .............................................................................

0.7 -1.0 s.............................................................................

1.0 -2.0s.............................................................................

2.0 -3.3 s .............................................................................

No. of reflections on the profile..... .....................................

Reflection continuity (percentage of total profile) ..........

Reflection two-way traveltime (milliseconds) ..................

Stacking velocity for reflections (meters/second) ...........

Depth to reflection (meters) ...............................................

Bonneville 

Golf Course 

(SLC 5)

4.5

o o

1.8

2.0

1

39

60

600

18

Forest Dale 

Golf Course 

(SLC 8)

4.5

4.2

4.7

6.6

1

100

30

1,000

15

Liberty Park 

(SLC 12)

8.0

9.0

8.5

6.0

3

100

40 

40 

30

50 

70 

915

1,475 

1,660 

14

37 

58

Hobble Creek 

(SSF 3)

8.5

5.5

3.5

1.6

2

66

25 

20

45 

550

600 

8

14

Spanish Fork 

Airport 

(SSF 4)

12.1

9.0

12.0

6.9

1

51

80

1,370

55

REFERENCES CITED

Adhidjaja, J.I., Cook, K.L., and Serpa, L.F., 1981, Salt Lake Valley 

gravity map and data: Utah Geological and Mineral Survey, 

Open-File Report 39.

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 Special Publication, 552 p.

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.

Borcherdt, R.D., 1970, Effects of local geology on ground motion 

near San Francisco Bay: Bulletin of the Seismological Society of America, v. 60, p. 29-61.

 ed., 1975, Studies for seismic zonation of the San Francisco Bay region: U.S. Geological Survey Professional 

Paper 941-A, 102 p.

Borcherdt, R.D., and Gibbs, J.F., 1976, Effects of local geological 

conditions in the San Francisco Bay region on ground 

motions and the intensities of the 1906 earthquake: Bulletin 

of the Seismological Society of America, v. 66, p. 467-500.

Carver, D.L., Cunningham, D.R., and King, K.W., 1986, Calibration and acceptance testing of the DR-200 digital seismograph with S-6000 and the L-4C seismometers: U.S. 

Geological Survey Open-File Report 86-340, 28 p.

Cloud, W.K., and Hudson, D.E., 1961, A simplified instrument for 

recording strong motion earthquakes: Bulletin of the Seismological Society of America, v. 51, p. 159-274.

Evernden, J.F, and Thomson, J.M., 1985, Predicting seismic 

intensities, in Ziony, J.I., ed., Evaluating earthquake hazards 

in the Los Angeles region An earth-science perspective: 

U.S. Geological Survey Professional Paper 1360, p. 151-202.

Gutenberg, B., 1957, Effects of ground on earthquake motion: 

Bulletin of the Seismological Society of America, v. 47, p. 

221-250.

Hays, W.W., and King, K.W., 1982, Zoning of the earthquake 

ground-shaking hazard along the Wasatch fault zone, Utah: 

International Earthquake Microzonation Conference, 3d, 

Seattle, 1982, Proceedings, p. 1307-1318.

Hays, W.W., Algermissen, S.T., Miller, R.D., and King, K.W., 1978, 

Preliminary ground response maps for the Salt Lake City 

area: International Conference on Microzonation, 2d, San 

Francisco, 1978, Proceedings, v. 2, p. 497-508.

Kanai, K., 1952, Relation between the nature of the surface layer 

and the amplitude of earthquake motion: Tokyo University 

Earthquake Research Institute Bulletin, v. 30, p. 31-37.

Knapp, R.W., 1985, Using half-integer source offset with split 

spread CDP seismic data: Geophysics: The Leading Edge of 

Exploration, v. 4, no. 1, p. 66-69,108.

Mayne, W.H., 1962, Common reflection point horizontal data 

stacking techniques: Geophysics, v. 27, p. 952-965.
N24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

McGregor, E.E., Van Horn, Richard, and Arnow, Ted, 1974, Map 

showing the thickness of loosely packed sediments and the 

depth to bedrock in the Sugar House quadrangle, Salt Lake 

County, Utah: U.S. Geological Survey Miscellaneous Investigations Map I-766-M, scale 1:24,000.

Medvedev, J.V., 1962, Engineering seismology: Moscow, Academia Nauk. [Translated into English by Israel Program for 

Scientific Translations, 1965, 260 p.]

Miller, R.D., 1980, Surficial geologic map along part of the 

Wasatch Front, Salt Lake Valley, Utah: U.S. Geological Survey Miscellaneous Field Studies Map MF-1198, scale 

1:100,000, 2 sheets, pamphlet.

Mime, J., 1908, Seismology (2d ed.): London, K. Paul, Trench, 

Trubner, and Col, 320 p.

Mower, R.W., and Van Horn, Richard, 1973, Map showing minimum depth to water in shallow aquifers (1963-1972) in the 

Sugar House quadrangle, Salt Lake County, Utah: U.S. Geological Survey Miscellaneous Investigations Map I-766-I, 

scale 1:24,000.

Murphy, J.R., Weaver, N.L., and Davis, A.H., 1971, Amplification 

of seismic body waves by low-velocity surface layers: Bulletin of the Seismological Society of America, v. 61, p. 109- 

145.

Reid, H.F., 1910, The California earthquake of April 18, 1906 

The mechanics of the earthquake: Carnegie Institute of 

Washington Publication 87, v. 2,192 p.

Rogers, A.M., and Hays, W.W, 1978, Preliminary evaluation of 

site transfer functions developed from nuclear explosions

and earthquakes: International Conference on Microzonation, 2d, San Francisco, 1978, Proceedings, v. 2, p. 753-764.

Rogers, A.M., Covington, P.A., Park, R.B., Borcherdt, R.D., and 

Perkins, D.M., 1980, Nuclear event time histories and computed site transfer functions for locations in the Los Angeles region: U.S. Geological Survey Open-File Report 80- 

1173, 207 p.

Rogers, A.M., Tinsley, J.C., and Borcherdt, R.D., 1985, Predicting 

relative ground response, in Ziony, J.I., ed., Evaluating 

earthquake hazards in the Los Angeles region An earthscience perspective: U.S. Geological Survey Professional 

Paper 1360, p. 221-247.

Rogers, A.M., Tinsley, J.C., Hays, W.W, and King, K.W, 1979, 

Evaluation of the relation between near-surface geological 

units and ground response in the vicinity of Long Beach, 

California: Bulletin of the Seismological Society of America, 

v. 61, p. 1603-1622.

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.

Van Horn, Richard, 1972, Surficial geologic map of the Sugar 

House quadrangle, Salt Lake County, Utah: U.S. Geological 

Survey Miscellaneous Investigations Map I-766-A, scale 

1:24,000.

Widess, M.B., 1973, How thin is a thin bed?: Geophysics, v. 38, 

p. 1176-1180.
In Situ Poisson's Ratio Measurements Near Provo, Utah

By RICHARD D. MILLER andDON W. STEEPLES, Kansas Geological Survey, 

KENNETH W. KING, U.S. Geological Survey, and RALPH W. KNAPP, Kansas 

Geological Survey

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-O

CONTENTS

Abstract............................................................................................................................................................. Ol

Introduction...................................................................................................................................................... 1

Techniques........................................................................................................................................................ 2

Field Methods................................................................................................................................................... 2

Provo Airport.......................................................................................................................................... 3

Springville Cherry Orchard.................................................................................................................... 4

Data Processing................................................................................................................................................ 4

Discussion......................................................................................................................................................... 4

Provo Airport.......................................................................................................................................... 4

Springville Cherry Orchard.................................................................................................................... 5

Conclusions...................................................................................................................................................... 8

References Cited............................................................................................................................................... 10

ILLUSTRATIONS

FIGURES 1,2. Twelve-fold CDP stacks from the Provo Airport showing:

1. P waves acquired with the .50-caliber rifle....................................................................................................................................................... O5

2. S waves acquired with the rotational arrow...................................................................................................................................................... 6

3-5. Twelve-fold CDP stacks from the cherry orchard showing:

3. P waves acquired with the .30-06 rifle.............................................................................................................................................................. 7

4. P waves acquired with the .50-caliber rifle....................................................................................................................................................... 8

5. S waves acquired with the rotational arrow...................................................................................................................................................... 9

III

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK 

ALONG THE WASATCH FRONT, UTAH

IN SITU POISSON'S RATIO MEASUREMENTS 

NEAR PROVO, UTAH

By RICHARD D. MILLER, DON W. STEEPLES, KENNETH W. KING, and RALPH W. KNAPP

ABSTRACT

Intensity variations of several units on the Modified Mercalli scale 

at sites only a few kilometers apart have been well documented. Classification of sites according to response to earthquake energy would enable 

engineers to accurately determine minimum structural requirements for 

individual sites in zones of high to moderate earthquake risk. It is possible to produce shallow P-wave and S-wave reflection data containing the 

necessary information to allow direct in situ determination of Poisson's 

ratio (which is directly related to site response). During two separate 

visits to Provo, Utah, and the surrounding area, high-resolution shallow 

P- and S-wave surveys were conducted. The P- and S-wave data from 

Provo airport are of excellent quality and yield average velocity values 

accurately enough to allow determination of Poisson's ratio as a function of depth in the upper 50 m of the geologic section. Poisson's ratio at 

the Provo airport (a high-response site) is calculated to be 0.44, whereas 

a low-response site near a cherry orchard has a Poisson's ratio of 0.27.

INTRODUCTION

Intensity of ground shaking during an earthquake is 

determined in part by local near-surface geologic conditions. Intensity differences of three to four units on the 

Modified Mercalli scale at sites no more than a few 

kilometers apart have been well established for nearly a 

century (Lawson, 1908). Because of these differences, 

prediction of individual site response is an important part 

of urban earthquake-hazard mitigation. To estimate such 

specific site responses in the San Francisco Bay area, 

Borcherdt and others (1975) used intensity maps from the 

1906 San Francisco earthquake and data obtained from 

recording far-field ground motion in the San Francisco 

Bay area from explosions at the Nevada Test Site (NTS).

The use of NTS explosions in other urban areas is not 

feasible now because of the decreasing size of NTS explosions in the past few years and the distance of many of 

these areas from NTS (for example, Seattle, Los Angeles, 

Salt Lake City, Memphis, St. Louis, Evansville, Charleston, 

and Boston). Although moderate to strong future earthquakes in these areas will provide an energy source for 

evaluating ground motions, it is the hazards of these very 

earthquakes that we wish to mitigate.

Knowledge of site response would allow engineers 

the data necessary to better calculate minimum structural 

requirements for development of individual sites in zones 

of high to moderate earthquake risk. It also would help to 

determine more accurately the present risk to structures 

already completed and occupied.

Shallow P- and S-wave reflection data contain the 

necessary information for direct in situ determination of 

some dynamic properties of materials within the upper 

100 m of the Earth's surface. It has long been known that 

the ratio of the acoustic impedances of juxtaposed layers 

theoretically relates to amplification. Medvedev (1965) 

empirically demonstrated the use of seismic shear impedance (the product of density and S-wave velocity) to 

predict relative ground response. The potential for amplification of shaking increases as the impedance contrast 

between layers increases, provided other parameters are 

constant.

Producing shallow P- and S-wave reflection profiles 

along the same surface line allows interpretation of more 

geological information than using P waves or S waves 

alone. Once interval velocities have been determined, it is 

possible to determine variations in Poisson's ratio with 

depth. Equation 1 uses average P and S wave velocities 

from the surface to a reflecting interface to determine 

Poisson's ratio.

« lr-2p CT = o 5 7 2 a2-e2 (D

where CT is Poisson's ratio, a is P-wave velocity, and Pis Swave velocity.

Although it would be possible to produce similar 

results with an expensive drilling program that included

Ol
02 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

cross-hole, downhole, or uphole P- and S-wave velocity 

measurements, logistical problems that accompany 

drilling are sometimes significant, particularly in urban 

areas. High-resolution seismic surveys can be safely performed by shallow-reflection techniques in areas such as 

parks and school yards, essentially without causing any 

environmental damage. For example, we performed one 

such survey across the baseball fields at Sylmar High 

School in the San Fernando Valley of California while the 

students were practicing baseball and softball.

TECHNIQUES

Seismic-reflection techniques have been used in 

petroleum prospecting for about half a century. Although 

examples are occasionally cited in the literature, the use 

of shallow-reflection techniques in engineering and 

ground-water applications has met with only limited success because of the difficulty encountered in identifying 

reflections from layers at shallow depths. The shallowest 

reflections documented in the literature are from 10 m 

(Steeples and Knapp, 1982). The amplitudes of the 

reflected waves at commonly recorded frequencies, when 

present, are smaller than the amplitudes of unwanted 

waves, particularly ground roll composed of Rayleigh 

waves and Love waves. Further, the generation and 

recording of frequencies much above 100 Hz have not 

been done routinely on land.

Typical energy sources that have been tried include a 

sledge hammer (Hunter and others, 1981; Meidav, 1969), 

weight drop (Doornenbal and Helbig, 1983), and small 

explosive charges (Pakiser and Warrick, 1956). These 

energy sources normally produce seismic waves having 

unfiltered dominant frequencies less than 100 Hz. In order 

to provide better resolution (both precision and accuracy) of shallow reflective interfaces, high frequencies are 

needed. Resolution of "thin beds" is theoretically limited 

to about Vs wavelength (V4 wavelength by conventional 

methods) of the recorded seismic energy (Widess, 1973), 

so improvement in resolution and imaging of shallower 

reflectors can be obtained directly by increasing the frequency of the energy recorded.

A worthy goal of shallow-reflection surveys is to provide bed resolution in the approximate dimension of 0.5 

m. Typical velocities of near-surface soil and (or) alluvial 

materials range from about 300 to about 1,200 m/s for P 

waves and from 100 to about 300 m/s for S waves. Using 

classic ^-wavelength methods, the desired 0.5-m resolution cannot be obtained without frequencies greater than 

75 Hz for a P-wave velocity of 300 m/s and 25 Hz for Swave velocity of 100 m/s. We have been able to routinely 

produce dominant P-wave frequencies greater than 200 

Hz and S-wave frequencies greater than 75 Hz.

The source for a shallow high-resolution reflection 

survey must possess a high-frequency signature, and it 

must be field durable and time efficient enough for costeffective seismic data acquisition. Most of our P-wave 

reflection research has involved the use of common hunting rifles modified for safe and quiet operation when fired 

vertically into the ground. Our most productive source 

has been a steel arrow forced hydraulically into the 

ground, rotated by hammer impact, and then removed 

hydraulically. The arrow is equipped with an arm protruding laterally from its base. This arm is impacted by a 

sledge hammer, causing a rotational motion about the 

center of the arrow.

High-resolution compressional-wave surveys have 

generally had the best success when severe low-cut filtering and high-natural-frequency geophones were used 

(Hunter and others, 1984; Steeples and Knapp, 1982; 

Miller and Steeples, 1986). As previously mentioned, the 

higher the dominant frequency of the seismic data, the 

greater the bed resolution. Our most commonly used lowcut filter suppresses energy down 3 dB/octave at 220 Hz 

and down 24 dB/octave below 220 Hz. We have had success with single 100-Hz geophones spaced as closely as 

0.25 m. In certain environments, higher low-cut filters 

(340 or 480 Hz) have proven useful in improving near-surface bed resolution (Treadway and others, 1985).

Field parameters for obtaining good shallow shearwave reflections are extremely site dependent. Usually, 

the low-frequency spectrum of S waves in comparison 

with that of P waves from the same site dictates the use of 

lower natural frequency geophones sensitive to ground 

motion perpendicular to the direction of wave propagation. Recently, we have had the best results with 30-Hz 

horizontal geophones. The most frequently used low-cut 

filter has a -3-dB point at 110 Hz.

S-wave reflection surveys have an added potential for 

suppressing undesirable noise. Owing to the nature of S 

waves, the polarity of reflections recorded is dependent 

on the direction of initial particle displacement. Independently recording shots having initial clockwise source 

rotations and subsequent counterclockwise rotations and 

then subtracting the recorded signal from the two shots 

improve the signal-to-noise ratio.

Geophone spacing is dictated by the degree of lateral 

resolution needed and the desired accuracy of the velocity function. In most instances when the target depth 

range is from 5 to 50 m, we space single geophones at 

1.25-m intervals.

FIELD METHODS

We use an Input/Output, Inc., DHR-2400 seismic 

recording system to amplify, filter, and digitize the data in
IN SITU POISSON'S RATIO MEASUREMENTS NEAR PROVO, UTAH 03

the field and to record the data on digital tape. Our amplifier gains can be adjusted from 42 to 120 dB for the individual channels, depending upon the distance from the 

shot point to the geophone. The upper limit of amplifier 

gains to avoid clipping of the signal is determined by digital work size rather than by the amplifier gains available. 

Our low-cut filters have a 24-dB/octave rolloff to decrease 

the amplitude of ground roll. The data sample interval on 

each channel can vary from 0.25 to 4 ms, the total record 

length being 1,000 samples per channel. Anti-alias filters 

are used to attenuate energy at frequencies above the 

Nyquist frequency (signal down 60 dB at that frequency). 

Relatively severe low-cut filters have the beneficial 

effect of eliminating substantial amounts of cultural 

noise. Cultural noise in urban areas is relatively severe at 

frequencies below 50 Hz, but vehicular traffic and other 

vibratory sources do not produce much energy above 100 

to 150 Hz. The noise that is produced at these high frequencies attenuates rapidly with distance from the source 

because of the low-pass nature of the Earth's transfer 

function. Low-cut filtering also allows limited control 

over the dominant frequency of the recorded energy. 

Boosting the dominant frequency increases the possible 

bed resolution and, therefore, the accuracy of the survey. 

Care must be taken, however, when increasing the lowcut filter. Overly severe filtering of lower frequencies 

decreases the signal-to-noise ratio and causes a ringiness 

in the data, the general result being a poorer quality section and a loss of bed resolution.

PROVO AIRPORT

Compressional-wave data were recorded at the 

Provo airport in the spring of 1985. The line for the Pwave data consisted of 65 shot locations occupied by two 

different sources. The line was set up with a 1.25-m shot 

interval and single 100-Hz geophones spaced at 1.25-m 

intervals. A .30-06 hunting rifle with a specially designed 

airblast containment device was used. Recording was 

done with 220-Hz low-cut filters, and the dominant frequency of the .30-06 seismic data was about 150 Hz. The 

second source was a .50-caliber single-shot sport rifle 

equipped with its own airblast and particle containment 

apparatus. Recording was done with 110-Hz low-cut filters, and the dominant frequency of the .50-caliber data 

was about 100 Hz.

The .50-caliber source was fired twice at each source 

location, the second projectile being fired into the hole 

made by the first projectile. The repetitious firing of projectiles into the same hole allows the energy from later 

shots to dissipate more efficiently into more competent 

material. The two shots were recorded on tape separately 

and processed through a common depth point (CDP)

stack as independent data sets. Then the two stacked sections were cross-correlated and CDP stacked into a single 

twelve-fold CDP seismic section.

The S-wave survey conducted in the spring of 1985 at 

the Provo airport was the first real test of the arrow shearwave source in a twelve-fold CDP production mode of 

operation. The source was a quad-winged arrow pointing 

vertically downward, each wing being oriented either parallel or perpendicular to the survey line. After insertion 

into the ground, the arrow was forced (by sledge hammer) to rotate clockwise and then counterclockwise, and 

each impact was recorded independently. The receivers 

were single 40-Hz horizontal geophones. The low-cut filter used had a -3-dB point at 55 Hz. Source and receivers 

were laid out in a split-spread orientation, the offset from 

source to closest receiver being 5 m. Owing to the lower 

bandpass recorded, the dominant frequency of the CDP 

stacked section was only about 50 Hz. Therefore, resolution was poor, and surface waves overpowered the early 

portions of the seismograms.

As a result of the inappropriate choice of field 

recording parameters in the spring, a second shear-wave 

survey was conducted at the Provo airport during the 

summer of 1985. After analysis of the spring survey data 

and slight modifications to the acquisition technique, new 

field parameters were chosen to improve the previous 

data quality. The new low-cut filter had a -3-dB point at 

110 Hz. Because the S-wave velocity was slower than 

expected and the S-wave radiation pattern was complex, 

the end-on source/receiver chosen had an 11-m source-toclosest-receiver offset.

A new source was designed for the summer trip. 

Although similar to the previous quad-winged arrow, the 

new arrow had only two wings separated by 180 ° and by 

0.25 m from the base. The new arrow design exploited 

some basic principles in an attempt to boost the frequency spectrum as well as reduce the generation of Love 

waves. The surface area of the second arrow head was 

reduced by about 70 percent by cutting off the base half of 

the arrow and replacing it with a 0.15-m long shaft 2.5 cm 

in diameter. The smaller surface area of the doublewinged arrow decreased the energy source volume and 

increased the dominant frequency. Also, the addition of 

the shaft allowed the arrow to penetrate more deeply into 

the soil layer, the result being a noticeable reduction in 

recorded Love waves.

It has been well documented that shear waves have a 

direction-dependent radiation pattern. Onsite studies 

showed that the radiation pattern of S waves generated by 

the smaller arrow was strongly dependent on the orientation of the arrow's wings. A lobe maximum traveled in the 

direction of the line when the arrow's wings were oriented 45° to the line, so that orientation was used for the 

summer survey.
O4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

SPRINGVILLE CHERRY ORCHARD

Compressional-wave data were collected at a cherry 

orchard in Springville; field parameters were similar to 

those used at the airport in Provo. On both runs, the 

receiver (single 100-Hz geophones) and shot spacing was 

1 m. The low-cut filter on the .30-06 survey attenuated the 

signal by 3 dB at 220 Hz, the result being a dominant frequency on the twelve-fold CDP stack of about 130 Hz. The 

.30-06 survey required only a single shot per shot point in 

a split-spread source/receiver array, the source-to-closestreceiver spacing being 1.5 m. The .50-caliber rifle occupied about the first 50 of the 100 shot locations recorded 

for the .30-06 rifle. The low-cut filter for the .50-caliber 

survey attenuated the signal by 3 dB at 110 Hz. These field 

and recording parameters resulted in a dominant frequency on the stacked section of about 90 Hz. The .50- 

caliber data set at the cherry orchard had two shots per 

shot point, the second projectile traveling down the hole 

made by the first projectile. The data were collected by 

using a split-spread array and a source-to-closest-receiver 

spacing of 4.5m.

The shear-wave survey at the Springville cherry 

orchard was conducted by using the same field parameters used in the spring S-wave survey at the Provo airport. 

The cherry orchard survey included the use of single 30- 

Hz horizontal geophones, 55-Hz low-cut filters, and the 

quad-winged arrow.

DATA PROCESSING

The processing flow of the data was similar to ordinary oil company routines. The seismic computer software that was used limits flexibility in engineering 

applications because it was originally intended for largescale petroleum exploration surveys. However, techniques normally used to reduce 10 s of data can be scaled 

down to work on 0.1 s of data by processing in units of 

Poisson's and tenths of seconds instead of meters and 

seconds.

The 24-channel compressional-wave field data were 

reduced to twelve-fold stacked CDP sections. If more 

than a single shot was fired at a location, a time shift was 

generally necessary to align the same CDP's from different shots. This time mismatch was the result of sourcesensor irregularities and delays in the energy transfer of a 

second projectile fired into the hole made by a previous 

projectile. Determining the value of this time shift was 

accomplished by cross-correlating traces from the same 

CDP and shot location for successive projectiles.

Shear-wave data were processed quite similarly to 

the compressional-wave data up to the point where the 

same CDP's from two different shots at the same source 

location were mixed to form a single CDP trace. To compensate for possible time inconsistencies between shot 1 

(straight polarity) and shot 2 (reverse polarity), the same 

cross-correlation performed on compressional-wave data 

was used (after the signal from shot 2 was reversed in 

polarity). The resulting calculated time shift necessary to 

align reflecting events was then applied to the individual 

trace in the same fashion as with the two-shot compressional-wave process. When the data were time shifted, 

polarity matched, and stacked, a pseudo twenty-four-fold 

S-wave reflection section resulted.

DISCUSSION

PROVO AIRPORT

The seismic line at the Provo airport was occupied 

during two separate visits in 1985. The line was located in 

the flat, grassy runway median, where an apparently uniform soil layer was underlain by lacustrine sediments. 

Geophone and source coupling with the ground on both 

the spring and summer trips were excellent and yielded 

high-quality seismic data.

The P-wave seismic section from the Provo airport, 

using the .50-caliber rifle, is shown in figure 1. Excellent 

reflections were detected to 450 ms, a depth of roughly 

300 m. Depth estimates are difficult to make for these 

data because the maximum source-to-receiver offset was 

only 20 m. The short sampling of the normal-moveout 

curve restricted the capability to do velocity analysis. The 

data seemed to process equally well at velocities as low as 

450 m/s and as high as 2,000 m/s. This example illustrates 

the shallow reflection seismologist's dilemma: close 

source-to-receiver offsets are necessary to detect shallow-reflections, but, if deep reflections are present as a 

bonus on the data, their depth cannot be accurately calculated because of velocity uncertainties.

Two other facets of this P-wave section should be 

pointed out. First, there is a lack of reflections between 

150 and 250 ms. The choice of offsets was such that this 

time window was nearly saturated with surface waves, 

obliterating any reflections that might have been present. 

During the CDP stacking process, the surface waves were 

attenuated but not to the extent of showing any reflections. In this case, absence of evidence is not necessarily 

evidence of absence of reflections. Second, the top two 

coherent wavelets on the section above 50 ms may be 

refractions that stacked in during processing. Overall, 

however, the P-wave data are excellent at this locality.

The S-wave seismic section from the second survey 

at Provo airport is shown in figure 2. There are reasonably 

good reflections in the time window between 250 and 500 

ms. The stacking velocity used was 170 m/s for the S-wave 

data as compared withJ350 m/s for P-wave data This comparison suggests possible correlation of the S-wave reflec-
IN SITU POISSON'S RATIO MEASUREMENTS NEAR PROVO, UTAH O5

0.0 

0.1 

o 

o

LU

en

0.2 

0.3 

0.4 

220 240

 kin ' i i i'M ' i , ,

i(;&^l!l!ili[i,i.!i;;!ii!l:!

260

,1,,,

CDP NUMBERS

280 300 320 340

!l»t»H»ft

,'»>»

tyii/v'^^^ttM^^^ _

i i' "i F l r' t h ' i 11K k ' I » i ' r '' I I * i i I l' bh ' ' " i

^N!lJt!li 1| 'l | |i > V'ii( 1 |^il ; ^:^'^VtMV>t M^|tM»^;\^

> » »»>WM>kM>»» I h \ I »»>HI >»>H.»^,»»n^>> ^v>*' >

^

>L

kVk'W

20 

I

40 FEET 

I

FIGURE 1. A twelve-fold P-wave CDP stack from the Provo airport that was acquired with the 50-caliber 

rifle and possesses several reflection events of excellent quality.

tion at 400 to 450 ms with the P-wave reflection at 140 ms. 

The S-wave reflection at 340 to 350 ms could correlate 

with the P-wave reflection at 110 ms. The S-wave reflection at 270 to 280 ms could correlate with the P-wave 

reflection at 90 to 100 ms.

Although the above-mentioned time correlations are 

reasonable with a compressional-wave velocity (Vp) to 

the shear-wave velocity (Vs) ratio of 3.1 which, using 

equation 1, yields a Poisson's ratio of 0.44 within the 

upper 50 m of sediment, the character of the reflections is 

different between the P-wave section and the S-wave section. This difference can be explained in part by noting 

that the frequency is almost identical between the two 

sections. Because the frequency is the same and the 

velocity differs by a factor of 3, the resolution is better by

a factor of 3 on the S-wave sections. In other words, what 

one sees on the seismic sections is resolution dependent 

to some degree. In any event, the combined data sets from 

the Provo airport represent our first successful effort to 

record shallow (less than 50 m) P- and S-wave reflections 

along the same line.

