Segmentation of the Wasatch Fault Zone, UtahSummaries, Analyses, and Interpretations of 

Geological and Geophysical Data 

By RUSSELL L. WHEELER and KATHERINE B. KRYSTINIK 

Descriptions, with supporting evidence, of segment boundaries 

as they are expressed in gravity, aeromagnetic, seismological, 

fault-geometric, topographic, and structural data 

U.S. GEOLOGICAL SURVEY BULLETIN 1827 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U. S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade names is for descriptive purposes only and does not imply 

endorsement by the U.S. Geological Survey. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Wheeler, Russell L. 

Segmentation of the Wasatch fault zone, Utah-summaries, analyses, and 

interpretations of geological and geophysical data. 

(U.S. Geological Survey bulletin ; 1827) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1827 

1. Faults (Geology)-Utah. I. Krystinik, Katherine B. II. Title. Ill. Title: 

Wasatch fault zone, Utah. IV. Series. 

QE75.B9 no. 1827 557.3 s [551.8' 7' 097922] 87-600430 

[QE606.5.U6] 
CONTENTS 

Abstract 1 

Introduction 1 

Criteria that the data must satisfy 2 

Bouguer gravity data 3 

Introduction 3 

Procedures 4 

Results 6 

Aeromagnetic data 6 

Introduction 6 

Procedures 7 

Results 9 

Earthquake epicenters 10 

Introduction 10 

Procedures 11 

Results 12 

Salients 13 

Introduction 13 

Procedures 15 

Results 15 

Topographic data 18 

Introduction 18 

Procedures 19 

Results 19 

Pre-Cenozoic structures 21 

Introduction 21 

Procedures 23 

Uinta aulacogen 24 

Evidence for existence of the aulacogen 24 

Evidence for faults in the aulacogen 26 

Defining a structural anomaly at the aulacogen 28 

Faults of the Late Proterozoic passive margin 28 

Cretaceous thrust faults north of the Uinta Mountains 30 

Geometries of the thrust faults 30 

Structural anomaly at a lateral ramp 32 

Cretaceous thrust faults south of the Uinta Mountains 34 

Geometries of the thrust faults 34 

Structural anomaly at a lateral ramp 36 

Results 37 

Summary and questions 38 

References cited 40 

Contents Ill 
FIGURES 

1. Index map of the Wasatch fault zone 2 

2. Map showing transverse anomalies from complete Bouguer gravity field 5 

3. Map showing transverse anomalies from total aeromagnetic intensity 8 

4. Graph showing changes in earthquake abundance along Wasatch Front 11 

5. Graphs showing overestimation and underestimation of widths of gap in 

seismicity 12 

6. Map showing salients of the Wasatch fault zone 14 

7. Map showing simplified topography of footwall of Wasatch fault zone 20 

8. Graphs showing degrees of reproducibility of topographic changes 21 

9. Map showing selected tectonic elements of the Uinta aulacogen 23 

10. Schematic cross section illustrating widths of structural anomaly at Uinta 

aulacogen 29 

11. Map showing Mesozoic extensional faults of Newark-Gettysburg basin 30 

12. Map showing selected structural elements north of the Mount Raymond 

fault and Uinta Mountains 31 

13. Generalized cross section showing northward view through structures 

mapped in figure 12 31 

14. Spherical projection of estimates of orientation of Willard thrust fault 32 

15. Block diagrams showing two possible geometries of lateral ramps at south 

nose of ramp anticlin~ 33 

16. Map showing selected structural elements south of Deer Creek fault 35 

17. Graphs showing distribution of anomalies along Wasatch fault zone 38 

TABLES 

1. Transverse anomalies from gravity data 7 

2. Transverse anomalies from aeromagnetic data 10 

3. Seismological anomalies 13 

4. Anomalies from fault geometry 16 

5. Topographic anomalies 21 

6. Structural anomalies 37 

CONVERSION FACTORS 

For readers who wish to convert measurements from the metric system of units to the inch-pound 

system of units, the conversion factors are listed below. 

Metric unit 

meter (m) 

kilometer (km) 

square kilometer (km2) 

IV Contents 

Multiply by 

3.281 

0.6214 

0.3861 

To obtain inch-pound unit 

foot (ft) 

mile (mi) 

square mile (mF) 
Segmentation of the Wasatch Fault Zone, UtahSummaries, Analyses, and Interpretations of 

Geological and Geophysical Data 

By Russell L. Wheeler and Katherine B. Krystinik 

Abstract 

The Wasatch fault zone of central Utah has been 

hypothesized to be segmented into lengths that tend to rupture independently of each other, with large seismic ruptures 

tending not to cross boundaries between segments. Validity 

of the segmentation hypothesis would affect estimates of seismic 

hazard for the urban corridor that contains most of Utah's 

population. Our evaluation of the hypothesis concentrates on 

detecting segment boundaries. To form a basis for this search, 

we examined six data types for anomalies that might record 

the presence and locations of segment boundaries that could 

influence the next few large seismic ruptures on the fault 

zone. 

Mapped Bouguer gravity values define saddles, easttrending gradients, ends of north-trending gradients, and ends 

of north-trending highs and lows that together identify 10 

transverse anomalies. Mapped aeromagnetic intensities show 

east-trending gradients and belts of small, intense highs that 

define three other transverse anomalies. Earthquake and 

microearthquake epicenters identify another four anomalies, 

as places where recent seismic activity along and near the trace 

of the fault zone changed markedly northward or southward 

along the zone. Geometry of the fault zone defines four more 

anomalies, which are large footwall salients around which the 

fault trace bows westward. Smoothed topography of the upthrown Wasatch Range and San Pitch Mountains defines two 

topographic anomalies. Consideration of pre-Cenozoic structures and geologic history identifies the Uinta aulacogen and 

two large, north-dipping lateral ramps in thrust sheets as three 

structural anomalies. 

Each data type and its anomalies satisfy seven criteria that 

insure that the anomalies are pertinent to identification of segment boundaries and the evaluation of seismic hazard, and are 

reliable. The 26 anomalies appear to be concentrated in two 

narrow sections and one wide section of the fault zone, each 

of which might contain one or more segment boundaries. 

This report is the second of three. The first described 

numerical methods for evaluating anomaly concentrations like 

those described in this report. The third report describes the 

application of the numerical methods to these concentrations, 

and the use of the results to help evaluate seismic hazard of 

the Wasatch fault zone. 

INTRODUCTION 

About two-thirds of Utah's 1.5 million people live 

within 25 km of the trace of the north-striking Wasatch 

fault zone (fig. 1; 1980 Census data). Large, young fault 

scarps along the fault zone indicate that seismic slip in 

the fault zone has continued through the late Pleistocene 

and Holocene and will probably continue into the future 

(Cluff and others, 1970, 1973, 1974, 1975; Cluff and 

Slemmons, 1972; Bucknam and others, 1980; Algermissen 

and others, 1982). The potential for future damaging 

earthquakes constitutes a hazard that needs to be assessed 

(Hays and Gori, 1984). 

Swan and others (1980) and Schwartz and Coppersmith (1984) hypothesized that the fault zone is 

segmented into lengths that tend to rupture independently 

of each other. If their segmentation hypothesis is valid, 

large ruptures would tend not to cross boundaries between segments, so that most ruptures would stop at segment boundaries. The strained rock in a boundary at the 

end of a recent rupture would be a likely place for the 

next rupture to nucleate, so ruptures would also tend to 

start at segment boundaries (King and Nabelek, 1985). 

If ruptures tend not to cross boundaries, then segment 

lengths would provide an upper limit on potential rupture length, and hence on earthquake size. Thus, the 

validity of the segmentation hypothesis will influence 

estimates of possible mainshock locations and maximum 

sizes, both of which are important parameters in the probabilistic estimation of ground motion (Algermissen and 

others, 1982). Maps of estimated ground motion are widely used components of hazard assessment. The maps are 

often incorporated into building codes, seismic design 

criteria, analyses of potential losses, and other applications (Hays, 1979, p. 2, 20). 

Accordingly, evaluation of the segmentation 

hypothesis as it applies to the Wasatch fault zone is an 

important part of hazard assessment for Utah (Wheeler, 

1984). Our strategy for this evaluation is to seek evidence 

of segment boundaries that have persisted through 

Introduction 
42o,_ __ 11r3_0 

__________ ~ __ 1T12~------------~~11_1°~ 

UTAH 

EXPlANATION 

--- Trace of Wasatch 

fault 

Collinston 

segment 

(~30 km) 

Ogden 

segment 

(70 km) 

~ Salt Lake 

Salt City segment 

Lake (35 km) 

City 

---

\\ Uta~)\! Lak'v:_\ava 

r-1-

1 Nephi 

segment 

Nephio{ (35 km) 

---j.c: (1.) 

. !:# ·~ Levan 

{ ll.: c segment 

--- Proposed segment 

boundary 

\Jj (40km) 

( 40 km) Length of proposed 

segment 

I 

25 

25 

I I 

50 MILES 

I 

50 KILOMETERS 

I 

L 

Provo 

segment 

(55 km) 

Figure 1. Index map of the Wasatch fault zone in northern 

central Utah, showing selected features along the Wasatch 

Front. The Wasatch Front is usually taken to include the 

populated valleys that lie along and immediately west of the 

trace of the Wasatch fault (Mabey, 1987) and which include 

most of the population of Utah. The high ground and mountains that border the fault on the east are called the Wasatch 

Range south to Nephi and the San Pitch Mountains south of 

there. Lat 42° N. marks the Utah-Idaho border. Segment 

boundaries are those of Schwartz and Coppersmith (1984), from 

whose figure 1 this map is modified. 

enough of the geologic past that they are likely to continue to affect large ruptures through the next few centuries or millenia, the time span of interest in hazard 

assessment. We examined six types of data along and near 

the fault zone, seeking the kinds of anomalies that a persistent segment boundary should produce in each data 

type. Other processes than segmentation might produce 

similar anomalies in one or more data types. Where more 

2 Segmentation of Wasatch Fault Zone, Utah 

anomalies coincide than would be expected by chance, 

and where these other processes can be ruled out on 

geologic grounds, we will have identified a persistent segment boundary. 

The area studied stretches along the north-trending 

fault zone from central Utah at lat 39 ° N. to the IdahoUtah border at lat 42 ° N. The study area has no fixed 

width, because for each data type the examined area 

extends east and west of the trace of the fault zone 

as far as is necessary to identify and characterize 

anomalies. 

In this report we cannot present all aspects of our 

investigations. Here we describe the anomalies in each 

data type, demonstrate their validity, and estimate the 

locational uncertainty of each anomaly. Wheeler and 

Krystinik (1987a) described the statistical and related 

methods that we used to analyze the set of anomaly locations, and Wheeler and Krystinik (1987b, c, d) applied 

the methods to the anomalies and interpreted the results 

in terms of segment boundaries. 

This report uses more diverse kinds of information 

than two authors could pretend to understand alone. We 

are unusually deeply indebted to topical and regional 

specialists for discussions, preprints, sharing of unpublished data and maps, and informal reviews of parts 

of the manuscript. These generous colleagues include 

W.J. Arabasz, R.L. Bruhn, Bruce Bryant, P.A . 

Cashman, T.G. Hildenbrand, R.M. Kligfield, D.R. 

Mabey, M.N. Machette, A.R. Nelson, C.G. Oviatt, S.F. 

Personius, W .E. Scott, Alain Villi en, and M.L. Zoback. 

The entire manuscript was improved by the comments 

of M.N. Machette, A.R. Nelson, and K.M. Shedlock. 

CRITERIA THAT THE DATA MUST SATISFY 

Each of the six types of data that we examined, or 

the anomalies in a data type, must meet seven criteria. 

Criteria 1-4 were imposed by the geological aspects of 

the segmentation hypothesis. Criteria 5-7 were imposed 

by requirements of the statistical and related analyses of 

Wheeler and Krystinik (1987a). 

The first criterion is that data must have a resolution that would enable them to show something the size 

of a segment boundary. For example, the distribution of 

earthquakes of magnitude 5 or greater along the Wasatch 

fault zone could not resolve segment boundaries because 

there have been too few such earthquakes in historic 

times. However, the many hundreds of closely spaced 

measurements of Bouguer gravity that have been collected 

by many workers and plotted and contoured by Zoback 

(1983) constitute data that have sufficient resolution 

potentially to detect segment boundaries. 

Second, data must be capable of delineating whatever structures are likely to be responsible for large 
earthquakes along the Wasatch fault zone. We do not 

require that the data actually identify the causative structures of large earthquakes, but only that the data are likely to be useful in some future identification. For example, 

some measure of the youthfulness or abundance of fault 

scarps could be used, if such a measure could be made 

continuously or at intervals of a few kilometers along the 

fault zone. However, estimates of depth to the base of 

the crust are unlikely to be useful because most earthquakes along the fault zone take place at shallower depths 

(Arabasz and others, 1980). 

Third, data must be likely to indicate the presence 

of a segment boundary by forming an anomaly whose 

relationship to the boundary can be explained. For example, Zoback (1983) noted that the contoured gravity 

values show several closed, elongate lows along the west 

side of the Wasatch fault zone. She argued that these lows 

correspond to low-density, sedimentary fillings of young 

basins that formed in the hanging-wall block during 

normal faulting. At several places along the fault zone 

such closed gravity lows are separated by gravity saddles 

or steep, east-trending gravity gradients that Zoback 

(1983) interpreted as indicating boundaries between adjacent basins of different depths. 

The explanation of the gradients as indications of 

segment boundaries begins with the assumption that 

sediments of about the same age will have about the same 

densities in adjacent basins. Adjacent basins that have 

had similar depositional and subsidence histories will have 

similar gravity signatures, and thus will not be separated 

by a gravity gradient. Steep gradients between basins 

therefore record different times of basin formation, different amounts of downdropping of the basin floors, or 

both. Because the basins formed by motion on the 

Wasatch fault and associated faults, and because that motion can be assumed to have been mostly seismic, the steep 

gradients between closed gravity lows are likely to indicate 

boundaries between parts of the fault zone with different 

seismic histories. These parts would be segments. 

However, if the gravity data contained no steep, easttrending gradients, or if the gradients could not be explained in terms of possible segment boundaries, then the 

gravity data would not be a useful tool with which to 

evaluate segmentation. 

Fourth, the data must record processes that operate 

over times that allow inferences to be drawn about the 

expected seismicity of the next decades to millenia. Accordingly, information on Precambrian structures would 

not be useful unless it can be shown that such structures 

were active during the late Pleistocene and Holocene, or 

influenced such young activity. Recently active structures 

are more likely to continue their activity into the near 

future than are structures that have been inactive for a 

long time. Elevations of the Bonneville shoreline east of 

the Wasatch fault zone could record segment boundaries 

because such elevations record uplift of the east wall of 

the fault zone since the shoreline formed about 15 ka. 

If adjacent segments had different amounts of uplift since 

the shoreline formed, then shoreline elevations east of the 

fault zone would vary more across segment boundaries 

than within the segments themselves. Unfortunately remnants of the shoreline on the east wall are too few to be 

able to resolve segment boundaries (Currey, 1982; 

Wheeler, 1984). 

Fifth, identification of anomalies is a subjective 

procedure, so the anomaly locations must be shown to 

be reliable. For most data types we demonstrated reliability by showing that identical anomaly locations have been 

inferred by two independent workers. Underlying such 

a demonstration is the assumption that independent 

observers are unlikely to make the same error of interpretation by inferring the existence of an anomaly at the 

same place, if none exists there. Less preferably, if no 

second worker has analyzed the data, we demonstrated 

reliability by describing the identification of the anomalies 

in sufficient detail that an independent worker would be 

likely to reproduce the anomalies. 

Sixth, the identification of the anomalies in any one 

data type must occur without reference to any other data 

type. If identification of anomalies in one data type were 

influenced by examination of a second type of data, then 

the anomalies in the first data type would contain information from the second. However, the statistical and 

related procedures of Wheeler and Krystinik (1987a) require that each data type be examined separately for 

anomalies, in order to determine where along the fault 

zone the data types have coincident anomalies. 

Seventh, the anomalies must be identifiable without 

reference to the proposed segment boundaries of 

Schwartz and Coppersmith (1984). Otherwise, efforts to 

identify anomalies would be distorted by this prior inspection of the boundaries (Hicks, 1973, p. 15-16; Freedman and others, 1978, p. 494; Moore, 1979, p. 294-295; 

Wheeler, 1985). This seventh criterion does not duplicate 

the sixth, because we did not use the proposed segment 

boundaries as a data type. 

BOUGUER GRAVITY DATA 

Introduction 

Zoback (1983) mapped, contoured, and interpreted 

complete Bouguer gravity data that had been collected, 

reduced, and compiled by many previous workers (for 

example, Cook and Berg, 1961; Cook and Montgomery, 

1972, 1974; Cook and others, 1975). The area studied by 

Zoback spans long 111 °-l13°W. and lat 39000'-42°15' 

N. This area includes the Wasatch Front and Wasatch 

fault zone (fig. 1). Zoback (1983, fig. 3) interpreted her 

Bouguer Gravity Data 3 
gravity map in terms of fault-bounded basins filled by 

low-density Cenozoic sediments and sedimentary rocks. 

The basins inferred from the gravity data correspond to 

those known from geologic mapping and subsurface investigations. The inferred basins trend north-south, and 

their gravity expressions are interrupted by transverse 

zones that Zoback (1983) interpreted as ends of basins 

or abrupt changes in basin depth. Several of the transverse 

zones also have recognizable expressions in surface 

geology (Zoback, 1983, p. 9, figs. 3-4). 

In the study of gravity and magnetic data, the term 

anomaly has several established meanings (Gary and 

others, 1972; Bates and Jackson, 1980). We use the term 

anomaly to refer to unusual aspects of various types of 

geological or geophysical data that characterize some 

small parts of the Wasatch fault zone and distinguish 

them from nearby, less anomalous lengths of the fault 

zone. To avoid confusion between this usage of anomaly and those usages that are common in the study of gravity and magnetic data, this section and the section on 

aeromagnetic data refer to anomalies as transverse 

anomalies, following Zoback (1983). The word transverse 

does not imply that a transverse anomaly has an east-west 

extent that is larger than its north-south extent along the 

Wasatch fault zone. Some transverse anomalies are 

elongate east-west, but others represent just the north or 

south ends of gravity features that extend for several tens 

of kilometers along the fault zone. In particular, the 

presence of a transverse anomaly by itself does not justify 

the inference of an east-striking fault (Zoback, 1983, p. 

3, 9). 

The complete Bouguer gravity map of Zoback 

(1983) constitutes one type of data that can be examined 

along the Wasatch fault zone for transverse anomalies 

that might coincide with the proposed segment boundaries of Schwartz and Coppersmith (1984). The transverse 

zones of Zoback (1983) are candidates for these transverse 

anomalies, and indeed both Zoback (1983, p. 3) and 

Schwartz and Coppersmith (1984, p. 5688) noted that 

some transverse zones appear to coincide with some proposed segment boundaries. 

Procedures 

M.L. Zoback (written and oral communs., 1984) 

supplied copies of the gravity map used by Zoback (1983). 

The map, which was machine contoured at intervals of 

2.5 mGal, is at a scale of 1:500,000 and shows only the 

isogals, station locations, and latitude and longitude 

marks at intervals of 0.5 °. Thus, geologic, geographic, 

or geophysical information that might bias interpretation 

is not shown. In the several months before our analysis , 

of the gravity map we took care not to compare the maps 

4 Segmentation of Wasatch Fault Zone, Utah 

of Zoback (1983) and Schwartz and Coppersmith (1984), 

in order to reduce distortion of our interpretation by prior 

inspection. 

Zoback (1983) and Mabey (1987) interpreted much 

the same gravity data and produced different sets of 

transverse anomalies. In order to determine which, if 

either, set to use, we must determine whether we can independently reproduce one of the sets. We demonstrate 

the credibility of our analysis by describing it in enough 

detail that a sufficiently determined reader could likely 

reproduce our results. Without the next three paragraphs 

a critical reader would have no reason to accept our 

set of transverse anomalies and therefore no basis for 

choosing the set of Zoback (1983) or that of Mabey 

(1987). 

Using only the 1 :500,000 gravity map as a base, we 

prepared four colored working maps (R.L. Wheeler, unpublished maps, 1986). The working maps cannot be 

reproduced here because color and large size are essential to their use. However, examples of most of the kinds 

of features described below are visible in figure 2 and in 

figures 2 and 4 of Zoback (1983). The procedures by 

which the working maps were prepared are more important than the maps themselves. If the procedures are accepted as reliable, then their results are reliable and the 

working maps are unnecessary for the acceptance of the 

transverse anomalies of figure 2. 

The area covered, which is the central half of that 

mapped by Zoback (1983), spans long 111 OJO' -112 OJO' 

W., a strip about 85 km wide that is roughly centered 

on the north-trending Wasatch fault zone (fig. 1). The 

working maps were designed to abstract and emphasize 

aspects of the complete Bouguer gravity field that best 

identify and locate structures like steep normal faults, 

low-density sediments that fill basins bounded by such 

faults, and north-south changes in the geometries of such 

faults and basins. 

Of the four working maps, map 1 is a colored version of the contoured Bouguer gravity map and shows 

general features of the gravity field such as regional gradients and groups of connected lows and highs. Features 

seen on map 1 aided preparation of map 2, which shows 

shapes of inferred basins and intervening horsts. Features 

that we drew first on map 2 are trough and crest lines 

of elongate gravity lows and highs. We infer the lows to 

represent basins filled by low-density sediments, so the 

trough lines are interpreted as running approximately 

along the deepest parts of the basins. We infer the highs 

to represent bedrock horsts between basins, so the crest 

lines are interpreted as running approximately along the 

highest parts of the horsts. Map 2 also identifies saddles 

in the contoured surface that is represented by the gravity values of map 1. To avoid saddles that are minor, 

caused by single spurious values, or artifacts of the 

contouring algorithm, map 2 shows only saddles that are 
EXPlANATION 

-

~ 

-200- Contour, in mGal, on complete Bouguer gravity field. Contour interval is 10 mGal. 

Hachures identify contour around closed gravity low. Gravity contours were 

traced and simplified from an enlarged clear-film version of the gravity map of 

Zoback (1983, fig. 2), which M.L. Zoback supplied at a scale of 1:500,000 and a 

contour interval of 2.5 mGal 

c:::=::=:::::::J Transverse zone as drawn by Zoback (1983, fig. 3). Only those transverse zones 

that lie along the Wasatch fault zone are shown 

[=-==-=-~ Transverse anomaly drawn by procedures described in text 

Section of Wasatch fault zone that is intercepted by a transverse anomaly or its 

G 1 eastward projection. Fault traces simplified for clarity 

1----...::.....:....--l North-south extent of transverse anomaly G1, as projected east or west onto long 

112• W. Projections of anomalies G3, G4, G6, G7, and G10 are offset from long 

112•w. for clarity 

I 

10 

10 

I I 

20 

I 

20 

I 

30 MILES 

I 

30 KILOMETERS 

Figu~e 2: Transverse anomalies (G) identified from map of complete Bouguer 

g~~v1ty f1eld. The north-trending alignment of closed gravity lows, which have 

Cities_ and towns along their eastern edges, marks the sediment-filled valleys 

that he at the western base of the Wasatch Range and San Pitch Mountains. 

Bouguer Gravity Data 5 
controlled by more than three stations and outlined by 

more than two isogals, or which are defined by a trough 

line that crosses a crest line. We interpret saddles to represent either ends of basins or horsts, or places where basin 

depth or horst height is less than to the north and south 

in the same basin or horst. Map 3 shows inferred faults, 

as drawn along the steepest parts of long, moderately 

straight gradients in the gravity field. If the faults are 

assumed to be steep normal faults, then the sides toward 

the bounding gravity lows have dropped down. Ends of 

pairs of inferred faults that face each other and ends of 

gravity lows that are controlled by several stations were 

interpreted as representing the ends of fault-bounded 

basins. 

Comparison of colored maps 1-3 allowed inference 

of block-faulted structure. Separate and explicit representation of the various features shown on maps 1-3 greatly aided identification of ends of fault-bounded basins 

and of abrupt steps along strike in basin depth or horst 

height. Inferred ends of basins and horsts, inferred steps, 

and saddles were compiled on map 4. Thus, map 4 shows 

gravity saddles, north- and south-facing gravity gradients, 

north and south ends of east- or west-facing gravity gradients, and north and south ends of elongate gravity highs 

and lows. Map 4 is simplified as figure 2. 

Results 

The structure interpreted by Zoback (1983, fig. 3) 

and that interpreted independently by us are essentially 

identical (fig. 2). All large fault-bounded basins mapped 

in either analysis were also mapped in the other. There 

are small differences in the degree to which basins are 

connected and in details of the shapes of inferred bounding faults. Each analysis mapped one small basin not 

mapped by the other; neither small basin lies along the 

fault zone. Along the fault zone, Zoback mapped transverse zones or groups of transverse zones at ten places. 

We mapped ends or steps in fault-bounded basins at these 

same ten places, and only there. In an independent study 

of much the same Bouguer gravity data, Mabey (1987) 

inferred block boundaries at six of these ten places and 

nowhere else along the fault zone. 

The transverse zones of Zoback (1983) identify 

centers of transverse anomalies. However, we identified 

the boundaries of transverse anomalies. Boundaries have 

estimated accuracies of 1-2 km in most cases and less than 

5 km in all cases. Most boundaries of transverse 

anomalies have estimated accuracies of 11-22 percent of 

the median width of the transverse anomalies. Transverse 

anomalies as identified in map view were projected onto 

a north-south line running along long 112 ° W. (fig. 2, 

table 1). Such a projection aids comparison of several 

data types and does not distort shapes of transverse 

6 Segmentation of Wasatch Fault Zone, Utah 

anomalies much because the Wasatch fault zone also runs 

north-south (fig. 1). Schwartz and Coppersmith (1984) 

defined their segments only in Utah. However, our northernmost transverse anomaly extends 10 km north of the 

Utah-Idaho border at lat 42° N. (fig. 2); data north of 

the border will not be used. 

The transverse anomalies obtained from analysis 

of the gravity data satisfy all seven of the criteria 

previously listed. First, the transverse anomalies have the 

necessary resolution to detect segment boundaries because 

transverse anomalies and segment boundaries (Schwartz 

and Coppersmith, 1984) have similar sizes and spacings 

along the Wasatch fault zone. Second, the transverse 

anomalies can show whatever structures are responsible 

for large earthquakes along the Wasatch Front, because 

scarps from such earthquakes are present on the Wasatch 

fault zone, and the transverse anomalies record different 

blocks in the hanging wall of the fault zone. Third, the 

transverse anomalies can indicate the presence of a segment boundary in an explainable way. The transverse 

anomalies delineate boundaries between basins of different depths in the hanging wall of the fault zone. Differences along strike in the amplitudes of the gravity lows 

record different thicknesses of sedimentary fillings of the 

basins, and different sedimentary thicknesses record different ages or rates of faulting. The faulting occurred 

seismically. Therefore, the transverse anomalies may be 

interpreted as separating segments of the fault zone whose 

hanging walls have had different seismic histories. 

Fourth, the transverse anomalies are products of faulting 

and sedimentation that have occurred more or less continually over the last several million years, that continue 

today as expressed in seismicity, and that therefore may 

be expected to continue into the next decades to millenia. 

Fifth, the transverse anomalies are reproducible because 

Zoback (1983) and we separately produced transverse 

zones or anomalies that are essentially identical (fig. 2). 

Mabey (1987) separately reproduced six of the ten 

transverse anomalies. Sixth, the transverse anomalies 

were derived without reference to any other data type of 

this report. Seventh, the transverse anomalies were derived without reference to the proposed segment boundaries of Schwartz and Coppersmith (1984). 

AEROMAGNETIC DATA 

Introduction 

As is the case for gravity data, the term anomaly 

has several established usages in the study of magnetic 

data (Gary and others, 1972; Bates and Jackson, 1980). 

