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CONSENSUS PREFERRED RECURRENCE- INTERVALAND VERTICAL SLIP-RATE ESTIMATES Review of Utah Paleoseismic-Trenching Data by the Utah Quaternary Fault Parameters Working Group

by William R. Lund Utah Geological Survey

The U.S. Geological Survey, Department of the Interior, supported the research leading to the publication of this Utah Geological Survey Bulletin under U.S. Geological Survey external grants award number 03HQGR0033. The views and conclusions contained in this document are those of the Utah Geological Survey and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

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Printed on recycled paper 1/05 CONTENTS ABSTRACT ...... 1 INTRODUCTION ...... 1 PREVIOUS WORK ...... 2 UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP ...... 2 PALEOSEISMIC-TRENCHING DATABASE ...... 2 Utah Quaternary Faults ...... 2 Paleoseismic-Trenching Investigations ...... 4 Woodward-Clyde Consultants ...... 6 Wasatch Front Regional Earthquake Hazards Assessment ...... 6 USGS National Earthquake Hazards Reduction Program ...... 6 U.S. Bureau of Reclamation ...... 6 Other Studies ...... 6 WORKING GROUP REVIEW PROCESS ...... 6 Review Process Tasks ...... 7 Consensus Process ...... 7 ISSUES RELEVANT TO THE WORKING GROUP REVIEW ...... 8 Sources of Uncertainty ...... 8 Epistemic Uncertainty ...... 8 Aleatory Uncertainty ...... 9 Data Adequacy ...... 9 Wasatch Fault Zone ...... 9 Other Quaternary Faults ...... 9 Constraining Age Estimates ...... 9 Numerical Ages ...... 9 Radiocarbon ages ...... 9 Luminescence ages ...... 10 Relative Ages ...... 10 Lake Bonneville chronology ...... 10 Soil-profile development ...... 11 Net Vertical-Displacement Data ...... 11 Measurement Uncertainty ...... 11 Scarp profiles ...... 11 Measurements in trenches ...... 11 Sparse Data ...... 11 Incomplete Documentation ...... 11 PALEOSEISMIC PARAMETERS ...... 12 Earthquake Timing ...... 12 Wasatch Fault Zone ...... 12 McCalpin and Nishenko (1996) ...... 12 New paleoseismic trenching information ...... 13 Original Data ...... 13 Nephi segment ...... 13 Other Quaternary Faults ...... 13 Recurrence Intervals ...... 13 Wasatch Fault Zone ...... 14 Other Quaternary Faults ...... 14 Vertical Slip Rates ...... 14 PALEOSEISMIC DATA GAPS ...... 15 Recommended Paleoseismic Investigations ...... 15 Investigation Summaries ...... 17 Wasatch Fault Zone ...... 17 Nephi segment ...... 17 Weber segment MRE ...... 18 Weber segment megatrench ...... 18 Collinston and Clarkston Mountain segments ...... 18 Levan segment ...... 18 Wasatch Front/Northern Utah Exclusive of the WFZ ...... 18 West Valley fault zone ...... 18 Utah Lake faults ...... 18 Great Salt Lake fault zone ...... 18 East Cache fault zone ...... 18 Clarkston fault ...... 19 Wasatch Range back-valley fault ...... 19 Faults beneath Bear Lake ...... 19 Eastern Bear Lake fault ...... 19 Central/Southern Utah ...... 19 Sevier/Toroweap fault ...... 19 Washington fault zone ...... 19 Cedar City-Parowan monocline/Paragonah fault ...... 19 Enoch graben ...... 19 Hurricane fault zone ...... 20 Gunnison fault ...... 20 Scipio Valley faults ...... 20 SUMMARY ...... 20 CONCLUSIONS ...... 21 ACKNOWLEDGMENTS ...... 21 REFERENCES ...... 22

APPENDICES Appendix A. Working Group Consensus Earthquake Timing and Preferred Recurrence-Interval and Vertical Slip-Rate Estimates with Supporting Information ...... 27 Appendix B. Summary Data Forms for Consensus Recurrence-Interval and Vertical Slip-Rate Estimates ...... 41 Appendix C. Examples of Fault/Fault Section Synopsis Form and Paleoseismic Study Summary For ...... 105 Appendix D. Sources of Uncertainty in Fault-Activity Studies ...... 106 Appendix E. Glossary and List of Abbreviations ...... 108

FIGURES Figure 1. Locations of Quaternary faults/fault sections for which paleoseismic-trenching data are available ...... 5 Figure 2. Locations of high-priority Quaternary faults/fault sections recommended for future paleoseismic investigations ...... 16

TABLES Table 1. Summary of Working Group consensus values for timing of most recent surface faulting and preferred recurrence-interval and vertical slip-rate estimates ...... 3 Table 2. Members of the Utah Quaternary Fault Parameters Working Group ...... 4 Table 3. Utah Quaternary faults/fault sections that have paleoseismic-trenching data ...... 4 Table 4. Timing of events related to the transgression and regression of Lake Bonneville ...... 10 Table 5. Example of determining earthquake timing and approximate 2-sigma confidence limits using earthquakes Y and Z, Brigham City segment, Wasatch fault zone ...... 12 Table 6. Example of determining mean recurrence intervals and 2-sigma confidence limits for the Brigham City segment of the Wasatch fault zone ...... 14 Table 7. Faults/fault sections by region that require additional study to adequately characterize Utah’s earthquake hazard ...... 17 Table 8. Paleoseismic investigations required to adequately characterize Utah’s earthquake hazard in order of decreasing statewide priority ...... 17 CONSENSUS PREFERRED RECURRENCE- INTERVALAND VERTICAL SLIP-RATE ESTIMATES Review of Utah Paleoseismic-Trenching Data by the Utah Quaternary Fault Parameters Working Group

by William R. Lund Utah Geological Survey

ABSTRACT transportation, utility, and pipeline corridors critical to Utah’s citizens and economy. The Utah Quaternary Fault Parameters Working Consensus RI and VSR estimates are necessary to Group, a panel of experts convened in 2003-04, has (1) update the National Seismic-Hazard Maps, (2) char- completed a comprehensive evaluation of paleoseismic- acterize seismic sources, (3) perform probabilistic seis- trenching data available for Utah’s Quaternary faults, mic-hazard analyses, and (4) provide data for research and where the data permit have assigned consensus pre- into other earthquake topics. The Working Group’s con- ferred recurrence-interval (RI) and vertical slip-rate sensus RI and VSR estimates are currently the best avail- (VSR) estimates for the faults/fault sections under able data to meet those needs. review. Trenching data are available for 33 (16%) of Utah’s 212 Quaternary faults/fault sections and related structures. The available paleoseismic-trenching data INTRODUCTION are most abundant on the six central, active segments of the Wasatch fault zone coincident with the populous This report presents the results of the Utah Quater- Wasatch Front, and typically are much less abundant for nary Fault Parameters Working Group (hereafter faults elsewhere in Utah. referred to as the Working Group) review and evaluation The general paucity of paleoseismic-trenching data, of Utah’s Quaternary fault paleoseismic-trenching data. combined with large uncertainties associated with some The purpose of the review was to (1) critically evaluate of the data, prevented using rigorous statistical tech- the accuracy and completeness of the paleoseismic- niques to determine RI and VSR values. Consequently, trenching data, particularly regarding earthquake timing the Working Group relied on the broad experience and and displacement, (2) where the data permit, assign con- best professional judgment of its members to assign pre- sensus, preferred recurrence-interval (RI) and vertical ferred RI and VSR estimates to the faults/fault sections slip-rate (VSR) estimates with appropriate confidence under review. For some faults/fault sections, the trench- limits to the faults/fault sections under review, and (3) ing data were insufficient for the Working Group to identify critical gaps in the paleoseismic data and rec- make RI and VSR estimates. The Working Group also ommend where and what kinds of additional paleoseis- determined “best estimate” confidence limits for the RI mic studies should be performed to ensure that Utah’s and VSR estimates that reflect both epistemic and earthquake hazard is adequately documented and under- aleatory uncertainties associated with each fault/fault stood. It is important to note that, with the exception of section. Until superseded by information from new the Great Salt Lake fault zone, the Working Group’s paleoseismic investigations, the Working Group’s pre- review was limited to faults/fault sections having paleo- ferred RI and VSR estimates and associated confidence seismic-trenching data. Most Quaternary faults/fault limits represent the best available information regarding sections in Utah have not been trenched, but many have surface-faulting activity for the faults/fault sections RI and VSR estimates based on tectonic geomorphology reviewed, and can be considered as approximating aver- or other non-trench-derived studies. Black and others age RI and VSR values and 2-sigma variability about (2003) compiled the RI and VSR data for Utah’s Qua- those mean values. ternary faults, both those with and without trenches. The Working Group recommends additional paleo- Although used extensively by researchers and geo- seismic study of 20 faults/fault sections to characterize logic and engineering practitioners, prior to this review, Utah’s earthquake hazard to a minimally acceptable Utah’s Quaternary fault paleoseismic-trenching data had level. The Working Group considered NEHRP mini- not been critically evaluated to establish consensus fault mum slip-rate criteria and specific fault priorities for parameter values and confidence limits. Consequently, urban areas in Utah when evaluating which faults to rec- users unfamiliar with the database and unaware of ommend for additional study. However, the Working important caveats often did not recognize variations in Group selected some faults precisely because so little is the quality and completeness of the data. Consensus RI known about their recurrence or slip history. Others, and VSR estimates are a critical component in four areas while not located adjacent to urban areas, are near major directly related to reducing losses from earthquakes in 2 Utah Geological Survey

Utah: (1) updating the National Seismic-Hazard Maps, abilities, 1988, 1990, 1999) have successfully employed (2) characterizing seismic sources, (3) performing prob- the concept of working groups composed of technical abilistic seismic-hazard analyses, and (4) providing con- experts in a field of interest to critically evaluate various sensus paleoseismic data for research into other earth- datasets and arrive at consensus decisions regarding data quake topics. With a widely distributed consensus data- values and reliability. The UGS employed a similar set, all users can have access to expert-reviewed paleo- strategy and convened the Utah Quaternary Fault Para- seismic-trenching data that are qualified with appropri- meters Working Group composed of experts in the fields ate caveats, and from which they can make informed of paleoseismology and seismology in 2003-04. The judgments regarding their own research and projects. paleoseismologists on the Working Group collectively Table 1 presents a summary of the Working Group’s represent many decades of experience in conducting results. An expanded table in appendix A contains addi- paleoseismic investigations in Utah as well as through- tional critical background information regarding the out the United States and around the world. Likewise, paleoseismic data considered in the Working Group the seismologists on the Working Group are familiar review. Appendix B summarizes the paleoseismic data with Utah tectonics, and have worked directly with available for each fault/fault section, and lists the paleo- Utah’s paleoseismic data. seismic source documents consulted for this review. The Working Group included two categories of experts, all serving in a volunteer capacity. The first category consisted of paleoseismologists having direct PREVIOUS WORK knowledge of Utah’s Quaternary fault dataset. These individuals have investigated one or more of Utah’s Hecker (1993) made a comprehensive compilation Quaternary faults, and are responsible for much of the of information regarding Quaternary (<1.6 million paleoseismic-trenching data reviewed by the Working years) tectonic features in Utah, particularly information Group. The second category consisted of knowledge- relevant to earthquake hazards. That compilation built able experts capable of providing critical analysis of the upon Anderson and Miller’s (1979) Quaternary Fault paleoseismic data, but who have not conducted paleo- Map of Utah, and was subsequently updated by Black seismic studies in Utah and therefore have no vested and others (2003), who incorporated the results of pale- interest in the Utah data; this group includes both paleo- oseismic and Quaternary mapping studies from the suc- seismologists and seismologists. Table 2 lists the mem- ceeding 10 years. bers of the Utah Quaternary Fault Parameters Working The Black and others (2003) Quaternary Fault and Group and their affiliations. Fold Database and Map of Utah includes all presently In addition to being a member of the Working recognized Quaternary tectonic features in Utah. It is Group, William Lund, UGS, served as principal investi- unlikely that any large Quaternary faults or folds remain gator for the National Earthquake Hazards Reduction unidentified, although smaller features may be discov- Program (NEHRP)-funded Utah Quaternary Fault Para- ered as the Utah Geological Survey (UGS) and others meters Working Group project, and as the Working continue systematic 1:24,000-scale geologic mapping of Group Coordinator. As coordinator, Mr. Lund was Utah. The Black and others (2003) compilation summa- responsible for Working Group logistics, made an initial rizes the paleoseismic-trenching data for Utah’s Quater- review of all relevant paleoseismology source docu- nary faults. Those data are presented as reported in the ments, summarized the paleoseismic-trenching informa- paleoseismic source documents from which the database tion in those documents for the Working Group’s con- was compiled, and have not been evaluated for accuracy, sideration, moderated and recorded Working Group completeness, or associated uncertainty. meetings, and prepared this final report on the Working McCalpin and Nishenko (1996) reviewed the Group results. numerical-age data (chiefly radiocarbon [14C] and thermoluminescence [TL]) then available for the five central segments of the Wasatch fault zone (WFZ) having evi- PALEOSEISMIC-TRENCHING DATABASE dence for recurrent Holocene surface faulting. They identified 89 ages as closely limiting the timing of past Utah Quaternary Faults surface faulting on those segments. The McCalpin and Nishenko (1996) study is discussed in detail below. There are 212 Quaternary faults, fault sections, and Youngs and others (2000) and Wong and others fault-related folds in Utah (Hecker, 1993; Black and oth- (2002) performed probabilistic analyses of the Wasatch ers, 2003). They are chiefly normal-slip faults or are Front region and Salt Lake City metropolitan areas, related to normal-slip deformation. Utah includes parts respectively. Both studies include estimates of activity of three physiographic provinces: the Basin and Range, rates for faults within their study areas. Colorado Plateau, and Middle Rocky Mountains. Qua- ternary faults are present in all three provinces; however, the greatest number of faults is in the Basin and UTAH QUATERNARY FAULT PARAME- Range Province, which comprises roughly the western TERS WORKING GROUP half of Utah. Over the past approximately 30 years, a time span encompassing the entire history of paleoseis- Various seismic-hazard-evaluation initiatives in Cal- mic investigations on normal-slip faults worldwide, ifornia (Working Group on California Earthquake Prob- investigators have conducted paleoseismic-trenching Consensus preferred recurrence-interval and vertical slip-rate estimates 3

Table 1. Summary of Working Group consensus values for timing of most recent surface faulting and preferred recurrence-interval and vertical slip-rate estimates.

Fault Timing of Most Recent Preferred Recurrence Preferred Vertical Fault Section/Segment1 Earthquake Interval (kyr)2 Slip Rate (mm/yr)2 Wasatch fault zone Brigham City segment 2100±800 cal yr B.P. 0.5-1.3-2.8 0.6-1.4-4.5 Weber segment 0.5±0.3 ka/950±450 cal yr B.P.3 0.5-1.4-2.4 0.6-1.2-4.3 Salt Lake City segment 1300±650 cal yr B.P. 0.5-1.3-2.4 0.6-1.2-4.0 Provo segment 600±350 cal yr B.P. 1.2-2.4-3.2 0.6-1.2-3.0 Nephi segment ≤1.0±0.4 ka4 1.2-2.5-4.8 0.5-1.1-3.0 Levan segment ≤1000±150 cal yr B.P. >3, <125 0.1-0.65 Joes Valley fault zone6 Not constrained 5-10-50 No estimate West Valley fault zone 1.3-1.7 ka No estimate 0.1-0.4-0.6 West Cache fault zone Clarkston fault 3600-4000 cal yr B.P. 5-205 0.1-0.4-0.7 Junction Hills fault 8250-8650 cal yr B.P. 10-255 0.05-0.1-0.2 Wellsville fault 4400-4800 cal yr B.P. 10-255 0.05-0.1-0.2 East Cache fault zone central section 4.3-4.6 ka 4-10-15 0.04-0.2-0.4 Hurricane fault zone Anderson Junction section 5-10 ka 5-505 0.05-0.2-0.4 Great Salt Lake fault zone7 Fremont Island segment 3150+235/-211 cal yr B.P. 1.8-4.2-6.6 0.3-0.6-1.6 Antelope Island segment 586+201/-241 cal yr B.P. 1.8-4.2-6.6 0.3-0.6-1.6 Oquirrh fault zone 4.8-7.9 cal yr B.P 5-20-50 0.05-0.2-0.4 Southern Oquirrh Mountains fault zone Mercur fault Shortly after 4.6+0.2 ka 5-20-50 0.05-0.2-0.4 Eastern Bear Lake fault southern section <2.1±0.2 ka, but >0.6+0.08 ka 3-8-15 0.2-0.6-1.6 Bear River fault zone 2370±1050 yr B.P.8 1-1005 0.05-1.5-2.5 Morgan fault zone central section <8320±100 14C yr B.P. 25-1005 0.01-0.02-0.04 James Peak fault >30-70 ka 10-50-100 0.01-0.03-0.07 Towanta Flat graben6 >130-150 ka 25-50-200 No estimate Bald Mountain fault >130 ka No estimate No estimate Strawberry fault ≥1.5 ka 5-15-25 0.03-0.1-0.3 Hansel Valley fault C.E. 19349 15-25-50 0.06-0.12-0.24 Hogsback fault southern section Not constrained No estimate No estimate North Promontory fault Latest Pleistocene/Holocene No estimate 0.1-0.2-0.5 Sugarville area faults Not constrained No estimate No estimate Washington fault zone northern section Not constrained No estimate No estimate Fish Springs fault <2280±70 14C yr B.P No estimate No estimate

1“Section” refers to a portion of a fault defined on the basis of static geologic criteria (geomorphic or structural), but for which no evidence presently exists to show that its history of surface faulting is different from adjacent parts of the fault. “Segment” refers to a portion of a fault, typically also defined on the basis of geomorphic or structural criteria, but for which historical surface ruptures or paleoseismic data show that the history of surface faulting is different from adjacent portions of the fault, and therefore that the seismogenic behavior of the segment is independent from that of the remainder of the fault. 2Consensus preferred recurrence-interval and vertical slip-rate estimates (bold) with approximate 2-sigma confidence limits; see section on Consensus Process for a discussion of the process used to determine these values. 3Two most recent earthquakes are reported for Weber segment; no consensus among investigators regarding the 0.5 ka event. 4Most recent surface-faulting earthquake may be as young as 0.4 ka. 5Due to limited data, parameter is reported as a range rather than as a central value with approximate 2-sigma confidence limits. 6Seismogenic origin of structure is uncertain. 7Information derived from high-resolution geophysics and drilling information; there are no trench data for this fault. 8Calendar calibrated but no mean residence correction applied 9Historical surface-faulting earthquake; C.E. = Current Era. 4 Utah Geological Survey

Table 2. Members of the Utah Quaternary Fault Parameters Working Group.

Category 1: Paleoseismologists who have conducted paleoseismic investigations in Utah. Suzanne Hecker – U.S. Geological Survey; Menlo Park, California Michael Hylland – Utah Geological Survey; Salt Lake City, Utah William Lund – Utah Geological Survey; Cedar City, Utah Michael Machette – U.S. Geological Survey; Denver, Colorado James McCalpin – GEO-HAZ Consulting; Crestone, Colorado Alan Nelson – U.S. Geological Survey; Denver, Colorado Susan Olig – URS Corporation; Oakland, California Dean Ostenaa – U.S. Bureau of Reclamation; Denver, Colorado Stephen Personius – U.S. Geological Survey; Denver, Colorado David Schwartz – U.S. Geological Survey; Menlo Park, California

Category 2: Subject-matter experts who have not conducted paleoseismic investigations in Utah. Craig dePolo – Nevada Bureau of Mines and Geology; Reno, Nevada Kathleen Haller – U.S. Geological Survey; Denver, Colorado Philip Pearthree – Arizona Geological Survey; Tucson, Arizona James Pechmann – University of Utah Seismograph Stations; Salt Lake City, Utah Mark Petersen – U.S. Geological Survey; Denver, Colorado Robert Smith – University of Utah Dept. of Geology and Geophysics; Salt Lake City, Utah Ivan Wong – URS Corporation; Oakland, California

Table 3. Utah Quaternary faults/fault sections that have paleoseismic-trenching data.

Wasatch fault zone1 Great Salt Lake fault zone2 Brigham City segment Fremont Island segment Weber segment Antelope Island segment Salt Lake City segment Oquirrh fault zone Provo segment Southern Oquirrh Mountains fault zone Nephi segment Mercur fault Levan segment Eastern Bear Lake fault Joes Valley fault zone southern section East Joes Valley fault Bear River fault zone West Joes Valley fault Morgan fault zone Intragraben faults James Peak fault West Valley fault zone Towanta Flat graben Taylorsville fault Bald Mountain fault Granger fault Strawberry fault West Cache fault zone Hansel Valley fault Clarkston fault Hogsback fault Junction Hills fault southern section Wellsville fault North Promontory fault East Cache fault zone Sugarville area faults central section Washington fault zone Hurricane fault zone northern section Anderson Junction section Fish Springs fault

1See appendix E for a complete list of fault/fault section abbreviations. 2Paleoearthquake information is from detailed seismic reflection surveys and drilling. studies on 33 (16%) of Utah’s Quaternary faults or fault Paleoseismic-Trenching Investigations sections. Much of that effort was directed at the six central segments of the Wasatch fault zone (WFZ; see Paleoseismic-trenching investigations in Utah fall appendix E for a full list of fault/fault section abbrevia- into one of five categories: (1) U.S. Geological Survey tions used in this report) that have evidence of Holocene (USGS)-funded studies performed by Woodward-Clyde surface faulting. Table 3 lists the Quaternary faults in Consultants (WCC), (2) studies performed during the Utah that have paleoseismic-trenching information and “Wasatch Front Regional Earthquake Hazards Assess- figure 1 shows their locations. ment,” cosponsored by the USGS and the UGS, (3) other Consensus preferred recurrence-interval and vertical slip-rate estimates 5

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Figure 1. Locations of Quaternary faults/fault sections for which paleoseismic-trenching or geophysical and drilling data are available: WVFZ = West Valley fault zone, GF = Granger fault, TF = Taylorsville fault, WCFZ = West Cache fault zone, CF = Clarkston fault, JHF = Junction Hills fault, WF = Wellsville fault, WS = Weber segment of the Wasatch fault zone, SLCS = Salt Lake City segment of the Wasatch fault zone. 6 Utah Geological Survey

USGS-funded studies under NEHRP, (4) U.S. Bureau of ers, 2000; Lund and others, 2001; Olig and others, Reclamation (USBR) studies related to water impound- 2001). NEHRP also funded the detailed mapping ment or conveyance structures, and (5) other studies per- (1:50,000 scale) of the Nephi segment (NS) of the WFZ formed chiefly by universities and geotechnical consult- (Harty and others, 1997), the West Cache fault zone ants. Black and others (2003) show the location of all (WCFZ; Solomon, 1999), and the Levan segment (LS) paleoseismic-trenching studies conducted on Utah’s of the WFZ (Hylland and Machette, 2004). NEHRP is Quaternary faults. presently supporting mapping of the Fayette segment (FS) of the WFZ by the UGS, trenching on the PS of the Woodward-Clyde Consultants WFZ (Olig and others, 2004), and a geophysical and drilling investigation of the Great Salt Lake fault zone Beginning in the 1970s and extending to the mid- (GSLFZ) beneath Great Salt Lake (Dinter and Pech- 1980s with funding from the USGS, WCC pioneered the mann, 2004a, 2004b). paleoseismic study of normal-slip faults by first mapping and then trenching young scarps on the WFZ. The WCC U.S. Bureau of Reclamation investigations (Swan and others, 1980, 1981a, 1981b; Hanson and others, 1981, 1982; Schwartz and others, Between 1982 and 1992, the USBR conducted both 1983; Schwartz and Coppersmith, 1984) were the first regional and project-specific paleoseismic-trenching performed on normal-slip faults anywhere, and much of investigations in support of construction and operation what is now known regarding the study of normal faults of USBR dams, reservoirs, and water-conveyance struc- in trenches was first developed on the WFZ by WCC. tures in Utah (Nelson and Martin, 1982; Martin and oth- Conducted early in the history of normal-fault paleoseis- ers, 1985; Nelson and Weisser, 1985; Foley and others, mology, the WCC studies predate more recent advance- 1986; Nelson and VanArsdale, 1986; Sullivan and oth- ments in paleoseismology and geochronology. ers, 1988a, 1988b; Ostenaa, 1990; Nelson and Sullivan, 1992; Sullivan and Nelson, 1992). These studies consti- Wasatch Front Regional Earthquake Hazards tute the bulk of the paleoseismic-trenching investiga- Assessment tions performed in the Middle Rocky Mountains and Colorado Plateau in Utah. Beginning in 1983 and continuing until 1989, the USGS targeted the Wasatch Front region for intense Other Studies study under the auspices of the Regional Earthquake Hazards Assessment element of NEHRP. The “Wasatch Universities and geotechnical consulting firms have Front Regional Earthquake Hazard Assessment” con- also conducted fault-trenching studies in Utah. West ducted in cooperation with the UGS resulted in the first (1994) trenched the Bear River fault zone (BRFZ) and detailed (1:50,000-scale) geologic maps of the Brigham Hogsback fault (HF) as part of his Ph.D. studies at the City (BCS), Weber (WS), Salt Lake City (SLCS), and Colorado School of Mines (project originally initiated as Provo segments (PS) of the WFZ (Personius, 1990; Per- a USBR investigation). As recognition of earthquake sonius and Scott, 1992; Machette, 1992; Nelson and Per- hazards in Utah has increased, some local jurisdictions sonius, 1993), as well as the East Cache fault zone have adopted ordinances requiring earthquake-hazard (ECFZ; McCalpin, 1989). Additionally, both USGS and evaluations. This is particularly true in Salt Lake Coun- other investigators performed paleoseismic-trenching ty, where geotechnical consultants have trenched the studies, chiefly on the WFZ and other faults in northern SLCS of the WFZ (Robison and Burr, 1991; Korbay and Utah (McCalpin, 1985; Keaton and others, 1987; McCormick, 1999; Simon and Shlemon, 1999). Other Machette and Lund, 1987; Nelson and others, 1987; faults investigated by geotechnical firms include the Schwartz and Lund, 1988; Keaton and Currey, 1989; Washington fault zone (WaFZ) and Hurricane fault zone Forman and others, 1991; McCalpin, 1990, 1994, 2003; (HFZ; Earth Sciences Associates, 1982) in southwestern Jackson, 1991; Lund and others, 1991; McCalpin and Utah and the Sugarville area faults (SAFs; Dames and Forman, 1991; Personius, 1991; Machette and others, Moore, 1978) in Utah’s Sevier Desert. 1992; McCalpin and others, 1992).

USGS National Earthquake Hazards Reduction WORKING GROUP REVIEW PROCESS Program Although the Utah paleoseismic-trenching database Following the end of the Wasatch Front Regional is small compared to California’s, where similar evalua- Earthquake Hazard Assessment in 1989, the USGS fund- tions of paleoseismic data have been conducted, it was ed additional paleoseismic-trenching studies in Utah neither reasonable nor practical to expect Working through the External Research Program of NEHRP. Group members serving in a volunteer capacity to While performed chiefly on the WFZ and other nearby review each of the more than 60 paleoseismic source faults (McCalpin and Forman, 1993, 2002; McCalpin documents available for Utah’s Quaternary faults. To and others, 1994; Black and others, 1996; Lund and expedite the process, the Working Group Coordinator Black, 1998; McCalpin and Nelson, 2000; McCalpin, summarized the available paleoseismic-trenching data 2002; Olig and others, 2004), NEHRP-funded trenching and forwarded the summary information to Working studies expanded to other areas of Utah as well (Olig and Group members for their review. The Working Group others, 1996; Stenner and others, 1999; Black and oth- convened three times to evaluate the data, and to come Consensus preferred recurrence-interval and vertical slip-rate estimates 7 to consensus decisions regarding preferred RI and VSR 7. Second Working Group meeting in Salt Lake estimates for the faults under review. The Working City, Utah, on September 4 and 5, 2003, to Group Coordinator then summarized the paleoseismic evaluate the paleoseismic-trenching data data and the results of the Working Group’s deliberations available for Quaternary faults/fault sec- on a “Consensus Recurrence-Interval and Vertical Slip- tions, exclusive of the WFZ. Rate Estimate” form for each fault/fault section (appen- 8. Incorporation of the Working Group’s rec- dix B). The consensus forms represent the principal ommendations regarding earthquake timing, result of the Working Group review, and should be con- RI, and VSR into Consensus Recurrence- sulted for details of each fault/fault section and of the Interval and Vertical Slip-Rate Estimate review process. forms for the WFZ segments and other Qua- Results of the Working Group review were present- ternary faults/fault sections (appendix B). ed at local, regional, and national professional society 9. Distribution of the draft consensus forms to meetings (2004 Utah Earthquake Conference, 2/26/04; Working Group members for review and Seismological Society of America 2004 Annual Meet- comment. ing, 4/17/04; Western States Seismic Policy Council Basin and Range Province Seismic Hazards Summit II, 10.Third Working Group meeting in Salt Lake 5/17/04). The UGS used the Working Group’s results to City, Utah, on February 27, 2004, to finalize revise the Quaternary Fault and Fold Database and Map RI and VSR estimates. of Utah (Black and others, 2003). The intent of this UGS 11. Presentation of the Working Group’s results Bulletin is to make the Working Group results widely and recommendations at professional socie- available to users who require expert-reviewed paleo- ty meetings, and to geological and engineer- seismic-trenching data. ing groups in Utah. 12.Preparation of a USGS Final Technical Review Process Tasks Report contract deliverable and this Bulletin presenting the Working Group’s results and The Working Group review consisted of the follow- recommendations. ing principal tasks: 13.Update of the Quaternary Fault and Fold Database and Map of Utah (Black and oth- 1. Preliminary Working Group meeting to ers, 2003) with the new consensus RI and establish review parameters and process. VSR values. Due to delays in approval of the federal FY 2003 budget, this initial meeting was Consensus Process replaced by e-mail and telephone contacts to facilitate project start-up. The Working Group review showed that the paleo- 2. Detailed review by the Working Group seismic-trenching data for Utah’s Quaternary faults are Coordinator of published and unpublished generally not adequate to permit rigorous statistical paleoseismic-trenching data available for the analysis of the data, or to constrain RI and VSR values six central segments (BCS, WS, SLCS, PS, within rigidly quantifiable bounds. Therefore, the Work- NS, LS) of the WFZ; preparation of summa- ing Group relied on the expertise and collective judg- ry data forms for each paleoseismic source ment of its members to assign preferred RI and VSR document, and of a synthesis form for each estimates to the faults/fault sections under review. The segment as a whole (see appendix C for preferred values represent the Working Group’s best col- example forms). lective judgment regarding a “mean” RI and VSR for the 3. Distribution of completed summary and syn- fault/fault section, based on paleoseismic-trenching data thesis forms to the Working Group for their available at the time of the review. review. The Working Group also assigned confidence limits 4. First Working Group meeting in Salt Lake to the RI and VSR estimates. Although much of the City, Utah, on June 4 and 5, 2003, to evalu- trenching data are not amenable to statistical analysis, ate the paleoseismic-trenching data for the the Working Group kept in mind the concept of 2-sigma six central WFZ segments. variability (5th and 95th percentiles) about the preferred RI and VSR estimates as they assigned upper and lower 5. Detailed review of published and unpub- bounds to their confidence limits (table 1, appendices A lished paleoseismic-trenching data pertain- and B). The goal was to capture both the uncertainty ing to the remaining Quaternary faults/fault associated with incomplete knowledge of the fault/fault sections in Utah that have paleoseismic- section (epistemic uncertainty – for example, data avail- trenching data; preparation of data forms able from only a single trench site along a many kilome- summarizing the information in each paleo- ter-long fault) and natural variation in the seismogenic seismic source document, and of a synthesis process through time (aleatory uncertainty – for exam- form for each Quaternary fault/fault section. ple, variations in the length of interevent intervals). The 6. Distribution of completed data and synthesis confidence-limit distribution around the preferred RI and forms to the Working Group for their review. VSR estimates is in some cases skewed to capture appar- 8 Utah Geological Survey ent variability in fault/fault section behavior. pared to the timing of earthquakes farther Establishing preferred RI and VSR estimates and north on the segment, indicate either partial associated confidence limits often generated spirited dis- segment rupture of the PS, or rupture over- cussion among Working Group members, and in several lap from surface faulting on the NS to the instances considerably stretched their comfort levels. south. Conversely, recent scarp mapping Although individual members of the Working Group and diffusion modeling on the NS indicates may retain reservations regarding some RI and VSR esti- that surface rupture may propagate from the mates or associated confidence limits, the reported val- PS to the NS during some large earthquakes. ues represent the final consensus of the Working Group. • Both paleoseismic-trenching investigations Therefore, until superseded by information from new performed on the NS produced conflicting paleoseismic investigations, the Working Group’s pre- sets of numerical ages on samples from the ferred RI and VSR estimates and confidence limits rep- same geologic units resulting in significant resent the best available fault activity information for uncertainty regarding paleoearthquake tim- those faults/fault sections, and can be considered as ing; as a result, the surface-faulting chronol- approximating mean RI and VSR values and 2-sigma ogy for the NS can vary depending on which variability about those mean values. ages are selected to constrain earthquake timing. • Over 300 numerical age determinations, ISSUES RELEVANT TO THE WORKING chiefly 14C and TL accumulated over 30- GROUP REVIEW plus years, constrain the timing of surface faulting on the WFZ; the 14C ages represent Sources of Uncertainty a wide variety of sampling, dating, and calibration techniques, thus injecting variability Epistemic Uncertainty into the absolute-age dataset. • A key component of the Working Group review was The Working Group considers many of the identification of “sources of uncertainty” within Utah’s confidence limits originally reported with paleoseismic-trenching data. Hecker and others (1998) the timing of surface-faulting earthquakes as compiled possible sources of uncertainty in fault-activi- too narrow, and as not fully accounting for ty studies for the Long Beach, California 30′ x 60′ quad- the geologic (aleatory) uncertainty associat- rangle fault map and database. A modified form of that ed with earthquake timing. list (appendix D) was used to evaluate epistemic uncertainty resulting from incomplete or imperfect knowledge For Utah’s other Quaternary faults, sources of epis- regarding Utah’s paleoseismic-trenching data. temic uncertainty include the following: Principal sources of epistemic uncertainty for the six central, active segments of the WFZ include the follow- • Seismogenic capability of fault uncertain. ing: • Zone of deformation wider than the zone of study – not all scarps trenched. • Investigators identified two different most • recent surface-faulting earthquakes (MRE) Time period too long or too short to represent at the two trench sites on the BCS, even contemporary conditions. though the two sites are within a few kilo- • Studies limited to a single strand or section of meters of each other. a complex fault zone. • Timing of older earthquakes on the BCS have • Number of surface-faulting earthquakes ± uncertainties that equal or exceed the uncertain. interevent intervals between the earth- • Surface-faulting earthquake timing only quakes. broadly constrained (thousands to tens of • Multiple investigators differ in their interpre- thousands of years) or unknown. tation of the timing of the MRE on the WS, • Vertical displacement per earthquake and/or raising the possibility of partial segment rup- cumulative vertical displacement poorly ture or rupture overlap from adjacent seg- constrained or unknown. ments. • Interevent intervals open at one or both ends. • Latest Pleistocene and early Holocene sur- • Number of interevent intervals may be too few face-faulting earthquakes on the SLCS are to yield representative mean recurrence. identified on the basis of a retrodeformation analysis of a trench exposure; the earth- • Earthquake recognition based on indirect quakes lack direct stratigraphic and structur- stratigraphic or structural evidence. al evidence of their occurrence. • Selected paleoseismic parameter conflicts • Differences in the number and timing of sur- with other data at the site. face-faulting earthquakes near the southern • Uncertain correlation of earthquakes between end of the PS (Water Canyon), when com- fault strands. Consensus preferred recurrence-interval and vertical slip-rate estimates 9