SPRINGVILLE CHERRY ORCHARD

During the spring 1985 trip to Provo, high-resolution 

P- and S-wave reflection surveys were conducted at a 

cherry orchard in Springville, Utah. Compressional-wave 

surveys were done with both a .30-06 rifle and a .50- 

caliber rifle. The shear-wave survey used as its source the 

quad-winged arrow discussed in the previous section. All
O

TWO-WAY TIME, IN SECONDS
CDP NUMBERS

0.0 

CO

Q

Z

o 

o

LU 

CO

0.1 

220 240 260 280 340 360 

FIGURE 3. A twelve-fold P-wave CDP stack from the cherry orchard that was acquired with the .30-06 rifle and a target time less than 40 ms. An antic

between CDP 210 and CDP 290.
O8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

0.0 

O

0.1 

220 240

CDP NUMBERS 

260 280 300 320

, i rfrjf. ' ', F" 1 j k.' r I / r ''l lifi

rVkWkVii'^' i'fT'"''rM^i (i-''l>"-r>p?P»'4^ ' li 1 * 1 " LT'. '' '' v" 'j'1 ' ' / »i- ' O'i 1 rF>ti " -'.^(^""-m* H^ ,^^1^^ ysiisipliiii^s MH'W>,^^

^w!T^>^y^^ t

. 9}

M

FEET

FIGURE 4. A twelve-fold P-wave CDP stack from the cherry orchard that was acquired with the .50-caliber rifle 

and possesses coherent reflection to times in excess of 140 ms. The same anticlinal feature present on figure 3 

can be interpreted on the deeper reflection on this line.

cessing software. The anticlinal nature of the profile is 

similar in both studies.

The processing of the data from the shear-wave survey was very similar to that of the Provo airport S-wave 

data. Two shots of opposite polarity were recorded separately at each shot location. The individual shots were 

time shifted, the second shot being reversed, and then 

they were stacked "together. The resulting section possesses dominant frequency around 40 Hz. The processed 

section lacked the good-quality, continuous reflecting 

events observed on the summer S-wave survey at the 

Provo airport.

The VP/VS ratio at this section is lower than that at the 

Provo airport, the ratio being about 1.8 in the near-surface 

sediments at the cherry orchard. Using this value in equation 1 yields a Poisson's ratio of 0.27.

Although we do not have .50-caliber data beyond the 

middle of figures 4 and 5 owing to equipment malfunction, the data quality from CDP 290 to CDP 340 is poor on 

the S-wave section and even worse on the .30-06 P-wave 

section. Beyond CDP 340, the data quality improves somewhat on both sections.

In brief, the cherry orchard data provide another 

example of using P- and S-wave reflection data to determine the dynamic properties of the near-surface materials 

to a depth of about 50 m. The S-wave reflection technique 

has been improved since these data were shot, and the 

.50-caliber rifle would most certainly give better seismic 

sections for P waves than did the .30-06. These two factors lead us to conclude that, although the data from the 

cherry orchard discussed here are useful, considerable 

improvement can be expected in the future.

CONCLUSIONS

The shallow-seismic P-wave reflection method is 

now a well-developed technique. The seismic P-wave sections shown in this chapter are typical of the resolution 

that can be obtained by proper selection of recording 

parameters. The seismic sections at the cherry orchard 

demonstrate the repeatability of the technique. The 

results obtained with the .30-06 source and the .50-caliber 

source were essentially the same in the upper 60 ms. Fur-
IN SITU POISSON'S RATIO MEASUREMENTS NEAR PROVO, UTAH O9

220 240

CDP NUMBERS 

300

0.0 

0.1 

0.2 

0.3 

0.4 

inirr, WmmM^^. ''/(' <'\ i ' "' vl ^ tli\f'rn

Wm\$ti$$M^w&w

inww{>unm>(\vMii(,)w.*;\.) y 

Wplll%^ OTwKW^IWW W$ WWm %'WmWki^^ &>M^! Mm, »>WM :V'L! fe^2*>f«:.^vife^ V'%MfflP^'W-)(iMp/Hil'i;^)!^l^ y$s*i$^%$$$$M !WfflfriOTViWH(^Wl!/iA/))WM iif«^»fl!iM®i!'lliiPi^§ff^!^i?![iitl

FIGURE 5. ^A twelve-fold S-wave CDP stack from the cherry orchard that was acquired with the rotational arrow. An anticMnal feature is interpreted

that is consistent with the P-wave data from this site.

thermore, the USGS used its own crew and equipment to 

shoot the same line for comparison purposes. The 

prominent features of the two lines are identical, even 

though the data were processed by different people using 

different software. In particular, the anticlinal feature 

between CDP 210 and CDP 290 is obvious on all three Pwave sections as well as on the S-wave section.

The shallow S-wave reflection technique has room 

for improvement. At the Provo airport, the second survey 

provided excellent reflection data. The dominant frequency of the S-waves using the arrow source appears to 

be somewhere between 60 Hz and 100 Hz a bit higher 

than expected. We have not yet been able to record Swave reflections shallower in time than 100 ms. The problem involves getting rid of substantial amounts of Lovewave energy. Because Love waves are essentially multiply 

reflected S waves trapped in near-surface layers, they 

exhibit many of the same characteristics as the desired Swave reflections. We have made progress toward solving 

that problem by moving the S-wave source below the surface and by increasing its frequency.

The actual calculation of Poisson's ratio is simple 

arithmetic once the P- and S-wave velocities are known. 

For example, at the Provo airport, P-wave velocity (determined from the normal-moveout equation) is approximately 550 m/s, and S-wave velocity approximately 170 m/ 

s within the upper 100 m of lake sediments, for an average 

Poisson's ratio of 0.44. At the cherry orchard, P-wave 

velocity is approximately 355 m/s, and S-wave velocity 

approximately 200 m/s in the upper 30 m, for an average 

Poisson's ratio in the upper 100 m of 0.27. We are not able 

to correlate individual reflections between the P- and Swave seismic sections well enough to allow direct calculation of Poisson's ratio, depth interval by depth interval. 

Part of this difficulty stems from the fact that the resolution of the S-wave reflections was greater than that of the
O10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

P-wave reflections at the Provo airport by a factor of 3 

and greater than that at the cherry orchard by a factor 

of 2.

Finally, the ultimate goal of this project is to relate 

the engineering properties of near-surface materials to 

amplification of ground shaking at specific sites.

REFERENCES CITED

Borcherdt, R.D., Joyner, W.B., Warrick, R.E., and Giws, J.F., 

1975, Response of local geologic units to ground shaking, in 

Borcherdt, R.D., ed., Studies for seismic zonation of the San 

Francisco Bay region: U.S. Geological Survey Professional 

Paper 941-A, p. A52-A68.

Doornenbal, J.C., and Helbig, K, 1983, High-resolution reflection 

seismics on a tidal flat in the Dutch Delta Acquisition, processing, and interpretation: First Break, May, p. 9-20.

Hunter, J.A., Burns, R.A., and Good, R.L., 1981, Optimum field 

techniques for bedrock mapping with the multichannel 

engineering seismograph [abs.]: Geophysics, v. 46, p. 451.

Hunter, J.A., Pullan, S.E., Burns, R.A., Gagne, R.M., and Good, 

R.L., 1984, Shallow seismic reflection mapping of the overburden-bedrock interface with the engineering seismograph: Some simple techniques: Geophysics, v. 49, p. 1381- 

1385.

Lawson, A.C., 1908, The California earthquake of April 18, 1906: 

Report of the State Earthquake Investigation Commission, 

v. I: Washington, D.C., Carnegie Institute of Washington, 

451 p.

Medvedev, S.V., 1965, Engineering seismology. Springfield, Va., 

National Technical Information Service, Report TT65- 

50011,260 p.

Meidav, T, 1969, Hammer reflection seismics in engineering geophysics: Geophysics, v. 34, p. 383-395.

Miller, R.D., and Steeples, D.W., 1986, Shallow structure from a 

seismic reflection profile across the Borah Peak, Idaho, 

fault scarp: Geophysical Research Letters, v. 13, p. 953-956.

Pakiser, L., and Warrick, R., 1956, A preliminary evaluation of 

the shallow reflection seismograph: Geophysics, v. 21, p. 

388-405.

Steeples, D.W., and Knapp, R.W., 1982, Reflections from 25 feet 

or less, [exp. abs.]: Society of Exploration Geophysicists 

program, p. 469-471.

Treadway, J.A., Steeples, D.W, and Miller, R.D., 1985, Seismic 

reflections within the upper 100 feet near Borah Peak, 

Idaho, earthquake scarp, in Technical Program, Abstracts, 

and Biographies [exp. abs.]: Society of Exploration Geophysicists annual meeting, 55th, Washington, D. C., p. 161- 

163.

Widess, M. B., 1973, How thin is a thin bed?: Geophysics, v. 38, 

p. 1176.
Earthquake Losses in Central Utah

By S.T ALGERMISSEN andE.P. ARNOLD, U.S. Geological Survey,

K.V. STEINBRUGGE, Consulting Structural Engineer, El Carrito, Calif., and

MARGARET G. HOPPER andP.S. POWERS, U.S. Geological Survey

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-P
IV CONTENTS

Page

11-13. Summaries of estimated losses in the four-county area from a strong (Ms=6.5) earthquake on the Wasatch fault:

11. On me Weber segment......................................................................................................................................^ P23

12. On the Salt Lake segment.............................................................................................................................................................................. 23

13. On the Provo segment.................................................................................................................................................................................... 24

14-16. Summaries of estimated losses in the four- county area from a moderate (ML=5.5) earthquake on the Wasatch fault:

14. On the Weber segment.....................................................................................^^ 24

15. On the Salt Lake segment.............................................................................................................................................................................. 25

16. On the Provo segment.................................................................................................................................................................................... 25

17-19. Summaries of estimated losses in Salt Lake City from a major (Ms=7.5) earthquake on the Wasatch fault:

17. On the Salt Lake segment.............................................................................................................................................................................. 26

18. On the Weber segment................................................................................................................................................................................... 26

19. On the Provo segment...................................................................................................................................................................... 27

20. Summary of expected maximum losses in the four-county area over a 50-year period with a 10-percent chance of

exceedance.............................................................................................................................................................................................................. 27

21-23. Variations of losses with intensity on the Salt Lake segment of the Wasatch fault zone for:

21. A major (Ms=7.5) earthquake....................................................................................................................................................................... 28

22. A strong (Ms=6.5) earthquake....................................................................................................................................................................^ 28

23. A moderate (ML=5.5) earthquake.................................................................................................................................................................. 28
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK 

ALONG THE WASATCH FRONT, UTAH

EARTHQUAKE LOSSES IN CENTRAL UTAH

By S.T. ALGERMISSEN, E.P. ARNOLD, K.V. STEINBRUGGE, MARGARET G. HOPPER, and P.S. POWERS

ABSTRACT

Estimates of earthquake losses in central Utah have been made for 

a series of simulated earthquakes modeled as both deterministic (scenario) and probabilistic events. Losses from scenario earthquakes of 

Ms=7.5 and 6.5 and M£=5.5 on the Provo, Salt Lake, and Weber segments 

of the Wasatch fault were considered, as was an M s=7.5 earthquake on a 

hypothetical fault about 30 km west of Salt Lake City. Expected maximum losses in the Salt Lake City urban corridor in 50 years with a 10- 

percent chance of exceedance were also calculated. Inventory of building stock was obtained for all classes of construction. Expected losses 

range from $830 million for a M£=5.5 shock on the Provo segment to 

$5.5 million for a Ms=7.5 quake on the Salt Lake segment. The sensitivity 

of these loss estimates to changes in intensity, vulnerability relationships, and surficial materials is discussed.

INTRODUCTION

Only limited attention has been given in the technical 

literature to the potential for earthquake losses in central 

Utah. The most important previous study was done by 

Rogers and others (1976), who dealt primarily with the 

severity of damage to facilities that would be critical in 

the aftermath of an earthquake, such as hospitals and 

related medical services, dams, State buildings, transportation, public utilities, food supplies, and debris removal. 

Casualties were also estimated. No estimates of monetary 

loss were given. In their study, the effects of M s=7.5 earthquakes located both on the Wasatch fault and approximately 10 km west of the fault in the vicinity of Salt Lake 

City, Ogden, and Provo were simulated.

Algermissen and Steinbrugge (1984) provided estimates of monetary losses for Salt Lake City for two probabilistic ground-motion models and one deterministic 

scenario-type (Ms=7.5) earthquake located on the 

Wasatch fault in Salt Lake City. Site response, based on 

the work of Hays and others (1978), was included in the 

deterministic models.

Improvements in our understanding of the seismotectonics of the Wasatch fault, recent studies of site 

response, and an improved and expanded inventory of 

buildings in Utah, Salt Lake, Weber, and Davis Counties 

(Steinbrugge, 1986) provided incentive for undertaking 

the present work.

ACKNOWLEDGMENTS

The authors wish to acknowledge the help of Richard 

M. Lockwood of the Salt Lake Board of Realtors, Linda 

Pollock of the Ogden Board of Realtors, and Neil Olson of 

the Salt Lake City Planning Commission.

PURPOSE AND SCOPE OF THIS STUDY

The purpose of this study is to estimate primary monetary losses associated with an ensemble of earthquakes 

to a range of buildings in Utah, Salt Lake, Davis, and 

Weber Counties (fig. 1). These four counties contain 77 

percent of the population of Utah and over 90 percent of 

the buildings exceeding $1.0 million in original cost. The 

five classes of buildings (referred to as classes I-V) considered in the study are described in table 1. These 

classes encompass the majority of buildings in the fourcounty area, with the exception of schools, government 

buildings, and heavy industrial complexes. Estimated 

earthquake losses to the water-supply system and to certain selected types of structures are contained in other 

papers in this volume.

Both deterministic (scenario) and probabilistic loss 

studies were undertaken. Losses were estimated for scenario earthquakes on the Provo, Salt Lake City (or Salt 

Lake), and Weber segments of the Wasatch fault. Earthquakes of magnitude (Ms) 7.5, 6.5, and (M^) 5.5 were simulated on each fault segment. Additionally, an earthquake

Pi
P2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 

112°30' 112° 1H°30' 111°

41°30' 

40°30' 

20 

I

40 50 MILES 

_|_____I

I I \ 1 I I

0 10 20 30 40 50 KILOMETERS

FIGURE 1. Location of the four-county area.
EARTHQUAKE LOSSES IN CENTRAL UTAH P3

TABLE 1. Building classifications used in this study

Class I Wood frame

Class I-A

1. Wood-frame and frame stucco dwellings regardless of area 

and height.

2. Wood-frame and frame stucco buildings, other than dwellings, 

that do not exceed three stories in height and do not exceed 

3,000 ft2 in ground-floor area.

3. Wood-frame and frame stucco habitational structures that do

not exceed three stories in height regardless of area 

Class I-B

Wood-frame and frame stucco buildings not qualifying under 

class I-A

Class II All-metal buildings

Class II-A

One-story all-metal buildings that have a floor area not exceeding 

20,000 ft2

Class II-B

All-metal buildings not qualifying under class II-A

Class III Steel-frame buildings

Class III-A

Buildings having a complete steel frame that carries all loads. 

Floors and roofs shall be of poured-in-place reinforced concrete or of concrete fill on metal decking welded to the steel 

frame (open-web steel joists excluded). Exterior walls shall be 

of poured-in-place reinforced concrete or of reinforced unit 

masonry placed within the frame. Buildings shall have at leastwidth-to-height-above-ground (or above any setback) ratio not 

exceeding 1:4. Not qualifying are buildings having column-free 

areas greater than 2,500 ftr (such as auditoriums, theaters, 

public halls, and so on).

Class III-B

Buildings having a complete steel frame that carries all loads. 

Floors and roofs shall be of poured-in-place reinforced concrete or metal or any combination thereof, except that roofs on 

buildings over three stories may be of any material. Exterior 

and interior walls may be of nonload-carrying material.

Class III-C

Buildings having some of the favorable characteristics of class 

III-A but otherwise falling jnto class III-B.

Class III-D

Buildings having a complete steel frame, floors and roofs of any 

material, and walls of any nonload-b earing material.

Class IV Reinforced concrete, combined reinforced concrete, 

and structural steel frame1

Class IV-A

Buildings having a structural system as defined by footnote 1 and 

poured-in-place reinforced concrete exterior walls or reinforced unit masonry exterior walls placed within the frame. 

Buildings shall have a least-width-to-height-above ground (or 

above any setback) ratio not exceeding 1:3. Not qualifying are 

buildings having column-free areas greater than 2,500 ft 2 (such 

as auditoriums, theaters, public halls, and so on).

Class IV-B

Buildings having a structural system as defined by footnote 1 and 

exterior and interior nonbearing walls of any material.

Class V Mixed construction

Class V-A

1. Dwellings, not over two stories in height, constructed of 

poured-in-place reinforced concrete and having roofs and second floors of wood frame.

2. Dwellings, not over two stories in height, constructed of adequately reinforced brick or hollow concrete-block masonry 

and roofs and floors of wood.

Class V-B

One-story buildings having superior earthquake-damage-control 

features including exterior walls of (1) poured-in-place reinforced concrete and (or) (2) precast reinforced concrete and 

(or) (3) reinforced hollow concrete-block masonry. Roofs and 

supported floors shall be of wood or metal diaphragm assemblies. Interior bearing walls shall be of wood frame or any one 

of a combination of the aforementioned wall materials.

Class V-C

One-story buildings having construction materials listed for class 

V-B but having ordinary earthquake-damage-control features,

Class V-D

1. Buildings having reinforced concrete load-beating walls and 

floors and roofs of wood and not qualifying for class IV-E.

2. Buildings of any height having class V-B materials of construction, including wall reinforcement; also included are 

buildings having roofs and supported floors of reinforced concrete (precast or otherwise) not qualifying for class IV.

Class V-E

Buildings having unreinforced solid-unit masonry of unreinforced brick, unreinforced concrete brick, unreinforced stone, 

or unreinforced concrete, where the loads are carried in whole 

or in part by the walls and partitions. Interior partitions may be 

wood frame or any of the aforementioned materials. Roofs and 

floors may be of any material. Not qualifying are buildings having nonreinforced load-carrying walls of hollow tile or other 

hollow-unit masonry, adobe, or cavity-wall construction.

Class V-F

1. Buildings having load-carrying walls of hollow tile or other 

hollow-unit masonry construction, adobe, or cavity-wall construction.

2. Any building not covered by any other class.

Classes VI-A, B, C, D, and E Earthquake-resistive 

construction

Any building or structure having any combination of materials 

and earthquake-damage-control features equivalent to those 

found in class I through V buildings. Alternatively, a qualifying 

building or structure may be classed as any class from I 

through V (instead of VI-A, B, C, D, or E) if the construction 

resembles that described for one of these classes and if the 

qualifying building or structure has an equivalent damageability.

1 Class IV-A, B, and C buildings shall have all vertical loads carried by a structural system consisting of one of or a combination of the following: (1) poured-inplace reinforced concrete frame, (2) poured-in-place reinforced-concrete bearing 

walls, (3) partial structural steel frame and 1 or 2. Floors and roofs all be of pouredin-place reinforced concrete, except that materials other than reinforced concrete 

may be used for the roofs on buildings over three stories.
P4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

HAZARD 

ASSESSMENT

Estimation of 

Ground Motion

Deterministic

Probabilistic

BUILDING 

INVENTORY

VULNERABLITY

ECONOMIC 

LOSS

FIGURE 2. Risk-assessment procedure and its parameters.

of magnitude Ms=7.5 was simulated on a hypothetical 

fault on the western side of the Oquirrh Mountains, 

approximately 30 km west of Salt Lake City. The expected 

maximum losses in 50 years with a 10-percent chance of 

exceedance were also calculated by using a slightly modified version of the probabilistic model described by 

Algermissen and others (1982).

ASSESSMENT OF LOSSES

INTRODUCTION

A general flow diagram for earthquake ground-shaking loss (risk) studies is shown in figure 2. The following 

sections describe each parameter in the loss-estimation 

procedure shown in figure 2 (hazard assessment, building 

inventory, and vulnerability) as it applies to this study.

GROUND-SHAKING HAZARD ASSESSMENT

Both deterministic and probabilistic ground-motion 

hazard models were developed for estimating earthquake 

losses. Deterministic (scenario) ground-shaking models 

were developed for hypothesized earthquakes of magnitude (Ms) 7.5, 6.5, and (M^) 5.5 located on the Provo, Salt 

Lake, and Weber segments of the Wasatch fault (Machette 

and others, 1992) and a Ms=7.5 shock wave west of Salt 

Lake City. All of the earthquakes were assumed to have 

surface ruptures, which begin at the center of each fault 

segment and rupture bilaterally over a distance log 

L= 1.915+0.389 M (Mark, 1977), where L is the fault rupture length (in meters) and M is taken to be ML for magnitudes of less than 6.5 and Ms for magnitudes of 6.5 and 

larger. The earthquakes are not allowed to rupture out of 

the fault segment in which they occur. Because some of
EARTHQUAKE LOSSES IN CENTRAL UTAH P5

TABLE 2. Scenario earthquake parameters

[-, no losses calculated for earthquakes of M s=6.5 and Mi=5.5 for this

fault]

Length of faulting (km)

Magnitude IQ Provo Salt Lake Weber Oquirrh 

(maximum) 1 segment segment segment Mountains2

7.5 Us

6.5 Ms

5.5 ML

Z+(10.3)

IX (9.0)

VII (8.0)

18

18

11

30

28

11

50

28

11

30

-

-

1The number in parentheses is the actual maximum intensity (/0) used in the 

calculation of intensity in equation 1.

Hypothetical fault on the western side of the Oquirrh Mountains (see fig.

TABLE 3. Site-response factors

Material

Silt and clay 

Sand and gravel 

Rubble

Rock

Mean relative

site response 

(0.2-0.7 s) 1

6.2 

3.7 

2.8

1.0

A/

2.6 

1.9 

1.4

0

rf

0 

-.7 

-1.2

-2.6

1 Computed by Rogers and others (1984). The mean relative response is the 

log normal mean of the mean spectral ratios in the ground-motion period band 0.2 

to 0.7 s.

2 Correction term for intensity attenuation in equation 1. K reduces the simulated intensity at a site if that site is underlain by sand and gravel, rubble, or rock. 

No correction to equation 1 is needed (that is, the correction is zero) if the site is 

underlain by silt and clay.

the Wasatch fault segments are much shorter than the 

fault rupture lengths given by Mark's (1977) regression 

equation, the length of the fault rupture for the larger simulated earthquakes is generally controlled by the length of 

the Wasatch fault segment. The assumed magnitudes and 

rupture lengths are summarized in table 2.

A magnitude Ms=7.5 shock was selected as the largest earthquake to be simulated on the Wasatch fault (and 

west of Salt Lake City) because (1) earthquakes of this 

magnitude have occurred historically in the Basin and 

Range province (in the Nevada seismic zone) and (2) paleoseismic evidence supports the idea that earthquakes of 

this magnitude are possible on segments of the Wasatch 

fault (Machette and others, 1992). The maximum 

Modified Mercalli (MM) intensities assigned to the earthquakes to be simulated are based on (1) consideration of 

maximum intensities observed in recent earthquakes 

characterized by surface faulting and (2) the high amplification ratios (amplitudes on alluvium to amplitudes on 

rock) observed in the Salt Lake urban corridor. All of the

IQ values in table 2 represent an estimate of the most 

intense shaking that might occur on surficial materials for 

the greatest site response. For this study, this material is 

taken to be silt and clay (as classified by Rogers and others, 1984). An IQ of intensity X+ is considered appropriately conservative for the maximum shaking on silt and 

clay for a Ms=7.5 earthquake. Maximum intensities of IX 

were widely observed in the Ms=6.2 Managua, Nicaragua, 

earthquake of 1972 (Algermissen and others, 1974; Dewey 

and Algermissen, 1974) and the Ms=6.5 San Fernando, 

Calif., earthquake of 1971 (Murphy, 1973). Both of these 

earthquakes had surface faulting. A maximum intensity of 

VIII was used for the maximum shaking for the M^=5.5 

earthquake simulated. Intensity VIII was widely observed 

after the 1986 Ms=5.4 earthquake that occurred at very 

shallow depth beneath the city of San Salvador (Rios and 

others, 1986). The above examples suggest that reasonable choices for maximum MM intensity have been made 

for the earthquakes simulated in the Utah urban corridor.

The ground-shaking attenuation in terms of MM 

intensity was calculated for the simulated earthquakes by 

combining an attenuation curve derived from 475 intensity observations in Utah reported by Hopper (this volume) with site-response data generalized by Rogers and 

others (1984). The equation used is

10 exp [-4.21873+0.604413 In ( x)]+K (1)

where IQ is the maximum MM intensity for each earthquake (see table 2) and / is MM intensity at x, the closest 

distance from the surface trace of the fault to the center 

of housing of each census tract in the four-county area. 

The standard deviation of one intensity observation, 07, is 

1.038. Kisa correction factor for site response (table 3).

The Kterm in equation 1 modifies the intensity attenuation, making use of the site response data of Rogers 

and others (1984), who used recordings by King and others (1983) of small earthquakes and blasts at 42 locations 

in the Wasatch Front urban corridor to derive the log normal mean of the mean spectral ratios in two period bands 

for three lithologic categories (silt and clay, sand and 

gravel, and rubble) with respect to rock. The two period 

bands over which the spectral ratios were averaged are 

0.2 to 0.7 and 0.7 to 1.0 s. For two reasons, only the log 

normal means for the short-period band (0.2-0.7 s) are 

used here. First, most of the structures of interest in this 

loss study are best represented by short-period band 

amplification; that is, the fundamental periods of vibration of the majority of buildings in the four-county area 

are best represented by the site-response data in the 0.2- 

to 0.7-s period band. Second, Rogers and others (1984) 

did not find the amplification data in the longer period 

band (0.7-1.0 s) to be statistically significant.
P6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

It has been found (Borcherdt, 1970; Borcherdt and 

others, 1975) that a factor of two in mean spectral velocity response level corresponds roughly to a change of 

about one MM intensity unit. A similar relationship has 

been shown to hold for peak velocity. For values of intensity up to X, Rosenblueth (1964) found that the relationship /=log 14Mog 2 (where / is MM intensity and v is the 

maximum ground velocity in centimeters per second) was 

a reasonable approximation. On the basis of the above 

relationships, the amplification of intensity with respect 

to rock was approximated for each of the three lithologic 

categories selected by Rogers and others (1984). The 

lithologic categories are listed in table 3, together with 

their mean relative responses in the period band 0.2 to 0.7 

s, the incremental increase of intensity with respect to 

rock (A/), and the K factor in equation 1. Because the 

maximum intensity IQ in equation 1 is assumed to occur 

on sand and silt, the K site correction is zero for sites with 

this category of lithology. For sites located on the other 

lithologic categories (sand and gravel, rubble, and rock), 

the K factor is a negative correction in the intensity attenuation relationship.

In summary, the distributions of intensities for the 

scenario earthquakes listed in table 2 were obtained by 

using equation 1 and the intensity correction factors listed 

in table 3. Topical intensity attenuation curves for silt and 

clay, sand and gravel, rubble, and rock are shown in 

figure 3. The site-response amplification factor of Rogers 

and others (1984) together with the associated K values 

for equation 1 are given in table 3. The distributions of 

surface materials listed in table 3 are shown in figure 4, 

together with the fault segments assumed to rupture in 

the earthquakes simulated on each fault. The simulated 

distribution of MM intensity for a portion of the area 

shaken by a Ms=7.5 earthquake on the Salt Lake segment 

of the Wasatch fault is shown in figure 5. An isometric 

view of the same simulated ground shaking is shown in 

figure 6.

