Segmentation of the Wasatch Fault Zone, Utah­ Summaries, 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 Multiply by To obtain inch-pound unit meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) square kilometer (km2) 0.3861 square mile (mF) IV Contents Segmentation of the Wasatch Fault Zone, Utah­ Summaries, Analyses, and Interpretations of Geological and Geophysical Data By Russell L. Wheeler and Katherine B. Krystinik Abstract INTRODUCTION The Wasatch fault zone of central Utah has been About two-thirds of Utah's 1.5 million people live hypothesized to be segmented into lengths that tend to rup­ within 25 km of the trace of the north-striking Wasatch ture independently of each other, with large seismic ruptures fault zone (fig. 1; 1980 Census data). Large, young fault tending not to cross boundaries between segments. Validity scarps along the fault zone indicate that seismic slip in of the segmentation hypothesis would affect estimates of seismic the fault zone has continued through the late Pleistocene hazard for the urban corridor that contains most of Utah's and Holocene and will probably continue into the future population. Our evaluation of the hypothesis concentrates on detecting segment boundaries. To form a basis for this search, (Cluff and others, 1970, 1973, 1974, 1975; Cluff and we examined six data types for anomalies that might record Slemmons, 1972; Bucknam and others, 1980; Algermissen the presence and locations of segment boundaries that could and others, 1982). The potential for future damaging influence the next few large seismic ruptures on the fault earthquakes constitutes a hazard that needs to be assessed zone. (Hays and Gori, 1984). Mapped Bouguer gravity values define saddles, east­ Swan and others (1980) and Schwartz and Cop­ trending gradients, ends of north-trending gradients, and ends persmith (1984) hypothesized that the fault zone is of north-trending highs and lows that together identify 10 segmented into lengths that tend to rupture independently transverse anomalies. Mapped aeromagnetic intensities show of each other. If their segmentation hypothesis is valid, east-trending gradients and belts of small, intense highs that large ruptures would tend not to cross boundaries be­ define three other transverse anomalies. Earthquake and tween segments, so that most ruptures would stop at seg­ microearthquake epicenters identify another four anomalies, ment boundaries. The strained rock in a boundary at the as places where recent seismic activity along and near the trace end of a recent rupture would be a likely place for the of the fault zone changed markedly northward or southward next rupture to nucleate, so ruptures would also tend to along the zone. Geometry of the fault zone defines four more start at segment boundaries (King and Nabelek, 1985). anomalies, which are large footwall salients around which the If ruptures tend not to cross boundaries, then segment fault trace bows westward. Smoothed topography of the up­ thrown Wasatch Range and San Pitch Mountains defines two lengths would provide an upper limit on potential rup­ topographic anomalies. Consideration of pre-Cenozoic struc­ ture length, and hence on earthquake size. Thus, the tures and geologic history identifies the Uinta aulacogen and validity of the segmentation hypothesis will influence two large, north-dipping lateral ramps in thrust sheets as three estimates of possible mainshock locations and maximum structural anomalies. sizes, both of which are important parameters in the prob­ Each data type and its anomalies satisfy seven criteria that abilistic estimation of ground motion (Algermissen and insure that the anomalies are pertinent to identification of seg­ others, 1982). Maps of estimated ground motion are wide­ ment boundaries and the evaluation of seismic hazard, and are ly used components of hazard assessment. The maps are reliable. The 26 anomalies appear to be concentrated in two often incorporated into building codes, seismic design narrow sections and one wide section of the fault zone, each criteria, analyses of potential losses, and other applica­ of which might contain one or more segment boundaries. tions (Hays, 1979, p. 2, 20). This report is the second of three. The first described Accordingly, evaluation of the segmentation numerical methods for evaluating anomaly concentrations like those described in this report. The third report describes the hypothesis as it applies to the Wasatch fault zone is an application of the numerical methods to these concentrations, important part of hazard assessment for Utah (Wheeler, and the use of the results to help evaluate seismic hazard of 1984). Our strategy for this evaluation is to seek evidence the Wasatch fault zone. of segment boundaries that have persisted through Introduction 0 0 anomalies coincide than would be expected by chance, 42 o,___ 11r3_ __________~ __1T12~ ------------~~11_1°~ and where these other processes can be ruled out on Collinston geologic grounds, we will have identified a persistent seg­ segment (~30 km) ment boundary. The area studied stretches along the north-trending fault zone from central Utah at lat 39 ° N. to the Idaho­ Utah border at lat 42 ° N. The study area has no fixed width, because for each data type the examined area Ogden extends east and west of the trace of the fault zone segment (70 km) I as far as is necessary to identify and characterize anomalies. L In this report we cannot present all aspects of our investigations. Here we describe the anomalies in each Salt Lake data type, demonstrate their validity, and estimate the Salt City segment locational uncertainty of each anomaly. Wheeler and Lake (35 km) City Krystinik (1987a) described the statistical and related ---~ methods that we used to analyze the set of anomaly loca­ \\ tions, and Wheeler and Krystinik (1987b, c, d) applied Provo the methods to the anomalies and interpreted the results Uta~)\! segment Lak'v:_\ava (55 km) in terms of segment boundaries. This report uses more diverse kinds of information r-1- than two authors could pretend to understand alone. We are unusually deeply indebted to topical and regional 1 Nephi segment specialists for discussions, preprints, sharing of un­ Nephio{ (35 km) UTAH published data and maps, and informal reviews of parts --- of the manuscript.