To avoid confusion with these established usages, we 

define a transverse anomaly in the magnetic data as a 

feature, usually a gradient or belt of narrow highs, and 
Table 1. Locations of transverse anomalies interpreted from gravity data 

[n.a., not applicable. Study area is 332 km long, north to south] 

Transverse North end of South end of Transverse Distance to next 

anomaly transverse transverse anomaly transverse anomaly 

number anomaly1 anomaly 1 width (km) to south (km) 

Gl 2-10 2 22 10 

G2 12 17 5 40 

G3 57 63 6 5 

G4 68 78 10 45 

GS 123 135 12 13 

G6 148 157 9 9 

G7 166 175 9 11 

G8 186 192 6 19 

G9 211 237 26 42 

GlO 279 292 13 34o 

Sum n.a. n.a. 98 234 

ln. 1stance south of Utah-Idaho border, in kilometers. 

2zoback (1983, fig. 2) showed that anomaly Gl extends 10 km north of 

Utah-Idaho border (north end of anomaly shown as negative number). 

Data north of border lie outside study area and are not used in this 

report, so width of Gl is taken as 2 km. 

3nistance to south edge of study area at lat 39° N. 

usually east-trending, that distinguishes a small part 

of the Wasatch fault zone from adjoining parts of the 

zone. 

Various workers have mapped total magnetic intensity for north-central Utah and surrounding regions 

(Mabey and others, 1964; Zietz and others, 1976; Stewart 

and others, 1977; Mabey and others, 1983). The digital 

magnetic data set of Mabey and others (1983) has its 

eastern edge as much as 30 km west of the Wasatch fault 

zone, and so cannot be used here, but the other maps 

show a general northward rise in total magnetic intensity along the Wasatch Front. Superimposed on this rise 

are east-trending gradients and east-trending belts of highamplitude, short-wavelength magnetic highs. Three of 

these east-trending features cross the Wasatch fault zone 

or abut it from the west. 

Total magnetic intensity can be examined along 

the Wasatch Front for transverse anomalies that might 

coincide with proposed segment boundaries and with 

transverse anomalies in other data types. The easttrending magnetic features are candidates for transverse 

anomalies. 

Procedures 

Mabey and others (1964) and Stewart and others 

(1977) noted a south-facing magnetic gradient near the 

south end of Great Salt Lake and attributed the small, 

intense magnetic highs north of the gradient to Precambrian metamorphic rock at shallow depths. This Precambrian rock is exposed at several places in and around the 

Lake (Hintze, 1980). West of the Wasatch fault zone the 

small, intense magnetic highs and exposures of Precambrian rock continue northward about to Brigham City 

(figs. 1, 3; Zietz and others, 1976). This broad belt of 

small, intense magnetic highs constitutes transverse 

anomaly M1 of figure 3. 

Aeromagnetic Data 7 
39°00' c__ ___ ____,_ __ ___L_ ___ _l_ __ _L____I 

8 Segmentation of Wasatch Fault Zone, Utah 

EXPLANATION 

-10.800- Contour, in gammas, on total magnetic intensity, traced and simplified from map of 

~ Zietz and others (1976). Contour interval is 100 gammas relative to arbitrary 

datum. Hachures identify contour around closed magnetic low. Where contours 

are close together north of Provo and northwest of Nephi, parts of some are 

omitted for clarity 

--- Part of north or south edge of a transverse anomaly, drawn by inspection of map of 

Zietz and others (1976) 

---- Part of trace of Wasatch fault zone that is intercepted by north and south edges of a 

e transverse anomaly 

1-----4 Location oftransverse anomaly M1 as projected from Wasatch fault zone east or 

west onto long 112" W. 

I 

10 

10 

I 

I 

20 

I 

20 

I 

30 MILES 

I 

30 KILOMETERS 

Figure 3. Transverse anomalies (M) identified from total aeromagnetic intensity. For more detail, see colored map of Zietz and others (1976), most parts 

of which have contour intervals of 20 gammas. 
Farther south, two other transverse anomalies are 

defined by narrow: east-trending belts of small, intense 

magnetic highs that are attributed to Tertiary intrusive 

and extrusive rocks (Mabey and others, 1964; Cook and 

Montgomery, 1972; Stewart and others, 1977, fig. 5). The 

Oquirrh-Uinta belt of Cook and Montgomery (1972) 

(Bingham-Park City zone of Mabey, 1987) crosses the 

Wasatch fault zone at about lat 40.5 ° N., and the Deep 

Creek-Tintic belt of the same authors (Tintic zone of 

Mabey, 1987) abuts the fault zone from the west at about 

lat 39.8 ° N. (fig. 3). 

The north and south edges of these three belts of 

magnetic highs were mapped by Stewart and others 

(1977). We transferred these six east-trending edges to the 

more detailed magnetic map of Zietz and others (1976) 

and redrew them to conform to the more detailed 

magnetic contours of that map and the map of Mabey 

and others (1964). The east-trending edges meet the 

Wasatch fault zone at points, which we projected west 

onto a north-south line at long 112 ° W. 

The magnetic data were collected along flight lines 

oriented east-west and spaced 2-4 mi (3.2-6.4 km) apart 

(Mabey and others, 1964; Zietz and others, 1976). Matching errors between adjacent east-trending flight lines can 

introduce east-trending artifacts into the mapped data. 

However, the east-trending belts that constitute the three 

transverse anomalies probably are not artifacts of matching errors between flight lines, for the following reasons. 

First, each belt is wide enough to involve several flight 

lines, and the edges of the belts cross flight lines obliquely 

in places. Second, belts are defined by closed magnetic 

highs with magnetic relief of 200-500 gammas, which is 

large enough to survive matching errors. Third, such 

closed highs are abundant inside the magnetic belts, 

but scarce outside the belts, indicating a real difference 

between magnetic susceptibilities, depths of burial 

of magnetic rocks, or both, inside and outside the 

belts. 

The Wasatch Front lies at middle magnetic latitudes, so magnetic inclination causes a magnetic high to 

appear south of a magnetized rock body that causes the 

high (Vacquier and others, 1951, fig. 3; Nettleton, 1971, 

p. 74-77, figs. 55-57). Model calculations by T.G. 

Hildenbrand (oral commun., 1985) for a simple magnetic 

source with steep sides and uniformly induced magnetization (Vacquier and others, 1951, p. 3-15) indicate that 

this offset is about 8 km. For magnetic models that are 

equidimensional in map view and for the southward offset of 8 km, the dimensionless diagrams of Vacquier and 

others (1951, figs. A53-A60, A67-A70) imply depths to 

magnetic rock of 0-6 km, which could represent small 

intrusions that are exposed or buried beneath several 

kilometers of graben-filling sediments. For sources that 

are elongate east-west, implied depths are 6-14 km, which 

could represent upper- or mid-crustal igneous bodies from 

which shallow or now-exposed intrusions rose. These 

depths are consistent with local geology. Accordingly, 

locations of transverse anomalies were offset 8 km northward to approximate the true locations of causative rock 

masses and structures. 

Results 

Six points along long ll2 ° W. represent the 

latitudes of the intersections of the Wasatch fault zone 

with the north and south edges of the three belts of 

magnetic highs (fig. 3, table 2). On inspection of the 

magnetic map of Zietz and others (1976), we found no 

other magnetic features that could constitute transverse 

anomalies in the study area. We estimate the accuracy 

of the distances shown in table 2 to be within 2-4 km, 

or 8-16 percent of the median width of the transverse 

anomalies, based on shapes and orientations of magnetic 

contours and on orientations and spacings of flight 

lines. 

The transverse anomalies of table 2 meet the seven 

criteria listed above. First, the transverse anomalies and 

segment boundaries are similar enough in size that all 

could be represented on maps of the same scale (figs. 1, 

3). The edges of the transverse anomalies have estimated 

locational accuracies of 2-4 km, which is accurate enough 

to detect segment boundaries. 

Second, aeromagnetic data delineate the geometries 

of magnetic bodies of rock, and so are commonly used 

to identify igneous rocks and faults. Such structures are 

of the same sizes and at the same depths as the upper parts 

of faults that are likely to generate large earthquakes. 

Third, the aeromagnetic data are likely to record 

the presence of a segment boundary in some explainable 

way. For example, transverse anomaly M1 (table 2) extends from about Salt Lake City to about Brigham City 

(fig. 3). Mabey and others (1964) attributed the small, 

intense magnetic highs within M1 to Precambrian 

metamorphic rock that is exposed (Hintze, 1980) or 

buried at shallow depths. Then the east-trending magnetic 

gradient that defines the south edge of M 1 might arise 

from a fault, south of which the Precambrian rocks are 

dropped down. Such a fault might have formed during 

Tertiary extension, as a lateral ramp during Cretaceous 

thrusting of the Precambrian rocks, or earlier. Whatever 

its age and origin, the gradient extends eastward to the 

Wasatch fault zone, and so could record segmentation 

of the hanging wall of the fault. Similar explanations 

could apply to the more poorly expressed northern edge 

of M1. Similarly, the other two transverse anomalies are 

attributed to buried and exposed igneous rocks of Tertiary age (Mabey and others, 1964; Stewart and others, 

1977; Hintze, 1980). Localization of igneous activity in 

elongated belts might indicate that the crust there was 

Aeromagnetic Data 9 
Table 2. Locations of transverse anomalies interpreted from aeromagnetic data 

[n.a., not applicable. Study area is 332 km long, north to south] 

Transverse North end of South end of Transverse Distance to next 

anomaly transverse transverse anomaly transverse anomaly 

number anomaly1 anomaly1 width (km) to south (km) 

Ml 54 130 76 18 

M2 148 173 25 54 

M3 227 246 19 286 

Sum n.a. n.a. 120 158 

1Distance south of Utah-Idaho border, in kilometers. Values are 

offset 8 km northward to correct for effect of magnetic declination. 

2Distance to south edge of study area at lat 39° N. 

unusually weak and perhaps fractured to depths at which 

magma was generated. If so, then at these two transverse 

anomalies the hanging wall of the Wasatch fault zone 

might still be weak and thus be segmented. 

Fourth, the aeromagnetic data reflect processes that 

operate over times that allow inferences to be drawn 

about near-future seismicity, because the transverse 

anomalies can be interpreted to identify faults or other 

fractures that could still exist as weak zones and could 

divide the hanging wall of the Wasatch fault zone into 

blocks that might slip independently in the next large 

earthquakes. 

Fifth, the transverse anomalies are reproducible 

because they were all identified by Mabey and others 

(1964), Stewart and others (1977), and us. 

Sixth, the transverse anomalies in the aeromagnetic 

data were derived solely from the aeromagnetic maps of 

Mabey and others (1964), Zietz and others (1976), and 

Stewart and others (1977). This lack of influence from 

other data types holds even though the transverse 

aeromagnetic anomalies, once identified, were verified 

by comparison with gravity, geological, and mineraldeposit data (Mabey and others, 1964; Stewart and 

others, 1977; Hintze, 1980). 

Seventh, the transverse anomalies were derived 

without reference to the proposed segment boundaries 

of Schwartz and Coppersmith (1984), because the 

transverse anomalies were identified by Mabey and 

others (1964) and Stewart and others (1977) before 

Swan and others (1980) and Schwartz and Coppersmith 

( 1984) applied the segmentation hypothesis to the 

Wasatch fault zone and selected their particular proposed 

boundaries; Schwartz and Coppersmith (1984) do not 

10 Segmentation of Wasatch Fault Zone, Utah 

mention magnetic data in their analysis of the fault 

zone. 

EARTHQUAKE EPICENTERS 

Introduction 

Smith (1972, 1974) observed that two parts of the 

Wasatch Front that lie north and south of Salt Lake City 

had fewer earthquakes than did the parts that lie north 

of Ogden, around Salt Lake City, and south of Utah Lake 

(fig. 1). Cook and Smith (1967), Sbar and others (1972), 

and Smith and Sbar (1974) had noted the southern gap 

in seismicity, but Smith documented both gaps. Smith 

(1972) mapped epicenters of earthquakes with magnitudes 

of 3 or greater that occurred from 1961 to 1969. Smith 

(1974) mapped epicenters of earthquakes with magnitudes 

greater than 2 that occurred from July 1962 to September 

1974 and showed a space-time plot of those epicenters 

that fell near the Wasatch fault zone. All three figures 

show both gaps, as do more recent epicentral maps and 

space-time plots for various time intervals from 1962 to 

July 1986 (Arabasz and others, 1980; Arabasz, 1984; 

Smith and Richins, 1984; W.J. Arabasz, oral communs., 

1986). 

We use the term gap in seismicity, because such 

gaps are what is observed in the figures cited here. We 

do not use the term seismic gap, because it connotes a 

seismically quiescent part of a fault zone that is more 

likely to yield in a large earthquake than are adjacent, 

more seismically active parts of the same fault zone. 

Earthquake epicenters can be examined along and 
near the Wasatch fault zone for anomalies. The four ends 

of the two gaps in seismicity are candidates for anomalies 

and mark places in the fault zone across which the level 

of earthquake activity near the zone changed markedly. 

Procedures 

Seven versions of the two gaps in seismicity show 

the gaps to be centered about 100 km and about 200 km 

south of the Utah-Idaho border (fig. 4). Arabasz and 

others (1980) mapped epicenters from an earthquake 

catalogue that was revised from the one available to Smith 

(1972, 1974). Arabasz and others (1980) summarized the 

two gaps as originally shown by Smith (1974). Column 

1 of figure 4 and Smith's (1972, 1974) three versions of 

the gaps all match the more recent versions and will not 

be used further. The similarity between all these versions 

of the ends of the gaps indicates that the existences and 

locations of anomalies Se1-Se4 are insensitive to differences between authors, earthquake catalogues, time intervals, and whether epicenters are plotted on a map or 

in a space-time diagram. 

Each column in figure 4 represents the anomalies 

as points, but the anomalies have widths of several 

kilometers, and these widths can be estimated. The 

anomalies separate parts of the Wasatch Front that have 

had different rates of earthquake occurrence, at least over 

the two decades of record. If these differences in occurrence rates reflect differences in longer term rates of 

seismic slip, then the anomalies could represent changes 

in the amounts of seismic slip on the Wasatch fault zone, 

which might occur on cross faults. Any departures of the 

cross faults from east-west strikes, vertical dips, and 

perfectly planar shapes will give the anomalies widths exceeding zero. If the anomalies do not represent cross 

faults, then they might represent strained or complexly 

faulted rocks that link hanging wall blocks that have 

dropped down different amounts. In this case, the anomalies would also have widths exceeding zero. Therefore, 

the range in locations of a particular anomaly in columns 

2-7 of figure 4 may be taken to represent some combination of the width of the anomaly and locational uncertainty of anomaly ends. 

One or more of columns 2-7 might overestimate 

the width of a gap. Overestimation would happen if no 

earthquakes occurred near but outside a gap during the 

time interval on which one of columns 2-7 is based (fig. 

5A). Then the gap and the associated anomaly would appear wider than they are. Alternatively, one or more columns might underestimate the width of a gap, because 

the gaps are not completely aseismic; several of the 

epicenter maps and space-time plots show isolated earthquakes that occurred within gaps. Even with these isolated 

earthquakes, the gaps remain markedly less seismically 

2 3 4 5 6 7 o.-.-~--r-.-~--r-.-----------~4T 

62 42 69 78 66 60 59 High seismicity 

ffi 50- /;. o _.,. ',, ~------ ANOMALY Se1 25 ....... -:.,... ......... m 64 91 59 o6 78 62 61 Low seismicity 

~ 100f-

< _ ANOMALY Se2 - 41' T 2-T9X2Jr', ,.If" 21} High seismicity 

~ 150>-- 'J,., -:J..,~F1} .. - ANOMALY Se3 

~ 65 79 75 67-71 70 72 Low seismicity 

0 

F= 200f-

:::> -----.. /--- ANOMALY Se4 

~ ........... r---.1/ 

(/) 250 r- ~ 

- 40' 

~ ~14 111 106 94 90 118 119 High seismicity 

::.:E g 300fS2 -+--r-~~--+-~~~332 km - 39' 

Figure 4. Changes in earthquake abundance along Wasatch 

Front, from north (top) to south (bottom). The seven vertical, 

broken lines depict seven estimates of the locations of changes 

in abundance of instrumental earthquakes. Solid lines show 

parts of the Front with many earthquakes located on or near 

the trace of the Wasatch fault zone, and gaps in these lines 

show parts of the Front with few or no earthquakes. Numerals 

to left of the solid lines and in the gaps show lengths of these 

parts of the fault zone, in kilometers, as projected onto a northsouth line along long 112° W. Dashed lines connect the seven 

different estimates of the locations of each anomaly (change 

in earthquake abundance; Se). 

Large numerals above the solid lines give sources of the 

seven estimates. 1, map of epicenters of earthquakes with 

magnitudes greater than 2, for july 1962-September 1974, as 

shown by Smith (1974, fig. 4) and interpreted by Arabasz and 

others (1980, fig. 3). 2, map of epicenters for October 

1974-june 1978, as shown by Arabasz and others (1980, fig. 

3) and interpreted by us. 3, revised map of epicenters for july 

1962-September 1974, as shown by Arabasz and others (1980, 

fig. 4) and interpreted by us. 4, space-time plot of epicenters 

within 10 km oftrace of Wasatch fault, for july 1962-june 1978, 

as shown and interpreted by Arabasz and others (1980, fig. 5). 

5, same as 4 except interpreted by us. 6, map of epicenters 

for july 1978-December 1983, as shown by Arabasz (1984, fig. 

5) and interpreted by us. 7, space-time plot of epicenters within 

10 km oftrace of Wasatch fault, for july 1962-December 1983, 

as shown by Smith and Richins (1984, fig. 10) and interpreted 

by us. 

active than the surrounding parts of the fault zone. However, if such an isolated earthquake occurred in a gap but 

near its end, then an earthquake catalogue for a few years 

that contained the time of the isolated earthquake could 

underestimate the width of the gap (fig. 5B). If the 

resulting column in figure 5 were combined with other 

columns that did not include such isolated earthquakes 

within gaps, the anomaly at the end of the gap would 

appear wider than it should. 

Earthquake Epicenters 11 
NORTH SOUTH 

A 

* * * * * * 

2 

* * * * * * 

Gap Anomaly I 

3 

* * * * 

Gap Anomaly 

B 

1 

* * * * * * 

2 * * * * * * 

Gap I Anomaly I 

3 

* * * * * 

Gap Anomaly 

Figure 5. Overestimation and underestimation of widths of 

gap in seismicity and their effects on anomaly width. A, 

Overestimation of gap width causes overestimation of anomaly width. Schematic vertical cross-sections along Wasatch Front 

show locations of earthquakes (asterisks) during time intervals 

1, 2, and 3. Gaps and anomalies inferred from these sequential locations are represented below 2 and 3. Time intervals 

1 and 2 produce two different estimates of location of south 

end of gap. Assume these estimates are correct. The range of 

these estimates is the true width of the anomaly, as shown 

below 2. During the later time interval (3) no earthquakes are 

detected in the northern end of the seismically active part of 

the Wasatch Front, so the south end of the gap appears to be 

too far south and the gap appears wider than it is. The width 

of the anomaly is overestimated from 1, 2, and 3, as shown 

below 3. 8, Similarly, underestimation of gap width also causes 

overestimation of anomaly width. During the later time interval (3) an isolated earthquake is detected inside the previously 

12 Segmentation of Wasatch Fault Zone, Utah 

Thus, both overestimation and underestimation of 

gap width cause anomalies to appear wider than they 

should. To guard against such errors we deleted any 

estimates of the location of the anomaly that make the 

anomaly seem unusually wide. These aberrant estimates 

were identified by inspecting histograms of the six 

estimates for each anomaly. For each anomaly, one or 

two estimates fall far above or below the others, are 

separated from them by a wide interval that contains no 

intermediate estimates, and would double or nearly 

double the anomaly width if the aberrant estimate or 

estimates were retained. For example, columns 2-7 place 

anomaly Se1 at 42, 59, 60, 66, 69, and 78 km south of 

the Utah-Idaho border (fig. 4). The value of 42 km is 

from column 2. The other five estimates are more tightly clustered, and give a width of 19 km for anomaly Sel. 

Adding the estimate from column 2 would widen the 

anomaly another 17 km, so this estimate is deleted and 

anomaly Se1 is taken as 19 km wide. Similarly, anomalies 

Se2-Se4 are taken to be 13, 10, and 13 km wide, 

respectively, by deleting the estimates of columns 4 

and 5. 

Once aberrant estimates of the location of an 

anomaly have been deleted, the remaining estimates 

define not only the width of the anomaly but also its north 

and south ends. For example, for anomaly Se1 five 

estimates remain after deleting that of column 2. The 

five estimates span 59-78 km south of the Utah-Idaho 

border, because column 7 places the north end of the 

anomaly a minimum of 59 km south of the border and 

column 4 places the south end at a maximum of 78 km 

(fig. 4). 

Results 

The ends of seismic anomalies (table 3) were identified from earthquakes that have occurred since 1962 and 

mostly since 1974. The location of each anomaly end was 

estimated from one of the columns of figure 4. Each column's estimate of the location of the end of a gap in 

seismicity was controlled by one or a few epicenters. 

Arabasz and others (1980, p. 1482) estimated from 

various tests that calculated locations of epicenters might 

be in error by 5 km or less. Accordingly, values in table 

6 are assumed to be accurate to within 5 km, or within 

38 percent of the median anomaly width. 

The anomalies of table 3 meet the seven criteria 

listed above. First, the epicentrallocations have estimated 

accuracies small enough to give them the resolution to 

inactive gap, near its south end. The south end of the gap appears to be too far north and the gap appears narrower than 

it is. 1, 2, and 3 cause the width of the anomaly to be 

overestimated, as shown below 3. 
Table 3. Locations of seismological anomalies 

[n.a., not applicable. Study area is 332 km long, north to south] 

Anomaly North end South end Anomaly Distance to next 

number of anomaly 1 of anomaly 1 width (km) anomaly to south (km) 

Sel 59 78 19 42 

Se2 120 133 13 8 

Se3 141 151 10 62 

Se4 213 226 13 2106 

Sum n.a. n.a. 55 218 

1Distance south of Utah-Idaho border, in kilometers. 

2Distance to south edge of study area at lat 39° N. 

detect something the size of a segment boundary. Second, 

the earthquakes occurred from 1962 to 1983 and were 

mostly of magnitude 4 or smaller (Arabasz and others, 

1980; Arabasz, 1984; Smith and Richins, 1984). These 

ranges of time and magnitude are large enough, and the 

epicentrallocations are numerous and accurate enough, 

that the epicentral patterns of small earthquakes along 

the Wasatch Front are capable of delineating the locations and map shapes of whatever structures are likely 

to be responsible for large earthquakes. We do notrequire that the observed earthquakes actually are related 

to likely causative structures of large earthquakes, but 

only that the observed earthquakes be able to delineate 

the causative structures if a relationship exists. Third, 

spatial and temporal patterns of instrumental epicenters 

are likely to change in some way across segment boundaries. The ends of the gaps in seismicity could represent 

segment boundaries by recording differences in rates of 

seismic slip or in ratios of seismic to aseismic slip between 

fault blocks that constitute adjacent segments. Fourth, 

the instrumental earthquake data span about two 

decades, and the gaps appear consistently in catalogues 

that cover 1962-1974, 1974-1978, and 1978-1983 (fig. 4). 

This duration and stationarity of the gaps indicates that 

the gaps are likely to persist at least for the next few 

decades. Fifth, the gaps are reproducible, because they 

have been recognized by several workers. Sixth, the gaps 

were derived from epicentral locations alone, without 

reference to any other data type of this report. Seventh, 

the gaps were derived without reference to the proposed 

segment boundaries because the gaps were noted by 

Smith (1972, 1974) before segmentation was proposed 

for the Wasatch fault (Swan and others, 1980) and 

before Schwartz and Coppersmith (1984) proposed their 

boundaries, and also because suggested locations of gap 

ends have changed little since 1972 (fig. 4). 

SALIENTS 

Introduction 

At four places along the Wasatch fault zone large 

projections of exposed footwall bedrock extend westward 

into the hanging wall. These projections are called salients 

(fig. 6; spurs of Gilbert, 1928; Zoback, 1983, p. 9; 

Bruhn, Gibler, and Parry, 1987; Bruhn, Gibler, 

Houghton, and Parry, 1987). The Pleasant View and 

Salt Lake salients consist of fault-bounded blocks that 

have dropped down with respect to the main part of the 

footwall that flanks them on the east, but have dropped 

less than has the hanging wall that flanks them on the 

west. At the Traverse Mountains salient the fault zone 

bulges sharply westward, with a partly down dropped 

block adjacent to the bulge on the southwest. The Payson 

salient consists of two south-trending ridges across which 

fault slip is distributed discontinuously as the fault zone 

steps southwestward across the ridges. 

At these four salients the trace of the Wasatch fault 

zone departs from its usual aspect of a single, comparatively narrow fault zone with a straight or simply 

curved trace that runs along the west foot of the range 

front of the Wasatch Range (figs. 1, 6). King (1983) suggested that fractured rocks around sharp bends or forks 

in faults could initiate or halt seismic ruptures. At the 

salients the fault trace bends or forks, and rock exposed 

in the salients is broken by unusually abundant small 

Salients 13 
GREAT SALT 

LAKE 

Pleasant View 

salient 

Salt Lake 

salient 

39°00' l___ ______ _l__ ______ _j 

14 Segmentation of Wasatch Fault Zone, Utah 

EXPlANATION 

Mapped or inferred normal fault, bar and ball on downthrown side. Traced and 

simplified from Hintze (1980), with modifications after Van Horn (1972), Miller 

(1980, 1982), Zoback (1983, figs. 1, 4, p. 9), Davis (1983a, b, 1985), Crittenden 

and Sorensen (1985), Scott and Shroba (1985), Bruhn, Gibler, Houghton, and 

8 Parry (1987), and M.N. Machette (oral common., 1986) 

1- - - - ---l Westward projection of anomaly Sal into long 112 o W. 

Geologic contact of Little Cottonwood stock, from Hintze (1980) 

I 

10 

10 

I 

I 

20 

I 

20 

I 

30 MILES 

I 

30 KILOMETERS 

Figure 6. Salients (Sa) of the Wasatch fault zone. 
faults (fig. 6 and maps and papers cited there). Accordingly, the geometry of the Wasatch fault in map view is 

a type of data that can be examined for anomalies. The 

salients are candidates for these anomalies. 

Procedures 

The four salients are large enough that the 

1: 500,000-scale Geologic Map of Utah (Hintze, 1980) 

shows them. Larger scale maps and topical reports provide details of the fault geometries in and around the 

salients. The expressions of salients in fault geometry and 

in exposed bedrock geology are distinctive enough that 

probably no large exposed salients have been missed. Support for this statement comes from consideration of three 

other westward-projecting footwall spurs that Gilbert 

(1928) discussed together with the salients of figure 6. The 

Madsen spur (Gilbert, 1928) nestles in a small reentrant 

of the range front and covers about 5 km2 (Davis, 

1985). Evidence for the existence of a fault-bounded 

bedrock salient at the Madsen spur is the issuance of hot 

springs from beneath an outcrop of limestone conglomerate or of mixed limestone and conglomerate, about 

3 km west of the range front (Gilbert, 1928; Davis, 1985). 

From this observation Gilbert inferred the presence of 

breccia on a fault at the west end of the Madsen spur. 