The probabilistic hazards model used to estimate 

losses in the Wasatch Front urban corridor is essentially 

the model described by Algermissen and others (1982). 

Earthquakes are exponentially distributed with regard to 

magnitude and randomly distributed with regard to time 

in this model. The exponential magnitude distribution is 

an assumption based on empirical observation. The 

distribution of earthquakes in time is assumed to be Poissonian. The model has essentially four parts (fig. 7):

1. A seismic source model that serves to define the spatial distribution of earthquakes within the region of 

concern.

2. An occurrence model of earthquakes that describes the 

recurrence of events in time within the seismic source 

zones (fig, IE). In practice, the magnitude distribution 

of earthquakes described by the relationship log^o Nl

unit area=a-6m, where N is the number of earthquakes 

greater than magnitude m and a and b are constants 

determined for each source zone.

3. A ground-motion-attenuation model that is a description of the manner in which earthquake ground 

motions decrease with distance from an earthquake 

source for various magnitude levels (fig. 7 G).

4. A probability model for calculating the expected maximum amplitude of ground motion within a given period 

of time corresponding to a chosen probability level for 

a number of individual sites in the region (fig. 7D, N). 

Figure ID shows an idealized cumulative distribution 

function for intensity at a site, and figure IE depicts the 

probability of occurrence (for a Poisson model) of 

an expected maximum intensity / for various exposure 

times T.

The seismic source zones used in the model differ 

from those of Algermissen and others (1982) in that the 

eight fault segments described by Machette and others 

(1992) have been assigned individual rates of occurrence 

of large earthquakes (Ms=6.4-7.6) based on new paleoseismic data. The attenuation of MM intensity used in the 

probabilistic model is that given in equation 1, the same 

attenuation used for the deterministic (scenario) earthquake modeling. The maximum MM intensity in a 50-year 

period with a 10-percent chance of exceedance is for a 

portion of the study area as shown in figure 8; an isometric view of the same ground shaking is shown in figure 9.

INVENTORY DEVELOPMENT

The inventory development consists of two parts: 

(1) dwellings (classes I-A, I-B, V-B, V-E (table 1) and (2) all 

other classes of buildings.

Buildings other than dwellings. The inventory for 

all buildings other than dwellings is based on a procedure 

developed by Steinbrugge (1986). The nondwelling inventory includes structures valued at $1,000,000 or more at 

the time of construction; replacements values at 1985 levels are not considered.

In addition to the largest cities namely, Salt Lake 

City, Ogden, and Provo and some sections of Salt Lake 

County most of the outlying areas have major buildings 

costing in excess of $1,000,000; generally, these are 

located in either shopping malls, shopping centers, or 

Church of Jesus Christ of the Latter Day Saints (LDS) 

church complexes. Malls are defined as large structures 

having several entries and exits and housing major 

department stores and many smaller stores all under one 

roof structure. Shopping centers are similar, consisting of 

some major buildings but not being part of a single covered complex.

These malls and shopping centers are generally 

located on or near major State roads, usually near the
EARTHQUAKE LOSSES IN CENTRAL UTAH P7

DC 

LLJ

H 

(0

z

LJ

H

10 50

DISTANCE x (km)

FIGUEE 3. Intensity attenuation curves, where IQ is the maximum Modified Mercalli intensity for each earthquake (table 2), / is the intensity at a 

distance x from the surface fault rupture, and K is a correction for site response (table 3).

intersections of major traffic routes. Although the malls 

are few in number, there are numerous shopping centers, 

only a few of which feature structures valued at more 

than $1,000,000. Most of the buildings are small stores and 

shops, located in residential areas where few other major 

structures exist.

The remote sections of the study area are unique in 

that usually the only constructions exceeding $1,000,000 

in value are LDS church structures. These buildings are 

generally of high-quality mixed construction designed to 

resist major seismic forces. They occur in most of the 

small outlying communities and farming areas where no 

other large structures exist. Most are in residential areas. 

There are literally hundreds of such buildings within the 

study area, many costing in excess of $1,000,000. Most of 

these structures are of recent construction and are 

included in the total square footage areas of table 4.

Schools, government buildings, and heavy industrial 

complexes are not included in the inventory.

A number of data sources were used to derive the 

nondwelling inventory. The major sources of data were 

(1) building permit records, (2) LDS records, (3) Sanborn 

maps, (4) reports to the F.W. Dodge-McGraw Hill Company, published as building statistics, (5) census tract 

maps, and (6) field investigations.

Possibly the most effective method of locating, listing, and describing buildings exceeding $1,000,000 in 

value not listed in county or other records was extensive 

field investigation. Because records for much of the fourcounty area are incomplete, considerable on-site inspection was required. Many of the available records list buildings that have been demolished and consequently cannot 

be included in the inventory. The location of these buildings can generally be determined only by field inspection.

A summary of the building inventory for all classes of 

buildings other than dwellings is given in table 4. Replacement cost values for nondwelling buildings were taken 

from Steinbrugge (1986) and are based on a review of
P8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

actual costs of replacement in Utah in 1985. The replacement costs per square foot are summarized in table 5.

Dwelling inventory. The inventory of housing is 

taken as of July 1,1985 for consistency with the nondwelling inventory. The dwelling or housing inventory was constructed from the 1980 Census of Housing as published by 

the Bureau of the Census (1983a, b) augmented by other 

information from Bureau of Census unpublished data and 

the various Boards of Realtors and county and city planning commissions. Details of these calculations are given 

below.

Tooele County, house trailers, and other nonpermanent, year-round housing units were not included in this 

inventory. Tooele County, although included by the 

Bureau of the Census in the Salt Lake Standard Metropolitan Statistical Area (SMSA), was excluded both because 

it is sparsely populated and because information is difficult to obtain, especially for valuations. Trailers were 

excluded from the counts simply because the vulnerability of house trailers is difficult to quantify. As a result, 

total calculated losses will be slightly less conservative 

than the actual ones. There were 9,229 trailers in the Salt 

Lake SMSA and 2,153 in the Provo SMSA in 1980 in comparison with 223, 097 and 42,104 single-family dwellings, 

respectively; thus, the error will be small.

A dwelling or housing unit is loosely defined as a selfcontained structure or partial structure serving as a yearround dwelling for a family. Dwellings are divided into 

two classes according to the number of housing units per 

structure those with four units or less per structure and 

those with five or more. The former are mostly singlefamily detached dwellings and probably large, older 

houses that have been divided into apartments as well as 

the more modern town houses. The larger buildings are 

mostly apartments in the category of 10 to 49 units per 

building. The apportionment between these two classes 

was made by using the ratios given by the Bureau of the 

Census (1983a, b) for 1980. Because the amount of housing erected between 1980 and 1985 is small, these ratios 

should still be fairly accurate.

The total number of housing units in each census 

tract was updated to 1985 by obtaining the number of 

building permits issued in the years 1980 to 1984 from the 

Bureau of the Census and, assuming that building was 

uniform over each county, prorating that number over the 

census tracts.

Housing units are further divided into wood-frame 

and masonry construction and are listed as either class I 

or class V of the building classification given by Algermissen and others (1978) (table 1). Unfortunately, no good 

estimate of the ratio of wood-frame to masonry housing 

units is available from the Bureau of the Census or elsewhere. In this case, the figure of approximately 60 percent

masonry for the region was used after consultation with 

staff members of the Salt Lake City Planning Commission.

All housing units in the masonry category were then 

subdivided into class V-E or V-B, depending on whether 

they were built before or after 1950, respectively. Again, 

the ratios are the ones given by the Bureau of the Census 

by county for 1980. No consideration was given to brick 

veneer on wood-frame construction; it was assumed that 

all of the veneer dwellings were constructed after 1949 

and had vulnerability similar to that of class V-B structures. The replacement cost of individual units was determined in the following manner. For one- to four-unit 

structures, the current value was obtained from the Salt 

Lake Board of Realtors, which has divided Salt Lake 

County into 12 regions. An average sale price in 1985 for 

each of the 12 regions was obtained. The Ogden Board of 

Realtors has divided Davis and Weber Counties into 18 

regions, and the Provo Board of Realtors has provided 

similar divisions for Utah County. Seventy-five percent of 

the average cost of dwellings in each of the areas sampled 

was taken as the replacement cost, 25 percent being 

taken as the land value.

In the case of structures having five or more units, 

the area of the average unit was taken to be 950 ft 2, a figure considered average in the estimation of each realty 

board consulted. The price of wood-frame structures was 

taken to be $55/ft2 and that of the masonry units $63/ft 2 

(Steinbrugge, 1986) (table 5). Any units in class III or IV 

structures will be slightly underpriced by this calculation, 

but damage will be calculated as if they were class V-B, 

the higher result having a compensating effect.

In addition to obtaining a value for all housing in a 

tract, it is necessary to locate the housing spatially by 

assuming it to exist at a "center of housing." The Bureau 

of the Census gives a center of population for each tract, 

which can reasonably be taken to be the center of housing. In addition, because of the irregular boundaries of 

many municipalities, there are many tracts that are "split" 

between a number of municipalities. This split necessitated combining the partial tracts by using means 

weighted by numbers of housing units. The centers 

of housing are expressed in geographic coordinates.

FIGURE 4. Distribution of site-response amplification factors 

(adopted from Rogers and others, 1984). Intensity corrections (K) 

used in conjunction with equation 1 are shown in table 3. The 

fault segments assumed to rupture in the simulated earthquakes 

are shown by heavy black lines. Assumed lengths of rupture for 

each magnitude earthquake are given in table 2. A, Weber 

segment. B, Salt Lake segment The heavy dashed line represents 

the fault rupture length for a Ms=7.5 earthquake on a hypothetical 

fault located approximately along the western flank of the 

Oquirrh Mountains. C, Provo segment
41.7 1 12.25

EARTHQUAKE LOSSES IN CENTRAL UTAH

1 12.00

P9

111.75

41.50 'jM

41.25 -

Silt and Clay 

Sand and Gravel 

Rubble 

I I Rock

0 5

righam City
P10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112.25 112.00 111.75

41.00 -

40.75 

40.50

Silt and Clay 

Sand and Gravel

__ Rubble

I I Rock

FIGURE 4. Continued.
EARTHQUAKE LOSSES IN CENTRAL UTAH 

11,75

Pll

111

Silt and Clay 

Sand and Gravel

__ Rubble

II Rock

5

40.25-

50

40.00 

No Data

FIGURE 4. Continued.
P12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

41.00 -

40.75 -

40.50

Salt Lake City 

Corporate Limits

Salt Lake Segment 

Wasatch Fault

FIGURE 5. Isoseismals for a simulated Ms=7.5 earthquake on the Salt Lake segment of the Wasatch fault. The intensities have 

been slightly smoothed. The emphasized line segment is the Salt Lake segment of the Wasatch fault.
EARTHQUAKE LOSSES IN CENTRAL UTAH P13

Salt Lake Segment 

Wasatch Fault

o

FIGURE 6. Isometric view of the distribution of Modified Mercalli intensity for a simulated M s=7.5 earthquake on the Salt Lake segment of the

Wasatch fault as seen from the northeast. The data are the same as in figure 5.
P14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

Source 2 

(POINT)

Source 1 (FAULT)

Source 3 (MIXED)

1.0

0.5

B

MAGNITUDE

Curves for various 

maximum 

intensities

2 4 6 10 20 40 100 

DISTANCE (MILES)

INTENSITY INTENSITY

FIGURE 7. Model for calculating probabilistic maximum Modified Mercalli intensity. A, Typical source areas and 

grid of points at which the hazard is to be computed. Circled grid point represents a typical location for 

intensity calculation. B, Statistical analysis of seismicity data and, C, typical attenuation curves. The straight 

lines labeled 1, 2, and 3 represent maximum likelihood regressions applied to historical earthquakes that have 

occurred in typical sources shown in A. D,F(i) is the cumulative conditional probability distribution of 

intensity for each geographic location where expected Modified Mercalli intensity is computed. F(i) is the 

cumulative number of intensities i<I (some T) for I(j>Imin (some minimum maximum Modified Mercalli 

intensity). E, Fmax> t (T) is the extreme probability for various levels of intensity ( 7) and exposure times (T); $ is 

the average rate of occurrence per unit time ( f) of intensity level I>Imin_
EARTHQUAKE LOSSES IN CENTRAL UTAH P15

41.00 -

40.75 -

40.50

Salt Lake City 

Corporate Limits

Salt Lake Segment 

Wasatch Fault

FIGURE 8. Distribution of maximum Modified Mercalli intensity in the Salt Lake area in 50 years with a 10-percent chance

of exceedance.
P16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

Salt Lake Segment 

Wasatch Fault

FIGURE 9. Isometric view of the distribution of maximum Modified Mercalli intensity in the Salt Lake area in 50 years with a 10-percen t chance of 

exceedance. The perspective is the same as in figure 6, and the data are the same as in figure 8.
EARTHQUAKE LOSSES IN CENTRAL UTAH P17

TABLE 4. Building classes other than dwellings in the four-county area 

[-, buildings having a replacement value of less than $1,000,000, which are not included in these classes. Building areas in square feetxl,000]

County

All metal Steel Reinforced concrete Mixed construction

Class II-A.B Class III-A Class III-B, C, D Class IV-A Class IV-B, C, D, E Class V-B.C Class V-D,E,F

Weber County

Davis County

Salt Lake County

Utah County

Total area

_

700.0

1,950.2

-

2,650.2

_

-

3,394.8

-

3,394.8

204.0

445.0

6,252.8

426.2

7,328.0

1,105.0

739.0

16,025.2

172.0

18,041.2

1,413.0

880.0

8,929.1

954.1

12,176.2

1,636.8

2,284.4

19,646.6

2,564.6

26,132.4

1,090.7

-

860.0

752.0

2,702.7

The replacement value for all classes of buildings 

(used to estimate losses) is taken as the total cash 

value required to fully repair or rebuild in kind 

any building of a particular class.

Vulnerability. Vulnerability as used in this study is 

taken to mean the relationship between percentage of 

loss and MM intensity. Percentage of loss is defined 

as the average percentage of the total actual cash 

value required to fully repair or rebuild in kind 

any building of a particular class experiencing 

ground motion represented by a particular level of 

the MM intensity scale.

The two sets of vulnerability relationships used are 

(1) those developed by Steinbrugge (1979), further 

reviewed by Steinbrugge (1986), and (2) those developed 

by the Applied Technology Council (1985). The Steinbrugge vulnerability relationships are shown graphically 

in figures 10 and 11 and numerically in table 6. It should 

be clearly understood that both vulnerability relationships used are for ground-shaking damage only.

LOSS CALCULATIONS

Losses were calculated in a totally automated manner on microcomputers. The only exception to the above 

statement is the hand digitization of geologic features, 

such as areas of varying soil amplification and fault 

traces. These exceptions are still less time consuming 

than previously used methods. The flow of the calculations is outlined below.

Calculations of losses to dwellings. Inputs are 

coordinates of the centers of housing for the 229 census 

tracts in the four-county urban area; a map of the various 

surficial materials underlying the urban corridor; a map of 

the Wasatch fault zone; numbers of housing units in the 

various construction classes in each tract along with estimates of value; floor areas of nondwelling structures in 

each tract and an estimate of values; a function giving the

TABLE 5. Replacement cost (1985) per 

unit area for building classes other 

than dwellings (Steinbrugge, 1986)

Class Price per square foot 

(1985 dollars)

I

II

III

IV

V

55

41

71

72

63

attenuation of intensity with respect to distance from a 

fault rupture; and a set of vulnerability curves giving 

losses as a function of intensity.

The two maps are first digitized and the points converted to the State plane (north) projection in this case, 

a Lambert conformal conic projection having standard 

parallels at 40°43' N. and 41°47' N. and a central meridian 

at 111 030' W. The centers of housing are transformed into 

the same projection.

The distances from the active fault segment to each 

center of housing are then computed. The distance is 

measured along the normal from the housing centers to 

the fault or, for housing centers beyond the ends of the 

active fault segment, the distance from the housing center 

to the closest end of the fault segment. The centers of 

housing and the soil amplification map are also plotted. A 

utility program is then used to produce a list of tract 

identification numbers together with the soil types underlying each center.

As previously explained, each soil type and earthquake magnitude correspond to a specific maximum 

intensity, IQ, in equation 1, repeated here for convenience:

(/0-7)=10 exp M.21873+0.604413 In (1)
P18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

100%

O

< . . «. XY £V-I.::.

X- '-* ' iC'.'r^* »«1

Xv'iGeologic^

.'; :".'. effects rD

IV VI VII VIII IX X XI XII.

MODIFIED MERCALLI INTENSITY

FIGURE 10. Characteristic intensity loss curve showing inflection points tabulated in table 6.

where x is the distance from the fault to the housing center (in kilometers). Once a list of tracts, distances, and IQ'S 

has been compiled by using equation 1, the intensity / at 

each housing center is calculated and rounded to the 

nearest 0.1 intensity unit.

The only step not yet completely automated in the 

calculation of losses is finding whether the fault trace 

intersects the tract. The present calculation makes this 

determination by overlying a tract map giving actual 

boundaries on the fault map and flagging the intersected 

tracts by hand, thus creating a list to augment the one giving tract identification and intensity.

The number of units in each tract that belong to each 

of the four dwelling construction classes are then calculated. Total numbers of housing units are obtained from 

the Bureau of the Census (1983c) on magnetic tape. The 

ratios of one to four dwelling units per structure to one of 

five or more dwelling units per structure were obtained 

from the Bureau of the Census (1983a, b), as were the 

ratios of pre-1950 housing units to post-1950 units. The 

total number of units added from 1980 to 1985 was 

obtained from the Bureau of the Census' unpublished 

records of building permits. The total units were adjusted 

to reflect 1985 numbers and then divided into wood frame
EARTHQUAKE LOSSES IN CENTRAL UTAH P19

10 1A,1B,2B 20

10

i i i i i

12

I I I

IV VI VIII X XII 

MODIFIED MERCALLI INTENSITY

IV VI VIII X

MODIFIED MERCALLI INTENSITY

XII

c/o

V)

o

30

20

10

100

80

60

40

20

D

I I I I

IV VI VIII X

MODIFIED MERCALLI INTENSITY

XII IV VI VIII X

MODIFIED MERCALLI INTENSITY

XII

FIGURE 11. Vulnerability relationships used in this study (Steinbrugge, 1986). The inflection points of the curves are shown 

in figure 10 and tabulated in table 6. A, Classes 1 and 2 (I and II); B, class 3 (III); C, class 4 (IV); D, class 5 (V).

and masonry (assuming 60 percent masonry) and further 

apportioned into large and small structures. The masonry 

structures were further divided into classes V-E and V-B, 

depending on whether the unit was built before or after 

1950.

Dwelling values for 1985 were then calculated. For 

all structures having one to four units, the value of each 

dwelling unit is the one given by the board of realtors in 

its area. That is to say, the census tracts were allotted to 

each realtor's areas, the latter always being larger than 

the former; no distinction was made between wood frame 

and masonry.

For all housing in structures having four units or 

fewer, an arbitrary 25 percent of the value was subtracted 

to account for the value of the land, which, of course, is 

not generally damaged.

The loss calculations for housing units were computed by using the vulnerability curves of Steinbrugge 

(1986) and those developed by the Applied Technology 

Council (1985). This comparison provides some degree of 

sensitivity analysis with regard to vulnerability. As a special case, for those tracts through which a fault trace 

passes, total destruction due to ground disruption was 

assumed to occur in a path 50 m wide along the fault. On a
P20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 6. Coordinates of loss-intensity curves for figure 11 (Steinbrugge, 1986)

[N/A, not applicable]

Building 

class

I-A

I-B

II-A

II-B

III-A

III-B

III-C

III-D

IV-A

IV-B

IV-C

IV-D

IV-E

V-A

V-B

V-C

V-D

V-E

V-F

Point A

Loss 

(percent)

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Modified 

Mercalli

V

V

VI

VI

VI

V

VI

V

VI

V

VI

V

V

VI

VI

V

V

V

IV

Point B

Loss 

(percent)

0.80

.80

.60

.80

1.00

1.75

1.25

1.75

1.25

2.50

1.75

3.00

2.75

.75

1.25

1.75

2.25

10.00

10.00

Modified 

Mercalli

VI

VI

vn

VII

VII

VI

VII

VI

VII

VI

VII

VI

VI

VII

VII

VI

VI

VI

V

Point C

Loss 

(percent)

8.00

8.00

6.00

8.00

10.00

17.50

12.50

17.50

12.50

25.00

17.50

30.00

27.50

7.50

12.50

17.50

22.50

100.00

100.00

Modified 

Mercalli

IX

IX

DC

IX

IX

IX

IX

IX

IX

IX

IX

IX

IX

IX

IX

IX

IX

XII

XII

Points D and E

Loss 

(percent)

9.00

9.00

6.50

9.00

12.00

20.00

15.00

20.00

15.00

27.50

20.00

32.50

30.00

9.00

14.00

20.00

25.00

N/A

N/A

Modified 

Mercalli

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

X/XII

N/A

N/A

typical tract, which has a length and width of about 1 km, 

about 5 percent of the tract, on average, experiences 100 

percent loss. This part of the loss was computed on the 

total valuation including land. Values for nondwelling 

structures were computed by multiplying the area of 

structures in each construction class by the 1985 value 

per unit given by Steinbrugge (1986). These replacement 

costs are given in table 5.

Calculation of losses to nondwellings. Losses for 

nondwelling structures were computed by using only the 

Steinbrugge (1986) vulnerability relationships because 

there were no comparable Applied Technology Council 

(1985) ones for the Wasatch urban region.

Summary of loss estimations. The earthquake loss 

estimates for all the scenario earthquakes and the

expected maximum losses in a 50-year period with a 10- 

percent chance of exceedance are listed in tables 7 

through 20. The loss estimates are organized in the following manner. Tables 7 through 16 give losses in the 

four-county area for Ms=7.5 and 6.5 and M^=5.5 scenario 

earthquakes on the Weber, Salt Lake, and Provo segments 

of the Wasatch fault and for a Ms=7.5 earthquake striking 

north-south 30 km west of Salt Lake City and located 

approximately on the western flank of the Oquirrh Mountains. Tables 17 through 19 list losses only in Salt Lake 

City for Ms=7.5 scenario earthquakes on the Weber, Salt 

Lake, and Provo segments of the Wasatch fault. These 

losses were computed to provide additional data for 

earthquakes that might occur at some distance from a 

particular urban area and also to facilitate comparison
EARTHQUAKE LOSSES IN CENTRAL UTAH P21

TABLE 7. Summary of estimated losses in the four-county area from a TABLE 8. Summary of estimated losses in the four-county area from a

major (Ms=7.5) earthquake on the Weber segment of the Wasatch major (Ms=7.5) earthquake on the Salt Lake segment of the Wasatch

fault fault

XT , Value Number ($x!06) Losses Losses 

(SxlO6) 1 ($x!06)2

Dwellings

Wood frame: 

1-4 units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

and V-E): 

1-4 units 

>5 units

Total

122,695 6,308.6 

21,824 1,140.3

184,042 9,462.9 

32,736 1,959.2

361,297 18,871.0

689.6 489.7 

131.9 89.2

1,848.7 2,960.9 

441.7 764.9

3,111.9 4,304.7

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3 

2,175.7

1,782.3 

4,867.9

3.5 

8.6 

111.5 

403.8

347.4 

874.8

Dwellings and nondwellings

Total 23,738.9 5,179.5

Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

Losses computed from vulnerability relationships shown in figure 11 and

Value Losses 

Number ($x!06) ($xl06)l

Losses 

($x!06)2

Dwellings

Wood frame: 

1-4 units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

and V-E): 

1-4 units 

>5 units

Total

122,695 6,308.6 850.3 

21,824 1,140.3 172.5

184,042 9,462.9 2,143.8 

32,736 1,959.2 537.4

361,297 18,871.0 3,704.0

550.4 

103.3

3,094.6 

802.7

4,551.0

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3 

2,175.7

1,782.3 

4,867.9

3.6 

8.5 

119.7 

417.1

362.9 

911.8

Dwellings and nondwellings

Total 23,739.9 5,462.8

1Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2Losses computed from vulnerability relationships shown in figure 11 and

table 6. table 6.

with earlier work (Algermissen and Steinbrugge, 1984). 

Table 20 gives expected maximum losses in the fourcounty area in 50 years with a 10-percent chance of 

exceedance.

DISCUSSION

Estimates of primary losses for 10 simulated 

earthquakes on four different faults together with a 

probabilistic estimate of maximum losses in 50 years have 

been presented for the Utah, Salt Lake, Davis, and Weber 

County urban area The total replacement cost or value of 

the buildings surveyed in the four-county area (1985 values) is $23.7 billion. Losses for all earthquakes considered

ranged from $830 million (M^=5.5 on the Provo segment) 

to $5.5 billion (Ms=7.5 on the Salt Lake segment), a range 

of from 3 to 22 percent of the total value at risk. The 

expected maximum loss in a 50-year period with a 10- 

percent chance of exceedance is $3.2 billion. Dwellings 

constituted between 82 and 88 percent of the total losses 

for all of the earthquakes considered, including the probabilistic model.

The relatively high loss estimates for the area are a 

result of two important factors: (1) the relatively high proportion of brick dwellings in the four-county area and 

(2) the high site response associated with the surface and 

near-surface materials underlying the area.

Extensive damage to unreinforced brick buildings is 

a well-documented result of severe earthquakes. Recent
P22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 9. Summary of estimated losses in the four-county area from a TABLE 10. Summary of estimated losses in the four-county area from

major (Ms=7.5) earthquake on a hypothetical fault segment 30 km a major (Ms=7.5) earthquake on the Provo segment of the Wasatch

west of Salt Lake City fault

Number Value Losses Losses 

($x!06) (SxlO6) 1 ($x!06)2

Dwellings

Wood frame:

1-4 units (class I-A)

>5 units (class I-B)

Masonry (classes V-B

andV-E):

1-4 units

>5 units

122,695

21,824

184,042

32,736

6,308.6

1,140.3

9,462.9

1,959.2

490.5

97.0

1,477.8

363.5

454.4

84.0

2,792.3

737.1

Total 361,297 18,871.0 2,428.8 4,067.8

Nondwellings

Wood frame (class I-B)

All metal (class II)

Steel frame (class III)

Concrete frame (class IV)

Mixed construction

(class V)

Total

40.1

108.5

761.3

2,175.7

1,782.3

4,867.9

3.3

8.2

107.2

375.2

330.2

824.1

Dwellings and nondwellings

Total 23,739.9 4,891.9

Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

Losses computed from vulnerability relationships shown in figure 11 and

Number Value Losses Losses 

($x!06) (SxlO6) 1 ($x!06)2

Dwellings

Wood frame:

1-4 units (class I-A)

>5 units (class I-B)

Masonry (classes V-B

andV-E):

1^1 units

>5 units

122,695

21,824

184,042

32,736

6,308.6

1,140.3

9,462.9

1,959.2

413.8

84.8

1,279.7

326.0

426.8

81.3

2,563.1

699.2

Total 361,297 18,871.0 2,104.3 3,770.4

Nondwellings

Wood frame (class I-B)

All metal (class II)

Steel frame (class III)

Concrete frame (class IV)

Mixed construction

(class V)

Total

40.1

108.5

761.3

2,175.7

1,782.3

4,867.9

3.0

6.4

97.0

321.6

298.9

726.9

Dwellings and nondwellings

Total 23,739.9 4,497.3

table 6.