Aleatory Uncertainty Holocene. Two segments, BCS and SLCS, also have information on surface faulting extending to the latest Uncertainty due to inherent variability of the seis- Pleistocene; however, the timing of the older earth- mogenic process is largely unknown for the faults/fault quakes is not as well constrained, and in some instances sections reviewed by the Working Group. All of the direct physical evidence of surface faulting (colluvial faults/fault sections lack the definitively complete and wedges, fault terminations, fissures and fissure-fill sufficiently long paleoseismic records required to illus- deposits) is lacking. A NEHRP-funded paleoseismic- trate the full range of variability in the seismogenic trenching investigation conducted cooperatively be- process. This is true even for the five central segments tween URS Corporation and the UGS in 2003 (Olig and of the WFZ (BCS, WS, SLCS, PS, NS), which are the others, 2004) is designed to extend the surface-faulting most studied faults in Utah, but where McCalpin and record on the PS to the latest Pleistocene; however, final Nishenko (1996) note that “The small number of results of that investigation are not yet available. observed recurrence intervals from individual fault segments (1 to 3) during the past 5.6 kyr [thousand years] Other Quaternary Faults precludes the unequivocal demonstration of a particular type of recurrence behavior (i.e., random versus period- Paleoseismic-trenching data for Utah’s other Qua- ic).” The coefficient of variation (COV; ratio of the stan- ternary faults are more limited than for the WFZ. Data dard deviation to the mean) provides a measure of the limitations are related to four principal causes: (1) periodicity of earthquake recurrence intervals (Norman reduced fault activity, (2) remote fault locations away Abrahamson, Pacific Gas and Electric Company, written from large population centers, (3) typically shorter fault communication to Susan Olig, Working Group member, lengths, and (4) difficulty identifying older earthquakes. 2000). The smaller the COV (<0.3) the more periodic is Less active faults produce fewer earthquakes over a earthquake recurrence, while a large COV (>1) indicates given time period; consequently, unless the deposits be- earthquakes are not periodic. The limited long-term ing trenched are old, a typical 3- or 4-meter-deep paleo- recurrence information available for Utah faults/fault seismic trench exposes evidence for fewer earthquakes. sections (BCS, SLCS, West Valley fault zone [WVFZ], The remote location of many faults equates to lower Southern Oquirrh Mountain fault zone [SOMFZ], earthquake risk and consequently to less intensive study. Hansel Valley fault [HVF]; appendices A and B) indi- Off the Wasatch Front, most faults have only a single cates that large variations in earthquake repeat times and trenching investigation, even on faults with evidence of size are possible, likely representing large COV values. possible segmentation or other complexities. Short The Working Group recognized the potential effect faults typically produce smaller earthquakes with small- of aleatory uncertainty on their RI and VSR estimates, er displacements, which can make recognizing the geo- and attempted to incorporate the effects of that variabil- logic record of their occurrence more difficult. Finally, ity when assigning confidence limits to their preferred where trenches expose evidence for early to middle Qua- RI and VSR values. However, the Working Group ternary surface faulting, recognition of individual sur- acknowledges that due to a lack of data, they may have face-faulting earthquakes has proven difficult; investiga- underestimated the effects of process variability for tors typically report evidence of surface faulting, but are some faults/fault sections. unsure of the exact number of earthquakes. This prob- lem becomes more acute for older earthquakes that were also small. Data Adequacy The paleoseismic investigation of the GSLFZ pre- Closely associated with data uncertainty is the issue sented a unique challenge since that fault zone is entire- of data adequacy – are the available paleoseismic- ly submerged beneath Great Salt Lake. Because trench- trenching data sufficiently abundant to make reliable RI ing was not possible on this important structure, investi- and VSR estimates for the faults or fault sections under gators used a combination of high-resolution geophysi- review? Utah’s paleoseismic-trenching data divide nat- cal techniques and detailed drilling and sampling to urally into two groups: (1) data for the WFZ, Utah’s develop a credible earthquake chronology for two of the longest, most active, and most studied fault, and (2) data three GSLFZ segments. Although no paleoseismic- for Utah’s other Quaternary faults that have been studied trenching data are available for the GSFFZ, the Working in trenches or natural exposures. Group felt that the paleoseismic data resulting from the geophysical and drilling studies were sufficiently credible to include in their review. Wasatch Fault Zone The WFZ, by virtue of its collocation with the pop- Constraining Age Estimates ulous Wasatch Front and abundant geomorphic evidence of geologically recent surface faulting, is the most stud- Numerical Ages ied and best understood Quaternary fault in Utah. Inves- tigators have performed multiple paleoseismic investiga- Radiocarbon ages: Paleoseismic-trenching studies in tions on the six active central segments of the WFZ, and Utah have resulted in more than 300 numerical ages. although significant questions remain unanswered (see The majority are 14C ages, which are of two principal above) the surface-faulting histories of most segments types: (1) ages from charcoal obtained by standard gas are generally well understood to at least the middle proportional counting techniques, or ages obtained using 10 Utah Geological Survey an accelerator mass spectrometer for samples too small mic-trenching investigations in Utah. Thermolumines- for conventional counting methods, and (2) apparent cence dating is the most common. Most TL ages were mean residence time (AMRT) ages on bulk organic sam- obtained during the 1980s on the central segments of the ples, usually collected from buried soils, tectonic crack- WFZ. There is no recognized need or procedure to cal- fill material, or colluvial-wedge deposits. Bulk organic ibrate TL or other luminescence ages and they are samples contain carbon of different ages, and the 14C assumed to be calendar ages. ages obtained from them must be corrected to account for this “carbon-reservoir” effect. Machette and others Relative Ages (1992) and McCalpin and Nishenko (1996) include dis- cussions of AMRT ages and their proper correction for Lake Bonneville chronology: Much of the WFZ and carbon age spans and carbon mean residence time. many other Quaternary faults in northern and western Production of 14C in the upper atmosphere has var- Utah lie below the highstand of Lake Bonneville, a late ied through time due to fluxes in the Earth’s magnetic Pleistocene pluvial lake (Gilbert, 1890) that occupied the field, and more recently due to open-air nuclear weapons Bonneville basin from about 32.5 to 13.9 ka (Donald testing. The variable rate of production means that 14C Currey, University of Utah Geography Department, writ- has been incorporated into living organisms (plant and ten communication to the UGS, 1996; verbal communi- animal) in different proportions to 12C at different times cation to Working Group, 2004). At its highest elevation 14 14 (Bonneville shoreline, 1551 m [5090 ft]), Lake Bon- in the past. Therefore, C ages ( C yr B.P.) must be 2 calibrated to adjust for the different production rates. neville had a surface area in excess of 50,000 km (20,000 mi2) and a maximum depth of more than 305 m Correction of 14C years to calendar years (cal yr B.P.) (1000 ft). Lake Bonneville lacustrine deposits and post- relies chiefly on the radiometric dating of tree rings and Bonneville alluvium and colluvium dominate the Qua- marine coral of otherwise known age, and comparing the ternary geology of the Bonneville basin. ages of those materials to the resulting 14C ages. Cali- brating 14C ages beyond about 20,000 years ago (ka; Four prominent shorelines, two transgressive appendix E) remains difficult. Once a properly correct- (Stansbury and Bonneville), one regressive (Provo), and one related to the post-Bonneville highstand of Great ed and calibrated calendar age is obtained, it remains for Salt Lake (Gilbert), provide well-documented time lines the paleoseismic investigator to interpret the age within against which the timing of surface faulting can be com- the sample’s geologic context and determine how close- pared. However, Lake Bonneville deposits also bury ly the age constrains the timing of surface faulting. older Quaternary deposits in the basin, making it diffi- Since the inception of paleoseismic-trenching stud- cult to decipher the history of older surface faulting. The ies in Utah, significant advances have been made in 14 details of Lake Bonneville chronology continue to methodologies for calendar-calibrating C ages, and in evolve through time (Oviatt and Thompson, 2002; Don- our understanding of how to properly sample for, cor- ald Currey, University of Utah Geography Department, rect, and interpret AMRT ages on bulk organic samples. verbal communication to Working Group, 2004), and The science of paleoseismology also has advanced over many early paleoseismic studies relied on age estimates that same time period, and our understanding of how to of Bonneville deposits and shorelines that were subse- conduct paleoseismic-trenching investigations and inter- quently revised. Additionally, early paleoseismic- pret their results has also improved. The result is a trenching investigations used Lake Bonneville age esti- dataset of 14C ages that are calibrated to a variety of stan- mates reported in 14C years. Donald Currey (University dards, if at all; sampled by a variety of techniques; ana- of Utah Geography Department, written communication lyzed by different laboratories; and interpreted by inves- to UGS, 1996; verbal communication to Working Group, tigators having varying levels of experience and expert- 2004) calendar-calibrated key Lake Bonneville ages, and ise. showed that Lake Bonneville events and features are as Luminescence ages: Investigators have employed a much as 4.5 kyr older than indicated by 14C ages. Table variety of luminescence dating techniques in paleoseis- 4 presents Currey’s Lake Bonneville chronology.

Table 4. Timing of events related to the transgression and regression of Lake Bonneville (modified from Donald Currey, University of Utah, written communication to the UGS, 1996; verbal communication to Working Group, 2004).

Lake Stage Radiocarbon Years Calendar Years (14C yr B.P.) (cal yr B.P.) Start of Lake Bonneville 28,000 ~32,500 Stansbury shoreline 21,000 – 20,000 24,400 – 23,200 Bonneville shoreline 15,500 – 14,500 18,000 – 16,800 Start/end Bonneville flood 14,500 16,800 Provo shoreline 14,500 – 14,000 16,800 – 16,200 Gilbert shoreline 11,000 – 10,000 12,800 – 11,600 Consensus preferred recurrence-interval and vertical slip-rate estimates 11

When possible, the Working Group used the calen- reliable net vertical-displacement estimates from trench dar-calibrated ages in table 4 to revise RI and VSR esti- exposures. mates for paleoseismic-trenching investigations that relied on 14C years for the ages of Lake Bonneville fea- Sparse Data tures and events. Soil-profile development: Relative age estimates based Net vertical-displacement measurements are point on soil-profile development play an important part in values made at individual locations along a fault. Slip many paleoseismic-trenching investigations in Utah, distribution during a surface-faulting earthquake varies particularly reconnaissance investigations off the along strike, rising to a maximum at one or more points Wasatch Front. Information presented in paleoseismic and decreasing to zero at the ends of the rupture (Crone source documents seldom permits an independent evalu- and others, 1985). Characterizing slip distribution along ation of relative soil age. Therefore, unless there was a a fault requires careful geologic mapping and the mak- compelling reason to do otherwise, the Working Group ing of numerous displacement measurements along the accepted relative age estimates based on soil develop- fault trace. With the possible exception of the WS of the ment as reported by original investigators, while recog- WFZ, no Quaternary faults/fault sections in Utah have nizing that uncertainties associated with soil-profile age sufficient displacement data to fully characterize their estimates may be thousands to tens of thousands of slip distribution. years. Net vertical-displacement information is most abundant for the BCS, WS, SLCS, and PS of the WFZ. These data represent a combination of measurements made Net Vertical-Displacement Data during paleoseismic-trenching investigations from both Net vertical-displacement data for Utah’s Quater- scarps and trenches, and scarp-profile measurements nary faults come from two principal sources: (1) topo- made as the USGS mapped these segments. With few graphic profiles measured across scarps, with or without exceptions, the net vertical-displacement data are sparse- an accompanying trench, and (2) measurements made in ly distributed along the segments, and their interpreta- trenches. Uncertainties in net vertical-displacement data tion is complicated by complex rupture patterns, poorly are of three principal types: (1) measurement uncertain- constrained deposit ages, and the presence of non-cor- ty, (2) sparse data, and (3) incomplete documentation. relative geologic units on either side of many scarps. The exception is the WS, where the USGS measured 375 Measurement Uncertainty scarp profiles (77 in the field and 298 using a photogrammetric plotter and aerial photographs); however, Scarp profiles: Scarp profiles are commonly used to only about 30 of those measurements are included on the determine scarp height and net vertical displacement geologic map of the WS (Nelson and Personius, 1993). across fault scarps. Profiling techniques range from Off the WFZ, net vertical-displacement information highly accurate, computer-assisted surveying, to sequen- is commonly limited to one or two points along a fault, tial measurements of slope angle along a profile line and represents “best available” data for the fault/fault using a meter stick lying on the ground and an Abney section. Where the measurements lie within the slip-dis- level resting on the stick to measure slope angles. Both tribution curve for the faults is almost always unknown. methods, and others, produce accurate profiles; uncertainty with the resulting net displacement data relates Incomplete Documentation chiefly to issues of erosion and deposition on and adjacent to the scarp, effects of near-field deformation (for Incomplete documentation of net vertical-displace- example – graben formation, back-tilting, and warping), ment measurements is common in many paleoseismic failure to profile all scarps at a site, and difficult site con- source documents. As discussed above, measurements ditions. Where unmodified pre-faulting surfaces on both of net vertical displacement, whether from scarp profiles sides of a scarp can be accurately projected to the fault, or trenches, frequently include important caveats that topographic profiles provide a reliable measurement of require explanation. The net vertical-displacement data cumulative net vertical displacement. However, where reviewed by the Working Group ranged from detailed complicating factors are present, uncertainty enters into explanations of how displacement was measured and the measurements, and considerable experience is associated uncertainty evaluated, to cursory statements required to interpret profile results and arrive at reliable of displacement values, commonly reported to the near- net vertical-displacement estimates. est meter, with no accompanying explanatory informa- Measurements in trenches: Correlative stratigraphy tion. The Working Group review showed that for some displaced across a fault zone and exposed in a trench can investigations not all scarps were trenched or profiled, so provide a direct measure of fault displacement. Howev- reported net vertical-displacement values are minima, er, many trenches lack correlative stratigraphy, and net while at other sites antithetic scarps, even when recog- vertical-displacement measurements from trenches are nized, were not included in the net displacement budget, often estimates based on secondary stratigraphic and and the resulting net vertical-displacement measure- structural relations, thickness of colluvial-wedge ments are too large. Consequently, where explanatory deposits, retrodeformation reconstructions, and trench details are lacking, the accuracy of the net vertical-dis- depth. As is the case with scarp profiles, in the absence placement information for Utah’s Quaternary faults is of a best-case scenario, experience is required to obtain often questionable. 12 Utah Geological Survey

PALEOSEISMIC PARAMETERS dataset (CALIB v. 3.0; Stuiver and Reimer, 1993) while applying a consistent methodology for assigning carbon Earthquake Timing age span, carbon mean residence time, and other calibration parameters. The result was a set of consistently cal- The timing of surface-faulting earthquakes reported ibrated, closely limiting 14C ages and associated TL ages in paleoseismic-source documents typically is con- for surface-faulting earthquakes on the central WFZ cur- strained by either numerical or relative ages and in sev- rent for investigations done up to about 1995. McCalpin eral instances by a combination of both. Depending on and Nishenko (1996) used the revised absolute ages to the number of ages available and their geologic context, calculate weighted means for the timing of surface-fault- the timing of surface faulting can be constrained in the ing earthquakes on the five WFZ segments. The ± con- best cases to within a few hundred years. More often, fidence limits reported for the weighted means (see resolution of earthquake timing is less precise, in some McCalpin and Nishenko [1996] table 1) reflect cumula- instances tens of thousands of years or more. Because tive laboratory uncertainty associated with the calibrated the WFZ is Utah’s most intensely studied Quaternary ages used to calculate the weighted means, but do not fault, and therefore has the greatest number of numerical incorporate geologic uncertainty associated with earth- ages, the timing of surface-faulting earthquakes on the quake timing (James McCalpin, GEO-HAZ Consulting, six active central segments of the WFZ is better con- verbal communication to Working Group, 2003). strained, at least to the middle Holocene, than are earth- With the exceptions noted below, McCalpin and quakes on other faults in Utah. Because earthquake tim- Nishenko’s (1996) revised 14C and associated TL ages ing is critical to determining RI and VSR, the Working remain the best available numerical-age data for the WS Group made a careful review of information relevant to and PS. On those segments, the Working Group re- earthquake timing on Utah’s Quaternary faults (appen- determined surface-faulting timing by calculating the dices A and B). simple mean of the McCalpin and Nishenko (1996) closely limiting absolute ages for each earthquake Wasatch Fault Zone (appendices A and B). The means were then rounded to the nearest half-century. In nearly every instance, the McCalpin and Nishenko (1996): Recognizing the vari- results were within 100 years of the corresponding ability inherent in the WFZ numerical-age dataset, McCalpin and Nishenko (1996) weighted means. To bet- McCalpin and Nishenko (1996) re-evaluated the 276 14C ter accommodate geologic uncertainty associated with and TL ages then available for the five central segments earthquake timing, the Working Group revised the ± of the WFZ having evidence for multiple Holocene sur- confidence limits assigned to each earthquake. The face-faulting earthquakes (BCS, WS, SLCS, PS, NS). Working Group determined revised confidence limits by Based on stratigraphic criteria, they identified 89 limit- dividing the range between the youngest and oldest ing ages (76 maximum and 13 minimum) as closely con- bounding age limits resulting from calibration of the straining the timing of surface faulting on those seg- closely limiting ages for each earthquake by 2, and ments (see McCalpin and Nishenko [1996] table 1). rounding the result to the nearest half-century (table 5). They recalibrated the 14C ages, using a single calibration The Working Group confidence limits are significantly

Table 5. Example of determining earthquake timing and approximate 2-sigma confidence limits using earthquakes Y and Z, Brigham City segment, Wasatch fault zone.

McCalpin and McCalpin and Working Group Limiting Earthquake Nishenko (1996) Nishenko (1996) Mean 14C or TL age1 Calibrated Weighted-mean Earthquake Timing Ages1 Earthquake Timing1 1720±90 Z 1691(1412)1142 1.7±0.2, 2.1±0.3 Z 1900±300 (TL) 2320±70 Z 2251(2020)1801 2125±104 cal yr B.P. 21002±8003 cal yr B.P. 2580±60 Z 2680(2513)2200 2630±90 Z 2767(2571)2187 3320±80 Y 3615(3344)3085 3430±70 Y 3687(3462)3166 3434±142 cal yr B.P. 3450±300 cal yr B.P. 3430±60 Y 3700(3476)3261

1McCalpin and Nishenko (1996) table 1; 2(1412+1900+2020+2513+2571)/5 = 2083, rounded = 2100; 3(2767-1142/2) = 813, rounded = 800; Approximates 2-sigma variability and includes analytical and sample context uncertainties. Consensus preferred recurrence-interval and vertical slip-rate estimates 13 broader than those of McCalpin and Nishenko (1996), Nephi segment: The NS exhibits evidence of multiple and are thought to better incorporate both the aleatory Holocene surface-faulting earthquakes, but earthquake and epistemic uncertainty associated with earthquake timing on the NS is the least well understood of any of timing. the central WFZ segments. Two paleoseismic-trenching New paleoseismic trenching information: Trenching investigations (Hanson and others, 1981; Jackson, 1991) information on the timing of surface-faulting earth- produced conflicting sets of numerical ages for horizons quakes obtained subsequent to McCalpin and Nishenko critical to determining the surface-faulting history of the (1996) is available for the BCS and SLCS. McCalpin NS. McCalpin and Nishenko (1996) re-evaluated the and Forman (2002) presented an updated interpretation ages used by the original investigators to define their of their trenching investigation on the BCS originally surface-faulting chronologies, but did not consider the performed in 1992-93, and first reported in McCalpin alternative ages, or comment regarding the suitability of and Forman (1993). Table 4 in McCalpin and Forman the alternate ages to constrain surface faulting. Addi- (2002) revises the 14C and TL ages both as reported in tionally, McCalpin and Nishenko (1996) used five previ- the original investigation and in McCalpin and Nishenko ously unpublished 14C ages from the southern part of the (1996). Differences in ages between McCalpin and PS to help constrain the timing of the MRE and second Nishenko (1996) and McCalpin and Forman (2002) are oldest (penultimate) event (PE) on the NS. The Working related chiefly to older earthquakes (T, U, V). The tim- Group believes that in the absence of supporting paleo- ing of earthquakes U and V remains the same, but the ± seismic information from the northernmost trace of the confidence limits are broader in McCalpin and Forman NS, it is premature to use 14C ages from the PS to deter- (2002). Event T is constrained by a single 14C age, mine the timing of surface faulting on the NS. Lacking which McCalpin and Nishenko (1996) reported in radio- new paleoseismic-trenching information to better define carbon years, but which McCalpin and Forman (2002) earthquake timing, the Working Group used the pre- calendar calibrated and then reported as a range ferred surface-faulting chronologies of the original in- (>14,800±1200 cal yr B.P., <17,100 [16.8 ka; see table vestigators to establish a composite chronology for the 4]) using the time of the Bonneville flood as the upper NS, but acknowledges a high level of uncertainty regard- bound for the timing of event T. The Working Group ing earthquake timing. broadened the ± confidence limits for event U by using the new limiting ages reported in McCalpin and Forman Other Quaternary Faults (2002) and employing the same methodology described above (table 5) for the McCalpin and Nishenko (1996) The timing of surface faulting generally is not as ages. well constrained for Utah’s other Quaternary faults. Trenching by Black and others (1996) constrained Reasons include: (1) fewer earthquake-limiting absolute the timing of the four youngest earthquakes (W, X, Y, Z) ages are available, (2) many investigations were recon- on the SLCS, and McCalpin (2002) identified three older naissance in nature and either lack numerical ages entire- earthquakes (T, U, V) on the basis of a retrodeformation ly, or the available ages only confine surface faulting to analysis of his “megatrench” exposure at Little Cotton- broad time intervals, and (3) the primary purpose of the wood Canyon. The Working Group judged the results of study was not to determine earthquake timing. these two new investigations credible, and combined the A comprehensive reinterpretation and recalibration results of the two studies to create a composite surface- of numerical ages similar to that performed by McCalpin faulting chronology for the SLCS. The Working Group and Nishenko (1996) for the central WFZ segments has re-evaluated the Black and others (1996) earthquake ± not been made for Utah’s other Quaternary faults. The confidence limits as described above. The Working principal reasons for not doing so are that: (1) many Group believes that the revised limits account for both studies lack information about the geologic context of the laboratory and geologic uncertainty associated with material dated or the manner in which samples were col- younger surface faulting on the SLCS, but timing of the lected, processed, and analyzed, and (2) where available three older earthquakes can be constrained only to broad ages are only sufficient to constrain earthquake timing to time intervals. broad time intervals, variations of a few tens to hundreds Original data: In two instances, the Working Group of years resulting from recalibration are inconsequential. chose to adopt earthquake timing on the WS and PS as Those studies that contain sufficient information to per- reported by the original investigators prior to the mit a re-evaluation of their absolute ages were carefully McCalpin and Nishenko (1996) re-evaluation. They scrutinized during the Working Group review process. include (1) the third-oldest (antepenultimate) earthquake on the PS as originally reported by Machette and others (1992), and (2) the MRE on the WS as reported by Swan Recurrence Intervals and others (1981b) and Machette and others (1992); McCalpin and Nishenko (1996) discounted a late Holo- Active faults generate repeated surface-faulting cene surface-faulting earthquake at about 0.5 ka on the earthquakes through time, and the time span between WS. Additionally, the Working Group chose to include those earthquakes is called the recurrence interval (RI). the LS in their deliberations and accepts the timing of the Recurrence interval is a fundamental descriptor of fault MRE as reported by Jackson (1991) and later confirmed activity (McCalpin, 1996), and defining earthquake by the UGS (Hylland and Machette, 2004; table 1, ap- recurrence is a major goal of most paleoseismic-trench- pendices A and B). ing investigations. A RI is typically reported in one of 14 Utah Geological Survey two ways: (1) as the interval between two individual dence limits for the interevent interval distribution. This paleoearthquakes or (2) as an average RI encompassing method was not applicable to the LS, where scarp-pro- several paleoearthquakes. Considerable variation is pos- file evidence (Hylland and Machette, 2004) indicates the sible between individual interevent intervals on some possibility of two surface-faulting earthquakes on the faults. An average RI smoothes out individual interevent southern part of the LS in latest Pleistocene/Holocene variations resulting in a mean value that is useful for time, although only one earthquake has been positively earthquake-hazard analysis. However, average recur- identified and its timing constrained on that segment. rence, especially determined over a long time period, can After a careful review of the available information mask large variations in individual recurrence, some of regarding earthquake timing, interevent interval lengths, which may represent fundamental changes or large irreg- and data variability for each segment, the Working ularity in fault behavior. For example, the average RI for Group assigned preferred Holocene RI estimates for the SOMFZ determined for five to seven earthquakes each segment along with “approximate” 2-sigma (5th over a nearly 100-kyr period is 12 to 25 kyr (Olig and and 95th percentile) confidence limits (table 1, appen- others, 2001). However, information on earthquake tim- dices A and B). However, limited data restricted the ing for the SOMFZ indicates individual interevent inter- Working Group’s preferred RI estimate for the LS to a vals may be as long as 46 kyr or as short as a few kyr. broadly defined range. Similarly large variations in interevent intervals over long time periods are seen on some other Utah Quater- Other Quaternary Faults nary faults, and are of particular concern on the WFZ, where evidence suggests that post-Bonneville (late Few of the other Quaternary faults/fault sections Pleistocene/Holocene) and particularly mid- to late- considered by the Working Group have sufficient infor- Holocene RIs are significantly shorter and more regular mation on earthquake timing to permit calculation of than recurrence prior to or during Lake Bonneville time even a single, well-constrained interevent interval. Typ- (Machette and others, 1992; McCalpin, 2002; McCalpin ically, the timing of bracketing earthquakes is poorly and Forman, 2002). constrained, and resulting interevent intervals are broad. The Working Group evaluated the information on Wasatch Fault Zone earthquake timing available for each fault/fault section, and again employing a consensus process, assigned a Surface-faulting chronologies for the five central preferred RI with “approximate” 2-sigma confidence segments of the WFZ that have multiple Holocene sur- limits to each fault/fault section where the data permitted face-faulting earthquakes are relatively well constrained (table 1, appendices A and B). However, because the through the middle Holocene (appendices A and B), and data are limited, most RI confidence limits are broad to permit calculation of interevent intervals between paleo- reflect high uncertainty. Additionally, the Working earthquake pairs (table 6, appendix B). Additionally, Group review showed that existing paleoseismic infor- longer surface-faulting chronologies on the BCS and mation for several faults/fault sections is insufficient to SLCS define less well-constrained interevent intervals to make even a broadly constrained RI estimate (table 1, the latest Pleistocene (Lake Bonneville and immediate appendices A and B). post-Bonneville time). The Working Group determined mean RI for the five Vertical Slip Rates central WFZ segments by calculating the weighted mean of the individual interevent intervals (rounded to the Vertical slip (displacement) represents the vertical nearest 100 years) and then calculating 2-sigma confi- component of total dip slip on a fault. Vertical slip is

Table 6. Example of determining mean recurrence intervals and 2-sigma confidence limits for the Brigham City segment of the Wasatch fault zone.

Earthquake Timing Interevent Recurrence Mean Recurrence Interval Interval

Z 2100±800 Y-Z = 1350±9001 Y 3450±300 X-Y = 1200±600 W-Z = 13002±2003 X 4650±500 W-X = 1300±600 W 5950±250 V-W = 1500±1000 V 7500±1000 U-V = 1000±1800 U-Z = 13002±4003 U 8500±1500

1±confidence limits equal the square root of the sum of the squares of the individual ± confidence limits for each bracketing earthquake. 2Weighted mean rounded to the nearest 100 years. 32-sigma standard deviation rounded to the nearest 100 years. Consensus preferred recurrence-interval and vertical slip-rate estimates 15 always smaller than dip slip unless the fault is vertical, fault/fault section where the data permitted (table 1, in which case vertical slip and dip slip are the same. appendices A and B). However, because the data are Accurately calculating dip slip requires knowing the limited, many of the Working Group’s confidence limits fault dip, which is generally poorly constrained for most are broad to reflect high uncertainty. Additionally, the Utah faults. Vertical slip rate (VSR) is calculated by nor- Working Group review showed that existing paleoseis- malizing net vertical displacement at a point on a fault mic information for several faults/fault sections is insuf- over time (net vertical displacement/time), and is a sec- ficient to make even broadly constrained VSR estimates. ond fundamental descriptor of fault activity (McCalpin, Special cases in that regard are the Joes Valley and 1996). In a manner similar to RIs, VSRs typically are Towanta Flat grabens, which have no measurable net reported in one of two ways: (1) as the slip rate between vertical displacement across them and therefore may not two individual paleoearthquakes, or (2) as the average be seismogenic structures. slip rate over a longer time period that encompasses slip from several to possibly hundreds of paleoearthquakes. In the first instance, the net vertical displacement from PALEOSEISMIC DATA GAPS the more recent of the two earthquakes is divided by the time interval between the earthquakes. In the second, Recommended Paleoseismic Investigations cumulative net vertical displacement and time are required parameters, but knowing the number of earth- Effective earthquake-hazard characterization relies quakes that produced the displacement is not necessary. on information regarding the paleoseismic parameters of For a VSR to be well constrained, both the net ver- seismic sources. To date, only 16 percent of Utah’s Qua- tical displacement and the time interval must be bracket- ternary faults/fault sections have paleoseismic-trenching ed (closed) by surface-faulting earthquakes (Wong and data available for them. A small number of other Qua- Olig, 1998). A common source of uncertainty in paleo- ternary faults/fault sections (less than an additional 5%) seismic-source documents reviewed by the Working have well-constrained VSRs obtained from geomorphic Group was the use of open time intervals when calculat- studies. The Working Group was charged with identifying slip rates. Intervals open to the present include time ing critical gaps in Utah’s paleoseismic database and rec- that is not represented by corresponding displacement, ommending future paleoseismic investigations to fill and thus produce slip rates that are too small (too much those gaps. time and not enough displacement). Intervals open to Table 7 lists the faults/fault sections by region with- the past typically include displacement that is not fully in Utah, which the Working Group considers of highest represented by time, and thus result in slip rates that are priority for additional paleoseismic investigation to too large (too much displacement and not enough time). ensure that Utah’s earthquake hazard is adequately char- Intervals open at both ends can produce slip rates that are acterized to a minimally acceptable level. These either too small or too large depending on the ratio of faults/fault sections are shown on figure 2. The Working time not accounted for in the past compared to extra time Group considered NEHRP criteria for the National/Inter- included since the most recent surface faulting. Howev- mountain West region regarding minimum slip rates (0.1 er, the greater the interval length and the more earth- mm/yr near urban areas and 0.2 mm/yr in other areas) quakes it represents, generally the smaller is the effect of and the specific fault priorities for urban areas in Utah open-ended intervals. listed on the USGS External Research Web site Because net vertical displacement is an essential (http://erp-web.er.usgs.gov/) when compiling table 7. component of slip-rate calculations, and because net ver- The faults/fault sections recommended for additional tical displacement produced by a surface-faulting earth- study in each region of the state are listed in order of de- quake varies along strike of a fault, so does the VSR. creasing priority. Like the net vertical-displacement measurements from Table 7 represents the faults/fault sections that the which they are derived, VSRs are point values that Working Group believes require additional study consid- reflect the rate of vertical displacement at a particular ering the current status of Utah development and a rea- location on a fault. Whether a slip rate is a maximum or sonable projection of future growth. It is important to some lesser amount depends on the nature of the corre- note, however, that we continually learn more about sponding net vertical-displacement measurement. what we do not know, and as our knowledge base regard- Well-constrained net vertical-displacement meas- ing Quaternary faults in Utah increases, other important urements are limited on the faults/fault sections consid- issues and data gaps may present themselves that either ered by the Working Group; therefore, well-constrained are not yet identified or currently not recognized as VSRs are similarly limited. This is particularly true for important. More than 150 Quaternary faults remain for faults/fault sections off the Wasatch Front where net ver- which the Working Group makes no recommendation at tical-displacement and slip-rate data may come from as this time. Paleoseismic information regarding those few as one or two locations on a fault/fault section that faults may become critical in the future as Utah’s popu- is tens of kilometers long. lation continues to grow and development encroaches on The Working Group evaluated available information ever more remote areas of the state. Likewise, a major on earthquake timing and net vertical displacement for project for which paleoseismic information is a neces- each fault/fault section under their review, and employed sary siting criterion (for example, waste-disposal or a consensus process to assign a preferred VSR with power-generating facilities) may require future study of “approximate” 2-sigma confidence limits to each specific faults or fault sections not listed here. 16 Utah Geological Survey

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Table 7. Faults/fault sections by region that require additional study to adequately characterize Utah’s earthquake hazard.

Wasatch fault zone (1) Nephi segment - trench north and south strands to confirm faulting history and segment boundary relations. (2) Weber segment – trench multiple locations to confirm MRE timing. (3) Weber segment - megatrench to determine a long-term surface-faulting history. (4) Collinston and Clarkston Mountain segments – detailed field reconnaissance and geologic mapping, to determine the timing of the MRE. (5) Levan segment - trench to extend faulting history.

Wasatch Front/Northern Utah (exclusive of WFZ) (1) West Valley fault zone – trench to better constrain faulting history and compare with WFZ earthquake history. (2) Utah Lake faults – geophysical surveys and possibly drilling to determine relation to WFZ; another WVFZ? (3) Great Salt Lake fault zone – drilling to obtain samples to determine earthquake timing on the Promontory segment. (4) East Cache fault zone – trench to determine faulting history on northern and southern sections. (5) Clarkston fault – trench to determine multiple earthquake record (megatrench?). (6) Wasatch Range back-valley fault – trench one back-valley fault to constrain earthquake timing and use as a model for similar back-valley faults. (7) Faults beneath Bear Lake – geophysical surveys and possible drilling to determine relation to Eastern Bear Lake fault. (8) Eastern Bear Lake fault – trench to determine faulting history on northern fault sections.

Central/Southern Utah (1) Sevier/Toroweap fault - determine number and timing of surface-faulting earthquakes; segmented? (2) Washington fault zone - trench to determine faulting history in a rapidly urbanizing area. (3) Cedar City-Parowan monocline/Paragonah fault – geomorphic studies to investigate possible Holocene deformation. (4) Enoch graben– geomorphic studies and possibly trenching to determine faulting history in this rapidly urbanizing area; determine relation to nearby HFZ. (5) Hurricane fault zone - trench to confirm faulting history on Cedar City section. (6) Gunnison fault - investigate possibly very young activity and large displacements in Sevier Valley. (7) Scipio Valley faults – geomorphic studies and possibly trenching to investigate possible very young activity and determine seismic hazard to nearby major transportation corridors.