However, mapping by C.G. Oviatt of pre-Cenozoic 

bedrock and Cenozoic sediments at and around the spur 

led Oviatt to conclude that the spur is a large rockslide 

(Oviatt, 1985, p. 53; C.G. Oviatt, written and oral 

communs., 1985, 1986). The geomorphic expression of 

the Honeyville spur led Gilbert (1928) to conclude that 

it is probably a landslide, covering less than 2 km2 ~ instead of a bedrock salient. Recent mapping confirms this 

(C.G. Oviatt, written commun., 1986; S.F. Personius, 

oral and written communs., 1985). Finally, Gilbert (1928) 

mentioned a small unnamed spur about 5 km north of 

Ogden. The spur, which exposes gneiss over most of its 

extent, is about 0.5 km wide and extends about 0.5 km 

west from the range front; the mapped trace of the 

Wasatch fault bulges westward around the west side of 

the spur (Crittenden and Sorensen, 1985). This spur is 

too small to constitute a salient in the sense of anomalies 

Sa1-Sa4 of figure 6. Therefore, these three other spurs 

are either too small or too likely to be of surficial origin 

to be grouped with the four salients shown in figure 6. 

However, other salients might be hidden beneath 

the upper Cenozoic sediments that bury most of the hanging wall of the Wasatch fault zone. For example, Cook 

and Berg (1961, plate 13) inferred from a west-facing 

gravity gradient that the Warm Springs fault (fig. 6) extends north about 14 km from the west tip of the Salt 

Lake salient before rejoining the main trace of the 

Wasatch fault. Similarly, Van Horn (1972; see also review 

by Scott and Shroba, 1985) suggested that for the past 

5,000 yr most faulting south of Salt Lake City has occurred on the East Bench fault instead of on the Wasatch 

fault along the range front (fig. 6). Thus, the Salt Lake 

salient might be flanked by two other unrecognized 

salients, one on the north that has dropped down too far 

to be expressed in exposed bedrock geology and one on 

the south that is too young for such expression. Such 

hidden salients might eventually be detected by combined 

use of gravity, seismic reflection or refraction, and 

geologic data. Such combined investigations are beyond 

the scope of this report. Also, the statistical analysis of 

Wheeler and Krystinik (1987a, d) requires that the different data types that are used must be examined independently of each other, in order that anomalies in any 

one data type are independent of anomalies in any other 

data type. 

Results 

Anomaly Sal (fig. 6, table 4) is the Pleasant View 

salient of Eardley (1944), also called the Pleasant View 

spur by Gilbert (1928) and the Warm Springs salient by 

Morisawa (1972, p. D13). This salient is nearly surrounded by mapped and inferred normal faults. It exposes Cambrian rocks over 3-4 km2 that are separated 

from the Wasatch Range by a 1-2-km-wide cover of 

Quaternary sediments (Gilbert, 1928; Davis, 1985; Crittenden and Sorensen, 1985; Personius, 1986). The thin 

veneer of sediment that covers most of the salient contains an unusually wide zone of numerous short, discontinuous, poorly expressed fault traces, scarps, grabens, 

photolineaments, and related features all of diverse orientations (Cluff and others, 1970; Miller, 1980; Crittenden 

and Sorensen, 1985). 

Anomaly Sa2 is the Salt Lake salient of Eardley 

(1944), also called the City Creek spur by Gilbert (1928) 

and the Ensign Peak salient by Snay and others (1984, 

p. 1115), and termed another Warm Springs salient by 

Morisawa (1972, p. D13). This salient is mostly bounded 

by mapped and inferred normal faults. Within the salient, 

Cambrian to Eocene rocks are exposed almost continuously to their eastern fault contact with the Wasatch 

Range (Gilbert, 1928; Hintze, 1980; Davis, 1983a; Bryant, 

1984). Scarps of latest Quaternary age (Scott and Shroba, 

1985) are present in two areas on the south side of the 

salient and are interspersed with less prominent scarps 

and inferred fault traces elsewhere around the salient 

(Cluff and others, 1970; Miller, 1980). 

Anomaly Sa3 is the Traverse Mountains salient, 

also called the Traverse spur by Gilbert (1928), at which 

upper Paleozoic sedimentary rocks and Eocene to Oligocene intrusive rocks in the footwall (Hintze, 1980; Davis, 

1983b) adjoin upper Cenozoic basin-filling sediments 

Salients 15 
Table 4. Locations of anomalies interpreted from fault geometry 

[n.a., not applicable. Study area is 332 km long, north to south] 

Anomaly North end South end Anomaly Distance to next 

number of anomaly 1 of anomaly 1 Width (km) anomaly to south (km) 

Sal 70 76 6 48 

Sa2 124 134 10 31 

Sa3 165 169 4 48 

Sa4 217 230 13 2102 

Sum n.a. n.a. 33 229 

1Distance south of Utah-Idaho border, in kilometers. 

2Distance to south edge of study area at lat 39° N. 

(M.N. Machette, unpublished mapping, I986) across 

the Wasatch fault zone. The fault zone bows sharply 

west to pass around the Little Cottonwood stock (Lone 

Peak salient of Gilbert, I928) in the footwall. Traverse 

Mountains salient itself is bounded by faults on the northwest and southeast sides and is in fault contact on its 

northeast with the Little Cottonwood stock of the 

Wasatch Range. Where the Wasatch fault crosses bedrock 

at the salient, the fault becomes degraded (Cluff and 

others, I973) and loses the steep, little-modified scarps 

that led Anderson and Miller (I979) to suggest a Holocene 

age for scarps of the Wasatch fault zone north and south 

of the salient. 

Anomaly Sa4 is here named the Payson salient, 

which Gilbert (I928) and Loughlin (1913) observed to 

comprise a pair of north-trending, north-sloping bedrock 

ridges west of Dry Mountain, south of Payson and south 

of Santaquin. Between the ridges and north and south 

of them the Wasatch fault zone is continuous and has 

well-defined scarps of Holocene age (Cluff and others, 

I973; Anderson and Miller, I979; Machette, I984). The 

ridges expose complexly faulted, folded, and tilted rocks 

of Precambrian through Tertiary ages. Four to five 

kilometers southwest of Santaquin bedrock exposures are 

nearly continuous across the northward projection of the 

Wasatch fault zone (Hintze, I980; Davis, I983b). The 

nearly continuous exposures reveal the presence of a partly buried bedrock ridge trending southwest from 

Santaquin, similar to the Traverse Mountains salient. Different authors drew the trace of the main fault through 

the bedrock in the two ridges and under the adjacent 

Quaternary sediments in markedly different ways (reconnaissance map of Morisawa, I972, fig. I; compilation of 

Anderson and Miller, I979; bedrock geologic map of 

16 Segmentation of Wasatch Fault Zone, Utah 

Metter, 1955, as compiled by Davis, I983b; M.N. 

Machette, unpublished mapping, 1986). 

The locations of anomaly ends (table 4) are uncertain for two reasons. First, cartographic errors from 

drafting and measurement during our compilation might 

move each anomaly end as much as I km north or south. 

Second, larger uncertainties in the locations of north and 

south ends of anomalies arise from ignorance of the subsurface geometries of the salients. However, these uncertainties can be estimated. Because the faults that bound 

the salients formed in extension, dip slip has been normal 

and the faults are unlikely to dip inwards under the 

salients. The north-south widths of the anomalies in the 

subsurface will be no smaller than the exposed widths of 

table 4, but they could be larger. Smith and Richins (I984, 

p. 97, figs. 9, 11) estimated that large normal-faulting 

earthquakes of the Basin and Range province nucleate 

at depths of about I5 km. If planar salient-bounding 

faults dip 45 °-60°, their down-dip lengths must be 

I7-2I km to reach depths of I5 km. In map view large 

changes in fault geometry can take place over horizontal 

distances of I7-2I km (fig. 6). If such large geometric 

changes can also take place down the dips of the faults 

that bound the salients, then it would be difficult to 

estimate the subsurface widths of the salients from exposed widths. · 

However, down-dip changes in geometry of the 

Wasatch and related faults likely are smaller than alongstrike changes. From oil well, seismic reflection, gravity, 

and topographic data at six places along the Wasatch 

fault zone, Zoback (I983, fig. 6) estimated that minimum 

structural relief across the fault zone is 2.6-4.0 km. Parry 

and Bruhn (1986, I987) examined fluid inclusions in fault 

rock at the southern and western edges of the Little 
Cottonwood stock and a stratigraphic reconstruction, and 

they concluded that uplift on the fault zone was at least 

11 km. Dips of 45 °-60° and uplifts of 2.6-4.0 km and 

of 11 km would require dip slip of 3-16 km. Dip slips 

this large might have broken asperities and otherwise 

smoothed some irregularities in the down-dip geometry 

of the Wasatch fault zone. Little such smoothing of the 

along-strike geometry is likely to have occurred, because 

the irregular shape of much of the fault trace around the 

salients (fig. 6) argues against large amounts of strike slip. 

Thus, the fault zone is likely to change its geometry less 

down its dip than along its strike. Accordingly, it is worthwhile to estimate subsurface widths of salients from 

exposed widths. 

The Pleasant View salient nestles in a southwestfacing embayment in the footwall of the Wasatch fault 

zone. The north and south ends of the embayment are 

about 5 km beyond the ends of the salient (Davis, 1985; 

Crittenden and Sorensen, 1985). If the salient at depth 

merges smoothly into the ends of the embayment, then 

the salient could be as much as 10 km wider at depth than 

its exposed width of 6 km. With cartographic errors of 

1 km or less at each end, the Pleasant View salient might 

be as narrow as 4 km or as wide as 18 km. 

The Salt Lake salient might be several times as wide 

in the subsurface as at ground level. Van Horn (1972), 

Scott and Shroba (1985), and earlier workers mapped the 

East Bench fault on the south side of the salient (fig. 6) 

about 5 km west of the main trace of the Wasatch fault 

and extending southward about 15 km beyond the south 

end of the salient. Van Horn (1972) estimated that the East 

Bench fault moved within the last 5,000 yr. Scott and 

Shroba (1985, p. 8-9) give evidence for middle to late 

Quaternary movement on this fault. From a shallow 

seismic-reflection profile, Crone and Harding (1984, 

p. 249-252) estimated that the East Bench fault had as 

much as 85 m of dip slip during Quaternary time. For 

the north side of the salient, Pavlis and Smith (1980) compiled several estimates of the northward extent of the 

Warm Springs fault (fig. 6). The compilation indicates that 

the Warm Springs fault might extend in the subsurface 

about as far north of the salient as the East Bench fault 

extends south of it. Therefore, the Salt Lake salient might 

be bounded on north and south by two blocks that have 

dropped down enough that they are buried today. The two 

buried blocks and the comparatively high salient might 

act together to control large seismic ruptures. If each 

buried block has a north-south width of 15 km, then with 

cartographic uncertainties the Salt Lake salient might be 

as narrow as 8 km or as wide as 42 km in the subsurface. 

The Traverse Mountains salient is bounded by 

border faults that dip northwest and southeast (fig. 6). 

Zoback (1983, p. 9) inferred the existence of the fault on 

the northwest side of the salient from a steep, northwestfacing gravity gradient and from drilled depths to 

consolidated rock (Mattick, 1970). Hunt and others (1953, 

p. 38, plate 1) mapped a fault along the southeast side 

of the salient. M.N. Machette (unpublished mapping, 

1986) found neither fault to be exposed nor to show 

evidence of late Pleistocene slip. Southwest-plunging slip 

vectors on the Wasatch fault zone north of the salient, 

together with the northeast strike of the northwest-dipping 

border fault, constrain the border fault to dip steeply 

(Bruhn, Gibler, Houghton, and Parry, 1987; Bruhn, 

Gibler, and Parry, 1987). Dip magnitude of the southern 

border fault is unknown. We assume that both border 

faults dip 45 Q.60 ~ to obtain a conservatively large estimate 

of the maximum subsurface width of the salient. The 

northern border fault strikes N. 61° E., and the southern 

border fault strikes N. 68 ° W. (Miller, 1982; Davis, 1983b ). 

For these strikes and dips, both faults would reach depths 

of 15 km between 9 and 15 km northwest or southeast 

of their traces. By projecting these distances into a northsouth line, the northern border fault is calculated to reach 

a depth of 15 km between 8 and 13 km farther north than 

the point at which the trace of the border fault meets the 

trace of the Wasatch fault (fig. 6). Similarly, the southern 

border fault could extend at depth 8-14 km farther south 

than its outcrop at the Wasatch fault. With cartographic 

uncertainties the northern end of anomaly Sa3 might be 

as much as 14 km farther north than shown in figure 6, 

and the southern end might be as much as 15 km farther 

south than shown. Anomaly width could then be as small 

as 2 km or as large as 33 km. 

At the Payson salient, geologic maps (Metter, 1955; 

Hintze, 1980; Davis, 1983b) show small normal faults that 

extend south along the west side of Dry Mountain, with 

bedrock exposed on both sides of the small faults. 

Stratigraphic separation on the largest of these faults 

decreases southward. Within 5 km south of the southern 

edge of the anomaly some of the normal faults are lost 

in a thick, undifferentiated mass of Pennsylvanian and 

Permian strata, and the other normal faults bend 

southwestward to merge with the main strand of the 

Wasatch fault zone. North of Payson, these normal faults 

disappear under the Quaternary sediments around Utah 

Lake. The north end of the anomaly is taken as the north 

end of the large normal fault on the west side of Dry 

Mountain (fig. 6). Mapping by M.N. Machette (oral 

commun., 1986) shows that this fault extends northeast 

in Quaternary sediments to the southeastern outskirts of 

Payson, where it turns toward the north and borders a 

bedrock-cored ridge. Thus, the anomaly is 13 km wide 

along a north-south line. To protect against unmapped 

faults at the south end of the anomaly and buried fault 

tips at both ends, we arbitrarily assumed that the anomaly 

might extend another 10 km beyond its ends as shown 

in figure 6. Then, with cartographic uncertainty of 1 km, 

the anomaly could be 2 km narrower or 22 km wider than 

shown in table 4, for a width of 11-35 km. 

Salients 17 
The geometric anomalies meet the seven criteria 

listed above. First, the data can detect something the size 

of a segment boundary, because the four salients are of 

about the same sizes as are the segment boundaries of 

Schwartz and Coppersmith (1984). The salients are 

recognized from the geologic compilation of Hintze 

(1980), which depicts and distinguishes many features as 

small as 1 km. 

Second, the data are able to reflect whatever structures are responsible for large earthquakes along the 

Wasatch Front. The salients are defined by the shapes 

of the traces of the Wasatch and related faults, which 

have structural reliefs of hundreds to thousands of meters 

(for example, Zoback, 1983, fig. 6). Motion on these 

faults was mostly seismic, because young scarps are present along most of the fault traces, and because where 

measured in trenches across such faults, vertical displacements of ancient ground surfaces are typically about 2 m 

per scarp-forming event along the Wasatch fault zone 

(Schwartz and Coppersmith, 1984). A compilation of 

characteristics of historic surface faulting in the Basin and 

Range province (Bucknam and others, 1980) indicates 

that most surface ruptures in the province have been 

associated with earthquake magnitudes of at least 6, and 

usually of at least 7. The smallest historic surface rupture was produced by an earthquake of magnitude 5.6. 

Surface offsets greater than 1.0 m are known only from 

earthquakes whose magnitudes exceeded or probably exceeded 7 (Bucknam and others, 1980; Crone and 

Machette, 1984), so the earthquakes that formed the 

salients incrementally were mostly large. 

Third, the data are likely to reflect the presence of 

a segment boundary by an explainable anomaly. For example, a segment boundary might produce a salient as 

a nonconservative barrier to rupture propagation: each 

salient involves large bends or forks in the trace of the 

Wasatch and related faults. If slip vectors change orientations across such geometric irregularities, the bends and 

forks are termed nonconservative barriers (King and 

Yielding, 1984). King (1983) used geometric arguments 

to conclude that nonconservative barriers should be 

embedded in networks of small interlocking faults of 

many orientations. King and Nabelek (1985) called these 

networks process zones. Process zones form soft volumes 

of shattered rock that absorb the energy of incoming ruptures by dispersing it among many small faults. Where 

the small faults intersect larger faults, motions on the 

small faults offset the larger faults. The offsets lock the 

larger faults to form asperities and destroy the throughgoing continuity of the larger faults. Then the incoming 

ruptures cannot find easy paths through the process 

zones, so these soft rock volumes have high fracture 

toughness. Accordingly, large seismic ruptures should 

tend to stop at nonconservative barriers. When small 

earthquakes within a process zone have broken the 

18 Segmentation of Wasatch Fault Zone, Utah 

asperities, one or a few asperities at a time, a later large 

rupture can initiate in the edge of the process zone and 

propagate beyond the zone. Thus, large seismic ruptures 

should tend to be restricted to lengths of a fault between 

nonconservative barriers. Such lengths would be segments, and the nonconservative barriers would be 

segment boundaries. King and Yielding (1984) and King 

and Nabelek (1985) gave examples of strike-slip and 

reverse faults that are, or appear to be, segmented by 

bends that are interpreted as nonconservative barriers. 

Bruhn, Gibler, Houghton, and Parry (1987) and Bruhn, 

Gibler, and Parry (1987) applied these ideas to the 

Wasatch fault zone and interpreted the Salt Lake and 

Traverse Mountains salients as nonconservative barriers. 

Bruhn and coworkers measured orientations of slip 

vectors for the fault strands that meet in forks at the south 

end of the Salt Lake salient and the north end of the 

Traverse Mountains salient, and showed that these slip 

vectors change orientations across the forks. They interpreted the forks as nonconservative barriers; the barriers 

could be segment boundaries. 

Fourth, the data reflect faulting processes that have 

operated continually over the past few million years. 

There is no geological reason to expect these processes 

to change their long-term behavior over the next few 

millenia; probably salients will stay where they are. 

Fifth, the anomalies are reliable and probably 

reproducible. Gilbert (1928) recognized the four salients 

of figure 6 and no others that are comparably large and 

deeply rooted. Neither we nor the reports and maps cited 

earlier change this list of four large salients. 

Sixth, the anomalies were derived without reference 

to any other data types in this report. Gravity data were 

used to support estimates of the northward extent of the 

Warm Springs fault (Pavlis and Smith, 1980) and of the 

amount of dip slip on the Wasatch fault zone (Zoback, 

1983, fig. 6), but were not used to identify the anomalies 

themselves. 

Seventh, the anomalies were derived without reference to the segment boundaries of Schwartz and Coppersmith (1984), because the anomalies were derived solely 

from maps and descriptions of the trace of the Wasatch 

and related faults and of the Cenozoic and older bedrock 

geology that is exposed along and near these faults. 

TOPOGRAPHIC DATA 

Introduction 

Schwartz and Coppersmith (1984, fig. 10, 

p. 5688-5689) examined elevations at 37 points along the 

crest of the Wasatch Range and San Pitch Mountains 

(fig. 1) and observed that crestal elevation changes across 

their proposed segment boundaries. They suggested that 
segment boundaries divide the footwall of the westdipping Wasatch fault zone into blocks that have 

experienced different amounts of uplift and presumably 

different seismic histories. However, Wheeler (1984) 

showed that the changes in crestal elevation across the 

proposed segment boundaries are not significantly larger 

than changes between the boundaries. The implication 

is that spot elevations might be influenced too much by 

local variations in lithology and erosion rates to reflect 

clearly any segmentation that might be present along the 

fault zone. 

However, Wheeler (1984) suggested that other 

representations of topography might be able to reflect 

segmentation. A footwall block that has been uplifted 

more than its neighbors to the north or south should be 

higher than the neighboring blocks, wider, or both. The 

topographic expression of differential uplift can be 

obscured by many local factors, including irregular fault 

geometry, lithologic variation between and within stratigraphic units, variation in the structure of the thrust 

sheets and related folds that make up most of the footwall, variation in erosion rates, and streams that cut 

through the high ground of the footwall. Accordingly, 

the topography should be generalized in some way, to 

try to smooth out effects of these obscuring factors. A 

simple way to generalize topography is to inspect a smallscale topographic map with a large contour interval (U.S. 

Geological Survey, 1976), to select and trace a few contours that most clearly summarize the overall shape of 

the footwall block and to smooth the selected contours 

during tracing by omitting small irregularities caused by 

minor drainages (fig. 7). Places where elevation or width 

of the high ground appears to change along the Wasatch 

Range and San Pitch Mountains are candidates for 

anomalies that might coincide with the proposed segment 

boundaries of Schwartz and Coppersmith (1984). 

Procedures 

Generalizing topography to produce figure 7 was 

subjective. To minimize this subjectivity, we selected 

every fourth 500-ft topographic contour, instead of 

unevenly spaced contours. In smoothing contours to 

eliminate small irregularities caused by small drainages, 

we used about the same degree of smoothing everywhere 

and for each selected contour. The distortion introduced 

in generalizing topography is probably less than the 

variability among the 12 persons who examined the 

smoothed topography for anomalies, as described next. 

Topographic anomalies were identified by interpreting figure 7, but the interpretation was subjective. To 

minimize effects of subjectivity and to demonstrate 

reproducibility of any interpretations, 12 independent interpreters (not including us) were given copies of the 

topographic map of figure 7 containing only topography, 

legend, scale bar, and latitude-longitude marks. The three 

panels of the figure were taped together into a single strip 

map. The interpreters were asked to pick places where 

the height, width, or both of the Wasatch Range or San 

Pitch Mountains appeared to change abruptly northward 

or southward. Interpreters were asked to ignore features 

that looked like river valleys that cross the ranges and 

interrupt their overall height and width. 

Interpretations differed markedly. No change was 

picked by all12 interpreters. Fourteen changes were each 

picked by one or more interpreters. Five of the 12 interpreters are familiar with various aspects of the geology 

of the Wasatch Front and with the segmentation 

hypothesis (fig. SA). The other seven have little or no such 

familiarity (fig. 8B). A Chi-squared test showed that the 

two groups do not differ significantly (P = 0.56) in 

numbers of interpreters that picked each of the 14 

changes, so results from the two groups of interpreters 

were combined (fig. 8C). 

Despite the variations between interpreters, the 14 

changes fall into two distinct groups according to their 

popularities (fig. 8C). Only two of the 14 changes were 

picked by half or more of the interpreters, and these two 

are taken as anomalies in the sense of this report. 

Only three of the 12 interpreters picked these two 

changes and no others. Thus, the two anomalies of figure 

7 might seem obvious now, but no one interpreter could 

have demonstrated in advance that these two changes, 

and only these two, should be chosen. 

Results 

Perusal of the topographic map from which figure 

7 was derived (U.S. Geological Survey, 1976) indicates 

that values in table 5 are accurate to within about 2 km, 

or about 24 percent of the median anomaly width. 

The topographic data and anomalies meet the seven 

criteria set out previously. First, figure 7 has sufficient 

resolution to detect a segment boundary. Local uncertainties of the original topographic map are less than 

those introduced by tracing contours to produce figure 

7. These tracing errors are less than 3 km and generally 

less than 1 km. The errors consist mostly of smoothing 

irregular contours that reflect numerous small drainages. 

In contrast, the proposed segments are tens of kilometers 

long and their boundaries are several kilometers or more 

wide (Schwartz and Coppersmith, 1984, fig. 1, p. 5688). 

Thus, segments and their boundaries are larger than the 

likely errors and the limit of resolution of the topographic 

map. 

Second, figure 7 can record the presence of likely 

causative structures of large earthquakes, because topography can record long-term differences in uplift between 

adjacent parts of the footwall of the Wasatch fault zone. 

Topographic Data 19 
41° A 

DO' 

8 

BrighamO 

\ 9 2 

City 

Ogden 0 

10 

10 20 

20 30 

30 40 KILOMETERS 

40 MILES 

T2 

390 c 

DO' 

I 

EXPlANATION 

Above 10,000 feet (3048 m) 

8,000-10,000 feet (2438-3048 m) 

Figure 7. Strip map showing simplified topography of footwall of the Wasatch fault zone. Three panels of figure show 

northern (A), central (8), and southern (C) parts of Wasatch 

Front. Fault zone passes through or near cities shown in the 

figure and is generally below the 6,000-ft contour. Footwall 

of the west-dipping normal fault includes the high ground 

(Wasatch Range and San Pitch Mountains) east of the cities. 

Selected topographic contours were traced from map with contour interval of 500ft and scale of 1:500,000 (U.S. Geological 

Survey, 1976). Numerals that are enclosed by ovals show 14 

Presumably this uplift was partly or mostly seismic, so 

the uplift differences could record long-term differences 

in seismicity, including occurrences of large earthquakes, 

along the Wasatch Front. 

Third, figure 7 is likely to show an explainable 

anomaly at a segment boundary. One expression of a 

boundary could be an uplift difference between footwall 

blocks that border the boundary. Such a difference could 

be large enough to be expressed as differences in heights 

and widths of parts of the Wasatch Range and San Pitch 

Mountains. Differences in uplift are large enough along 

the Wasatch fault zone to be expressed topographically: 

20 Segmentation of Wasatch Fault Zone, Utah 

'----'------'-----'------' 6,000 feet (1829 m) topographic 

contour, with hachures pointing 

downhill 

places where one or more of 12 independent interpreters 

picked a change in elevation, width, or both of the Wasatch 

Range or San Pitch Mountains. Numeral inside each oval shows 

how many of the 12 interpreters picked that change. Changes 

picked by at least three-quarters of the interpreters are taken 

as topographic anomalies. Two vertical bars labeled T1 and 

T2, west of Salt Lake City and Nephi, respectively, show 

anomalies as projected into a north-south line along long 112° 

W. Five east-pointing arrows locate the segment boundaries 

of Schwartz and Coppersmith (1984, fig. 1 ). 

topographic relief is almost 2 km greater in the center of 

the Wasatch Range than at the north end or in the San 

Pitch Mountains (U.S. Geological Survey, 1976). Similarly, 

measured and estimated depths of the sediment-filled 

valleys that flank the ranges on the west are greatest in 

the center of the Wasatch Front (Zoback, 1983, fig. 6). 

Therefore, structural relief varies along the Wasatch fault 

zone by amounts that are large enough to be shown by 

topographic data. If these changes in structural relief are 

concentrated in zones as narrow as segment boundaries, 

then the zones are likely to be detectable with topographic 

data. 
5 f. A ... ~... . .. !... ...; ... 2 l::::::::::: y::::::::::: 2::::::::::: 2 

(!) :::::. :::::. ·::::::::::::::::::. :::::. w 1 Em::·:·:·:·: ::::::::::: 1 ::::::::::=:·:·:·:·:·:·:·:·:=::::::::: fill::·:·:·:·: 

~ yGWologo~o~Off 

o B 6 

5 ~ 5 

a.. 

~ 

~ 0 

a.. 

~ 

~ 

TOPOGRAPHIC CHANGES 

NORTH SOUTH 

Figure 8. Degrees of reproducibility of 14 topographic 

changes of figure 7. From left to right are numbered the 14 

changes that are shown as numerals enclosed in ovals in figure 

7. Popularity of a change is the number of interpreters who 

picked the change, which is shown by the numerals atop each 

bar. A, Results for five interpreters who are familiar with the 

geology of the Wasatch Front and with the segmentation 

hypothesis. 8, Results for seven interpreters who lack such 

familiarity. C, Combined results for all 12 interpreters. 

Fourth, the topographic data record processes that 

operate over times that allow inferences to be drawn 

about expected near-future seismicity. The high ground 

(fig. 7) arose from normal slip on the Wasatch fault zone. 

Table 5. Locations of topographic anomalies 

Naeser and others (1983) used fission-track ages of apatite 

to estimate that normal slip between Ogden and the vicinity of Salt Lake City has continued for 10 m.y. or more, 

at a long-term rate of 0.4 m/ka. Slip rates for various parts 

of late Quaternary time near Brigham City (Personius, 

1985) and between Provo and Nephi (Machette, 1984) vary 

from 0.12 m/ka to 1.8 m/ka or more (Machette, 1984; Personius, 1985). If these values are taken as estimates of the 

rate of seismic slip and as illustrations of the variability 

of this rate, then the implication is that such rates and 

associated seismicity have been occurring for 10 m.y. or 

longer, and so are likely to persist for the next few millenia. 