1 Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2Losses computed from vulnerability relationships shown in figure 11 and 

table 6.

large, damaging earthquakes have established the importance of site response in earthquake damage and have 

also shown that site response can, at least in a general 

way, be estimated from analysis of small earthquakes and 

blasts. Examples are the large site-response factors that 

radically affected the patterns of ground shaking in the 

large 1985 earthquakes in Chile (Algermissen, 1985) and 

Mexico (Stone and others, 1987; Cassaro and Romero, 

1986) and the small but damaging 1986 shock in El Salvador (Rios and others, 1986).

A source of possible uncertainty in the estimation of 

losses to brick buildings in the four-county area is the

ratio of brick to wood-frame dwellings. The ratio used in 

this study (60 percent brick) was based on limited sampling in the study area and expert opinion from staff of 

the Salt Lake City Planning Commission. We believe that 

additional sampling may be warranted in future studies, 

particularly because of the large site-response factors that 

exist throughout the area.

Sensitivity studies of the effect of variability in MM 

intensity on the loss estimations are included in tables 21 

through 23 for all of the scenario earthquakes on the Salt 

Lake segment of the Wasatch fault. The losses for each of 

these scenario earthquakes have been recomputed by
EARTHQUAKE LOSSES IN CENTRAL UTAH P23

TABLE 11. Summary of estimated losses in the four-county area from TABLE 12. Summary of estimated losses in the four-county area from

a strong (Ms=6.5) earthquake on the Weber segment of the Wasatch 

fault

a strong (Ms=6.5) earthquake on the Salt Lake segment of the 

Wasatchfault

XT , Value Number ($xl()6) Losses Losses 

(SxlO6) 1 ($x!06)2

Dwellings

Wood frame: 

1-4 units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

and V-E): 

1-4 units 

>5 units

Total

122,695 6,308.6 

21,824 1,140.3

184,042 9,462.9 

32,736 1,959.2

361,297 18,871.0

218.0 301.2 

40.7 55.1

699.4 1,713.3 

168.9 472.6

1,127.0 2,560.2

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3 

2,175.7

1,782.3 

4,867.9

2.3 

5.2 

64.2 

236.6

198.1 

506.4

Dwellings and nondwellings

Total 23,739.9 3,066.6

Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

Losses computed from vulnerability relationships shown in figure 11 and 

table 6.

Value Losses 

Number ($xl06) ($xl()6)l

Losses 

($x!06)2

Dwellings

Wood frame: 

1^4 units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

and V-E): 

1-4 units 

>5 units

Total

122,695 6,308.6 330.0 

21,824 1,140.3 68.7

184,042 9,462.9 1,025.6 

32,736 1,959.2 273.1

361,297 18,871.0 1,697.4

396.4

77.8

2,214.3 

629.2

3,317.7

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3 

2,175.7

1,782.3 

4,867.9

3.0 

5.9 

89.6 

301.1

236.3 

635.9

Dwellings and nondwellings

Total 23,739.9 3,953.6

1 Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2 Losses computed from vulnerability relationships shown in figure 11 and 

table 6.

assuming a one-unit increase and decrease in the MM 

intensity throughout the four-county area. The changes in 

loss levels are striking and are a representative measure 

of the change in loss estimates when the MM intensity is 

varied over a realistic range. These sensitivity studies 

reveal some interesting characteristics of the loss estimates. For example, for a Ms=7.5 earthquake on the Salt 

Lake segment (table 21), a decrease of one intensity unit 

decreases losses by 20 percent ($1.03 billion), whereas an 

increase of one intensity unit reverses the losses by only 6 

percent ($353 million). This variation in rate of change is 

associated with saturation of the vulnerability curves at

high intensities. The variation in rate of change is still evident, but to a lesser extent, for the Ms=6.5 earthquake on 

the Salt Lake segment (table 22). For the M£=5.5 earthquake, the vulnerability curves are in a more linear range, 

and the change in losses from A/=-l to A/=+l is much 

more linear (-61.9 percent) for A/=-l and +89.4 percent 

for A/=+l) than it is for the larger shocks.

We also provide some information in this study on 

variation in loss estimates with changes in vulnerability. A 

measure of the sensitivity of loss estimations to changes 

in vulnerability assumptions is provided by comparing the 

losses calculated for dwellings by using the vulnerability
P24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 13. Summary of estimated losses in the four-county area from TABLE 14. Summary of estimated losses in the four-county area from

a strong (Ms=6.5) earthquake on the Provo segment of the Wasatch a moderate (M^S.5) earthquake on the Weber segment of the

fault Wasatchfault

XT i. Value Number ($x!06) Losses Losses 

($x!06) 1 ($x!06)2

Dwellings

Wood frame: 

1^4 units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

andV-E): 

1^4 units 

>5 units

Total

122,695 6,308.6 

21,824 1,140.3

184,042 9,462.9 

32,736 1,959.2

361,297 18,871.0

157.9 242.9 

34.7 50.1

489.5 1,307.7 

135.4 392.4

817.5 1,993.1

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3

2,175.7

1,782.3 

4,867.9

1.8 

2.0 

49.2 

148.5

136.3 

337.8

Dwellings and nondwellings

Total 23,739.9 2,330.9

Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2Losses computed from vulnerability relationships shown in figure 11 and

XT Value Number ($x!06) Losses Losses 

($x!06) 1 ($x!06)2

Dwellings

Wood frame: 

1^4 units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

andV-E): 

1^4 units 

>5 units

Total

122,695 6,308.6 

21,824 1,140.3

184,042 9,462.9 

32,736 1,959.2

361,297 18,871.0

87.2 132.3 

14.4 24.8

215.4 702.2 

52.8 199.5

369.8 1,058.8

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3

2,175.7

1,782.3 

4,867.9

1.2 

1,2 

22.4

87.7

66.4 

178.9

Dwellings and nondwellings

Total 23,739.9 1,237.7

table 6.

1 Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2Losses computed from vulnerability relationships shown in figure 11 and 

table 6.

relationship developed by the Applied Technology Council (1985) with those calculated by using Steinbrugge's 

(1986) relationship. These results are shown in tables 7 

through 20. The differences in losses computed by using 

the two different vulnerability curves are not very significant for wood-frame dwellings; however, they are quite 

significant for masonry dwellings. This difference suggests that further review and research on the vulnerability 

of brick dwellings are important. Loss comparisons for 

structures other than dwellings made by using the 

Applied Technology Council (1985) and the Steinbrugge 

(1986) vulnerability relationships were not possible

because of differences in the way nondwellings are classified in the two vulnerability studies.

Most of the areas covered by this study is underlain 

by silt and clay having a very high site-response factor 

(that is, 2.6 intensity units greater than rock) (compare 

with table 3), so losses may appear very large. The effects 

of site response can be deduced from the data in tables 21 

through 23. Decreasing the intensities by one unit is the 

equivalent of decreasing the soil amplification to slightly 

more than that associated with rubble in table 3; this 

decrease, in turn, reduces the losses very significantly, 

especially at low intensities. In the case of the
EARTHQUAKE LOSSES IN CENTRAL UTAH P25

TABLE 15. Summary of estimated losses in the four-county area from TABLE 16. Summary of estimated losses in the four-county area from

a moderate (Mi=5.5) earthquake on the Salt Lake segment of the 

Wasatchfault

a moderate (Mi=5.5) earthquake on the Provo segment of the 

Wasatchfault

M , Value Losses 

umber Qxl(fy ($xiQ6)l

Losses 

($x!06)2

Dwellings

Wood frame: 

1-4 units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

and V-E): 

l-b units 

>5 units

Total

122,695 6,308.6 139.3 

21,824 1,140.3 27.4

184,042 9,462.9 395.3 

32,736 1,959.2 105.7

361,297 18,871.0 667.7

210.0 

40.1

1,081.1 

329.1

1,660.3

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3

2,175.7

1,782.3 

4,867.9

1.8 

2.0 

44.2 

137.8

91.8 

277.6

Dwellings and nondwellings

Total 23,739.9 1,937.9

1Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

Losses computed from vulnerability relationships shown in figure 11 and 

table 6.

XT , Value Losses 

Number ($x!06) ($x!06)l

Losses 

($x!06)2

Dwellings

Wood frame: 

\-b units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

and V-E): 

1^1 units 

>5 units

Total

122,695 6,308.6 66.3 

21,824 1,140.3 15.6

184,042 9,462.9 129.7 

32,736 1,959.2 45.3

361,297 18,871.0 256.9

91.0 

22.9

446.5 

154.4

714.8

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3

2,175.7

1,782.3 

4,867.9

0.7 

.3 

17.2 

46.5

45.3 

110.0

Dwellings and nondwellings

Total 23,739.9 824.8

1 Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2Losses computed from vulnerability relationships shown in figure 11 and 

table 6.

earthquake on the Salt Lake segment, the losses are 

reduced from $1,937.9 million to $735.7 million. This latter 

estimate is still larger than the $385 million loss sustained 

in the intensity VIII Whittier Narrows, Calif., earthquake 

of October 1, 1987 (Hauksson and others, 1988), but this 

difference may be attributed to the comparatively high 

proportion of very vulnerable masonry buildings in the 

Salt Lake area.

Thus, these analyses provide considerable additional 

information about the structure of the losses and how 

they change.

Losses associated with liquefaction and landsliding 

have not been considered in this report because of uncertainty in the area! relationships for these types of geologic 

hazards. Rojahn and others (1977) have recently provided 

information on areas of possible liquefaction damage in 

Salt Lake County and discussed a general methodology 

for incorporating estimates of damage caused by liquefaction and landsliding into damage estimates based on 

shaking and fault rupture. The losses associated with geological hazards are, however, not simply additive, the total 

damage to a structure being some complex combination 

of damage from shaking, landsliding, and liquefaction.
P26 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 17. Summary of estimated losses in Salt Lake City from a 

major (Ms=7.5) earthquake on the Salt Lake segment of the Wasatch 

fault

TABLE 18. Summary of estimated losses in Salt Lake City from a 

major (Ms=7.5) earthquake on the Weber segment of the Wasatch 

fault

ttanta Value Losses Losses 

($x!06)2

XT , Value Losses 

Number ($xl06) ($xlQfl)l

Dwellings

Wood frame:

1^1 units (class I-A)

>5 units (class I-B)

Masonry (classes V-B

andV-E):

1^1 units

>5 units

Total

21,506

8,409

32,260

12,614

74,789

1,115.7 223.7

439.4 88.4

1,673.5 684.5

754.9 309.5

3,983.5 1,306.1

112.9

46.2

1,047.0

473.4

1,679.5

Wood frame:

1^1 units (class I-A)

>5 units (class I-B)

Masonry (classes V-B

andV-E):

1^1 units

>5 units

Total

Nondwellings

Wood frame (class I-B)

All metal (class II)

Steel frame (class III)

Concrete frame (class IV)

Mixed construction

(class V)

Total

40.1

79.8

551.6

1,692.4

1,013.7

3,377.6

3.6

6.4

84.9

327.3

178.1

600.3

Dwellings and nondwellings

Total 7,361.1 2,279.8

Wood frame (class I-B)

All metal (class II)

Steel frame (class III)

Concrete frame (class IV)

Mixed construction

(class V)

Total

Losses 

($x!06)2

Dwellings

21,506

8,409

32,260

12,614

74,789

1,115.7 153.9

439.4 61.2

1,673.5 535.4

754.9 241.9

3,983.5 992.4

94.0

36.9

1,026.0

460.4

1,617.3

Nondwellings

40.1

79.8

551.6

1,692.4

1,013.7

3,377.6

3.5

6.3

80.1

311.8

171.9

573.6

Dwellings and nondwellings

Total 7,361.1 2,190.9

Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2 Losses computed from vulnerability relationships shown in figure 11 and

table 6.

1 Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2Losses computed from vulnerability relationships shown in figure 11 and 

table 6.
EARTHQUAKE LOSSES IN CENTRAL UTAH P27

TABLE 19. Summary of estimated losses in Salt Lake City from a TABLE 20. Summary of expected maximum losses in the fourmajor (Ms=7.5) earthquake on the Provo segment of the Wasatch county area over a 50-year period with a 10-percent chance of 

fault exceedance

Value Losses

($x!06) ($x!06) 1

Losses

($x!06)2

Dwellings

Wood frame:

1-4 units (class I-A)

>5 units (class I-B)

Masonry (classes V-B

and V-E):

1-4 units

>5 units

Total

21,506

8,409

32,260

12,614

74,789

1,115.7 73.5

439.4 28.4

1,673.5 343.1

754.9 151.8

3,983.5 596.8

79.7

31.0

914.1

407.0

1,431.8

Nondwellings

Wood frame (class I-B)

All metal (class II)

Steel frame (class III)

Concrete frame (class IV)

Mixed construction

(class V)

Total

40.1

79.8

551.6

1,692.4

1,013.7

3,377.6

3.0

5.2

66.2

249.6

143.2

467.2

Dwellings and nondwellings

Total 7,361.1 1,899.0

XT . Value Number ($xl()6) Losses Losses 

($x!06) 1 ($x!06)2

Dwellings

Wood frame: 

1-4 units (class I-A) 

>5 units (class I-B) 

Masonry (classes V-B 

and V-E): 

1-4 units 

>5 units

Total

122,695 6,308.6 

21,824 1,140.3

184,042 9,462.9 

32,736 1,959.2

361,296 18,871.0

128.0 221.4 

25.7 43.0

416.0 1,214.5 

109.0 352.6

678.7 1,831.4

Nondwellings

Wood frame (class I-B) 

All metal (class II) 

Steel frame (class III) 

Concrete frame (class IV) 

Mixed construction 

(class V)

Total

40.1 

108.5 

761.3 

2,175.7

1,782.3 

4,867.3

1.7 

2.6 

44.8 

150.1

125.7 

324.9

Dwellings and nondwellings

Total 23,738.9 2,156.3

1 Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2Losses computed from vulnerability relationships shown in figure 11 and 

table 6.

1 Losses computed from vulnerability relationships developed by the Applied 

Technology Council (1985).

2 Losses computed from vulnerability relationships shown in figure 11 and 

table 6.
P28 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 21. Variation of losses with intensity for a major (Ms=7.5) earthquake on the Salt Lake segment of the Wasatch fault

[N/A, not applicable]]

Class

I

II

III

IV 

V

Total

Value

($x!06)

7,489.0 

108.5

761.3

2,175.7 

13,204.4

23,739.9

Loss 1 

(SxlO6) 1

530.8 

6.8

100.9

336.7 

3,443.7

4,418.9

Loss0 

($x!06)2

657.3 

8.5

119.7

417.1 

4,260.2

5,462.8

Loss+1 

($x!06)3

712.2 

8.8

122.7

438.0 

4,546.2

5,827.9

Loss 1 

(percent) 1

-19.2 

-20.0

-15.7

-19.3 

-19.2

N/A

Loss0 

(percent)2

0 

0

0

0 

0

N/A

Loss"1" 1 

(percent)3

8.4 

3.5

2.5

5.0 

6.7

N/A

1 Loss 1 ($xlO 6) and loss 1 (percent) are the losses and change in losses, respectively, when all intensities have been reduced by one intensity unit.

2 Loss0 ($xl(T6) and loss0 (percent) are the same losses given in table 8.

3 Loss"1" 1 ($xl(T6) and loss"1" 1 (percent) are the losses and change in losses, respectively, when all intensities have been increased by one 

intensity unit.

TABLE 22. Variation of losses with intensity for a strong (Ms=6.5) earthquake on the Salt Lake segment of the Wasatch fault

[N/A, not applicable]]

Class

I 

II

III

IV 

V

Total

Value

($x!06)

7,489.0 

108.5

761.3

2,175.7 

13,204.4

23,739.9

Loss 1 

(SxlO6) 1

301.6 

2.4

51.7

163.3 

1,711.8

2,230.8

Loss0 

($x!06)2

477.2 

5.9

89.6

301.1 

3,079.8

3,953.6

Loss"1" 1 

($x!06)3

620.4 

8.2

114.5

399.4 

4,052.0

5,194.5

Loss 1 

(percent) 1

-19.2

N/A

Loss0 

(percent)2

0 

-59.3

-42.3

-45.8 

^4.4

N/A

Loss* 1 

(percent)3

8.4 

0

0

0 

0

N/A

1 Loss 1 (SxlCT6) and loss 1 (percent) are the losses and change in losses, respectively, when all intensities have been reduced by one intensity unit.

2 Loss0 (SxlCT6) and loss0 (percent) are the same losses given in table 8.

3 Loss"1" 1 (SxlCT6) and loss"1" 1 (percent) are the losses and change in losses, respectively, when all intensities have been increased by one intensity unit.

TABLE 23. Variation of losses with intensity for a moderate (Mi=5.5) earthquake on the Salt Lake segment of the Wasatch

fault

[N/A, not applicable]

Class

I 

II

III

IV 

V

Total

Value

($x!06)

7,489.0 

108.5

761.3

2,175.7 

13,204.4

23,739.9

Loss 1 

(SxlO6) 1

105.7 

.3

15.1

43.8 

570.8

4,418.9

Loss0 

($x!06)2

251.9 

2.0

44.2

137.8 

1,502.0

5,462.8

Loss"1" 1 

($x!06)3

426.0 

5.4

81.4

274.6 

2,844.8

5,827.9

Loss 1 

(percent) 1

-58.0 

-85.0

-65.8

-68.2 

-61.9

N/A

Loss0 

(percent)2

0 

0

0

0 

0

N/A

Loss"1" 1 

(percent)3

69.1 

170.0

84.2

99.3 

89.4

N/A

1 Loss 1 ($xlO 6) and loss 1 (percent) are the losses and change in losses, respectively, when all intensities have been reduced by one intensity unit.

2 Loss0 (SxHT6) and loss0 (percent) are the same losses given in table 8.

3 Loss"1" 1 ($xlO~*) and loss"1" 1 (percent) are the losses and change in losses, respectively, when all intensities have been increased by one intensity unit.
EARTHQUAKE LOSSES IN CENTRAL UTAH P29

REFERENCES CITED

Algermissen, S.T., ed, 1985, Preliminary report of investigations 

of the central Chile earthquake of March 3, 1985: U.S. Geological Survey Open-File Report 85-542,180 p.

Algermissen, S.T., and Steinbrugge, K.V., 1984, Seismic hazard 

and risk assessment: Some case studies, in The Geneva 

papers on risk and insurance: Association International 

pour 1'Etude de 1'Economie de 1'Assurance, v. 9, no. 30, p. 8- 

26.

Algermissen, S.T., Dewey, J.W., Langer, C.J., and Dillinger, W.H., 

1974, The Managua, Nicaragua, earthquake of December 23, 

1972: Location, focal mechanism, and intensity distribution: 

Bulletin of the Seismological Society of America, v. 64, p. 

993-1004.

Algermissen, S.T., Perkins, D.M., Thenhaus, PC., 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, 99 

p., 6 pis.

Algermissen, S.T., Steinbrugge, K.V., and Lagorio, H.L., 1978, 

Estimation of earthquake losses to buildings (except single 

family dwellings): U.S. Geological Survey Open-File Report 

78-441, 174 p., 38 figs.

Applied Technology Council, 1985, Earthquake damage evaluation data for California: Redwood City, Calif., Report ACT13, 492 p.

Borcherdt, R.D., 1970, Effects of local geology on ground motion 

near San Francisco Bay: Bulletin of the Seismological Society of America, v. 60, p. 29-61.

Borcherdt, R.D., Joyner, W.B., Warrick, R.E., and Gibbs, J.F., 

1975, Response of local geologic units to ground shaking, in 

Borcherdt, R.D., ed., Studies for seismic zonation of the San 

Francisco Bay region: U.S. Geological Survey Professional 

Paper 941-A, p. A52-A67.

Cassaro, M.A., and Romero, E.M., eds., 1986, The Mexico 

earthquakes 1985: New York, American Society of Civil 

Engineers, 473 p.

Dewey, J.W., and Algermissen, S.T., 1974, Seismicity of the Middle American arc-trench system near Managua, Nicaragua: 

Bulletin of the Seismological Society of America, v. 64, p. 

1033-1048.

Hauksson, E., Jones, L.M., Davis, T.L., and others, 1988, The 1987 

Whittier Narrows earthquake in the Los Angeles metropolitan area, California: Science, v. 239, p. 1409-1412.

Hays, W.W., Algermissen, S.T., Miller, R.D., and King, K.W., 1978, 

Preliminary ground response maps for the Salt Lake City, 

Utah, area: International Conference on Microzonation, 2d, 

San Francisco, 1978, Proceedings, 12 p.

King, K.W., Hays, W.W., and McDermott, P.J., 1983, Wasatch 

Front urban area seismic response data report: U.S. Geological Survey Open-File Report 83-452, 68 p.

Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone: A summary of recent

investigations, interpretations, and conclusions, in Gori, 

PL., and Hays, W.W., eds., Assessment of regional earthquake hazards and risks along the Wasatch Front, Utah: U.S. 

Geological Survey Professional Paper 1500-A, p. A1-A71.

Mark, R.K., 1977, Application of linear statistical models of 

earthquake magnitude versus fault length in estimating 

maximum expectable earthquakes: Geology, v. 5, p. 464- 

466.

Murphy, L, 1973, San Fernando, California earthquake of February 9, 1971: Washington, D.C., U.S. Department of Commerce, v. Ill, 432 p.

Rios, R., Grases, J., Ponce, L., Weidmann, N., and Zegarra, L, 

1986, El terremoto de San Salvador 10 de Octubre 1986: 

Lima, CERESIS/UNESCO, 56 p., 3 apps.

Rogers, AM., Algermissen, S.T., Hays, W.W., Perkins, D.M., Van 

Strein, D.O., Hughes, H.C., Hughes, R.C., Lagorio, H.J., and 

Steinbrugge, K.V., 1976, A study of earthquake losses in the 

Salt Lake City, Utah, area: U.S. Geological Survey Open-File 

Report 76-89, 375 p.

Rogers, AM., Carver, D.L., Hays, WW, King, K.W, and Miller, 

R.D., 1984, Preliminary estimates of geographic variation in 

relative ground shaking in the Wasatch Front urban corridor: U.S. Geological Survey Open-File Report 84-763, p. 

547-556.

Rojahn, C., King, S.A., Scholl, R.E., Kiremidjian, AS., Reaveley, 

L.D., and Wilson, R.R., 1997, Earthquake damage and loss 

estimation methodology and data for Salt Lake County, 

Utah (ATC-36): Earthquake Spectra, v. 13, p. 623-642.

Rosenblueth, E., 1964, Probabilistic design to resist structures: 

Proceedings of the American Society of Civil Engineers, 

Journal of the Engineering Mechanics Division, v. EM-90, p. 

182-219.

Steinbrugge, K.V., 1979, Estimation of loss estimation techniques 

to metropolitan Salt Lake City, Utah: Report to the U.S. Geological Survey under contract 14-08-0001-17423, 55 p.

 1986, Building inventory data and loss estimation methods for Salt Lake City-Ogden-Provo, Utah: Report to the 

U.S. Geological Survey under contract 14-08-0001-22408, 

48 p., 2 apps.

Stone, W.C., Yokel, F.Y., Celebi, M., Hanks, T, and Leyendecker, 

E.V., 1987, Engineering aspects of the September 19, 1985 

Mexico earthquake: Washington, D.C., National Bureau of 

Standards, 215 p.

Bureau of the Census, 1983a, Census tracts, Provo-Orem, Utah 

Standard Metropolitan Statistical Area: Washington, D.C., 

Report PHC80-2-294.

 1983b, Census tracts, Salt Lake City-Ogden, Utah Standard Metropolitan Statistical Area: Washington, D.C., 

Report PHC80-2-317.

 1983c, Census of population and housing: Washington, 

D.C., Master Area Reference File (MARF) 2 (magnetic 

tape).

Isoseismals of Some Historical Earthquakes Affecting the 

Wasatch Front Area, Utah

By MARGARET G. HOPPER

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-Q,

CONTENTS

Page 

Abstract............................................................................................................................................................. Ql

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

Isoseismals........................................................................................................................................................

1900 Eureka Earthquake........................................................................................................................

1910 Salt Lake City Earthquake.............................................................................................................

1914 Odgen Earthquake.........................................................................................................................

1915 Provo Earthquake..........................................................................................................................

1934 Hansel Valley Earthquake.............................................................................................................

1943 Salt Lake City Earthquake............................................................................................................. 4

1958 WaUsburg Earthquake................................................................................................................... 4

1962 Cache Valley Earthquake .............................................................................................................. 4

1962 Salt Lake City Earthquake............................................................................................................. 16

1972 Midway Earthquake ...................................................................................................................... 16

1978 Magna Earthquake ........................................................................................................................ 16

1981 Orem Earthquake .......................................................................................................................... 16

1983 West Valley City Earthquake......................................................................................................... 16

Maximum Historical Intensity Map .................................................................................................................... 16

References Cited.................................................................................................................................................. 25

ILLUSTRATIONS

Page

FIGURES 1,2. Maps showing epicenters of historical Utah earthquakes:

1. Having maximum Modified Mercalli intensities of VI or greater ................................................................................................................ Q2

2. Used in this study.......................................................................................................................................................................................... 5

3-15. Isoseismal maps showing:

3. August 1, 1900, Eureka earthquake .............................................................................................................................................................. 6

4. May 22, 1910, Salt Lake City earthquake..................................................................................................................................................... 7

5. May 13, 1914, Ogden earthquake ................................................................................................................................................................. 8

6. July 15, 1915, Provo earthquake................................................................................................................................................................... 9

7. March 12, 1934, Hansel Valley earthquake................................................................................................................................................... 10

8. February 22, 1943, Salt Lake City earthquake.............................................................................................................................................. 12

9. February 13, 1958, Wallsburg earthquake..................................................................................................................................................... 13

10. August 30, 1962, Cache Valley earthquake................................................................................................................................................... 14

11. Septembers, 1962, SaltLake City earthquake............................................................................................................................................. 17

12. October 1, 1972, Midway earthquake........................................................................................................................................................... 18

13. March 9, 1978, Magna earthquake................................................................................................................................................................ 19

14. February 20, 1981, Orem earthquake............................................................................................................................................................ 20

15. October 8, 1983, West Valley City earthquake.............................................................................................................................................. 21

16. Map showing maximum historical intensities for the Utah counties of Weber, Davis, SaltLake, and Utah....................................................... 22

17, 18. Isoseismal maps showing:

17. March 28, 1975, Malad City and Pocatello Valley, Idaho, earthquakes........................................................................................................ 23

18. Sevier County, Utah, earthquakes of November 14, 1901, September 29,1921, September 30, 1921, and

October 1, 1921..................................................................................................................................................................^ 24

III
IV CONTENTS

TABLES

Page 

TABLE 1. Earthquakes shown in figure 1 .....................................................................................................................................................................................^ Q3

2. Earthquakes used in this study........................................................................................................................................................................................ 4
ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK 

ALONG THE WASATCH FRONT, UTAH

ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING

THE WASATCH FRONT AREA, UTAH

By MARGARET G. HOPPER

ABSTRACT

Isoseismals were drawn for 13 historical earthquakes that caused 

damage-level Modified Mercalli intensities (MM/greater than or equal to 

VI) in the four Wasatch Front counties of Weber, Davis, Salt Lake, and 

Utah. The highest historical intensity within any of the four counties was 

found to be MMIVII. Davis, Salt Lake, and most of Weber Counties were 

found to be in an area commonly reporting intensity VI. However, historical intensities in Utah County have been below damage level, with the 

exception of those in the Provo area to the northeast of Utah Lake and in 

the Eureka mining district in the southwestern corner of Utah County 

and adjoining Juab County. High historical intensities in the northern 

part of the four-county study area are the result of the 1934 Hansel Valley 

earthquakes and large earthquakes in southern Idaho.