Table 8 ranks the recommended paleoseismic inves- Table 8. Paleoseismic investigations required to adequately characterize Utah’s earthquake hazard in order of decreasing state- tigations in table 7 on a statewide-priority basis. The wide priority. Working Group considers these investigations to be of highest priority and recommends them for future (1) Nephi segment WFZ NEHRP funding. (2) West Valley fault zone (3) Weber segment WFZ – MRE Investigation Summaries (4) Weber segment WFZ - megatrench (5) Faults beneath Utah Lake The faults/fault sections listed in table 7 are those that the Working Group believes require additional (6) Great Salt Lake fault zone (Promontory section) investigation to ensure that Utah’s earthquake hazard is (7) Collinston and Clarkston Mountain segments WFZ characterized to a minimally acceptable level. However, (8) Sevier/Toroweap fault details of the recommended investigations such as site (9) Washington fault zone selection, land ownership, and investigation methods (10) Cedar City-Parowan monocline/Paragonah fault remain to be determined. Therefore, the following dis- (11) Enoch graben cussion summarizes only the need for the investigations, (12) East Cache fault zone (northern and southern sections) and not details of how or where the investigations should be performed. (13) Clarkston fault (14) Wasatch Range back-valley fault Wasatch Fault Zone (15) Hurricane fault zone (Cedar City section) (16) Levan segment WFZ Nephi segment: The NS is the southernmost of the five (17) Gunnison fault central segments of the WFZ having documented evi- (18) Scipio Valley faults dence of recurrent Holocene surface faulting. The NS consists of two distinct fault strands: the northern San- (19) Faults beneath Bear Lake taquin strand and the southern Nephi strand, separated (20) Eastern Bear Lake fault 18 Utah Geological Survey by a 4- to 5-kilometer right step in the segment trace second earlier earthquake near the southern end of the (DuRoss and Bruhn, 2003). Existing paleoseismic data segment in early Holocene or latest Pleistocene time. from two previous trench studies on the Nephi strand are The Working Group recommends trenching near the not closely constrained; both investigations produced southern end of the segment to determine a long-term conflicting sets of numerical ages on samples from the surface-faulting chronology for the LS. same geologic units resulting in significant uncertainty regarding paleoearthquake timing. No paleoseismic- trench data are available for the rapidly urbanizing San- Wasatch Front/Northern Utah taquin strand. New scarp-modeling information indi- Exclusive of the WFZ cates that the paleoearthquake histories on the Nephi and Santaquin strands may be different (DuRoss and Bruhn, West Valley fault zone: The WVFZ lies entirely with- 2003), and yet present seismic-hazard analyses assume in the densely urbanized Salt Lake Valley. As many as that the two strands form a single segment, and therefore six or seven surface-faulting/flexuring earthquakes are rely entirely on the poorly constrained record of Nephi- proposed for the WVFZ (appendices A and B). Earth- strand earthquakes. Understanding the individual rup- quake timing is poorly constrained, and the WVFZ may ture histories of the two strands is critical, as asynchro- or may not have ruptured coseismically with some or all nous ruptures would result in more frequent moderate- to of the surface-faulting earthquakes on the nearby SLCS large-magnitude earthquakes, whereas synchronous rup- of the WFZ. The Working Group recommends trenching tures would generate less frequent but larger earth- the various strands of the WVFZ to better constrain sur- quakes. A minimum of two trench sites on the NS are face-faulting timing and to determine the relation be- required to (1) clarify the history of paleoearthquakes on tween surface faulting on the WVFZ and the SLCS. the Nephi strand, (2) determine the timing of paleo- Utah Lake faults: Several poorly understood, ques- earthquakes on the Santaquin strand, (3) accurately char- tionable Holocene faults and folds beneath Utah Lake in acterize the seismic-source potential of the NS, and (4) the densely urbanized Utah Valley have been identified compare the NS paleoearthquake parameters with the from widely spaced seismic-reflection data (Brimhall adjacent PS and LS to evaluate the possibility of rupture and Merritt, 1981). Displacements of <2 to 5 m in the overlap between the segments. past 16.8-18 kyr (period of the Bonneville highstand) Weber segment MRE: Different paleoseismic-trench- indicate slip rates from <0.1 to about 0.4 mm/yr. Little ing investigations have identified different MREs on the is known regarding these sub-lacustrine structures; they WS (table 1, appendices A and B), raising the possibili- may represent east-dipping faults antithetic to the PS of ty of (1) partial-segment rupture during some earth- the WFZ, and therefore be analogous to the WVFZ in quakes, or (2) rupture overlap from adjoining segments, Salt Lake Valley. The Working Group recommends ad- on one of Utah’s most densely urbanized faults/fault sec- ditional geophysical investigations and, if warranted, tions. Trenching at multiple locations along the WS is drilling to assess the seismic potential of these faults. recommended to document MRE timing and resolve Great Salt Lake fault zone: A NEHRP-funded seis- these possible rupture scenarios. mic-reflection study of the Promontory section of the Weber segment megatrench: The current surface- Great Salt Lake fault zone (GSLFZ) is scheduled for faulting chronology for the WS extends to the middle summer 2004 (Pechmann and Dinter, University of Holocene. Long-term earthquake histories are important Utah, verbal communication to Working Group, 2004). when performing Probabilistic Seismic Hazard Analyses If the survey identifies evidence of paleoearthquakes in (PSHAs), especially along the heavily urbanized the sediments beneath Great Salt Lake, the Working Wasatch Font. A megatrench similar to those excavated Group recommends drilling and sampling lake-bottom on the SLCS and PS, or a series of smaller trenches sim- sediments at critical locations to provide carbon samples ilar to those excavated on the BCS, is required to deter- necessary to determine earthquake timing. mine the long-term (since the latest Pleistocene) earth- East Cache fault zone: The East Cache fault zone quake history of the densely urbanized WS. (ECFZ) consists of three sections (northern, central, and Collinston and Clarkston Mountain segments: The southern) based on fault zone complexity, tectonic geo- Collinston and Clarkston Mountain segments are the two morphology, and expression of fault scarps (appendix northernmost WFZ segments in Utah. They are the only B). Paleoseismic-trenching data are available only for WFZ segments in Utah that lack geologic strip maps. the central section, which shows evidence of Holocene Relatively modern 1:24,000-scale geologic maps are surface faulting. Geomorphic evidence for Holocene available along the segment, but detailed paleoseismic faulting is not evident on the northern and southern sec- studies of scarps and geologic relations along the seg- tions, but the east-dipping West Cache fault zone ment have not been performed. The Working Group rec- (WCFZ) on the opposite (west) side of Cache Valley is ommends that such studies be performed, and if evi- segmented, and all three segments have experienced dence of Holocene or latest Pleistocene surface faulting Holocene surface faulting (table 1, appendices A and B). is discovered, the segments be trenched to determine The ECFZ trends along the base of the precipitous Bear earthquake timing, RIs, and VSRs. River Range, where numerous well-developed faceted Levan segment: Previously only one Holocene surface- spurs indicate a high rate of tectonic activity. Cache Val- faulting earthquake was recognized on the LS (appendix ley is rapidly urbanizing, and the Working Group rec- B). Additional scarp-profile analysis (Hylland and ommends that both the northern and southern ECFZ sec- Machette, 2004) indicates that there may have been a tions receive additional paleoseismic evaluation to Consensus preferred recurrence-interval and vertical slip-rate estimates 19 assess their seismic potential. Central/Southern Utah Clarkston fault: The Clarkston fault (CF) is the northernmost and longest of the three segments comprising Sevier/Toroweap fault: The Sevier/Toroweap fault the WCFZ (appendices A and B). Black and others trends in a north-south direction for more than 167 kilo- (2000) trenched the CF and identified Holocene surface meters in northern Arizona and southern Utah. The fault faulting. The trench was not sufficiently deep to expose is designated the Sevier fault (SF) in Utah and the evidence for older earthquakes, so little is known about Toroweap fault (TF) in Arizona (Black and others, the long-term behavior of the CF; however, geomorphic 2003). Pearthree (1998) subdivides the TF into three relations indicate a minimum of two surface-faulting subsections; the SF has not received similar paleoseis- earthquakes since the Bonneville highstand (past 18 kyr). mic study and therefore is not subdivided. However, the Prominent scarps along the CF, particularly where the fault’s long length in Utah (88 km end to end) indicates fault trends northward into southern Idaho, indicate the likelihood of more than one seismogenic section. A recurrent late Quaternary surface faulting. The Working displaced basalt flow (200 m net vertical displacement) Group recommends that the long-term history of surface at Red Canyon provides a late Quaternary VSR estimate for the SF of 0.36 mm/yr (Black and others, 2003). The faulting on the CF be investigated by excavating addi- UGS began a reconnaissance investigation of the SF in tional, deeper trenches across multiple-earthquake the second half of 2004 to look for evidence of fault seg- scarps, possibly in Idaho if a suitable site is not available mentation, and sites where additional RI and VSR infor- in Utah. mation can be obtained. If potential sites are found, Wasatch Range back-valley fault: The USBR investi- detailed mapping and geomorphic studies should be per- gated several Wasatch Range back-valley faults as part formed to assess their potential for paleoseismic trench- of seismic-hazard evaluations for USBR dams and ing, and if found suitable, trenching should be per- water-conveyance structures (appendices A and B). formed. These studies were chiefly reconnaissance in nature and Washington fault zone: The Washington fault zone lack sufficient numerical ages to narrowly constrain the (WaFZ) trends in a north-south direction for more than time of surface faulting. Consequently, present RI and 40 kilometers (end to end) from northern Arizona into VSR estimates for back-valley faults have high uncer- the St. George basin of southern Utah (appendix B). tainties (appendices A and B). Considering their loca- Earth Sciences Associates (1982) trenched the WaFZ tion in or close to rapidly urbanizing areas of the near four flood-control dikes in Utah. The trenching Wasatch Front, and their potential inclusion in future produced net vertical-displacement estimates, but no PSHAs, the Working Group recommends that one back- information on the number or timing of paleoearth- valley fault be selected for detailed paleoseismic study to quakes. The Working Group recommends that addition- serve as a model for all such faults until detailed study of al paleoseismic investigations be performed on the other structures becomes warranted. WaFZ to assess its seismic potential and the earthquake Faults beneath Bear Lake: Several normal faults ap- hazard it presents to the rapidly urbanizing St. George pear on seismic-reflection profiles across Bear Lake basin. (Skeen, 1976; Coleman, 2001). These sub-lacustrine Cedar City-Parowan monocline/Paragonah fault: faults are downthrown both to the east and west, and The Cedar City-Parowan monocline (CC-PM) is a com- some on the eastern side of the lake displace the lake plex zone of deformation that may form a structural bottom, indicating possible Holocene movement bridge between the Paragonah fault (PF) to the north and (McCalpin, 2003). The relation of these faults to the the Hurricane fault zone (HFZ) to the south (Threet, Eastern Bear Lake fault (EBLF) and to faults on the west 1963). Hecker (1993) suggested the possibility that a side of Bear Lake is unknown. The Working Group rec- blind, plateau-bounding normal fault with significant ommends additional geophysical investigation and, if seismic potential underlies the main mountain-front warranted, drilling to assess the seismic potential and monocline. Both normal and strike-slip faults deform the hazard these faults present to nearby communities on monocline and form numerous closed range-front basins both sides of the Utah/Idaho border. only partially filled with sediment. Stream downcutting Eastern Bear Lake fault: The EBLF can be subdivid- exposes faults in late Holocene deposits, and a geodetic ed into northern, central, and southern sections on the network (Anderson and Bucknam, 1979) indicates sig- basis of fault-rupture patterns, youthfulness of fault nificant horizontal and vertical movement in directions scarps, and subsurface geophysical data (appendix B). opposite to the topographic gradient, suggesting tectonic Only a portion of the southern section is in Utah, and deformation consistent with right-lateral fault slip. Mod- only that part of the fault has been trenched. Multiple- ern deformation has not been accompanied by seismici- earthquake scarps in geologically recent unconsolidated ty above a threshold of about ML 3.0, suggesting ongo- deposits along the central section of the EBLF in Idaho ing, aseismic deformation. The Working Group recom- indicate recurrent late Quaternary movement. Ground mends additional geodetic monitoring and detailed geo- shaking related to a large earthquake on the central sec- logic mapping of the CC-PM and PF to determine if tion of the EBLF would affect nearby communities in rapid tectonic deformation is continuing and, if so, what Utah. The Working Group recommends that the central hazard the CC-PM and PF represent to nearby commu- section of the EBLF be trenched to assess the fault’s nities in the Cedar City area. seismic potential and the hazard it presents to nearby Enoch graben: The Enoch graben (EG) is a poorly communities on both sides of the Utah/Idaho border. understood late Pleistocene to Holocene structure that 20 Utah Geological Survey bounds the southern end of the Red Hills and extends cation to Suzanne Hecker, 1989). The sequence of into the valley north of Cedar City. Scarps in the town deposits appears to be monoclinally folded, with an of Enoch on unconsolidated alluvium are 5- to 7-meters apparent dip that is parallel to the face of the scarp and high and have been trenched in numerous places by local fairly uniform throughout the section. These relations residents to stimulate spring flow. Anderson (1980) suggest locally intense, recent deformation along this reports a soil layer (paleosol) in one such exposure that part of the range front (Hecker, 1987). The Working separates faulted coarse alluvium below from well-bed- Group recommends that geologic mapping and a geo- ded sandy clay above. The relation of faulting to the soil, morphic investigation be conducted of the GF to better which yielded an age of 9,500 14C yr B.P., is uncertain, constrain its VSR and to investigate the unusual geolog- although subjacent strata are faulted (Hecker, 1993). ic relations reported at Birch Creek. Five kilometers north of Enoch, faults with up to 50 Scipio Valley faults: The Scipio Valley faults (SVF) are meters or more of throw displace Quaternary basalt northeast-trending normal faults along the west side of flows. Some of these bedrock faults likely have moved northern Scipio Valley. The faults show a total displace- recently as indicated by the Enoch alluvial scarp (Heck- ment greater than 11.1 meters in alluvium, and evidence er, 1993). The Working Group recommends detailed for two periods of fault movement (pre-Holocene and geologic mapping and geomorphic investigations be per- Holocene). Alluvium is displaced 2.7 meters by the formed on the EG to better constrain the rate of slip and younger earthquake (Hecker, 1993). The morphology time of most recent surface faulting on this structure. If and degree of dissection of the young scarps are similar it is warranted by the reconnaissance investigations, the to the Fish Springs fault scarps, which formed 2-3 ka Working Group recommends trenching one or more (Bucknam and others, 1989); scarp-profile data are bounding faults to obtain detailed paleoseismic informa- insufficient to constrain the age of the pre-Holocene tion for earthquake-hazard analysis. scarps. The SVF are adjacent to major transportation, Hurricane fault zone: The HFZ is the longest and like- utility, and pipeline corridors. The Working Group rec- ly the most active fault in southwestern Utah and north- ommends that geologic mapping and a geomorphic western Arizona (appendix B). Previous attempts to investigation be conducted of the SVF to better constrain trench the HFZ in Utah have proven unsuccessful due to their slip rates and the timing of the MRE. a limited number of suitable trench sites, landowner constraints, and scarps formed on deposits containing boulders too large to excavate with locally available track- SUMMARY hoes (Lund and others, 2001). Single- and multiple- earthquake scarps identified at Coyote Gulch on the Ash The Utah Geological Survey convened the Utah Creek section of the HFZ (Lund and others, 2001) Quaternary Fault Parameters Working Group, a panel of remain the best potential site for determining the surface experts in paleoseismology and seismology, to critically faulting history of the northern HFZ. That site is cur- review Utah’s Quaternary fault paleoseismic-trenching rently unavailable due to landowner restrictions; howev- data, and to establish consensus preferred RI and VSR er, should that situation change in the future, the Work- estimates and confidence limits for those faults/fault sec- ing Group recommends that the Coyote Gulch scarps be tions where the data permit. The Quaternary Fault and trenched. Fold Database and Map of Utah (Black and others, 2003) A large scarp (≥10 m net slip) on the Cedar City sec- indicates that 33 of Utah’s 212 Quaternary faults or tion of the HFZ at Shurtz Creek is formed on very coarse fault-related structures have paleoseismic-trenching data bouldery alluvium. An attempt to trench the Shurtz available for them. The six active, central segments of Creek scarp was unsuccessful due to boulders in the the WFZ, collocated with the most populous part of trench too large to excavate with available trackhoes Utah’s Wasatch Front, account for the greatest number of (Lund and others, 2001). The displaced alluvial surface investigations and best quality paleoseismic data. How- at Shurtz Creek is covered with numerous large basalt ever, even for those segments, well-constrained informa- boulders. Preliminary cosmogenic isotope dating (36Cl, tion on surface faulting generally extends only to the 3He) of the boulders resulted in an age for the Shurtz middle Holocene, with less reliable information to the Creek surface of between 30 and 60 kyr. The Working latest Pleistocene for two segments, and new long-term Group recommends additional efforts to determine the information pending for a third segment. Paleoseismic- age of the Shurtz Creek surface to better constrain the trenching data for Utah’s other Quaternary faults are late Quaternary VSR for the northern part of the HFZ. generally less abundant and not as well constrained. Gunnison fault: The Gunnison fault (GF) is marked by Those data are typically limited to a single location northwest- to northeast-trending scarps in alluvium in along a fault/fault section, including many suspected western Sanpete Valley along the east side of the Gunni- segmented faults or faults/fault sections exhibiting other son Plateau. The MRE may be as young as late tectonic complexities. Numerical ages available to con- Holocene and likely produced less than a meter of dis- strain the timing of paleoearthquakes on faults/fault sec- placement (Black and others, 2003). Progressively older tions off the Wasatch Front are commonly much less alluvial surfaces have greater displacements, and old abundant, and several trenching investigations resulted Quaternary (Tertiary?) surfaces have tens of meters of in no numerical ages at all. Consequently, significant displacement across steep, high scarps. At Birch questions remain to be answered, including questions Canyon, 2-4 ka fluvial and debris-flow deposits underlie pertaining to some comparatively well-studied WFZ a 10- to 15-m-high scarp (Elliott Lips, verbal communi- segments, to ensure that Utah’s earthquake hazard is Consensus preferred recurrence-interval and vertical slip-rate estimates 21 characterized to the minimum level necessary for accu- and pipeline corridors critical to Utah’s economic well rate hazard evaluation. being. The faults/fault sections recommended for study Issues related to data uncertainty and adequacy reflect the current status of Utah development and a rea- weighed heavily upon the Working Group’s delibera- sonable projection of future growth. However, with tions. The combined result of limited data and data future population increases and expansion into previous- uncertainties for many faults prevented rigorous statisti- ly undeveloped areas, additional faults/fault sections will cal analysis of most paleoseismic-trenching data or con- undoubtedly become critical to future earthquake-hazard straint of RI and VSR estimates within rigidly quantifi- reduction and will require study and characterization at able bounds. Consequently, the Working Group relied that time. on its collective experience and best professional judgment to determine consensus preferred RI and VSR estimates and confidence limits for the faults under review. CONCLUSIONS For several faults, the data were too sparse or too uncertain to make meaningful estimates. The Utah Quaternary Fault Parameters Working The preferred RI and VSR estimates presented in Group has completed a comprehensive evaluation of the this report are typically bracketed by upper and lower paleoseismic-trenching data available for Utah’s Quater- bounds that represent the Working Group’s best estimate nary faults, and where data permitted determined preferred RI and VSR estimates with approximate 2-sigma of 2-sigma confidence limits for the estimated values. confidence limits. Although not based on rigorous sta- The confidence limits are approximations, and were not tistical analysis, the consensus values and confidence derived in a statistically rigorous manner. Instead, they limits represent the best professional judgment of a panel again represent the Working Group’s best collective of experts thoroughly familiar with Utah’s paleoseismic judgment regarding the range over which recurrence and data. Until superseded by information from new paleo- slip is expected to vary for a particular fault. They are seismic investigations, the Working Group’s preferred intended to incorporate both epistemic and aleatory RI and VSR estimates and associated confidence limits uncertainty, and to approximate 2-sigma (5th and 95th represent the best available information regarding sur- percentile) confidence limits. In a few instances, the face-faulting activity for the faults/fault sections re- available data were not sufficient to determine individual viewed. These data can be considered as approximating preferred RI or VSR values. In those cases, the Working average RI and VSR values and 2-sigma variability Group’s consensus estimates are reported as a range of about those mean values. values rather than as a central value with associated con- With paleoseismic-trenching performed on only 16 fidence limits. In other instances, the trenching data percent of Utah’s Quaternary faults, clearly much were insufficient to allow the Working Group to make remains to be done to characterize Utah’s earthquake fault parameter estimates. hazard. Future paleoseismic investigations will un- The Working Group recommends additional paleo- doubtedly result in new data that will refine some Work- seismic study of 20 faults/fault sections to characterize ing Group estimates, answer outstanding questions, and Utah’s earthquake hazard to a minimally acceptable fill data gaps. The Working Group looks forward to the level. The faults/fault sections include segments of the completion of those studies and the clarity they will WFZ that have already received considerable study, but bring to earthquake-hazard evaluation in Utah. for which significant questions remain regarding earthquake timing and/or fault segmentation; other previously investigated faults for which questions remain regard- ACKNOWLEDGMENTS ing their seismic behavior; and faults that have not yet received detailed study. The Working Group considered The Utah Geological Survey thanks the members of NEHRP minimum slip-rate criteria and specific fault pri- the Utah Quaternary Fault Parameters Working Group. orities for urban areas in Utah when evaluating which Without their volunteer efforts, this study could not have faults to recommend for additional study. However, the been completed. The UGS also thanks Bill Black (West- Working Group selected some faults/fault sections ern GeoLogic, LLC), Christopher DuRoss (University of specifically because so little is known about their recur- Utah), and Kathryn Hanson (Geomatrix Consultants, rence or slip history, and others, while not located adja- Inc.) for providing information on fault studies that was cent to urban areas, are near major transportation, utility, not otherwise available to the Working Group. 22 Utah Geological Survey

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M1-M67. Consensus preferred recurrence-interval and vertical slip-rate estimates 27

APPENDIX A

WORKING GROUP CONSENSUS EARTHQUAKE TIMING AND PREFERRED RECURRENCE-INTERVALAND VERTICAL SLIP- RATE ESTIMATES WITH SUPPORTING INFORMATION 28 Utah Geological Survey 5 P r e f e r r e d C V o n e s r t e i c n T i n F i a F i W n i T h s n t n o e W l i t r h u t v t e o u e e r e S r e r s v r o r e e r r v k v a v l r e m m a i i k m a l a n p l i m o o l a l g n o a a s s v a R g s o v G t t v e 4 v t s 5 1 e 1 G e a r r r e r r t 1 0 3 e 5 1 e r a 1 o r e r t a r r 3 a 0 0 c 0 6 c g a 0 u e o c e g 0 e e g - 0 0 0 e 0 g p u e c 1 e 0 n n e - 0 e I p e n r 4 1 n P t t r e I n t r 0 r 3 t n P e r ( e e c e t ( ( e U 0 0 t c r W W c u r e c ( f e - 0 u v u W e r u r 2 t f - a r r v o r r e r 4 t t 2 r e r r l a r o o e r P e t 0 e 8 n e Z c r l o n c c c c n 0 d Z Z 0 e c n ) a r a a c 7 a a c e Z 0 7 7 d c a e ) 7 7 l i y R e l l n l l e : e b ) y i f r y y y R : n y y t e : e : r ) i e r r r r r n t c e 6 4 e 2 4 i r r n t 1 u c e r 0 0 + + e r 0 0 + + t 4 r + u e e v 6 6 0 0 6 6 e r 0 0 r 6 0 e r v e e d r r e 0 v e n n e C e n v t R c n n e t e c o e t n e n t c s u e r r n W g 1 S 1 i N l c Y X U W o W n n d L P n o e h o l 4 . f e e o V W X s - w - e e s a n 3 W W F - i Z Y n r B r . a p n 1 l i r - - u X l c g a o 5 l t n s - Z s W Y b a r o g m . h o c o o X i - 1 n n + 5 s o g o a r t s e n W r r e 0 o 0 . 0 m o e - d 1 s n n l k k i 0 a 1 1 r a n 1 t . n . e . l r 0 m i i i i s 6 o 6 - / n 6 . . C a n u e m t . I n n l g y 0 4 / 1 l e 6 n - t g d - a - o n s v g g r a h r 0 – m - - . 0 u r 3 a 1 y r i t 9 r d s 1 1 m m n e l . 0 i o ( . d p G G e a l n - n 9 e l - 9 . . 1 m e a y i . t 0 0 . P d 6 9 m 4 i p g c r - n 8 h r r n - 9 o s - n P . . 1 a - - o o v e . 1 e p e r t - e 6 6 A e / l 9 B n 3 4 3 d r e i . 1 r t y u u t . a r h a p b - - r 4 0 e e o 6 . . o f s s r . 1 1 s s p p r m 5 1 n a r t o l 4 . ) d r v e t n o e r o o Z m . . s e s u a ( 2 4 o P P m 3 B m m m c o n n s m u m 1 L b R e R t n t m - - - o o - f e i r r C t e 9 a / 4 4 p m m m u a s e e d a a h m f r d y v t 0 s / 8 k 9 . . h s d l s h f f a t t r r y n i 3 5 / / / S i e . e e e e o e / p 1 l u 5 a y y y e 2 i r r l ( y e p h a n l e r r f y r ) r r p 0 m m s 1 i 4 r B r r v t p o i B - L e g e e + t r 9 s P t a a u s m m s o e m o i e h d d i 9 R Z t l g r l n d i r n p u t 8 i t l o / / 3 e p m l l 1 a a e s g e a V V y y n o s e e v ) ? , . t l r r p s e r e e a c s / 3 e e d o C ) Z X W s t y a o v a r r c e 6 0 s e - 2 h V f r n t t s a s a i r l r d : i i . p . l T o X W X U c c o 1 l o n d l m 5 Y Y i n 0 g y i e t o w p a a s 3 m s e ( + 7 6 4 y r e u m s p m > - l l s . 0 a o 4 3 5 1 5 s 8 B 3 2 8 r i p 5 a S S 1 t 0 a r n / 6 9 0 0 0 m 5 t 4 4 s . 9 r y h 4 l l + P t t 5 5 0 0 0 t 0 t 5 5 i i e r o 5 / h 1 i p p e a , + 0 0 + + a y 0 . 0 0 8 0 e 2 t r + + l 7 7 7 + + + 0 + 2 0 s 1 3 5 1 5 5 0 0 0 8 0 4 6 0 5 0 e 5 0 0 0 5 5 0 0 0 0 . 5 5 + 0 , 3 g 0 0 0 0 0 0 0 0 0 c c c < m 0 0 c k a a a c c c c 1 c c c a a e a a a a c l l l c a a a 7 l S n a y y y l l a l l l l l , y 0 r r r t l y y y y l y y y L r t 0 r r r r y Y y B B B r r r r i B r 0 a r n B B B B B B B . . . . P P P B B i P e ...... g P P P P P P P . . . . 3 . . P P h ...... 0 . . t 0 0 S + T u ( h r r k t f a 2 m a c g c e ) e n e n L o i t c T i n 1 h e t e r E e r S v e a a r m t l t l a h o v s u q e t 1 u r a r 3 a e a 0 g c F k 0 e e / e n t r e l t T c ( W u u i m r r a t e i o n n F W Z c g B S W e ) a : r a i s e i n g l a t b t h e t L e c a r a r e h m k s v f e e e a C g n u C i m t t l i t y t e y z s n o s e t n e g e g m m e e n n t t 3 3 5 5 9 6 . 5 4 6 4 6 1 0 Z 2 1 0 0 + H s l a i N G R W p r - o e t a i r y l r d r a l t o d a h t C w e n n C

e 29 Consensus preferred recurrence-interval and vertical slip-rate estimates a d e C r n r s e o r C y t e e t i o m h r e k e n e k a e r 0 t s k 0 e 0 . . 8 s ( 6 . 1 0 7 : - - 1 9 . - 1 5 1 . 9 . 2 - 0 . 7 0 0 m ) . m 7 m 5 m m m m m i / d / y m y / d y r r l / r e y 0 r H . 1 o l m o c m P e / n y r e r e f e r r e d C V o N 1 n e 9 s o r W 8 M t r e i t 4 o c h i W n d ) 1 r a : i T k d C s . n o 1 W i l w l t u n r e r - e e S k o g 1 s o r H e i . v n r l m G 2 k i k a o g p 0 i r o l l m ( n o . o G S a 1 6 s R g c m u t v 2 - c 4 r ~ e 1 p o G e a h r 0 1 / n 2 e . y u r w 0 2 2 t P r e 5 a r c R p e o - 0 - a r 0 e g 2 I 3 u 1 e a 0 n r P 0 n e 5 . p t . f t - t 0 z 2 r e t e 0 2 e I r e n P 7 r ( e 0 4 r m a f r X t v - r e c - 0 e e n 1 e a 4 m u r 0 d r d t . f r l 8 o v r 3 e - / e r V a 0 y C 3 6 r c P e d Z r r 0 l 2 e a + o c n e ) 7 r R 0 r a l p c y d e 7 t i 0 n y i l e p e r c f R r t y c e : e y a e r u r e r l r 2 3 r s r e c S r 1 0 + + r m u v e e 6 6 l 0 0 i e r i n p d 0 t r n h c e C t , e R n c o e e n c s u C A o H C E S r k e e r a f P P a m p o o a : P P P W p r n , s o o a t b n e < 0 o e o o o t s r 0 t s s n o o b y e . r n r s s s . 1 u t t o o t i r n l i 8 s - t t - t s s e c k c 8 f - - - n P B s w u 0 - 2 h a B B P i e - t 1 P n s C r o l . . o h 0 n t o o r 6 o 2 F . : g r i n r o o e 1 . n I n n o 8 v - e 1 F o n n v d - 2 G g o n n v - a e 9 r 1 o e o 0 0 0 . k t o 2 e e r C g k 4 . i r e t v . . . n m o 4 k - i v v t : 4 6 7 e - C i m b a i r u l 2 i i m - - 6 l m a m l l C e v n a e o 1 0 l l p . e A e e - 7 e t y n a a f . . / l m 0 w t a 2 7 y P o y t t i n t . R m l m r i i t h - - e / 8 n o r m m e y y 1 4 3 e a e e 3 m . n o e r . . e e f t P n 4 0 : n e e m m / S l y : r S e m m c r o r m i e l o s r m m a d / a t t y + t t o i / / e r n V y y ( c 0 1 e r r a e e 0 9 n n r a 0 t 8 3 d e i s . c 9 7 5 Z S T U V S a s 1 ) Z X k l l l 8 ~ i ( i X p a Y S ~ Y d ? ~ - b 6 1 2 < 5 e 7 ) 9 l W e r 0 7 i 6 a > p 3 . ~ 1 2 f 0 5 k o t k 3 0 3 7 8 + e a r a . 0 k . – 5 e 9 9 a + ( 2 0 + 7 s + 5 6 3 0 + ( 0 0 h 5 0 3 . a 5 0 1 . . o 0 . 5 k 5 5 f 0 0 4 5 5 - r t a 0 5 . . e c t k k c c k l a . r y 1 c 9 2 a a a a 3 a l a 8 . S , a l l 6 4 0 y k l . y y < f L 8 + r t a t y r r r e 5 - i B r ) a 9 n B B r . 3 B . i 2 T a q 2 i V . i 9 P e . . n s 1 g + P P . 0 n u W w - . t P k . W ~ 5 e h 0 d i k . . o e y . o - 8 r a 2 t r s 9 v l r m ; a ) a c k k a b . S . t t 9 n e i y o h l e u n 5 T u d r n . a s e s t k g 9 ( , c r t v t a r k U 7 i T 5 f a e G n 2 e r P ) m a 2 - 3 5 e - d r c V l r U d 4 a c 0 c e o i e ) c u 0 e g 0 i e i u s m a n r 0 n e - p i t t 1 t n o t e i e I r n 3 n P g c ( a e r g X 0 t v e r n c e L e e 0 a n u s r t a e i f - l o e v u n r e s 2 r r a n l r t r e 4 Z i t e a f r e l i g a n m 0 e ) r r s c e e c t 0 d i t e h n e 1 v e e H y R t - e : ( e f a r E M o n a e r c l t t t e u c a c o h i u v l C n n c n t r e i r r e t n t a e e r o e h n e n g l u r t p q n e v m m g i c n a u h g g e , l l a y e e k s s e i o h T v p i o m e r i P N n F g a u l t / F a u l t S e c t i o n 30 Utah Geological Survey 5 9 P r e f e r r e d C V o n e s r t e i c n a s l u W W S s P o a S o o l i n c r r l l p e i k k c p e i i i u s n n R v r r t 4 g g a e r o a e t n c G G e d t e t 0 e r r i n Y . d s o o 0 5 e u u u . b - 1 ( r 1 p p H t a - i i n . 0 m s y 1 P P g R R . e l - 6 e r r l 3 a 9 d e e a a e . m n o f f t t a 0 o e e e e P d n r m n r r l m r r r y a t e e / h e t m y n h H d d e f r d e / e o V V y L M l l r r o S i e e k r c a r r e . e t t e c i i l d c c i n h h a a e C e o R l l t o o S S t o e e d r l l n , i i c p p l t a 2 s h u t 0 e a e r 0 t s r n 4 e t s ) n u c s u i r C w E M W E n e e n W a J i o t M c t J e c V h r n I u V t o e n F n i s F h r n F r r t r e t q 3 i k t e o e a b q u - i n n n i r r u a n ( ( ( o c g a v ~ ~ ~ e k A t e a l y a 1 < 1 R 2 3 3 n e l G - y d . 0 0 5 6 0 0 i t l e n r l t b - - y 0 0 t i o c 3 t m k k 2 1 i r , m e u u 5 a a o k k 0 5 t r i p r - h n a y a e v r 1 r r k k r e g e d a e e C r 0 i y y n n l e c c r r W i - o t s t e c o o o 5 c e n e s o r r 0 o c r s r d d t r v e r o i I e d k ) ) k m n a f n n y ) l i t l e n s a s r e s Y c t g . t u r < r e t v a u s 2 o l r G a h a i u n i 0 n f s P l i t r p Z k g e 0 e o r s n t h s e 4 d u u e o t f ) p r g e o . w e P ’ m r s n n l r 1 p e l e e y 5 o b i d s n 0 s u t t s o t c 0 ( i c l b H a i e 3 k 1 l l y e e n 0 J v i t t L J a r n s T t F v y S n l h h h e l l e V V e c e 0 a n o a o h r a y L t e e e m a ; t r r e 0 c d r l F l F e n B o r e t t i t e r p k g + i i J W s n a r h Z Z d a p c c . n t y a e i a r V e P h i e n . a a e s g i n a r o f a a r a i g e F r . g o l l l s t s t i r n n t o n s i e e Z r s m s h k a s n d t n e o d r d s l l i J h l e t e i i n o e b , u p p t V M s i Q o i g s g g s t S e d s t H e 5 F h t r t a a u e v m a . e h r T a G t g o u c e Z c r u i e a n c ( s e m t d 5 m o o l r r r r , N e c r k r o h t i p r o n a o i c 2 n g e t f 2 t s n e a c e e m i i o a s u n u t c s e r c p g . c e e t n n s ( r f a o p i t c n e a o e d 1 u e n ) e t a t l t t i t e s p r 9 e n h , e r c h , h r s b t e t a y 8 r d e a i e e l o i c t e e c i b 6 m a n o p d a n n o d p J i ) m d r l e p a L a m V a r e i r a t e p a e t ( t y c p e G e u m 2 e s o b n r s o p o r n o e ) , s i t i e e r o l n f a s n i a t a i g n t t r e W g t l s y c o s n t h t d d c q l e f h d n e 1 t o e o a s o u h t f o r f o l n h q o 1 e e o J t k E t h f 0 s e u r n V s n i h n J a e a e e t t t g e F J i V h g t n o s t r : V Z n e t F s t t n G h ( i h G . u o e 1 Z e > r s q n ) , , o 3 m u u a g p a n I e k n P d s t e r e e < r f T n v 1 e a 2 a i r m r l v e k B M E W d y i e n a r i a F d L R J W e l g ( d s d o T i a s e n t e l e t M a e c u t J s s r J y u o a o t l M l o t r V o e g u V r / e o e r F s a r n a s s a u n l t l a v V l a b l n e c V e i u i a e l e t y n a l y a e l n l l l f i t e f l f ) a n e a a f y a S u y u u f u l a f l l e t f a t t l u a s t z u c z l u o t l o t t l n i t n o e e n 3 5 5 4 7 7 1 1 5 6 3 8 6 9 1 1 1 4 9 4 c I o f i e ( T n n a J o f x h V d u n p 2 2 e i l G s e v t 5 ( T T s e J G t i r 0 ) a h d r V i a h a e F e y o u k r F a i n l t ) n a n y o h W Z v c r l a e r q t e ; e s e h n d V f u t d o v a e e d h . F a i r r l a e a m l Z k W t a e h c e W m s h i q z a f s n a o u i a s J c e n i u n a a n V l x i l u l m e k o t t p F c t d e n h ( e u o h e T a g e r m t f s F i i n P , e m s p d ) n t l n h o a h a i a a n c o f i t e s n r e n e g r 4 t r t t a d s d o r , ~ i e a u u s w l 3 G a e a g b . n 0 s r r r p g m t o a a T s h a r k t b n a h i y q r n w e g a b e r u i e . m n e l e a l E e l r n l k u - J l f d e m a V e s u F f l i t n e d F b r o o l e a s m K y d o e e a u I r a a n e n t t h n d c o d e i u n v r o r e r i n r a d t c e h n u u W n e d a r c r r V l s e e o F F n t ( i h a 1 Z n c e u 9 e t . e r l 8 t s o r 6 K R v f ( ) a 1 e e 1 d l 9 a c . e 8 e 8 u t o s - t 7 r e 2 n t r ) i e r . m 2 m a d n a n e c i k n t d t e y e e e r s C r d m f o u o i r r f n : r t e e h y d e a T f K o a e l l y a o l t w o o r i n s n v g a i l n v l e d e r f o a t i t c u h a l e t l r s s < l 1 i ( p 1 2 9 r k a 8 y t 7 e r ) s d f e o t r e t r h m e i n W e V d F t h Z e : r r r y y y / / / r r y m m m y k k m m m + 3 7 3 6 5 2 1 . . . 4 1 0 0 0 ) - - - t t e 5 4 1 l l . . . r i u u t 0 0 0 a a 31