However, within these next millenia, rates and associated 

seismicity could vary, perhaps from about one-third to 

as much as four or five times the long-term average. 

Fifth, the anomalies are reproducible, because each 

anomaly has been picked by nine or ten of twelve independent interpreters. 

Sixth, the anomalies were derived from topographic 

data alone without reference to any other data type of 

this report. 

Seventh, the anomalies were derived without apparent reference to the proposed segment boundaries of 

Schwartz and Coppersmith (1984). Interpreters familiar 

with the segmentation hypothesis did not produce 

significantly different anomalies than did interpreters 

without such familiarity. 

PRE-CENOZOIC STRUCTURES 

Introduction 

The Wasatch fault zone formed in a part of the 

earth's crust that had already been damaged by a long 

series of compressional and extensional deformations, 

mainly episodes of Precambrian rifting and Late Cretaceous thrusting. 

[n.a., not applicable. Study area is 332 km long, north to south] 

Anomaly North end South end Anomaly Distance to next 

number of anomaly! of anomaly! width (km) anomaly 

Tl 130 140 10 

T2 247 254 7 

Sum n.a. n.a. 17 

1Distance south of Utah-Idaho border, in kilometers. 

2Distance to south edge of study area at lat 39° N. 

to south (km) 

107 

278 

185 

Pre-Cenozoic Structures 21 
During Late Proterozoic time, rifting and associated crustal extension and thinning formed a northtrending, west-facing, passive continental margin that 

extended from Alaska through Utah to Mexico (Stewart, 

1972). The sediments of the Cordilleran miogeocline accumulated on the extended and thinned continental crust 

of this margin (Stewart and Poole, 1974). By analogy with 

modern passive margins that rim the North Atlantic 

Ocean, the Red Sea, and similar oceans and seas, the 

extension and thinning probably formed a network of extensional faults in the upper crust throughout a belt that 

was tens to hundreds of kilometers wide (Bollinger and 

Wheeler, 1982, p. 22-23). Stewart and Poole (1974, p. 52) 

suggested that the thinning and extension occurred as far 

east as the Wasatch Front. If so, then the faults that 

accomodated extension and thinning would also have 

formed this far east. 

The Late Proterozoic passive margin cut across an 

older, east-trending depositional trough of Middle Proterozoic age (Stewart, 1972). The trough lay on the site 

of what would later become the Uinta Mountains (fig. 

9). We follow Forrester (1937, p. 635, fig. 1) in calling 

the trough and its Phanerozoic successors the Uinta 

trough. Stewart and Poole (1974) reported an oral suggestion by Paul Hoffman in 1971 that the Uinta trough 

might have been a Proterozoic aulacogen. Burke and 

Dewey (1973), Crittenden and Wallace (1973), and Sears 

and Price (1978) regarded the trough as one of several 

aulacogens in western North America that formed as 

precursors to the Late Proterozoic rifting. 

After Proterozoic rifting and deposition of the 

subsequent miogeoclinal sediments, in Late Cretaceous 

time north-trending, east-verging thrust sheets of the 

Sevier orogeny reached the Wasatch Front from the west 

(Armstrong, 1968). Presumably the thrust sheets overrode and buried the older passive margin. By analogy with 

other complexes of thrust sheets, this thrust complex can 

be expected to contain numerous faults of diverse strikes, 

dips, and degrees of interconnection (Boyer and Elliott, 

1982; Perry and others, 1984), although most faults strike 

northerly and dip at low angles. Oil well, seismicreflection, and stratigraphic data along the Wasatch Front 

indicate that the thrust complex is typically 5-10 km thick 

(for examples, see Smith and Bruhn, 1984), although 

Bruhn and others (1983) review petrological evidence 

from which they suggest that the deepest thrust sheets of 

northern Utah might have extended as deep as 16 km, 

somewhere west of the trace of the Wasatch fault zone. 

We assume that these tectonic events produced a 

crust that still contains numerous faults of diverse orientations, histories, and mechanical properties. Beginning 

in Cenozoic time, this damaged crust was extended to produce the Wasatch and other faults of the Basin and Range 

province. 

The various pre-Cenozoic faults might control 

22 Segmentation of Wasatch Fault Zone, Utah 

earthquakes along the Wasatch Front. Almost all small 

earthquakes in the area of the Wasatch fault zone occur 

at depths shallower than 20 km, and most are shallower 

than 10 km (Arabasz and others, 1980, p. 1493, fig. 8A). 

By analogy with large, normal-faulting earthquakes that 

have occurred in the Basin and Range province, Smith 

and Richins (1984, p. 97, figs. 9, 11) suggested that a large 

earthquake on the Wasatch fault zone would nucleate at 

a depth of about 15 km, at or near the base of the brittle 

upper crust (Smith and Bruhn, 1984, fig. 18). Such depths 

lie beneath or in the base of the thrust complex. 

Therefore, Arabasz (1984) and Arabasz and Julander 

(1986) suggested that most small, and perhaps some large, 

earthquakes along the Wasatch Front might be controlled 

by thrust-belt structures. Control could take the form of 

modern extensional reactivation of Cretaceous compressional faults or of earthquakes being constrained to occur 

within specific thrust sheets. However, large earthquakes 

might also occur by modern extensional reactivation of 

faults in structural basement beneath the thrust sheets. 

These subthrust faults might be north-striking, Late Proterozoic normal faults of the ancient passive margin. 

Extensional reactivation of such faults should produce 

mostly normal slip. Near the Uinta Mountains, subthrust 

faults might also include east-striking faults of the suggested Uinta aulacogen. Extensional reactivation of faults 

of the aulacogen should produce mostly strike slip. 

The suggestion that subthrust faults might be reactivated in modern extension has precedents elsewhere. 

Winslow (1981) suggested reverse reactivation of subthrust faults for the southern Andes Mountains in and 

near Tierra del Fuego. The subthrust faults formed before 

the thrusting occurred, as normal faults associated with 

rifting. Reverse reactivation of the subthrust faults is inferred from folds, faults, and thickness variations of 

Jurassic to Tertiary strata (Winslow, 1981, p. 522-523), 

so is not known to have been seismic. However, seismic, 

compressional reactivation of subthrust faults that 

originally formed as normal faults during prethrusting 

rifting is inferred from focal mechanisms and geological 

data for southwestern Iran (Jackson and others, 1981; 

Ni and Barazangi, 1986), and from focal mechanisms, 

well-constrained hypocentral depths, and geological data 

for southwestern Virginia and eastern Tennessee (Bollinger and Wheeler, 1982, 1983; Bollinger and others, 

1985; Munsey and Bollinger, 1985). 

If subthrust faults that formed during prethrusting 

extension can be reactivated in compression, then reactivation in extension should be even easier, for two reasons. 

First, rocks are typically weaker in extension than in compression. Second, extensional reactivation of old normal 

faults would be aided by gravity, because any dip slip 

would involve the hanging wall dropping down. Compressional reactivation would be resisted by gravity because 

any dip slip would involve the hanging wall moving up. 
UINTA MOUNTAINS 

Uinta basin 

30 60 MILES 

30 60 KILOMETERS 

Figure 9. Map of selected tectonic elements of the Uinta 

aulacogen, east of the Wasatch fault zone. Line representing 

the trace of the north-striking Wasatch fault has bars and balls 

on downthrown side (footwall). Upthrown block on east side 

of fault zone forms Wasatch Range. The present Uinta Mountains are roughly coextensive with area that is labeled Yu. Outcrops of east-directed thrust faults, and inferred subcrops of 

these faults beneath Upper Cretaceous and Tertiary rocks and 

sediments, have sawteeth on hanging walls. Westward projection of Uinta Mountains to Wasatch fault is bounded by Parleys 

In summary, compressional faults of Mesozoic age 

and probably deeper extensional faults of Proterozoic age 

permeate the rock volume within which the Wasatch fault 

zone formed. Cenozoic extensional reactivation of some 

of these faults is likely. The geometries of the reactivated 

faults could have controlled the nucleation, growth, and 

linking of large seismic ruptures that integrated to form 

the Wasatch fault zone. In particular, changes along the 

fault zone in the geometries of pre-Cenozoic faults could 

localize the nucleation points of large extensional earthquakes, could guide the propagation of ruptures, and 

could limit the lengths of surface ruptures, the areal 

dimensions of ruptures (and thus the magnitudes of earthquakes), or both. That is, pre-Cenozoic faults could impose segmentation on the Wasatch fault and its large _ 

earthquakes (Zoback, 1983, p. 9-12; Smith and Bruhn, 

1984, p. 5740-5741; Wheeler, 1984). The Borah Peak 

earthquake of Idaho appears to illustrate at least some 

aspects of such segmentation of modern extension imposed by old faults in the Basin and Range province 

(Crone and Machette, 1984; Skipp and Harding, 1985; 

Wheeler and Krystinik, 1987d). 

Accordingly, structures of the Cretaceous thrust 

complex and of the rifted crust that probably underlies 

the thrust complex can be examined along the Wasatch 

fault zone for along-strike changes (structural anomalies) 

that might coincide with the proposed segment boundaries of Schwartz and Coppersmith (1984). The 

Canyon syncline (PC) and by Mount Raymond fault (MR) and 

Deer Creek fault (DC) that connect with and presumably are 

parts of the larger thrust faults that bound the Absaroka and 

Charleston thrust sheets. Points a and b correspond to a and 

b of figure 10. Location of aulacogen is shown by locations of 

exposures of Tertiary intrusives (Ti), of aulacogen-filling, Middle 

Proterozoic Uinta Mountain Group (Yu), of correlative Big Cottonwood Formation (Ybc), and of east-trending arch that 

separates Absaroka and Charleston thrust sheets. Simplified 

from Hintze (1980) and Tweto (1979). 

pre-Cenozoic structures of most interest are faults, but 

folds and other features will be useful to the degree 

that they indicate the presence and geometries of buried 

faults. 

Procedures 

Four considerations guide the following summary 

of pre-Cenozoic structures. First, Smith and Bruhn (1984, 

p. 5740-5741, fig. 4) and Bruhn and Smith (1984) have 

already observed that the proposed segment boundaries 

of Schwartz and Coppersmith (1984) appear to coincide 

with large changes along strike in the geometries of thrustbelt and older structures. This observation constitutes 

prior inspection of the segment boundaries. If we used 

the structural changes that were noted by Smith and 

Bruhn, such prior inspection would distort results of the 

statistical analysis of Wheeler and Krystinik (1987a). 

Therefore, we must summarize pre-Cenozoic structures 

in enough detail, and with enough citations of reports 

that predate publication of the segmentation hypothesis, 

to make it clear that our choice of structural anomalies 

is not influenced by prior inspection. 

Second, the thrust belt of central Utah is structurally and stratigraphically complex, is the subject of a 

voluminous literature that is partly proprietary, and contains abundant candidates for structural anomalies of 

Pre-Cenozoic Structures 23 
many sizes. Therefore, we make no attempt to summarize 

all structures. Instead, we show that the structural 

anomalies chosen are reliable, reproducible, and derivable 

without reference to the other data types or to the proposed segment boundaries. 

Third, only large structural anomalies are chosen, 

because only large anomalies are likely to extend to the 

depths at which large earthquakes are thought to nucleate 

in the Basin and Range province (about 15 km: Smith 

and Richins, 1984). Here, a large anomaly is defined as 

one with map dimensions exceeding 5-10 km, for two 

reasons. The upper crust along the Wasatch Front is a 

thrust belt, and thrust-related structures smaller than 

5-10 km are unlikely to penetrate the thrust sheets to the 

nucleation depths of large earthquakes. Also, a shock of 

magnitude 7 might occur on a rectangular fault as short 

as 22 km in map view, as calculated from values of shear 

modulus, slip, hypocentral depth, and fault dip that are 

typical of large earthquakes in the Basin and Range province. A map dimension of 5-10 km is considerably less 

than 22 km. A practical guide for us is that any structural anomalies must be visible at the 1 :500,000 scale of 

the geologic map of Hintze (1980). For example, Wheeler 

(1984) suggested that a small lateral ramp exists about 

8 km east-northeast of the proposed boundary between 

the Provo and Nephi segments (fig. 1). A ramp is a part 

of a thrust fault on which the fault changes stratigraphic 

level (Butler, 1982). At frontal ramps, which strike 

parallel to the strike of the thrust belt, the change in level 

takes place across strike, so that the ramp undergoes 

reverse slip. At lateral or oblique ramps, which cut across 

the strike of the thrust belt, the change in level takes place 

along or oblique to strike, so that the ramp undergoes 

strike slip or oblique slip. Lateral ramps are one kind of 

structural anomaly that might control segmentation 

(Wheeler, 1984). However, this lateral ramp is too small 

for our purposes. 

Fourth, the following discussion concentrates on 

the exposed footwall of the Wasatch fault zone, near the 

fault-zone trace. Structures of thrust belts change 

markedly across strike, so that thrust-belt structures more 

than about 30 km east or west of the trace of the fault 

zone are unlikely to bear directly on structures at seismogenic depths near the trace. For example, Lamerson 

(1982) described a large lateral ramp about 100 km east 

of the fault zone. The ramp is large enough for consideration here but is too far east of the trace of the fault zone 

for our use. However, the west-striking faults that bound 

the Uinta Mountains are considered, even though they 

lie many tens of kilometers east of the Wasatch fault 

zone, because these faults might continue westward in the 

subsurface to the fault zone. 

The following sections summarize four groups of 

faults that might form structural anomalies. 

24 Segmentation of Wasatch Fault Zone, Utah 

Uinta Aulacogen 

Evidence for Existence of the Aulacogen 

The region occupied by the east-trending Uinta 

Mountains (fig. 9) has been the locus of repeated deformation since Late Archean or Early Proterozoic time. 

Forrester (1937) called this region the Uinta trough. The 

trough lies along the east-trending southern edge of an 

Archean craton (Bryant, 1985). The edge appears to have 

formed in Late Archean time, because at least 4-7 km 

of miogeoclinal rocks accumulated there as the Late 

Archean to Early Proterozoic Red Creek Quartzite. This 

thick sequence was thrust northward onto the Archean 

craton by latest Early Proterozoic time (Hansen, 1965, 

p. 22-32; Sears and others, 1982, p. 993-994; Bryant, 

1985). 

In Middle Proterozoic time the red, medium- to 

coarse-grained sandstones and quartzites with shales and 

conglomerates of the Uinta Mountain Group accumulated unconformably on the Red Creek Quartzite; these 

Middle Proterozoic rocks now constitute most of the exposed rocks in the Uinta Mountains (fig. 9; Hansen, 1965, 

p. 32-38). The Uinta Mountain Group is thick: in the 

eastern Uinta Mountains, Hansen (1965, p. 33) measured 

a partial thickness of 6.3 km and estimated its total 

thickness at slightly over 7.3 km. Richard (1986) reported 

a thickness of about 1. 7 km in a well drilled about 44 km 

southeast of exposed Uinta Mountain Group rocks in 

Colorado. From wells and seismic-reflection profiles in 

the same area, Stone (1986) estimated partial thicknesses 

of 4.6-6.1 km, 1. 7 km, and 0.8 km of the group in successively more easterly buried fault blocks. In the western 

Uinta Mountains, Wallace and Crittenden (1969, p. 120) 

measured thicknesses of 3.8-4.1 km in incomplete sections, and Crittenden and others (1967, p. I8, I22) used 

aeromagnetic data to estimate a total thickness for the 

group of 6.1 km. Farther west, in the footwall of the 

Wasatch fault zone, the correlative Middle Proterozoic 

Big Cottonwood Formation (Crittenden and others, 1972; 

Crittenden and Wallace, 1973; Crittenden, 1976) is about 

4.9 km thick (fig. 9; Hansen, 1965, p. 33). The depositional patterns and great thickness of coarse clastic 

material in the Uinta Mountain Group led Hansen (1965, 

p. 34-38, 167) and Wallace and Crittenden (1969) to conclude that the clastic material was eroded from a nearby 

source of high relief in and northeast of the present Uinta 

Mountains. The material was transported southward and 

accumulated in the rapidly subsiding Uinta trough. 

Woodward (1976, p. 89) noted that the Big Cottonwood 

Formation (fig. 9) contains more mature sediments than 

does the Uinta Mountain Group, and that this difference 

is consistent with both units having been derived from 

a northeastern source. Wallace and Crittenden (1969) 

concluded that a strandline and hinge line trended 
east-west, about along the present center of the Uinta 

Mountains, and that the lines separated the exposed 

metamorphic and plutonic rocks of the northeastern 

source area from the rapidly subsiding trough. Lower 

parts of the Uinta Mountain Group were deposited in a 

shallow marine environment, but gradually these conditions yielded to westward-flowing rivers throughout the 

trough. 

Bryant and Nichols (in press) compiled thicknesses 

of Late Proterozoic strata that are exposed in and near 

the footwall of the Wasatch fault zone. The Big Cottonwood Formation (fig. 9) is overlain by about 1.2 km of 

Late Proterozoic strata that contain unconformities. 

About 20 km south the thickness is about 700 m. 

However, nine other localities within 60 km north and 

south of these two expose less than 100m of beds of correlative rocks (Hintze, 1980; Davis, 1983a, b, 1985). Thus, 

in Late Proterozoic time the trough of the Uinta Mountain Group might have continued to subside, at least 

where it crossed the future site of the Wasatch fault zone 

(Bryant and Nichols, in press). At the eastern end of the 

Uinta trough, Stone (1986) concluded from oil well and 

seismic-reflection data that the Uinta Mountain Group 

dropped down along the south edge of its present-day 

subcrop by Late Proterozoic to Cambrian, north-facing, 

normal faulting. From angular discordance between the 

group and overlying strata, Hansen (1986) inferred southfacing normal faulting of the same age, on the north edge 

of the trough. The inward-facing normal faulting at both 

edges of the east end of the Uinta trough indicates extension and probably subsidence at about the same time 

that the west end of the trough might also have been 

subsiding. 

Forrester's (1937) isopach map of combined Paleozoic and Mesozoic strata shows the Uinta trough as an 

eastward projecting arm of the north-trending Cordilleran 

miogeocline, with greater thicknesses in the trough than 

in adjoining areas to the north, south, and east. However, 

isopach, lithofacies, and paleogeologic maps of shorter 

time intervals show more complex behavior of the trough. 

The area of the trough formed a linear high in Cambrian time (Lochman-Balk, 1976), the Uinta peninsula 

of Lochman-Balk (1972). However, tectonic activity of 

the Uinta trough during Ordovician to Mississippian time 

is less clearly established. Ross (1976, figs. 5, 9, 11) 

thought that the site of the trough was exposed in Ordovician time, when a westward extension of the Uinta 

penninsula into eastern Nevada was inferred to have been 

high (Tooele arch of Webb, 1958; Uinta-Gold Hill arch 

of Roberts, 1960; Cortez-Uinta axis of Roberts and 

others, 1965, p. 1928). However, Foster (1972) showed 

the Tooele arch trending northeast during Ordovician 

time and lying well south of the westward projection of 

the Uinta Mountains, separated from the Uintas by areas 

of more typical thicknesses of Ordovician strata. During 

Devonian time the site of the Uintas was exposed (Craig, 

1972), and a Late Devonian unconformity was interpreted 

as defining a wide, flat-topped high that extended 

70-80 km west of the present Wasatch fault zone, on 

trend with the Uinta Mountains (Rigby, 1958, 1959; 

Roberts, 1960; Morris and Lovering, 1961; Baars, 1972; 

Sandberg and others, 1982, figs. 8 and 9, p. 704). However, the size, shape, and orientation of this Late Devonian high might not have fit those of a linear westward 

projection of the Uintas. Maps of Rigby (1959, figs. 1, 

3), Morris and Lovering (1961, fig. 33), and Baars (1972) 

can be interpreted as showing that the Late Devonian 

uplift consisted of one small, north~trending high 

60-80 km west of the Wasatch fault zone and a second, 

much larger high that was a tilted block, sloping southward, with the site of the Uinta Mountains lying along 

its northern edge. Sandberg and others (1982, p. 704-707) 

inferred earliest Mississippian uplift and erosion at the 

west end of the Uinta trough. The Uinta trough received 

sediments in Late Mississippian time (Hansen, 1965, fig. 

14; Craig, 1972; Welsh, 1979, p. 103, 105; Sandberg and 

others, 1982, p. 714, fig. 17), although this trough might 

have been just one part of a larger trough that covered 

much of northeastern Utah then (Craig, 1972; Rose, 1976, 

figs. 9, 10). West of the present Wasatch fault zone during Late Mississippian time, the westward projection of 

the Uinta Mountains separated two deepening basins 

(Roberts and others, 1965, p. 1931 and 1934; Chamberlain, 1984). 

Tectonic activity of the Uinta trough is better 

established for times after the Mississippian. In Pennsylvanian and Permian time, the Uinta trough was low 

again, being shallow, east-trending, and west-deepening 

(Hansen, 1965, p. 39, figs. 16, 17, 20). However, the westward projection of this trough continued to separate two 

lobes of the deep Oquirrh basin, as it had during Late 

Mississippian time (Roberts and others, 1965, figs. 7, 16). 

Finally, in Late Jurassic to Early Cretaceous time, the 

eastern part of the Uinta trough was a depocenter with 

a well-defined, east-trending south edge and east end but 

with an unknown northward extent (Hansen, 1965, 

fig. 30). 

After the Early Cretaceous, uplift changed the site 

of the Uinta Mountains from a sink to a source of 

sediments. Conglomerates, dated unconformities, and 

south-directed paleocurrents led Hansen (1965, p. 108), 

Crittenden (1976, p. 378), Bruhn and others (1983, p. 70), 

and Bradley (1984) to suggest that uplift began in latest 

Cretaceous time. For the northeastern Uintas, Hansen 

(1965, p. 108) concluded that there were 2.4-3.4 km of 

uplift by early Paleocene time. Sedimentological, 

stratigraphic, oil well, seismic-reflection, and other data 

show major uplift along the length of the Uinta Mountains in Paleocene and especially Eocene time (Hansen, 

1965, p. 108; Robinson, 1972; Gries, 1983, p. 13, fig. 7; 

Pre-Cenozoic Structures 25 
Bruhn and others, 1983, p. 72-74; Clement, 1983). 

Oligocene time saw stocks emplaced and volcanics 

erupted in the gap between the western Uinta Mountains 

and the Wasatch fault zone (fig. 9; Crittenden and others, 

1973; Hintze, 1980), as well as elsewhere in Utah (Hintze, 

1980). In late Oligocene time the eastern Uinta Mountains underwent north-south extension. The Oligocene 

Bishop Conglomerate was tilted inward toward the center 

of the range to form an east-trending basin in which the 

late Oligocene and Miocene Browns Park Formation was 

deposited (Hansen, 1984). 

Thus, the stratigraphic record shows that the Uinta 

Mountains lie on the east-trending site of repeated tectonic activity, perhaps localized by structures of a rifted 

margin of an Archean continent. The concept of a Uinta 

aulacogen arose from consideration of this stratigraphic 

record. Specifically, the characteristics of the Uinta 

Mountain Group led Stewart and Poole (1974, p. 34), 

Burchfiel and Davis (1975, p. 365), Stokes (1976, p. 15), 

Bruhn and others (1983, p. 63, 85), Zoback (1983, 

p. 10-11), and Pricha and Gibson (1985) to interpret the 

group as having been deposited in a Middle Proterozoic 

aulacogen. Thus, the aulacogen would have been connected on the west to a Middle Proterozoic continental 

margin, whose location and orientation remain unknown. 

Presumably the same stratigraphic evidence led Burke 

and Dewey (1973), Sears and Price (1978), Welsh 

(1979, p. 93), Sears and others (1982, p. 990), and Gries 

(1983, p. 11) to call the Uinta trough an aulacogen, 

although these reports cite no evidence to justify this 

inference. 

Evidence for Faults in the Aulacogen 

Whether an aulacogen underlies the Uinta Mountains is of more than semantic interest, because of the 

implications for the generation of large earthquakes. An 

aulacogen is a marginal cratonic trough that is known 

or inferred to be bounded by faults (Gary and others, 

1972; Hoffman, 1973; Hoffman and others, 1973; Burke 

and Dewey, 1973), and indeed Bates and Jackson (1980) 

gave graben and rift as synonyms for aulacogen. The 

great thicknesses of aulacogen fillings imply large 

amounts of dip slip on the bounding faults. These faults 

could penetrate to the base of the brittle upper crust where 

large earthquakes are likely to be generated (Meissner and 

Strehlau, 1982; Sibson, 1982, 1984; Chen and Molnar, 

1983; Smith and Bruhn, 1984). The large map sizes of 

aulacogens means that their bounding faults are likely to 

be long enough in map view to generate long ruptures 

and large earthquakes. The repeated activity of the Uinta 

aulacogen, from Middle Proterozoic and perhaps 

Archean through Oligocene time, suggests that any such 

faults might still be weak enough to slip, or to decouple 

26 Segmentation of Wasatch Fault Zone, Utah 

parts of the Wasatch fault zone from each other to form 

independent segments. The Oligocene intrusions between 

the western Uinta Mountains and the Wasatch fault zone, 

the correlation of the Big Cottonwood Formation with 

the Uinta Mountain Group, and the stratigraphic 

evidence of repeated uplift or depression along the 

westward projection of the Uinta Mountains, west of the 

Wasatch fault zone, all combine to show that the inferred 

aulacogen intersects the fault zone. 

Accordingly, the next paragraphs will argue three 

things: first, that the Uinta aulacogen probably is 

bounded by deep faults; second, that the inferred 

faults are too poorly defined to be used in this report 

as structural anomalies in themselves; and third, that 

what is known of the aulacogen and thrust belt structures near it allows us to define one usable structural 

anomaly. 

The repeated rises and falls of the crustal block that 

carries the Uinta Mountains could occur by regional or 

local flexing of the crust in isostatic compensation or by 

movement on faults. The block is too narrow for local 

isostatic compensation to be likely (Behrendt and Thiel, 

1963, p. 860-861). The contours and contacts that define 

the block on paleogeologic, isopach, and lithofacies maps 

show that the edges of the block are too narrow for the 

vertical motions to have occurred by regional flexing. The 

sharp boundaries of the uplifts and downdrops that are 

revealed on the maps suggest that the block is bounded 

by faults. 

Other evidence also suggests that the aulacogen is 

fault bounded. Mapping, well data, and seismic-reflection 

profiles have shown the exposed Precambrian core of the 

Uinta Mountains to be bounded on the north and south 

by inward-dipping reverse faults (for examples, see 

Hansen, 1965, p. 38, 137-138; Ritzma, 1969; Campbell, 

1975; Hintze, 1980, section GH; Sears and others, 1982; 

Gries, 1983; Bruhn and others, 1983; Clement, 1983; Osmond, 1986; Stone, 1986; Hansen, 1986). Most of these 

data deal with faults that are exposed or faults that are 

shallower than the Precambrian floors of the basins north 

and south of the Uinta Mountains (8-11 km deep: Gries, 

1983) and with Cenozoic motion on these faults. 

However, Sears and others (1982, p. 995) documented 

as much as 10 km of Early Proterozoic dip slip on faults 

along the northeast corner of the present range. Along 

the edge of the southwestern Uinta Mountains, faults 

experienced at least 1.8 km of dip slip before Late Devonian time (Wallace and Crittenden, 1969, p. 128, 140) and 

mostly between deposition of the Middle Proterozoic 

Uinta Mountain Group and Middle Cambrian time 

(Bryant, 1985, p. 117). 

Behrendt and Thiel (1963) mapped Bouguer gravity and residual magnetic intensity over the Uinta Mountains. Both potential fields show steep gradients above 
the northern and southern edges of the range, suggestive 

of large buried faults. Steenland (1969) interpreted an 

aeromagnetic low over the range, and steep aeromagnetic 

gradients at the north and south edges of the range, as 

reflecting a fault-bounded basin that underlies the range 

and contains at least 11 km of the Uinta Mountain 

Group. Smith and Cook (1985) mapped Bouguer gravity over northeastern Utah and modeled free-air gravity 

along two north-south profiles across the Uinta Mountains. The models include vertical to steeply inward dipping faults at the north and south edges of the range, 

bounding a basin that contains 8-11 km of the Uinta 

Mountain Group. 