INTRODUCTION

Simulations of earthquake shaking in the Wasatch 

Front area require a knowledge of intensities generated 

by historical earthquakes affecting the area Epicenters 

for all historical shocks in or near Utah having maximum 

Modified Mercalli intensities (MM/0) of at least VI (Wood 

and Neumann, 1931) are shown in figure 1 and listed in 

table 1. For this study, isoseismal maps were made showing a four-county study area (Weber, Davis, Salt Lake, and 

Utah Counties). The set of 13 earthquakes mapped in this 

study (table 2, fig. 2) all caused damage (MMI >VI) within 

this four-county area. Most were small local shocks 

(MM/0=VI), but a few were larger, more distant shocks 

that produced MMI>VL within the study area. There was 

available for each earthquake at least enough intensity 

information at locatable points for rough isoseismals to 

be contoured.

ISOSEISMALS

1900 EUREKA EARTHQUAKE

The Eureka earthquake of August 1, 1900 (fig. 3), 

twisted the deep shaft of the Mammoth Mine so that the

cage could not be lowered. At Santaquin, an adobe house 

was split in two (Townley and Alien, 1939).

1910 SALT LAKE CITY EARTHQUAKE

The Salt Lake City earthquake of May 22, 1910 (fig. 

4), on the Wasatch fault damaged several buildings in Salt 

Lake City, shaking plaster from ceilings and toppling 

chimneys on poorly constructed buildings but leaving 

well-constructed buildings unharmed. The earthquake did 

minor damage at Bingham and Garfield but was not felt 

more than 50 mi from Salt Lake City. It was followed by 

two lighter shocks (Williams and Tapper, 1953).

1914 OGDEN EARTHQUAKE

The Ogden earthquake of May 13, 1914 (fig. 5), on the 

Wasatch fault cracked walls, toppled a few chimneys, and 

broke plate glass windows in Ogden. In Salt Lake City, 

dishes were broken. The shock was followed by a second, 

lighter shock (Williams and Tapper, 1953).

1915 PROVO EARTHQUAKE

The Provo earthquake of July 15, 1915 (fig. 6), on the 

Wasatch fault cracked ceilings at Provo (Williams and 

Tapper, 1953).

1934 HANSEL VALLEY EARTHQUAKE

The Hansel Valley earthquake of March 12, 1934 (fig. 

7A, J5), occurred in a sparsely populated area and did very 

little damage except for fissures, ground cracks, and liquefaction. At Kosmo, a poorly constructed brick building 

was severely cracked; at Monument, all the chimneys 

were thrown down. Beyond the immediate meizoseismal 

area, chimneys fell at Bacchus, Blue Creek, Hooper,

Qi
Q2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

114° 113° 112° 111° 110° 109°

IDAHO

Pocatello Valley

1934|March 12 15:05 VIlH

193 4 March 12 18:02 VII >

1934 March 15VIJ 

1934 May 6 ^

i

i

1978 March i i

1962

1943

1

1 1915

1

i

£

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2

i

l

i

1901 NovHIX^ I

1975 March 28

7 1909 Oct 6

> __

i ^VJ Hanse

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6 1958

Aug 11 1915

1981 Feb

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Eu

1963 July 7

1910 Jan 10 VI ^ 1945Nov18 ,

6 1960 Aug 7

Cache '

Valley

1 962 Aug 30 

__Z _y__

1884Nov10

I

Valley i

l

roOgden

\1894July18 VI

1914 May 13 VII

 ^1983 Oct 8

./Salt Lake City

> OflTl910May22VII

1949 March 7 VI

Feb 13^6 6-1972

July 15 6

20 6 Provo o

1900 Aug 1

 eka

_ 1876 March 22 

6 6

6 B

1961 April 16 °

UTAH

1989 Jan 30

S -*.

1910 Jan 12 VI f~\~~ ^ ° **

1 1921 Sept 29 VIII JBg

1921 Sept 30 VII 

1921 Oct1 VIM

l 1972 Jan 3 VI

1

1

 L.

1 

1

1982 May 24 VI ^

1942 Aug 30 VI _ 

1942 Sept 26 VI B

<»1902Nov 1

® | 55 1902 Dec 5

w

1967 Oct 4 7

1959 Feb 27

61933 Jan 20

7 VIII

VI

ISDD 1 1

Aug 16

1

Pine Valley

61891 April 20

1887 Dec 5

6

Oct1

^

6

1977 Sept 30

^Area of

figure 2

1963 March 12

6

1977 Feb 9

S 1988 Aug 14

SEVIER COUNTY

WYOMING

_

l

i

i

. __ _______ i 

i 

O 

O

i O

5

O

0

l

i

1

1 

1

i

i

1

1

42'

41

40C

39 C

38 C

50 100 MILES

50 100 150 KILOMETERS

FIGURE 1. Epicenters of historical earthquakes having maximum Modified Mercalli intensities (MM/0) of VI or greater. Numbers are plotted 

at epicenters and indicate the MM/0's. The earthquakes shown are listed in table 1.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH Q3

TABLE 1. Earthquakes shown in figure 1

[All information in this table is from Stover and others (1986a, b), except as noted. This list includes all earthquakes with MM/0>VI located between 

37.0° and 42.5°N. and between 108.5° and 114.5°W. , data not available. All locations in Utah unless otherwise indicated]

Year

1876

1884

1887

1891

1894

1900

1901

1902

1902

1909

1910

1910

1910

1914

1915

1915

1921

1921

1921

1933

1934

1934

1934

1934

1942

1942

1943

1945

Date

03/22

11/10

12/05

04/20

07/18

08/01

11/14

11/17

12/05

10/06

01/10

01/12

05/22

05/13

07/15

08/11

09/29

09/30

10/01

01/20

03/12

03/12

03/15

05/06

08/30

09/26

02/22

11/18

Time

(UTC)

08:50

15:30

13:55

22:50

07:45

04:32

19:50

02:41

13:00

03:00

14:28

17:15

22:00

10:20

14:12

02:30

15:32

13:05

15:05

18:20

12:01

08:09

23:08

15:50

14:20

01:07

Lat

(°N.)

39.50

42.00

37.10

37.10

41.20

40.00

38.70

37.40

37.40

41.80

38.70

38.70

40.70

41.20

40.40

40.50

38.70

38.70

38.70

37.80

41.50

41.50

41.50

41.50

37.70

37.70

40.70

38.80

Long

(°W.)

111.60

111.30

112.50

113.60

112.00

112.10

112.10

113.50

113.50

112.70

112.10

112.10

111.80

112.00

111.60

112.70

112.10

112.10

112.10

112.80

112.50

112.50

112.50

113.00

113.10

113.10

112.00

112.00

MM/0

VI

VII

VI

VI

VI

VII

IX

VIII

VI

VII

VI

VI

VII

VII

VI

VI

VIII

VII

VIII

VI

VIII

VII

VI

VI

VI

VI

VI

VI

Location

Moroni

Near Paris,

Idaho

Kanab

St. George

Ogden

Eureka

Richfield

Pine Valley

Pine Valley

Hansel Valley

Richfield

Elsinore

Salt Lake City

Ogden

Provo

Josepa

Elsinore

Elsinore

Elsinore

Parowan

Kosmo, Hansel

Valley

Kosmo, Hansel

Valley

Utah

Kosmo, Hansel 

Valley

Cedar City

Cedar City

Near Salt Lake

City

Glenwood

Year

1949

1949

1958

1959

1960

1961

1962

1962

1963

1963

1966 1

1967

1972

1972

1975

1977

1977

1978

1981

1982

1983 *

1988 1

19891

Date

03/07

11/02

02/13

02/27

08/07

04/16

08/30

09/05

03/12

07/07

08/16

10/04

01/03

10/01

03/28

02/09

09/30

03/09

02/20

05/24

10/08

08/14

01/30

Time

(UTC)

06:50

02:29

22:52

22:19

16:27

05:02

13:35

16:04

23:47

19:20

18:02

10:20

10:20

19:42

02:31

00:42

10:19

06:30

09:13

12:13

11:57

20:03

04:06

Lat

(°N.)

40.70

37.00

40.50

38.00

42.40

39.33

42.04

40.72

39.51

39.53

37.40

38.54

38.65

40.51

42.06

39.29

40.47

40.76

40.32

38.71

40.75

39.13

38.82

Long

(°W.)

111.80

113.50

111.50

112.50

111.50

111.65

111.74

112.09

110.66

111.91

114.20

112.16

112.17

111.35

112.52

111.11

110.47

112.09

111.74

112.04

111.99

110.87

111.61

MM/0

VI

VI

VI

VI

VI

VI

VII

VI

VI

VI

VI

VII

VI

VI

VIII

VI

VI

VI

VI

VI

VI

VI

VI

Location

Salt Lake City

St. George

Wallsburg

Panguitch

Near Soda

Springs,

Idaho

Ephraim

Cache Valley,

Idaho

Salt Lake City

Columbia

(rockburst)

Levan

Nevada

Marysvale

Elsinore

Midway

Ridegdale,

Pocatello

Valley, Idaho

Elmo

Mountain

Home

Magna

Orem

Anabella

West Valley

Citjr

Utah

Utah

'U.S. Geological Survey National Earthquake Information Center Earthquake Data

Base Sy:stem.

2 Stover (1987).
Q4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 2. Earthquakes used in this study 

[All information in this table is from Stover and others (1986a), except as noted. All locations in Utah except as noted. , data not available]

Year

1900

1910

1914

1915

1934

1943

1958

1962

19622

1972

1978

1981

19836

Date

08/01

05/22

05/13

07/15

03/12

02/22

02/13

08/30

09/05

10/01

03/09

02/20

10/08

Time (UTC)

07:45

14:28

17:15

22:00

15:05

14:20

22:52

13:35

16:04

19:42

06:30

09:13

11:57

Lat 

(°N.)

40.00

40.70

41.20

40.40

41.50

40.70

40.50

42.04

40.72

40.51

40.76

40.32

40.75

Long

(°W.)

112.10

111.80

112.00

111.60

112.50

112.00

111.50

111.74

112.09

111.35

112.09

111.74

111.99

Number of 

sites1

9

3

3

6

154

20

6

240

55

23

19

9

25

MMI0

VII

VII

VII

VI

VIII

VI

VI

VII

VI

VI

VI

VI

VI

Magnitude

35.7

35.7

35.7

35.0

46.6

35.0

35.0

55.7

55.2

54.3

53.2

53.9

54.3

Felt area 

(km2)

1,500

9,100

20,700

12,900

440,000

Local

3,100

168,000

23,300

6,500

Location

Eureka

Salt Lake City

Ogden

Provo

Kosmo, Hansel Valley

Salt Lake City

Wallsburg

Cache Valley, Idaho

Salt Lake City

Midway

Magna

Orem

West Valley City7

Number of located sites having assigned Modified Mercalli intensities. 

2Coffman and others (1982). 

3Magnitude estimated from MM/0 (Arabasz and McKee, 1979).

Gutenberg-Richter magnitude.

University of Utah magnitude.

6U.S. Geological Survey National Earthquake Information Center Earthquake Data Base System. 

7Stover (1987).

Kelton, Locomotive Springs, Promontory, and Snowville 

(Neumann, 1936).

1943 SALT LAKE CITY EARTHQUAKE

The Salt Lake City earthquake of February 22, 1943 

(fig. 8), was felt most strongly in the Salt Lake Valley west 

of Salt Lake City. It shook down much plaster from the 

new brick Bingham High School; cracked plaster, windows, and chimneys at Magna; and cracked plaster and 

windows in Salt Lake City (Bodle, 1945). The shock is presumed to have been located at depth on a branch of the 

Wasatch fault (Williams and Tapper, 1953).

1958 WALLSBURG EARTHQUAKE

The Wallsburg earthquake of February 13, 1958 (fig. 

9), caused minor damage at Wallsburg and Provo. At 

Provo, a wall was cracked and plaster fell (Brazee and 

Cloud, 1960).

1962 CACHE VALLEY EARTHQUAKE

The Cache Valley earthquake of August 30, 1962 (fig. 

10A, B~), on the East Cache Valley fault caused the most 

severe damage at Richmond, where numerous houses lost

walls, a large church was damaged beyond repair, and 75 

percent of the older chimneys fell. Buildings in downtown 

Logan suffered severe damage from cracked and distorted walls, and parapets fell at the junior high school. At 

Lewiston, there was much chimney damage, and a large 

parapet and the second-story part of a brick wall fell. At 

Hyde Park, there was considerable chimney damage but 

no damage to newer brick structures. At Smithfield, 

plaster, walls, and chimneys cracked, and bricks fell from 

most old chimneys. At Franklin, Idaho, plaster, walls, and 

chimneys cracked, chimneys twisted and fell, and some 

foundations cracked. At Preston, Idaho, a few chimneys 

fell, and a few homes had cracked walls and fireplaces 

(Lander and Cloud, 1964).

FIGURE 2. Epicenters of historical earthquakes used in this study. 

Solid circles show the locations of the epicenters; the roman 

numerals indicate their maximum Modified Mercalli intensities 

(MM/0). Each of these earthquakes caused damaging intensities 

(MMf>VL) within the four-county study area. In addition, the Cache 

Valley earthquake (August 30, 1962), north of this map, caused 

damage within the study area. All earthquakes used in this study 

are listed in table 2. Where there are multiple shocks at a single 

location, only the largest shock was used in this study. Thus, the 

smaller shocks at the sites of multiple shocks appear in table 1 and 

figure 1 but not in table 2 or figure 2.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH

112°30' 112°00' 111°30' 11 TOO'

Q5

41°30'

41°00'

40°30'

40°00'

25 50 MILES

25 50 75 KILOMETERS
Q6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112°30' 112°00'

41°30

41W

25 50 MILES

0 25 50 75 KILOMETERS

FIGURE 3. Isoseismal map for the earthquake of August 1,19.00, Eureka, Utah. The maximum Modified Mercalli intensity ( MM/0) is 

VII. The star indicates the epicenter. Arabic numbers indicate site intensities (MM/), and roman numerals indicate isoseismal 

intensities. F indicates report of "felt" that was not assigned a MM/. Isoseismals are dashed where uncertain.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH Q7

112"30'

41°30'

41°00'

40°30'

40°00'

25 50 MILES

0 25 50 75 KILOMETERS

FIGURE 4. Isoseismal map for the earthquake of May 22, 1910, Salt Lake City, Utah. The maximum Modified Mercalli intensity 

(MM/0) is VII. The star indicates the epicenter. Arabic numbers indicate site intensities (MM/), arid roman numerals indicate 

isoseismal intensities. F indicates a report of "felt" that was not assigned a MM/. Isoseismals are dashed where uncertain.
Q8 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

M2°30' 112°00 r 111°30 r 111W

40

40W

25 50 MILES

25 50 75 KILOMETERS

FIGURE 5. Isoseismal map for the earthquake of May 13,1914, Ogden, Utah. The maximum Modified Mercalli intensity ( MM/0) is VII. 

The star indicates the epicenter. Arabic numbers indicate site intensities (MM/), and roman numerals indicate isoseismal 

intensities. F indicates a report of "felt" that was not assigned a MM/. Isoseismals are dashed where uncertain.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH Q9

112°30' 112W 111°30' 11 TOO'

41°30'

41°00'

40°30'

40°00'

WEBER COUNTY

Ogden

V

Salt Lake City

\ I-"'

1 \

IV I 

LAKE COUNTY X N

x

\

WYOMING

25 50 MILES

0 25 50 75 KILOMETERS

FIGURE 6. Isoseismal map for the earthquake of July 15,1915, Provo, Utah. The maximum Modified Mercalli intensity ( MMI(j) is VI. 

The star indicates the epicenter. Arabic numbers indicate site intensities (MMF), and roman numerals indicate isoseismal 

intensities. F indicates a report of "felt" that was not assigned a MMI. Isoseismals are dashed where uncertain.
Q10 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112 r30' 112°00' 1

40°30

40°00'

75 KILOMETERS

FIGURE 7. Isoseismal map for the earthquake of March 12, 1934, Hansel Valley, Utah (near Kosmo, Utah). The maximum Modified 

Mercalli intensity (MM/0) is VIII; ML is 6.6. Arabic numbers indicate site intensities (MM/), and roman numerals indicate 

isoseismal intensities. Isoseismals are from Neumann (1936). A, Within the four-county study area. The star indicates the 

epicenter. B, Entire isoseismal map. Area of A is indicated.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH Ql 1

A S H IN G TON !- "

100 200 MILES

i i i r~

0 100 200 300 KILOMETERS 

FIGURE 7. Continued.
Q12 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

!12°30' 112W 111°30' 1'

41°30'

41°00'

25 50 MILES

0 25 50 75 KILOMETERS

FIGURE 8. Isoseismal map for the earthquake of February 22,1943, Salt Lake City, Utah. The maximum Modified Mercalli intensity 

(MM/0) is VI. The star indicates the epicenter. Arabic numbers indicate site intensities (MMI), and roman numerals indicate 

isoseismal intensities.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH Q13

112 D 30' 112W 111°30' 11 TOO'

41°30'

41W

40°30'

40W

25 50 MILES

25 50 75 KILOMETERS

FIGURE 9. Isoseismal map for the earthquake of February 13, 1958, Wallsburg, Utah. The maximum Modified Mercalli intensity 

(MM/0) is VI. The star indicates the epicenter. Arabic numbers indicate site intensities (MM/), and roman numerals indicate 

isoseismal intensities. F and H indicate reports of "felt" and "heavy," respectively, that were not assigned MMTs. Isoseismals are 

dashed where uncertain. Berg and Resler (1958) developed a similar isoseismal map.
Q14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112°30'_________________ 112°00'

41°30'n ^

41°00'

40°30'

40W

_S_5_^ C

\

^5 ?

(* ^

Salt Lake City

V

SALT LAKE COUNTY 

H

\ (>

1_

55

WYOMING

5

50 MILES

0 25 50 75 KILOMETERS

FIGURE 10. Isoseismal map for the earthquake of August 30, 1962, Cache Valley, Utah. The maximum Modified Mercalli intensity 

(MM/0) is VII; ML is 5.7. Arabic numbers indicate site intensities (MM/), and roman numerals indicate isoseismal intensities. 

Isoseismals are from Lander and Cloud (1964). A, Within the four-county study area. Epicenter is off the map to the north. 

B, Entire isoseismal map. Star indicates the epicenter. Area of A is indicated.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH Q15

40'

38'

100 200 MILES

100 200 

FIGURE 10. Continued.

I

300 KILOMETERS
Q16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

1962 SALT LAKE CITY EARTHQUAKE

The Salt Lake City earthquake of September 5, 1962 

(fig. 11), damaged three older houses in Salt Lake City. In 

one, the outside front wall cracked and fell through the 

ceiling; portions of inside walls and ceiling plaster fell in 

the other two. There were slight interior cracks in some 

large buildings in Salt Lake City and cracks and loosened 

acoustical tile "in some of the schools. An old parapet 

facade pulled 2 in. away from one school building, and the 

parapet had to be removed. At the newspaper office, 16 

windows were cracked, a light fixture fell, and there were 

many small cracks in walls and ceilings.

In North Salt Lake, plaster, windows, walls, and 

chimneys were cracked. At the Bingham Canyon Mine, 

there were slight cracks in freshly painted concrete-block 

walls. At Draper, plaster, walls, and cement were cracked. 

At Lark, walls cracked. At Magna, plaster, windows, walls, 

and chimneys cracked. At Midvale, plaster cracked. At 

Morgan, there were slight wall and foundation cracks. At 

Murray, plaster cracked. At Provo, bricks fell from chimneys (Lander and Cloud, 1964).

1972 MIDWAY EARTHQUAKE

The Midway earthquake of October 1, 1972 (fig. 12), 

shook down some bricks from chimneys and cracked 

plaster at two schools and other places in Midway (Coffman and von Hake, 1974).

1978 MAGNA EARTHQUAKE

The Magna earthquake of March 9, 1978 (fig. 13), 

caused damage only at Magna, where there were broken 

windows in several houses, cracked plaster, and cracks in 

the exterior walls of a fire station (Cook, 1979; Stover and 

von Hake, 1980).

1981 OREM EARTHQUAKE

The Orem earthquake of February 20, 1981 (fig. 14), 

cracked a foundation and caused hairline cracks in plaster and drywall in Orem (Stover, 1984).

1983 WEST VALLEY CITY EARTHQUAKE

The West Valley City earthquake of October 8, 1983 

(fig. 15), caused damage only in West Valley City and 

Granger, which are western suburbs of Salt Lake City. In 

West Valley City, chimneys were cracked, and one chimney fell. In Granger, bricks fell from a chimney (Stover, 

1987).

MAXIMUM HISTORICAL INTENSITY MAP

A map was made of the highest intensities experienced within the four-county study area (fig. 16). This 

map, which was derived from the isoseismals and site 

intensities of the 13 earthquakes listed in table 2, shows 

the maximum intensity experienced at any point due to 

any one of the 13 shocks.

Intensities from two of the earthquakes (1934 Hansel 

Valley, 1962 Cache Valley) override most of the map. 

These two large shocks determine the maximum historical intensities for wide areas. Small areas of higher intensities within these wide areas are caused by local shocks.

Three other sets of earthquakes deserve mention in 

connection with the maximum intensity map:

1. The 1975 Pocatello Valley, Idaho, earthquake (fig. 17) 

caused high intensities across the northern part of 

Utah (Stover and others, 1986b; Coffman and Stover, 

1977). However, nowhere within the study area did its 

isoseismals exceed those of the 1962 Cache Valley 

earthquake, and nowhere did its damage-level isoseismals (MMI>VI) intrude upon the four-county study 

area. Therefore, the 1975 shock was not used in the 

development of the maximum intensity map.

2. The group of shocks in Sevier County, Utah (fig. 18), 

includes shocks having maximum intensities estimated 

by various authors to range from MMIo=VH to IX (one 

in 1901 and three in 1921) (Townley and Alien, 1939; 

Williams and Tapper, 1953; Coffman and others, 1982; 

Stover and others, 1986a; Hopper, 1988). All these 

shocks had very rapid intensity attenuation and frequently were not felt much beyond the county borders; 

they were probably extremely shallow events. The 

1901 event, which was reported felt in Salt Lake City, 

could have produced isolated damaging intensities 

within the southern part of the study area, but there is 

not enough information about the 1901 earthquake's 

effects to know for certain.

3. Several small local earthquakes were not included in 

the maximum historical intensity map because little is 

known about their intensity distributions and because 

their epicentral intensities are no higher than other, 

better reported shocks at nearby epicenters. For example, shocks in 1894 and 1949 produced MMIVL at their 

epicenters at Ogden and Salt Lake City, respectively, 

but the effects of the shocks beyond those cities are 

unknown. Neither earthquake affects the maximum 

intensity map (fig. 16) because both locations already 

show MMI VII from earthquakes in 1914 (fig. 5, VII at 

Ogden) and 1910 (fig. 4, VII at Salt Lake City).
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH Q17

112*30' 112°00'

41°30'

41°00'

40°30'

40°00'

WEBER COUNTY

J\ IV

3

25 50 MILES

0 25 50 75 KILOMETERS

FIGURE 11. Isoseismal map for the earthquake of September 5,1962, Salt Lake City, Utah. The maximum Modified MercaUi intensity 

(MM/0) is VI; ML is 5.2. The star indicates the epicenter. Arabic numbers indicate site intensities (MM/), and roman numerals 

indicate isoseismal intensities.
Q18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112°30' 112°00' 11T30'

41

SALT LAKE COUNTY V 

UTAH

25 50 MILES

0 25 50 75 KILOMETERS

FIGURE 12. Isoseismal map for the earthquake of October 1,1972, Midway, Utah. The maximum Modified Mercalli intensity ( MM/0) 

is VI; ML is 4.3. The star indicates the epicenter. Arabic numbers indicate site intensities ( MMI), and roman numerals indicate 

isoseismal intensities.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH

112=30' 112W 111°30' 111°00'

Q19

25 50 MILES

0 25 50

FIGURE 13. Isoseismal map for the earthquake of March 9,1978, Magna, Utah, 

VI; MX, is 3.2. The star indicates the epicenter. Arabic numbers indicate 

isoseismal intensities. Isoseismals are dashed where uncertain.

75 KILOMETERS

The maximum Modified Mercalli intensity ( MM/0) is 

site intensities ( MM/), and roman numerals indicate
Q20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112"30'_________________ 112°00' 

41°30 r

41W

25 50 MILES

0 25 50 75 KILOMETERS

FIGURE 14. Isoseismal map for the earthquake of February 20,1981, Orem, Utah. The maximum Modified Mercalli intensity ( MMI(j) 

is VI; ML is 3.9. The star indicates the epicenter. Arabic numbers indicate site intensities (MM/), and the roman numerals 

indicate isoseismal intensities. F indicates a report of "felt" that was not assigned a MMI.
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH

112 n 30' 112°00' 111°30' 11 TOO'

Q21

41°30'

41°00

Salt Lake City

West \ 

Valley City \

25

I

50 MILES

0 25 50 75 KILOMETERS

FIGURE 15. Isoseismal map for the earthquake of October 8,1983, West Valley City, Utah. The maximum Modified Mercalli intensity 

(MM/0) is VI; ML is 4.3. The star indicates the epicenter. Arabic numbers indicate site intensities (MM/), and roman numerals 

indicate isoseismal intensities.
Q22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112"30'_________________112°00'_________________111°30'_________________ 111W r^7 i i ^**^ i

41°30

41°00

40°30

40°00

SALT LAKE COUNTY

VI 

6 UTAH

25 50 MILES

25 50 75 KILOMETERS
ISOSEISMALS OF SOME HISTORICAL EARTHQUAKES AFFECTING THE WASATCH FRONT AREA, UTAH Q23

FIGURE 16. Maximum historical intensity map for the Utah counties of Weber, 

Davis, Salt Lake, and Utah. Contours and roman numerals indicate areas of 

highest historical isoseismals (MM/) based on the maps in figures 3 through 

15. Arabic numbers indicate historical site intensities strong enough to cause 

damage (MM/>VI). Outlying high intensities (for example, 7's in the VI area) 

are all due to the larger distant earthquakes (Hansel Valley and Cache Valley).

117° 116° 115° 114° 113° 112° 110° 109° 108° 107°

45°

44°

43° II

42°

41'

40°

39°

100 200 MILES

0 100 200 300 KILOMETERS

EXPLANATION

^ Epicenter 3 Intensity o Not felt

FIGURE 17. Isoseismal map for the earthquake of March 28,1975, Pocatello Valley, Idaho. The maximum Modified Mercalli intensity ( MM/0) is 

VIII; ML is 6.0. The star indicates the epicenter. Arabic numbers indicate site intensities (MM/), and roman numerals indicate isoseismal 

intensities. The map is from Coffman and Stover (1977). The area of this study is indicated by the rectangle.

Seismic-Risk Methods and Estimates for Utility Systems and 

State-Owned Buildings Along the Wasatch Front

By CRAIG E. TAYLOR, Dames and Moore, Inc., andDELEEKT B. WARD, Structural 

Facilities, Inc.