Consensus preferred recurrence-intervaln and vertical slip-rate estimates f f e r r ( e e g g Z n n F a a V r r G W G W T t i c S n o h h l f W i V i o p r G G G e 5 e e F r - r o f r r r m a r f e l Z a a a l a y r e f k n n n a o t c e f a e i g g g t r n r t f i s o k e o e e e g i h e n m o , n n a r r r a 0 t i f G g e t g t f f f . o o h a w 0 a a a a 0 g h r m n h 0 r 0 r 0 e n i . u u u 1 t o e m h . 1 . . a e u d 0 l l l 1 0 u k m o o c o t t t - r m a i n c p r 3 t 0 - 2 p m o P l i t e 0 t c s o - m . - n i n o t 6 8 1 e f 0 4 o P o . e 0 r n s l a 3 R f b e 0 0 4 e . / - n r n r i r . r 1 y d e r 0 0 r r p b t W e e + + 0 a 1 m f e a o e e r r t e f . 4 h e f i t + 0 2 2 3 m a o 6 i a e e n l m n o h i i u e a 0 0 0 r r c i n c m d t r . k m c i m t r . c r n t r h / t y f l a y e k k k a e e e y r h y m e t m f / : m e r i l o y y y r n v y d n e y d t l r – c , C / u r r r i r d t / y n b m a o k V i s y W s i o r V o w n n a d r i e e n t d n s t e V 1 s n r e s i e a r a l i i t t e s l a F h 1 s r i l r l e f v t c - a l a o a Z t n y e o h d a i t i i e r v o c n f o m s n l b c r t p r t a t r e l a S o h r e e s e r o n d d a e m e l i 6 l t u d i a . p e t - S d e f n e c i s d l i r i i l u W s c w d e T s l h o - c s o i l o t n e a e h i p i i W m e t F p n f n p c t r e h Z e t f - w s c n e f h R o i o i e a r i r n t F d r a 4 r n u m q i o r r c t m s e a e k . e t a b u V e s e ) i i s s n n i i t r a d n r p Z i n n e o 9 n n c e r e e o k g g t e I l W u a F e t s y d e 8 u e e s G d . t l o a s l V n i 9 c o e e 0 i r m a n m r a t t e r . o 1 e i h h i d 1 m o a i t ( B t G p W r i c u t t - n t h s d o e 0 p e a e i u . s f s i 4 P l R o r P n l l p d y - f r r c t a 9 o 0 l s e r e . a n e o b o t t r e l . e h f 7 r e c e 7 u r n - o e Z v S t - e h o f o e S s h l k l r d , a t s e m d 3 y Z F a m 6 r i e s h C . W l a ; e C n e r e s d - m b n S F f e o e l r d L - 1 C . u s e L a g l W o h t e o e i l t n i o / k t V e s o F G r V y t S q T n r ) t S d r d c k o l i a e r , c w b r e h r o h E o i e d 7 U a r n n y W h C a e J t e u h r e a l a R e f e n t m h t i r t g H 8 f f e R a b i r i o t l q s d o w d t h c r h t e e e o r e a F 9 t t h t G l r t a m h d n n h c f r e u M t a h c a , i o n t e t a 1 a l e o t r e e t i s a s r h k d a ( o u r o s i a S e e h r n k e h p F e n s g s u a t n r s w l i n e i r c , e i s 4 g s a d d e r p l r n i p . w n a e i n . a d r G r e h i o ’ y u e s w c b e h s e t f l e 2 e t i t l n ) g 5 h Z u d q - Z u d p c h , c h a n i i c u e t 9 r c n r s t n h F 0 r p F i i e d s e t e . t a b y 8 o r o a m r n f n h d V V n n s g 2 k - 9 r t I s a u g o m a i n o w d o n F r e 1 a s W W e 3 n o c ( t e n o i , s d c e 4 T g o e F m l 1 t e s e y - a e e n a f y l e p r s . a m m T t f e g o m i v h r i k h a a e r e u t A t h n u e s t h c a e r t a e . t v ) u n o s o r o r d d y a r i s l i r n l u t n h d t s s t 8 o n i u l t o p t o w 3 v i q c a f o r y t e e a i 9 o G t l n y 2 C i y r h e l e t e o n n h d t 9 t n s s t i f . e i f t u g p n h h o e r s K d n 1 r t u e e a e t h i o Z ( n s i s e h a r t e r n , a i p k , k w t t , l F e h r d i n y e f u k a e r u a n a i d r s o m r c r e k n o c o q e u o u W e n h , m t g u n o v i e F r n v q d q s Z 2 o a s n g r o m s e e s i p e m e t h e h c k W G 1 t t e i F n u t o t l a l h e i d w s a l e r w t t s r t i n ~ c V i u 7 u e i e d o a e o h v a o o a . c a f m a a i a h h n n f B e K m i e t h n S d o 1 t f T e o r W H i t 5 2 N H Z t E H n T h o h a o o i e l r 3 l e c l s m o t W 6 h r W c e ( 0 a J e f C n 0 l a C H n c F f 1 1 - u a F e 4 F h 1 1 Z l u t 0 Z ) 3 s , e , l i 6 ( 0 t s u 5 a c v k a s 0 1 r 2 n o i : a n m f d a d c n t d e h s a c s ) n e e e W e l i s c a i y s C t e f e c s r a m h l l B l o u a o s o L f l r h . f v t g k P i o a e i t n s e l h . l l s S g e t d n n r o . e e e L i g t f n c r e a r x i t a n s p u ( e h 1 e C e e i l e a t g a g s F r r s i t h m e ) h ( t , - W n t q e d J c u S n t i u F e l p a t n ) T u d p u k . o c i r r e n f t a f a i s f g o a , c n r c e e e g b r e G i b T c n n e o s r e W h r t o p a t u e o e i r c a o o m l r u d o v t d r r p i t G a k a n m e n F ’ t W C i t s l s o n d e s o i t a n l g . s p r t e a r a g a u e r b s r G f s T e i k o f t n e l o E l h t f s r r e r e C f e / o a t e d c F a t d o o r a u n h r e r t n l e r a p e c g e d w t 5 h I t e f h n d e u e f h - o i P i C a r g 2 e t t s r r m l h q e r r u h 0 t e F e e e f i r a l u n i a , , v c t S n a f u k n e u t a a u J e y r b n h d e r t l H l r r d k t r c h s r e r c o e a e e F z q e o a W d r t n o r , u e i r t d o c n T a a a R o c e i e i n k n t n r i o n e i - m k e m d t i u t c e n i y s n u l e W i n . t d g n e e r t r i F r o i g e o t v h n n n a a e l c a y l r r e e 5 l l ? y 1 7 0 32 Utah Geological Survey 5 < r r y y / / m m m m 2 2 . . P 0 0 r - - e 1 1 f . . e y 0 0 0 r - - k r . 4 e 5 5 5 5 d 0 0 C . . m 2 V o - 0 0 m n e 0 / s y r 1 t r e i , c n s a l s o l B u w S 0 e s i n . c l 2 i g a p m u t o s R m e 4 a / i y n t r e f o s r o m m a e t t i o i m n r e P y o 9 9 b n k r e e e f 5 f o a e 2 r r - r e t - t h r 0 e 3 q s a 5 u 1 d o 0 a C p e k R t k e o a n e a i . n c c i s s u d e r e n r n k i e s a n . . s u u c s n P P e q . . o i h B B t I t n r a r r 5 l t a y y e - e e l l r r v A . a a o a c e c c i l w a h t 3 m 0 0 i k p f t 5 0 r o 5 6 8 o e a l 2 8 4 l k i - - m m o v 0 0 o u 2 t e 5 0 e 2 m 5 n 2 4 g i 1 > n 8 4 t n i o u b m B Z Z Y Y 5 1 2 3 5 0 ( 2 2 k 2 m ) L e S n L g t r i t a n h t 1 l i e g u h a t t f l S u s l T u a l f i r r f a H a e l c l c i e n e v o i s t l l c e n u W J F E H a a u A s c s r u r e e t n i l c c n E C t d / a t t e i a r F a o n a r c n r a e s l h t o s u h e f n a e l q t f u c J a u t l S u t i u o a n z l e n t c o k z c t n e i o o t e i n n o T e n i m i n g 4 1 2 6 5 1 4 6 i e o s T e Z Y t H d s s s E M s r t e e e a n v a h e C o r Z Y c o i c c c r e a t e n l d C F a 4 t b o t t t n d h c e i i i o Z . A > e a c o o o l d h a 5 3 q y n f e t l n n n 5 n l r - - p u i w c e s 6 c n n e 1 4 s - d i a e f w 1 s i o e n t e . 0 . o e t i o k a 6 e 0 d n e i f r r t t ( e s v T k o h s s h e s n n 1 k w k u s a a r , h o e t n o a 9 a r ; g 1 r i h e b n 3 l t a r 8 f U a h e i 6 a i t o a f i c 9 h c J s b i n o o . c t t e n h 2 , a e e u u h l e w m e e d d 1 h n r n r d p a e . i n f o f 9 / t n c H < a a n a A r v , t r t 9 a 2 c o l d p u F e i A c r e o 4 e l 5 i h l r Z e z o r b t 1 n ) s , - - i i i n o f s n z r 5 c 8 e s e s a o s t n e o g 0 c u e r a e u e k a a i n a , t b s c r v l a k c i d t a l t o a m t d , i b i a t h i d n , i n n o t i a o o q i i v e g p d m c n n n r u i i n s r d d s d t a i o e c ( s e r e . t k A v e e h s d r i t i e n n J T o e d h a o t t S c u e e w t e h n f o i h t ) m r e d o h n o i v n e l n i a i y n g s r l l y g s n : u i t u i l r r o a E l l a o E d a m S e n n a e a h e e v n n n s c q n u W W v a i t t r n r i e c c n e s c e e n c c g g u d r i r g d u u r t g i o o i t u r e e n n i e e i h n e g h m t v e a r r Y l r r r r r t t h l e e g r r f n q o k k , i i y n g t t v o e e o o u . a a i e i i u c a t c e i A t s r n n n n o n n m h n i i h d a e a e n n r T g g c c J n a a e t 1 l i e l , k i t t a e h e e . S t k l l 1 f y y 2 G G e o l t l s o t i r e B - - y y e h : : . n 1 i . e I l i i o r 7 r r r i n n f t t l e o t . b b p v e o o y i h n T e 6 a t t m a n e r r k - e e u u f e c s r h 4 ~ o o r o n k y n v t r r i p p o t a e - h n 4 a a r h v v i W y e r a t I 1 I n n t e . i g n n d d P P a i a r e v l r W i r 0 k f , , n o d t t l i i d l i b r r y i n d - e e l t t i t m t e e r s l e e o o n o 1 e h e r e e k t e r r e f f , s e s f r 5 v v t e a p r e e i a n r r o a k w n t t v t w r a a e e x o r r r i i r c r i e r k c g m a m a r r n e l l t i f f m t h o e m y h e e v o l l h l n g e e e i a G r a r e q d d n c a ( q l l s n u c c t i Y t y G n h u f t r m u e e g t t m R R i i o t e - d a o c a r r h h i Z i u i a e e m e o v i e k o n t s k n i i 1 s ) p r c c g g e e s u n n , e : t e p 3 b u u ’ s h h n e p a c s a s ( l . a d i r r i t e r t ’ r 7 X n o e r r s s r v e s i e e d a d n ) s a l e k X i n n i m l y n d i c c s r e i e e , t d s Q t f ~ i S i t t l s c 1 i P T d Z n n n i h l o h m l o o 9 i 7 h c o i u d t s a r W r p r w ~ e 9 r 0 a e o s e p i a i r t e - t c r 2 4 - r e s u s c s o r l t v s m f 1 p a a e a 5 ) i s u g l r b a l e , ( a 2 t c i k r t - p i r h p L e p l e l n n c 5 p e s 5 r i e o n r u e l t a i s 0 r m t s e r o m e n i g k o n n l a h i r m l a n f g s d i 0 k a y e d d i c t u p i G l r g t e g n a i e 0 . e n u e g i m n a 0 d a m t s h e r . . r s t c h e g d r 0 a 4 e o i n d , e e o n a s u e ( 4 t T - p u d w d m l e S f t i 0 t a n t p - n a o > e h p e i o o s 0 0 . o f t r c a o c s r e 0 f e g u s . . P t v a e e t x e o s i 2 r 5 7 R h n h t e l l a r e r r i r i i s d f - - - m e t n p e e t e t e a l f 1 0 0 s , a v h 0 s r o e - n p f t . u . . . a a s i i W r , e . e e 4 n r 2 n 1 r c r 1 a m , n r e a r o D t e t 0 a i m - t r o m 2 e m y d a , n e d 0 e , s n i r : 0 n m t t s d i u . d k a m 0 i d h d r 3 i o ( 0 t p c n i a n i m M . d e a n o V / n 1 l 1 o e m n a y d l n g i n s ) / e c c - e t n g r c y h 0 m a a C o i r e t G e e r i n e e . t n l v n v 4 i a / l d d c r r y r y o e f e s o l s e i i a p r m f b r c d b n l , u v : a l i i a i a e p r n t n e 1 m p S g o t s n , Z n 9 ’ e e r l s a / a c i a y 9 t p d l e Y r t t 9 e - ) Consensus preferred recurrence-interval and vertical slip-rate estimates 33 d p d r a e e a r o s h r i t r c ; e s , b f e s t d e y w l a r m e l t o i o P l a m r r f i 5 f n t y e m s t / i t l s o l u n i a a t e m s o s : i s i n R n t e y m a t e t t e a t p b n i a n 4 s l i n . r m i e n - d r a S 0 t o - p m e e o l r i f P r l 2 c e C a e . s n a r c a i c c e l 0 i p a - s t s n f p l ’ n t r u e i 5 u s p p o e o i r 0 r s r u m . h d i V y i e l l o 0 G g d f d r C i d e e h g n d i V o G c t n a n h n e i n g c c a s k r s e n t e r e i l i e d d g c f l o n i k i e f n a r e f s i r s n l W o o u a r S m o s i o t W b p c t r l i y p k R 4 0 a 5 t - e 5 P 9 r e f X e r > r e 2 d 6 C , R 4 0 o e 0 n c s u e r r n e s n u c s e I n 6 t e - r v A a l C 3 1 y 4 r B O O . P l F i g . Z a r u n p d t ? u o r t a h e k e s r C c s 0 o 1 ( y 5 4 s 2 - r e 0 B 5 i 0 s . 2 m 1 P ) > . i c b l e y S l X i w e L t r i v i t a n h e i e t t g h h h e a t t t S h e T u ( r r k f a 2 m a c c e ) e t f i i o R m a S n n o o v n i e t t r i n e e e n e r c L t g e r r r i h u m a v v p l f e e e l r a a g o e r u n e l l e o i c W . m n g e n t r t 1 l o s T c e h t y 1 7 h t 1 r e h r i i g 2 k c . e m e 0 i i o h . E v n n 4 r a n e k e g f u a t o s k y n f e n o G r r t y r t r m c . t r a r a i h r e e , n r o a i , e n r m t q u t t e c e i a u p o i a r o n d i v ’ n n x a n s t a e i i t f m k n i y l r i s n d e : t e ( u t e Y e r c i o m e r n u - T e n s Z c r v 1 i t a 2 r ) e m r e 2 e : l 1 i l c n n l y . i i t 6 t m c s F s A n e b F e e e G O z r d n k g i r e t - g g o o a t y s r q t m e m m n e a r o u u l , e a d o o e e i l r t p t n n n r / h S e t F t t I a f I s a a s W i t v B o 0 T Z d l n l t o a u l u e t . i h e a t s L h 0 r o n l r e a e l c e t n p a e t 9 t r d r i a c d z k l c k r v - s m a S s u u 0 o i a e a i n c n , i s r . n l e l n g e 1 r , g f 1 e e n e a i m 4 c t l m 9 G e e n h u i - t n 9 e t 0 , c e l i u r t o f 6 o n e . s p m o 2 c ) u t n l o r i i 2 o , p m p p n o o n m r ’ t f m r f s a e o f l o 2 i y a t r d d m s r i . v x o 0 e c u l t a i / i n o h p - n 3 3 c m y l 2 s n e e r - c 5 0 o . u r . , e s 2 7 a n O a m a t 1 t r l m e n r i F a s , m e d l Z i a e i n c ( p i n t s O u a e i s - s d t r r n d i l a r m i f r g e o a e i t e n e a n r v v a s t e t c t e e t r n e h r e a r 2 n r d i a e c e n a 2 t a g t v g r Z e n e e e e d d n Y o t - f Y Z Y Z X X Y Z f O a O a t F l u 5 3 6 6 9 < i t b b g Z l w 8 1 1 4 8 1 t e e i 6 5 7 1 a 9 1 o n a t t + 0 w 0 2 w n 8 , g n 2 s + 2 + + d + e e d i 4 e t 0 2 2 2 2 e e e o 7 a f 1 3 3 0 4 n n o s t r / 5 h 6 9 7 + t u - 4 h 2 o / e 2 / / / 6 n - - - - 8 q 0 n r 4 2 0 2 2 3 d s 0 u , 1 1 5 3 1 0 3 t 0 a h e ( 1 / 4 1 2 0 c 1 - k e v a 4 0 a c 9 e c c c i d n 9 n l a a a a 9 s a e d 9 o y l : l l l 6 n n r r y y y y ) 7 c d t c r B r r r h a 9 e e 2 B e B B B . l 0 x P 6 r . y f c . . . 0 P n o P P P . , r a 4 r c . . . . v B p 0 a t a o h . 0 l P t r r e y t e . i r d o e B n t s r . e o u P n f r f c t a h h c e e e s - Y X X Y W W - - - - Z o o Z Y Y r r k k 5 3 3 < i i 5 n n 2 7 5 8 g g 6 2 0 4 2 8 1 G G + + + 5 2 r r 1 2 + o o 1 1 1 5 2 5 u u . . 9 8 8 1 3 8 p p / - - - / / I I 7 4 4 - - 1 n n P P 1 / 2 . . 7 t t - 2 2 r r 8 e e 8 4 e e 2 - - 4 r r 5 2 6 6 f f v v e e y 4 . . a a y 6 6 r r r y r r r l l y r e e k k r d d y y r r R R e e c c u u r r r r e e n n c c e e W W o o r r k k i i n n g g G G 0 0 r r . . o o 3 3 u u - - 0 0 p p . . 6 6 P P R R - - r r 1 1 e e a a . . f f t t 6 6 e e e e r r m m r r e e m m d d / / V V y y r r e e r r t t i i c c a a l l S S l l i i p p p i l S l a c i t h r g r i e y h / V t m d c e e m l r b H f r y o 4 e e e . w u r t f

34 n Utah Geological Survey e 0 a e - d o a i v v r n t e e v e R 2 t P e r e . t r e t d , r r i 0 c c a e t p a - h a t g v e u i o 5 l e e s d r d n o 0 s v i r . b a t s l e i n i p 0 p n : r G y t t l l t i a y i l r t e c t h a c g a r a e t e 5 v n e n l i n t m a i s i c a m l k o l s e t i i r p f , t r a n a o p y t r n e u a r o l c b o e t t W v t i e 0 e d n n e . u n g o 5 a r i u c 2 f 8 f t b i f h 2 > n e P e m e . g c 1 n r t m p e a e e m a a n d / f y e s t e o h r t c . r f 5 r n n e e e d r C t r V o u n e c s r e t e f r t G S O v m k i H i n h a a R c a y n W O o r l d e u u t i r a r o e g s w , i c i l M d o v r a t O u l u h s s e e a i r t e y p F d S u k F i v s s l n r o t a ’ u k Z i g o s Z e t r n d l n s a i a i g m r g p l r s e r t , l v 0 i o 4 e e e f o k a r e r G t i 6 s c f 5 e R n e h e t l a t t e - u e e 4 l r r h c i e k t y r n i o a r c s n 0 e u t r y r P g u h n t r s 5 t e t r r a I 2 u e n e p q , r t - n o - ( h e r p r 2 I p o u o 2 e c n P r e a n t 5 0 r a r d 0 u u e t v t t r c t - e k h e a 0 r e - h e 5 o e p e i e r , r s 1 f n e r n 0 v o - e s s a i ) t o t a n s s r e s W k n G c f b c r i l h t s a r - n y 1 e e d o o e v m i o m t b P 0 r 0 d r g s u a l e r t v i i . a . h e t e e l l l r r 2 n 2 R a d i g v i t e e i a v a s l y f e a k s n k e k s b n o f m . W a c l r i y e e d r s b t a t u . r u i h . o r o c e b a r s d r f a l r r t o e y l s e w e k e i t b t d c O e t W p w o i h n d w t e o n e a i - h l f t c i n e f g t r a i 1 C g e i t w e t R c s n h l h 4 a u t e o n q e r a n l e a u V n u t M c y n d n i m a n s u d c c t p a t ( r r c i B T k o W e t b e o n e i a 2 e r C o r o m e o c t h 4 t a a d e e t r W r n 0 f n h r c n e s o s a m e . l r . s e n r e r e g 0 6 e o o s s f r l e e l e o s c w d : i o p c v n i 1 W + r U u i m d u n l a d h b h s r n e t a a f i ) e n e c n k s g n v o l e s m o r t l e e o o l ( d i d n s i e e c l n r s 2 r t n ( a f n e t z o c h e k e 2 r r l . g o 0 ( l g r a a o t e y d i e I s r s 0 t t t n e 0 v n t r h g m h l G h a t s 0 a 0 a g e g r l 1 O a q e i e 7 t o o t m u 3 i . m r f e r ) e b e n 0 u 0 t - o G l m r ) a a e i p a ~ i l c t t 5 i g a . r v u n i u l b f t y n r b 0 r r a s o e y 9 v s - a k A e p C o e n e a h d o 9 0 s C 7 n n 0 r n e a r p a u s d e c h f e t n e v . i P 5 s o o o s a t 2 o f e p R l a k a g e n d 4 d o i t + a t r r t t i d - r e y r ’ r v c a 3 e e ; a i s v 0 t c t t c o o t b e r 0 r e o s a a h r f h t o e , o . ? o n 1 e 1 t d e s t u r . X Y 4 m i q o h 1 r > O r n b ) r l s 4 r . i 6 t t a r n i u r n 4 w e r 4 h t m p e e 9 e e p l e g y i a g i k i + r i u e - e n s e s 2 g b l n l s o m s d m e a r a k a ( r s n C v e + h o a t t o f e e s p e t a t h e ( e m r V e / s e t e o t E r l 2 a y s i o r s n w e e r i l n l r o M e r p i e 5 y / r u 0 s m , w d w p t y c e b r e l v t l 0 y t t r t r i e i h a o s 0 r h e i t i . t t e 1 c 1 n h a r h r n 8 a t b t . r n a ) e a i h r 0 g 2 o t m f l i t e e e s 2 n k l l y t e u o e k f k a e s a a S 0 c u a r r g n a r a b O m o s r o d h r t 0 l q h – t g a t e i 4 e n m ) h p h – a . l e t - n , i 1 2 t o m q g q O n w a 5 d m . + u u d k n a a e t a h 5 a a a n Z o y n e t w r r k 0 k y a e d n 7 o e e u e l e c k o t r s s l o h 9 E s o u s l a h r m t o 2 h t , u 1 V r V s h m r u + o r ( 4 u e a e f c g a r a S e r n y e t k o h s y l c c L a y h n t e S t e r ( e t i a 2 - r a n v f a o f 0 a e i t e l f g e 0 u n t 1 r 1 r h l t t e 2 ) 4 i V t n n . d t 6 g S c o e + h e t T u 2 e ) ( a 5 r r r k o r f m a 2 t r m a k h c i y W c n q r e ) e u e b d a a k a s e e s L d e n g t h 1 E a r t h q u a k e T i m i n F M S E g a o a o s u s o u u ( M t t u n e l h t t t e r e / h a n F r r e i c n n B a r u s n O e u r f a s q f l a a t e r u u u c L S i l r t l t a t r i e ) z o h k o n c e n t f i e a o u n l t 2 2 4 3 5 2 6 7 ~ t M f H s a S n a O M M o o o n o n 9 O e l e c e l i u l d r d 7 2 g o C r r M t e t + c h c w h w L s a a x u u F e e u a i l n c e r n r p r Z r r k , a n d s g n f f i . e n a a v W t , ) o T s c s c a u c ( t e e h o e e t l 2 e h o t e x s r m c - 0 n e f e ( f d c t t a M 0 t r i K p e a r o E s u t 3 a o i w F v n l a t l ) l ( a s t r a , ) 1 o i g e d a i r n t a t e 9 n l e n i e n g e t v n x 9 r e d c d e i d p e 9 e H d y h n a o a d ) a e e s i f c c l s r e a r d i l s o t n , h r t e t u h h o u e c e S ( t d l q h q e s t t l r s h o u s u m s e u a e e l d o a a d s v i s i E n r e n n k t k i i n t d , e B d r e e e t t a e c r h s d h L s t t e n n C e e h i e F m t s n d c c e h a i e t s i t n i e r n n i n o a c o f y t g o o n l e o f , : r s n t h . 5 , e e v t i M i n n e a a d t r c r e r m C i i t v a r h v a i b m q d a l l u p u e l e s a i a , n a ) k l r n o a e ( i n 2 f n s r t 0 e 7 g e Y 0 c . i r n 6 e u 3 a g v r ) k n r e d y e f d r n r e n o . t Z t c m e i e H n t r o m t ~ o ( e 5 2 w > i r n v . i e 9 n e a v t d l k e s e y r a r a e r , r l v b o e e e n n h t g w t i - g e h e l y n Consensus preferred recurrence-interval and vertical slip-rate estimates 35 5 P r e f e r r e d C V o n e s r t e i c n a s l u S s r T a f a a i i m V l n n a e i p s b h e p t v u m W f e s p o e l r e l e i t t r r / u R g o s i - e y c o W c 4 b t n t r r t v p a i a k s e s g e o e d r l i u h t a n i d r t n e s a e h k r a t g t l p f t e c i i e v a e n p i e o d G n i c 0 l g - o p n y n t r e . r a r e o a d 0 f o G b - i n r s m t i 5 d f u v n r e a r d s o - e p o a o g u 1 i e a n b a u l d l . P o t s P d c i 5 s r p R e i l 9 a t n r i n e . s - ’ t i l e r a s 2 m y s n g u e t l f t , . g i h m e e s o 5 a m r f a e e l e r e f t i p r n m p i e c t e t f r s t s d w - s r u i r d m c r o o W r e a o a f t a n m r o V h / t r e d r y s e p e r e p e n r > 0 o C r m t c a i e W l e B a b l T t e r e R c a s i h 3 e h . t t k o s s a l u l W 5 h s e i e R o r t a a 1 e f e c o e r e t e n o g r t t - e . n a e i p d + a F i t n 2 r l o m r f m Y r e n Y c h i t k p s i e l n h e Z W . Y i r d s p , o a q - 3 i u - a e k S o a a o , n , Z Z e r i e t 1 l u i o t d n r t r e t o e l t g n o e n > e i t i h 0 a r n r p f i v f e d s g n n t c r k 5 e e e 5 G k r e i e s d e m e i t e t . e r o 0 G n n e h 0 q V U r r u f f a f o g o e e l r a l e + c u c r y i e s e s a m u o f d u l r e e G s c u v p p u > a > l a n y i B i t v t s e 1 o t n r ’ p n s - 3 3 o s I t s a o t . n b - b s c I n e 1 P 9 . u 1 e n l b P u o t e s r e r u 8 r + + t n 0 . a e o t o p l i s i h r , e e - e d b n g t 0 e r c a t . a h i r t h i r t r n u f w d l h f v e e v a e k i e A v a T t d r a q p i r c i y y g r r r a u o t h v i M r l u e a o o h r c e l r e o a e l t a n n s a g R d f ~ l m r f s t 3 k c g . t n o 0 e h i E a 2 i e e n e R i o b r . f a z n - 8 o s - g d i s e e t e t i r l r h n i e a i o x k c s e t p t g 0 e t i h l t c a u r e e , r m h . t a f o e r 0 h s B r t o e r h t v d a e 3 3 , 8 e e r R r a e 1 t u d a n h t k k t 3 k F l c s h h n c 1 e a a a e Z e a f T a d 0 e o s , v n h r X a e w i l s a h < o b g i 3 c u l r 1 e h e t + h . + a 1 p e 0 t 3 6 6 0 6 1 0 a e 6 r . . . 9 n 2 5 l r k k k 5 e k 0 2 0 a a a o s k a k , , , < e a y s y , a b b b r e c , r b , u t u u i b B B u s i b S o t t t u . Z m . t u P Y n l P < < t L i t m t k i f c ( r . 3 3 > . c i o i T l e a a s n i u 9 9 4 0 k r h l t l c i y + + 6 e . r e t e c g e 6 h e h 2 n l n a s y + e h n 0 d o r > e c b n t + a o t 1 h o a o r n S m 5 i n g t n l c . y T u u 2 e m g a m ( c + s s r r l k u d i f h a e 2 e b m a a c a c a r c g r h t a c t n e a e ) r i t o e e e r o a n a e d n r t s , e e i b b d r L u u e l e t n k a n c t v r e W T b o h r e e a g s e W h e r r f o g r t t o e l t t i i e w n i r o m a h a 1 m u k c c o r e n t g i a k i t o e n t E e o i h c i t u n ) c g n e n n s a n f o t g s i i G a e d c r m a r r G i t r a e e r n n e h e e o r r n d r c t t : q u i o i c h a n t n a p e u e u q i t t n 3 e r ’ d p e u s a e l t - r I i n y a 8 n m e P k r t k e - t v d i r e e i 1 o e e c t e u s r 5 n s u f n v e 2 ( T T e a r + a t a k X 3 h r r l t i s 6 i r l y e l o m 7 e n y e s a r 9 n 0 t d i p b Y n e 0 c g i + n o r d r / e - n R F - B o v Z s g - 1 e a Y e a i s a e n 0 d i l c d i . a n u b t 5 l u e t r t e l t 0 o l e e r o r t n R r ) v r / e t g e l F a i h y a v n t v l r e h r a e c s e g r e u . n e f t l a E t i u n v S l t e t e i a i W T i l v i B b e n n n n e n z r a s a h e t v c t n d o W o s r s e t c e a a Z g i h n i t r o l v a k n i i r l o k t e e c p o i c - t b t h d i i r L i t i o s n i m o k n l i n a u t e l a o n g h t i n e 2 t a n i i , k f f e n r n a t 2 i G l g o e r d t d g g l e v i 5 i u l m e n r . y G o e f 0 o w i g n l t 0 s r e e r e b r u A h i c t . o t c b r i r 2 p h e b d c e o u t t y - ’ i i a e d 3 s v a 0 n m p p l i l u t i 5 e d . m g t o w s 6 e s P i n n o R l s l t - i : e b i m r t i t o p n 1 s p e a s e o i - i a . e n f t b r n t 6 r p a e e h l e a a t l l e e e s y r e f m s t r v l , e s r r e a u e l v a m i s a e c g r r d e a r l t e r a t i n i / h s l g p a 9 V y s d r u e t q e b t 3 r i e r n h m d a u t i a e r v l c q r a i t t a t e t c a e i u e o k y c t t r r a s e e o h a i t i t a n a k s h n i l f a g t e o i e i S i n t a c r h o f s r B s a s R m W l . h r e o a l t e n l l o u n i Z l y e R y e i p t g e u s e p d m r h s s g F a u f l s t o a u r e e s l Z 2 t e t u c l u i d 3 t ( a n s e g s i 1 r s 7 c c g - h a e 9 f i 0 a t n n a d a i 9 f a v r r d u + 4 p o i 1 y a r i t l ) s c o t m r h 1 t i i i . a n e u o g i r s t g x n g n a d W e c c g e t a e o i e a a o e o r a ( n a v n s p l n m o r r a t t a e t g d p h h t i c n w e n i i r q q o c t o d i c ) u u m u a n f h n a n i a s l c l u l e o y k d e k l e u m s e r e e v x r m d s e e r p o i e o l s n c y a o n o d f i e l s n n p t t t n f r u t w g h o a e h t t r e s h o u n a e t n s h e l c t , B t o i r h b t a B u r R h e m l n s e R e F s d t a F , Z o l - Z n . , u v i G c n a o n r t W r n e o c i e a f n u o i d r t t p r i i t o e o k ’ a s i n n n i n n s s c a g t l e l y 0 i i 36 l Utah Geological Survey p n y G . l a - 0 i b m r r s s 1 o a r l s i - i o u t p t o 0 e s a p . c t d 0 e a h i P a 2 R s s r t r t o o t - s e e a i 0 u m i d f t g r . g e e e 0 a n w h r f 4 t e r l e e e i t d t m i c h d m a t t m 5 r o p V e t e h o / . e t y e h s r r t s e i c 2 i b W a 0 l e l 0 o S r k k l i y p i n r ) g , P r e f e r r e d C V o n e s r t e i c n a s l u S s d 2 B m m S l i 0 e e u p m i n 0 p c l l i / o i a R m v t y 4 o s u a r a u i s t n b 4 m s e t a 0 e a i s 0 t s a n h e v d k e p d e y o N a r r o o a . g e n r g e l l s y e 4 o o c P 9 s m n f o l r t i n ( p h e g o 1 s e f f r 9 n e t , a e e r d r 9 d r a o t r i h l 2 i e e a s i e s r t n ) p a p e h o d e r s r l l t l a f d d e a s o p o c 0 p c t f C d r ( i e R . e o f + 0 e a s m d o r e o 1 e t c d o n e c i a c r t s n p o s v n u e t i t e e 0 l r e i b d n r n . d u d 0 e i ) s m 2 o a n d v f u 8 u c w e e s 8 t n o e T 9 a d ; N n I 1 d . e n g ( i . s e l w 9 s i s t n d t T s e - h h i s o o e o a Y t h r n d e r s n k t p e - A v m a e o e s ? i M n k i a x a r c m t t h a e r o n u F e t e l o c r m g c u n i u Z e u r q 3 E o n e f s n t a c n i d r h a d f s k R n a e o t d i c u v d d v n r l r a h a C n l i o o 3 4 M s e d n w a i n c 1 w e l a . u a d e r s t h e o o c n t h a n b t Z i n o t e n c r r l x e i l f o o e d n h F a e k r a i e e t u n . t l a e i a v l p g i M r E i y l a Z C s t u e a h l m h m l a i 1 e r v F e R q t q l l u y 4 i h a m o u o h r h e . t m S M f M F t s s t e a B r e m e o k 1 y . P i P a n e s u 0 n e l m l . 0 e e y e t i h m ( i m i i s c t r ~ s c t e x 9 - i o o t e + n t t c . r c a 3 u d g 0 e t s l e 3 i i n o b 7 2 c n m o 2 c n e a e , 3 l S h l , a o e n 8 i k L P n t f h a r < g i e w t a n ) e l d o i e i n g d n v h e k d c p a r c s B e e m o r d e d a e d r m S o i t e e e h o u o r n Z s a c e r a e a v a i W e u s e e c s p S c n r a n r t o r v r r c l p w u m p a a e u f u t f t t t r l o u s e i f f a i h a e h h h e T u l l v i n 7 r r l a a r r t d b t l c n ( q m r u r q q q 5 s r o a k r i r r e r b c k 1 n e r a e e r e u s e u u u i f e p n a 2 o e i e n n l g n m i - a d n e t a n a a e a d n m a f c m g c c a c e a s k t k k k c e i u o d t e s d n e e ) e v n l e u h e e e e G e d i r v t - a a p n d h r s l e s s s 8 r n i m l , e t r a n t i e l t , . . i e m t u o a a , t n N r i t e n t n i f h a B i e i e d u n t g 2 m t a s a i g e a c h e i n t h r p d a t d 5 L s c r e e l w r v e e d i 0 e s I l s t e i n W - . t e n e P h s a s a i . o u 1 e o r t g d 5 5 p t q t l a i r h r n n a 0 o d t 1 e f n a i 0 e t a o e u t s o l m h 0 r r 2 g s t h ~ - f ( s o m s k p a e v n f e q 1 1 u l 5 e t 2 t a t k i n a l k u i r n a u i 0 h a 9 f . - 1 t r n m y - 2 t r e f l h 5 m g n a e s 0 c s 9 i e d r t 0 c s H a e 0 h a E i d e o k 2 d i g b i 0 G Q k v t n e a e o t m i i n ) k m e l f e a y 4 i R t - n n r s w u t d o y m h o r r p t o o e i r t a h a i e u r e e t i s a r t f o u n z e s t e t 4 c o o a h f v o s e c f p t o a r s t u 0 r f l e p q e l a r ’ i l r s e a r s l 0 n i f r o e p v p r u o u u , 8 s a e o e o k u r a a r n l r e r r y r f y l a t k c y d r a e , g e r e d n 4 t e a T n g e i r m c o i n F M J g a a m u e l t s / F P a e a u k l t f a S u e l t c t i o n 6 6 b B e e e T a t S r e a a a h e a u B f r r r e n s l o t t t l u i h h h e i c v r r l h e q q q d l a r L u u u e n a 3 o a a a a s c 0 n a k k k k u r - n e e e e o l s 7 t d s . s o s s 0 ( s i ~ o o r l i k - n a 1 a t c p a d h t 4 c r h . i 7 e o 0 u c e - r f a r m s r i k r l t e e a t e ( - h d 1 ) h d a t 9 i g w e a g n 8 l v f h o a t 8 o e e c s s ) n l i r o a c u e e 1 p a l r x f 1 l m o r a c a p 0 u a c r e - g t f e v 7 n w o e a - 0 t r f a , t m a e s k t u h d h e a l e t d a i f b n a t o u w g n n t . o e W f u B t s e m e e m S o h o s n a a a o e r u a e o t i k r r r i c l i c x a t t t l r m n t h h h a o k h i d w m o s q q q u i n a e n o a w h u u u s u t v g t e s o e a a a n h m e e r G k k k , e 4 a a l i t / o n e e e c m 0 r r r i o p e t d n s s o o h t l k i m ; i t i u n d n v i t q m e y n h o p f e i i e u r r m i d t i o d e ’ r n n e a a s u c w e v u c t g n k n o a r e n m o e , t e p e u l i 8 n c l v o s l r c l e l i e o 0 u t d c e m . n u a v - v i o r a n r l s k r i , i i i n r t t m d l a t e y i e l h a e n s y e l r p n q i b r t g t w s b r a i v c s u s n a a r r t e e e a t h o i f a e l n d - o n k e a t i b r t g n r e e c s v d e e t t e t a w e t t h t f t h i w o l a m o r o e o . e v r r f e i a g A n a e l u g s n e i s s f C W a a U N m Q a v B o e s n a a Q e u m l l o s r l i d s c l o k a t r F s e / i e h e k w t c S y o P s e d e i l a r n n i y s u r . W n l n g m o f l s g ( l a d a i n G v o v e G u i r s e s a u , n y a l l r p t r i n t 2 t o n b p h s l z a 0 u a a - ( e l o e i r 1 c 0 p n l p r a n s e 9 n 3 d c t e t c r m e 9 o i ) a m e o r 2 t m n e e t e o x n e a ) o c n t s m c e t t r o w t f t u e h e o i n u m m i r d e p n r s c n s m o a i o t 4 n i o ~ c w h r t n . o e n t e 1 a e 2 i s r n t a 4 t t h o f i i J m h d d o o 0 f m P . s t e n r h t k o F h r e t t T a y t s h f h e o a r t o h e n e , t n f e E h e N J 0 e l a t a P e . s t 0 J F l e t s P 3 : o F n p i l S l a c

Consensus preferred recurrence-intervali and vertical slip-rate estimates 37 t r r y e / V m d m e r 7 r 0 e e . t f 0 a e - r 5 R 3 P 0 . p 0 u - o 1 t u a a s t f H r h h o 0 e n c b o . e e u G r i a s w s o n 0 e T b g m s d e I g n l r F P n s e v a o c n n G s e b r i g e t t o h u o e r . e e k , o e f n n r f n m f N e f e i i g o c c a a e a t r r i s r n l k a c v e s e W e y a b e a n o 0 d p e r C t n n . a p t w 0 a n i e d c o V o a 9 s h b , t a a s n l o i t n i e v m l l t h p s d i l a t e s e d e r - y i m r r b t W , i e – s a t i W o i a / l c p e n c t y e e n f e n e a a l o r c s i a t o d . s c h l r l e c u n s k d e e s q n S e e i s e i n t s u m s T e r r i l g m h t p e i r r F ( t i p e r l s m i 1 a G a u G n t 9 u t c i R t a c r w e o . 8 4 o e c t a n e a 5 e u f m I c d e o n ) p t t r r R e r h e e o t r t i n e h s s n d y t e a i s i k e l c r s r 0 a o n e v 0 s o f r i s 1 t e P e - r a t a r i 0 P e n r y 5 I t f a - e p n v 0 i r u r 1 a o e e . t r r g d e r e G C R a m c l i o g t n e n n e c u i l h s u t k b g e i r r c u r n s o e e l o s s f r n u W o e h c s r p t e I n 0 t e 1 - r v a A l 3 S L t r i a n i e g h t S T u ( r r k f a 2 m a c c e ) e 5 L e n g t h 1 E 1 1 a 0 6 r t h q u a y t w o t t a b N h h r k o Z n o n e e i e r n e u d u , e n d s e n n p Y e c g o o T g d h a s , e i t s l e h e s h i - X s a u m p r t e s o , r r 2 t r n r f t o a h i s a h t 5 a n f n a c F e f T B S i 0 ( r a g l d n e 1 r e - o a t a o e u - 5 9 r f > 1 w l f w a a l d u o d 0 8 a t 3 1 w r e a u , 0 l 5 u s t 0 M 3 t v n h n 5 b c ) l / - t k 0 e t q F d a - e o i 1 e a a n k l u r r u o k 5 x a , g e m p r F a a n p c 0 y w v u s k l e , - a t m a i a l k f l e i d o a < o v a t t t a i e h s e a n n r n 2 u g S . n t n t g 5 h l o n r e F T t t c a e 0 q c o d g a o o e b - c u c r u n w 5 e t e u a a f t h l 0 a o a n r t i c k b r o r 0 r r n o e e e e t n h a t l s e n k d a l u t q . a . w v l u F e B M i a i l a a t a a k h a l s t s e i r t n e t s i n d 3 2 2 t p r e t u s B s 2 2 M r h h e e u i 5 o s n 5 W e n a r e f c t r 0 s k o l c c i r f u e t o m s n - a W o t u a e r c i 5 r i o n c g u r b a k t 9 0 o e 1 w e h s l i t u a 0 0 e n r n 3 e - e n k n n f t g c 0 4 k a i k l i , a 2 c d m t a n e - 3 y a u h G c r 1 e r g r e e o l e o f g 5 r t , a r o t i . t G n e o 0 n i a w 2 h n t r n i f u g i m 5 e d r i v s i t k n d o p t e - r a u h a t e i 1 e 5 I s u n n y r n P r . a 3 n 0 o 1 s 1 a N a Q w d M p i n f g t ( a t a a r i r 0 u c - e 1 5 c . l e ’ o o e e o t u o s t R 3 s 2 c s o e h - c r i r 9 - v n d f o a u n 1 f e s s 0 v 3 f e E u s r q e 8 a k a g l n t b g e o u a 5 0 0 c i i r r u e l m 5 c f m l n e o s r r s c c r l 0 > a l a a e r t y ) e i f k k d i p u i n h n u a 1 r i d i a a k - w y k d t y r n i p b m r g e f a s l v c 3 e i t e n r a r r t a a e s e r i i e R r , e 0 r p s d e d n o i m a u y r s e d n o w n o o y e n u g a s f l k t i c a f t a a d c s n r w c t i t s a d i a f o u t t s h n b i e u e a u h i o n l i f u g r s a e g e t - t l b p u r r n t o h i o n t n t r S e t i n a o l h h n m e e w e t c c e a n t s s F e e g a a n e ( i o d e e n c e a i 1 . t r r c r n m e y e a s d t l t v t e u y e h s o o T o n n a a a n o d o q f u t w l i o r f n l u n 4 n i e a b o d a ) g m u s a S a n k t e o o T T M m m e l s r i F t t o e s d e n e e i s i F F l n a m a t s t - - n t d d i G G p a i H - n r x d e m c i / f t n r n i d d c y i i i a o h m l s o n f s a l o r i e a r l e c p t a f , c o d t u t l h p i a u a s l h e l l a l a c u e a e l m e o n l t u q r n t e a v p e c t s p s d v u h p d n i c e o s i a s i 2 t a e a i o e r a d l s s s l P i o k s r t p l i . t m a h t s i e < o t f s e a n e r s a . t a 3 n g r y x s t i e i n a m ( s g n 1 u d a 9 c B 3 o N m f m r c 8 o 0 n e V o e e r 5 n c m o l s e a k t s ) s w I a l s a t n n o i t t e p u s 0 r o . e n s a s r s . r i r e u 0 o t i n e a a n i ( c 2 m f r d n 1 t e f u p e t i i d 9 t a d v h c r a o r 9 f t i r i l V d o e e a e e e 9 0 u r n d a n o n e ) . a c 0 n t d s t p h e e l 4 A e d o e l a i i r o m s a s r s m f t t s d i h a t 5 i a e i q b c – l d t u e l o e d a n t ( . a o k 1 o 1 t e 5 a 9 a s e 8 k a s s s 6 y r u t i r ) e i n b m b r c s p e a e a i o p d s t o 1 o i e e a r 5 r d l r t y y t a o o t b 0 d s N h l . f i y e i s 1 e p t l p t 7 h y s h r l e o a I a a e m n n c t s t e s a m o e s a u l t m u l f / a n u o f y m f l e d v r r i s c i n e t b a V l h i t i a l e p t a e p h s s n n e d e c e t t p A r r u a d e d d o r r r e n s i i p a o s s n a d c s t n s p g r h a a t i l r h 1 a b e l t e e – h q c d l p t e e e u o ( n r s . 1 m a e o e c 2 9 k s a a e 8 e e e m r r n 6 s . t n p h t ) t t o H q i r s o r m f e e u f o o c p a a 0 w n o o k t . l r e 0 e y e r d v t 4 s e p e a - d a r , r t l l p i l e y e o r t t e S w a a h l i t d t e a d m o e i i h c n t t n i n s t s o t g b r i e e c r u r e p s i e y s l a l e s / V f t s a a a e a m d t r r d s e - o a t m e r t i o p r d i t z l i 3 d e e m . i c t f s