Therefore, the term aulacogen adequately characterizes the depositional setting of the Uinta Mountain 

Group and the structural setting of the present Uinta 

Mountains. The implied bounding faults are probable but 

not certain along most of the north and south edges of 

the range and westward to the Wasatch fault zone. 

However, the number, locations, orientations, and sizes 

of the bounding faults remain too poorly known for the 

faults by themselves to constitute structural anomalies, 

for seven reasons. 

First, the paleogeologic, isopach, and lithofacies 

maps that comprise most of the evidence for the 

aulacogen and its repeated activity lack the control to 

define individual faults or fault zones. 

Second, published well and seismic-reflection profiles define the faults in the upper several kilometers of 

the Uinta and Green River basins but do not yet allow 

the faults to be followed down to the lower part of the 

brittle upper crust, nor west of the basins to the Wasatch 

fault zone. Similarly, geologic mapping has not defined 

such faults in most of the 40 km or so that separate the 

western Uinta Mountains from the Wasatch fault zone, 

because Cretaceous thrust sheets and Cenozoic strata 

cover much of this area (Hintze, 1980). 

Third, Sears and others (1982) mapped Proterozoic 

faults in the northeastern Uinta Mountains, and Wallace 

and Crittenden (1969} did the same in the southwestern 

part of the range. Even these faults might not be simply 

related to the main bounding faults of the aulacogen, 

because the northeastern faults predate the Middle Proterozoic sedimentary filling of the aulacogen, and the 

southwestern faults postdate it (Sears and others, 1982, 

p. 995; Bryant, 1985, p. 117). 

Fourth, large east-trending faults or fault zones at 

the north and south edges of the range can be inferred 

from gravity and magnetic data. These inferred faults are 

interpreted to limit the present northward and southward 

extent of the Uinta Mountain Group, but the faults 

are of unknown age. Therefore, they might postdate 

the Uinta Mountain Group and the aulacogen. If so, 

then the Uinta Mountain Group might originally have 

extended north and south beyond these inferred faults 

but might have been uplifted there by motion on the faults 

and eroded. The stratigraphic record documents such 

uplifts in Cambrian, Devonian, perhaps latest Cretaceous, Paleocene, and Eocene times. In fact, Stone (1986) 

offered this explanation for the region east-southeast of 

exposed Uinta Mountain Group rocks (fig. 9), where 

buried parts of this unit have been drilled and interpreted 

on seismic-reflection profiles. Reflectors of the group are 

truncated on the south against north-dipping faults, and 

internal reflectors parallel each other into the faults. The 

parallelism indicates that the group was eroded south of 

the faults as a result of postdepositional normal slip on 

the faults. 

Fifth, the present southward extent of the Uinta 

Mountain Group is unknown, aside from the justmentioned inference from gravity and magnetic maps. 

The sedimentological and stratigraphic descriptions and 

interpretations of the Uinta Mountain Group form one 

of the strongest arguments for the existence of a northern 

bounding fault that was active during deposition of the 

group. However, Bryant (1985, p. 117) noted that these 

descriptions and interpretations contain no hint of the 

existence of a southern limit to these strata (Hansen, 1965; 

Wallace and Crittenden, 1969). Sears and others (1982) 

mapped a graben in the northeastern Uinta Mountains, 

but it lies entirely on the north side of the range and so 

cannot bear on the southward extent of the Uinta Mountain Group. Small patches of Middle Proterozoic strata 

are exposed in the footwall of the Wasatch fault zone east 

of Provo and 13 km and 30 km north of Nephi (Hintze, 

1980; Davis, 1983a; Bryant, 1985). It is not known 

whether these rocks represent the southward extent of the 

aulacogen, the eastward extent of the continental margin 

to which the aulacogen presumably connected, or something else. 

Sixth, the Uinta Mountains and their environs 

might be underlain by many east-striking faults of 

Archean age, each responsible for some part of the evolution of the aulacogen and its several subsequent rises and 

falls. Bryant (1985) reviewed aeromagnetic, mapping, and 

other data that show or appear to show several large, eaststriking faults within about 300 km north of the Uinta 

Mountains. Some of the faults have long histories of activity. The Red Creek Quartzite, which unconformably 

underlies the Uinta Mountain Group in the northeastern 

Uinta Mountains, is interpreted to be a miogeoclinal 

sequence of Archean to Early Proterozoic age (Hansen, 

1965; Sears and others, 1982; Bryant, 1985). If the 

Red Creek Quartzite accumulated on a south-facing 

continental margin, then analogies with modern passive 

margins suggest that it should be underlain by numerous 

east-striking normal faults of Archean age. Such speculation suggests that the faults that might have controlled 

Pre-Cenozoic Structures 27 
the growth of the aulacogen and its subsequent rises and 

falls through the Phanerozoic might be many, widespread, and geometrically complex. 

Seventh, there are too few published data to force 

agreement on the dips, depths of penetration, relative 

sizes, or westward extents of the faults that are known 

to bound the Cenozoic uplift of the Uinta Mountains or 

are inferred to bound the aulacogen. The hypothesis that 

the bounding faults of the uplift steepen downward (for 

example, Campbell, 1975) has been replaced by the interpretation that these faults retain moderate inward dips, 

at least to depths that are illuminated by published 

seismic-reflection profiles (8-11 km or shallower: Gries, 

1983). However, below these depths, sparse data allowed 

Sears and others (1982, fig. 5) to model faults of equal 

size on both north and south sides of the uplift as dipping inward at 25 °-45 °. The same data also allowed 

Bruhn and others (1983, p. 85-88, fig. 21) and Bruhn, 

Picard, and Is by (in press, fig. 9) to model the same faults 

differently. Bruhn and coworkers suggested that the 

northern fault dips south at about 10° to a depth of about 

8 km, then steepens to dip about 30° down to 22-27 km 

depth. However, they interpreted the fault at the south 

edge of the Uinta Mountains to dip north at about 40 °, 

to end against the south-dipping fault and to die out 

westward before reaching the Wasatch fault zone. 

In summary, characteristics of the Uinta aulacogen 

indicate that it is large enough, and has been important 

enough in the geologic evolution of the region that includes the central Wasatch Front, that our structural 

anomalies should reflect the aulacogen. Unfortunately, 

faults of the aulacogen are too poorly known to be used 

directly. 

Defining a Structural Anomaly at the Aulacogen 

Other faults can be used instead of those of the 

aulacogen. Where the westward projection of the Uinta 

Mountains crosses the footwall of the Wasatch fault zone, 

the projection is bounded by two east-striking, outwarddipping parts of the Cretaceous thrust complex (fig. 9). 

On the south, the westward projection of the Uinta Mountains is separated from the Charleston thrust sheet by the 

south-dipping Deer Creek fault (Baker and Crittenden, 

1961; Hintze, 1980, section CD). On the north, the projection is separated from the Parleys Canyon syncline in 

the Absaroka thrust sheet and overlying sheets by the 

north-dipping Mount Raymond thrust fault (Crittenden, 

1965a,b; Crittenden and others, 1966). The intersections 

of the east-striking Deer Creek and Mount Raymond 

faults with the north-striking Wasatch fault are taken to 

bound a single structural anomaly, which represents the 

unknown fault system of the Uinta aulacogen. 

The observed width of this anomaly at ground level 

28 Segmentation of Wasatch Fault Zone, Utah 

is 21 km (fig. 10; Davis, 1983a,b). The width of the anomaly at seismogenic depths is uncertain because the subsurface geometry of the north and south edges of the fault 

system of the aulacogen is unknown. This uncertainty can 

be estimated by considering two extreme cases (fig. 10). 

At one extreme, the aulacogen and its fault system are 

assumed to be bounded in the upper few kilometers of 

the crust by the Deer Creek and Mount Raymond faults, 

which dip outward from the aulacogen. Examination of 

both faults as they appear in cross sections of Baker and 

Crittenden (1961), Crittenden (1965a,b), and Smith and 

Bruhn (1984) suggests outward dips of about 30~ typical 

of ramps in thrust belts. The Deer Creek and Mount Raymond faults might dip downward to the top of Precambrian gneisses and schists, or to the base of a thrust sheet, 

whichever is shallower. Smith and Bruhn (1984, figs. 5, 

6) estimate these depths as about 6 km. Therefore, the 

anomaly in the upper 6 km might be as wide as 42 km 

(fig. 10). Any anomaly-bounding faults below 6 km are 

assumed to be high-angle faults beneath the thrust sheets, 

and to project upward about to the outcrops of the Deer 

Creek and Mount Raymond faults. Such high-angle faults 

would give smaller widths than 42 km for the anomaly 

at depths below 6 km. 

At the second extreme, the aulacogen and its fault 

system are assumed to be bounded by steep extensional 

faults of Archean to Middle Proterozoic age, which project upward to and have controlled the positions of the 

outcrops of the Deer Creek and Mount Raymond faults. 

Such faults could dip either inward or outward, at dips 

of perhaps 60~ and could penetrate to the depth at which 

large earthquakes are suggested to nucleate (15 km: Smith 

and Richins, 1984). Outward dips indicate that the maximum width of the anomaly is distance gh of figure 10, 

which is less than cd and so is not considered further. 

However, inward dips indicate that the anomaly might be 

as narrow as 4 km. Accordingly, the anomaly that corresponds to the fault system of the Uinta aulacogen has 

a width of 21 km, and might range from 4 km to 42 km 

wide at seismogenic depths. 

Fau Its of the Late Proterozoic Passive Margin 

Normal and other extensional faults are inferred to 

have formed mostly during Late Proterozoic rifting, in 

crust that later became the passive continental margin on 

which the sediments of the Cordilleran miogeocline accumulated (for example, Christie-Blick, 1984; Pricha and 

Gibson, 1985; Christie-Blick and von der Borch, 1985). 

The inferred existence of such normal faults arises from 

analogies with modern passive margins. The inferred 

faults would now be buried under Cretaceous thrust 

sheets. The faults could be recognized in seismic-reflection 

profiles by determining thicknesses of sedimentary 
N 

42 km 

4 km 

~21km 

10 MILES 

10 KILOMETERS 

s 

Figure 10. Schematic cross section illustrating widths of structural anomaly formed by the inferred fault system of the Uinta 

aulacogen. Figure represents north-south section along Wasatch 

fault zone, looking east into footwall block that contains the 

fault system of the aulacogen. Horizontal lines represent ground 

level (labeled 0 km) and depths of 6 and 15 km below it. Eaststriking Mount Raymond fault crops out at point a, and eaststriking Deer Creek fault, at b. Rocks exposed north of a or 

south of b are involved in the thrust sheets above these two 

faults. Rocks that crop out between a and b include Big Cottonwood Formation and other rocks that are interpreted to be 

associated with the aulacogen. Lines that plunge 30° and 60° 

away from a and b represent extreme estimates of the subsurface geometries of faults that might form the north and south 

boundaries of the rocks and fault system of the aulacogen, at 

various depths below ground level. Deer Creek and Mount Raymond faults might dip outward 30° to depth of 6 km. Upward 

projections of these faults from 6 km to ground level generate 

points c and d. Other, deeper faults might dip inward (or outward) at 60° to 15 km depth, generating points e and f (or g 

and h). 

sequences of Late Proterozoic to early Paleozoic age. 

Thicknesses should increase abruptly from footwalls to 

hanging walls of such faults, because the sequences would 

be of the same ages as the normal faulting. 

For example, Pricha and Gibson (1985) suggested 

such a rifting origin for the north-striking ancient 

Ephraim fault of Moulton (1975), which lies about 30 km 

east of the Levan segment. Standlee (1982, fig. 14, p. 367) 

inferred from well and seismic-reflection data that the 

Ephraim fault, which dips west and offsets Cambrian to 

Jurassic strata, was active during but not after Middle 

Jurassic time. For a high-angle basement fault at about 

the same location, Villi en and Kligfield (in press, figs. 7, 

8, 11) inferred from regional relationships that the 

Paleozoic section is thicker in the hanging (west) wall than 

in the footwall. From seismic-reflection and related data 

Zoback (1987) suggested that south of Nephi the Wasatch 

fault zone is localized by a west-dipping normal fault of 

Late Proterozoic age, which has been reactivated in 

Cenozoic extension. 

This inferred system of Late Proterozoic normal and 

other extensional faults remains too poorly documented 

and defined for use in this report, for two reasons. First, 

the faults and the stratigraphic evidence of their expected 

Late Proterozoic activity remain too deeply buried to be 

illuminated by most published seismic-reflection profiles. 

Second, the parts of the system of Late Proterozoic extensional faults that would be most important for defining structural anomalies are likely to be small, to strike 

east-west roughly parallel to most reflection profiles, or 

both, and so are likely to be hard to detect. This second 

reason is based on examination of the system of early 

Mesozoic extensional faults that dissects the eastern 

United States from Massachusetts to South Carolina 

(King and Beikman, 1974). 

Most of these Mesozoic faults, and all of the long 

ones, in the eastern United States are normal faults that 

strike north-northeast to east-northeast, parallel to the 

Mesozoic-Cenozoic passive margin during whose evolution they formed. However, short cross faults or enechelon offsets of the northeast-striking, longitudinal 

faults are spaced at intervals that are typically several tens 

of kilometers. At these cross faults and offsets, the longitudinal faults and the Mesozoic basins that they bound 

have three kinds of map patterns. Some longitudinal faults 

and basins end abruptly (for example, fig. 11, locality 1). 

Some longitudinal faults end and are replaced en echelon 

by others, so that the same basin edge extends along both 

faults (for example, fig. 11, locality 2). Some basins change 

map width abruptly across a cross fault (for example, fig. 

11, locality 3). Numerous smaller examples of such fault 

geometries are shown on the geologic maps of New Jersey 

(Lewis and Ki.immel, 1910-1912), Pennsylvania (Berg and 

others, 1980), Maryland (Cleaves and others, 1968), and 

Virginia (Calver and others, 1963). 

For these map patterns to be visible at a scale of 

1:2,500,000 (King and Beikman, 1974), the cross faults and 

offsets that make up the patterns must have many meters 

and perhaps several kilometers of structural relief. 

Schwartz and Coppersmith (1984) suggested that typical 

scarp-forming earthquakes along the Wasatch fault zone 

each generate about 2 m of structural relief and occur at 

intervals of hundreds to thousands of years. The cross 

faults and offsets illustrated in figure 11 have enough 

structural relief that their formation occupied many such 

intervals. Map patterns like those represented in figure 11 

indicate that the longitudinal normal faults on which the 

Mesozoic extension occurred were segmented by cross 

faults and en-echelon offsets. If most of the faulting 

occurred seismically, then the cross faults and offsets 

might have acted as stress concentrators, nucleation sites, 

and propagation barriers for seismic ruptures, as appears 

to have occurred during the Borah Peak earthquake 

(Crone and Machette, 1984; Skipp and Harding, 1985; 

Wheeler and Krystinik, 1987d). 

Pre-Cenozoic Structures 29 
50 100 MILES 

I I I 

I I 

50 100 KILOMETERS 

Figure 11. Map of Mesozoic extensional faults that bound 

Triassic sedimentary filling of Newark-Gettysburg basin in 

eastern United States. Heavy lines denote faults and light lines 

denote unfaulted contacts. Triassic basin filling (R) and Paleozoic (Pz) rocks are overlapped from southeast by Mesozoic (Mz) 

strata of Coastal Plain province. Circled numerals identify 

localities cited in text. Traced and simplified from King and 

Beikman (1974). 

Therefore, an analogy between the Mesozoic extensional faults of the eastern U.S. and the inferred Late 

Proterozoic extensional faults beneath the thrust sheets 

of the Wasatch Front suggests that the Late Proterozoic 

faults might be segmented by short cross faults and by 

en-echelon offsets of north-striking normal faults. For 

defining structural anomalies the most important parts 

of the inferred system of Late Proterozoic extensional 

faults will be cross faults and offsets. The analogy with 

the eastern U.S. suggests that beneath the Wasatch Front, 

most such cross faults and offsets should be small compared to the longitudinal faults and should strike more 

or less east. However, published seismic-reflection 

profiles also strike roughly east at high angles to the 

Wasatch fault zone (most profiles lie entirely east of the 

fault zone), the thrust sheets, and the Late Proterozoic 

passive margin. These profiles are unlikely to show most 

of the expected cross faults or offsets, at least until the 

reflection data become much more numerous and widely available for reprocessing than they are now. 

In summary, the thrust sheets of the Wasatch Front 

are probably underlain by an extensional fault system of 

Late Proterozoic age. This system might be segmented. 

The parts of the system that are the most likely to cause 

segmentation and to control its expression in the properties of large earthquakes are unlikely to be detectable with 

data that are now available. To solve this problem would 

30 Segmentation of Wasatch Fault Zone, Utah 

require such complete coverage of the Wasatch Front by 

expensive reflection profiles that any cross faults and offsets are likely to remain undetected for many years. 

Accordingly, the analysis of segmentation in this report 

proceeds, but we remain mindful of our ignorance of subthrust structures. 

Cretaceous Thrust Faults North of the 

Uinta Mountains 

Geometries of the Thrust Faults 

Structural terminology of thrust belts varies between regions, organizations, and geologists. We use the 

terminology of Boyer and Elliott (1982) and Butler (1982), 

because it comes closer to being an accepted standard 

than any other terminology known to us. 

North of the Mount Raymond fault and the Uinta 

Mountains, the footwall of the Wasatch fault zone 

comprises a series of thrust sheets that extend east into 

Wyoming and north into Idaho. These thrust sheets 

reached the area of the Wasatch Front from the west in 

Cretaceous time, as part of the eastward-propagating 

Sevier orogeny (Armstrong, 1968). Thrusting progressed 

farther eastward until Eocene time (Armstrong and Oriel, 

1965; Oriel and Armstrong, 1966). The eastern thrust 

sheets, those not extensively deformed by Tertiary extension, have been objects of intensive petroleum exploration (Monley, 1971; Royse and others, 1975; Dixon, 

1982), and proprietary well data and seismic-reflection 

profiles have illuminated many of their structures. Most 

of this exploration took place several tens of kilometers 

east of the Wasatch fault zone, but some profiles extend 

west as far as the fault zone, where they support interpretations of thrust structure there (Royse and others, 

1975; Smith and Bruhn, 1984). 

The oldest large thrust fault is the Willard fault (fig. 

12), which bounds the base of the Cache allochthon of 

Crittenden (1972). This fault places Precambrian and 

Paleozoic rocks of the Cordilleran miogeocline over correlative rocks of the adjoining shelf, and so has carried 

the miogeoclinal rocks far from the west (Crittenden, 

1972, 1976). The younger Absaroka thrust fault then 

formed in the footwall of the Willard fault and carried 

the Willard fault and the Cache allochthon eastward to 

their present positions (fig. 13). En route, the Absaroka 

fault cut upsection in a frontal ramp, from Precambrian 

basement to Cambrian shales, at about the position of 

the present Wasatch fault zone. When the hanging wall 

of the Absaroka thrust climbed this ramp, the thrustingduplicated rocks formed a large ramp anticline with the 

Cache allochthon draped over its top and down its east 

limb (figs. 12, 13; Royse and others, 1975). The Precambrian rocks that are exposed in the core of this ramp 
41°00' & 

I 

\ 

\ 

~ \ 

\ 

I 

Cache 

allochthon 

I I 

10 

10 

I I 

20 MILES 

I 

20 KILOMETERS 

EXPLANATION 

aJ -·-·.-\l ~ 

--·-- 1 

l 

-·-·.-

1 

_ __.. __ Trace of Wasatch fault-Bar and ball on downthrown (west) 

++ Anti~i~~~al crest or synclinal trough-Dashed where covered by 

Upper Cretaceous or Cenozoic strata 

~ Anticline (B) or syncline (A, C) that is shown in figure 

13-Parleys Canyon syncline (C) extends northward between 

Utah-Wyoming border and eastern trace of Willard fault, but as 

a wide basin bounded on east and west by west-dipping thrust 

faults. Accordingly, the Parleys Canyon syncline is shown on 

figure 13, but its trough is too poorly defined to represent on 

this map much north of lat 41 oN. 

• + Trace of thrust fault-Dashed where covered by Upper Cretaceous 

or Cenozoic strata, sawteeth on hanging wall. All thrust faults 

involved eastward movement of hanging walls 

---- Generalized contact between Archean and Early Proterozoic 

Farmington Canyon Complex (AX), Late Proterozoic (Z), 

Paleozoic (Pz), and Mesozoic (Mz) strata-Dashed where ffi covered by Upper Cretaceous or Cenozoic strata 

1--•-•----l Approximate location of a published cross section that spans 

part or all of map area-Section ends that fall within map 

area are located approximately by a short line that crosses and 

terminates the dash-dot line. Numeral in triangle identifies 

section. 1, sectionAB of Hintze (1980); 2, section JJ' of Smith 

and Bruhn (1984); 3, section CC' of Crittenden (1972); 4, 

section YY' of Royse and others (1975); 5, section CC' of 

Smith and Bruhn (1984); 6, section AA' of Bruhn, Picard, and 

Isby (in press); 7-9, sections XX', YY', and ZZ' of Schirmer 

(;;\ (1985) 

~ Approximate depth in kilometers below ground level of top of 

west·dipping ramp in basal (Absaroka) thrust fault, as interpreted on sections 2, 4, 

Figure 12. Sketch map of selected structural elements, in and 

near footwall of Wasatch fault zone north of the trace of the 

Mount Raymond fault and Uinta Mountains. Location of east 

edge of Willard fault is uncertain north of about lat 

West 

ramp anticline 

Absaroka--..::... fault 

z 

5 

I 

I 

Pz 

10 MILES 

I 

10 KILOMETERS 

East 

Figure 13. Sketch of cross section showing northward view 

through structures mapped in figure 12. Generalized and 

simplified from various sources, mostly down-plunge viewing 

of figure 12 and section YY' of Royse and others (1975). 

Because sketch is generalized, it corresponds to no single line 

of section through figure 12, nor does it match details of mapped geology exactly along any one line of section. Structures 

plunge gently northward, so the rocks exposed along successively more southerly sections of figure 12 correspond to 

successively deeper horizontal cuts through this figure. 

Prethrusting rocks are Archean and Early Proterozoic (AX), late 

Proterozoic (Z), Paleozoic (Pz), and Mesozoic (Mz). Postthrusting strata are Cenozoic (Cz). Boxed letters A-C above section indicate approximate locations of the Parleys Canyon 

syncline (C) and two other large folds that trend northward 

toward or across the southern edge of the Cache allochthon 

(fig. 12). 

anticline define the east edge of the northern Utah 

highland of Eardley (1939; Northern Utah uplift of 

Eardley, 1969). 

Lastly, Cenozoic extension of the thrust complex 

produced the Wasatch and related faults. Numerous other 

named thrust faults are known in outcrop or subcrop farther east, but they can be regarded as eastern splays of 

the Willard and Absaroka thrust systems and so will not 

be treated here (Royse and others, 1975, sections XX', 

YY'). 

The thrust complex above the Absaroka thrust fault 

plunges gently north. Accordingly, south of Ogden the 

core of the ramp anticline is exposed as the Archean and 

Early Proterozoic Farmington Canyon Complex (Bryant, 

1984). Paleozoic and Mesozoic units dip east in the front 

limb of the anticline. The eastern dips identify rocks that 

were uplifted as they were carried up the frontal ramp 

and were tilted east at the top of the ramp. The tilted 

rocks are bounded on the southeast by the northeastward 

extension of the Parleys Canyon syncline (figs. 12, 13). 

Northward the ramp anticline plunges beneath the Cache 

41 °30' N. (for example, compare Royse and others, 1975, fig. 

1, with Dover, 1983). Compiled and interpreted from Stokes 

(1963), Crittenden (1972, 1974), Royse and others (1975), 

Sorensen and Crittenden (1979), Hintze (1980), Lamerson 

(1982, p. 320), Bruhn and others (1983), Dover (1983), Bryant 

(1984), and Smith and Bruhn (1984). 

Pre-Cenozoic Structures 31 
allochthon. However, the presence of the ramp anticline 

beneath the allochthon is revealed by a paired anticline 

and syncline that begin in the exposed Farmington Canyon Complex and continue northward through the Cache 

allochthon (figs. 12, 13). 

The northward deepening of the Willard thrust 

fault might define a structural anomaly for this report 

(Bruhn, Picard, and Isby, in press, fig. 8). Bruhn and 

others (1983) summarized differences in metamorphic 

grade and structural style across the Willard fault. These 

differences between the hanging wall and footwall of the 

fault might cause the two walls to respond differently to 

Neogene extension, and so might have favored segmentation of the Wasatch fault zone. A structural anomaly 

could extend from the intersection of the traces of the 

Willard and Wasatch faults, northward to the latitude 

at which the Willard fault reaches the depth at which large 

normal-faulting earthquakes are suggested to nucleate 

(15 km, according to Smith and Richins, 1984). 

However, such a structural anomaly would be too 

wide to be useful because the Willard fault deepens northward only gradually. The trace of the fault has been 

mapped at scales of 1:24,000 and 1:50,000 for a straightline distance of about 26 km southeastward from its intersection with the trace of the Wasatch fault (Sorensen and 

Crittenden, 1976, 1979; Bryant, 1984; Crittenden and 

Sorensen, 1985). Within this mountainous area the trace 

of the Willard fault is sinuous and offset by steep young 

faults. By choosing small areas between such offsets, the 

orientation of the Willard fault can be determined by 

three-point problems. Each determination averages the 

orientation of the fault over about 1 km2 • Under the 

Willard fault is a recumbent, isoclinal syncline (Crittenden and Sorensen, 1985; Crittenden, 1972). The orientation of the axial surface of the syncline, determined as 

was the fault orientation, parallels the Willard fault. 

West of the north-trending anticline and syncline 

shown in figure 12, the Willard fault is folded about an 

axis that plunges northwest at 6 ° (fig. 14). The tightness 

of the clustered points in figure 14 and the scatter of their 

source areas over a distance of about 26 km indicate that 

this low plunge can be used as a regional estimate of the 

northward deepening of the Willard fault. If the axis of 

the folded fault plunges northwest at 6 °, the axis will 

reach a depth of 15 km at a point Ill km north of its 

outcrop. Perusal of the maps of Hintze (1980) and Stokes 

(1963) showed no evidence that the fault increases its 

northward deepening anywhere south of the Utah-Idaho 

border. Instead, the map patterns indicate a gradual 

northward deepening of the fault. This indication is consistent with estimates by Crittenden (1972) that the Cache 

allochthon reaches thicknesses of 35,000 ft (10.6 km) or 

more in northernmost Utah. The indication of gradual 

northward deepening is also consistent with the shapes 

of the ramp anticline as it is shown in the successively 

32 Segmentation of Wasatch Fault Zone, Utah 

more northerly cross sections CC' of Smith and Bruhn 

(1984), YY' ofRoyseandothers(l975),andJJ' ofSmith 

and Bruhn (1984). These sections show the ramp anticline 

gradually becoming wider and lower northward, which 

would allow the overlying Cache allochthon to plunge 

gradually northward also. 

Therefore, the Willard fault might divide the upper crust into nearly horizontal layers with different 

seismogenic properties, but the fault is unlikely to dip 

steeply enough northward to segment either wall of the 

Wasatch fault zone. 