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS 

AND RISK ALONG THE WASATCH FRONT, UTAH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-R

CONTENTS

	Page 

Abstract............................................................................................................................................................. Rl

Introduction...................................................................................................................................................... 1

Acknowledgments.................................................................................................................................. 2

Methods and Assumptions................................................................................................................................ 2

General Results................................................................................................................................................. 18

State-Owned Buildings Subjected to a Maximum Credible Event.................................................................. 19

Overall Earthquake Damage Expected to Water and Gas Conduits................................................................. 20

Summary........................................................................................................................................................... 25

References Cited............................................................................................................................................... 27

ILLUSTRATIONS

Page

FIGURE 1. Map of study areas showing exposures in Weber, Davis, Salt Lake, and Utah Counties ...................................................................................... R3

2. Flow chart showing hazard assessment procedures ............................................................................................................................................... 4

3. Plan view of the Wasatch fault system underlying facilities in Salt Lake County................................................................................................. 6

4. Diagram showing two three-dimensional rectilinear fault planes intersecting at (Xf, Yf, h) for some depth h.................................................. 1

5-8. Maps showing:

5. Isocontours indicating estimated distances from sites to the Salt Lake segment of the Wasatch fault zone ................................................ 8

6. Expected relative ground-shaking response in the central part of the Wasatch Front urban area................................................................. 10

7. Thickness of low-density sedimentary rocks in Wasatch Front valleys and Cache Valley inferred from gravity data................................. 12

8. Microzones assigned the highest liquefaction susceptibility ........................................................................................................................ 14

9. Flow chart showing procedures for estimating earthquake loss for insurance purposes ...................................................................................... 16

10. Map of Salt Lake City showing number of State-owned buildings having assumed (above-threshold) occupancies

by microzone...............................................................................................................................................................^ 17

11. Flow chart showing damage estimation procedures used for natural gas and culinary water systems.................................................................. 18

12, 13. Graphs showing:

12. Percentage of pipeline surveyed estimated to be subjected to permanent ground displacement in worst-case earthquake events .............. 22

13. Break rate variations versus fault rupture center locations, Ms 6.1, Davis County ...................................................................................... 26

TABLES

TABLE 1. Assumed geometric characteristics of the Salt Lake segment of the Wasatch fault....................................................................................................... R5

2. Alternative magnitude scaling factors derived from equations by Youd and Perkins (1987), Joyner and Fumal (1985), and

Campbell (1987).................................................._ 13

3. Property values for State-owned buildings by microzone and and seismic building quality.......................................................................................... 20

4. Mean building loss, contents loss, and downtime estimates to State-owned buildings in a hypothetical Ms 7.3 earthquake on the

Salt Lake segment of the Wasatch fault zone.................................................................................................................................................................. 21

5. Average break rates estimated for utility systems surveyed for maximum credible and worst-case Ms 6.7 events simulated ...................................... 23

6. Percent of estimated repairs attributed to different hazard modes in worst-case events................................................................................................. 24

7. Ratio comparisons of pipe repair rates based on diverse assumptions for maximum credible earthquakes affecting water and gas

systems in Utah and Weber Counties...........................................................................................................................................................^ 24

8. Sensitivity of worst-case repair rates at lower magnitude levels to assumptions on magnitude scaling factors............................................................ 25

III

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK 

ALONG THE WASATCH FRONT, UTAH

SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS 

AND STATE-OWNED BUILDINGS ALONG THE WASATCH FRONT

By CRAIG E. TAYLOR and DELBERT B. WARD

ABSTRACT

This paper summarizes the methodology and results of two seismic-risk projects on Utah facilities. The topics selected in these projects 

are explained in relation to past seismic-risk projects on Utah facilities 

and in relation to potential uses of seismic-risk estimates. Results are 

highlighted with reference to project goals. A longer discussion of methods used and technical obstacles addressed indicates the iterative 

nature of seismic-risk projects. Finally, a more detailed presentation of 

results is made. Detailed results in the form of computer printouts of 

expected earthquake damage have been presented to specific users 

namely, local culinary water and natural gas utility officials and to the 

State of Utah Risk Administrator.

INTRODUCTION

In early 1985, we began a 1-year U.S. Geological Survey (USGS)-sponsored project to examine (1) expected 

losses and casualties in State-owned buildings as a result 

of a maximum credible earthquake and (2) expected damage to culinary water and natural gas facilities in Salt Lake 

and Davis Counties as a result of a broad range of possible earthquakes affecting them. In early 1987, we began a 

second 1-year USGS-sponsored project to examine 

expected damage to water and gas facilities in Weber and 

Utah Counties as a result of a broad range of possible 

earthquakes affecting them.

Project topics were selected so that (1) they did not 

duplicate previous work or work in progress (including 

studies by the J.H. Wiggins Company, 1979; Algermissen 

and others, 1976; Liu, 1977; Bresko and others, 1981; Taylor and Ward, 1979, 1981; Highland, 1987; Algermissen and 

Steinbrugge, 1984); (2) project results could illuminate 

some of the more recent methodological and data developments in the geosciences and engineering as they pertained to the region of study; and (3) project results might 

assist in advancing earthquake hazard reduction programs. As a result of this third consideration, for instance, 

we decided not to reexamine average annual loss estimates for various classes of buildings. Although new findings would likely change and improve those estimates (as 

Taylor and Ward (1979) found, for instance), we did not 

believe that significant policy conclusions would deviate 

from those developed by the Utah Seismic Safety Advisory Council (1981) and the Utah Geological and Mineral 

Survey (1983). Furthermore, this third consideration 

implied that the primary objective outputs detailed computer printouts of damage and casualty estimates 

should be designed for those who might use them (that is, 

the State of Utah Risk Administrator and utility officials 

whose systems were analyzed).

Results of the first project reaffirm that shaking 

losses and potential casualties would be severe for Stateowned buildings affected by a maximum credible earthquake (whether from the Salt Lake or the Ogden segment 

of the Wasatch fault). Principal factors affecting results 

are the magnitude of these maximum credible earthquakes (Ms 7.0-7.5), the close proximity of exposures to 

the Wasatch fault at depth, and the seismic vulnerability 

of many State-owned buildings. Further study of the seismic vulnerability of State-owned structures especially 

with regard to life-safety factors could greatly assist in 

identifying those structures most vulnerable and hence in 

need of long-term removal, replacement, or occupancy 

reduction. Whether the current State of Utah practice of 

self-insurance is adequate to respond to expected damage 

to State-owned buildings deserves careful review based 

on the results of this study.

Although building damage from these maximum 

credible events is expected to be severe, so is damage to 

and disruption of culinary water and natural gas systems. 

With the possible exception of portions of the natural gas

Rl
R2 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

systems, local gas or water systems can be expected to 

have little or no immediate reliability once they have been 

affected by a maximum credible Wasatch fault event in 

their county. Mountain Fuel Supply representatives have 

used detailed computer printouts of damage estimates 

made in this study to confirm this expectation for their 

systems. The damage estimates for culinary water systems, which contain much more seismically vulnerable 

facilities, were many times more severe than those for the 

natural gas systems.

In addition to the comparative vulnerabilities of facilities, primary factors affecting these results include the 

magnitudes of these maximum credible events, the close 

proximity of the Wasatch fault at depth to exposures, 

high, soft soil site amplification factors for many exposures, and severe potential liquefaction problems, especially near the Jordan River and the Great Salt Lake. 

Although fault-zone crossings are significant, detailed 

results indicate that damage from fault displacement is 

generally only a small proportion of the overall expected 

damage to water and gas facilities.

An important characteristic of these projects is that 

we have emphasized smaller magnitude events (from 5.5- 

7.0) as much as maximum credible ones for two major 

reasons. First, owing to high liquefaction susceptibilities 

in the four counties, local water utility catastrophes can 

occur from low-magnitude earthquakes (possibly as low 

as 5.5 in some instances). Second, utility damage estimates from maximum credible earthquakes tend to be so 

high that no obvious limited measures can be used to 

improve immediate system reliability. In contrast, smaller 

magnitude earthquake scenarios can help utility officials 

to define those portions of the system that can be seismically enhanced to provide increased system reliability. 

Measures designed to increase system reliability in these 

smaller magnitude events also tend to (1) improve 

expected system performance (long-term reliability) 

through reduced expected damage in larger magnitude 

earthquakes as well and (2) concentrate efforts on the 

more seismically vulnerable portions of systems those 

portions expected to be damaged by a wide variety of 

potential earthquakes.

Some of the more important methodological problems encountered in these projects are highlighted below. 

Methodological revisions from the first project to the second have included incorporations of changes made by 

K.W. Campbell in defining attenuation functions for the 

Wasatch Front region, simplification of the fault models 

used as a consequence of findings in the first project, and 

adjustment of damage algorithms for pipelines subjected 

to severe ground shaking. To estimate probabilities of liquefaction-induced ground failure, makeshift methods had 

to be devised for both projects. Our study findings 

strongly suggest the need to develop a research program

to assess probabilities. The program should include an 

empirical and (or) experimental reassessment of multipliers used to adjust critical accelerations to account for the 

effects of different numbers of cycles of shaking. 

Although seismic-risk methods used often require 

working assumptions or imperfect data, these limitations 

are best approached by (1) sensitivity and other statistical 

analyses that determine the impact of alternative assumptions and (2) continued investigations in science and engineering to improve pertinent information and algorithms 

used.

ACKNOWLEDGMENTS

We thank J. Keaton (Dames and Moore), L. Reaveley 

(Reaveley & Reaveley Engineers), D. Ford (Ductile Iron 

Pipe Research Association), and S.T. Algermissen, 

L. Highland, and D. Perkins (U.S. Geological Survey) for 

their careful reviews. We also thank the American Society 

of Civil Engineers for permission to reproduce figures 2, 

6, 8, 11, and 12 in this paper from an earlier publication. 

These projects were supported by U.S. Geological Survey 

contracts 14- 08-001-22013 and 14-08-0001-G1394. This 

support is gratefully acknowledged.

METHODS AND ASSUMPTIONS

A significant consideration in structuring a database 

for these projects is the selection of a basic geographic 

reference system. The township system was selected for 

the primary study areas (fig. 1). In the course of these 

projects, many advantages and disadvantages of this system became apparent. A very compelling advantage for 

these risk projects has been the widespread and customary used of the township system by those who select pertinent data and by potential users of data and results. Two 

examples are the culinary water systems of GrangerHunter (in Salt Lake County) and Orem (in Utah County), 

which are laid out on the township system. Disadvantages 

arise because the ideal microzone size varies with particular purposes, Thus, township sections (1x1 mi) used as 

microzones are, at times, too large or too small. This 

study areas was 2,504 microzones. In some cases, especially in Weber and Utah Counties, liquefaction susceptibility data developed by Utah State University and Dames 

& Moore are too sparse to make meaningful estimates for 

each microzone. In other cases, these data are diverse 

enough to suggest that smaller microzones might be desirable. On the whole, however, given the large number of 

microzones used, the township system was adequate for 

study purposes.

Figure 2 outlines the primary hazard steps in these 

projects. Unique to this approach in Utah seismic-risk 

studies is the choice of many earthquake scenarios repre-
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R3

R2W R1W R1E R2E

T5N

T4N

T4S

T5S

FIGURE 1. Study areas showing exposures in Weber, Davis, Salt Lake, 

and Utah Counties.

senting the gamut of possible magnitudes and the range of 

rupture locations that may damage exposures in the study 

area. The annual probabilities of magnitude and rupturelocation occurrences can be assessed to place risk results 

within a probabilistic framework. First, we explain why 

we selected multiple scenarios rather than either 

selecting maximum ones only or selecting methods to 

develop probabilistic estimates of hazards affecting individual sites. We then describe how these earthquake 

scenarios were selected to accommodate a probabilistic 

framework.

A severe limitation of using only maximum credible 

earthquakes has been that, because this method greatly 

underestimates long-term primary losses expected from 

earthquakes, it consequently underestimates the benefits 

of seismic-risk reduction measures as well. To draw an 

example from California, exclusive concern with potential losses from major San Andreas faulting scenarios

(affecting either San Francisco and Los Angeles) would 

ignore losses from the Whittier, Coalinga, Imperial Valley, 

San Fernando Valley, Long Beach, and Hayward events, to 

name but a few. Total estimated benefits from earthquake 

hazard reduction measures comprehend the entire range 

of potential earthquakes.

One traditional weakness of procedures designed to 

estimate total primary losses from all earthquakes potentially damaging to facilities has been that these procedures ignore or provide a poor account of secondary 

losses, including systematic consequences of individual 

earthquakes. These consequences include business interruption losses, governmental discontinuities, fires, losses 

of heating and lighting, water and gasoline shortages, and 

problems in caring for those left homeless. These sorts of 

losses are clearly illustrated with respect to lifeline 

networks, for which direct dollar loss is not linearly 

related to system performance. In a highly redundant portion of a system, service may continue at pre-disaster levels even though direct dollar loss is large. In a portion of a 

system that has no alternative service paths or supply 

sources, damage to a single facility may cause extensive 

service outages over a long period of time. Traditional 

probabilistic approaches also have difficulties treating 

resources that are limited, such as inventories of piping, 

local medical personnel, local capital for reconstruction, 

and insurance reserves available for paying the insured. 

Using individual scenarios, to evaluate earthquake risks 

has the advantage of indicating not only whether specific 

critical levels of resources will be surpassed for given 

events but also how soon critical infrastructure services 

will be available for use in response, recovery, and 

reconstruction.

Accordingly, these projects have used a large number 

of individual scenarios within a probabilistic framework. 

This approach overcomes the limitations of using traditional probabilistic methods and of emphasizing only 

a small set of mainly maximum credible earthquake 

scenarios.

To accommodate this probabilistic framework, we 

first treated major fault traces as sequences of straight 

lines. The faults used include the Granger, the Hansel Valley, the Cache Valley, and the Taylorsville faults and various segments of the Wasatch fault (Keaton and others, 

1987; Robinson, 1986). For the sake of simplicity, we have 

followed Wheeler and Krystinik (1987) in modeling the 

Wasatch fault as consisting of four persistent segment 

boundaries and five main segments: a northernmost, the 

Weber, the Salt Lake, the Provo, and a southernmost. Relative to exposures examined in this project, the chief controversial aspect of this treatment is that it allows the 

possibility of larger magnitude events on the Provo segment than the smaller American Fork (22.5 km) and 

Provo (18.5 km) segments would allow if they were used
R4 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

MICROZONE INPUTS STEPS MICROZONE OUTPUTS 

FOR EACH EVENT

Define earthquake events 

(magnitudes, seismogenic 

zones, return intervals)

Contains/does not contain 

surficial fault traces /

Dynamic site 

amplification factor

Liquefaction 

susceptibility parameters

Estimate maximum 

fault displacement

Derive distances from 

seismogenic zones to 

microzones

Estimate peak horizontal 

ground acceleration (PHA) 

and Modified Mercalli 

intensity

Maximum fault 

displacement

Modified 

Mercalli intensity

PHA for each microzone 

and each event

Estimate liquefaction 

probabilities

Liquefaction 

probability

FIGURE 2. Hazard assessment procedures used. (Reproduced with permission of the American Society of Civil Engineers.)

to define maximum earthquakes in this region (Machette 

and others, 1987).

As the 1987 Whittier Narrows and 1983 Coalinga, 

Calif., earthquakes have shown, not all earthquakes occur 

on fault zones known to be active, and few normal or

thrust zones have been mapped at depth (Hutton, 1987; 

Alfors, 1983). Accordingly, our treatment of possible 

earthquakes permits the development of a random source 

zone and the designation of a dip angle for whatever fault 

system is used. Some random source zones for the
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R5

TABLE 1. Assumed geometric characteristics of the Salt Lake segment of the Wasatch fault 

[Origin is defined as the intersection of the base line and the Salt Lake meridian]

End 

Section point

~ ,. ... Intersection points 

Coordinates (mi) ..- /. -j / >> Strike angle Dip angle at 7.46 mi deep (mi)

index" < "^UCB'

Draper

Cottonwood

Olympus

Foothill

University

City Creek

1

2

3

4

5

6

7

3.36

5.86

6.24

2.29

2.29

3.22

3.76

-16.82 4.35 035

-13.26 4.35 005

-8.93 7.46 328

-2.61 3.11 360

.50 1.86 030

2.11 6.22 005

8.30

Xi 1)i

65 3.72 -20.88

40 -3.38 -16.96

31 -7.26 -10.76

38 -7.26 -7.43

60 -6.40 -5.94

40 -5.88 -.03

Calculated 

trend (deg)

Yt*

232

232

230

230

230

230

Calculated 

plunge (deg)

£.*

31

31

31

31

31

31

Wasatch fault region have included very narrow zones 

between 10 and 20 km on each side of the Wasatch fault 

(Taylor and Ward, 1979; Algermissen and others, 1982). 

Other accounts of background seismicity (earthquakes 

having magnitudes below 6.5) have employed a very wide 

random source zone (Arabasz and others, 1987). Our 

method is to develop earthquakes along lines parallel to 

and at any specified distance from either a known fault 

trace (such as the Wasatch) or a north-south line. To 

account for rupture lengths relative to specified magnitudes, we have used empirical work (Bonilla and others, 

1984) relating magnitude and rupture length for all fault 

systems. Given these rupture lengths, a number of earthquakes can be specified from north to south along the 

designated sequence of lines (representing fault traces, 

lines parallel to fault traces, or lines parallel to northsouth lines). Our method is therefore compatible with 

either very narrow or very wide source zones and with the 

view that either an entire Wasatch fault segment must 

rupture in a Wasatch fault event or that only a portion of a 

given Wasatch fault segment may rupture.

Designation of a dip angle enables us to provide a 

crude account of distances for exposures to the Wasatch 

fault at depth. Currently, models of the Wasatch normal 

fault are either listric or planar (Smith and Richins, 1984). 

In our first project, we explored the use of a complex planar model hypothesized by Ronald Bruhn (written commun., 1985) and others but modified by Bruhn and others 

(1987). That model suggests that the Salt Lake segment of

the Wasatch fault can be modeled as a series of rectilinear 

planes whose surface projections are shown in figure 3.

Table 1 indicates the assumed geometric characteristics of this segment, and figure 4 clarifies the variables 

used in table 1. This model comes from an attempt to 

derive a fairly constant slip vector for the planes composing the Salt Lake segment, so that a single slip can be 

modeled along all such planes.

This model suggests that sites in Salt Lake County 

are much closer to the dipping planes of the Salt Lake segment of the Wasatch fault than an estimate based on the 

distances to the surface expression of this zone would 

indicate. Figure 5 shows the estimated shortest distances 

to these Wasatch fault planes as modeled in figure 3. This 

figure suggests that, for exposures to the west of the 

Wasatch fault trace (Salt Lake segment), distances to the 

surface trace are approximately twice the distances to the 

fault at depth.

These findings, along with the presence of alternative 

Wasatch fault models (including listric ones) confirming 

the above result, have persuaded us that a designated dip 

angle is currently adequate for seismic risk purposes to 

model each Wasatch fault segment at depth. (In our current program, this dip angle applies uniformly to each 

straight-line section of the segment; variations in dip 

angles for each section, as table 1 implies, might be 

slightly preferable.) Modeling distances to the surface 

expression of the fault has the disadvantage of ignoring the Wasatch fault dipping planes underlying most
R6 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

R3W R2W R 1 W R1 E R2E R3E

T5N

SALT LAKE 

COUNTY

T3S

T4S -

FIGURE 3. Plan view of Wasatch fault system (Salt Lake segment, 12 km deep) underlying facilities in Salt Lake County. Dashed 

lines indicate projection of fault planes to depth below the ground surface.
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R7

EXPLANATION

<t> Strike angle

fi Dip angle Defined normal to each surface segment

<t>* Trend angle

p* Plunge angle Calculated with respect to the line normal to <j>*

/ Length of surface trace

FIGURE 4. Two three-dimensional rectilinear fault planes intersecting at the (Xi*, Y^*, h) for some depth h (no elevations are assumed on the Earth's 

surface). <j>, strike angle; 6, dip angle (defined normal to each surface segment); <(>*, trend angle; 6*, plunge angle (calculated with respect to the 

line normal to <(>*); /, length of surface trace.
R8

T5N

T4N

T3N

T2N

T1 N

T1 S

T2S

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

R3W R2W R1W R1E R2E R3E

T3S

T4S

WASATCH 

RANGE

Surface trace of 

segment of the 

Wasatch fault zone

SALT LAKE 

COUNTY

Exposures within \ 

4 miles of the N 

Wasatch fault zone

f Within 

8 miles K

OQUIRRH 

MOUNTAINS

FIGURE 5. Isocontours indicating estimated distances (in miles) from sites to the Salt Lake segment of the Wasatch fault zone

(modeled in three dimensions).
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R9

exposures; more refined fault models worth exploring 

for their implications nonetheless have the disadvantage of not being fully corroborated.

Figure 5 illustrates how larger magnitude (6.5 and 

above) earthquakes from the Wasatch fault (especially the 

Salt Lake, Ogden, and Provo segments) are expected to 

affect many exposures in the near field. Because the problem of defining attenuation patterns in the Wasatch region 

includes this near-field aspect and because large error 

terms hence arise from using attenuation functions based 

on hypocentral distance, Campbell's (1984, 1987) work 

has been used as a basis for developing both peak horizontal accelerations and Modified Mercalli intensities. 

For assessing peak horizontal acceleration (PHA) in the 

first project, the following equation from Campbell (1984) 

was used:

PHA=exp {-2.817+0.702M-1.02 In [tf+0.0921 exp (0.584M)]} (1)

where M is earthquake magnitude (Ms when M>6 and M 

otherwise), R is the shortest distance to the seismogenic 

surface (in kilometers), and PHA is the mean of the 

two horizontal components of peak accelerations (in 

units of #).

For the second project, following Campbell (1987), 

we used the equation

PHA=exp {-3.303+0.850M-1.25 In [E+0.0872 exp

(2)

where Kj is 0 for strike-slip faults and 1 for reverse and 

thrust faults, K% is 0 if the rupture is not directed toward 

the site (or directivity is unknown) and 1 if the rupture is 

directed toward the site, and Kg is 0 if soils are over 10 m 

deep and 1 otherwise.

In programming these additional variables, we have 

permitted options to analyze the sensitivity of results to 

various selections of values for KI, K%, and K$. For somewhat conservative results for the natural gas system, we 

have used Kj=l, K%=0, and K$=.\ (rupture directivity is 

largely unknown). For less conservative results, we have 

used Kj=0, Kg=Q, and Kg=L For sensitivity analyses, we 

have K!=Q, K2=0, and K3=Q.

To derive estimates of Modified Mercalli shaking 

intensities, we first developed estimates of peak horizontal velocities and derived estimated intensities from both 

relative response data for the region and an equation 

relating shaking intensities to peak horizontal velocity.

In the first project and following Campbell (1984), we 

used the following equation to derive estimates of peak 

horizontal velocity (PHV):

PHVrocfc=exp {0.798+ 1.02M-1.26 In [tf+0.015 exp (0.812M)]} (3)

where PHV is the median value of the mean of two horizontal components of peak velocity (in centimeters per 

second) ("rock" refers to basement rock).

To derive surficial estimates of peak horizontal velocity, the following relationship was used:

(4)

where DAFSite indicates the soft surficial soil dynamic 

amplification factors.

Spectral values used in equation 4 were derived for 

the short-period band (0.2-0.7 s) results of Rogers and 

others (1984) and are as shown for Salt Lake, Davis, and 

Weber Counties in figure 6. Spectral values tend to be 

slightly higher for the long-period band (0.7-1.0 s). Nonetheless, as figure 6 shows, extensive portions of the study 

area were assigned spectral dynamic amplification factors of 6.2. The chief rationale for this procedure was that 

it yielded intensity increases of slightly over 2 relative to 

basement rock estimates.

In the second project, following Campbell (1987), we 

used the following equations to derive estimates of PHV:

PHCc=exp {-1.584+ 1.18M-1.24 In [E+0.00907 exp 

(0.951M)]-0.0059E+0.49#;+0.99^+0.53^- 

tanh (0.41£)+0.60(l-J<Qr) tanh (0.88J9) (5)

where PHVC is peak horizontal velocity normalized primarily to California earthquake data, D is the depth (in kilometers) of sediments beneath the site (that is, the depth 

to basement rock), and K3 - is 0 for buildings over four 

stories and 1 otherwise.

For estimates of depth to basement rock, we used 

maps developed by Mabey (1987) and shown in figure 7. 

Following Campbell (1987), we have used K^--\ to establish upper bound median estimates of PHV; for establishing estimates of PHVSMrjace, we adapted remarks by 

Campbell in the following equation:

PHVSttr/ace=[(PHVc)(DAFsite)]/4.0 (6)

This equation assists in adjusting for the high relative site 

response factors found along the Wasatch fault. We again 

used KI=\ for somewhat conservative results (for the natural gas system). The value of 4.0 in the denominator of 

equation 6 is used because KS-=\ already includes site 

amplification factors typical of California soft soils, relative site response factors being generally between 3.6 and 

4.5 (see Campbell, 1987).

To derive estimates of shaking intensities (7), we 

used the following equation from Wiggins and others 

(1981):

=(log PHVsttr/ace+1.484)/0.366 (7)
RIO REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112.25 112.00 111.75

41.00

40.75

Silt and Clay 

Sand and Gravel

__ Rubble

II Rock

40.50

FIGURE 6. Expected relative ground-shaking response in the central part of the Wasatch Front urban area. Numbers indicate mean 

ground response relative to bedrock locations. (Reproduced with permission of the American Society of Civil Engineers.)
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS Rll

The notion of intensity used here is that of shaking intensity only. Ground-failure effects such as liquefaction are 

accounted for with reference to input strong-motion values, such as PHA, and hence are excluded from the shaking-intensity scale implied here. Given equations 4 and 7, 

sites having soft surficial soil amplification values of 6.2 

are assumed to have intensities exceeding basement rock 

values by over 2 intensity increments. Given equations 6 

and 7, sites having amplification values of 6.2 exceed normal soft soil sites (chiefly from California earthquake 

data) by 0.5 intensity increment. These simplifying relationships should be used for general planning purposes 

only, inasmuch as effects of high strain, surface faulting, 

and seismic radiations are not accounted for.

Obviously, values derived by using these equations 

depend significantly on how distances from sites to the 

seismogenic source are estimated. In comparison with 

results using a three-dimensional model, illustrated in 

figure 1, one may as a rule of thumb state that distances in 

the near field of an earthquake from the Salt Lake segment of the Wasatch fault are overestimated by a factor of 

roughly 2 if they are calculated from the surface trace of 

this fault. More generally, if the dip angle is IS, the distance 

is overestimated by a factor of I/cos IS. On the basis of 

equation 2, the relative decrease in PHA to near-field sites 

(resulting from estimating distances to the surficial trace 

of the fault zone can be expressed as

(A+B-C-D) (8)

where ^=-1.25 hi [(R/cos J3)+0.0872 exp (0.678 M)], B=- 

0.0059 R/cos 13, C=-1.25 hi [E+0.0872 exp (0.678 M], and 

D=-0.0059 R. For instance, given magnitude 7.3 and a site 

6 km west of the fault assumed to have a dip angle IS of 

60°, this ratio is 0.68.

To express the underestimate in intensities derived 

from using distances to the surface trace, the difference 

in intensity estimates is a convenient mode of expression. 

On the basis of equations 5, 6, and 7, this difference can 

be expressed as

diff/=CF+G-F'-G')/0.366 In 10 (9)

where F=-L24 hi [E+0.00907 exp (0.951 M)], G=-0.0059 R, 

and F=-L24 hi[(E/cos JS)+0.00907 exp (0.951 Jlf)], and 

G'=-0.0059 R/cos IS. When the same example presented 

for PHA (60° dip angle, magnitude 7.3, and 6-km distance 

to the dipping plane) is used, there is an underestimate of 

0.53 intensity increment if distance to the surficial fault 

trace is used. These findings, along with the definition of 

distance required to use Campbell's equations, confirm 

that significant errors may arise if one estimates distances 

relative to the surface fault trace only. In spite of the deficiencies inherent in ignoring dip angles, our studies also

indicate that Salt Lake County results are most affected 

by use of more complex fault models. In Weber, Davis, 

and Utah Counties, less space is available between the 

Wasatch Range and the Great Salt Lake or Utah Lake for 

suitable development of exposures. As a consequence, in 

these counties, most exposures are extremely close to the 

surface trace of the Wasatch fault.

The general approach of simulating many earthquake 

scenarios within a probabilistic framework also has 

advantages in estimating liquefaction probabilities. To 

estimate these probabilities, we first used raw data provided by Utah State University and Dames and Moore. 