38 n Utah Geological Survey d l i e 0 . t a e g a - i s r e F m ’ o R o 1 c s P m p r . 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I a d 2 n o d P r a k g o r e b r t r 5 t y l o h r e s n h e e t l c e r e e - . o e s n n e n r s 5 l t u f , s t i v i h c a e b c o r 0 e t W f t t a r e e h e r e i l h k m r e n r l e p k o f e e o e e l t d r i r y o e s h n a l 0 r d l a a n r c s c i a k r p u t t m t 3 o r e e o s t - i R c a g n s l - u r i n s u i i T k a k 7 f b e M e i b e a e e v t n g o s g 5 f e a a n n x 2 a a 1 h e l a r c o i d c r u y h d d e p , , G c d s r e I 1 u l C i k k s t m p 1 n e v e o e a a a h a r t r a 2 < a - p a i i n a r r f s d n t o t s u d q r m , e t 6 n c o e l e c u d l u a u p l u u o e o n l n t t d A r e e r o i r p i k u i i a a t t a n e q p n p o c n v . i t n a ’ s a l k n l s e s t l o e h a d e a , y p e a l e e a g t e t s n a t l a t n a l e r o s e u i o m d n r d r 3 i n s a a v E a c t t b h h i e i r i e c o a l a n e n a p P s a p W M t y q R q m u a h t l g e i i t r n a d b h u l u n r e r c a r W t q e e l o a l o r a t M e a a c a C e r e n u f i h e e r s r e s p p k e r k e o k k n o t s > a q d a s r t d e h t r e e r e i r E d o t s l s k n e u l r ~ k i q a s a p ( o s e e m s c e c g t a i 1 1 o d M u r i e n n . 6 d e i y s b n i k t a s 9 7 a i s a g ) G d h g c s . n L c e e m t d 9 6 t s b k ( q l h p k e e C n a u t r e v d s G l 2 e e i w 0 a n o i i u i l a H / d n e c p u k s s a a ) H t e . r r u a . e a w k b . r 0 - r c a o l o 1 m l d f p p b o f r k n o e n i e e a 5 6 n a u t u e T a i a T t e i , a l . g n o t t n m u - o s c n p e t h o w t n h a 0 l 2 e g w a ’ 1 l e e c a d n 7 e r s t e e c e . 0 P e e e 4 . e l t t n 1 R e 2 o f n f y 1 0 k 1 e l d W r V n o a 1 . - a s s m m e t a m 5 3 4 n m 0 a r e a a u 3 a t t a f o o n t ) i e 0 . n u m r e l e p t m l t 5 2 u o t s f r l h k g o r g d o e i r k a 8 n o e l f u a a e u r q m i u m l y c i 1 p e n 5 g - d i n n . t e l h e e H 4 e d l d m 8 i g i t y r n s y c v i a r l v V f p V / a a G S o y t F a n e l t e l e r a r m L i u r t o r o ( e d o c r b t r t w M a a i t n h h a a u n e i a t s c i e c e e n d p m i t s e a g C o t d g e e H h H l e e h a d e S h v n V V s l t i i a p l t o r s F F i f p i r S n n r : . i o a T u a m t T ( i r n o r k i m f a 2 d n > m a c e o i c n e ) t b e h i e n e t t r w e s r e L e ( e 1 v e n 9 e n 9 n e g 2 t a ) t i r n h 1 t r h t e e q E p r u o v a a a r t r k l s t e h s o q n u a k e T i m i n F H H N g a a o o n g r u t s s h s l t e b o / P l a F u r V c t o a h k a m e u l f l r a e o l n t u y n l s S t t f o e a e r c u y c t l i t f o t a i n o u l n t 1 3 2 6 3 9 1 2 2 0 2 7 3 a e t s U W N 2 r o a n . e t o 5 e a u r d n - 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a o i u e e p 1 d 8 e t o . h u r W t h a 0 c d l t i o r G e r e s s t o m h ’ f g G a a s n T i n i e i g . m e e t p i k t l n a s t a r i a 5 a k s o k o r m r e e C i 7 l t p o W c i 1 a n s l W p a M ~ s e - P e l , d c a r n e e e a v f g r e f w e r o e t m a t e o e c r u r t a u r n h n s e e n e s t l i k m e l v d s - C i r b n h a t a r a e t V o a u s u c u o n d n e n e d c n n i b s n u r i o e e t e v , o a r i a i r c s r . d n r i n d a s e n e s m e u l n l p v u e s i a – e c S b r k u e s . a f e e i a l r a o i b o F s w s t p r m l u e o s a P g F u R q G k e f o d 4 H l n N P h i a a g t g p t b i t r . u e N n n e m n i i s i s a q h t t e n t e k s e i h r n t a h o r d f c r t o e i e p o n o f a f r f f m a W m e o s o e f u e r r P t e a g c s e e y s a r h t n a e n t b b i e e l I t e m f l i k i u p e m m d r l t u a s r u i u n s a a r a e v v n d n f a m e d C R o e n t c n s i u u d e e - r a b e t e r n g t p , e t s h r r s m s ) s t s n n a u a a a a e l r e c e e t p c s b k d s e k e d M s a h s s a i o ) t l l a i e e i a s e s 5 o e p t I r ? w c s p 2 n r o h t u n 1 s o t ( w h n o t l i r t a u t a p 1 q t e q l n < e t o e c s - e ( u e n h g u p r n ( e a r o h i r t a 1 v n m m c n s n r o s h e A g k 9 i i a i t e e e e t e F e t 7 a v t n l d d l l c s k v n 8 f a e a e P u 3 u . a l t ) u e o a o e u l l v o r a t o e N a g e l u o f h t g o r r f y e m t p i n q t c l i n n r , r t e H f i s t o a a e n h ’ e l i a e t n v n c s t o a l - y l r b u e a e c n l e e i g e i l r ) n u a r s m a u ) l c . 3 r f t t m n . p a d a a e e l s i 2 - e . f . e u 8 e u n o e s g u m e 9 l e b s s c g h m t s r a w a e s t c h h 9 u i n e a n f y t d a i l o o r a 1 o t h t o d e d r g f i t f ( l i y p D f n f r t T t C e e r h n o n l c o h u a k e t o f s u u 1 a e n a a o m T y r l a 4 . i n l a s s m d f r r e e e m e e e 7 d - u s a n . u B i y r c s m c t c 0 e h a i i e l l . e g s k a t s o t t s P n n a l i s c l t t p n b n o s i h p i e e i v h 0 s o a t d r i u t e f l F e g d k d 0 d p i r i i r a s e u S c P l 1 n v n e e o v c u L a t a s m e l P o d h A n e s ~ N r i a n I i e n g I s h n u t s f u f S i f c T u f i i ( e c r r k f n a i 2 m a e t p c n c d o e ) t p e a s d o t s a a s i b t s – a i l L b e n – e . l o e n n . o e g s t e t h 1 s i m t E i m a a t a e r t t e h q u a k e T I n i m I s n u s i f n u f F S W F i g f c i a u f s a i i g e c h u s a n i h l e S t r t p n i / v n n p F d o o i t p g l r r l a s i a e t d o t n o t s h u g a a s a n i e b s t s l r r – a t f e i l n a f b e a a S n – u . s l u o e l f e e n t l a . t c c o e z u t s o t i l o e t i t n s o n s i e m n t i m a t a e t e 3 3 4 6 0 4 2 1 3 0 3 d s s S i B f I S E i d n e n h t i Z c r i i s r e d a d n g o o a i p n i e t i g h ~ a v r v 2 t u l t t n i l t 2 a i d i g i 2 e r d d f c f e t k c i r 8 - a u r u e c e a a e e s e 0 u a o a d s p m v n + l a ( l r l ; t e h m c A r n s e e t e t n y h r u r s d n a a e l t a e a s e r t r x n f f f c s t o t s a a a i o h i c e m o c t u e c u n q h a i n e a u l x l a l u e b t t y s p - n s m t a s l . s f e i , o d a s w k c s e b s l h u e a i i > x m e t u ( e l s r h t c 1 d p t i d i n a c 9 t L n i l g o v f n 8 i a o a q a u g 2 k e u u t e l ) e a d e l a e v t d r d g f B i n t t a d o h i e o m b o c e c q t y n t u n i u i n b n o m c E a g e e n e a k e . v e r o n A i t l s h f t l e e . d I n s u f f i c i e n t p d o a s t s a i b – l e n . o e s t i m a t e I n s u f f i c i e n t p d o a s t s a i b – l e n . o e s t i m a t e f f o o r c i n o t n f n f o i n o o e t i s e s g u y s c r s o b u e o u , r t c m c a s t s s i i i s i s r i h d e o e d t s s i a t m r a i r e c r t e o h l o f t a h f a t t t h r t t x a u g x t e h n e t w t c i t r t o u t r e r u h r r t o s d o s o p f p r o h e e t t e r o r r g s e n c t n i i n h i s t e i h l . s x n p s d s r t e e o n e r i o n 40 t Utah Geological Survey c a y u e l c t m o t , e t r p o l m n s u P e u u e r s c , s g a l s f s o e e f e a u r r t e d o e s h u p t o c h t n 0 t s e t o i a e e e 1 t r c l f s r ( r c s c c P o a e u n n e n l b h m s s o e f o t e e s u s u n r d r e w s e C i o t r e o n i s c m o i ) e h c v i h a t m t ] i t a n n n t c o h h i e t a r w r r o a p a e e o h f h N m n o ) - o a o t k h f n p ] s f o r o t [ s r o n o t p a i t c g n l n o w i i s y t x g k i x t o t e [ o d o i n e n h e m a n k a i t t h t t s n h o r r e o i t p C t e n c s c t t o u e s r m e i r r n S e s a i e d o o g c . o o e a e s r h f r t l e n p s f p p e a h a e r g e o t j r o e s a w t o e t i d t r r s t g p e p d n d p s a n r n s y i n i n ; a e e n e o i a x i d o s a i e e a r o r c n l s s f . t e r s i a s t . r o a , s n s i j f h s l ; t t b s e m t o i y s S o e i r d l e i f d t e c n s t a u t i e u m a b m h t c h e t e i n c e i n n t m h b t l r o y t p e s e m r b m r m h l o e o r e , g i s l s u o m r l u e n e ) P r s a i h e t l i s n t l a c C s o n l r e l c p r s o m s o e a c e a p y o o n b i l n d u t e r a e i d o c y 3 s a e o e c m n u p e r c s u t e n c e n d 0 e f c n t o o i i y t p d n s y c r r r f e 1 n t c i e i e u t f c r c ( s f 3 e e t e e r P t u , s e a d n u n l d d i t s n m r d a m n f o i r s m l f a o i t e t s l v e e o u o f A u r u r r n C m e s e s s a n c c d l n e e h t e r i a o n o f t n n f r a o f n f o e c o r i f e e i c a a m o t n i n o o C - y s c d a r s l h g a i l f d o n n i t h N a y m l s o r c e i ( a r n i f o a o c i a i ; i . . s g k c s p t u t i o i m a o i 0 C l h h g o m n c m p s s e 5 s u c g n m e w e p e r . i i y r l i o a e - n g g 9 a t , t r i s r k l i e o a f r a s t 1 t h 2 , o n d g s u s o o l p - s e t t i i e y . a f l e t a n t t - a ; l n a e f 2 l a d u e h t n r i a D s m i t m . t 2 s . t u a u m n u i , m m e o p f s h f ; o t e r A a t b d A i a c i c o t a o o s e m p p f h i a e e , i t f o t a r o n t l i t ) m r a f f t m m f h l a f i h r i g e r g i a l e a t g s r e m a o e ( n e s i u i d n t r x r i d l m l r c a m n e p i s c e m n i t u n o b d l o e n o o i i a c t t f e f x o a i g y r u c e n a i n a c a n c x f o e i e r i t N d p o d n r p u r r e s l n c d i ( i r m e o r g u e y a f r l o d t ” s r t e a p r r u t i l a e t n r p i o f s i s p a r a e d e r 0 t o l s a n i p r m o r t f n l f p r . h e 5 e a c d e e s o f c e p o a e t n d i o t e r a s d l e h c v r c 9 r n o e h o o t t a a e i e c t c a r n u i c d e r e i f 1 r f h g a h g ” e m o t i r r y h t t p s a o a d s r n g . a o i h “ g r p ; r u f m p . i t f e t e o n n r ) e a r , s e n o g l s m o f t D a w i l r m h o - 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m t l e r r t i , o g m a e s t g g t t o o f g g i a h n l u d i g o i r r d e f a t d c e a t Z u e e s a c c a i e r s n n f f t t i o e r r n r i i f c d ; e i a c o r f n l W r i F e d e n o n o o o c h u r u i l b r u n a p p n n t h e t e i i e c t o n r t t e m m f a t l o e c f l s f C i m b f o i i l r c o u u a d a r n n r e t t i i k a n e s l a f e o f r k G a l u d c o i o o e t d u o o i l G r u o W n n l c t a i i e i r r m g d o s ” c a t e e i t o s m a m a n r i a o a e o ” f a s s g n a a r l n i k k t f h t g o i e G G e n t u a s u y a f t s s d g u l g r o m v h t d - t i t a r e n a a i i n m r n , h t c l n a i m o u u q o t a c e g g t o n d q o o i u u g i e g m h e h h l e t s r e i t c m c b t i h g e t k o t u n n n h m o i q q - m c i i k i e h u e h s c r s s s n s o g t r r l t r c m v o g g l i i i t i t e c r s i i h h - k k h e a h g l f r i o a e t t o g r m a r r i n e f s h r d t N d o p a a a a e o a f r r e e a r o e f c e f n e r a o o e e u i t a L c u w n r t a a h I w r S I C e L S C e S S W h E 0 1 2 3 S E W W W T E D “ u r ” S E W W T D e 0 2 3 1 u h h 1 2 3 4 5 6 7 8 9 1 1 1 1 1 2 3 4 5 6 7 8 9 1 1 1 1 s t t a s w b d n e l a c t i m r e p t o n o d a t a d e l b . ) a l 3 i 8 a v 9 a 1 Consensus preferred recurrence-interval and vertical slip-rate estimates 41

APPENDIX B

SUMMARY DATA FORMS FOR CONSENSUS RECURRENCE-INTERVAL AND VERTICAL SLIP-RATE ESTIMATES

Wasatch fault zone ...... 42 Brigham City segment ...... 42 Weber segment ...... 45 Salt Lake City segment ...... 49 Provo segment ...... 52 Nephi segment ...... 55 Levan segment ...... 59 Joes Valley fault zone ...... 61 West Valley fault zone ...... 63 West Cache fault zone ...... 66 East Cache fault zone ...... 69 Hurricane fault zone ...... 71 Great Salt Lake fault zone ...... 73 Oquirrh fault zone ...... 76 Southern Oquirrh Mountains fault zone ...... 79 Eastern Bear Lake fault ...... 81 Bear River fault zone ...... 83 Morgan fault zone ...... 85 James Peak fault ...... 87 Towanta Flat graben ...... 89 Bald Mountain fault ...... 91 Strawberry fault ...... 92 Hansel Valley fault ...... 94 Hogsback fault ...... 96 North Promontory fault ...... 98 Sugarville area faults ...... 100 Washington fault zone ...... 101 Fish Springs fault ...... 103 42 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Brigham City segment (BCS), Wasatch fault zone (WFZ), Box Elder County, Utah

Paleoseismic Data Source Documents: Personius, S.F., 1990, Surficial geologic map of the Brigham City segment and adjacent parts of the Weber and Collinston segments, Wasatch fault zone, Box Elder and Weber Counties, Utah: U.S. Geological Survey Miscella- neous Investigations Series Map I-1979, scale 1:50,000. Personius, S.F., 1991, Paleoseismic analysis of the Wasatch fault zone at the Brigham City trench site, Brigham City, Utah, and Pole Patch trench site, Pleasant View, Utah: Utah Geological and Mineral Survey Special Study 76, 39 p. McCalpin, J.P., and Forman, S.L., 1993, Assessing the paleoseismic activity of the Brigham City segment, Wasatch fault zone, Utah - Site of the next major earthquake on the Wasatch Front?, in Jacobson, M.L., compiler, Summaries of Technical Reports, v. XXXIV: U.S. Geological Survey Open-File Report 93-195, p. 485-489. McCalpin, J.P., and Forman, S.L., 2002, Post-Provo paleoearthquake chronology of the Brigham City segment, Wasatch fault zone, Utah: Utah Geological Survey Miscellaneous Publication 02-9, 46 p.

Age of Youngest Faulting: Holocene

Discussion: The BCS is 35.5 km long end-to-end, has a cumulative (trace) length of 40 km (Machette and others, 1992), and is the northernmost segment of the WFZ that exhibits clear evidence of recurrent Holocene faulting along its entire length (Machette and others, 1992). West-facing scarps along the western base of the Wellsville Mountains and Wasatch Range characterize the BCS along most of its length. Scarps on the valley floor between Willard and Brigham City may be associated with incipient lateral spreads, but have orientations and relief consistent with a faulting origin. In the southern part of the segment, 15- to 20-m-high scarps formed on a Provo-level delta suggest that as many as 6-10 surface- faulting earthquakes may have occurred since about 16 ka (assuming an average displacement per event of 2+ m). Only a few short, discontinuous scarps are in upper Holocene deposits near the southern segment boundary, which is in contrast to the abundance of Holocene scarps on the Weber segment (WS) to the south. The northern boundary of the BCS with the Collinston segment (CS) is defined by a change in fault trend, differences in displacement of similar aged pre- Bonneville deposits, and a lack of geomorphic evidence for Holocene faulting on most of the CS (Machette and others, 1992).

Earthquake Timing Earthquake timing for the BCS is based on the results from two paleoseismic-trenching studies; Personius (1991) and McCalpin and Forman (1993, 2002). The two trench sites are only a few kilometers apart on the central part of the segment, but both studies identified a surface-faulting earthquake that was not recognized at the other site. Event Z was only identified by McCalpin and Forman (1993, 2002), and event X was only identified by Personius (1991). Both paleoearthquakes are well documented at their respective trench sites and therefore were included in the composite surface-faulting chronology for the BCS prepared by McCalpin and Nishenko (1996) and reported in McCalpin and For- man (2002). Additionally, although the interevent interval between them is comparatively short and their associated confidence limits are large and overlap, McCalpin and Forman (2002) concluded that events V and U are separate, dis- cernable paleoearthquakes.

The Working Group determined earthquake timing on the BCS by calculating a simple mean for each earthquake using the earthquake-limiting, calendar-calibrated radiocarbon (14C) ages and thermoluminescence (TL) ages presented in McCalpin and Nishenko (1996, table 1) and rounding the results to the nearest half-century. The ± confidence limits have been increased beyond those reported in McCalpin and Nishenko (1996, table 3) to accommodate the full range of limiting 14C and TL ages used to constrain earthquake timing as described in the section on Earthquake Timing in the accompanying report. The Working Group believes that the resulting ± confidence limits account for both epistemic and aleatory uncertainty associated with the timing of each earthquake.

Z 2100±800 cal yr B.P. Y 3450±300 cal yr B.P. X 4650±500 cal yr B.P. Consensus preferred recurrence-interval and vertical slip-rate estimates 43

W 5950±250 cal yr B.P. V 7500±1000 cal yr B.P. U 8500±1500 cal yr B.P. T >14,800±1200, <17,000 cal yr B.P.

Surface-Faulting Recurrence: Interevent recurrence intervals with associated confidence limits are:

Elapsed time since event Z 2100±800 cal yr Y-Z interval 1350±9001 cal yr X-Y interval 1200±600 cal yr W-X interval 1300±600 cal yr V-W interval 1550±1000 cal yr U-V interval 1000±1800 cal yr T-U interval ~3.6-10 kyr

The weighted mean recurrence for the three most recent interevent intervals (X-Z) rounded to the nearest century is:

13002±2003 cal yr

The weighted mean recurrence for the five most recent interevent intervals (V-Z) rounded to the nearest century is:

13002±4003 cal yr

1±confidence limits equal the square root of the sum of the squares of the individual ± confidence limits for each bracketing earthquake rounded to nearest 100 years. 2weighted mean rounded to nearest 100 years. 32-sigma confidence limits rounded to nearest 100 years.

The elapsed time since event Z exceeds the length of both mean-recurrence intervals. Additionally, a possible 3.6 to 10-kyr gap between events T and U indicates an extended period of seismic quiescence on the BCS in latest Pleis- tocene and early Holocene time. However, McCalpin and Forman (2002) indicate that an unknown number of unrecognized sublacustrine earthquakes may have occurred in the interval between events U and T.

Based on currently available information on earthquake timing and variability in the length of individual interevent intervals, the Working Group’s preferred recurrence-interval estimate and confidence limits for the BCS are:

500-1300-2800 yr

Vertical Slip Rate: Slip-rate information for the BCS is sparsely distributed along the segment. Combining the length of the interevent intervals above with per-event displacements from Personius (1991) results in the following vertical slip rates at the Per- sonius (1991) trench site:

X-Y (1 m/1200±500 yr) = 0.6-0.8-1.4 mm/yr W-X (2.5 m/1300±500 yr) = 1.4-1.9-3.1 mm/yr V-W (2.5 m/1550±1000 yr) = 1.0-1.6- 4.5 mm/yr

Long-term vertical slip rates for the BCS from scarp-profile measurements (Personius, 1990) vary depending on the deposit age and profile location along the segment trace. Slip rates recorded in Provo-age deposits range from a low of 0.24 mm/yr near the north segment boundary south of Two Jump Canyon to 1.36 mm/yr south of Pearsons Canyon near Willard. Bonneville-cycle delta deposits at the mouth of Willard Canyon record a vertical slip rate of 1.5-1.6 mm/yr.

Based on currently available information on earthquake timing and displacement, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the BCS are:

0.6-1.4-4.5 mm/yr

Summary: For the past five interevent intervals (six surface-faulting earthquakes) on the BCS, surface faulting mean recurrence has been approximately 1300±400 years and 1300±200 years for the three most recent, and better constrained, interevent 44 Utah Geological Survey intervals. The elapsed time since event Z exceeds both recurrence intervals. The BCS experienced a possible long period (~3.6-10 kyr) of seismic quiescence during the latest Pleistocene and early Holocene, although McCalpin, (2002) states that unrecognized sublacustrine earthquakes may fill part of the gap. These two possible extended periods with no surface faulting indicate that the long-term record of paleoearthquake recurrence on the BCS is likely irregular.

Additional References: Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone – A summary of recent investigations, interpretations, and conclusions, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-A, p. A1-A71. McCalpin, J.P., and Nishenko, S.P., 1996, Holocene paleoseismicity, temporal clustering, and probabilities of future large (M>7) earthquakes on the Wasatch fault zone: Journal of Geophysical Research, v. 101, no. B3, p. 6233-6253. Consensus preferred recurrence-interval and vertical slip-rate estimates 45

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Weber segment (WS), Wasatch fault zone (WFZ), Weber and Davis Counties, Utah

Paleoseismic Data Source Documents: 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, no. 5, p. 1431-1462. Swan, F.H., III, Schwartz, D.P., Hanson, K.L., Knuepfer, P.L., and Cluff, L.S., 1981b, Study of earthquake recurrence intervals on the Wasatch fault at the Kaysville site, Utah: U.S. Geological Survey Open-File Report 81-228, 30 p. Nelson, A.R., Klauk, R.H., Lowe, M., and Garr, J.D., 1987, Holocene history of displacement on the Weber segment of the Wasatch fault zone at Ogden, northern Utah [abs.]: Geological Society of America Abstracts with Programs, v. 19, no. 5, p. 322. Nelson, A.R., 1988, The northern part of the Weber segment of the Wasatch fault zone near Ogden, Utah, in Machette, M.N., editor, In the footsteps of G.K. Gilbert - Lake Bonneville and neotectonics of the eastern Basin and Range Province, Guidebook for Field Trip Twelve: Utah Geological and Mineral Survey Miscellaneous Publication 88-1, p. 33-37. Forman, S.L., Nelson, A.R., and McCalpin, J.P., 1991, Thermoluminescence dating of fault-scarp-derived colluvium - Deciphering the timing of paleoearthquakes on the Weber segment of the Wasatch fault zone, north-central Utah: Journal of Geophysical Research, v. 96, no. B1, p. 595-605. Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone - A summary of recent investigations, interpretations, and conclusions, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-A, p. A1-A71. Nelson, A.R., and Personius, S.F., 1993, Surficial geologic map of the Weber segment, Wasatch fault zone, Weber and Davis Counties, Utah: U.S. Geological Survey Miscellaneous Investigations Series Map I-2199, 22 p. pamphlet, scale 1:50,000. McCalpin, J.P., Forman, S.L., and Lowe, M., 1994, Reevaluation of Holocene faulting at the Kaysville site, Weber segment of the Wasatch fault zone, Utah: Tectonics, v. 13, no. 1, p. 1-16.

Age of Youngest Faulting: Holocene

Discussion: West-facing scarps along the western base of the Wasatch Range characterize the WS, the second-longest segment (56 km end-to-end; 61 km cumulative trace) of the WFZ (Machette and others, 1992). The southern segment boundary is a prominent bedrock salient (Salt Lake salient) where the WFZ makes a 2 km right step to the Warm Springs fault at the north end of the Salt Lake City segment (SLCS), but fault scarps in that area are easily confused with nontectonic features and scarp identification and distribution is less certain (Machette and others, 1992). The northern segment boundary is at another bedrock salient (Pleasant View salient). Scarp heights along the northern part of the WS suggest a high rate of late Holocene surface faulting (Machette and others, 1992).

Earthquake Timing: Information about surface-faulting recurrence on the WS comes from three trench sites: Garner Canyon (Nelson and others, 1987; Nelson, 1988; Machette and others, 1992), East Ogden (Nelson and others, 1987; Nelson, 1988; Forman and others, 1991; Machette and others, 1992), and Kaysville, where investigators performed two independent trenching studies more than a decade apart (Swan and others, 1980, 1981b; McCalpin and others, 1994). Study results from East Ogden and the first investigation at Kaysville both identified two surface-faulting earthquakes post ~1.5 ka. The second investigation at Kaysville identified the most recent surface-faulting earthquake (MRE) at 0.6-0.8 ka and the second-oldest event (penultimate event [PE]) at 2.8±0.7 ka, and discounted the occurrence of a second earthquake younger than 1.5 ka at either Kaysville or East Ogden.

McCalpin and Nishenko (1996) report four surface-faulting earthquakes for the WS since mid-Holocene time (past ~6 kyr) based on their reevaluation of earthquake-limiting 14C and TL ages from the Garner Canyon, East Ogden, and Kaysville trench sites. As part of their reevaluation, they also discounted the very young (~0.5 ka) paleoearthquakes 46 Utah Geological Survey previously identified at East Ogden and Kaysville. However, the investigators at East Ogden and for the first Kaysville study remain confident in their results, and believe the WS, or portions of it, have experienced two earthquakes within the past ≤1.5 kyr. Following their review of the available paleoseismic information for the East Ogden and Kaysville sites, the Working Group concluded that a fifth (youngest) earthquake on the WS at about 0.5±0.3 ka is possible.

Evidence for a fifth earthquake is limited and indicates that it was small (≤0.6 m displacement) and therefore possibly did not rupture the entire 61-km-long WS. This raises questions regarding whether the young earthquakes at East Ogden and Kaysville are the same or different, and therefore the possibility of partial segment rupture on the WS. Nel- son and Personius (1993) and McCalpin and others (1994) both discussed the possibility of partial segment rupture either from small, non-characteristic earthquakes on the WS, or from overlap of surface rupture from a large earthquake on an adjoining segment.

Based upon currently available paleoseismic information for the WS, the Working Group’s consensus surface-faulting chronology for the WS is:

Za 0.5±0.3 ka (partial segment rupture?) Zb 950±450 cal yr B.P. Y 3000±700 cal yr B.P. X 4500±700 cal yr B.P. W 6100±700 cal yr B.P.

The Working Group determined the timing of earthquakes W, X, Y, and Zb from the earthquake-limiting, calendar- calibrated 14C ages and TL ages presented in McCalpin and Nishenko (1996, table 1), by calculating a simple mean for each earthquake and rounding the results to the nearest half-century. The ± confidence limits have been increased beyond those reported in McCalpin and Nishenko (1996, table 3) to accommodate the full range of limiting 14C and TL ages used to constrain earthquake timing as described in the section on Earthquake Timing in the accompanying report. The Working Group believes the resulting ± confidence limits account for both epistemic and aleatory uncertainty associated with the timing of each earthquake. The timing and associated confidence limits for event Za are from Machette and others (1992).

Surface-Faulting Recurrence: The two presently proposed models for surface-faulting recurrence on the WS allow for either three or four interevent intervals since the middle Holocene. The McCalpin and Nishenko (1996) four-earthquake model permits three interevent intervals: Y-Zb interval 2050±8001 cal yr X-Y interval 1500±1000 cal yr W-X interval 1600±1000 cal yr

The weighted mean recurrence for the three most recent interevent intervals (X-Zb) rounded to the nearest century is:

16002+6003 cal yr

The five-earthquake model proposed by Swan and others (1980, 1981b) and Machette and others (1992), creates a fourth interevent interval of about 450 years.

Zb-Za 450±5001 cal yr Y-Zb interval 2050±800 cal yr X-Y interval 1500±1000 cal yr W-X interval 1600±1000 cal yr

The weighted mean recurrence for the four most recent interevent intervals (X-Za) rounded to the nearest century is:

11002±14003 cal yr 1±confidence limits equal the square root of the sum of the squares of the individual ± confidence limits for each bracketing earthquake rounded to nearest 100 years. 2Weighted mean rounded to nearest 100 years. 32-sigma confidence limits rounded to nearest 100 years.

500-1400-2400 yr Consensus preferred recurrence-interval and vertical slip-rate estimates 47

Vertical Slip Rate: Net vertical-slip data per earthquake from trench sites on the WS are generally poorly constrained (not all scarps trenched) or are estimates based on stratigraphic and structural relations in trenches (no direct measurement of displaced strata across the fault). Swan and others (1981b) estimated minima of 1.7 and 1.8 m of displacement for events Y and Za, respectively, at Kaysville. McCalpin and others (1994) reported 1.7-1.9 m for event Zb, 2.3-3.4 m for event Y, and 1.4 m for event X.

Using the interevent intervals above, and the net vertical-slip measurements of McCalpin and others (1994), results in the following interevent slip rates at Kaysville:

Y-Zb (1.7-1.9 m/2050±700 yr) = 0.6-0.9-1.4 mm/yr X-Y (2.3-3.4 m/1500±700 yr) = 1.0-1.9-4.3 mm/yr W-X (1.4 m/1600±700 yr) = 0.6-0.9-1.6 mm/yr

Net vertical displacement across the main scarp at Kaysville is 11.5 m in middle Holocene to uppermost Pleistocene “Fan alluvium 2” (Nelson and Personius, 1993). Swan and others (1981b) estimated the fan age at 6±2 kyr; McCalpin and others (1994) estimated the age as post-Provo (≤16.2 ka). If the fan is ~ 6 kyr old, the mid-Holocene to present slip rate at the Kaysville site would be 1.9+1.0/-0.5 mm/yr. If the slip occurred in post-Provo time (≤16.2 ka), the late Pleis- tocene to present slip rate would be ≥0.7 mm/yr, depending on the actual fan age.

A.R. Nelson (U.S. Geological Survey, written communication to Working Group, 2003) reports a minimum vertical displacement of Bonneville-Provo-phase deposits at the East Ogden trench site of 23.7 m. Assuming a maximum age for the Bonneville-phase deposits of 18 ka results in a long-term, minimum slip rate at East Ogden of 1.3 mm/yr.

Nelson and Personius (1993) measured 375 scarp profiles (77 measured in the field and 298 measured from aerial photos using a photogrammetric plotter) along the WS. They estimate that approximately 15% of the 375 profiles gave surface-displacement values that were within ±10-30% of the total net vertical displacement across the fault zone. How- ever, uncertainties in estimating the age of the deposits, particularly Holocene deposits, in which many profiles were measured, could be ±50-100%. Additionally, the Lake Bonneville chronology used to constrain the age of lacustrine deposits for the slip-rate calculations is dated. Presently reported calendar-calibrated ages for the Bonneville and Provo shorelines (D.L. Currey, University of Utah, written communication to the UGS, 1996; verbal communication to Work- ing Group, 2004) are ≤4.5 kyr older than those used by Nelson and Personius (1993).

Despite the limitations of the scarp-profile data, the slip rates reported by Nelson and Personius (1993) clearly demonstrate that latest Pleistocene and Holocene slip is greatest along the central WS and decreases toward the segment ends.

Using an updated Lake Bonneville chronology and net-vertical-displacement values from Nelson and Personius (1993) measured in Lake Bonneville lacustrine deposits and alluvial-fan deposits graded to Lake Bonneville shorelines, provides scattered point data along the segment that indicates that slip rates recorded in Bonneville-phase deposits are as high as 2.0 mm/yr, and up to 1.3 mm/yr in Provo-phase deposits.

Based on currently available information on earthquake timing and displacement, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the WS are:

0.6-1.2-4.3 mm/yr Summary: Although one of the most intensively studied of the six central, active segments of the WFZ, significant questions remain regarding the paleoseismic history of the WS. These questions limit the Working Group’s ability to establish closely constrained recurrence-interval and slip-rate values for the WS. Chief among the questions are:

1. Has the MRE on the WS been accurately identified – is event Za real?

2. Is the MRE different on different parts of the WS; is the WS subject to partial segment rupture, and if so, on which parts of the segment and how often?

3. What is the long-term (past ~18 ka) history of surface faulting on the WS? Present information on surface-faulting recurrence only extends to the middle Holocene; the number and timing of earlier surface-faulting earthquakes is unknown, as is any corresponding variability in long-term recurrence and slip similar to that reported for the adjacent BCS and SLCS. 48 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Salt Lake City segment (SLCS), Wasatch fault zone (WFZ), Salt Lake County, Utah

Paleoseismic Data Source Documents: Swan, F.H., III, Hanson, K.L., Schwartz, D.P., and Knuepfer, P.L., 1981a, Study of earthquake recurrence intervals on the Wasatch fault, Utah - Little Cottonwood Canyon site: U.S. Geological Survey Open-File Report 81-450, 30 p. Hanson, K.L., Swan, F.H., III, and Schwartz, D.P., 1982, Study of earthquake recurrence intervals on the Wasatch fault, Utah: San Francisco, Woodward-Clyde Consultants, Seventh Semi-Annual Technical Report prepared for the U.S. Geological Survey, contract no. 14-08-0001-19842, 26 p. Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and characteristic earthquakes - Examples from the Wasatch and San Andreas fault zones: Journal of Geophysical Research, v. 89, no. B7, p. 5681-5698. Schwartz, D.P., and Lund, W.R., 1988, Paleoseismicity and earthquake recurrence at Little Cottonwood Canyon, Wasatch fault zone, Utah, in Machette, M.N., editor, In the footsteps of G.K. Gilbert - Lake Bonneville and neotectonics of the eastern Basin and Range Province, Guidebook for Field Trip Twelve: Utah Geological and Mineral Survey Miscellaneous Publication 88-1, p. 82-85. Robison, R.M., and Burr, T.N., 1991, Fault-rupture hazard analysis using trenching and borings - Warm Springs fault, Salt Lake City, Utah, in McCalpin, J.P., editor, Proceedings of the 27th Symposium on Engineering Geology and Geotechnical Engineering: Boise, Idaho Department of Transportation, p. 26-1 - 26-13. Lund, W.R., 1992, New information on the timing of earthquakes on the Salt Lake City segment of the Wasatch fault zone - Implications for increased earthquake hazard along the central Wasatch Front: Utah Geological Survey, Wasatch Front Forum, v. 8, no. 3, p. 12-13. Personius, S.F., and Scott, W.E., 1992, Surficial geology of the Salt Lake City segment and parts of adjacent segments of the Wasatch fault zone, Davis, Salt Lake, and Utah Counties, Utah: U.S. Geological Survey Miscellaneous Inves- tigation Series Map I-2106, scale 1:50,000. Black, B.D., Lund, W.R., Schwartz, D.P., Gill, H.E., and Mayes, B.H., 1996, Paleoseismic investigation on the Salt Lake City segment of the Wasatch fault zone at the South Fork Dry Creek and Dry Gulch sites, Salt Lake County, Utah: Utah Geological Survey Special Study 92, 22 p. Korbay, S.R., and McCormick, W.V., 1999, Faults, lateral spreading, and liquefaction features, Salt Palace Convention Center, Salt Lake City [abs.]: Association of Engineering Geologists, 42nd Annual Meeting Program with Abstracts, p. 73. Simon, D.B., and Shlemon, R.J., 1999, The Holocene “Downtown Fault” in Salt Lake City, Utah [abs.]: Association of Engineering Geologists, 42nd Annual Meeting Program with Abstracts, p. 85. McCalpin, J.P., and Nelson, C.V, 2000, Long recurrence records from the Wasatch fault zone, Utah: U.S. Geological Survey, National Earthquake Hazards Reduction Program Final Technical Report, contract no. 99HQGR0058, 61 p. McCalpin, J.P., 2002, Post-Bonneville paleoearthquake chronology of the Salt Lake City segment, Wasatch fault zone, from the 1999 “Megatrench” site: Utah Geological Survey Miscellaneous Publication 02-7, 37 p.