Structural Anomaly at a Lateral Ramp 

Near Salt Lake City the ramp anticline (fig. 13) 

plunges south steeply enough that a usable structural 

anomaly can be defined. The structural anomaly is 

I!JA 

I!JB 

I!JC 

N 

Figure 14. Equal-area, lower hemisphere projection of 

estimates of orientation of Willard thrust fault. Estimates are 

planes, although their poles are plotted here. Estimates are 

three-point solutions to orientation of fault (16 dots) and to 

orientation of axial surface of recumbent, isoclinal syncline 

beneath and parallel to fault (one circled dot). Great circle 

A was fit by eye to the nine northwestern poles, and C, to 

the seven southeastern poles. Pole marked with an asterisk 

was discarded as anomalous, because its orientation is so 

far from the other orientations. Boxed dots are poles to great 

circles A-C, with pole B determined as midpoint of an arc 

joining poles A and C. Great circle B was drawn from pole 

B, and represents the best fit to all 16 retained estimates of 

the fault orientation. Pole B plunge-s 6° toward N. 39° W. 

Estimates were calculated from maps of Sorensen and Crittenden (1976, 1979), Bryant (1984), and Crittenden and 

Sorensen (1985). 
associated with the southern nose of the ramp anticline. 

The Archean and Early Proterozoic core of the anticline 

is bounded on the east by Paleozoic and Mesozoic strata 

and by the northeast-trending syncline that lies at the bottom of the east limb of the anticline (fig. 13). The eastdipping strata and the syncline sweep south and then west 

into the Parleys Canyon syncline, which bounds the nose 

of the anticline on the south (fig. 12). The Archean and 

Early Proterozoic rocks in the ramp anticline and the 

Mesozoic rocks in the trough of the Parleys Canyon 

syncline both have moved up the frontal ramp, in order 

to be exposed at the present ground level. If the two rock 

masses had moved up the ramp separately, they would 

be separated by a large, east-striking, strike-slip fault. No 

such fault is mapped (Crittenden, 1974; Davis, 1983a), 

so the two rock masses moved up the ramp together. 

The structural anomaly could have either of two 

subsurface geometries, each involving a large lateral ramp 

(fig. 15). Both geometries can be visualized by noticing 

that the Precambrian core of the ramp anticline has more 

structural relief than does the Parleys Canyon syncline. 

Before the thrusting that formed both folds, the strata 

that are now involved in the anticline and nearby syncline 

were probably flat lying. Because the anticline and 

syncline are within about 10 km of each other, the strata 

involved in both folds were probably of comparable 

thicknesses before thrusting and remain so now. Total 

stratal thicknesses involved in the two folds probably differ by an amount much less than the difference in structural relief between the two folds, which Smith and Bruhn 

(1984, fig. 5B) estimated as at least 6 km. As an example 

of such a difference, strata of Late Proterozoic age are 

absent in the ramp anticline (Bryant, 1984) but are exposed south of the Parleys Canyon syncline (Davis, 

1983a, b) and so might be present under the syncline. If 

the difference in structural relief exceeds likely thickness 

differences between the areas of the two folds, then the 

Absaroka thrust fault on which both areas moved up the 

frontal ramp together must have risen from deeper 

stratigraphic levels west of the ramp anticline than west 

of the Parleys Canyon syncline. Then the ramp west of 

the anticline is higher than the ramp west of the syncline. 

· Published cross sections that are based on seismicreflection profiles represent the top of the frontal ramp 

at various locations under or a few kilometers west of 

the Wasatch fault zone (fig. 12). Dixon (1982, fig. 14, 

p. 1579) interpreted the top of the ramp to be at the western edge of a prominent reflector above the basement. 

If other authors have inferred the position of the top of 

the ramp from similar evidence, then westward-worsening 

quality of data and the subjectivity inherent in such inferences could explain the east-west scatter of the three 

circled numerals on sections 2, 4, and 5 of figure 12. 

Atop the frontal ramp the Absaroka thrust fault 

is in Cambrian shales (Monley, 1971; Royse and others, 

Figure 15. Unsealed block diagrams illustrating two possible 

geometries of lateral ramps at south nose of ramp anticline of 

figure 13. Projections are orthographic. Both diagrams represent geometries at the time that the Absaroka thrust fault 

formed, but before eastward transport had occurred. Hanging 

wall block is drawn dashed as if transparent. Parts of Absaroka 

fault are horizontal flats (F) on which motion paralleled beds, 

frontal ramp (FR) on which reverse motion cut upsection to 

east, and lateral ramp (LR) on which motion was strike slip. 

Lateral ramp is drawn perpendicular to frontal ramp for simplicity but might strike obliquely to frontal ramp. Stratigraphic levels 

of flats are in Archean and Early Proterozoic metamorphic rocks 

of Farmington Canyon Complex (AX), in late Proterozoic 

metasedimentary strata of early part of Cordilleran miogeocline 

(Z), and in Cambrian shales of miogeocline (€). Open arrows 

represent eastward transport of parts of hanging wall block up 

frontal ramp. Arrow 1 represents motion of ramp anticline that 

is cored by W rocks, and arrow 2, of rocks in and under Parleys 

Canyon syncline (figs. 12, 13). A, Lateral ramp is west of frontal 

ramp. 8, Lateral ramp is east of frontal ramp. 

1975; Dixon, 1982). The ramp anticline exposes Archean 

and Early Proterozoic rocks (fig. 12), and so must have 

risen at least from this stratigraphic level (arrows 1, 

Pre-Cenozoic Structures 33 
figs. 15A, B). Because rocks in the Parleys Canyon 

syncline have moved up less of a ramp than have rocks 

in the ramp anticline, this lesser ramp has either a 

shallower bottom (arrow 2, fig. 15A) or a deeper top (arrow 2, fig. 15B). The first alternative requires a lateral 

ramp west of the frontal ramp (fig. 15A). The second requires a lateral ramp east of the frontal ramp (fig. 15B). 

The flat at intermediate depth (labeled Z in fig. 15) could 

be in late Archean and Early Proterozoic rocks or in Late 

Proterozoic strata of the Cordilleran miogeocline. In the 

case shown in figure 15A, the Absaroka thrust would 

have ramped from an Archean level to a Late Proterozoic 

level somewhere west of arrow 2 and the area represented 

by the block diagram. In the case shown in figure 15B, 

ramping from Late Proterozoic level to Cambrian level 

would have taken place somewhere east of arrow 2 and 

the block diagram. 

We favor the model of figure 15A over that of 

figure 15B, because of a tilted frontal ramp that is exposed along the trace of the Mount Raymond fault over 

a distance of about 12 km eastward from the Wasatch 

fault zone (R.L. Bruhn, oral commun., 1984; Crittenden, 

1965b, c; Crittenden and others, 1966; Bruhn, Picard, 

and Isby, in press, figs. 5, 6; Bradley, 1986). Exposed 

strata range from Late Proterozoic to Jurassic in age and 

dip steeply northward off the dome that was formed by 

Oligocene (Crittenden and others, 1973) intrusion of the 

Little Cottonwood stock (figs. 9, 12). Thus, map view 

is nearly a cross-sectional view of pre-Oligocene structure. Down-plunge viewing of geologic maps (Crittenden, 

1965b, c; Crittenden and others, 1966; Bruhn, Picard, 

and Isby, in press) reveals that for about 6 km eastward 

from the Wasatch fault zone, the Mount Raymond thrust 

fault is in footwall rocks of the shaly Middle Cambrian 

Ophir Formation and Mississippian limestones and 

dolomites. Below this part of the thrust fault lies an unbroken sequence of Middle and Late Proterozoic and 

Lower Cambrian strata. Eastward, however, the thrust 

fault cuts progressively up section, so that its footwall 

exposes successively younger rocks ending in Middle 

Jurassic Twin Creek Limestone. Where the fault cuts up 

section, the angle between the fault and undragged beds 

in the footwall is about 30°, as calculated from mapped 

bed orientations and fault traces of Crittenden (1965b). 

The doming has uplifted and exposed a cross section of 

a frontal ramp, on which the Mount Raymond thrust 

fault has ramped from Cambrian up to Jurassic levels. 

Also exposed is part of the flat that lies at the western 

foot of the frontal ramp, in the Cambrian and Mississippian strata; the flat lies south along trend from the nose 

of the ramp anticline (fig. 12). 

The Mount Raymond thrust fault is interpreted to 

be part of the basal thrust fault, the Absaroka fault (fig. 

12; Crittenden, 1974; Hintze, 1980; Bradley, 1986). The 

Absaroka fault underlies the ramp anticline at Cambrian 

34 Segmentation of Wasatch Fault Zone, Utah 

level (Royse and others, 1975) and is at Cambrian level 

at the Mount Raymond fault. The ramp anticline and 

Mount Raymond fault are separated by only 10-20 km, 

a distance that includes the Parleys Canyon syncline, so 

presumably the deepest thrust fault under the Parleys 

Canyon syncline is also at Cambrian level. If the deepest 

thrust fault atop the frontal ramp lies in Cambrian rocks 

under the ramp anticline, under the Parleys Canyon syncline, and at the Mount Raymond fault, then the model 

of figure 15A must be preferred over that of figure 15B. 

Thus, the structural anomaly that has caused the 

southern nose of the ramp anticline is best interpreted 

to be a north-dipping lateral ramp that extends westward 

from the frontal ramp, and which underlies the hanging 

(west) wall of the Wasatch fault zone. This conclusion 

is consistent with independent interpretations of Smith 

and Bruhn (1984, section DD') and Schirmer (1985, 

p. 136-137). 

The location of the lateral ramp can be estimated 

from the southward decrease in structural relief of the 

ramp anticline, by recalling that structural relief increases 

with the height of the frontal ramp that produced the 

relief. The south end of the anticlinal nose should be east 

of the top of the lateral ramp of figure 15A. Between the 

anticlinal nose and the Parleys Canyon syncline lies a 

small anticline-syncline pair (fig. 12). The south end of 

the nose of the ramp anticline is the north limb of the 

syncline of the fold pair. The bottom of the lateral ramp 

is harder to locate, because the shape of the ramp anticline is obscured by Tertiary cover, the complex internal structure of the Archean and Early Proterozoic rocks, 

and extensional faults (Bryant, 1984). A dip of about 30° 

is common among ramps, so the lateral ramp is assumed 

to dip 30 ° north. Such a ramp would reach a depth of 

15 km at about 26 km north of the small syncline. Perusal 

of the map pattern of the ramp anticline (Stokes, 1963; 

Bryant, 1984) suggests that the nose of the anticline might 

extend as far as 38 km north of the small syncline, so 

the lateral ramp might dip shallowly enough north (22 ") 

to be 38 km wide in map view. Accordingly, the structural anomaly is taken as 26 km wide, with its south edge 

at the trough of the small syncline. The width of the 

anomaly is uncertain and could be as much as 38 km, 

assuming that the south edge of the anomaly is fixed at 

the synclinal trough. 

Cretaceous Thrust Faults South of the 

Uinta Mountains 

Geometries of the Thrust Faults 

South of the Deer Creek fault and the Uinta Mountains, the footwall of the Wasatch fault zone exposes a 

thrust complex much like that in northern Utah. The best 
exposed thrust sheet in central Utah is the Charleston 

thrust sheet (fig. 16). Like the Cache allochthon of northern Utah, the Charleston sheet contains rocks of the Cordilleran miogeocline that moved eastward over rocks of 

the coeval shelf facies (Crittenden, 1961, 1972, 1976; 

Tooker, 1983). From thrust offsets of isopachs of lower 

Paleozoic, Mississippian, and Pennsylvanian to Permian 

units, Crittenden (1961) estimated eastward transport of 

about 40 mi (64 km) each for the Cache allochthon and 

Charleston sheet. From a balanced cross section through 

the northern part of the Charleston sheet, Riess (1985, 

pCM, IPP, and Mz 

• CH ...--......_ 

"""-=-~. "" 

IPP ffi ·-·-·ksv-& 

·-·~ '\~?~~) pCM J Mz f 

CHARJESTON I 

rH'Rusr 1 

;SHEET I 

.'-y./ , 

[rP£M;I ·"'· I ,. I I Mz \i 

0z r;IPP / xl'· & .-L.;::d./'1 \ 

-- I \ 

Nephiot NB X \ & ...,, I I 

LC/y 0z --J Mz I IP~Leaming- ffi J \ 0 ton "·........._ + 

---j ·-...........I 

pCM Mz & !-............./--.. Mz 'i-.. • ........._ 

\-·---.~.i~·--J·~-~~' \ -· ·-·-.·':::;:t 'I 

I Cz -{ / &-·-·-·-.___, 

\ & 

Gunnison\0 

I I 

10 

10 

I I 

20 MILES 

I 

20 KILOMETERS 

Figure 16. Sketch map of selected structural elements, in and 

near the footwall and part of the hanging wall of the Wasatch 

fault zone, south of the Deer Creek fault. Compiled and interpreted from Christiansen (1952), Stokes and Madsen (1961), 

Stokes (1963), Hintze (1980), Standlee (1982), Smith and Bruhn 

(1984), Villien and Kligfield (in press), and Lawton (1985). 

Deer Creek fault bounds Charleston thrust sheet on the 

north and connects eastward to Charleston thrust fault, but 

underwent Tertiary extensional motion over about 1 0 km of 

its length east of the Wasatch fault zone (Baker and Crittenden, 

p. 21) estimated 41 km of eastward transport. Paleontological, stratigraphic, and structural data have established 

that most thrusting was of Cretaceous age in both northern and central Utah (Armstrong and Oriel, 1965; Oriel 

and Armstrong, 1966; Royse and others, 1975; Standlee, 

1982; Fouch and others, 1983; Villien and Kligfield, in 

press; Lawton, 1985; Heller and others, 1986; Bryant and 

Nichols, in press; Bruce Bryant, oral and written 

communs., 1985). 

The Charleston sheet is bounded in map view by 

traces of the Deer Creek, Charleston, Strawberry Valley, 

EXPlANATION 

_......~...._ Trace of Wasatch fault-Dashed where uncertainly located. 

Bar and ball on downthrown (west) side 

++ Anticlinal crest or synclinal trough-Dashed where covered 

by Upper Cretaceous or Cenozoic strata 

--f(l-T+- Overturned syncline 

+ + Trace of thrust fault-Dashed where covered by Upper 

Cretaceous or Cenozoic strata, sawteeth on hanging 

wall. All thrust faults involved eastward movement of 

hanging walls. DC, Deer Creek fault; CH, Charleston 

fault; NB, Nebo fault; LC, Leamington Canyon fault; SV, 

Strawberry Valley fault. 

--- Generalized contact between Precambrian to Mississippian 

(pC M), Pennsylvanian to Permian (IPP), and Mesozoic 

(Mz) strata that are involved in thrusting-Dashed 

where covered by postthrusting- Upper Cretaceous or 

Cenozoic strata (Oz). Location of eastern edge of 

youngest thrust strata is uncertain and is taken as about 

at the easternmost structures shown here. East of these 

structures, rocks are mostly Mesozoic and Cenozoic. 

Structures not compiled for northwest or southwest parts 

& of map area 

f-·-·~ Approximate location of a published cross sectionNumeral in triangle identifies section. 1, section EE' of 

Smith and Bruhn (1984); 2, section DD' of Lawton 

(1985); 3, section FP of Smith and Bruhn (1984); 4, 

section CC' of Lawton (1985); 5, section in figure 14 of 

Standlee (1982); 6, section AA' of Villien and Kligfield 

(in press); 7, section AA'A" of Riess (1985, plate II). 

Section 6 extends west beyond edge of figure 

1961, section AA'; Riess, 1985). This Tertiary extension on the 

Deer Creek fault might have reactivated an east-striking lateral 

ramp that formed during Cretaceous thrusting on the Charleston 

fault, but such reactivation is not proven. Here the Deer Creek 

fault is drawn as an extension of the Charleston fault for simplicity. Geometry of connection between Strawberry Valley and 

Nebo faults is uncertain (Lawton, 1985, figs. 2, 6, p. 1156). 

Northwestward dip of Leamington Canyon fault and its connection with Nebo fault are controversial (Higgins, 1982; 

. Sprinkel and others, 1982, p. 303-306), as discussed in text. 

Pre-Cenozoic Structures 35 
and Nebo faults, and by their mapped and inferred connections. At the Wasatch fault zone, along the west edge 

of the exposed part of the thrust sheet, one or two large 

ramp anticlines expose east-dipping Precambrian and 

Paleozoic strata and have been dismembered by the irregular trace of the Wasatch fault zone. Tertiary extensional reactivation of some Mesozoic thrust faults (Royse, 

1983; Hopkins and Bruhn, 1983; Riess, 1985) might have 

contributed to this dismemberment. 

The thrust sheet has internal structural complexities (Smith and Bruhn, 1984, p. 5740-5741; Wheeler, 

1984; Lawton, 1985), but its north and south edges stand 

out as candidates for structural anomalies. At the north 

edge, the Deer Creek fault has already been used to define 

the south end of an anomaly that is associated with the 

Uinta aulacogen. At the south edge, the Nebo fault has 

been inferred to extend southwest as the Leamington Canyon fault (Christiansen, 1952; Morris and Shepard, 1964; 

Morris, 1983; Smith and Bruhn, 1984, p. 5741; Bruhn, 

Picard, and Is by, in press, fig. 8). The Nebo and Leamington Canyon faults are southern limits of exposures of 

thick, Pennsylvanian and Permian rocks of the miogeocline. The Nebo and perhaps the Leamington Canyon 

faults transported these basin-facies rocks eastward, overriding or deforming Upper Cretaceous synorogenic strata 

(Christiansen, 1952, plate 1; Jefferson, 1982; Sprinkel and 

others, 1982, p. 303-306; Lawton, 1985). Both faults can 

be regarded as parts of the same northeast-striking 

oblique ramp (Smith and Bruhn, 1984, p. 5741), up which 

the Charleston sheet moved as its basal thrust fault cut 

up section eastward. 

Several map relations are consistent with this interpretation. On the southeast side of the Leamington Canyon fault, the north end of a large syncline bends to the 

northeast (fig. 16). Where this syncline trends north, beds 

in the west limb are upright and dip moderately east, but 

where trends swing to the northeast, the northwest limb 

is overturned (Christiansen, 1952, plate 1; Higgins, 1982, 

fig. 7). Where trends change from north to northeast in 

the west limb of the syncline, several small strike-slip 

faults cut Cambrian units at high angles (Higgins, 1982, 

fig. 7, p. 54). The angles at which these small faults cut 

beds in map view, and the senses of the offsets of the 

faults, indicate that the faulting would have caused the 

overturned limb of the syncline to undergo northeastsouthwest extension. The northeastward bend of the 

syncline, the overturning of the northwest limb, and the 

northeastward extension are all consistent with the 

hypothesized overriding of the north end of the syncline 

by the Charleston thrust sheet as it moved up an oblique 

ramp, and with deformation of the north end of the 

syncline by drag in the footwall of the ramp. 

However, other map relations are inconsistent with 

the interpretation that the Leamington Canyon and Nebo 

faults are parts of the same oblique ramp. In its few small 

36 Segmentation of Wasatch Fault Zone, Utah 

exposures the Leamington Canyon fault dips southeast 

instead of northwest (Morris, 1983, p. 77; Higgins, 1982, 

fig. 7). Slickenlines and hinges of small folds record 

northwestward or southeastward movement on the Leamington Canyon fault (Higgins, 1982, p. 50, fig. 12), but 

Higgins does not mention the sense of this movement. 

If the sense of movement was northwestward, then the 

southeast-dipping Leamington Canyon fault might be a 

small backthrust (Sprinkel and others, 1982, p. 305-306), 

subsidiary above some larger, east- or southeast-directed 

thrust fault. If so, then apparently this hypothetical larger 

fault is not exposed anywhere. If the sense of movement 

on the Leamington Canyon fault was southeastward, then 

the southeast dips of this fault might indicate that the 

fault has been folded within a ramp anticline that formed 

as a younger, deeper, unexposed thrust fault moved up 

the hypothesized oblique ramp. 

Examination of the maps and sections of Higgins 

(1982) and Christiansen (1952) suggest these and other 

structural interpretations of these apparently contradictory relations, but all such interpretations are speculative. 

It remains unclear whether the Leamington Canyon fault 

is the main thrust fault northeast of Leamington, which 

way the main fault dips or moved, and whether the main 

fault and the Nebo fault are part of the same oblique 

ramp. Accordingly, the following calculations assume 

only that there is an oblique or lateral ramp in the Nebo 

fault where it crosses the Wasatch fault; the calculations 

are independent of whether the Leamington Canyon and 

Nebo faults are connected. 

Structural Anomaly at a Lateral Ramp 

The width of the structural anomaly near Nephi 

must be estimated from map relations there, excluding 

relations near Leamington. Near the_ intersection of the 

Nebo and Wasatch faults, the Charleston thrust sheet 

(above the Nebo fault) has been interpreted to have been 

distorted by evaporite diapirism of Cretaceous to Tertiary 

age, which was intense locally (Witkind, 1982; 1983, fig. 

4) but not regionally in the thrust sheet (Standlee, 1982; 

Lawton and others, 1982). Map patterns of the trace of 

the Nebo fault and of vertical to overturned units in its 

hanging wall (Johnson, 1959; Black, 1965) can be interpreted to indicate that diapiric distortion is confined south 

of a point that lies 7 km north of Nephi. The effect of 

the distortion takes the form of a south-facing half 

window in the thrust sheet (fig. 16). However, cross sections of Johnson (1959) and Black (1965) can also be interpreted to attribute this half window to irregularities 

in the original shape of the Nebo thrust fault, perhaps 

locally accentuated by diapirism. Whether the half window is attributed to diapirism, to the original shape of 

the Nebo fault, or to both, elsewhere the fault remains 

more or less horizontal where exposed in the hills 0-7 km 
north of Nephi. If the Nebo fault dips north or northwest 

on a lateral or oblique ramp, it must do so farther than 

7 km north of Nephi. Because the thrust sheet is distorted 

only south of this point, the top of the ramp is assumed 

to lie 7 km north of Nephi. 

If the ramp is lateral, it strikes east. If it is oblique, 

it probably strikes N. 70 ° E., to pass along the south 

edges of exposures of basin-facies Pennsylvanian to Permian rocks southwest of Nephi (Hintze, 1980). A lateral 

ramp that dips north at 30 ° would reach a depth of 15 km 

at a point 26 km north of the top of the ramp. An oblique 

ramp would reach this depth 24 km north of its top. The 

mean of these two estimates is 25 km, measured along 

a north-south line. 

Thus, a structural anomaly that represents a lateral 

or oblique ramp in the Nebo fault most likely spans the 

area between 7 and 32 km north of Nephi. Cartographic 

errors and the 2-km difference between the northward 

extents of lateral and oblique ramps combine to indicate 

that parts of the ramp might lie buried as little as 6 or 

as much as 34 km north of Nephi. 

Southward, other thrust sheets emerge from 

beneath the Charleston sheet (Standlee, 1982; Villien and 

Kligfield, in press; Lawton, 1985). Cross sections through 

the other sheets are based on geologic mapping and on 

well and seismic-reflection data. The sections differ in the 

relative sizes of several structures and in the inferred 

eastern limit of thrusting, but not in the main features 

shown in the sections. Neither these sections, nor their 

accompanying geologic and structural maps, nor the 

mapped surface geology (Hintze, 1980; Stokes, 1963) 

shows any candidates for structural anomalies between 

the Nebo fault and the southern edge of the study 

area. 

Table 6. Locations of structural anomalies 

Results 

Anomaly St1 corresponds to the north-dipping 

lateral ramp that we infer to lie west of the southern nose 

of a ramp anticline between Salt Lake City and Ogden 

(figs. 12, 15A). The south end of the anomaly is where 

the trace of the Wasatch fault zone intersects the trough 

of a small syncline about 5 km north of the Parleys Canyon syncline. The north end of the anomaly might be as 

much as 38 km north of this point, but the anomaly has 

a most likely width of 26 km. 

Anomaly St2 corresponds to the fault system of the 

Uinta aulacogen. The anomaly extends 21 km along the 

Wasatch fault zone between its intersections with the 

traces of the Deer Creek and Mount Raymond faults 

(figs. 9, 10). North-south width could vary from 4 to 

42 km. Regardless of width, the anomaly is centered on 

the midpoint of distance ab of figures 9 and 10. 

Anomaly St3 corresponds to the north-dipping, 

lateral or oblique ramp that is inferred to connect to the 

Nebo fault (fig. 16). The anomaly spans the area from 

7 to 32 km north of Nephi, and might extend as little as 

6 km or as much as 34 km north of Nephi. 

The three anomalies were plotted on the map of 

Hintze (1980) and projected west onto long 112 ° W. 

(table 6). 

The anomalies meet the seven criteria set out previously. First, the anomalies were identified using geological data at various scales but were located using 

mapping that was published mostly at 1 :24,000, which 

affords sufficient resolution to detect a segment boundary. Second, the structural data characterize large structures in the top part of the brittle upper crust, where large 

earthquakes are likely to nucleate. Third, the structural 

[n.a., not applicable. Study area is 332 km long, north to south] 

Anomaly North end South end Anomaly Distance to next 

number of anomaly 1 of anomaly 1 width (km) anomaly to south (km) 

Stl 112 138 26 8 

St2 146 167 21 53 

St3 220 245 25 287 

Sum n.a. n.a. 72 148 

1Distance south of Utah-Idaho border, in kilometers. 

2Distance to south edge of study area at lat 39° N. 

Pre-Cenozoic Structures 37 
data are likely to reflect the presence of a segment boundary by an explainable anomaly. The anomalies correspond to faults or fault systems that could divide one or 

both walls of the Wasatch fault zone into blocks. The 

faults or fault systems could have low enough strength 

that they could partly or entirely decouple blocks that they 

separate, thereby confining seismic ruptures to single 

blocks (as suggested independently from gravity data by 

Zoback (1987) for St3). Such confining would amount 

to segmentation, and such faults or fault systems would 

be segment boundaries. Alternatively, the faults that cross 

the Wasatch fault zone at anomalies Stl-St3 could form 

nonconservative barriers (King and Nabelek, 1985), at 

which large seismic ruptures might tend to start and stop. 

Fourth, the structural data allow inferences about expected near-future seismicity, because the pre-Cenozoic 

structures involved have stopped their developments and 

their characteristics will not change over the next decades 

to millenia. Fifth, the structural anomalies are reproducible because Smith and Bruhn (1984) and Bruhn and Smith 

(1984) have independently observed apparent spatial 

associations of anomalies Stl-St3 and other structures 

with the segment boundaries that were proposed by 

Schwartz and Coppersmith (1984). Also, the anomalies 

have been described in enough detail that an independent 

reader should be able to reproduce them by reference to 

the cited sources. Sixth and seventh, the structural 

anomalies were derived from structural, stratigraphic, 

and other geological data, but without reference to any 

other data types of this report or to the proposed segment boundaries of Schwartz and Coppersmith (1984). 

SUMMARY AND QUESTIONS 

The six data types discussed above have 2-10 

anomalies each and together form 26 anomalies along the 

length of the Wasatch fault zone in Utah (fig. 17A). The 

distribution of anomalies along the fault zone defines a 

pattern of clustered anomalies. The north and south ends 

of each anomaly in the pattern have various locational 

uncertainties, which are small compared to anomaly 

widths and do not greatly distort most aspects of the 

anomaly pattern (figs. 17B, C). 

Each data type and its anomalies have been shown 

to satisfy criteria-that insure that the anomalies are pertinent to identifying segment boundaries. Each anomaly 

was identified by methods that are partly or wholly subjective. Subjectivity introduces operator variability, which 

can decrease the reliability of results, so the criteria also 

insure that the individual anomalies and their locations 

are reliable enough to be worth interpreting. 

Each anomaly might indicate the presence of a segment boundary. Where several anomalies in different 

data types fall at about the same place along the fault 

38 Segmentation of Wasatch Fault Zone, Utah 

LU 

c 

::;) 

1-

i= 

s 

42° 

00' 0 

50 

100 

41° 

00' ffi 

c 

c:::: 

0 

c:c 

0 

:I: 

<t 

c 

± 150 <t 

1-

:::l 

u... 