These data include critical blow counts, water table 

depths, and critical accelerations for various sites examined (see Anderson and others, 1982). Critical accelerations are those peak horizontal accelerations that may 

lead to liquefaction relative to a magnitude 7.5 earthquake. This definition does not suggest that liquefaction 

will occur if the critical acceleration is equaled or 

exceeded in a magnitude 7.5 event. To ach'ust critical 

accelerations for earthquakes of other magnitudes, the 

following magnitude scaling factors have been proposed 

to account for cyclic effects (Seed and Idriss, 1983):

Magnitude

7.5

6.75

6.0

5.25

Magnitude scaling 

factor (MSF)

1.0

1.13

1.32

1.5

Hence, if the critical acceleration for magnitude 7.5 event 

is 0.10 g for a given site, then the above magnitude scaling 

factors suggest that 0.113-, 0.132-, and 0.15-0 PHA's must 

occur in earthquakes having magnitudes of 6.75, 6.0, and 

5.25, respectively, for liquefaction possibly to occur. Our 

general approach assists in incorporating such multipliers 

for earthquakes having magnitudes other than 7.5. Moreover, if these magnitude scaling factors and equation 2 are 

used and if it is assumed that ^=0, ^=0, and -K#=0, then 

a site having a critical acceleration of 0.10 g for a magnitude 7.5 earthquake (an ach'usted critical acceleration of 

0.15 g for a magnitude 5.25 earthquake) can have this 

acceleration exceeded as far as 8 km from a magnitude 

5.25 event. Although these magnitude scaling factors may 

be too low, this finding partially confirms our later conclusions that liquefaction-induced ground failure may be a 

major problem even in smaller magnitude earthquakes.

It has been suggested that these standard magnitude 

scaling factors are too low. As a consequence, we 

examined the sensitivity of our pipe break estimates to 

alternative magnitude scaling factors. To develop these 

alternative factors, we used the following equation from
R12

113°

REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

112°

0 10 20 30 KILOMETERS
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R13

TABLE 2. Alternative magnitude scaling factors derived from equations by Youd and Perkins (1987), Joyner and Fumal (1985), and

Campbell (1987)

[LSI, severity of liquefaction (in inches)]

Calculated critical acceleration 

at specified magnitude (percent <?)

Derived magnitude scaling factors 

relative to M 7.5 event

Magnitude (Mw) 7.5 6.75 6.0 5.25 7.5 6.75 6.0 5.25

Equations 11 and 12

LSI=1

LSI=4

0.027 

0.084

0.07 

.164

0.13 

.2361

0.174 

.210 1

1.0 

1.0

2.6 

2.0

4.8 

2.8 1

6.4 

2.51

Equation 2

LSI-1 

LSI=4

0.0237

.075

0.056

.135

0.099

.21

0.16 

.29

1.0 

1.0

2.4 

1.8

4.2

2.7

6.5 

3.8

Value demonstrates that combining equations 11 and 12 yields paradoxes (recognized by T.L. Youd in his review of study findings).

Youd and Perkins (1987) relating severity of liquefaction 

to magnitude for Western U.S. earthquakes:

log LSI=-3.49-1.86 log fl'+0.98 M (10)

where LSI is the severity of liquefaction (in inches), R* is 

the distance to the energy source (in kilometers), and M w 

is moment magnitude.

For estimating the onset of liquefaction, we used two 

alternative assumptions: LSI=1 in and LSI=4 in. To 

develop magnitude scaling factors, it is necessary to use 

only a suitable attenuation relation relating PHA to magnitude and distance. Equation 2 is only partially suitable. If 

M^M^; and R=R\ equation 2 can be used. (The values 

ascribed to KI make no difference to the magnitude scaling factors derived.) However, more directly suitable 

equations from Joyner and Fumal (1985) are

log PHA=0.49+0.23 (Mw-6)-log Z)'-0.0027Z)' (11)

(12)

From these alternative equations for PHA and alternative 

assumptions for LSI at the boundary threshold, we developed four sets of magnitude scaling factors in table 2. Values derived are much higher than those normally used. 

We examined, for instance, consequences of using the

FIGURE 7. Thickness of low-density sedimentary rocks in Wasatch 

Front valleys and Cache Valley inferred from gravity data (Mabey, 

1987). Contour interval is 1 km.

multipliers 1.3 for magnitude 6.75 events, 2.55 for magnitude 6.0 events, and 4.1 for magnitude 5.25 events (half 

the values derived from using LSI= 1 in and equations 

11 and 12. These results greatly reduced the expected 

liquefaction-induced ground failures and consequently 

reduced the expected pipe breaks associated with these 

failures for smaller magnitude (<6.4) scenarios. Accordingly, further research is required to determine the merits 

of the objection that standard magnitude scaling factors 

(Seed and Idriss, 1983) are too low.

Because the exceedance of adjusted critical accelerations by calculated PHA's only implies that liquefaction 

may occur, actual estimates of liquefaction probabilities 

also remain a problematic area In the first project, we 

distinguished among microzones having high, medium, 

and low susceptibilities:

Liquefaction susceptibility 

of microzone Characterization of average site

High

Medium

Low

Critical blow counts below 12 and 

water table depths below 3 m.

Critical blow counts below 25 and 

water table depths below 6 m.

All other average sites.

Figure 8 indicates the highly susceptible zones (generally 

near the Great Salt Lake or Jordan River) in Salt Lake and 

Davis Counties.

hi the first project, we assumed that no liquefactioninduced ground failure occurs in microzones having high 

liquefaction susceptibilities or in microzones where calculated PHA's were below adjusted critical accelerations.
R14 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

R3W R2W R1 W R1 E R2E R3E

T5N

T4N

T3N

T2N

T1 N

T1 S

T2S

T3S

T4S

Gr 

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La

eat 

alt

ke

OQUIRRH 

MOUNTAINS

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EXPLANATI

Average critical acce 

average blow cou 

average water tab

Average critical acc< 

or average blow 

average water tab

WAS

" RAf

ATCH

SIGE

ON

lerations - 

nts <12, < 

le depth *= _ .

so. 13s, 

jnd

slerations >0. 13g, 

:ounts >12, or 

le depth >3 m
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R15

Otherwise, we assumed that highly susceptible microzones had probabilities of liquefaction-induced ground 

failure of 50 percent; for medium susceptible zones, this 

value dropped to 10 percent.

These makeshift procedures were replaced with others in the second project, in which we used the following 

general function for estimating the probability of 

liquefaction:

liquefaction probability=/(CSReS£, 0.011 Nlt MSF, A) (13)

where CSRes ^ is the calculated cyclic shear ratio for a 

given microzone, Nj is the critical blow count (average 

for sites in the microzone), MSF is the magnitude scaling 

factor (based on earthquake magnitude), and A is a function dependent on the amount by which CSRest exceeds

NI.

In the standard magnitude 7.5 earthquake, we 

assumed that no liquefaction occurs if A^>25 or 

PHA<ac*MSF or if CSRes^<0.011 N1} where ac is the critical acceleration and MSF is the adjustment for earthquake magnitude. Otherwise, we assumed that the 

probability of liquefaction is a function of A, the amount 

by which CSRes^ exceeds 0.011 Nj. On the basis of limited 

data from Youd and Perkins (1987), we drew a working 

analogy between the degree of liquefaction severity that 

may occur and the probability of liquefaction-induced 

ground failure. We assumed that the probability is 15 percent if A=0.03, 60 percent if A=0.09, and 99 percent if 

A=0.22. For other magnitude-level earthquakes simulated, 

we used the same procedure, except that CSRes^/MSF is 

used in place of CSRes^. (Similar methods have been 

reported by Liao (1986), except that magnitude scaling 

factor problems are ignored.)

Previous National Science Foundation (NSF) studies 

and earthquake insurance studies were adapted to 

develop procedures for making loss estimates for Stateowned buildings. Summaries of data and methods have 

been given by Wiggins and Taylor (1986) and Taylor and 

others (1986a); Steinbrugge (1982) has also provided useful data summaries. No published comprehensive raw 

loss data are available for U.S. structures (see National

FIGURE 8. Microzones assigned the highest liquefaction susceptibility (based on data furnished by Utah State University and 

Dames and Moore). Shaded boxes indicate an average critical 

acceleration of 0.13 g or less, an average blow count of 12 or 

less, and a water table depth of 3 m or less. Open boxes indicate 

an average critical acceleration greater than 0.13 g, an average 

blow count greater than 12, and a water table depth greater than 

3 m. (Reproduced with permission of the American Society of 

Civil Engineers.)

Research Council, 1989), in contrast to the routine practice of collecting systematic loss data in the Balkan region 

as reported by Wiggins and Taylor (1985). The loss estimation tools used employ probabilistic methods (for 

example, Monte Carlo simulations, beta distributions to 

estimate probability of degree of damage relative to a 

sample mean) as opposed to deterministic ones. They 

were adapted from methods approved by the State of California in fulfillment of the annual requirement that earthquake insurance carriers report expected maximum 

credible losses and other insurance estimates for their 

California portfolio. Principally developed and refined for 

proprietary insurance studies (and from proprietary computer codes), the probabilistic methods were derived 

from the deterministic ones by probabilistic modeling of 

the sources of uncertainty (for example, distance to the 

fault plane, intensity, site dynamic amplification, seismic 

building quality, exposed population) in loss estimation. 

Properly speaking, the methods employed are probabilistic, not deterministic. Even though one fixed rupture zone 

and magnitude are assumed, the method used implies that 

many loss outputs are conceivable from these assumptions. These loss outputs were calculated through Monte 

Carlo simulations to yield statistics such as means and 

variances.

The general procedures for earthquake loss estimation are outlined in figure 9. Inventory data were provided 

by the Utah State Risk Administrator and the Utah State 

Division of Facilities Construction and Management. 

These data include facility name, address, building frame 

system, number of stories, construction date(s), total 

square footage, and replacement value of buildings and 

contents. To these data, we added occupancy usage, 

township section, and building and contents vulnerability 

categories. Four major building vulnerability categories 

were used, defined at the minimum level with reference 

to the Uniform Building Code (International Conference 

of Buildings Officials, 1988) earthquake zone for which a 

structure is designed. Contents vulnerability categories 

are similar to those given by Steinbrugge (1982). Procedures to estimate deaths and injuries are based on severity of expected building damage, where this severity is 

treated probabilistically in terms of the aforementioned 

beta distributions applied to deterministic loss estimates. 

The chief bases for estimating casualties relative to severity of building damage are derived from methods 

explicated by Whitman and others (1975) and critiqued by 

Wiggins and Taylor (1985).

Given what at first appears to be the limited seismic 

resistance of many State-owned buildings (as indicated by 

the seismic codes governing them), the concentration of 

these buildings in Salt Lake County (see fig. 10), and the 

high shaking intensities expected in a M s 7.0 to 7.5 earthquake on the Salt Lake segment of the Wasatch fault, we
R16 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

Define maximum 

event(s) affecting 

exposure

Input (mean) 

Modified Mercalli 

intensities for 

maximum event/

STEPS

Inventory State-owned 

buildings (location, valuation, occupancy, 

rate assumptions, 

seismic vulnerability, 

ratings)

Develop procedures for 

estimating building and 

content losses and mean 

and 90th centile levels

Develop procedures for 

estimating deaths and 

casualties at mean and 

90th centile levels

Derive building losses 

(mean and 90th centile 

estimates)

Derive contents losses 

(mean and 90th centile 

estimates)

Derive downtime estimates (mean and 90th 

centile)

Derive estimates of 

deaths and casualties 

(during working hours) 

at mean and 90th 

centile levels

Aggregate estimates 

of earthquake losses

FIGURE 9. Procedures for estimating earthquake loss for insurance purposes.

originally expected property and casualty losses to be 

significant likely larger than those losses estimated 

from previous studies.

Our only surprise in this regard arose during the first 

project, when we decided to distinguish between economic building vulnerability ratings and ratings for lifesafety estimates. We discovered that many State-owned 

buildings contain features such as numerous interior partitions and are in good physical condition, so that they are 

more seismically safe than economic damageability factors such as age (indicative of building code used) and 

frame system alone would indicate. As a result, we recommend further study to formalize life-safety factors for 

Utah buildings so that future casualty estimates can be 

based on explicit procedures like those developed for California structures (see Wiggins and Taylor, 1985). Those 

life-safety visual inspection and rating procedures originally were developed to estimate potential casualties in 

assessing risks and retrofit needs in Long Beach, Calif.

We adapted previous NSF lifeline research (Eguchi 

and others, 1983; Taylor and others, 1986b) to define procedures for making earthquake risk estimates to culinary

water and natural gas systems. These procedures are outlined in figure 11. These projects inventoried the natural 

gas system for each of the four main counties and the following culinary water systems: Clinton (Davis County), 

Granger-Hunter Improvement District (Salt Lake County), 

Weber Basin Conservancy District (Weber and Davis 

Counties), Salt Lake County Conservancy District, South 

Ogden City (Weber County), Ogden City, Provo City (Utah 

County), and Orem City (Utah County). The Salt Lake 

Department of Public Utilities was not inventoried 

because it has been the subject of several past seismic 

risk studies (Algermissen and others, 1976; Bresko and 

others, 1981; Taylor and Ward, 1981; Taylor and Bigler, 

1982; Highland, 1987). These inventories included types of 

component, locations distributed by township sections, 

pipe material, pipe joint, pipe length and diameter, and 

percentage of pipe in the Wasatch fault zone of deformation. To characterize this last input, we used the broad 

conservative definition of the fault zone of deformation 

given by Algermissen and others (1976): 457 m on the 

downthrown side and-61 m on the upthrown side of the 

fault trace. Further details of the inventory process for
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R17

62nd North Street

41st North Street

21st North Street

O North /O South

21st South Street

41st South Street

62nd South Street

86th South Street

1 10th South Street

134th South Street

1 58th South Street

182nd South Street

£

5>

Great 

Salt 

Lake

OQUIRRH 

MOUNTAINS

R3W R2W

3 17 2

z

R1 W

85

R1 E

WASATCH 

RANGE

R2E R3E

T2N

T1N

T1 S

T 2 S

T 3 S

T 4 S

Pumphouses, roadsheds etc. were excluded, as were buildings 

for which square footage data were unavailable.

FIGURE 10. Number of State-owned buildings having assumed (above-threshold) occupancies by microzone. Pumphouses, roadsheds, and so on

were excluded, as were buildings for which square footage data were unavailable.

the natural gas system were developed by McDonough 

and Taylor (1986). In the second project, R. Campbell and 

M. Salmon of EQE made engineering site inspections of 

various wells, booster stations, and filtration plants in the 

four counties to develop earthquake fragility models (failure probabilities as a function of PHA) and seismic-risk 

reduction recommendations.

Figure 11 distinguishes among three hazard modes: 

vibrations, liquefaction, and fault displacement. In the 

first project, we found that pipeline vulnerability models
R18 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

MICROZONE AND EVENT

INPUTS (FROM HAZARD

ESTIMATION PROCEDURES)

STEPS

Inventory natural gas and 

culinary water systems 

(component locations, vulnerability, functions, 

lengths, relations to surface fault traces)

OUTPUTS FOR EACH

EVENT, COMPONENT,

AND MICROZONE

Expected maximum 

displacement

Z

7

Expected liquefaction 

probability

7

7

Expected Modified 

Mercalli intensity

7

Estimate component damageability given fault 

displacement

I

Estimate component damageability given liquefaction

i

Estimate damageability 

given Modified Mercalli 

intensity

For each event, estimate 

combined expected damage 

by component, microzone, 

and utility company

Expected damage from / 

fault displacement /

/

Expected damage 

from liquefaction

Expected damage from / 

vibration alone /

FIGURE 11. Damage estimation procedures used for natural gas and culinary water systems. (Reproduced with permission of the American Society

of Civil Engineers.)

developed by Eguchi and others (1983) yielded estimates 

of breakage that were excessive at higher shaking intensities (Modified Mercalli intensities of IX and above). As a 

consequence, with the assistance of the project consultant, R. Eguchi of EQE, we revised those pipeline vulnerability models to reflect first project findings. Revised 

models are shown in the final report for the second 

project. (Taylor and others, 1988). Empirical data on 

which pipeline vulnerability models are based lead us to 

believe that results developed at lesser intensities and for 

the hazard failure modes examined are meaningful. Data 

summaries forming the basis for pipeline vulnerability 

models especially for the 1971 San Fernando Valley, 

1983 Coalinga, and 1987 Whittier earthquakes have been

given by Eguchi and others (1983), Isenberg and Taylor 

(1984), Taylor and others (1986), Trautmann and others 

(1986), and Schiff (1988).

GENERAL RESULTS

The probabilistic approach used in this study has 

provided new materials to address unresolved issues on 

the validity of Wasatch fault loss estimates. In general, 

results are as expected hazard estimates are higher than 

previous estimates, and worst-case earthquakes yield very 

high damage estimates. Some specific results, however, 

were surprising.
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R19

Risk estimates are improved by employing a nearsource approach one that uses at least dip angles for the 

Wasatch fault and attenuation functions especially 

designed to account for near-field effects principally 

because the Wasatch fault is held to be the major source 

of larger (Ms>6.4) earthquakes and because so many 

Wasatch fault exposures are close to the fault itself, especially when defined in two or more dimensions (see fig. 5 

and its discussion for technical confirmation). Liquefaction susceptibility and dynamic amplification studies, 

combined with this fault modeling, show that risks are 

significantly greater than estimates obtained from simpler 

models, especially for Salt Lake County exposures.

Indeed, vibrational hazards are severe enough in the 

largest earthquakes postulated (Ms>6.4) that modeling 

ground-failure hazards initially made little difference to 

damage estimates in the near field except with respect 

to the most seismically resistant facilities exposed to 

ground-failure hazards. As a result of high pipe-breakage 

rates developed in the first project for these larger magnitude events, we were forced to revise (downwards) 

breakage estimates for piping subjected to strong ground 

motion so that these estimates did not exceed those for 

piping subjected to ground failures. Extrapolation from 

lower intensity data has long been a problem in pipe 

breakage algorithms, and ground-failure estimates serve 

as a useful upper boundary on those pipe breaks expected 

from strong ground motion.

At the high levels of ground motion expected in these 

larger earthquake scenarios, a high degree of uncertainty 

also applies to building loss and casualty estimates 

derived for the least seismically resistant classes of building (see Wiggins and Taylor (1986) for a discussion of different estimates among investigators for the worst classes 

of buildings and for preliminary loss estimates and loss 

distributions for unreinforced masonry construction the 

1983 Coalinga, Calif., earthquakes). In other words, such 

high levels of strong motion test the vulnerability and 

damageability models principally developed from Modified Mercalli intensity VIII data.

Liquefaction hazards, moreover, appear to be significant enough to yield potential ground-failure hazards and 

consequent pipeline breaks even in some near-field events 

of ML 5.5. For instance, modeling earthquake events of 

comparatively low magnitudes (5.5-6.4) beneath selected 

portions of Salt Lake County or Davis County initially suggests that these localized hypothetical earthquakes could 

yield significant ground-failure problems and resulting 

pipeline breaks in local systems such as the GrangerHunter Improvement District or the Clinton Water Department. However, these results raise questions about 

research tools used. Our analysis of the sensitivity of pipe 

repair rates to magnitude scaling factors (MSF's) shows 

that nonstandard multipliers (derived from table 2)

reduce expected repair estimates considerably. Even if 

research is appropriately directed to this topic, another 

topic remains the development of liquefaction probabilities for those sites and (or) microzones affected by earthquakes so that adjusted critical accelerations are 

exceeded by PHA's. Assignment of probabilities in these 

cases also greatly affects study results in both large- and 

small-magnitude earthquakes simulated.

The risk estimates produced in this study have been 

presented to potential users the State of Utah Risk 

Administrator and representatives of local utilities inventoried and analyzed. Currently, the most significant use 

has been to assist in the natural gas system seismic mitigation program. Key elements in this use involve participation by gas utility representatives in providing facility 

data, checking vulnerability models and other kinds of 

models, and using detailed damage estimates from several postulated earthquakes to make system flow analyses 

and hence to identify weak system links. Thus, full-scale 

seismic risk analyses have been developed to estimate 

consequences for various categories of customers (industrial, commercial, and residential) and of reduced losses 

through alternative mitigations. These risk analyses, 

moreover, have been the basis for mitigation planning and 

actions.

STATE-OWNED BUILDINGS SUBJECTED TO A MAXIMUM 

CREDIBLE EVENT

The high intensities estimated in maximum credible 

events (Ms=7.3) generated from the Salt Lake segment of 

the Wasatch fault zone indicate that State-owned buildings in Salt Lake County are expected to suffer very high 

property (building and content) losses; additional losses 

are expected to result from potentially long downtimes. 

Table 3 summarizes replacement values at risk by microzone (township sections) and seismic building qualities. 

Table 4 summarizes mean estimates of building losses, 

contents losses, and downtime by microzone. For all 

buildings surveyed, mean loss estimates are 39 percent of 

the replacement value ($480 million) of buildings 

surveyed; downtime estimates are two-thirds of a year for 

many buildings.

Population exposure estimates in State-owned buildings vary considerably, depending primarily on university 

and governmental schedules. Nonetheless, the severe 

building damage estimated resulted in high estimates of 

death (3 percent), severe injuries (4 percent), and moderate injuries (10 percent) (expressed as ratios of the population exposed). For the least seismically resistant class 

of buildings surveyed, these property and casualty loss 

estimates were much higher. The upper limit of mean 

deaths estimated was 20 percent of occupants. (See the 

project report by Taylor and others (1986a) for details.)
R20 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 3. Property values (in thousands of 1984 dollars} for State-owned buildings by microzone and seismic building quality

[Seismic building quality (Q) is the minimum level, without inspection and rating; Q=l refers to buildings designed to Uniform Building Code (UBC) 

seismic zone O-l standards; Q=2 refers to UBC seismic zone 2; Q=3 refers to UBC seismic zone 3]

Building replacement values

Microzone

T1NR1WS34

T1NR1WS35

T1NR1WS36

T1NR1ES31

T1SR1ES6

T1SR1ES4

T1SR1ES3

T1SR1WS12

T1SR1ES8

T1SR1ES9

T1SR1WS14

T1SR1WS35

T2SR1WS4

T2SR1WS3

T2SR1WS8

T2SR1WS13

T2SR1WS30

Total

(Percentage of total)

Q=l

0

4,573

2,636

53,056

3,276

46,380

196

467

5

2,906

0

843

192

0

0

157

0

114,687

(23.9)

Q=2

5,338

140

274

9,947

3,120

108,494

3,179

0

0

3,386

0

843

192

2,577

99

157

153

137,899

(28.7)

Q=3

5,022

0

0

0

19,283

158,854

5,235

0

0

7,237

2,557

0

0

29,526

99

0

0

227,813

(47.4)

Total

10,360

4,713

2,910

63,003

25,679

313,728

8,610

467

5

13,529

2,557

1,686

384

32,103

198

314

153

480,399

(100)

e=i

0

48

1,884

5,378

432

9,205

12

0

0

18

0

93

22

0

0

5

0

17,097

(9.3)

Contents values

Q=2

481

0

0

1,146

432

55,016

20

0

0

312

0

93

22

236

19

5

8

57,790

(31.4)

<2=3

443

0

0

0

5,091

89,991

7

0

0

900

6,246

0

0

6,611

19

0

0

109,308

(59.3)

Total

924

48

1,884

6,524

5,955

154,212

39

0

0

1,230

6,246

186

44

6,846

38

10

8

184,195

(100)

As high as these estimates are, they reflect a refinement in characterization of the hazards posed by these 

buildings. Our initial estimates, based on standard seismic 

categorization loss procedures that focus on construction 

features known to be seismically vulnerable, were still 

higher. Vulnerability categories were carefully reassessed 

through the adaptation of visual inspection and rating 

methods discussed by Wiggins and Taylor (1986), which 

emphasize both economic and life-safety building hazards. Two sets of seismic ratings were subsequently developed, one for property loss estimates and the other for 

casualty estimates. As a result of this reassessment, 

aggregate property and casualty loss estimates were 

reduced considerably. For instance, estimated deaths 

dropped by a factor of 3.7 once university and technical 

school buildings were reassessed more carefully. Loss 

results for this maximum credible event are clearly very 

sensitive to how buildings are evaluated for seismic property and safety issues. This sensitivity suggests that continued refinement of seismic inspection and rating 

procedures especially with respect to crucial safety 

issues should be encouraged for and applied to Wasatch 

Front buildings.

To our surprise, the dynamic amplification estimates 

used made less of a difference in property and casualty

estimates than we had expected. When dynamic amplification factors were ignored and only basement rock 

intensities were used, mean building loss estimates were 

reduced from 39 to 28 percent, and contents loss estimates were reduced from 39 to 20 percent. Estimated 

deaths were reduced from 3 to 2 percent, severe injuries 

from 4 to 3 percent, and moderate injuries from 10 to 

8 percent of the population exposed. The magnitude (M s 

7.3) of the earthquake postulated and the very short distances of buildings from the Salt Lake segment of the 

Wasatch normal fault zone (generally within 5 mi) imply 

very high shaking intensities even if dynamic amplification factors are ignored. We expect these factors to have 

more of an influence on risk estimates for earthquakes of 

lesser magnitudes.

OVERALL EARTHQUAKE DAMAGE EXPECTED 

TO WATER AND GAS CONDUITS

On the basis of vulnerability models developed by 

Eguchi and others (1983), we had originally anticipated 

that earthquake damage to buried conduits would be 

dominated by liquefaction-induced ground failure; lesser 

contributions could be expected from fault rupture and
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R21

TABLE 4. Mean building loss, contents loss, and downtime estimates to State-owned buildings in a hypothetical Ms 7.3 earthquake on the Salt

Lake segment of the Wasatch fault zone

[Seismic building quality (Q) is the minimum level, without inspection and rating; Q=l refers to buildings designed to Uniform Building Code (UBC) 

seismic zone O-l standards; Q=2 refers to UBC seismic zone 2; Q=3 refers to UBC seismic zone 3]

Buildings loss 

(thousands of 1984 dollars)

Contents loss 

(thousands of 1984 dollars)

Downtime

(years)

Microzone Q=l ?=2 Q=3 Q=l Q=2 G=l Q=2

T1NR1WS34

T1NR1WS35

T1NR1WS36

T1NR1WS31

T1SR1ES6

T1SR1ES4

T1SR1ES3

T1SR1WS12

T1SR1ES8

T1SR1ES9

T1SR1ES14

T1SR1WS35

T2SR1WS4

T2SR1WS3

T2SR1WS8

T2SR1WS13

T2SR1WS30

Total

Grand total

0

4,573

2,636

53,056

3,276

46,380

196

467

5

2,906

0

843

192

0

0

157

0

114,687

2,595

140

88

3,188

3,120

24,899

3,179

0

0

1,085

0

346

57

2,577

25

157

56

41,512

187,664

1,008

0

0

0

4,329

16,779

1,175

0

0

1,017

575

0

0

6,571

11

0

0

31,465

0

48

1,884

5,378

432

9,205

12

0

0

18

0

93

22

0

0

5

0

17,097

468

0

0

735

432

25,252

20

0

0

200

0

76

13

236

10

5

6

27,453

72,031

178

0

0

0

2,286

19,010

3

0

0

253

2,805

0

0

2,942

4

0

0

27,481

0

.66

.66

.66

.66

.66

.66

.66

.66

.66

0

.66

.66

0

0

.66

0

0.41

.66

.30

.32

.66

.29

.66

0

0

.31

0

.37

.30

.66

.12

.66

.33

0.09

0

0

0

.11

.05

.09

0

0

.06

.33

0

0

.11

.04

0

0

intensity shaking. The chief reason for this hypothesis 

was the greater extent of buried conduits exposed to 

potential liquefaction than to fault-related hazards. 