Age of Youngest Faulting: Holocene

Discussion: From north to south the SLCS is divided into three en echelon subsections: the Warm Springs fault, East Bench fault, and Cottonwood subsection (Personius and Scott, 1992). The Warm Springs fault forms a prominent escarpment for about 7 km along the western flank of the Salt Lake salient and then trends south into basin fill and dies out beneath downtown Salt Lake City. At the south end of the Warm Springs fault, the SLCS steps 2 km to the east to the East Bench fault. The East Bench fault forms prominent northwest- to southwest-facing intrabasin fault scarps that generally parallel 1100 East Street and Highland Drive from Salt Lake City south to Big Cottonwood Creek. The Cottonwood subsection forms a prominent (often wide and complex) zone of faulting along the range front from just north of Big Cot- tonwood Canyon to the Traverse Mountains. At the mouth of Little Cottonwood Canyon, the fault zone forms a complex 50-m-wide graben with main scarps as high as 25 m and antithetic scarps as high as 20 m. Farther south at South Fork Dry Creek, the graben is 400 m wide, and six en echelon scarps comprise the main fault zone.

Earthquake Timing: All surface-faulting recurrence and slip-rate information for the SLCS comes from the Little Cottonwood and South 50 Utah Geological Survey

Fork Dry Creek/Dry Gulch trench sites near the south end of the Cottonwood subsection in the southeastern part of Salt Lake Valley.

Following a review of the paleoseismic information available from those two sites, the Working Group’s consensus for the timing of surface faulting on the SLCS is:

Z 1300±650 cal yr B.P. Y 2450±550 cal yr B.P. X 3950±550 cal yr B.P. W 5300±750 cal yr B.P. V ~7.5 ka (after 8.8-9.1 ka but before 5.1-5.3 ka) U ~9 ka (shortly after 9.5-9.9 ka) T ~17 ka S(?) 17-20 ka

Timing for earthquakes W, X, Y, and Z is from Black and others (1996). The ± confidence limits have been increased for each earthquake to accommodate the full range of limiting 14C ages used to constrain the timing of the earthquakes as described in the section on Earthquake Timing in the accompanying report. The Working Group believes that the resulting ± confidence limits account for both the epistemic and aleatory uncertainty associated with the timing of each earthquake.

McCalpin (2002) identified earthquakes V, U, T, and S based on a retrodeformation analysis of stratigraphic and structural relations exposed in a trench at Little Cottonwood Canyon. No direct evidence (colluvial wedges, tectonic crack fills, fault terminations, etc.) was found to document these earthquakes, and consequently their timing can only be broadly constrained.

Surface-Faulting Recurrence: Interevent recurrence intervals with associated confidence limits for the SLCS are:

Elapsed time since event Z 1300±650 cal yr Y-Z interval 1150±9001 cal yr X-Y interval 1500±800 cal yr W-X interval 1350±900 cal yr

The weighted mean recurrence for the three most recent interevent intervals (W-Z) rounded to the nearest century is:

13002±4003 cal yr

1±confidence limits equal the square root of the sum of the squares of the individual ± confidence limits for each bracketing earthquake rounded to nearest 100 years. 2Weighted mean rounded to nearest 100 years. 32-sigma confidence limits rounded to nearest 100 years.

The timing of earthquakes U and V on the SLCS is broadly constrained. McCalpin (2002) reports the U-V and V- W interevent intervals are both about 2 kyr, resulting in a similarly broadly constrained mean recurrence for surface faulting of 2 kyr for mid- to early Holocene time.

The timing of event T is likewise broadly constrained. McCalpin (2002) reports a range in the interevent interval between events T and U of 7.1 to 9.6 kyr, with a mean of 8.4 kyr, indicating a long period of surface-faulting quiescence during earliest Holocene and latest Pleistocene time on the SLCS, similar to that noted on the Brigham City segment to the north (McCalpin and Forman, 2002). However, McCalpin (2002) noted, but considered the possibility unlikely, that the physical evidence of additional earthquakes in the gap has been removed by alluvial-fan erosion in the interval 9-10 ka and is lost from the stratigraphic record.

500-1300-2400 yr Vertical Slip Rate: Trenching investigations at Little Cottonwood Canyon and at South Fork Dry Creek/Dry Gulch did not provide Consensus preferred recurrence-interval and vertical slip-rate estimates 51 well-constrained net vertical-slip data for the SLCS. Swan and others (1981a) reported 14.5 (+10/-3) m of net vertical displacement across the WFZ determined from a scarp profile measured along the crest of the Bells Canyon glacial moraine a few hundred meters south of the Little Cottonwood Canyon site. Scott (1989) reports the age of the moraine as 18-26 ka. The resulting slip rate is:

(14.5 [+10/-3] m /18-26 kyr): 0.4-0.7-1.4 mm/yr

Complex rupture patterns, poorly constrained ages of faulted mixed lacustrine units, and difficulties associated with measuring topographic scarp profiles in a densely urbanized environment prevented using the net vertical-slip information reported by Personius and Scott (1992) to calculate additional late Pleistocene/early Holocene slip rates along the SLCS.

The slip rate reported for Bells Canyon (Little Cottonwood Canyon site) is a long-term rate extending from the latest Pleistocene. Well-constrained Holocene vertical slip-rate data are lacking for the SLCS. Therefore, the Working Group considered the slip data on the Weber segment to the north and the Provo segment to the south when arriving at a consensus Holocene slip-rate estimate for the SLCS. Because the Bells Canyon long-term slip-rate estimate includes a possible period of seismic quiescence on the SLSC in the late Pleistocene, the Working Group’s estimate for the Holocene is higher than the longer term rate at Bells Canyon.

Based on currently available information on earthquake timing and displacement, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the SLCS are:

0.6-1.2-4.0 mm/yr

Summary: Reliable paleoseismic-trenching data for the SLCS are limited to the Cottonwood subsection. Sites of opportunity along the more heavily urbanized Warm Springs and East Bench subsections have not produced reliable recurrence, net vertical-displacement, or vertical slip-rate information.

Investigators have identified four surface-faulting earthquakes on the SLCS since mid-Holocene time (~5.3 ka) on the basis of colluvial-wedge stratigraphy, tectonic crack fills, and fault terminations. The timing of these earthquakes is based on 14C ages on both charcoal and bulk organic samples (primarily paleosol A horizons and tectonic crack-fill deposits). The Working Group considers both the number of earthquakes and their timing well constrained, as is the resulting mid-Holocene mean recurrence between surface-faulting earthquakes of 1300±400 years.

Three older earthquakes (T, U, V) identified based on a retrodeformation analysis lack direct stratigraphic or structural evidence (colluvial wedges, crack-fill deposits, fault terminations) of their occurrence. The Working Group considers the evidence for these earthquakes compelling; however, event timing is only broadly constrained. Available data indicate two recurrence intervals of about 2 kyr in mid- to early Holocene time, preceded by a >7 kyr period of surface- faulting quiescence.

The elapsed time since the MRE (event Z) on the SLCS is equal to or greater than the Working Group’s preferred recurrence-interval estimate (500-1300-2400 yr) for the segment.

Despite being one of the most intensely studied of the six central segments of the WFZ, well-constrained vertical slip-rate information for the SLCS is limited to a single latest Pleistocene slip rate at the mouth of Bells Canyon on the Cottonwood subsection.

Additional References: McCalpin, J.P., and Forman, S.L., 2002, Post-Provo paleoearthquake chronology of the Brigham City segment, Wasatch fault zone, Utah: Utah Geological Survey Miscellaneous Publication 02-9, 46 p. Scott, W.E., 1989, Temporal relations of lacustrine and glacial events at Little Cottonwood and Bells Canyons, in Machette, M.N., editor, In the footsteps of G.K. Gilbert - Lake Bonneville and neotectonics of the eastern Basin and Range Province: Utah Geological and Mineral Survey Miscellaneous Publication 88-1, p. 78-81. 52 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES Fault/Fault Section: Provo segment (PS), Wasatch fault zone (WFZ), Utah County, Utah

Paleoseismic Data Source Documents: 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, no. 5, p. 1431-1462. Machette, M.N., and Lund, W.R., 1987, Trenching across the American Fork segment of the Wasatch fault zone, Utah [abs.]: Geological Society of America Abstracts with Programs, v. 19, no. 5, p. 317. Machette, M.N., 1988, American Fork Canyon, Utah – Holocene faulting, the Bonneville fan-delta complex, and evidence for the Keg Mountain oscillation, in Machette, M.N., editor, In the footsteps of G.K. Gilbert – Lake Bon- neville and neotectonics of the eastern Basin and Range: Utah Geological and Mineral Survey Miscellaneous Pub- lication 88-1, p. 89-95. Lund, W.R., Black, B.D., and Schwartz, D.P., 1990, Late Holocene displacement on the Provo segment of the Wasatch fault zone at Rock Canyon, Utah County, Utah [abs.]: Geological Society of America Abstracts with Programs, v. 22, no. 6, p. 37. Ostenaa, D., 1990, Late Holocene displacement history, Water Canyon site, Wasatch fault zone [abs.]: Geological Soci- ety of America Abstracts with Programs, v. 22, no. 6, p. 42. Lund, W.R., Schwartz, D.P., Mulvey, W.E., Budding, K.E., and Black, B.D., 1991, Fault behavior and earthquake recurrence on the Provo segment of the Wasatch fault zone at Mapleton, Utah County, Utah: Utah Geological and Min- eral Survey Special Study 75, 41 p. Machette, M.N., 1992, Surficial geologic map of the Wasatch fault zone, eastern Utah Valley, Utah County and parts of Salt Lake and Juab Counties, Utah: U.S. Geological Survey Miscellaneous Investigations Series Map I-2095, scale 1:50,000. Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone - A summary of recent investigations, interpretations, and conclusions, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-A, p. A1-A71. Lund, W.R., and Black, B.D., 1998, Paleoseismic investigation at Rock Canyon, Provo segment, Wasatch fault zone, Utah County, Utah: Utah Geological Survey Special Study 93, 21 p. Olig, S., McDonald, G., Black, B., DuRoss, C., and Lund, B., 2004, The Mapleton “Megatrench” – Deciphering 11,000 years of earthquake history on the Wasatch fault near Provo: Utah Geological Survey, Survey Notes, v. 36, no. 2, p. 4-6.

Age of Youngest Faulting: Holocene

Discussion: Machette and others (1992) tentatively subdivided the PS (as originally proposed by Schwartz and Coppersmith, 1984) into three subsections based on fault geometry and apparent recency of movement as indicated by scarp morphology: from north to south, they are the American Fork, Provo "restricted sense," and Spanish Fork subsections. The American Fork subsection has a 22.5-km-long trace length and ends on the south at a 2-km left step in the fault at the Provo River. The Provo “restricted sense” subsection extends from Provo Canyon south to Springville, Utah, a trace length of 18.5 km and includes the Springville fault, which continues into Utah Valley after the main WFZ makes a large southeast bend. The Spanish Fork subsection forms a major concave-to-the-west bend in the WFZ and extends 31.5 km along trace from Springville to Payson Canyon at the south end of Utah Valley. However, despite the distinctive geometry of the fault trace in Utah Valley, paleoseismic studies (Machette, 1988; Lund and others, 1991; Machette and others, 1992; Lund and Black, 1998) show that the entire length of the PS ruptured during at least the two most recent surface-faulting earthquakes.

Earthquake Timing: Five trenching studies (from north to south: American Fork Canyon, Rock Canyon, Hobble Creek, Mapleton, and Water Canyon) provide paleoseismic-trenching information for the PS. A sixth, cooperative study between URS Cor- poration and the Utah Geological Survey reoccupied the Mapleton North trench site in 2003 to extend the record of surface faulting on the PS beyond the three most recent earthquakes. The “megatrench” at Mapleton exposed evidence of older surface-faulting earthquakes (Olig and others, 2004); however, final results of that study are not yet available. Consensus preferred recurrence-interval and vertical slip-rate estimates 53

At American Fork Canyon, Machette and others (1992) identified three surface-faulting earthquakes at 500±200, 2650±250, and 5300±300 cal yr B.P. At Rock Canyon, Lund and Black (1998) identified the timing of the MRE at 650+50/-100 cal yr B.P. In an early study at Hobble Creek, Swan and others (1980) identified six to seven surface-faulting earthquakes based on a combination of strath terraces inset along streams on the upthrown side of the fault and colluvial wedges in trenches. However, no numerical ages were obtained as part of this study, so earthquake timing is only constrained to broad intervals. Lund and others (1991) determined the timing of the two most recent earthquakes at Mapleton as 600±80 and 2820+150/-130 cal yr B.P., respectively. Within the bounds of geologic uncertainty, the MRE (event Z) appears to have occurred at the same time at the American Fork Canyon, Rock Canyon, and Mapleton sites, as does the PE (event Y) at the American Fork Canyon and Mapleton sites. The trench and stream-cut exposure investigated at Rock Canyon were not deep enough to uncover evidence of the PE.

At Water Canyon near the boundary between the PS and Nephi segment (NS), Ostenaa (1990) identified 3 or more Holocene surface-faulting earthquakes. The timing of some Water Canyon earthquakes does not correlate well with earthquakes identified farther north on the PS. Ostenaa (1990) attributed this discrepancy to Water Canyon’s location close to the southern segment boundary, and the possibility that the Water Canyon site records surface faulting from both the PS and adjacent NS.

McCalpin and Nishenko (1996) reported preferred times for the three most recent surface-faulting earthquakes on the PS (rounded to the nearest half-century) of 600, 2850, and 5500 cal yr B.P. Their reevaluation of earthquake-limiting 14C and TL ages for the PS included five Water Canyon 14C ages, three of which they assigned to event Z and one of which they assigned to event X. They assigned the remaining Water Canyon ages to surface-faulting earthquakes on the NS.

Following their review of available paleoseismic information, the Working Group’s consensus for surface-faulting timing on the PS is:

Z 600±350 cal yr B.P. Y 2850±650 cal yr B.P. X 5300±300 cal yr B.P.

The Working Group determined the timing of events Y and Z by calculating a simple mean for each earthquake using the earthquake-limiting 14C ages and TL ages presented in McCalpin and Nishenko (1996, table 1) and rounding the results to the nearest half-century. The ± confidence limits have been increased beyond those reported in McCalpin and Nishenko (1996, table 3) to accommodate the full range of limiting 14C and TL ages used to constrain earthquake timing as described in the section on Earthquake Timing in the accompanying report. The Working Group believes that the resulting ± confidence limits account for both epistemic and aleatory uncertainty associated with the timing of each earthquake.

Event X was obtained from Machette and others (1992), who presented a comprehensive analysis of uncertainty affecting the timing of that earthquake.

Surface-Faulting Recurrence: Interevent recurrence intervals with associated confidence limits are:

Elapsed time since event Z 600±350 cal yr Y-Z interval 2250±7001 cal yr X-Y interval 2450±700 cal yr

The weighted mean recurrence for the two most recent interevent intervals (X-Z) rounded to the nearest century is:

24002±3003 cal yr

1±confidence limits equal the square root of the sum of the squares of the individual ± confidence limits for each bracketing earthquake rounded to nearest 100 years. 2 Weighted mean rounded to nearest 100 years. 32-sigma confidence limits rounded to nearest 100 years.

1200-2400-3200 yr 54 Utah Geological Survey

However, preliminary results from new trenching at Mapleton indicate that the interval from the middle Holocene to latest Pleistocene may include several surface-faulting earthquakes (Olig and others, 2004). If so, the Working Group’s preferred recurrence-interval estimate for the PS will likely require future modification.

Vertical Slip-Rate: Vertical slip-rate information for the PS is widely scattered along the segment trace, and is summarized below using the revised Lake Bonneville chronology of Currey (University of Utah Geography Department, written communication to the UGS, 1996; verbal communication to Working Group, 2004).

Hobble Creek1 Post-Provo time: 11.5-13.5 m/16.2-16.8 kyr = 0.68-0.76-0.83 mm/yr. Post-Bonneville time: 40-45 m/16.8-18.0 kyr = 2.2-2.4-2.7 mm/yr.

American Fork Canyon2 Post-Bonneville time: 15-23 m/16.8-18.0 kyr = 0.8-1.1-1.4 mm/yr.

1Net-vertical-slip data are from Swan and others (1980), net displacement in Bonneville shoreline deposits revised by Machette and others (1992). 2Net-vertical-slip data are from Machette (1988).

Slip-rate data are not available for the Mapleton, Rock Canyon, and Water Canyon sites.

Machette (1992) reported net displacements across scarps at several locations along the PS. Two locations provide displacement data that are sufficiently well age-constrained to calculate long-term vertical slip rates.

Mouth of Spanish Fork Canyon Stream alluvium related to the Provo phase of Lake Bonneville: 3 m/16.2-16.8 kyr = 0.18-0.19 mm/yr East of Provo between Slate and Slide Canyons: Gravel related to the Bonneville phase of Lake Bonneville: >20 m/16.8-18.0 kyr = minimum 1.1-1.2 mm/yr

Based on currently available information on earthquake timing and displacement, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the PS are:

0.6-1.2-3.0 mm/yr

Summary: Timing of the three most recent surface-faulting earthquakes (X, Y, Z) on the PS is well constrained and provides recurrence information for the segment since the mid-Holocene. Little is known about the timing of older earthquakes; however, a recent cooperative trenching project between URS Corporation and the Utah Geological Survey (Olig and others, 2004) at the Mapleton North site of Lund and others (1991) will provide additional information about the timing of earlier earthquakes. Results of that study are not yet available.

Vertical slip-rate information for the PS consists of widely distributed data, which generally show that slip rates on the segment since Provo time have been as high as 1.0-1.4 mm/yr, but likely average closer to 0.8-1.2 mm/yr along the central part of the segment. A slip rate of 2.2-2.7 mm/yr resulting from an anomalously high displacement measured in sediments of the Bonneville and Provo phases of Lake Bonneville, as documented by both Swan and others (1980) and Machette and others (1992) at Hobble Creek, remains unexplained. Reliable information regarding the long-tem rate of slip at the segment ends is not available.

Additional References: McCalpin, J.P., and Nishenko, S.P., 1996, Holocene paleoseismicity, temporal clustering, and probabilities of future large (M>7) earthquakes on the Wasatch fault zone: Journal of Geophysical Research, v. 101, no. B3, p. 6233-6253. 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. Consensus preferred recurrence-interval and vertical slip-rate estimates 55

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Nephi segment (NS), Wasatch fault zone (WFZ), Juab County, Utah

Paleoseismic Data Source Documents: Bucknam, R.C., 1978, Northwestern Utah seismotectonic studies, in Seiders, W., and Thompson, J., compilers, Sum- maries of technical reports, v. VII: Menlo Park, California, U.S. Geological Survey Office of Earthquake Studies, p. 64. Hanson, K.L., Swan, F.H., III, and Schwartz, D.P., 1981, Study of earthquake recurrence intervals on the Wasatch fault, Utah: San Francisco, Woodward-Clyde Consultants, Sixth Semi-Annual Technical Report prepared for the U.S. Geological Survey, contract no. 14-08-0001-16827, 22 p. Hanson, K.L., Swan, F.H., III, and Schwartz, D.P., 1982, Study of earthquake recurrence intervals on the Wasatch fault, Utah: San Francisco, Woodward-Clyde Consultants, Seventh Semi-Annual Technical Report prepared for the U.S. Geological Survey, contract no. 14-08-0001-19842, 26 p. Schwartz, D.P., Hanson, K.L., and Swan, F.H., III, 1983, Paleoseismic investigations along the Wasatch fault zone – An update, in Gurgel, K.D., editor, Geologic excursions in neotectonics and engineering geology in Utah: Utah Geo- logical and Mineral Survey Special Study 62, p. 45-48. 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. Jackson, M.E., 1991, The number and timing of Holocene paleoseismic events on the Nephi and Levan segments, Wasatch fault zone, Utah: Utah Geological Survey Special Study 78, 23 p. Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone - A summary of recent investigations, interpretations, and conclusions, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-A, p. A1-A71. Harty, K.M., Mulvey, W.E., and Machette, M.N., 1997, Surficial geologic map of the Nephi segment of the Wasatch fault zone, eastern Juab County, Utah: Utah Geological Survey Map 170, scale 1:50,000, 14 p. booklet.

Age of Youngest Faulting: Holocene

Discussion: The NS consists of two subsections identified by Machette and others (1992) as the main western (southernmost) strand and an eastern (northernmost) strand. The western strand bounds the Wasatch Range along the east side of Juab Valley and the eastern strand bounds the west side of Dry Mountain. Heavy vegetation and landslides in Mancos Shale bedrock obscure a suspected connecting fault between the two strands. The northern end of the eastern strand overlaps the Provo segment (PS) at the Payson salient. The Benjamin fault forms the west side of the salient and extends northward into Utah Valley where it dies out (Machette, 1992; Harty and others, 1997). Sediments of the Provo phase of Lake Bonneville are displaced up to 2 m along this fault (Machette, 1992). No paleoseismic-trenching investigations have been conducted on the eastern (northern) strand of the NS.

Scarps on the western strand exhibit very young scarp morphology, prompting Hanson and others (1981) and Schwartz and Coppersmith (1984) to suspect a very young age (latest Holocene) for the MRE on this part of the NS. Faults associated with young scarps on the western strand north of the town of Nephi are probably continuous with near- surface faults in town identified from seismic-reflection data (Crone and Harding, 1984). The southern boundary of the NS is at a 15-km gap in Holocene and latest Pleistocene surface faulting (5-km gap in Quaternary surface faulting) that separates the NS from the Levan segment to the south (Machette and others, 1992; Harty and others, 1997; Hylland and Machette, 2004).

Earthquake Timing: Hanson and others (1981) and Jackson (1991) identified three surface-faulting earthquakes on the western strand of the NS at the North Creek and Red Canyon trench sites, respectively. Two stacked colluvial wedges and a stream terrace inset into the upthrown block of the fault at North Creek, and three stacked colluvial wedges at Red Canyon provide evidence for repeated Holocene surface faulting on the NS. However, while the number of earthquakes is well constrained, earthquake timing remains uncertain. 56 Utah Geological Survey

North Creek At North Creek Hanson and others (1981) interpreted the three earthquakes as occurring after 4580±250 14C yr B.P. (5890 [5307] 4532 cal yr B.P.; calibrated in accordance with table 1 in McCalpin and Nishenko, 1996) based on a 14C age on charcoal from a burn layer in North Creek alluvium (Bucknam, 1978); the alluvium is displaced by all three earthquakes. Evidence for the two youngest earthquakes consists of scarp-derived colluvial-wedge deposits exposed in trenches, whereas the third (oldest) earthquake is represented by a tectonic strath terrace inset into the North Creek alluvial fan on the upthrown side of the fault. Event Z displaces an alluvial deposit and a soil, both containing charcoal dated at 1350±70 14C yr B.P. (1262 [1086] 828 cal yr B.P.; in accordance with McCalpin and Nishenko, 1996), and 1110±60 14C yr B.P. (1165 [983] 924 cal yr B.P.; in accordance with McCalpin and Nishenko, 1996), respectively. Based on the steep scarp angles at North Creek and the presence of a nickpoint in a stream channel just above the scarp, Hanson and others (1981) and Schwartz and Coppersmith (1984) prefer a timing for event Z of about 0.4±0.1 ka.

Event Y timing is constrained by 14C ages on both charcoal and soil organics from a soil formed on top of the event Y colluvial wedge. Conventional 14C ages on the soil organics and charcoal yielded ages of 3640±75 14C yr B.P. (4427 [4144] 3874 cal yr B.P.; in accordance with McCalpin and Nishenko, 1996) and 1650±50 14C yr B.P. (not calibrated by McCalpin and Nishenko, 1996), respectively. A second set of ages obtained using an accelerator mass spectrometer resulted in similar divergent ages. Given the choice of two sets of ages for the soil, Hanson and others (1981) concluded that the younger ages represent younger material incorporated into the soil prior to burial, and therefore consider the older ages as providing minimum-limiting constraints on the timing of event Y, making it older than 3640±75 14C yr B.P., but younger than 4580 14C yr B.P. Event X, represented by the tectonic strath terrace, must also have occurred within that interval, indicating that two surface-faulting earthquakes occurred in an approximately 2 kyr time period (3874 – 5890 cal yr B.P.).

Red Canyon Jackson (1991) estimated the timing of the three surface-faulting earthquakes at Red Canyon using a combination of 14C and TL ages on soils buried by colluvial wedges. The soil buried by the event Z wedge yielded TL ages of 1300±500 and 1400±400 yr B.P., and a conventional 14C age of 2900±90 14C yr B.P. Jackson reports the soil as having a low carbon content, and therefore preferred the two closely corresponding TL ages and determined the timing of event Z to be ~1400 yr B.P.

Jackson (1991) obtained two 14C and two TL ages from the soil buried by the event Y colluvial wedge and formed on top of the event X wedge. One 14C age (1380±120 14C yr B.P.) and one TL age (1700±200 yr B.P.) gave results that Jackson considered too young and subsequently disregarded. The second 14C sample yielded a 14C age of 3690±170 14C yr B.P. (4423 [3900]) 3429 cal yr B.P., in accordance with McCalpin and Nishenko, 1996), and the second TL age estimate was 7000±800 yr B.P. Jackson considered the TL age too old, likely due to incomplete bleaching prior to sediment deposition, and selected the 14C age as the preferred maximum time for event Y. Event X was consigned to be older than 3690±170 14C yr B.P., but younger than the alluvial-fan sediments it displaces. M.N. Machette (personal communication in Jackson, 1991) estimated the age of the faulted alluvial-fan surface as latest Pleistocene. However, based on the Bucknam (1978) 14C age of 4580±250 14C yr B.P. for the alluvial-fan deposits at North Creek, Jackson assigned a time to event X of between 4 and 4.5 ka.

McCalpin and Nishenko Earthquake-Limiting Ages McCalpin and Nishenko (1996) accepted the Hanson and others (1981) and Jackson (1991) choices of earthquake- limiting 14C and TL ages for earthquakes Y and Z. Additionally, they included five previously unpublished 14C ages from the Water Canyon site (Ostenaa, 1990) on the PS in their analysis of surface-faulting timing for the NS. Three Water Canyon ages were used to help constrain the timing of event Z, and the other two were used to help constrain the timing of event Y. By including the Water Canyon ages in their analysis of the NS, McCalpin and Nishenko (1996) implicitly accept the partial segment rupture model for the Nephi and Provo segments originally proposed by Ostenaa (1990). Rounded to the nearest half-century, the McCalpin and Nishenko (1996) estimates for the timing of events Y and Z on the NS are 3850 and 1150 cal yr B.P., respectively.

Working Group Evaluation Following review of the available paleoseismic data for the NS, the Working Group concludes that the NS has experienced a minimum of three surface-faulting earthquakes since ~5300 cal yr B.P. The Working Group’s consensus for the timing of surface-faulting earthquakes on the NS is:

Z <1.0±0.4 ka, possibly as young as 0.4±0.1 ka Y ~3.9±0.5 ka X >3.9±0.5 ka, <5.3±0.7 ka Consensus preferred recurrence-interval and vertical slip-rate estimates 57

The Working Group agrees with the McCalpin and Nishenko (1996) observation that for the WFZ as a whole, “Until more precise dating is available to evaluate this possibility [partial segment rupture], ‘the megaquake’ hypothesis needs to be considered as a valid end member scenario for planning purposes along the Wasatch Front.” However, in the absence of corroborating data regarding surface-faulting timing from the eastern strand of the NS, which is the strand closest to the PS and the Water Canyon trench site, the Working Group believes that it is premature to use information on earthquake timing from the PS to constrain the timing of surface faulting on the western (southernmost) strand of the NS.

Surface-Faulting Recurrence: Because the timing of surface faulting on the NS is not tightly constrained, interevent intervals are broad. Interevent recurrence intervals with associated confidence limits for the BCS are:

Elapsed time since event Z ≤1000±400 cal yr Y-Z interval ≤2900±6001 cal yr X-Y interval ≤1400±900 cal yr

The approximate weighted mean recurrence for the two most recent interevent intervals rounded to the nearest century is:

~25002±21003 cal yr

Based on the poorly constrained information available on earthquake timing and variability in the length of individual interevent intervals, the Working Group’s preferred recurrence-interval estimate and confidence limits for the NS are: 1200-2500-4800 yr Vertical Slip Rate: The western strand of the NS lies entirely above the highstand of Lake Bonneville, so lacustrine geomorphic features and stratigraphy do not help constrain the age of deposits along that portion of the NS. Hanson and others (1981) report 7.0±0.5 m of net vertical displacement at North Creek and a late Holocene vertical slip rate of 1.3±0.1 mm/yr. Schwartz and Coppersmith (1984) report a vertical slip rate for the same site of 1.27-1.36±0.1 mm/yr.

Harty and others (1997) calculated middle to late Holocene vertical slip rate ranges at four locations (North Creek, Willow Creek, Gardner Creek, and Red Canyon) on the western strand of the NS. At North Creek and Red Canyon the slip rates are based on net vertical-displacement estimates made from scarp profiles and the thickness of colluvial-wedge deposits exposed in trenches. They used earthquake timing and the net vertical-displacement values reported by Han- son and others (1981) and Jackson (1991). At Willow Creek and Gardner Creek, alluvial-fan ages were assumed to be the same as at North Creek based on the Bucknam (1978) 14C age of ~5300 cal yr B.P.

North Creek 0.8-1.2 mm/yr Willow Creek 0.7-1.0 mm/yr Gardner Creek 0.5-0.7 mm/yr Red Canyon 0.6-1.0 mm/yr

Machette (1984) described a soil just north of Gardner Creek on faulted upper Pleistocene alluvial-fan deposits. The fan deposits are displaced a minimum of 26-28 m, and based on soil-profile development the fan age is about 250 ka. The resulting vertical slip rate is 0.12 mm/yr, which is approximately 20 percent of the slip rate at Gardner Creek in more recent geologic time.

Based on currently available information on earthquake timing and displacement, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the NS are:

0.5-1.1-3.0 mm/yr Summary: Investigators have conducted two paleoseismic-trenching investigations on the western (southernmost) strand of the NS. The investigations revealed evidence for at least three surface-faulting earthquakes since middle Holocene time. 58 Utah Geological Survey

Both investigations produced conflicting sets of numerical ages on samples from geologic units critical to determining the timing of surface faulting. Consequently, the existing paleoseismic data for the NS are considered poorly constrained, and multiple surface-faulting chronologies are possible depending on which ages are accepted and which are discarded. Paleoseismic-trenching data are lacking from the eastern (northernmost) strand.

Because surface-faulting timing on the NS is the least well constrained of the five central WFZ segments with multiple Holocene surface faulting earthquakes, and because paleoseismic-trenching information is lacking from the northern (easternmost) strand of the NS, the Working Group recommends that additional investigations be conducted on both the eastern and western strands of the NS to better constrain earthquake timing, and that vertical slip-rate data along the segment be re-evaluated in light of that new earthquake information. Information about earthquake timing on the eastern subsection of the NS would help resolve questions regarding possible rupture overlap between the NS and the PS.

Additional References: Crone, A.J., and Harding, S.T., 1984, Near-surface faulting associated with Holocene fault scarps, Wasatch fault zone, Utah-A preliminary report, in Hays, W.W., and Gori, P.L., editors, A workshop on "Evaluation of regional and urban earthquake hazards and risk in Utah": U.S. Geological Survey Open-File Report 84-763, p. 241-268. Hylland, M.D., and Machette, M.N., 2004, Part III – Interim surficial geologic map of the Levan segment of the Wasatch fault zone, Juab and Sanpete Counties, in Christenson, G.E., Ashland, F.X., Hylland, M.D., McDonald, G.N., and Case, Bill, Database compilation, coordination of earthquake-hazards mapping and study of the Wasatch fault and earthquake-induced landslides, Wasatch Front, Utah: Utah Geological Survey Final Contract Report to the U.S. Geological Survey, contract no. 03HGAG0008, 30 p. Machette, M.N., 1984, Preliminary investigation of late Quaternary slip rates along the southern part of the Wasatch fault zone, central Utah, in Hays W.W. and Gori, P. L., editors, Proceedings of Conference XXVI, A workshop on “Evaluation of regional and urban earthquake hazards and risk in Utah”: U.S. Geological Survey Open-File Report 84-763, p. 391-406. Machette, M.N., 1992, Surficial geologic map of the Wasatch fault zone, eastern Utah Valley, Utah County and parts of Salt Lake and Juab Counties, Utah: U.S. Geological Survey Miscellaneous Investigations Series Map I-2095, scale 1:50,000. McCalpin, J.P., and Nishenko, S.P., 1996, Holocene paleoseismicity, temporal clustering, and probabilities of future large (M>7) earthquakes on the Wasatch fault zone: Journal of Geophysical Research, v. 101, no. B3, p. 6233-6253. Ostenaa, Dean, 1990, Late Holocene displacement history, Water Canyon site, Wasatch fault zone [abs.]: Geological Society of America Abstracts with Programs, v. 22, no. 6, p. 42. Consensus preferred recurrence-interval and vertical slip-rate estimates 59

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Levan segment (LS), Wasatch fault zone (WFZ), Juab and Sanpete Counties, Utah

Paleoseismic Data Source Documents: 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. Jackson, M.E., 1991, The number and timing of Holocene paleoseismic events on the Nephi and Levan segments, Wasatch fault zone, Utah: Utah Geological Survey Special Study 78, 23 p. Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone - A summary of recent investigations, interpretations, and conclusions, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-A, p. A1-A71. Hylland, M.D., and Machette, M.N., 2004, Part III – Interim surficial geologic map of the Levan segment of the Wasatch fault zone, Juab and Sanpete Counties, in Christenson, G.E., Ashland, F.X., Hylland, M.D., McDonald, G.N., and Case, Bill, Database compilation, coordination of earthquake-hazards mapping, and study of the Wasatch fault and earthquake-induced landslides, Wasatch Front, Utah: Utah Geological Survey Final Contract Report to the U.S. Geological Survey, contract no. 03HGAG0008, 30 p.

Age of Youngest Faulting: Holocene

Discussion: A 15-km-long gap in Holocene and latest Pleistocene surface faulting (5-km gap in Quaternary surface faulting) marks the boundary between the LS and the Nephi segment (NS; Machette and others, 1992; Harty and others, 1997; Hylland and Machette, 2004) of the WFZ. Landslide deposits occupy much of the gap, and likely obscure any fault scarps (Hylland and Machette, 2004). South of the gap, most fault scarps on Holocene deposits are less than 3 m high, while scarps on upper to middle Pleistocene alluvium are 5 to 10 m high or higher, suggesting recurrent late Quaternary surface faulting (Machette and others, 1992). Holocene ruptures on the LS extend from about 4 km north to 18 km south of the town of Levan, where the fault steps left about 0.5 km and enters bedrock. Clear evidence of faulting extends for another 5 km to the south. The geomorphic appearance of the scarps and new (2004) scarp-profile data (M.D. Hylland, Utah Geological Survey, verbal communication to Working Group, 2004) indicate that these scarps are probably Holocene.

Although the gap between the NS and LS lacks evidence of Holocene and latest Pleistocene surface faulting, it does contain older fault scarps formed on middle Pleistocene alluvial fans extending along the front of the San Pitch Moun- tains south of Nephi (Machette and others, 1992; Harty and others, 1997). The older fault scarps may indicate that the boundary between the two segments is nonpersistant, at least in earlier Quaternary time. To the south, the boundary between the LS and Fayette segment (FS) of the WFZ is marked by a 3.5-km step to the east and 5-km step to the south in late Quaternary faulting. Fault scarp morphology on the FS indicates that the most recent surface faulting is older than on the LS (Machette and others, 1992).

Earthquake Timing: Based on scarp morphology at Deep Creek and Pigeon Creek, Schwartz and Coppersmith (1984) concluded that the LS has experienced a single surface-faulting earthquake in Holocene time. At Pigeon Creek they obtained a 14C age on charcoal from faulted alluvial-fan sediments of 1750±350 14C yr B.P., which provides a maximum limiting time for the MRE. At Deep Creek a charcoal sample collected from the fault footwall yielded a 14C age of 7300±1000 14C yr B.P. These ages show that the MRE on the LS postdates 1750±350 14C yr B.P., and that the PE is older than 7300±1000 14C yr B.P.