0 

:I: 

1-

:::l 

0 

Cl) 

Cl) 

c:::: 

~ 200 LU 

::!! 

g 

40° :;;;;:: 

00' 

39° A 

00' 

u 

i= 

LU 

~~ 

~~ <t<t 

:ii!c >c:t 0 

~c c:::: 

LU 

(!J <t 

Ml 

_, 

<t 

u 

tEi 

g~ Cl) 

1-

~c z 

LU Cl) ::::; w (;§ Cl) 

Sal 

Se4 Sa4 

>-

:%: 

a... 

<t 

c:::: 

t!:l 

0 

a... 

0 

1-

Tl 

T2 

_, 

<t 

c:::: 

::;)~ 

t;<t 

::)C 

c:::: 

1-

Cl) 

Salt 

Lake 

City 

Provo 

Nephi 

Figure 17(above and facing page). Distribution of anomalies 

along Wasatch fault zone. Length of anomaly pattern is 332 

km, southward from Utah-Idaho border. Anomaly locations 

were projected east or west from trace of fault zone into lat 

112° W. Locations of anomaly ends along this meridian and 

anomaly names are from tables 1-6. Columns show anomalies 

in six data types discussed in text. Solid line in each column 

steps to right edge of column where there is an anomaly in 

the corresponding data type, and steps to left edge between 

anomalies. A, Each anomaly is drawn with its measured width 

(tables 1-6). B, Each anomaly is drawn with the minimum width 

that is allowed by the locational uncertainties of its edges. This 

procedure would give anomaly Se3 a width of 0 km, so its 

minimum width is arbitrarily set at 1 km. Uncertainties are 

described in the "Results" sections of the six parts of this report 

that describe the data types. C, Each anomaly is given its maximum allowable width. Anomalies Se2 and Se3 overlap by 2 

km, as do St1 and St2. 
LU 

c 

:::::1 

1-

i= 

:5 

42° 

00' 0 

41° 

00' a: 

LU 

c 

a: 

0 

cc 

0 

:I: 

<( 

c 

± 150 

~ :::::1 

.... 

0 

:I: 

1-

:::::1 

0 

Cl) 

Cl) 

a: 

~ 200 LU 

::::2!:: 

0 

..... 40° ::;;;:: 

DO' 

39° B 

00' 

~ 

i= 

LU 

~~ ~<( <(<( 

::::i!:c >!;;;: 0 

~c a: 

L.U 

t!J <( 

Gl 

G2 

Ml 

M3 

..... 

<( 

~ >- ..... (!)<( :I: <( c... glo<( 1-

Cl) 

a: 

<( ~~ ::::2!::0 ge3 z ~ 

Cl) LU 0 ::i c... a: w Ci5 0 Cl) 1- ~ 

Brigham 

City 

Sel Sal 

Ogden 

Se2 

Salt 

Tl Lake 

City 

Provo 

Se4 

Sa4 

T2 

Nephi 

zone, a segment boundary is a reasonable interpretation, 

provided other causes of the coinciding anomalies can be 

ruled out. For example, at and near km 70 (fig. 17A) 

anomalies G4, Ml, Sel, Sal, and perhaps G3 appear to 

coincide. Near km 230 anomalies G9, M3, Se4, Sa4, St3, 

and perhaps T2 similarly appear to coincide. Over a broad 

zone between km 110 and 180 as many as 12 anomalies 

might coincide. These three parts of the fault zone might 

each contain one or more segment boundaries. 

However, subjective evaluation of figure 17 can 

take us little farther than this. Probably most interpreters 

would agree that G3 coincides with Ml and Sel, and G4 

LU 

c 

:::::1 

1-

i= 

<( 

..... 

42° 

00' 0 

41° 

00' a: 

LU 

c 

a: 

0 

cc 

0 

:I: 

<( 

c 

± 150 <( 

1-

:::::1 

.... 

0 

:I: 

1-

:::::1 

0 

Cl) 

Cl) 

a: 

~ 200 LU 

::::2!:: 

0 

..... 40° ::;;;:: 

DO' 

390 c 

00' 

~ 

i= 

LU 

~~ 

~~ <(<( 

::::i!:c ><( 0 

a: ~c LU 

~ <( 

Ml 

..... 

<( 

~ >- ..... (!)<( :I: <( c... a: g!;;i Cl) <( 1- a: :::::1~ 

§lc z ~ t;<( LU 0 :::JC Cl) ::i c... a: w Ci5 0 Cl) 1- ~ 

Brigham 

City 

Salt 

Tl Lake 

City 

Provo 

T2 

Nephi 

with Ml, Sel, and Sal, but might disagree about whether 

G3 coincides with Sal or G4. Does T2 coincide with the 

five anomalies that lie immediately north of it (G9, M3, 

Se4, Sa4, and St3)? Probably most interpreters would 

consider M3 to coincide with G9 and St3, and G9 and 

St3 to coincide with Se4 and Sa4; does this mean that 

M3 coincides with Se4 and Sa4? Should the cluster of 12 

anomalies between km 100 and 180 (fig. 17A) be subdivided? If so, how? More subtly, how much of the visual 

impact of figure 17 is caused by the order in which the 

data types are listed from left to right? 

Chance also creates a problem for interpretation 

Summary and Questions 39 
of the observed anomaly pattern. Anomalies are abundant enough and wide enough that two or more might 

coincide by chance, even if the coincident anomalies share 

no underlying geological cause like a segment boundary. 

Experiments with anomalies that are scattered randomly 

along a fault zone show that chance coincidences can be 

surprisingly common and can look disturbingly convincing (Wheeler and Krystinik, 1987a). Anomaly coincidences that have a high probability of having arisen by 

chance should not be considered worth interpreting. How 

many anomalies in different data types must coincide 

before we may confidently assume that the pattern of 

coincidences is worth interpreting? 

Development and application of methods to answer 

these questions lie well beyond the scope of this report, 

which is restricted to data summaries, analyses, and interpretations. However, Wheeler and Krystinik (1987a) 

derived and described such methods, and Wheeler and 

Krystinik (1987b, c, d) applied them to the anomaly pattern of figure 17 and the values of tables 1-6. 

RE FE RE NCES CITED 

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., 6 folded plates. 

Anderson, L.W., and Miller, D.G., 1979, Quaternary fault map 

of Utah: Long Beach, Calif., Fugro, Inc., 35 p., 1 folded 

plate, scale 1:500,000. 

Arabasz, W.J., 1984, Earthquake behavior in the Wasatch 

Front area-Association with geologic structure, space-time 

occurrence, and stress state, in Hays, W.W., and Gori, 

P.L., eds., Workshop on "Evaluation of regional and urban earthquake hazards and risk in Utah", Salt Lake City, 

Utah, August 14-16, 1984, Proceedings: U.S. Geological 

Survey Open-File Report 84-7 63, p. 310-3 3 9. 

Arabasz, W.J., and Julander, D.R., 1986, Geometry of 

seismically active faults and crustal deformation within the 

Basin and Range-Colorado Plateau transition in Utah: 

Geological Society of America Special Paper 208, p. 43-74. 

Arabasz, W.J., Smith, R.B., and Richins, W.D., 1980, Earthquake studies along the Wasatch Front, Utah-Network 

monitoring, seismicity, and seismic hazards: Bulletin of the 

Seismological Society of America, v. 70, p. 1479-1499. 

Armstrong, R.C., and Oriel, S.S., 1965, Tectonic development 

of Idaho-Wyoming thrust belt: American Association of 

Petroleum Geologists Bulletin, v. 49, p. 1847-1866. 

Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and 

Utah: Geological Society of America Bulletin, v. 79, 

p. 429-458. 

Baars, D.L., 1972, Devonian system, in Mallory, W.W., ed.-

in-chief, Geologic atlas of the Rocky Mountain region: 

Denver, Colo., Rocky Mountain Association of Geologists, 

p. 90-99. 

Baker, A.A., and Crittenden, M.D., Jr., 1961, Geology of the 

40 Segmentation of Wasatch Fault Zone, Utah 

Timpanogos Cave quadrangle, Utah: U.S. Geological 

Survey Geologic Quadrangle Map GQ-132, scale 1 :24,000, 

1 sheet. 

Bates, R.L., and.Jackson, J.A., eds., 1980, Glossary of geology, 

2d ed.: Falls Church, Va., American Geological Institute, 

749 p. 

Behrendt, J.C., and Thiel, Edward, 1963, A gravity and 

magnetic survey of the Uinta Mountains: Journal of 

Geophysical Research, v. 68, p. 857-868. 

Berg, T.M., Edmunds, W.E., Geyer, A.R., Glover, A.D., 

Hoskins, D.M., MacLachlan, D.B., Root, S.l., Sevon, 

W.D., and Socolow, A.A., compilers, 1980, Geologic map 

of Pennsylvania: Pennsylvania Topographic and Geologic 

Survey Map 1, scale 1:250,000, 3 sheets. 

Black, B.A., 1965, Nebo overthrust, southern Wasatch Mountains, Utah: Brigham Young University Geology Studies, 

v. 12, p. 55-89. 

Bollinger, G.A., Teague, A.G., Munsey, JW., and Johnston, A.C., 

1985, Focal mechanism analyses for Virginia and eastern 

Tennessee earthquakes (1978-1984): U.S. Nuclear Regulatory 

Commission NUREG/CR-4288, 86 p., 8 microfiches. 

Bollinger, G.A., and Wheeler, R.L., 1982, The Giles County, 

Virginia, seismogenic zone-Seismological results and 

geological interpretations: U.S. Geological Survey OpenFile Report 82-585, 142 p. 

___ 1983, The Giles County, Virginia, seismic zone: Science, 

v. 219, p. 1063-1065. 

Boyer, S.E., and Elliott, David, 1982, Thrust systems: American 

Association of Petroleum Geologists Bulletin, v. 66, 

p. 1196-1230. 

Bradley, M.D., 1984, Demarcation of a duplex zone-Rockport 

Lake area, Utah [abs.]: American Association of Petroleum 

Geologists Bulletin, v. 68, p. 932-933. 

___ 1986, Thrust splay correlation in the Absaroka thrust 

system of northcentral Utah: Geological Society of America 

Abstracts with Programs, v. 18, no. 5, p. 343. 

Bruhn, R.L., Gibler, P.R., Houghton, Wendy, and Parry, 

W.T., 1987, Structure of the Salt Lake segment, Wasatch 

normal fault zone-Implications for rupture propagation 

during normal faulting, in Gori, P.L., and Hays, W.W., 

eds., Assessment of regional earthquake hazards and risks 

along the Wasatch Front, Utah: U.S. Geological Survey 

Open-File Report 87-585, v. I, p. H1-H57. 

Bruhn, R.L., Gibler, P.R., and Parry, W.T., 1987, Rupture 

characteristics of normal faults-An example from the 

Wasatch fault zone, Utah, in Coward, M.P., Dewey, J.F., 

and Hancock, P.H., eds., Continental extensional tectonics: Geological Society of London Special Publication 

No. 28, p. 337-353. 

Bruhn, R.L., Picard, M.D., and Beck, S.L., 1983, Mesozoic 

and early Tertiary structure and sedimentology of the central Wasatch Mountains, Uinta Mountains and Uinta 

Basin, in Gurgel, K.D., ed., Geologic excursions in the 

overthrust belt and metamorphic core complexes of the Intermountain region: Utah Geological and Mineral Survey 

Special Studies 59, p. 63-105. 

Bruhn, R.L., Picard, M.D., and Isby, J.S., in press, Tectonics 

and sedimentology of the Uinta arch, western Uinta Mountains and Uinta basin, in Petersen, James, ed., Sedimentation and tectonics in the Rocky Mountain region: 
American Association of Petroleum Geologists Special 

Publication. 

Bruhn, R.L., and Smith, R.B., 1984, Relation of Wasatch fault 

zone to ancient thrust belt structures-Implications for 

normal fault geometry and maximum expectable earthquake magnitudes [abs.]: Transactions of the American 

Geophysical Union, v. 64, no. 45, p. 842. 

Bryant, Bruce, 1984, Reconnaissance geologic map of the Precambrian Farmington Canyon Complex and surrounding 

rocks in the Wasatch Mountains between Ogden and Bountiful, Utah: U.S. Geological Survey Miscellaneous Investigations Map 1-1447, scale 1:50,000, 1 sheet. 

___ 1985, Structural ancestry of the Uinta Mountains, in 

Picard, M.D., ed., Geology and energy resources, Uinta 

basin of Utah: Utah Geological Association Publication 

12, p. 115-120. 

Bryant, Bruce, and Nichols, D.J., in press, Late Mesozoic and 

early Tertiary reactivation of an ancient zone of crustal 

weakness along the Uinta arch and its interaction with the 

Sevier orogenic belt, in Schmidt, C.J., and Perry, W.J., 

Jr., eds., Interaction of the Rocky Mountain foreland and 

the frontal Cordilleran thrust belt: Geological Society of 

America Special Paper. 

Bucknam, R.C., Algermissen, S.T., and Anderson, R.E., 1980, 

Patterns of late Quaternary faulting in western Utah and 

an application to earthquake hazard evaluation, in Andriese, P.D., compiler, Earthquake hazards along the 

Wasatch-Sierra Nevada frontal fault zones, 1979, Proceedings: U.S. Geological Survey Open-File Report 80-801, 

p. 299-314. 

Burchfiel, B.C., and Davis, G.A., 1975, Nature and controls 

of Cordilleran orogenesis, western United StatesExtensions of an earlier synthesis: American Journal of 

Science, v. 275-A, p. 363-396. 

Burke, Kevin, and Dewey, J .F., 1973, Plume-generated triple 

junctions-Key indicators in applying plate tectonics to old 

rocks: Journal of Geology, v. 81, p. 406-433. 

Butler, R.W.H., 1982, The terminology of structures in thrust 

belts: Journal of Structural Geology, v. 4, p. 239-245. 

Calver, J.L., Hobbs, C.R.B., Jr., Milici, R.C., Spiker, C.T., 

and Wilson, J.M., 1963, Geologic map of Virginia: Virginia 

Division of Mineral Resources, scale 1:500,000, 2 sheets. 

Campbell, J.A., 1975, Structural geology and petroleum potential of the south flank of the Uinta Mountain uplift, northeastern Utah: Utah Geology, v. 2, p. 129-132, 2 folded 

plates, approximate scales 1:703,000, 1:43,640, and 

1:69,800. 

Chamberlain, A.K., 1984, Shallow-water clastic sediments of 

Great Blue Formation and Manning Canyon Shale, 

Oquirrh basin, Utah [abs.]: American Association of 

Petroleum Geologists Bulletin, v. 68, p. 934. 

Chen, W.-P., and Molnar, Peter, 1983, Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the 

lithosphere: Journal of Geophysical Research, v. 88, no. 

B5, p. 4183-4214. 

Christiansen, F. W., 1952, Structure and stratigraphy of the Canyon Range, central Utah: Geological Society of America 

Bulletin, v. 63, p. 717-740. 

Christie-Blick, Nicholas, 1984, Latest Proterozoic and Early 

Cambrian rifting in California, Nevada and Utah-Support 

for Early Cambrian continental separation: Geological 

Society of America Abstracts with Programs, v. 16, no. 

6, p. 471. 

Christie-Blick, Nicholas, and von der Borch, C.C., 1985, Sequence boundaries in Proterozoic successions-A comparison of the Brigham Group (Utah) and Wilpena Group 

(South Australia), with implications for sea-level change 

and global correlation: Geological Society of America 

Abstracts with Programs, v. 17, no. 7, p. 546. 

Cleaves, E.T., Edwards, Jonathan, Jr., and Glaser, J.D., 1968, 

Geologic map of Maryland: Maryland Geological Survey, 

scale 1:250,000, 1 sheet. 

Clement, J.H., 1983, North flank of the Uinta Mountains, 

Utah, in Bally, A. W., ed., Seismic expression of structural 

styles-A picture and work atlas: American Association 

of Petroleum Geologists Studies in Geology Series No. 15, 

v. 3, p. 3.2.2-29 to 3.2.2-32. 

Cluff, L.S., Brogan, G.E., and Glass, C.E., 1970, Wasatch 

fault, northern portion-Earthquake fault investigation 

and evaluation: Oakland, Calif., Woodward-Clyde and 

Associates unpublished report to Utah Geological and 

Mineral Survey, 109 p. 

___ 1973, Wasatch fault, southern portion-Earthquake 

fault investigation and evaluation: Oakland, Calif., 

Woodward-Lundgren and Associates unpublished report 

to Utah Geological and Mineral Survey, 179 p. 

Cluff, L.S., Glass, C.E., and Brogan, G.E., 1974, Investigation and evaluation of the Wasatch fault north of Brigham 

City and Cache Valley faults, Utah and Idaho: Oakland, 

Calif., Woodward-Lundgren and Associates unpublished 

report to U.S. Geological Survey, 232 p. 

Cluff, L.S., Hintze, L.F., Brogan, G.E., and Glass, C.E., 1975, 

Recent activity of the Wasatch fault, northwestern Utah, 

U.S.A.: Tectonophysics, v. 29, p. 161-168. 

Cluff, L.S., and Slemmons, D.B., 1972, Wasatch fault zoneFeatures defined by low-sun-angle photography, in Hilpert, 

L.S., ed., Environmental geology of the Wasatch Front, 

1971: Utah Geological Association Publication 1, 

p. G1-G9. 

Cook, K.L., and Berg, J.W., Jr., 1961, Regional gravity survey 

along the central and southern Wasatch Front, Utah: U.S. 

Geological Survey Professional Paper 316-E, p. 75-89, 1 

folded plate, scale 1:500,000. 

Cook, K.L., and Montgomery, J.R., 1972, East-west transverse 

structural trends in eastern Basin and Range province as 

indicated by gravity data: Geological Society of America 

Abstracts with Programs, v. 4, no. 5, p. 315. 

___ 1974, Crustal structure and east-west transverse structural trends in eastern Basin and Range province as indicated by gravity data: Geological Society of America 

Abstracts with Programs, v. 6, no. 3, p. 158. 

Cook, K.L., Montgomery, J.R., Smith, J.T., and Gray, E.F., 

1975, Simple Bouguer gravity anomaly map of Utah: Utah 

Geological and Mineral Survey Map 37, scale 1:1,000,000, 

1 sheet. 

Cook, K.L., and Smith, R.B., 1967, Seismicity in Utah, 1850 

through June 1965: Bulletin of the Seismological Society 

of America, v. 57, p. 689-718,1 folded plate. 

Craig, L.C., 1972, Mississippian system, in Mallory, W.W., 

References Cited 41 
ed.-in-chief, Geologic atlas of the Rocky Mountain region: 

Denver, Colo., Rocky Mountain Association of Geologists, 

p. 100-110. 

Crittenden, M.D., Jr., 1961, Magnitude of thrust faulting in 

northern Utah: U.S. Geological Survey Professional Paper 

424-D, p. D128-D131. 

___ 1965a, Geology of the Dromedary Peak quadrangle, 

Utah: U.S. Geological Survey Geologic Quadrangle Map 

GQ-378, scale 1:24,000, 1 sheet. 

__ 1965b, Geology of the Mount Aire quadrangle, Salt Lake 

County, Utah: U.S. Geological Survey Geologic 

Quadrangle Map GQ-379, scale 1:24,000, 1 sheet. 

___ 1965c, Geology of the Sugar House quadrangle, Salt 

Lake County, Utah: U.S. Geological Survey Geologic 

Quadrangle Map GQ-380, scale 1:24,000, 1 sheet. 

___ 1972, Willard thrust and the Cache allochthon, Utah: 

Geological Society of America Bulletin, v. 83, 

p. 2871-2880. 

___ 1974, Regional extent and age of thrusts near Rockport 

Reservoir and relation to possible exploration targets in 

northern Utah: American Association of Petroleum 

Geologists Bulletin, v. 58, p. 2428-2435. 

___ 1976, Stratigraphic and structural setting of the Cottonwood area, Utah, in Hill, J.G., ed., Geology of the Cordilleran hingeline: Denver, Colo., Rocky Mountain 

Association of Geologists, p. 363-379. 

Crittenden, M.D., Jr., Calkins, F.C., and Sharp, B.J., 1966, 

Geologic map of the Park City West quadrangle, Utah: 

U.S. Geological Survey Geologic Quadrangle Map 

GQ-535, scale 1:24,000, 1 sheet. 

Crittenden, M.D., Jr., and Sorensen, M.L., 1985, Geologic map 

of the North Ogden quadrangle and part of the Ogden and 

Plain City quadrangles, Box Elder and Weber Counties, 

Utah: U.S. Geological Survey Miscellaneous Investigations 

Series Map I -1606, scale 1:24,000, 1 sheet. 

Crittenden, M.D., Jr., Stewart, J.H., and Wallace, C.A., 1972, 

Regional correlation of Upper Precambrian strata in 

western North America: International Geological Congress, 

24th, Montreal, Canada, 1972, Section 1, p. 334-341. 

Crittenden, M.D., Jr., Stuckless, J.S., Kistler, R.W., and Stern, 

T.W., 1973, Radiometric dating of intrusive rocks in the 

Cottonwood area, Utah: U.S. Geological Survey Journal 

of Research, v. 1, p. 173-178. 

Crittenden, M.D., Jr., and Wallace, C.A., 1973, Possible 

equivalents of the Belt Supergroup in Utah: Idaho Bureau 

of Mines and Geology, Belt Symposium, v. 1, p. 116-138. 

Crittenden, M.D., Jr., Wallace, C.A., and Sheridan, M.J., 

1967, Mineral resources of the High Uintas primitive area, 

Utah: U.S. Geological Survey Bulletin 1230-I, 27 p., 1 

folded plate, scale 1:125,000. 

Crone, A.J., and Harding, S.T., 1984, Relationship of late 

Quaternary fault scarps to subjacent faults, eastern Great 

Basin, Utah: Geology, v. 12, p. 292-295. 

Crone, A.J., and Machette, M.N., 1984, Surface faulting accompanying the Borah Peak earthquake, central Idaho: 

Geology, v. 12, p. 664-667. 

Currey, D.R., 1982, Lake Bonneville-Selected features of 

relevance to neotectonic analysis: U.S. Geological Survey 

Open-File Report 82-1070, 30 p., 1 folded plate, scale 

1:500,000. 

42 Segmentation of Wasatch Fault Zone, Utah 

Davis, F.D., compiler, 1983a, Geologic map of the central 

Wasatch Front, Utah: Utah Geological and Mineral Survey 

Map 54-A, scale 1:100,000, 2 sheets. 

___ 1983b, Geologic map of the southern Wasatch Front, 

Utah: Utah Geological and Mineral Survey Map 55-A, 

scale 1:100,000, 2 sheets. 

___ 1985, Geologic map of the northern Wasatch Front, 

Utah: Utah Geological and Mineral Survey Map 53-A, 

scale 1:100,000, 2 sheets. 

Dixon, J.S., 1982, Regional structural synthesis, Wyoming 

salient of western overthrust belt: American Association 

of Petroleum Geologists Bulletin, v. 66, p. 1560-1580. 

Dover, J.H., 1983, New data on thrusts in northeastern Utah 

and southwestern Wyoming: Geological Society of America 

Abstracts with Programs, v. 15, no. 5, p. 318-319. 

Eardley, A.J., 1939, Structure of the Wasatch-Great Basin 

region: Geological Society of America Bulletin, v. 50, 

p. 1277-1310. 

___ 1944, Geology of the north-central Wasatch Mountains, 

Utah: Geological Society of America Bulletin, v. 55, 

p. 819-894. 

__ 1969, Willard thrust and the Cache uplift: Geological 

Society of America Bulletin, v. 80, p. 669-680. 

Forrester, J.D., 1937, Structure of the Uinta Mountains: Geological Society of America Bulletin, v. 48, p. 631-666. 

Foster, N.H., 1972, Ordovician system, in Mallory, W.W., ed.-

in-chief, Geologic atlas of the Rocky Mountain region: 

Denver, Colo., Rocky Mountain Association of Geologists, 

p. 76-85. 

Fouch, T.D., Lawton, T.F., Nichols, D.J., Cashion, W.B., and 

Cobban, W.A., 1983, Patterns and timing of synorogenic 

sedimentation in Upper Cretaceous rocks of central and 

northeast Utah, in Reynolds, M.W., and Dolly, E.D., eds., 

Mesozoic paleogeography of the west-central United States, 

Rocky Mountain paleogeography symposium 2: Denver, 

Colo., Rocky Mountain Section, Society of Economic 

Paleontologists and Mineralogists, p. 305-336. 

Freedman, Davis, Pisani, Robert, and Purves, Roger, 1978, 

Statistics: New York, W. W. Norton and Company, 506 p. 

Gary, Margaret, McAfee, Robert, Jr., and Wolf, C.L., eds., 

1972, Glossary of geology: Washington, D.C., American 

Geological Institute, 857 p. 

Gilbert, G.K., 1928, Studies of Basin-Range structure: U.S. 

Geological Survey Professional Paper 153, 91 p. 

Gries, Robbie, 1983, North-south compression of Rocky Mountain foreland structures, in Lowell, J.D., and Gries, 

Robbie, eds., Rocky Mountain foreland basins and uplifts: 

Denver, Colo., Rocky Mountain Association of Geologists, 

p. 9-32. 

Hansen, W.R., 1965, Geology of the Flaming Gorge area, UtahColorado-Wyoming: U.S. Geological Survey Professional 

Paper 490, 196 p. 

___ 1984, Post-Laramide tectonic history of the eastern Uinta 

Mountains, Utah, Colorado, and Wyoming: Mountain 

Geologist, v. 21, no. 1, p. 5-29. 

___ 1986, History of faulting in the eastern Uinta Mountains, 

Colorado and Utah, in Stone, D.S., ed., New interpretations of northwest Colorado geology: Denver, Colo., 

Rocky Mountain Association of Geologists, p. 5-17. 

Hays, W.W., 1979, Program and plans of the U.S. Geological 
Survey for producing information needed in national 

seismic hazards and risk assessment, fiscal years 1980-1984: 

U.S. Geological Survey Circular 816, 40 p. 

Hays, W.W., and Gori, P.L., 1984, Summary and background, 

Workshop on "Evaluation of regional and urban earthquake hazards and risk in Utah", in Hays, W.W., and 

Gori, P.L., eds., Workshop on "Evaluation of regional 

and urban earthquake hazards and risk in Utah", Salt Lake 

City, Utah, August 14-16, 1984, Proceedings: U.S. 

Geological Survey Open-File Report 84-763, p. 1-47. 

Heller, P.L., Bowdler, S.S., Chambers, H.P., Coogan, J.C., 

Hagen, E.S., Shuster, M.W., Winslow, N.S., and Lawton, 

T.F., 1986, Time of initial thrusting in the Sevier orogenic 

belt, Idaho-Wyoming and Utah: Geology, v. 14, 

p. 388-391. 

Hicks, C.R., 1973, Fundamental concepts in the design of experiments: New York, Holt, Rinehart, and Winston, 349 p. 

Higgins, J.M., 1982, Geology of the Champlin Peak 

quadrangle, Juab and Millard Counties, Utah: Brigham 

Young University Geology Studies, v. 29, Part 2, p. 40-58, 

1 folded plate, scale 1:24,000. 

Hintze, L.F., 1980, compiler, Geologic map of Utah: Salt Lake 

City, Utah, Utah Geological and Mineral Survey, scale " 

1:500,000, 2 sheets. 

Hoffman, Paul, 1973, Evolution of an early Proterozoic continental margin-The Coronation geosyncline and 

associated aulacogens of the northwestern Canadian shield: 

Royal Society of London Philosophical Transactions, 

Series A, v. 273, p. 547-581. 

Hoffman, Paul, Dewey, J.F., and Burke, Kevin, 1973, 

Aulacogens and their genetic relation to geosynclines, with 

a Proterozoic example from Great Slave Lake, Canada: 

Society of Economic Paleontologists and Mineralogists 

Special Publication No. 19, p. 38-55. 