Figure 12 indicates provisionary estimates of the percentage of piping exposed to fault rupture and liquefactioninduced ground failure in worst-case events for various 

utilities surveyed. These estimates are provisionary principally because they are based on the liquefaction probabilities used for the first project (50 percent for highly 

susceptible microzones and 10 percent for the medium 

susceptible microzones) and on the broad fault zone definition used earlier and applied to maps by Cluff and 

others (1974) and Scott and Shroba (1985). Figure 12 

nonetheless illustrates how, in Salt Lake and Davis Counties, potential liquefaction-induced ground failure covers 

a much greater exposure than fault rupture and appears 

to be a more significant hazard on an aggregate level even 

for worst-case earthquakes. Moreover, fault rupture is not 

modeled for smaller magnitude (Ms<6.5) events, whereas 

liquefaction-induced ground failure can be extensive in 

smaller magnitude events. At the same time, our inventory has indicated critical facilities that traverse the 

Wasatch fault zone of deformation.

These original hypotheses were by and large confirmed with respect to natural gas system piping. The 

most serious damage to this piping, especially for the 

highly resistant piping surveyed, naturally takes place 

when permanent displacement occurs. (Details of the 

seismic vulnerability characteristics of natural gas piping 

have been discussed by McDonough and Taylor (1986) 

and Taylor and McDonough (1986); shaking vulnerabilities for both butt-welded polyethylene piping and weldedsteel piping have been treated as X-grade welded steel 

piping by Eguchi and others (1983).) Furthermore, our 

inventory showed that over one-fourth of the Weber and 

Davis aqueducts in the Weber Basin Conservancy District 

were contained in the Ogden segment of the Wasatch fault 

zone. Rupture of key sites in this segment would therefore likely stop ample Weber Basin Conservancy District 

water supplies from being conveyed to local utilities until 

repairs could be made. Several water-distribution storage 

facilities and at least one Davis County filtration plant
R22 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

100

80

60

O 40

EXPLANATION

In Wasatch 

fault zone

Subjected to 

liquefaction

Natural gas u 

pipelines

FIGURE 12. Percentage of pipeline surveyed estimated to be subjected 

to permanent ground displacement (fault rupture or liquefaction) in 

worst-case (Ms) earthquake events. (Reproduced with permission of 

the American Society of Civil Engineers.)

also appear to be located in the Wasatch fault zone of 

deformation. (Earlier evidence of water tanks in this zone 

were reported by B. Kaliser and found by Taylor and Ward 

(1981).)

We modified our original hypothesis with respect to 

aggregate culinary water piping in larger (M s>6.4) magnitude events. High shaking levels in these events contributed significantly to expected culinary water damage. At 

first, we speculated that significant damage could occur 

to seismically vulnerable pipelines even at Modified Mercalli intensities as low as VIII and when no permanent 

ground displacement occurred (see Isenberg and Taylor 

(1986) on the Coalinga earthquake; later estimates by 

Thiel and Zsutty (1987) suggest that intensity IX or X may 

have occurred in the city of Coalinga). Eventually, we 

determined that pipe vulnerability models need to be 

modified (downward) at these higher intensities. In the 

first project, it was only through an examination of an 

entire suite of earthquakes of varying magnitude and

with both distant and near-field rupture centers that a 

geographical distribution of damage dependent in liquefaction susceptibility as well as exposure became evident.

Table 5 provides a summary of piping damage 

expected in larger magnitude events postulated. For each 

utility and for each event evaluated, we aggregated estimated piping breaks. For a given utility, these total breaks 

were divided by total pipeline lengths to calculate piping 

break rates. We then selected the worst rates for any M s 

6.7 event simulated to construct table 5. To put these rates 

into perspective, approximately 2,800 miles (14.78 million 

lineal feet) of natural gas piping exists in Salt Lake and 

Davis Counties. Hence, a break rate of 10 breaks per 

million lineal feet would indicate approximately 148 

expected breaks to natural gas piping in those two 

counties.

Table 5 thus shows the widespread piping damage 

expected from events in which rupture occurs on either 

the Ogden or Salt Lake segment of the Wasatch fault zone. 

As a result of the comparative invulnerability of natural 

gas piping (either butt-welded polyethylene plastic or 

welded steel with arc-welded joints) to shaking damage, 

its pipe-break rates are much lower than those of culinary 

water piping. Much of the Provo and Orem city piping systems surveyed, moreover, consisted of flexible-jointed 

ductile piping. But even the natural gas system surveyed 

and evaluated would have limited immediate reliability in 

these larger magnitude near-field events except for the 

Utah County system, which is exposed to few potential 

ground-failure hazards.

As contrasts between Mt. Fuel supply break rates 

and culinary water system break rates partially indicate, 

breaks from strong ground motion are expected to be 

numerous. However, since the revision (downward) of 

pipe-break estimates ar higher shaking intensities, overall 

pipe-break estimates are currently especially sensitive to 

estimates of liquefaction probability. In the M s 7.45 earthquake from the Weber segment that affected Ogden city 

water, for example, only 18 percent of all breaks estimated were from strong ground motion; 23 percent were 

estimated to be from surficial fault displacement, and the 

remaining 59 percent were from liquefaction-induced 

ground failures. Still, the high levels of ground shaking 

also suggest significant potential damage to less resistant 

above-ground utility structures such as unanchored 

water-distribution storage tanks. Table 6 provides repair 

results in the second project broken down by the hazard 

that affected these results in worst-case events for each 

system surveyed. Standard assumptions are those hazard 

assumptions already explored, including standard magnitude scaling factors. The relative contribution of groundshaking to estimated breaks is higher for lower magnitude 

events and is higher in Utah County than it is in Weber 

County.
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R23

TABLE 5. Average break rates (per million lineal feet of pipe) estimated for utility systems surveyed for maximum

credible and worst-case M 6.7 events simulated

[NC, not calculated]

Weber segment

Utility system

Salt Lake County Conservancy District

Granger-Hunter Improvement District

Weber Basin Conservancy District

Clinton Water (Davis County)

Ogden City Water

South Ogden Water

Mt. Fuel Supply:

Weber County

Davis County

Salt Lake County

Utah County

Provo City Water

Orem City Water

s

190

350

260

280

5771

5221

401

70

25

NC

NC

NC

Worst case

Ms 6.7

100

210

150

160

512 1

494 1

371

50

20

NC

NC

NC

Salt Lake segment

s

240

390

100

110

NC

NC

NC

40

50

NC

NC

NC

Worst case

Ms 6.7

200

330

50

50

NC

NC

NC

35

40

NC

NC

NC

Provo segment2

s

NC

NC

NC

NC

NC

NC

NC

NC

NC

14 1

841

71 1

Worst case

MS 6.7

NC

NC

NC

NC

NC

NC

NC

NC

NC

II 1

76 1

57 1

1 Preliminary results from second project.

Both projects so far suggest that liquefaction-induced 

ground failure is the most significant hazard mode for 

earthquakes possibly damaging water and gas systems in 

the four-county area. Concentration on pipes traversing 

fault systems may in general ignore that breaks from fault 

displacements, when they occur, will be concurrent with 

many breaks from other hazard modes (liquefactioninduced ground failure, strong ground motion, and other 

hazard modes not studied explicitly here such as 

landsliding).

Table 7 examines the sensitivity of pipe repair rate 

estimates in maximum credible earthquakes to the various geoseismic hazard assumptions used. These assumptions consist of two alternative shaking hazard 

assumptions (the extreme assumption of basement rock 

estimates and the assumption that the rupture propagates 

toward all sites in the system) and one alternative 

assumption concerning liquefaction probability (10 percent as a maximum). Results for all systems are clearly 

sensitive to the maximum liquefaction probability assignment. Utah County water systems are especially sensitive 

to alternative assumptions concerning shaking hazards.

The results of this research indicate that earthquakes 

in Salt Lake, Davis, or Weber Counties having magnitudes 

perhaps as small as M 5.5 can also yield numerous breaks 

to local culinary water piping, especially for those systems having much nonductile piping. Preliminary results 

also suggest that smaller magnitude events (M s<6.5) are

expected to produce over half of all expected earthquakerelated breaks to culinary water piping in systems evaluated, largely because of high liquefaction potential for 

many sites even in smaller magnitude events. Table 8 

explores the sensitivity of repair rate results in smaller 

magnitude earthquakes to the selection of magnitude 

scaling factors. For systems having much nonductile piping, even strong motions from near-field smaller magnitude events can yield considerable breaks. Clearly, 

though, the magnitude scaling factors used make dramatic differences in pipe repair estimates ar lower magnitude levels.

Break estimates naturally vary considerably with the 

various earthquakes postulated in this study. Figure 13 

shows how break-rate estimates vary for a hypothetical 

Davis County system composed of welded-steel pipelines 

(not X grade, as in the more resistant natural gas piping). 

Aggregate system damage decreases with distance of the 

rupture center from the system. Moreover, on the basis of 

mathematical reasoning by Taylor and others (1985), a 

region of rupture centers, identified by the intersection of 

intervals Rx and Ry (fig. 13), contains rupture centers for 

which utility system reliability is minimum relative to the 

earthquake magnitude level examined.

For the natural gas system, which was sectionalized 

by county, rupture centers were determined for the magnitude levels 5.5, 6.1, 6.7, and 7.0 to 7.5, depending on the 

fault segment length and equations from Arabasz and
R24 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

TABLE 6. Percent of estimated repairs attributed to different hazard modes in worst-case events 

[Standard assumptions include 60° dip angle; K;=0 for water systems, K;=l for gas systems; Kg=0; Kg=l;

Hazards

System

Odgen City Water

South Odgen Water

Mt. Fuel Supply (Weber County)

Provo City Water

Orem City Water

Mt. Fuel Supply (Utah County)

Magnitude

7.45

6.7

6.1

5.5

7.45

6.7

6.1

5.5

7.45

6.7

6.1

5.5

7.39

6.7

6.1

5.5

7.39

6.7

6.1

5.5

7.39

6.7

6.1

5.5

Faulting 

(conservative 

definition) 

(percent)

23.4

16.8

0

0

8.0

5.4

0

0

21.7

15.3

0

0

28.7

21.6

0

0

18.7

15.0

0

0

43.7

34.2

0

0

Liquefaction 

(percent)

58.6

65.4

82.3

71.6

75.4

79.3

86.2

81.1

68.8

75.2

89.4

91.1

38.1

42.4

46.0

50.8

25.9

29.5

30.9

8.7

48.6

60.7

91.8

93.3

Shaking 

(percent)

18.0

17.8

17.8

28.4

16.6

15.3

13.8

18.9

9.4

9.6

10.6

8.9

33.2

36.0

54.0

49.2

55.4

55.5

69.1

91.3

7.8

5.1

8.2

6.7

Total 

(percent)

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

TABLE 7. Ratio comparisons of pipe repair rates based on diverse assumptions for maximum credible earthquakes affecting water and gas

systems in Utah and Weber Counties

Weber County systems

Standard assumptions 1

Standard but basement rock PHA and PHV estimates

used

Standard but directivity assumed

Standard but maximum liquefaction probability

of 10 percent

Ogden City

Water

1.0

.94

1.03

.61

South Ogden

Water

1.0

.94

1.02

.52

MtFuel

Supply

1.0

.95

1.0

.50

Utah County systems

Provo City

Water

1.0

.91

1.08

.79

Orem City

Water

1.0

.80

1.12

.86

Mt. Fuel

Supply

1.0

.92

1.01

.63

x Dip angle 60°; KI=O for water systems, KI=! for gas systems; ^2=°; ^3=1 ; K>3=1 
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R25

TABLE 8. Sensitivity of worst-case repair rates at lower magnitude 

levels to assumptions on magnitude scaling factors

Expected repairs per million lineal feet of piping

System

One-half

Magnitude Standard highest 

level multipliers multipliers 

in table 2

Mt. Fuel Supply (Weber

County)

South Ogden Water

Ogden City Water

Mt. Fuel Supply (Utah

County)

Provo City Water

Orem City Water

5.5

6.1

5.5

6.1

5.5

6.1

5.5

6.1

5.5

6.1

5.5

6.1

26.3

30.0

200

393

158

424

2.1

5.4

30.3

48.1

13.8

34.3

4.4

6.6

50.1

94.1

60.6

129.5

.3

1.0

16.4

30.0

12.7

24.4

others (1987), so that minimum system reliability could 

be assessed at each such magnitude level. The selection 

of rupture centers for magnitude 7.0 to 7.5 earthquakes 

was dependent only on the segment modeled a degenerate case of techniques used. System flow analyses were 

performed by the natural gas utility on the basis of 

detailed damage estimates for each scenario so selected. 

These analyses in turn were used to identify system weak 

links and to prioritize component retrofit and system 

enhancement alternatives.

Using smaller magnitude earthquakes proved to have 

several important practical advantages in this report. 

First, as is the case for the building industry (except for 

sites where seismicity is dominated by a single very active 

fault system or for structures designed to special standards), earthquake scenarios typically used for design or 

redesign are smaller in magnitude than maximum credible events. The maximum credible events are less probable and can be expected to cause only a small portion of 

overall long-term expected earthquake losses. Second, 

using smaller magnitude earthquake scenarios can provide greater assistance in identifying weak system links 

(for example, those elements that, if strengthened, could 

improve system reliability). Even many seismically resistant facilities (not necessarily weak links and not typically 

priorities in a seismic risk reduction program) tend to be 

damaged in maximum credible earthquakes. This tendency held in all systems surveyed except for the natural 

gas system in Utah County, which contains highly resistant piping and which is exposed to comparatively few 

permanent ground-failure hazards.

SUMMARY

The combined probabilistic and near-source 

approach to Wasatch Front risk estimation yields new 

insights and raises difficulties concerning the extent of 

losses and risks expected. Using many scenarios within a 

probabilistic framework assists in incorporating elements 

such as magnitude scaling factors and in developing damage statistics that can be used as a basis for examining 

systemic and secondary losses. Using many scenarios of 

varying magnitudes also assists in defining the types of 

earthquakes that are potentially dangerous and the 

parameters that could be studied further to improve risk 

estimates in these various earthquakes.

In maximum credible and other high-magnitude (M s 

6.7) localized events studied, high levels of ground shaking estimated indicated that previous pipe vulnerability 

models needed to be revised at intensities IX and above. 

Partly as a result of findings on soil effects, these high levels still suggest that significant culinary water piping damage and damage to above-ground utility structures (many 

of which have limited seismic resistance or have equipment having limited lateral resistance) are expected from 

strong ground motion, even in somewhat smaller magnitude earthquakes.

But, for natural gas and culinary water piping, estimates of expected damage are especially sensitive to liquefaction probabilities (estimated for diverse sites and 

(or) microzones as affected by a variety of simulated 

earthquakes). Methods have been improvised in these 

projects to estimate these probabilities, but improved 

methods would greatly assist in refining damage estimates to buried piping. Of special interest would be a 

review of this project's findings that even selected (nearfield) earthquakes having magnitudes perhaps as low as 

5.5 could cause extensive localized liquefaction-induced 

ground failures and consequent high levels of pipe 

breakage. This result has been demonstrated to be 

extremely dependent on the magnitude scaling factors 

used.

Results of liquefaction susceptibility studies are useful in delineating regions where overall earthquake damage to buried conduits is expected to be greater, 

particularly so with respect to smaller magnitude 

(Ms<6.5), more distant earthquake scenarios or more seismically resistant buried conduits. More detailed analysis 

of microzones having large exposures and high liquefaction susceptibilities would provide information valuable 

to Wasatch Front earthquake risk reduction problems. 

This analysis may involve gathering additional data on 

sites within these microzones, developing improved
R26 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

150

100

-A--

ft represents location of rupture centers causing highest 

damage to the systems (for x- and y- axes respectively)

FIGURE 13. Break rate variations versus fault rupture center locations, M s 6.1 (Davis County) (origin is intersection of base line and Salt Lake 

meridian). R represents the location of rupture centers causing the most damage to the system (for x and y axes).

procedures for defining liquefaction potentials for areas 

rather than sites, associating liquefaction susceptibility 

data with specific facilities, or using smaller cell grids.

Losses to less seismically resistant State-owned 

building are estimated to be extremely high even higher 

than the 39 percent estimated for all State-owned buildings in worst-case events. These losses include probable 

long-term downtime and consequent disruption of governmental and university operations. Because life-safety 

hazards may also be significant, we recommend a more 

detailed visual inspection and rating of the most seismically vulnerable and densely occupied facilities.
SEISMIC-RISK METHODS AND ESTIMATES FOR UTILITY SYSTEMS AND STATE-OWNED BUILDINGS R27

The approach used here has also proved to be practical. Its use by the natural gas utility in defining seismic 

risk reduction plans indicates that a probabilistic 

approach can be successfully applied to network problems as well as to building problems. Participation by utility representatives in study phases lends both credibility 

and meaning to results for those who can employ them in 

strengthening systems.

REFERENCES CITED

Alfors, J.T., 1983, Regional geology of the Coalinga, California 

area, in The 1983 Coalinga, California earthquakes, Sacramento, CA: California Department of Conservation, Division of Mines and Geology, Special Publication 66, p. 105- 

112.

Algermissen, S.T., and Steinbrugge, K.V., 1984, Seismic hazard 

and risk assessment: Some case studies: Geneva Papers on 

Risk and Insurance, v. 9, no. 30, p. 8-26.

Algermissen, S.T., Perkins, D.M., Thenhaus, PC., 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.

Algermissen, S.T., Rogers, A.M., Hays, W.W., Perkins, D.M., Van 

Strien, D.O., Hughes, H.C., Hughes, R.C., Lagorio, H.J., and 

Steinbrugge, K.V., 1976, A study of earthquake losses in the 

Salt Lake City, Utah area: U.S. Geological Survey Open-File 

Report 76-89, 357 p.

Anderson, L.R., Keaton, J.R., Audrey, K, and Ems, S.J., 1982, Liquefaction potential map for Davis County, Utah: Logan and 

Salt Lake City, Utah State University Department of Environmental Engineering and Dames and Moore, under USGS 

contract 14-08-0001-19127, 49 p.

Arabasz, W.J., Pechmann, J.C., and Brown, E.D., 1987, Observational seismology and the evaluation of earthquake hazards 

and risk in the Wasatch Front area, Utah, in Gori, PL., 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. D1-D58.

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, no. 6, p. 2379-2411.

Bresko, D., Hendrickson, D., and Oppenheim, I., 1981, Seismic 

risk analysis of an urban water system, in Lifeline earthquake engineering, Proceedings of the Second Specialty 

Conference: New York, American Society of Civil Engineers, 138 p.

Bruhn, R.L., Gibler, PR., Houghton, W, and Parry, W.T., 1987, 

Structure of the Salt Lake segment, Wasatch normal fault 

zone: Implications for rupture propagation during large 

earthquakes, in Gori, PL, and Hays, W.W., Assessment of 

regional earthquake hazards and risk along the Wasatch 

Front, Utah: U.S. Geological Survey Open-File Report 87- 

585, p. H1-H57.

Campbell, K.W., 1984, Near-source attenuation of strong ground 

motion for moderate to large earthquake; an updated and

suggested application to the Wasatch fault zone of northcentral Utah: U.S. Geological Survey Open-File Report 84- 

763, p. 483-499. 

-1987, Predicting strong ground motion in Utah, in Gori,

PL., and Hays, WW, eds., Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. 

Geological Survey Open-File Report 87-585, p. L1-L90.

Cluff, L.S., Glass, C., and Brogan, G., 1974, Wasatch fault investigation maps: Salt Lake City, Utah Geological and Mineral 

Survey, scale 1:2000.

Eguchi, R.T., Legg, M.R., Taylor, E.G., Philipson, L.L., and Wiggins, J.H., 1983, Earthquake performance of water and gas 

supply systems: Redondo Beach, Calif., J.H. Wiggins 

Company Report 83-1396-5 to the National Science Foundation, 206 p.

Highland, L.M., 1987, Earthquake hazards to domestic water distribution systems in Salt Lake County, Utah, in Gori, PL, 

and Hays, WW, eds., Assessment of regional earthquake 

hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Open-File Report 87-585, p. Q1-Q76.

Hutton, L.K., 1987,1987 Whittier Narrows earthquake: Prior seismicity and comparison with the 1919 Whittier sequence: 

California Institute of Technology Seismological Laboratory 

Report 252.21,8 p.

International Conference of Building Officials, 1988, Uniform 

building code: Whittier, Calif.

Isenberg, J., and Taylor, C.E., 1984, Performance of water and 

sewer lifelines in the May 2, 1983 Coalinga, California earthquake, in Lifeline earthquake engineering: New York, American Society of Civil Engineers, p. 190-211.

Joyner, W.B., and Fumal, T.E., 1985, Predictive mapping of earthquake ground motion, in Ziony, J.I., ed., Earthquake 

hazards in the Los Angeles region An earth-science perspective: U.S. Geological Survey Professional Paper 1360, 

p. 203-220.

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: Salt Lake City, Dames 

and Moore and University of Utah Department of Geography report under U.S. Geological Survey contract 14-08- 

0001-22048, 54 p.

Liao, S.S.C., 1986, Statistical modeling of earthquake induced liquefaction: Cambridge, Massachusetts Institute of Technology Department of Civil Engineering, unpublished Ph.D. 

dissertation, 470 p.

Liu, S.C., 1977, Earthquake protection of communications facilities, in World Conference on Earthquake Engineering, 6th, 

New Delhi, 1977, Proceedings, p. 3304-5309.

Mabey, D.R., 1987, Subsurface geology along the Wasatch Front, 

in Gori, PL., and Hays, WW, eds., Assessment of regional 

earthquake hazards and risk along the Wasatch Front, Utah: 

U.S. Geological Survey Open-File Report 87-585, p. C1-C39.

Machette, M.N., Personius, S.F., and Nelson, A.R., 1987, Quaternary geology along the Wasatch fault zone: Segmentation, 

recent investigations, and preliminary conclusions, in Gori, 

PL, and Hays, WW, 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.
R28 REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT

McDonough, P.W., and Taylor, C.E., 1986, Assessing seismic 

response of Utah gas systems: Earthquake Spectra, v. 2, 

no. 4, p. 747-766.

National Research Council Panel on Earthquake Loss Estimation Methodology, 1989, Estimating losses from future 

earthquakes: Washington, D.C., National Academy Press, 

231 p.

Robison, R.M., 1986, The surficial geology and neotectonics of 

Hansel Valley, Box Elder County, Utah: Logan, Utah State 

University, unpublished M.Sci. thesis.

Rogers, A.M., Carver, D.L., Hays, W.W., King, K.W, and Miller, 

R.D., 1984, Preliminary estimates of geographic variations 

in relative ground shaking in the Wasatch Front urban corridor, in Hays, W.W, and Gori, PL, 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. 547-556.

Schiff, A.J., 1988, Response of lifelines to the Whittier Narrows 

earthquake of 1987 and their effect on the emergency 

response: Earthquake Spectra, v. 4, no. 2, p. 367-376.

Scott, WE., 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.

Seed, H., and Idriss, I.M., 1983, Ground motions and soil liquefaction during earthquakes: Berkeley, Calif., Earthquake 

Engineering Research Institute, 134 p.

Smith, Robert, and Richins, William, 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.

Steinbrugge, K.V., 1982, Earthquakes, volcanos, and tsunamis: 

An anatomy of hazards: New York, Skandia American 

Group, 392 p.

Taylor, C.E., and Bigler, E., 1982, A preliminary study of possible 

earthquake problems to Salt Lake County water supply system: Report to the Governor of Utah.

Taylor, C.E., and McDonough, P.W, 1986, Earthquake risks to 

Utah water and gas systems, in Lifeline seismic risk 

analysis Case studies: New York, American Society of 

Civil Engineers, p. 70-37.

Taylor, C.E., and Ward, D.B., 1979, Seismic risk assessment of 

Utah primary and secondary schools and recommendations 

for risk reduction: Salt Lake City, Utah Seismic Safety Advisory Council, 91 p.

 1981, Seismic risk assessment of Utah culinary water supply systems and recommendations for risk reduction: Salt 

Lake City, Utah Seismic Safety Advisory Council, 94 p.

Taylor, C.E., Legg, M.R., Haber, J.M., and Wiggins, J.H., 1985, 

New lifeline multi-scenario seismic risk techniques with a 

model application: Civil Engineering Systems, v. 2, p. 77-83.

Taylor, C.E., Wiggins, J.H., Haber, J.M., and Ward, D.B., 1986a, A 

systems approach to Wasatch Front seismic risk problems: 

Long Beach, Calif., NTS Engineering Technical Report to

U.S. Geological Survey under contract 14-08-001-22013, 

169 p.

Taylor, C.E., Haber, J.M., Legg, M.R., Heubach, W.F., and Wiggins, 

J.H., 1986b, Earthquake vulnerability of water and natural 

gas supply systems: Long Beach, Calif., NTS Engineering 

Technical Report 86-1466 to the National Science Foundation.

Taylor, C.E., Salmon, M., Eguchi, R., Campbell, R., and Tillman, 

C., 1988, Continuing investigations of earthquake risks to 

Utah water and gas systems: Long Beach, Calif., NTS Engineering Technical Report to U.S. Geological Survey under 

contract 14-08-001-22013,192 p.

Thiel, C.C., and Zsuttys, T.C., 1987, Earthquake parameters and 

damage statistics: San Francisco, Forell/Elsesser Engineers, report under U.S. Geological Survey contract 14-08- 

0001-G1181,264p.

Trautmann, C.H., O'Rourke, T.D., Grigoriu, M.D., and Khayer, 

M.J., 1986, Systems model for water supply following 

earthquakes, in Lifeline seismic risk analysis Case studies: New York, American Society of Civil Engineers, p. 30- 

50.

Utah Geological and Mineral Survey, 1983, Governor's Conference on Geologic Hazards, Salt Lake City, Utah: Utah Geological and Mineral Survey Circular 74, 98 p.

Utah Seismic Safety Advisory Council, 1981, Earthquake safety 

in Utah and abbreviated recommendations for risk reduction: Utah Seismic Safety Advisory Council Report, 38 p.

Wheeler, R.L., and Krystinik, K.B., 1987, Persistent and nonpersistent segmentation of the Wasatch fault zone, Utah: Statistical analysis for evaluation of seismic hazard, in Gori, PL., 

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. B1-B124.

Whitman, R.V., Biggs, J.M., Brennan, J.E., III, Cornell, C.A., de 

Neufville, R.L., and Varmarcke, E.H., 1975, Seismic design 

decision analysis: Journal of the Structural Division, May 

1975, ST5, p. 1067-1084.

J.H. Wiggins Company, 1979, Considerations and alternatives for 

abatement of seismically hazardous existing buildings in 

Utah: Redondo Beach, Calif., report prepared for the Utah 

Seismic Safety Advisory Council.

Wiggins, J.H., and Taylor, C.E., eds., 1985, Earthquake rehabilitation manual for buildings, in Building construction under 

seismic conditions: Vienna, United Nations Industrial 

Development Programme, v. 4, 246 p.

 1986, Earthquake performance of low-rise construction: 

Long Beach, Calif., NTS Engineering Technical Report 86- 

1442, prepared for the National Science Foundation, v. 2, 

189 p.

Wiggins, J.H., Taylor, C.E., Eguchi, R.T., and Slosson, J.E., 1981, 

Earthquake hazards and risk mitigations: Identification and 

evaluation methods: Redondo Beach Calif., J.H. Wiggins 

Company Technical Report 1373-1, prepared for the Federal Emergency Management Agency, 438 p.

Youd, T.L., and Perkins, D.M., 1987, Mapping of liquefaction 

severity index: Journal of Geotechnical Engineering, v. 113, 

no. 11, p. 433-445.

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