Jackson (1991) collected a TL sample from the upper few centimeters of a soil buried beneath the MRE colluvial wedge exposed in a stream cut at Deep Creek. The soil yielded an age of 1000±100 yr B.P., which provides a closely limiting maximum time for the earthquake. Hylland and Machette (2004) revisited Deep Creek in 2003 and sampled the upper 5 cm of the same buried soil for 14C dating. The soil yielded an age of 1200±80 14C yr B.P., which when calendar calibrated and rounded to the nearest half-century results in an AMRT age of 1000±150 cal yr B.P. Therefore, 60 Utah Geological Survey based on both TL and 14C ages from the Deep Creek stream exposure, the MRE on the LS likely occurred shortly after 1000±150 cal yr B.P.

Jackson (1991) excavated a trench across a 3-m high scarp at Skinner Peaks south of Deep Creek and found colluvial-wedge evidence for a single surface-faulting earthquake in the late Holocene (between 1000 and 1500 yr B.P.). Evi- dence for a second surface-faulting earthquake was equivocal and was based on secondary stratigraphic relations exposed in the fault’s hanging wall. All that Jackson could say with confidence regarding the timing of the PE at Skin- ner Peaks is that it predates 3.1-3.9 ka.

Hylland and Machette (2004) report that scarp-profile data south of Chriss Canyon indicate the possibility of two surface-faulting earthquakes on the southern approximately 15 km of the LS. Scarp data alone are not sufficient to constrain PE timing. The PE may have ruptured only the southern part of the LS, or conversely, evidence for a northern continuation of the rupture is not expressed in the scarp morphology (Hylland and Machette, 2004).

The Working Group’s consensus for the timing of surface-faulting earthquakes on the LS is:

Z shortly after 1000±150 cal yr B.P. Y unknown but likely early Holocene to latest Pleistocene (possible partial segment rupture)

Surface-Faulting Recurrence: Because information on timing of surface faulting is limited for the LS, but recognizing the possibility of two Holocene earthquakes near the southern end of the segment, the Working Group’s preferred Holocene recurrence-interval estimate for the LS is reported as a range and not as a central value with approximate 2-sigma confidence limits.

>3 and <12 kyr Vertical Slip Rate: Hylland and Machette (2004) state that determining an accurate late Quaternary slip rate for the LS is presently not possible because the timing of only one surface-faulting earthquake is known. Using the mean diffusion age of scarps south of the Skinner Peaks trench site, Hylland and Machette (2004) calculated a vertical slip rate since the latest Pleis- tocene/earliest Holocene of 0.33-0.53 mm/yr for the southern part of the LS.

Because the paleoseismic information available for the LS is limited, the Working Group’s consensus Holocene preferred vertical slip-rate estimate for the LS is reported as a range and not as a central value with approximate 2-sigma confidence limits.

0.1-0.6 mm/yr

Summary: Geomorphic evidence and a natural stream-cut exposure indicate a minimum of one surface-faulting earthquake since the early to middle Holocene on the LS. However, scarp-profile analysis indicates a possible second earthquake in the early Holocene or latest Pleistocene near the south end of the LS. The Working Group recommends additional paleoseismic investigation (additional scarp profiles and trenching) to resolve this earthquake timing issue and to better resolve both surface-faulting recurrence and vertical slip rates on the LS.

Additional References: Harty, K.M., Mulvey, W.E., and Machette, M.N., 1997, Surficial geologic map of the Nephi segment of the Wasatch fault zone, eastern Juab County, Utah: Utah Geological Survey Map 170, scale 1:50,000, 14 p. booklet. Consensus preferred recurrence-interval and vertical slip-rate estimates 61

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Joes Valley fault zone (JVFZ), Sanpete County, Utah

Paleoseismic Data Source Documents: Foley, L.L., Martin, R.A., Jr., and Sullivan, J.T., 1986, Seismotectonic study for Joes Valley, Scofield, and Huntington North Dams, Emery County and Scofield Projects, Utah: Denver, U.S. Bureau of Reclamation Seismotectonic Report No. 86-7, 132 p., scale 1:60,000 and 1:155,000.

Age of Youngest Faulting: Holocene

Discussion: The JVFZ consists of parallel, en echelon, and locally overlapping, north- to northeast-trending faults which extend for 120 km on the east side of the Wasatch Plateau (Foley and others, 1986). The fault system contains two major structures, a southern and a northern graben, each of which has distinct geomorphic characteristics that may reflect differences in total displacement and recency of movement on bounding faults. The northern part of the fault zone is characterized by greater total stratigraphic throw within the graben, more continuous and linear bedrock escarpments that mark graben-bounding faults, and Quaternary scarps, which are generally absent to the south (Foley and others, 1986).

The northern part of the JVFZ extends for a distance of about 50 km and is bounded by the East and West Joes Val- ley faults (EJVF & WJVF) and contains several intragraben faults, the most prominent being the Middle Mountain fault (MMF) and the Bald Mountain faults. Distinct scarps mark both graben-bounding faults and intragraben structures on unconsolidated Quaternary deposits (Foley and others, 1986).

Despite the presence of scarps formed on Quaternary deposits along the northern JVFZ, a fundamental question remains regarding the nature of the Joes Valley graben (JVG), and the seismogenic capability of the associated JVFZ. Foley and others (1986) report no significant net displacement across the JVG, and the graben’s close association with the active Wasatch Plateau monocline suggests that the JVG may be a keystone graben formed along or near the monocline crest (S. Hecker, U.S. Geological Survey, verbal communication to Working Group, 2003). Foley and others (1986) proposed three additional possible origins for the JVG, only one of which involves faulting that extends to seismogenic depths, and conclude, “the origin of the long linear grabens on the Wasatch Plateau [JVG] cannot be resolved with presently available geologic and seismologic data.” They further state:

The main issue regarding the scarps in Joes Valley involves whether or not these faults formed in response to recurrent large-magnitude earthquakes. The available data on subsurface geometry of the Wasatch Plateau faults is equiv-ocal; therefore, the possibility that the Joes Valley faults and other similar faults extend to seismogenic depths cannot be precluded.

Due to uncertainty regarding the seismogenic nature of the JVFZ, the Utah Geological Survey intends to reclassify the Joes Valley faults (EJVF, WJVF, and intragraben faults) as “Suspected Faults” in the next revision of the Quaternary Fault and Fold Database and Map of Utah (M. D. Hylland, Utah Geological Survey, verbal communication to Working Group, 2003).

Earthquake Timing: Foley and others (1986) excavated six trenches across three faults in the north JVG (the EJVF, WJVF, and MMF) to investigate the timing and displacement of surface-faulting earthquakes. Trenching results showed that the EJVF has experienced a minimum of four surface-faulting earthquakes over the past ~250 ka, while the WJVF and MMF have each experienced a minimum of two surface-faulting earthquakes since ~30 ka. The ability to resolve the timing of individual earthquakes in the trenches was limited, and Foley and others (1986) state that the measured displacement of 1 to 5 m may record cumulative displacement from several earthquakes rather than from one or two larger earthquakes.

Surface-Faulting Recurrence: Due to the ambiguity of the paleoseismic data, Foley and others (1986) were able to provide only broadly con- 62 Utah Geological Survey strained recurrence-interval estimates for the faults comprising the JVFZ.

Fault Recurrence EJVF1 <60 kyr WJVF2 10-20 kyr MMF2 10-15 kyr

1 ~250 kyr period of record 2 ~30 kyr period of record

Vertical Slip Rate: Foley and others (1986) report no net vertical displacement across the JVG. Based on that observation, the Work- ing Group recommends that the JVFZ be considered a single integrated structure. Lacking net displacement, the JVFZ as a whole has no vertical slip rate, again raising questions about the seismogenic capability of the fault zone. Foley and others (1986) do not report vertical slip rates for individual faults comprising the JVFZ. However, Black and others (2003) do report vertical slip rates for the EJVF, WJVF and MMF, which they determined by dividing the maximum displacement recorded in Quaternary deposits along those faults by the estimated age of the displaced deposits. In some instances the Black and others (2003) vertical slip rates are as high as 1.1 mm/yr; however, all their slip rates are for seismic cycles open at both ends, and the maximum displacements were reported by Foley and others (1986) as “scarp heights,” not as net displacement. Because Foley and others (1986) distinguished between scarp height and net displacement on the EJVF where they measured scarp profiles, their “scarp height” values presumably do not represent net displacement.

Summary: Due to ambiguities regarding both the seismogenic capability of the JVFZ and the number and timing of surface- faulting earthquakes on individual faults, the Working Group recommends that the JVFZ be treated as a single integrated structure rather than as several independently seismogenic faults and that the combined fault zone be assigned a single consensus preferred recurrence interval. The Working Group’s preferred recurrence-interval estimate and confidence limits for the JVFZ are:

5-10-50 kyr

In the absence of measurable net displacement across the JVG, no vertical slip-rate estimate is possible for the JVFZ.

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: West Valley fault zone (WVFZ), Salt Lake County, Utah

Paleoseismic Data Source Documents: Keaton, J.R., Currey, D.R., and Olig, S.J., 1987, Paleoseismicity and earthquake hazards evaluation of the West Valley fault zone, Salt Lake City urban area, Utah: Salt Lake City, Dames and Moore, Final Technical Report for U.S. Geo- logical Survey, contract no. 14-08-0001-22048, 55 p.; published as Utah Geological Survey Contract Report 93-8, 1993. Keaton, J.R., and Currey, D.R., 1989, Earthquake hazard evaluation of the West Valley fault zone in the Salt Lake City urban area, Utah: Salt Lake City, Dames and Moore, Final Technical Report for U.S. Geological Survey, contract no. 14-08-001-G1397, 69 p.; published as Utah Geological Survey Contract Report 93-7, 1993. Solomon, B.J., 1998, New evidence for the age of faulting on the West Valley fault zone: Utah Geological Survey, Sur- vey Notes, v. 30, no. 3, p. 8 and 13.

Age of Youngest Faulting: Holocene

Discussion: The WVFZ is a north- to northwest-trending fault zone about 15 km long and 7 km wide that consists of generally east-dipping faults that form the western boundary of a fault-bounded basin in the center of Salt Lake Valley. The Salt Lake City segment (SLCS) of the Wasatch fault zone (WFZ; Machette and others, 1992) traverses the eastern side of the valley at the base of the Wasatch Range and forms the eastern side of the basin. The WVFZ is at about the midpoint of the valley and has scarps up to 6.1 m high formed on late Pleistocene Lake Bonneville lacustrine deposits. Whether the WVFZ is independently seismogenic or an antithetic fault that ruptures coseismically with the SLCS is unclear based on currently available information.

The southern portion of the WVFZ consists of two subparallel east-dipping faults, the Taylorsville fault (TF) to the east, and Granger fault (GF) to the west, whereas the northern portion is broader and characterized by many smaller, east- and west-dipping faults that form a broad, indistinct graben. Seismic-reflection data from an area on-trend with the fault zone at the south end of Great Salt Lake (north of the fault zone) indicate a buried, east-dipping fault that cuts the inferred base of the Quaternary section (Wilson and others, 1986).

The WVFZ shows evidence for Holocene surface faulting, but exposures are poor and often lack clear evidence of faulting, earthquake timing, and datable material.

Earthquake Timing: Taylorsville fault Keaton and others (1987) and Keaton and Currey (1989) excavated several trenches across the central of the three strands that comprise the TF, but were unable to identify discrete fault zones, evidence for individual surface-faulting earthquakes, or to determine earthquake timing. Displacement on the TF is chiefly monoclinal flexuring, which occurred at or close to the threshold for surface fault rupture. Keaton and others (1987) interpreted a minimum of two surface-faulting earthquakes in post-Gilbert shoreline time (~12 ka) based on the presence of a 1.2-1.5 m-high scarp they believe formed during the PE, which was then followed by the MRE that truncated the PE scarp.

Solomon (1998) described an exposure of the TF in a consultant’s trench near the north end of the fault trace, and identified the MRE based on the presence of a tectonic crack filled with organic-rich sediment. He obtained two 14C ages on organic bulk samples, one from the crack-fill material and the other from pre-faulting sag-pond sediments. The 14C ages indicate the MRE occurred shortly after 2.0-2.4 ka, which corresponds in a general way with the timing of the PE (2.45 ka) on the SLCS.

Granger fault Scarps up to 6.1 m high in Lake Bonneville lacustrine deposits mark the trace of the GF. Keaton and others (1987) excavated two trenches and drilled eight boreholes on the GF. The fault formed a prominent, discrete, planar trace in both trenches, but neither the number nor the timing of individual earthquakes could be determined from trench expo- 64 Utah Geological Survey sures. Keaton and Currey (1989) drilled 24 boreholes at three additional sites along the fault. They identified two earthquakes on the GF based on the presence in the boreholes of displaced Lake Bonneville deposits and calcareous playa deposits buried by scarp colluvium. Their interpretation is that the PE created the depression in which the playa formed and that the MRE scarp generated the colluvium. They also identified evidence for three additional surface-faulting earthquakes based on morphostratigraphic scarp relations, for a total of five earthquakes on the GF in 13 ka.

A 14C age on a bulk sample of organic fault-zone colluvium, obtained by the UGS from a consultant’s trench across the GF, indicates that the MRE occurred slightly after 1.3-1.7 ka (UGS unpublished data), which is similar to the timing of the MRE (1.3 ka) on the SLCS.

Entire WVFZ Keaton and Currey (1987) report six to seven surface-faulting/flexure earthquakes in the past 13 kyr for the WVFZ as a whole, the MRE being younger than ~ 12 ka, but were unable to determine the timing of individual surface-faulting earthquakes. Later work by Solomon (1998) and the UGS (unpublished data) shows that the MREs on the TF and GF are similar in age to the PE and MRE, respectively, on the SLCS.

Surface-Faulting Recurrence: Taylorsville fault Keaton and others (1987) report a mean recurrence of 6 kyr for the TF based on two earthquakes in ~12 kyr. How- ever, the timing of the two earthquakes is unknown, and therefore so is the length of the interval between them.

Granger fault Keaton and others (1987) report a mean recurrence of 2.6 kyr for the GF based on five earthquakes in 13 kyr. How- ever, earthquake timing is unknown, and therefore so is the length of the intervals between them.

Entire WVFZ For the WVFZ as a whole, Keaton and others (1987) report a total of six to seven surface-rupture/flexure earthquakes in the past 13 kyr producing a latest Pleistocene/Holocene mean recurrence of 1.8-2.2 kyr. Keaton and Currey (1989) focused their attention on the less well-defined scarps at the north end of the WVFZ and report a mean recurrence of surface faulting or folding on that part of the WVFZ of 6 to 11 kyr. They acknowledge the shorter recurrence intervals reported by Keaton and others (1987) for the southern part of the WVFZ, and state that the more prominent fault traces to the south are likely indicative of shorter recurrence intervals.

Solomon (1998) and the UGS (unpublished data) documented the MREs on the TF and GF at 2.0-2.4 ka and 1.3- 1.7 ka, respectively. These data indicate that a minimum of two surface-faulting earthquakes have occurred on the WVFZ in the past ~2.4 kyr, with an interval of 0.3-1.1 kyr between them. These two earthquakes may have been coseismic with the two most recent surface-faulting earthquakes on the SLCS. The timing of older surface-faulting/flexuring earthquakes on the TF and GF remains unknown and therefore the Working Group was unable to make a recurrence- interval estimate for the WVFZ.

Vertical Slip Rate: Vertical slip-rate estimates reported by Keaton and others (1987) for the WVFZ are based on a combination of displacements estimated from trench exposures and boreholes. The trend is for the rate of slip to increase in more recent geologic time.

Time Interval Vertical Slip Rate Taylorsville fault <12 kyr 1-0.2 mm/yr Granger fault 13 kyr 0.4-0.5 mm/yr WVFZ - entire 13 kyr 0.5-0.6 mm/yr Granger fault 47±20 kyr 0.1-0.3 mm/yr Granger fault 60±20 kyr 0.02-0.04 mm/yr Granger fault 80±30 kyr 0.03-0.1 mm/yr Granger fault 140±10 kyr 0.01 mm/yr

Available vertical slip-rate data for the WVFZ come chiefly from geomorphic fault-scarp studies and from drill-hole data. Those data indicate that slip on the WVFZ has increased in more recent geologic time (see table above).

Based on available information, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the WVFZ, as a whole, are:

0.1-0.4 -0.6 mm/yr Consensus preferred recurrence-interval and vertical slip-rate estimates 65

Summary: The Working Group believes that current paleoseismic data are insufficient to make a recurrence-interval estimate for the WVFZ. Exposures in consultant’s trenches allowed the UGS (Solomon, 1998; UGS unpublished data) to determine the timing of the MREs on the TF and GF. The timing of those earthquakes is in general agreement with the timing of the PE and MRE on the SLCS, respectively, indicating that for at least some surface-faulting earthquakes on the WFZ, all or part of the WVFZ may rupture coseismically. However, the relation between the WVFZ and the SLCS remains unclear, and additional paleoseismic investigations are required to determine if the WVFZ is persistently coseismic with the WFZ, or if it is independently seismogenic. Until such investigations are performed, the Working Group considers the WVFZ an independently seismogenic fault.

Additional References: Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone - A summary of recent investigations, interpretations, and conclusions, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-A, 71 p. Wilson, E.A., Saugy, Luc, and Zimmermann, M.A., 1986, Cenozoic tectonics and sedimentation of the eastern Great Salt Lake area, Utah: Bulletin de Société Géologique Francaise, v. 2, no. 5, p. 777-782. 66 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: West Cache fault zone (WCFZ), Cache County, Utah

Paleoseismic Data Source Documents: Black, B.D., Giraud, R.E., and Mayes, B.H., 2000, Paleoseismic investigation of the Clarkston, Junction Hills, and Wellsville faults, West Cache fault zone, Cache County, Utah: Utah Geological Survey Special Study 98, 23 p.

Age of Youngest Faulting: Holocene

Discussion: Cache Valley is a large north-south structural basin formed by repeated movement on the east-dipping WCFZ and the west-dipping East Cache fault zone. The valley was occupied by Pleistocene Lake Bonneville until sometime after 16.2 ka, when Lake Bonneville began to recede from the Provo shoreline. The WCFZ trends along the west side of Cache Valley in northern Utah and southern Idaho.

The WCFZ consists of three related east-dipping normal faults, which from north to south are the Clarkston (CF), Junction Hills (JHF), and Wellsville (WF) faults. The three faults are subparallel and extend for 70 km along the west side of Cache Valley at the base of the Malad Range, Junction Hills, and Wellsville Mountains, respectively. All three faults show evidence for recurrent late Quaternary activity. The timing of each fault’s MRE is different, as are their estimated vertical slip rates; therefore, Solomon (1999) and Black and others (2000) consider each fault an independent seismogenic segment of the WCFZ.

Earthquake Timing: Clarkston fault Black and others (2000) excavated a trench across a 4-m-high scarp that exposed a single fault trace and evidence for one surface-faulting earthquake. Radiocarbon age estimates on carbon from the upper few centimeters of a paleosol beneath the MRE colluvial wedge, from organic-rich wedge matrix near the middle of the wedge, and from organic-rich wedge matrix from the wedge heel immediately adjacent to the scarp result in an estimate of MRE timing of 3600 to 4000 cal yr B.P. The trench did not contain evidence of older events.

Junction Hills fault A stream-cut exposure provides the only conclusive evidence of late Quaternary faulting on the JHF (Solomon, 1999). Black and others (2000) logged the exposure and found evidence for two surface-faulting earthquakes. A 14C age obtained from slightly organic material from the bottom of the MRE colluvial wedge and the top of an underlying paleosol resulted in an estimate of MRE timing of 8250 to 8650 cal yr B.P. The timing of the PE could not be constrained other than that it was older than overlying Lake Bonneville transgressive deposits that Black and others (2000) believe were deposited about 22.5 ka.

Wellsville fault Black and others (2000) excavated a trench across a 7-m-high scarp of the western strand of the WF and exposed evidence for two surface-faulting earthquakes. Radiocarbon ages from bulk-soil samples collected from the MRE colluvial wedge and an underlying paleosol resulted in an estimate of MRE timing of 4400 to 4800 cal yr B.P. A 14C age from small pieces of degraded detrital charcoal in alluvial-fan sediments that predate the PE provides a maximum limiting constraint on the timing of that earthquake of about 25 ka. A post-earthquake loess deposit overlies the PE colluvial wedge. Black and others (2000) interpreted the loess as deposited about 15 ka following the desiccation of many pluvial lakes in the region and the retreat of glaciers in the mountains. Therefore, timing of the PE on the WF is constrained between broadly limiting ages of 15 and 25 ka.

Surface-Faulting Recurrence: Clarkston fault Lack of evidence for a PE prevented Black and others (2000) from directly determining the interevent interval between the PE and MRE on the CF. However, based on a minimum age for the Bonneville shoreline of 16.8 ka, they Consensus preferred recurrence-interval and vertical slip-rate estimates 67 report an estimated recurrence interval of 13.2 kyr (shoreline age minus MRE age) if there have been two post-Bon- neville shoreline earthquakes and 6.6 kyr if there have been three earthquakes in that interval. Both recurrence-interval estimates assume that the oldest earthquake occurred shortly after abandonment of the Bonneville shoreline, a supposi- tion for which no direct evidence exists, nor can the number of surface-faulting earthquakes on the CF in post-Bon- neville time be constrained. Therefore, current recurrence-interval estimates for the CF have high uncertainty.

Junction Hills fault Black and others (2000) estimated a minimum elapsed time of ~13.8 kyr between the MRE (8.7 ka) and PE (≥ 22.5 ka) on the JHF. However, timing of the PE has high uncertainty, and consequently there is high uncertainty regarding the length of the interevent interval, which could be much longer than 13.8 kyr.

Wellsville fault Black and others (2000) estimated the PE/MRE interevent interval on the WF as 10.2 to 20.6 kyr. However, PE timing is poorly constrained, so the actual length of the interevent interval is uncertain.

Vertical Slip Rate: Clarkston fault Black and others (2000) used the 9 m displacement of the Bonneville shoreline between the CF and JHF to make a vertical slip-rate estimate for the CF. Using the approximately 13 kyr interval between the minimum age of the Bon- neville shoreline (16.8 ka) and the MRE (3.8 ka), and the 9 m of displacement that occurred in that interval results in a late Pleistocene/Holocene vertical slip rate for the CF of 0.7 mm/yr. Black and others (2000) consider this value to be a high estimate and believe that the true vertical slip rate is likely lower, possibly in the range of 0.25 – 0.5 mm/yr; however, if the interval between events Y and Z is less than 13 kyr, the slip rate would be higher, not lower than 0.7 mm/yr.

Junction Hills fault Black and others (2000) report a vertical slip rate for the PE/MRE interevent interval on the JHF of 0.21 mm/yr based on 2.9 m of net vertical displacement during the MRE and a minimum elapsed time of ~13.8 kyr between the two earthquakes. However, PE timing is poorly constrained, and the 2.9 m of vertical net displacement may or may not represent the maximum MRE displacement on the JHF. Given the uncertainties in timing and net vertical displacement of the PE, 0.21 mm/yr is likely a near maximum value for the JHF.

Wellsville fault Black and others (2000) report a vertical slip rate of approximately 0.1-0.2 mm/yr for the PE/MRE interevent interval based on 1.9 m of MRE net vertical displacement and an interevent interval of 10.2-20.6 kyr.

Summary: Each of the three faults comprising the WCFZ has experienced a different Holocene surface-faulting earthquake, confirming that each fault is an independent seismogenic segment of the WCFZ. Penultimate earthquake timing on the segments either could not be determined or could be constrained only within broad time intervals with large uncertainties. As a result, preferred recurrence-interval and vertical slip-rate estimates for the segments also have high uncertainties. No information is available for earthquakes older than the PE on any of the segments, so the Working Group’s preferred recurrence-interval and vertical slip-rate estimates are based on a single, poorly constrained interevent interval between the PE and MRE and have high uncertainty. Due to the lack of well-constrained information on earthquake timing, the Working Group’s preferred recurrence-interval estimates represent a range of values and not a central value with approximate 2-sigma confidence limits.

Segment Recurrence Interval Vertical Slip Rate Clarkston fault 5-20 kyr 0.1-0.4-0.7 mm/yr Junction Hills fault 10-25 kyr 0.05-0.1-0.2 mm/yr Wellsville fault 10-25 kyr 0.05-0.1-0.2 mm/yr

Continued rapid growth is quickly converting Cache Valley from a rural farming region into an urban/industrial center. Despite the recent paleoseismic investigation by Black and others (2000), with the exception of the timing and displacement of the MRE on each of its three segments, the WCFZ remains poorly understood as a source of large, dam- aging earthquakes in Cache Valley and in northern Utah and southern Idaho in general. The Working Group recommends that additional paleoseismic investigations be performed on the WCFZ to better understand the slip distribution along the three fault segments and, that based on those investigations, additional trenching be performed to better constrain the timing of the PE and earlier earthquakes, particularly on the CF before urbanization makes such studies more difficult or impossible. 68 Utah Geological Survey

Additional References: Solomon, B.J., 1999, Surficial geologic map of the West Cache fault zone and nearby faults, Box Elder and Cache Coun- ties, Utah: Utah Geological Survey Map 172, scale 1:50,000, 21 p. pamphlet. Consensus preferred recurrence-interval and vertical slip-rate estimates 69

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: East Cache fault zone, central section (ECFZ), Cache County, Utah

Paleoseismic Data Source Documents: McCalpin, J.P., and Forman, S.L., 1991, Late Quaternary faulting and thermoluminescence dating of the East Cache fault zone, north-central Utah: Bulletin of the Seismological Society of America, v. 81, no. 1, p. 139-161. McCalpin, J.P., 1994, Neotectonic deformation along the East Cache fault zone, Cache County, Utah: Utah Geological Survey Special Study 83, 37 p.

Age of Youngest Faulting: Holocene

Discussion: The ECFZ is a roughly 80-km-long, generally north-trending range-front normal fault along the western base of the Bear River Range in eastern Cache Valley. McCalpin (1989, 1994) subdivided the ECFZ into three sections (northern, central, and southern) based on fault zone complexity, tectonic geomorphology, and expression of fault scarps. The central section is the only one that shows evidence of Holocene activity, and is the only section on which detailed paleoseismic-trenching studies have been performed.

The 16-km-long central section is typified by a single, straight fault trace. Fault scarps displace Bonneville-lake- cycle or younger deposits along the northern half (8 km) of the section, where scarps may diverge as much as 400 m from the range front. On the southern half of the section post-Bonneville faulting may have occurred, but no scarps are preserved, possibly due to mass movement at the base of faceted spurs (McCalpin, 1994).

McCalpin and Forman (1991) excavated two trenches on the central section of the ECFZ. The “Bonneville trench” on a Lake Bonneville-highstand delta exposed evidence for two surface-faulting earthquakes. Soil-profile development, TL ages, and a 14C age on a gastropod shell provide limits on event Y timing and broadly constrain the timing of event Z. The “Provo trench” was about 1 km north of the Bonneville trench on a Provo-age delta surface. This trench exposed stratigraphic evidence for event Z, and organic-rich bulk samples of crack-fill material, debris-facies colluvium, and a paleosol beneath the event Z colluvial wedge provided 14C age estimates to constrain the timing of this earthquake (McCalpin, 1994).

Earthquake Timing: Radiocarbon ages from three locations constrain the timing of event Z in the Provo trench. Organic matrix from the basal part of material filling a crack created by event Z yielded an age of 3100±80 14C yr B.P. Organic basal debris- facies colluvium from the event Z colluvial wedge yielded an age of 4240±80 14C yr B.P., and a soil buried by the colluvial wedge gave an age of 4040±60 14C yr B.P. McCalpin (1994) interpreted the crack-fill 14C age as erroneously young. He also considered the small age difference between the earliest debris-facies colluvial-wedge material and the top of the underlying buried soil to constrain the time of event Z to about 4.0-4.2 ka. However, calibration of those two 14C ages, assuming a carbon age span and a carbon mean residence time each of 200 years for both samples, results in a revised estimate for the timing of event Z of 4.3-4.6 ka.

Stratigraphic relations and numerical ages from the Bonneville trench bracket event Y between a TL age of 8.7±1.0 ka and the 14C age on the gastropod shell of 15,540±130 14C yr B.P. The scarp height at the Bonneville trench is roughly twice the height of the scarp at the Provo trench, implying that event Y predates formation of the Provo delta surface. McCalpin (1994) concludes that event Y occurred between about 13 ka (age of Provo delta) and 15.5 ka. However, Cur- rey’s revised Lake Bonneville chronology (D.L. Currey, University of Utah Department of Geography, written communication to the UGS, 1996; verbal communication to Working Group, 2004) places the age of the Provo delta between 16.8 and 16.2 ka. Calibration of the 14C age on the gastropod shell (not previously done) results in a calibrated age of 18.6+0.7/-0.6 cal yr B.P. (two sigma). Following McCalpin’s (1994) original reasoning, those revised ages place event Y between about 18 (maximum age of the Bonneville delta) and 16.2 ka (minimum age of the Provo delta).

Based upon currently available paleoseismic information, the Working Group’s consensus surface-faulting chronology for the central section of the ECFZ is: 70 Utah Geological Survey

Z 4.3-4.6 ka Y between 16.2 and 18 ka

Surface-Faulting Recurrence: Y/Z interevent interval (using the revised ages above): minimum 11.6 kyr, maximum 13.7 kyr, mean 12.7 kyr.

Evidence for an earlier surface-faulting earthquake (event X) during the Bonneville transgression is equivocal. However, McCalpin (1994) speculated that if a third earthquake did occur, the interval between events X and Y is likely only about 4 kyr, much shorter than the elapsed time between events Y and Z.

Because limited information regarding event X indicates possible large variations in the length of the intervals between events X and Y and Y and Z, the Working Group’s preferred recurrence-interval estimate for the ECFZ is intentionally broad to reflect possible large variations in time between surface-faulting earthquakes. Based on available information, the Working Group’s preferred recurrence-interval estimate and confidence limits for the central section of the ECFZ are:

4-10-15 kyr Vertical Slip Rate: McCalpin (1994) reports a long-term vertical slip rate for the central section of the ECFZ based on 8.5 m of displacement in pre-Bonneville alluvium of as high as 0.06 mm/yr, depending on the age assigned to the alluvium – but the actual alluvium age, and therefore the slip rate, is unknown.

The minimum (11.6 kyr), maximum (13.7 kyr), and mean (12.7 kyr) length of the Y-Z interevent interval, and a reported displacement for event Z of 0.5-1.2 m in the Provo trench (McCalpin, 1994), results in a range of vertical slip rates for the most recent interevent interval of 0.04-0.10 mm/yr with a mean slip rate of 0.07 mm/yr.

Because limited information regarding event X indicates a possible large variation in the length of the interevent intervals between events X and Y and Y and Z, the confidence limits for the Working Group’s preferred vertical slip- rate estimate for the ECFZ are intentionally broad to reflect possible large variations in time between surface-faulting earthquakes. Based on available information, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the central section of the ECFZ are:

0.04-0.2-0.4 mm/yr Summary: The ECFZ forms the eastern bounding fault of the Cache Valley graben. It consists of three sections in a manner similar to the West Cache fault zone (WCFZ), which forms the west side of the graben (Black and others, 2000). How- ever, unlike the WCFZ, only the central section of the ECFZ has been the subject of a paleoseismic-trenching investigation. Trenching studies conducted by Black and others (2000) documented Holocene surface faulting on all three segments of the WCFZ. In contrast, based on geologic mapping and geomorphic relations, McCalpin (1994) reports no evidence of Holocene surface faulting on the northern and southern sections of the ECFZ. McCalpin (GEO-HAZ Con- sulting, verbal communication, 2004) states that the range front geomorphology (faceted spurs) of the Bear River Range indicates a higher slip rate than has been documented on the ECFZ.

The earthquake hazard represented by the ECFZ to the rapidly urbanizing Cache Valley remains poorly understood. The occurrence of three different Holocene surface-faulting earthquakes, one on each of the three segments of the WCFZ on the west side of Cache Valley, raises questions regarding the timing of surface faulting on the three proposed sections of the ECFZ. The Working Group recommends additional paleoseismic investigations on the ECFZ to extend the surface-faulting chronology on the central section and to investigate the history of surface faulting on the northern and southern sections of the fault as well.

Additional References: Black, B.D., Giraud, R.E., and Mayes, B.H., 2000, Paleoseismic investigation of the Clarkston, Junction Hills, and Wellsville faults, West Cache fault zone, Cache County, Utah: Utah Geological Survey Special Study 98, 23 p. McCalpin, J.P., 1989, Surficial geologic map of the East Cache fault zone, Cache County, Utah: U.S. Geological Sur- vey Miscellaneous Field Studies Map MF-2107, scale 1:50,000. Consensus preferred recurrence-interval and vertical slip-rate estimates 71

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Hurricane fault zone (HFZ), Anderson Junction section (AJS), Washington County, Utah and Mohave County, Arizona

Paleoseismic Data Source Documents: Stenner, H.D., Lund, W.R., Pearthree, P.A., and Everitt, B.L., 1999, Paleoseismic investigation of the Hurricane fault, northwestern Arizona and southwestern Utah: Arizona Geological Survey Open-File Report 99-8, 137 p. Lund, W.R., Hozik, M.J., and Hatfield, S.C., 2005, Paleoseismic investigation of earthquake hazard and long-term movement history of the Hurricane fault in southwestern Utah: Utah Geological Survey Bulletin 134, compact disc. Stenner, H.D., Crosby, C.J., Dawson, T.E., Amoroso, L., Pearthree, P.A., and Lund, W.R., 2003, Evidence for variable slip from the last three surface-rupturing earthquakes along the central Hurricane fault zone [abs.]: Seismological Research Letters, v. 74, no. 2, p. 238.

Age of Youngest Faulting: Holocene

Discussion: The HFZ is a long (250 km), generally north-trending fault near the western margin of the Colorado Plateau in southwestern Utah and northwestern Arizona. From the Utah-Arizona border, the HFZ trends generally north along the steep Hurricane Cliffs, forming a narrow zone of subparallel, en echelon, west-dipping normal faults. Stewart and Tay- lor (1996) documented 450 m of stratigraphic separation in Quaternary basalt and a total separation of 2520 m across the HFZ near Anderson Junction in Utah. Displacement decreases southward; Pearthree (1998) indicated Cenozoic displacement of only 200-400 m along most of the HFZ in Arizona. Several swarms of historical seismicity have occurred adjacent to, but cannot be correlated directly with, the north end of the HFZ (Arabasz and Smith, 1979; Pechmann and others, 1995).

The AJS is one of six sections identified along the HFZ (Black and others, 2003). The AJS is near the center of the HFZ, extending approximately 45 km from north of Toquerville in Utah to south of Cottonwood Canyon in Arizona. The fault trace generally follows a high, north-trending, west-facing escarpment in Paleozoic bedrock. Scarps up to 30 m high with slopes up to 35° on late Pleistocene colluvium and alluvium mark the fault along the base of the escarpment.

Earthquake Timing: Stenner and others (1999) excavated two trenches on the AJS at Cottonwood Canyon in Arizona. One trench crossed a low fault scarp less than 1 m high formed on a stream terrace; the second trench crossed a 5-m-high scarp formed on an intermediate-age alluvial fan a few tens of meters to the south. Additionally, Stenner and others (2003) excavated a trench across a single fault scarp formed on an alluvial fan at Rock Canyon, approximately 4 km south of Cottonwood Canyon.

At Cottonwood Canyon, Stenner and others (1999) identified two surface-faulting earthquakes on the basis of stratigraphic displacement, shear fabric, and fault drag. Soil development on the faulted stream terrace implies a surface age of 8-15 ka. Based on stratigraphic relations in the trench across the low scarp on the terrace, Stenner and others (1999) estimated the timing of the MRE as occurring 5-10 ka (event Z). No carbon or other material suitable for dating was recovered from the first trench. Similarly, soil-profile development on the older alluvial-fan surface indicates an age of 25-50 ka. Charcoal from slope colluvium above fissure-fill material in the second trench yielded a 14C age of 870 years, which was interpreted as too young to be representative of the age of the colluvium and likely the result of bioturbation. Based on stratigraphic relations in the second trench, Stenner and others (1999) believe the timing of event Z is the same as in the first trench. The timing of event Y could not be determined in the second trench other than >5-10 ka and <25- 50 ka (age of the fan).

The trench at Rock Canyon revealed evidence for three surface-faulting earthquakes of variable displacement based on stratigraphic displacement, shear fabric, fault drag, fissuring, and minor graben formation. Laboratory results from bulk samples collected from the trench for 14C analyses are not yet available, so the timing of the three earthquakes is unknown. 72 Utah Geological Survey

The timing of surface-faulting earthquakes on the HFZ is poorly constrained; consequently, the Working Group’s consensus surface-faulting chronology for the Anderson Junction section of the HFZ, is limited to broad time intervals.

Z 5-10 ka Y >5-10 ka and <25-50 ka X >25-50 ka?

Surface-Faulting Recurrence: Stenner and others (1999, 2003) do not report recurrence intervals at either Cottonwood Canyon or Rock Canyon. However, the surface-age estimates at Cottonwood Canyon imply one surface-faulting earthquake in the past 8-15 kyr and likely within the past 5-10 kyr, and two earthquakes in 25-50 kyr. Neither age estimates for the surface of the alluvial fan, nor the three surface-faulting earthquakes at Rock Canyon are presently available. However, if events Y and Z at Rock Canyon are the same as at Cottonwood Canyon, then event X at Rock Canyon must be >25-50 kyr, since it was not recognized in the second trench at Cottonwood Canyon.

Because the timing of surface faulting on the AJS of the HFZ is poorly constrained, the Working Group’s preferred recurrence-interval estimate is reported as a range rather than as a central value with approximate 2-sigma confidence limits to reflect the large uncertainty associated with the data. Based on available information, the Working Group’s preferred recurrence-interval estimate for the AJS is:

5-50 kyr Vertical Slip-Rate: Based on scarp profiles measured at Cottonwood Canyon, Stenner and others (1999) calculated vertical slip rates of 0.1-0.3 mm/yr in ~70-125 ka deposits, and 0.1-0.4 mm/yr in ~25-50 ka deposits. Lund and others (in preparation) geo- chemically correlated and radiometrically dated (40Ar/39Ar) displaced basalt flows across the HFZ at the Ash Creek Sec- tion/AJS boundary, and at South Black Ridge, Pah Tempe Hot Springs, and Grass Valley, all on the AJS. These flows indicate a vertical slip rate since the middle Quaternary of ≥0.45 mm/yr, slowing to ≤0.2 mm/yr since 350 ka.