Hopkins, D.L., and Bruhn, R.L., 1983, Extensional faulting 

in the Wasatch Mountains, Utah: Geological Society of 

America Abstracts with Programs, v. 15, no. 5, p. 402. 

Hunt, C.B., Varnes, H.D., and Thomas, H.E., 1953, Lake 

Bonneville-Geology of northern Utah Valley, Utah: U.S. 

Geological Survey Professional Paper 257-A, 99 p., 7 

folded plates. 

Jackson, J.A., Fitch, T.J., and McKenzie, D.P., 1981, Active 

thrusting and the evolution of the Zagros thrust belt, in 

McClay, K.R., and Price, N.J., eds., Thrust and nappe 

tectonics: Geological Society of London Special Publication No. 9, p. 371-380. 

Jefferson, W.S., 1982, Structural and stratigraphic relations of 

Upper Cretaceous to Lower Tertiary orogenic sediments 

in the Cedar Hills, Utah, in Nielson, D.L., ed., Overthrust 

belt in Utah: Utah Geological Association Publication 10, 

p. 65-80. 

Johnson, K.D., 1959, Structure and stratigraphy of the Mount 

Nebo-Salt Creek area, southern Wasatch Mountains, Utah: 

Brigham Young University Geology Studies, v. 6, no. 6, 

49 p., 1 folded plate, scale about 1:29,333. 

King, Geoffrey, 1983, The accomodation of large strains in the 

upper lithosphere of the Earth and other solids by selfsimilar fault systems-The geometrical origin of b-value: 

Pure and Applied Geophysics, v. 121, p. 761-815. 

King, Geoffrey, and Nabelek, John, 1985, Role of fault bends 

in the initiation and termination of earthquake rupture: 

Science, v. 228, p. 984-987. 

King, Geoffrey, and Yielding, Graham, 1984, The evolution 

of a thrust fault system-Processes of rupture initiation, 

propagation and termination in the 1980 El Asnam 

(Algeria) earthquake: Royal Astronomical Society 

Geophysical Journal, v. 77, p. 915-933. 

King, P.B., and Beikman, H.M., 1974, Geologic map of the 

United States (exclusive of Alaska and Hawaii): U.S. 

Geological Survey, scale 1:2,500,000, 3 sheets. 

Lamerson, P.R., 1982, The Fossil basin and its relationship to 

the Absaroka thrust system, Wyoming and Utah, in 

Powers, R.B., ed., Geologic studies of the Cordilleran 

thrust belt: Denver, Colo., Rocky Mountain Association 

of Geologists, p. 279-340, 13 folded plates. 

Lawton, T.F., 1985, Style and timing of frontal structures, 

thrust belt, central Utah: American Association of 

Petroleum Geologists Bulletin, v. 69, p. 1145-1159. 

Lawton, T.F., Dickinson, W.R., and Jefferson, W.S., 1982, 

Inferred eastern extent, overthrust belt, central Utah [abs.]: 

American Association of Petroleum Geologists Bulletin, 

v. 66, p. 592. 

Lewis, J.V., and Kiimmel, H.B., 1910-1912, Geologic map of 

New Jersey: New Jersey Bureau of Geology and 

Topography Map No. 40, scale 1:250,000, 1 sheet (revised 

in 1931 by H.B. Kiimmel and in 1950 by M.E. Johnson). 

Lochman-Balk, Christina, 1972, Cambrian system, in Mallory, 

W.W., ed.-in-chief, Geologic atlas of the Rocky Mountain region: Denver, Colo., Rocky Mountain Association 

of Geologists, p. 60-75. 

___ 1976, The Cambrian section of the central Wasatch 

Mountains, in Hill, J .G., ed., Geology of the Cordilleran 

hingeline: Denver, Colo., Rocky Mountain Association of 

Geologists, p. 103-108. 

Loughlin, G.F., 1913, Reconnaissance in the southern Wasatch 

Mountains, Utah: Journal of Geology, v. 21, p. 436-452. 

Mabey, D.R., 1987, Subsurface geology along the Wasatch 

Front, in Gori, P.L., and Hays, W.W., eds., Assessment 

of regional earthquake hazards and risks along the Wasatch 

Front, Utah: U.S. Geological Survey Open-File Report 

87-585, v. I, p. C1-C39. 

Mabey, D.R., Crittenden, M.D., Jr., Morris, H.T., Roberts, 

R.J., and Tooker, E.W., 1964, Aeromagnetic and generalized geologic map of part of north-central Utah: U.S. 

Geological Survey Geophysical Investigations Map 

GP-422, scale 1 :250,000, 1 sheet. 

Mabey, D.R., Oliver, H.W., and Hildenbrand, T.G., 1983, 

Regional gravity and magnetic anomalies in the northern 

Basin and Range province, in The role of heat in the 

development of energy and mineral resources in the northern Basin and Range province: Geothermal Resources 

Council Special Report No. 13, p. 307-315. 

Machette, M.N., 1984, Preliminary investigations of late 

Quaternary slip rates along the southern part of the 

Wasatch fault zone, central Utah, in Hays, W.W., and 

Gori, P.L., eds., Workshop on "Evaluation of regional 

and urban earthquake hazards and risk in Utah", Salt Lake 

City, Utah, August 14-16, 1984, Proceedings: U.S. Geological Survey Open-File Report 84-763, p. 391-406. 

Mattick, R.E., 1970, Thickness of unconsolidated to semiconsolidated sediments in Jordan Valley, Utah: U.S. 

References Cited 43 
Geological Survey Professional Paper 700-C, 

p. C119-C124. 

Meissner, R., and Strehlau, J., 1982, Limits of stresses in continental crusts and their relation to the depth-frequency 

distribution of shallow earthquakes: Tectonics, v. 1, 

p. 73-89. 

Metter, R.E., 1955, The geology of a part of the southern 

Wasatch Mountains, Utah: Columbus, Ohio, Ohio State 

University Ph. D. dissertation, 242 p., 2 folded plates, scale 

1:24,000. 

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, 13 p. pamphlet. 

___ 1982, Surficial geologic map along the southern part of 

the Wasatch Front, Great Salt Lake and Utah Valleys, 

Utah: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-1477, scale 1:100,000, 1 sheet, 14 p. pamphlet. 

Monley, L.E., 1971, Petroleum potential of Idaho-Wyoming 

overthrust belt, in Cram, I.H., ed., Future petroleum provinces of the United States-Their geology and potential: 

American Association of Petroleum Geologists Memoir 15, 

p. 509-537. 

Moore, D.S., 1979, Statistics-Concepts and controversies: San 

Francisco, W.H. Freeman and Company, 313 p. 

Morisawa, Marie, 1972, The Wasatch fault zone-General 

aspects, in Hilpert, L.S., ed., Environmental geology of 

the Wasatch Front, 1971: Utah Geological Association 

Publication 1, p. D1-D17. 

Morris, H.T., 1983, Interrelations of thrust and transcurrent 

faults in the central Sevier orogenic belt near Leamington, 

Utah, in Miller, D.M., Todd, V.R., and Howard, K.A., 

eds., Tectonic and stratigraphic studies in the eastern Great 

Basin: Geological Society of America Memoir 157, 

p. 75-81. 

Morris, H.T., and Lovering, T.S., 1961, Stratigraphy of the 

East Tintic Mountains, Utah: U.S. Geological Survey Professional Paper 361, 145 p. 

Morris, H.T., and Shepard, W.M., 1964, Evidence for a concealed tear fault with large displacement in the central East 

Tin tic Mountains, Utah: U.S. Geological Survey Professional Paper 501-C, p. C19-C21. 

Moulton, F.C., 1975, Lower Mesozoic and Upper Paleozoic 

petroleum potential of the Hingeline area, central Utah, 

in Symposium on deep drilling frontiers of the central 

Rocky Mountains: Denver, Colo., Rocky Mountain 

Association of Geologists, p. 87-97 (reprinted in Hill, J.G., 

ed., 1976, Geology of the Cordilleran hingeline: Denver, 

Colo., Rocky Mountain Association of Geologists, 

p. 219-229). 

Munsey, J.W., and Bollinger, G.A., 1985, Focal mechanism 

analyses for Virginia earthquakes (1978-1984): Bulletin of 

the Seismological Society of America, v. 75, p. 1613-1636. 

Naeser, C.W., Bryant, Bruce, Crittenden, M.D., Jr., and 

Sorensen, M.L., 1983, Fission-track ages of apatite in the 

Wasatch Mountains, Utah-An uplift study, in Miller, 

D.M., Todd, V.R., and Howard, K.A., eds., Tectonic and 

stratigraphic studies in the eastern Great Basin: Geological 

Society of America Memoir 157, p. 29-36. 

Nettleton, L.L., 1971, Elementary gravity and magnetics for 

44 Segmentation of Wasatch Fault Zone, Utah 

geologists and seismologists: Society of Exploration 

Geophysicists Monograph Series No. 1, 121 p. 

Ni, James, and Barazangi, Muawia, 1986, Seismotectonics of 

the Zagros continental collision zone and a comparison with 

the Himalayas: Journal of Geophysical Research, v. 91, 

no. B8, p. 8205-8218. 

Oriel, S.S., and Armstrong, F.C., 1966, Times of thrusting in 

Idaho-Wyoming thrust belt-Reply: American Association 

of Petroleum Geologists Bulletin, v. 50, p. 2614-2621. 

Osmond, J.C., 1986, Petroleum geology of the Uinta 

Mountains-White River uplift, Colorado and Utah, in 

Stone, D.S., ed., New interpretations of northwest Colorado geology: Denver, Colo., Rocky Mountain Association of Geologists, p. 213-221. 

Oviatt, C.G., 1985, Preliminary notes on the Paleozoic 

stratigraphy and structural geology of the Honeyville 

quadrangle, northern Wellsville Mountain, Utah, in Kerns, 

G.J., and Kerns, R.L., Jr., eds., Orogenic patterns and 

stratigraphy of north-central Utah and southeastern Idaho: 

Utah Geological Association Publication 14, p. 47-54. 

Parry, W.T., and Bruhn, R.L., 1986, Pore fluid and seismogenic characteristics of fault rock at depth on the Wasatch 

fault, Utah: Journal of Geophysical Research, v. 91, no. 

B1, p. 730-744. 

___ 1987, Fluid inclusion evidence for minimum 11 km 

vertical offset on the Wasatch fault, Utah: Geology, v. 15, 

p. 67-70. 

Pavlis, T.L., and Smith, R.B., 1980, Slip vectors on faults near 

Salt Lake City, Utah, from Quaternary displacements and 

seismicity: Bulletin of the Seismological Society of America, 

v. 70, p. 1521-1526. 

Perry, W.J., Roeder, D.H., and Lageson, D.R., compilers, 

1984, North American thrust-faulted terranes: American 

Association of Petroleum Geologists Reprint Series No. 

27, 466 p. 

Personius, S.F., 1985, Preliminary assessment of late Quaternary displacement along the Wasatch fault zone near 

Brigham City, Utah: Geological Society of America 

Abstracts with Programs, v. 17, no. 4, p. 261. 

___ 1986, The Brigham City segment-A new segment of the 

Wasatch fault zone, northern Utah: Geological Society of 

America Abstracts with Programs, v. 18, no. 5, p. 402. 

Pricha, Frank, and Gibson, R.I., 1985, Cordilleran hingelineLate Precambrian rifted margin of the North American 

craton and its impact on the depositional and structural 

history, Utah and Nevada: Geology, v. 13, p. 465-468. 

Richard, J.J., 1986, Interpretation of a seismic section across 

the Danforth Hills anticline (Maudlin Gulch) and Axial 

Arch in northwest Colorado, in Stone, D.S., ed., New 

interpretations of northwest Colorado geology: Denver, 

Colo., Rocky Mountain Association of Geologists, 

p. 191-193, 2 folded unsealed plates. 

Riess, S.K., 1985, Structural geometry of the Charleston thrust 

fault, central Wasatch Mountains, Utah: Salt Lake City, 

Utah, University of Utah, M.S. thesis, 73 p., 2 folded 

plates, scales about 1:85,000 and about 1:24,000. 

Rigby, J.K., 1958, Geology of the Stansbury Mountains, eastern 

Toole County, Utah: Utah Geological Society Guidebook 

No. 13, p. 1-134. 

___ 1959, Upper Devonian unconformity in central Utah: 

Geological Society of America Bulletin, v. 70, p. 207-218. 
Ritzma, H.R., 1969, Tectonic resume, Uinta Mountains, in 

Lindsay, J .B., ed., Geologic guidebook of the Uinta Mountains, Utah's maverick range: Intermountain Association 

of Geologists, Annual field conference, 16th, September 

4-6, 1969, Guidebook, p. 57-63. 

Roberts, R.J., 1960, Paleozoic structure in the Great Basin 

[abs.]: Geological Society of America Bulletin, v. 71, 

p. 1955. 

Roberts, R.J., Crittenden, M.D., Jr., Tooker, E.W., Morris, 

H.T., Hose, R.K., and Cheney, T.M., 1965, Pennsylvanian and Permian basins in northwestern Utah, northeastern Nevada and south-central Idaho: American Association 

of Petroleum Geologists Bulletin, v. 49, p. 1926-1956. 

Robinson, Peter, 1972, Tertiary history, in Mallory, W.W., ed.-

in-chief, Geologic atlas of the Rocky Mountain region: 

Denver, Colo., Rocky Mountain Association of Geologists, 

p. 233-242. 

Rose, P.R., 1976, Mississippian carbonate shelf margins, 

western United States, in Hill, J.G., ed., Geology of the 

Cordilleran hingeline: Denver, Colo., Rocky Mountain 

Association of Geologists, p. 135-151. 

Ross, R.J., Jr., 1976, Ordovician sedimentation in the western 

United States, in Hill, J .G., ed., Geology of the Cordilleran 

hingeline: Denver, Colo., Rocky Mountain Association of 

Geologists, p. 109-133. 

Royse, Frank, 1983, Extensional faults and folds in the foreland 

thrust belt, Utah, Wyoming, Idaho: Geological Society of 

America Abstracts with Programs, v. 15, no. 5, p. 295. 

Royse, Frank, Warner, M.A., and Reese, D.L., 1975, Thrust 

belt structural geometry and related stratigraphic problems, 

Wyoming-Idaho-northern Utah, in Bolyard, D.W., ed., 

Deep drilling frontiers of the central Rocky Mountains: 

Denver, Colo., Rocky Mountain Association of Geologists, 

p. 41-54. 

Sandberg, C.A., Gutschick, R.C., Johnson, J.G., Poole, F.G., 

and Sando, W.J., 1982, Middle Devonian to Late Mississippian geologic history of the overthrust belt region, western 

United States, in Powers, R.B., ed., Geologic studies of 

the Cordilleran thrust belt: Denver, Colo., Rocky Mountain Association of Geologists, p. 691-719. 

Sbar, M.L., Barazangi, Muawia, Dorman, James, Scholz, 

C.H., and Smith, R.B., 1972, Tectonics of the Intermountain Seismic Belt, western United States-Microearthquake 

seismicity and composite fault plane solutions: Geological 

Society of America Bulletin, v. 83, p. 13-28. 

Schirmer, T.W., 1985, Basement thrusting in north-central 

Utah-A model for the development of the northern Utah 

highland, in Kerns, G.J., and Kerns, R.L., Jr., eds., 

Orogenic patterns and stratigraphy of north-central Utah 

and southeastern Idaho: Utah Geological Association 

Publication 14, p. 129-143. 

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

and characteristic earthquakes-Examples from the 

Wasatch and San Andreas fault zones: Journal of 

Geophysical Research, v. 89, no. B7, p. 5681-5698. 

Scott, W.E., and Shroba, R.R., 1985, Surficial geologic map 

of an area along the Wasatch fault zone in the Salt Lake 

Valley, Utah: U.S. Geological Survey Open-File Report 

85-448, scale 1:24,000, 18 p., 1 folded plate of 2 sheets. 

Sears, J.W., Graff, P.J., and Holden, G.S., 1982, Tectonic 

evolution of lower Proterozoic rocks, Uinta Mountains, 

Utah and Colorado: Geological Society of America 

Bulletin, v. 93, p. 990-997. 

Sears, J .W., and Price, R.A., 1978, The Siberian connection-A 

case for Precambrian separation of the North American 

and Siberian cratons: Geology, v. 6, p. 267-270. 

Sibson, R.H., 1982, Fault zone models, heat flow, and the depth 

distribution of earthquakes in the continental crust of the 

United States: Bulletin of the Seismological Society of 

America, v. 72, p. 151-163. 

___ 1984, Roughness at the base of the seismogenic zoneContributing factors: Journal of Geophysical Research, v. 

89, p. 5791-5799. 

Skipp, Betty, and Harding, S.T., 1985, Preliminary report on 

geology of Borah Peak area, Idaho, including interpretation of seismic and gravity data, in Stein, R.S., and 

Bucknam, R.C., eds., Workshop XXVIII On the Borah 

Peak, Idaho, earthquake, 3-6 October, 1984, Proceedings, 

v. A: U.S. Geological Survey Open-File Report 85-290, 

p. 657-671. 

Smith, J .T., and Cook, K.L., 1985, Geologic interpretation of 

gravity anomalies of northeastern Utah, in Picard, M.D., 

ed., Geology and energy resources, Uinta basin of Utah: 

Utah Geological Association Publication 12, p. 121-146. 

Smith, R.B., 1972, Contemporary seismicity, seismic gaps, and 

earthquake recurrences of the Wasatch Front, in Hilpert, 

L.S., ed., Environmental geology of the Wasatch Front, 

1971: Utah Geological Association Publication 1, p. 11-19. 

___ 1974, Seismicity and earthquake hazards of the Wasatch 

Front, Utah: Earthquake Information Bulletin, v. 6, no. 

4, p. 12-17. 

Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional 

tectonics of the eastern Basin-Range-Inferences on structural style from seismic reflection data, regional tectonics, 

and thermal-mechanical models of brittle-ductile deformation: Journal of Geophysical Research, v. 89, no. B7, 

p. 5733-5762. 

Smith, R.B., and Richins, W.D., 1984, Seismicity and earthquake hazards of Utah and the Wasatch Front-Paradigm 

and paradox, in Hays, W.W., and Gori, P.L., eds., 

Workshop on "Evaluation of regional and urban earthquake hazards and risk in Utah", Salt Lake City, Utah, 

August 14-16, 1984, Proceedings: U.S. Geological Survey 

Open-File Report 84-763, p. 73-112. 

Smith, R.B., and Sbar, M.L., 1974, Contemporary tectonics 

and seismicity of the western United States with emphasis 

on the Intermountain Seismic Belt: Geological Society of 

America Bulletin, v. 85, p. 1205-1218. 

Snay, R.A., Smith, R.B., and Soler, T., 1984, Horizontal strain 

across the Wasatch Front near Salt Lake City, Utah: Journal of Geophysical Research, v. 89, p. 1113-1122. 

Sorensen, M.L., and Crittenden, M.D., Jr., 1976, Preliminary 

map of the Mantua quadrangle and part of the Willard 

quadrangle, Box Elder, Weber, and Cache Counties, Utah: 

U.S. Geolological Survey Miscellaneous Field Studies Map 

MF-720, scale 1:24,000, 1 sheet. 

___ 1979, Geologic map of the Huntsville quadrangle, Weber 

and Cache Counties, Utah: U.S. Geological Survey Geologic Quadrangle Map GQ-1503, scale 1:24,000, 1 sheet. 

Sprinkel, D.A., Witkind, I.J., and Baer, J.L., 1982, The 

References Cited 45 
overthrust belt in central Utah (road log), in Nielson, D.L., 

ed., Overthrust belt in Utah: Utah Geological Association 

Publication 10, p. 301-334. 

Standlee, L.A., 1982, Structure and stratigraphy of Jurassic 

rocks in central Utah-Their influence on tectonic development of the Cordilleran foreland thrust belt, in Powers, 

R.B., ed., Geologic studies of the Cordilleran thrust belt: 

Denver, Colo., Rocky Mountain Association of Geologists, 

p. 357-382. 

Steenland, N.C., 1969, An aeromagnetic survey of the Uinta 

Mountains, in Lindsay, J.B., ~d., Geologic guidebook of 

the Uinta Mountains, Utah's maverick range: Intermountain Association of Geologists, Annual field conference, 

16th, September 4-6, 1969, Guidebook, p. 47-51, 1 folded 

plate, approximate scales 1:192,000 and 1:290,000. 

Stewart, J .H., 1972, Initial deposits in the Cordilleran 

geosyncline-Evidence of a late Precambrian (<850 m.y.) 

continental separation: Geological Society of America 

Bulletin, v. 83, p. 1345-1360. 

Stewart, J.H., Moore, W.J., and Zietz, Isidore, 1977, East-west 

patterns of Cenozoic igneous rocks, aeromagnetic 

anomalies, and mineral deposits, Nevada and Utah: 

Geological Society of America Bulletin, v. 88, p. 67-77. 

Stewart, J.H., and Poole, F.G., 1974, Lower Paleozoic and uppermost Precambrian Cordilleran miogeocline, Great 

Basin, western United States, in Dickinson, W.R., ed., Tectonics and sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication No. 22, 

p. 28-57. 

Stokes, W.L., 1963, compiler, Geologic map of Utah, northwest 

quarter: Salt Lake City, Utah, Utah State Land Board and 

University of Utah, scale 1:250,000, 1 sheet. 

___ 1976, What is the Wasatch Line?, in Hill, J .G., ed., 

Geology of the Cordilleran hingeline: Denver, Colo., Rocky 

Mountain Association of Geologists, p. 11-25. 

Stokes, W.L., and Madsen, J.H., Jr., 1961, Geologic map of 

Utah, northeast quarter: Salt Lake City, Utah, Utah State 

Land Board and University of Utah, scale 1:250,000, 1 

sheet. 

Stone, D.S., 1986, Seismic and borehole evidence for important pre-Laramide faulting along the Axial Arch in northwest Colorado, in Stone, D.S., ed., New interpretations 

of northwest Colorado geology: Denver, Colo., Rocky 

Mountain Association of Geologists, p. 19-36. 

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. 

Tooker, E.W., 1983, Variations in structural style and correlation of thrust plates in the Sevier foreland thrust belt, Great 

Salt Lake area, Utah, in Miller, D.M., Todd, V.R., and 

Howard, K.A., eds., Tectonic and stratigraphic studies in 

the eastern Great Basin: Geological Society of America 

Memoir 157, p. 61-73. 

Tweto, Ogden, 1979, compiler, Geologic map of Colorado: U.S. 

Geological Survey, scale 1:500,000, 1 sheet. 

U.S. Geological Survey, 1976, State of Utah: U.S. Geological Survey, scale 1:500,000, contour interval 500 feet, 

1 sheet. 

46 Segmentation of Wasatch Fault Zone, Utah 

Vacquier, Victor, Steenland, N.C., Henderson, R.G., and Zietz, 

Isidore, 1951, Interpretation of aeromagnetic maps: Geological Society of America Memoir 47, 151 p., 14 folded 

plates. 

Van Horn, Richard, 1972, Map showing relative ages of faults 

in the Sugar House quadrangle, Salt Lake County, Utah: 

U.S. Geological Survey Miscellaneous Investigations Map 

I -766-B, scale 1:24,000, 1 sheet. 

Villien, Alain, and Kligfield, Roy, in press, Thrusting and 

synorogenic sedimentation in central Utah, in Petersen, 

James, ed., Sedimentation and tectonics in the Rocky 

Mountain region: American Association of Petroleum 

Geologists Special Publication. 

Wallace, C.A., and Crittenden, M.D., Jr., 1969, The stratigraphy, depositional environment and correlation of the 

Precambrian Uinta Mountain Group, western Uinta Mountains, Utah, in Lindsay, J.B., ed., Geologic guidebook of 

the Uinta Mountains, Utah's maverick range: Intermountain Association of Geologists, Annual field conference, 

16th, September 4-6, 1969, Guidebook, p. 127-141. 

Webb, G.W., 1958, Middle Ordovician stratigraphy in eastern 

Nevada and western Utah: American Association of 

Petroleum Geologists Bulletin, v. 42, p. 2335-2377. 

Welsh, J.E., 1979, Paleogeography and tectonic implications 

of the Mississippian and Pennsylvanian in Utah, in 

Newman, G.W., and Goode, H.D., eds., Basin and Range 

symposium and Great Basin field conference: Denver, 

Colo., Rocky Mountain Association of Geologists, 

p. 93-106. 

Wheeler, R.L., 1984, A plan for evaluating hypothesized 

segmentation of the Wasatch fault, in Hays, W.W., and 

Gori, P .L., eds., A workshop on "Evaluation of regional 

and urban earthquake hazards and risk in Utah", Salt Lake 

City, August 1984, Proceedings: U.S. Geological Survey 

Open-File Report 84-763, p. 576-605. 

___ 1985, Evaluating point concentrations on a mapEarthquakes in the Colorado lineament: Geology, v. 13, 

p. 701-704. 

Wheeler, R.L., and Krystinik, K.B., 1987a, Evaluating coinciding anomalies along a fault trace-Simulations and 

statistical procedures: U.S. Geological Survey Bulletin 1802 

[in press]. 

___ 1987b, Persistent segment boundaries on the Wasatch 

fault zone, central Utah [abs.]: Seismological Research Letters, v. 58, no. 1, p. 31. 

___ 1987c, Persistent and nonpersistent seismic segmentation 

of the Wasatch fault zone, Utah: Geological Society of 

America Abstracts with Programs, v. 19, no. 5, p. 342. 

___ 1987, Persistent and nonpersistent segmentation of the 

Wasatch fault zone, Utah-Statistical analysis for evaluation of seismic hazard, in Gori, P.L., and Hays, W.W., 

eds., Assessment of regional earthquake hazards and risks 

along the Wasatch Front, Utah: U.S. Geological Survey 

Open-File Report 87-585, v. I, p. B1-B124. 

Winslow, M.A., 1981, Mechanisms for basement shortening 

in the Andean foreland fold belt of southern South 

America, in McClay, K.R., and Price, N.J., eds., Thrust 

and nappe tectonics: Geological Society of London Special 

Publication No. 9, p. 513-528. 

Witkind, I.J., 1982, Salt diapirism in central Utah, in Nielson, 
D.L., ed., Overthrust belt in Utah: Utah Geological 

Assocation Publication 10, p. 13-30. 

___ 1983, Overthrusts and salt diapirs, central Utah, in 

Miller, D.M., Todd, V.R., and Howard, K.A., eds., 

Tectonic and stratigraphic studies in the eastern Great 

Basin: Geological Society of American Memoir 157, 

p. 45-59. 

Viloodward, L.A., 1976, Stratigraphy of younger Precambrian 

rocks along Cordilleran hingeline, Utah and southern Colorado, in Hill, 1. G., ed., Geology of the Cordilleran 

hingeline: Denver, Colo., Rocky Mountain Association of 

Geologists, p. 83-90. 

Zietz, Isidore, Shuey, Ralph, and Kirby, J.R., Jr., 1976, 

*U.S. GPO:l988 )13-fJ!l/ffi,rEl 

Aeromagnetic map of Utah: U.S. Geophysical Investigations Map GP-907, scale 1: 1 ,000,000, 1 sheet. 

Zoback, M.L., 1983, Structure and Cenozoic tectonism along 

the Wasatch fault zone, Utah, in Miller, D.M., Todd, V.R., 

and Howard, K.A., eds., Tectonics and stratigraphy of the 

eastern Great Basin: Geological Society of America Memoir 

157, p. 3-27. 

___ 1987, Superimposed late Cenozoic, Mesozoic, and possible Proterozoic deformation along the Wasatch fault zone 

in central Utah, in Gori, P.L., and Hays, W.W., eds., 

Assessment of regional earthquake hazards and risks along 

the Wasatch Front, Utah: U.S. Geological Survey OpenFile Report 87-585, v. I, p. E1-E43. 

References Cited 47