Based on available information, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the AJS are:

0.05-0.2-0.4 mm/yr Summary: Three of the six fault sections identified along the HFZ lie entirely or partially in southwestern Utah. The AJS is the southernmost of the three sections and straddles the Utah/Arizona border. The AJS is the only Utah section of the HFZ that has been successfully trenched, and both trench sites are on the Arizona portion of the section. The trenches provide evidence for three surface-faulting earthquakes; however, earthquake timing can only be constrained to broad time intervals.

Vertical slip-rate estimates for the AJS come from profiles measured across scarps formed on unconsolidated late Quaternary deposits and from displaced basalt flows. These data show a decreasing rate of slip from middle Quater- nary time to the present.

Additional References: Arabasz, W.J., and Smith, R.B., 1979, The November 1971 earthquake swarm near Cedar City, Utah, in Arabasz, W.J., Smith, R.B., and Richins, W.D., editors, Earthquake studies in Utah, 1850 to 1978: Salt Lake City, University of Utah Seismograph Stations Special Publication, p. 423-432. Black, B.D., Hecker, S., Hylland, M.D., Christenson, G.E., and McDonald, G.N., 2003, Quaternary fault and fold database and map of Utah: Utah Geological Survey Map 193DM, scale 1:50,000, compact disk. Pearthree, P.A., 1998, Quaternary fault data and map for Arizona: Arizona Geological Survey Open-File Report 98-24, scale 1:750,000, 122 p. Pechmann, J.C., Arabasz, W.J., and Nava, S.J., 1995, Seismology, in Christenson, G.E., editor, The September 2, 1992 ML 5.8 St. George earthquake, Washington County, Utah: Utah Geological Survey Circular 88, p.1. Stewart, M.E., and Taylor, W.J., 1996, Structural analysis and fault segment boundary identification along the Hurricane fault in southwestern Utah: Journal of Structural Geology, v. 18, p. 1017-1029. Consensus preferred recurrence-interval and vertical slip-rate estimates 73

UTAH QUATERNARY FAULT PARAMETER WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Great Salt Lake fault zone (GSLFZ), Davis, Weber, and Box Elder Counties, Utah

Paleoseismic Data Source Documents: Dinter, D.A., and Pechmann, J.C., 2000, Paleoseismology of the East Great Salt Lake fault: U.S. Geological Survey, National Earthquake Hazards Reduction Program Final Technical Report, award no. 98HQGR1013, 6 p. Coleman, S.M., Kelts, K.R., and Dinter, D.A., 2002, Depositional history and neotectonics in Great Salt Lake, Utah, from high-resolution seismic stratigraphy: Sedimentary Geology, v. 148, p. 61-78. Dinter, D.A., and Pechmann, J.C., 2004a, Segmentation and Holocene displacement history of the East Great Salt Lake fault: Salt Lake City, PowerPoint presentation to the 2004 Utah Earthquake Conference, 26 February, 2004. Dinter, D.A., and Pechmann, J.C., 2004b, Holocene segmentation and displacement history of the East Great Salt Lake fault, Utah [ext. abs.]: submitted to Proceedings of the Basin and Range Province Seismic Hazards Summit II, Reno, Nevada, May 16-19, 2004, 5 p.

Age of Youngest Faulting: Holocene

Discussion: The GSLFZ is a major active system of normal faults that lies submerged beneath Great Salt Lake along the west side of Antelope and Fremont Islands and the Promontory Mountains peninsula (Coleman and others, 2002). The GSLFZ is north of and generally on trend with the Oquirrh fault zone (OFZ; Olig and others, 1996), which bounds the east side of Tooele Valley at the base of the Oquirrh Mountains. Dinter and Pechmann (2000, 2004a, 2004b) subdivided the GSLFZ into three segments on the basis of high-resolution seismic-reflection data. From south to north the three segments are the Antelope Island segment (AIS), Fremont Island segment (FIS), and Promontory segment (PS). The boundary between the AIS and FIS is a 1-2 km left step in the fault west of White Rock Bay on Antelope Island. The AIS is 35 km long, is marked by a lakebed scarp up to 3.6 m high, and bends sharply to the southwest at its southern end where it appears to merge with the OFZ. The FIS is 30 km long and has no fault scarps along most of its length. No high-resolution seismic-reflection data are presently available for the PS.

The high-resolution seismic-reflection data collected for both the AIS and FIS show evidence for three lake-sediment-displacing earthquakes in post-Lake Bonneville time. Subsequent drilling and sampling at critical locations along the two fault segments resulted in the recovery of carbon suitable for 14C dating.

Earthquake Timing: Dinter and Pechmann (2004a, 2004b) report the following earthquake timing for the AIS and FIS of the GSLFZ.

Event 14C yr B.P.1 cal yr B.P.2 Residence-corrected Residence-corrected cal yr B.P.3 cal yr before 2004 Antelope Island segment EH-A3 (Z) >804±38 >706+81/-40 586+201/-2415 640+201/-2415 <1027±44 <944+106/-147 EH-A2 (Y) 5711±50 6491+163/-135 6170+236/-234 6224+236/-234 EH-A1 (X) 9068±66 10,219+178/-234 9898+247/-302 9952+247/-302 Fremont Island segment EH-F3 (Z) 3269±47 3471+161/-90 3150+235/-211 3204+235/-211 EH-F2 (Y) 5924±44 6733+121/-90 6412+209/-211 6466+209/-211 EH-F1 (X) <10,155±72 <11,748+580/-406 <11,427+605/-449 <11,481+605/-449 1Before 1950, 1-sigma confidence limits. 2Before 1950, 2-sigma confidence limits, Stuiver and others (1998), terrestrial calibration (CALIB v. 4.3). 3Before 1950, 2-sigma confidence limits, correction for carbon residence time in provenance area prior to deposition = 321+191/-171 cal yr – the difference between the terrestrially calibrated 14C date of Mazama ash interval in the AIS core (7994 +170/-128 cal yr BP) and terrestrial calibration (7673+133/-86 cal yr BP) of published Mazama 14C age (6845 ± 50 14C yr B.P., Bacon, 1983). 42-sigma error bars. 5Age of event horizon interpolated from dates of material collected above and below it. 74 Utah Geological Survey

Based on available data, the Working Group adopts the Dinter and Pechmann (2004a, 2004b, written communication 2004) surface-faulting chronology presented above as their consensus chronology for the GSLFZ.

Surface-Faulting Recurrence: Dinter and Pechmann (2004a, 2004b) report the following earthquake recurrence intervals for the AIS and FIS of the GSLFZ. Their average single-segment recurrence interval of 4200±1400 yrs represents the mean, with 2-sigma confidence limits, for the three closed recurrence intervals.

Earthquake Pairs Timing Recurrence Interval (yr)4 (terrestrially calibrated1, residence corrected2, cal yr B.P.3)4

Antelope Island segment EH-A3 586+201/-241 5584+219/-172 EH-A2 6170+236/-234 EH-A2 6170+236/-234 3728+223/-285 EH-A1 9898+247/-302 Fremont Island segment EH-F3 3150+235/-211 3262+151/-184 EH-F2 6412+209/-211 EH-F2 6412+209/-211 <5015+587/-424 EH-F1 <11,427+605/-449 Average single-segment recurrence interval = 2800-4200-5600 years

1Radiocarbon years were converted to calendar years using Stuiver and others (1998) terrestrial calibration (CALIB v.4.3; Stuiver and Reimer, 1993) 2Correction for carbon residence time in provenance area prior to deposition = 321+191/-171 cal yr, the difference between the terrestrially calibrated 14C date of Mazama ash interval at site GSL00-3 (=7994+170/-128 cal yr B.P.) and terrestrial calibration (=7673+113/-86 cal yr B.P.) of published Mazama 14C age (6845±50 14C yr B.P.; Bacon [1983]). 3Calendar years before 1950 42-sigma confidence limits 5Calculated from surface rupture length using an empirical relation for normal faults from Wells and Coppersmith (1994); confidence limits are 1-sigma.

Vertical Slip Rate: J.C. Pechmann and D.A. Dinter (University of Utah, written communication to Working Group, 2004) report a slip rate applicable to both the AIS and FIS of 0.3-0.6-1.6 mm/yr. This slip rate is based upon net vertical tectonic displacements (NVTD) determined at 17 locations across the sublacustrine AIS scarp from high-resolution seismic profiles. Integration of the 17 NVTD measurements versus distance along the fault and dividing by the length of the AIS fault trace resulted in an average NVTD of 2.3±0.6 m. Pechmann and Dinter interpret the average NVTD along the AIS to be the average during the most recent surface-faulting earthquake on the segment, which occurred at 586 +201/-241 cal yr B.P. Their basis for this interpretation is (1) the observation that there is a prominent fault scarp along the AIS, but not along most of the FIS, which has not had a surface-faulting earthquake since 3150+235/-211 cal yr B.P., and (2) that their seismic-reflection data show no significant difference in the amount of sediment accumulation between the two sides of the fault since the last earthquake. The slip rate comes from dividing the average AIS NVTD of 2.3–0.6 m by the average recurrence interval of 4200–1400 yrs. They assume that the slip rate for the AIS is also applicable to the FIS given the similarities in sediment thickness along both segments (Viveiros, 1986).

Summary: Information on surface-faulting timing and NVTD for the GSLFZ comes from high-resolution seismic-reflection profiles and drilling information. Because the GSLFZ lies entirely submerged beneath Great Salt Lake no trench data are available for this fault.

Based on the available data, the Working Group adopts the values reported by Dinter and Pechmann (2004a, 2004b, written communication 2004) as their consensus surface-faulting chronology and average recurrence-interval and vertical slip-rate estimates for both the FIS and AIS of the GSLFZ. However, for consistency with the Working Group consensus parameters for other faults, the uncertainty limits on the average recurrence interval have been changed from two standard deviations of the mean to two standard deviations of the distribution (2400 yrs). The uncertainty limits on the slip rate were correspondingly adjusted.

Recurrence interval 1800-4200-6600 years Vertical slip rate 0.3-0.6-1.6 mm/yr Consensus preferred recurrence-interval and vertical slip-rate estimates 75

References: Bacon, C.R., 1983, Eruptive history of Mount Mazama and Crater Lake caldera, Cascade Range, USA: Journal of Vol- canology and Geothermal Research, v. 18, p. 57-115. Olig, S.S., Lund, W.R., Black, B.D., and Mayes, B.H., 1996, Paleoseismic investigation of the Oquirrh fault zone, Tooele County, Utah, in Lund, W.R., editor, Paleoseismology of Utah, Volume 6, The Oquirrh fault zone, Tooele County, Utah -Surficial geology and paleoseismicity: Utah Geological Survey Special Study 88, p. 22-64. Stuiver, M., and Reimer, P., 1993, University of Washington Quaternary Isotope Lab Radiocarbon Calibration Program Rev. 4.3: Radiocarbon, v. 35, p. 215-230. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., and Braziunas, T.F., 1998, Revised calibration dataset: Radiocarbon, v. 40, p. 1127-1151. Viveiros, J.J., 1986, Cenozoic tectonics of the Great Salt Lake from seismic reflection data: University of Utah, Salt Lake City, M.S. thesis, 99 p. Wells, D.L., and Coppersmith, K.J., 1994, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement: Bulletin of the Seismological Society of America, v. 84, p. 974-1002. 76 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Oquirrh fault zone (OFZ), Tooele County, Utah

Paleoseismic Data Source Documents: Olig, S.S., Lund, W.R., Black, B.D., and Mayes, B.H., 1996, Paleoseismic investigation of the Oquirrh fault zone, Tooele County, Utah, in Lund, W.R., editor, Paleoseismology of Utah, Volume 6, The Oquirrh fault zone, Tooele Coun- ty, Utah - Surficial geology and paleoseismicity: Utah Geological Survey Special Study 88, p. 22-64.

Age of Youngest Faulting: Holocene

Discussion: The OFZ is a north-trending, range-front normal fault bounding the east side of Tooele Valley at the western base of the Oquirrh Mountains. Scarps formed on alluvium and lake deposits range from 12 to 18 m high, have maximum slope angles of 24 to 32°, and surface displacements of 4.0 to 6.8 m. Everitt and Kaliser (1980) and Barnhard and Dodge (1988) divided the OFZ into two sections: a northern section expressed by Quaternary fault scarps formed on basin-fill sediments, and a southern section expressed as a prominent break in slope at the base of the range front. Large displacements documented on the northern section imply a rupture length greater than 12 km (the length of the northern trace), suggesting both sections of the fault probably form a single rupture segment extending from the town of Stock- ton to Great Salt Lake (Solomon, 1996).

Olig and others (2001) excavated trenches at two sites where the OFZ crosses Lake Bonneville deposits overlain by late Holocene alluvium/colluvium. Three trenches at the Big Canyon site were about 0.3 km west of the mouth of Big Canyon. Radiocarbon ages on bulk samples from debris-flow deposits directly overlain by colluvial-wedge material, and from unfaulted fluvial deposits that bury the fault scarp, constrain the timing of the MRE. The Big Canyon trenches did not expose evidence for older earthquakes.

A single trench at the Pole Canyon site 1.7 km northwest of the mouth of Pole Canyon lacked diagnostic stratigraphy and datable organic material necessary to resolve the timing of event Z beyond a post-Bonneville age. Radiocar- bon ages from charcoal in a Bonneville transgressive marsh deposit and an older fluvial deposit constrain the timing of the PE. A 14C age from charcoal in fluvial sediments that bury the eroded free face of the antepenultimate earthquake (event X) provides a broadly limiting minimum age for that earthquake.

Earthquake Timing: Olig and others (1996) identified the three most recent surface-faulting earthquakes on the OFZ. Evidence for events Y and Z consists of scarp-derived colluvial-wedge deposits. A buried scarp free face provides indirect evidence for event X; the trench was not deep enough to expose the event X colluvial wedge.

Event Z At Big Canyon, bulk samples from the youngest faulted deposit, an organic-rich debris flow, yielded 14C ages of 6840±100 and 7650±90 14C yr B.P. Because the 14C age from the debris flow came from a mix of detrital material entrained in the debris flow when it was active, Olig and others (1996) considered the younger age to better represent a maximum limit for the timing of event Z. Calendar calibrated and rounded to the nearest century, the younger age is 7600+300/-100 cal yr B.P. A bulk sample of an unfaulted, organic-rich, debris-flow deposit 0.5 m above the event Z colluvial wedge yielded a calendar-calibrated age of 4900±100 cal yr B.P., which provides a minimum limit on event Z timing. At Big Canyon, therefore, event Z timing is constrained within a 3100 yr interval between 4800 and 7900 cal yr B.P. Where within that time period the earthquake occurred is not known; however, Olig and others (1996) report event Z timing as 6350±1550 cal yr B.P., which is the middle of the interval.

The Pole Canyon trench provided neither stratigraphic relations nor datable organic material that allowed the timing of event Z to be constrained more closely than at Big Canyon.

Event Y At Pole Canyon, three 14C ages help constrain the timing of event Y. Two ages came from a faulted, charcoal-rich, Consensus preferred recurrence-interval and vertical slip-rate estimates 77 channel-fill deposit. A single, large charcoal fragment yielded an age of 33,950±1160 14C yr B.P., and several small detrital charcoal fragments combined for dating yielded an average age of 26,200±200 14C yr B.P. Both 14C ages are too old to calendar calibrate (Stuiver and Reimer, 1993), and Olig and others (1996) considered the younger age the best limiting age for the deposit, and a maximum limit on event Y timing, which must be younger than the deposits it displaces.

The third 14C age came from detrital charcoal recovered from a lake-marginal marsh deposit at the base of the Lake Bonneville transgressive sequence directly overlying the event Y colluvial wedge. The charcoal yielded a 14C age of 20,370±120 14C yr B.P., which is also too old to calendar calibrate.

Therefore, event Y occurred within an approximate 6100 14C yr interval between 26,400 and 20,300 14C yr B.P. Where within that time period the earthquake occurred is not known; however, Olig and others (1996) report event Y timing as 23,350±3100 14C yr B.P., which is the middle of the interval.

Event X The Pole Canyon trench provides indirect evidence to partially constrain the timing of the antepenultimate earthquake. The charcoal-rich, stream-channel deposit (see above) exposed in the trench unconformably overlies a stratigraphic package, which includes post-event X slope colluvium that mantles the event X free face. The event X colluvial wedge remained buried beneath the trench floor. The sedimentary units comprising the older stratigraphic package did not contain organics and could not be directly dated. However, the event X free face lies beneath the stream-channel deposit and therefore the earthquake is older than 26,400 14C yr B.P (see above). How much older is unknown; however, a paleosol formed on the colluvial unit overlying the event X free face includes strong Bt and Bk (stage III) horizons, implying a long period of soil formation. Therefore, event X could be several thousand to several tens of thousands of years older than event Y.

Based upon currently available paleoseismic information, the Working Group’s consensus surface-faulting chronology for the OFZ is:

Z between 4800 and 7900 cal yr B.P Y between 20,300 and 26,400 14C yr B.P. X > 26,400 14C yr B.P.

Surface-Faulting Recurrence: Data on surface-faulting recurrence for the OFZ are restricted to a single, poorly constrained interevent interval. Age estimates for earthquakes Y and Z constrain the most recent interevent interval on the OFZ as follows: minimum = 12.4 kyr, maximum = 21.6 kyr, mean = 17 kyr.

Because information on surface-faulting recurrence for the OFZ is poorly constrained, the confidence limits for the Working Group’s preferred recurrence-interval estimate are intentionally broad to reflect high uncertainty.

5-20-50 kyr

Vertical Slip Rate: Using the above maximum, minimum, and mean estimates for the length of the Y-Z interevent interval, and a net vertical displacement for event Z of 2.0-2.7 m as reported by Olig and others (1996) from the Big Canyon and Pole Canyon trench sites, results in a vertical slip rate for the most recent interevent interval of 0.09-0.14-0.22 mm/yr.

Because surface-faulting recurrence and net vertical displacement for the OFZ are poorly constrained, the confidence limits for the Working Group’s preferred vertical slip-rate estimate for the OFZ are intentionally broad to reflect high uncertainty.

0.05-0.2-0.4 mm/yr

Summary: Paleoseismic information available for the OFZ is limited. Timing of the two most recent earthquakes (Y and Z) is constrained to broad intervals, each thousands of years long. Indirect evidence for a penultimate earthquake (X) indicates that event occurred several thousand to tens of thousands of years prior to event Y, but the actual age of event X remains unknown. Likewise, vertical slip-rate estimates are based on net vertical-displacement measurements for event Z from just two locations along the fault trace. How representative those displacements are of the long-term slip distribution along the fault is unknown. Net vertical-displacement measurements for longer time intervals from scarp profiles are not possible because the OFZ scarps lie below the highstand of Lake Bonneville and were heavily modified as 78 Utah Geological Survey the lake transgressed and regressed across them.

Results of the Olig and others (1996) investigation show that the OFZ is a low-slip-rate fault typical of much of the Basin and Range Province. Comparison of earthquake timing on the OFZ with that on the Salt Lake City segment (SLCS) of the Wasatch fault zone (WFZ), approximately 45 km to the east, shows little or no correlation between surface-faulting earthquakes. The SLCS has had as many as six surface-faulting earthquakes in the Holocene; the OFZ has had one. Because the timing of event Z on the OFZ is only broadly constrained, it is not possible to correlate it with a particular Holocene earthquake on the WFZ.

The OFZ lies on trend with the Great Salt Lake fault zone to the north and the Southern Oquirrh Mountains fault zone (SOMFZ) to the south. All three faults exhibit Holocene surface faulting, and Olig and others (1996) and Olig and others (2001) speculated that the three faults may be part of a single, more than 200-km-long fault zone that parallels the Weber, Salt Lake City, and Provo segments of the WFZ to the east. Additionally, Olig and others (2001) believe that the OFZ and the SOMFZ have ruptured coseismically, at least since the late Pleistocene.

Additional References: Barnhard, T.P., and Dodge, R.L., 1988, Map of fault scarps formed on unconsolidated sediments, Tooele 1° x 2° quadrangle, northwestern Utah: U.S. Geological Survey Miscellaneous Field Studies Map MF-1990, scale 1:250,000. Everitt, B.L, and Kaliser, B.N., 1980, Geology for assessment of seismic risk in Tooele and Rush Valleys, Tooele Coun- ty, Utah: Utah Geological and Mineral Survey Special Study 51, 33 p. Olig, S.S., Gorton, A.E., Black, B.D., and Forman, S.L., 2001, Paleoseismology of the Mercur fault and segmentation of the Oquirrh - East Great Salt Lake fault zone, Utah: Oakland, California, URS Corporation, unpublished Techni- cal Report for U.S. Geological Survey, award no. 98HQGR1036, variously paginated. Solomon, B.J., 1996, Surficial geology of the Oquirrh fault zone, Tooele County, Utah, in Lund, W.R., editor, Paleo- seismology of Utah, Volume 6 ,The Oquirrh fault zone, Tooele County, Utah - Surficial geology and paleoseismicity: Utah Geological Survey Special Study 88, p. 1-17. Stuiver, M., and Reimer, P.J., 1993, Extended 14C database and revised CALIB 3.0 14C calibration program: Radiocar- bon, v. 35, no. 1, p. 215-230. Consensus preferred recurrence-interval and vertical slip-rate estimates 79

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Southern Oquirrh Mountains fault zone (SOMFZ), Tooele County, Utah

Paleoseismic Data Source Documents: Everitt, B.L., and Kaliser, B.N., 1980, Geology for assessment of seismic risk in the Tooele and Rush Valleys, Tooele County, Utah: Utah Geological and Mineral Survey Special Studies 51, 33 p. Barnhard, T.P., and Dodge, R.L., 1988, Map of fault scarps formed on unconsolidated sediments, Tooele 1° x 2° quadrangle, northwestern Utah: U.S. Geological Survey Miscellaneous Field Study Map MF-1990, scale 1:250,000. Olig, S.S., Gorton, A.E., and Chadwell, L., 1999, Mapping and Quaternary fault scarp analysis of the Mercur and West Eagle Hill faults, Wasatch Front, Utah: Oakland, California, URS Greiner Woodward Clyde, National Earthquake Hazards Reduction Program Final Technical Report, award no. 1434-HQ-97-GR-03154, variously paginated, scale 1:48,000. Olig, S.S., Gorton, A.E., Black, B.D., and Forman, S.L., 2000, Evidence for young, large earthquakes on the Mercur fault - implications for segmentation and evolution of the Oquirrh-East Great Salt Lake fault zone, Wasatch Front, Utah [abs.]: Geological Society of America Abstracts with Programs, 2000 Annual Meeting, v. 32, no. 7. —2001, Paleoseismology of the Mercur fault and segmentation of the Oquirrh - East Great Salt Lake fault zone, Utah: Oakland, California, URS Corporation, unpublished Technical Report for U.S. Geological Survey, award no. 98HQGR1036, variously paginated.

Age of Youngest Faulting: Holocene

Discussion: The SOMFZ consists of en echelon, down-to-the-west normal faults bounding the western flank of the southern Oquirrh Mountains. Olig and others (1999) defined the SOMFZ as including the previously recognized Mercur (MF), West Eagle Hill (WEHF), Soldier Canyon (SCF), and Lakes of Kilarney (LKF) faults. The MF and WEHF comprise 17 km of the total SOMFZ along-strike length of 25 km, and show evidence for repeated Quaternary displacement in late Pleistocene alluvial fans and terraces. The SCF and LKF comprise the remaining 8 km and are chiefly expressed in bedrock or as bedrock-alluvial contacts. Barnhard and Dodge (1988) report that MF scarps show displacements of 1.8 to 5.6 m. Faulted alluvium exposed in a mineshaft, and an uplifted bedrock pediment suggests a minimum of 60 m of Quaternary displacement on the MF (Everitt and Kaliser, 1980). Olig and others (1999) report that net vertical displacements of intermediate-age surfaces on the fault zone as a whole average 5.3 to 6.3 m, and are 1.0 to 2.0 m on the MF and WEHF, respectively. Maximum displacements on older surfaces are 21.7 and 2.8 m, respectively. Displace- ment patterns indicate faulting has shifted basinward and most Quaternary displacement was partitioned on the MF, although coseismic rupture on both faults remains a possibility.

Everitt and Kaliser (1980) excavated a trench near the southern end of the MF, about 4.5 km west of Fivemile Pass and just south of where the scarp intersects the Bonneville shoreline. Trench stratigraphy revealed evidence for repeated surface faulting during the late Pleistocene, and a 0.6-m-high scarp was interpreted as indicating post-Bonneville displacement. Barnhard and Dodge (1988) reinterpreted Everitt and Kaliser's trench data, analyzed fault-scarp morphology from 11 scarp profiles, and excavated a shallow trench just south of the Everitt and Kaliser (1980) trench; they found no evidence of post-Bonneville surface faulting. Neither study included numerical age dating. Olig and others (2001) trenched three traces of the MF where it crosses alluvial-fan deposits about 30 km south of Tooele, near the intersection of Utah Highway 73 and Mercur Canyon Road. The trenching revealed evidence for five to seven surface-faulting earthquakes since about 92±14 ka and as recently as about 4.6±0.2 ka.

Earthquake Timing: Olig and others (2001) identified evidence for five to seven surface-faulting earthquakes in three trenches (C-center, E-east, and W-west) across the MF. The evidence included stacked colluvial-wedge stratigraphy and differential displacement and crosscutting relations. Earthquake timing is summarized below:

Z shortly after 4.6±0.2 ka and well before 1.4±0.1 ka Y between 20 and 50 ka X shortly after 42±8 ka – may or may not correlate with earthquakes VC or WE 80 Utah Geological Survey

W shortly after 75±10 ka – may or may not correlate with earthquakes VC and WE, although event VC is probably older V around (shortly after?) 92±14 ka

Olig and others (2001) considered the timing of the above five earthquakes, although broadly constrained, to be well established. Uncertainty regarding the total number of earthquakes comes from difficulty in correlating earthquakes between trenches. Specifically, earthquakes VC and WE may correlate with one or more earthquakes in the west trench, or they may represent separate earthquakes, resulting in the possibility of at least five to as many as seven surface-faulting earthquakes on the SOMFZ. The above earthquake chronology is constrained by two 14C ages and six infrared spin luminescence ages from buried vesicular A horizon soils.

Based on available data, the Working Group adopts the Olig and others (2001) surface-faulting chronology presented above as their consensus chronology for the SOMFZ.

Surface-Faulting Recurrence: Olig and others (2001) calculated average recurrence intervals for the SOMFZ based on five to seven earthquakes between 92±14 and 4.6±0.2 ka, resulting in average recurrence intervals of 12 to 25 kyr. However, they state that due to uncertainty in earthquake timing, intervals between earthquakes could be as great as 46 kyr. Additionally, they note that the lack of soil development between earthquakes XW and YW in contrast to the strong soil development between earthquakes XW and ZW, suggests an order of magnitude or more difference in the length of individual interevent intervals.

Because available paleoseismic information indicates that interevent intervals on the SOMFZ may vary by an order of magnitude, the confidence limits for the Working Group’s recurrence-interval estimate for the OFZ are intentionally broad to reflect high uncertainty. Additionally, Olig and others (2001) believe that the SOMFZ likely ruptures coseismically with the OFZ to the north, and their recurrence-interval estimate reflects that possibility. Based upon available paleoseismic information, the Working Group’s preferred recurrence-interval estimate and confidence limits for the SOMFZ are:

5-20-50 kyr Vertical Slip Rate: Based on data for the past four to six interevent intervals on the SOMFZ, Olig and others (2001) report an average slip rate of 0.09-0.14 mm/yr for the past approximately 90 kyr.

The Working Group’s preferred vertical slip-rate estimate for the SOMFZ reflects the Olig and others (2001) long- term average above; however, the confidence limits have been increased to accommodate uncertainty resulting from possible large variations in slip through time. Additionally, Olig and others (2001) believe that the SOMFZ likely ruptures coseismically with the OFZ to the north, and consequently the fault’s vertical slip-rate estimates also reflect that possibility. Based upon available paleoseismic information, the Working Group’s preferred vertical slip-rate estimate and confidence limits for the SOMFZ are:

0.05-0.2-0.4 mm/yr Summary: The Olig and others (2001) study demonstrates that the SOMFZ is a low-slip-rate, long recurrence-interval normal fault typical of the Basin and Range Province. The timing of the surface-faulting earthquakes identified by trenching could only be broadly constrained even after careful paleoseismic study. The SOMFZ and the OFZ to the north lie on trend with each other along the western base of the Oquirrh Mountains. Based on range-front geomorphology and similarities in the timing of events X, Y, and Z on both the SOMFZ and OFZ, Olig and others (2001) believe that the two faults have ruptured coseismically, at least since the late Pleistocene. Consensus preferred recurrence-interval and vertical slip-rate estimates 81

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Eastern Bear Lake fault (EBLF) southern section, Rich County, Utah and Bear Lake County, Idaho

Paleoseismic Data Source Documents: McCalpin, J.P., 1990, Latest Quaternary faulting in the northern Wasatch to Teton corridor (NWTC): Final Technical Report for U.S. Geological Survey, contract no. 14-08-001-G1395, 42 p. 1993, Neotectonics of the northeastern Basin and Range margin, western USA, in Stewart, I., Vita-Finzi, C., and Owen, L., editors, Neotectonics and active faulting: Zeitschrift für Geomorphologie, Supplement Bd., p. 137-157. 2003, Neotectonics of Bear Lake Valley, Utah and Idaho; A preliminary assessment: Utah Geological Survey Mis- cellaneous Publication 03-4, 43 p.

Age of Youngest Faulting: Holocene

Discussion: The EBLF is a 78-km-long, west-dipping normal fault bounding the east side of the Bear Lake Valley half graben in Utah and Idaho (McCalpin, 1990, 1993, 2003). Seismic-reflection data show that the lake floor and reflectors within the underlying Neogene sediments dip eastward into the EBLF (Skeen, 1975). Total throw of the top of the Eocene Wasatch Formation across the fault is about 1.5 km at the north end of Bear Lake (McCalpin, 1990). McCalpin (2003) divides the EBLF into northern, central, and southern sections on the basis of fault-rupture patterns, morphology of fault scarps, and subsurface geophysical data. Only part of the southern section is in Utah. Whether these geomorphic sections define earthquake rupture segments cannot be determined from currently completed paleoseismic studies because paleoseismic data are only available for the southern section of the fault (McCalpin, 2003).

The southern section of the EBLF extends for 32 km from Laketown, Utah in the south to Bear Lake Hot Springs at the northeastern corner of Bear Lake in Idaho. Along its length, the southern section is marked by discontinuous fault scarps up to 13 m high in Quaternary deposits at the base of a steep escarpment of Mesozoic rocks on the east side of Bear Lake. Scarps are best developed where the fault crosses the mouths of major drainages such as at North Eden Creek, where McCalpin (1990, 1993, 2003) excavated two trenches.

Earthquake Timing: At North Eden Creek, McCalpin (2003) constrained the timing of surface faulting on the southern section of the EBLF as follows: West Trench Z <2.1±0.2 ka but >0.6±0.08 ka Y ~5 ka (2.5 ka TL age estimate considered erroneously young)

East Trench Y >5.0±0.5 ka, but likely not much greater X <31±6 ka but much >15.2±0.8 ka W >31±6 ka but <39±3 ka V >31±6 ka but <39±3 ka U >39±3 ka, but likely not much greater

Despite a young TL age (2.5 ka) in the west trench indicating that the two earthquakes recorded there occurred within the past 2.5 kyr, McCalpin (2003) questioned how both surface-faulting earthquakes can be younger than 2.5 ka, and why neither earthquake was recognized in the eastern trench only a few tens of meters away. He hypothesized that event Y in the western trench may be older than 2.5 ka, and that the soil sample from the western trench that gave the 2.5±0.5 ka TL age may have been inadvertently collected from younger material, possibly in an animal burrow. In that case, event Y in the western trench may correlate with event Y in the eastern trench (hence the same letter designation in the chronology above) and there would only be one earthquake younger than about 5 ka.

Based on available data, the Working Group adopts the McCalpin (2003) surface-faulting chronology presented above as their consensus chronology for the southern section of the EBLF. 82 Utah Geological Survey

Surface-Faulting Recurrence: McCalpin (2003) reports a mean recurrence for the southern section of the EBLF over the past five interevent intervals (events U to Z) of 7.6 kyr. However, although earthquake timing is generally poorly constrained on the EBLF, the elapsed time between individual surface-faulting earthquakes appears highly variable, ranging from about 2.9 kyr between earthquakes Y and Z to a minimum of 10.2 kyr between earthquakes X and Y.

Because available paleoseismic information indicates that interevent intervals on the southern section of the EBLF may be highly variable, the confidence limits for the Working Group’s preferred recurrence-interval estimate are intentionally broad to reflect high uncertainty. Based upon available paleoseismic information, the Working Group’s preferred recurrence-interval estimate and confidence limits for the southern section of the EBLF are:

3-8-15 kyr Vertical Slip Rate: McCalpin (2003) reports that all but 1.2 m of the net ≥23.3 m of vertical displacement measured at the North Eden site occurred since 39 ka. Therefore, over the past five interevent intervals, ≥22.1 m of net vertical displacement has been released in about 38 kyr for an average vertical slip rate of ≥0.58 mm/yr. However, this slip rate may be affected (made smaller) by undetected antithetic faulting beneath Bear Lake or by unmeasured tectonic back-tilting, both of which would decrease stratigraphic displacement and, if not accounted for result in net vertical-displacement values that are too large.

Paleoseismic data show that vertical slip rates for individual interevent intervals on the central section of the EBLF are highly variable, reflecting variability in the length of time between individual earthquakes and in the net vertical displacement per earthquake. Therefore, the confidence limits for the Working Group’s preferred recurrence-interval estimate are intentionally broad to reflect high uncertainty. Based upon available paleoseismic information, the Working Group’s preferred recurrence-interval estimate and confidence limits for the southern section of the EBLF are:

0.2-0.6-1.6 mm/yr Summary: The EBLF consists of three geometric sections that may or may not be independent seismogenic segments. Paleo- seismic data are only available for a single location on the southern section. Those data indicate that there have been six surface-faulting earthquakes on the southern part of the EBLF in the past ~40 kyr. Only the timing of the youngest earthquake is well constrained, and available net-vertical-displacement measurements are mostly minimum estimates.

Additional References: Skeen, R.C., 1975, A reflection seismic study of the subsurface structure and sediments of Bear Lake, Utah-Idaho: Salt Lake City, University of Utah, senior thesis, 25 p. Consensus preferred recurrence-interval and vertical slip-rate estimates 83

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Bear River fault zone (BRFZ), Summit County, Utah and Uinta County, Wyoming

Paleoseismic Data Source Documents: West, M.W., 1994, Seismotectonics of north central Utah and southwestern Wyoming: Utah Geological Survey Special Study 82, 93 p.

Age of Youngest Faulting: Holocene

Discussion: The BRFZ extends for about 40 km from southeast of Evanston, Wyoming to the north flank of the Uinta Moun- tains in Utah, where it ends at a complex juncture with the Laramide-age North Flank thrust fault. In general, the BRFZ consists of distinct individual scarps each about 3.0 to 3.5 km long arranged in a right-stepping, en echelon pattern. Major scarps trend N. 20° W. to N. 20° E., and show consistent down-to-the-west displacement. Scarps with lesser, down-to-the-east displacements are interpreted to be antithetic faults and trend N. 15-20° W. Near the south end of the fault zone, scarps show a strong angular discordance (70°) with the main north-northeast pattern of faulting, likely due to the buttressing effect of the Uinta Mountains. Scarp heights and tectonic displacements increase markedly from north to south along the BRFZ. Fault scarps are between 0.5 and 15 m high on upper Quaternary deposits; sag ponds, behead- ed drainages, and antithetic fault scarps are also present. Late Holocene to historical landsliding obscures evidence of faulting through an approximately 9-km-wide gap between the southernmost clearly defined scarps in Wyoming and the northernmost scarps in Utah.

Earthquake Timing: West (1994) excavated seven trenches, logged an irrigation ditch exposure, and measured 14 scarp profiles on the BRFZ. The irrigation ditch exposure and four of the trenches were on scarps in Wyoming; the other three trenches were in Utah. Results of the trenching showed that there has been a minimum of two surface-faulting earthquakes on the BRFZ based on stacked colluvial-wedge stratigraphy, although additional poorly constrained earthquakes are considered possible on some scarps.

Constraints on earthquake timing were provided by 14C ages and amino acid racemization ratios obtained from land snail shells. West (1994) reported timing for the two most recent surface-faulting earthquakes on the BRFZ as:

Z 2370 ±1050 yr B.P. Y 4620±690 yr B.P.

The timing estimates for both earthquakes are “best estimate” mean values calculated from the youngest and oldest constraining ages obtained for the two earthquakes, as determined from the trenches excavated across the BRFZ scarps. The “±” confidence limits reflect the greater of the differences between the mean value and the youngest or oldest possible earthquake timing, and are considered by West to incorporate both the analytical uncertainty of the 14C ages and the geologic uncertainty associated with earthquake timing. However, earthquake timing is reported as “yr B.P.” because, while all 14C ages on soil organics were calendar calibrated according to de Jong and others (1986), Linick and others (1986), and Pearson and Stuiver (1986), no mean residence corrections (Machette and others, 1992, appendix A) were subtracted from the calibrated ages to account for the age of the carbon in the soil at the time of burial. As a result, West (1994) states:

It is likely that the estimated ages [timing] of the surface-faulting events in the project area (4620±690 and 2370±1050 yr B.P.) may be too old by at least several hundred years.

The estimate of several hundred years too old reflects West’s belief that the soils in his study area are significantly better developed and therefore older than the soils studied by Machette and others (1992) along the Wasatch fault zone, where the average soil age was estimated as 200-400 years.

Recognizing that the timing of surface-faulting earthquakes reported by West (1994) may be too old by several hun- 84 Utah Geological Survey dred years, the Working Group accepts the earthquake timing as published as representing the currently “best available” information for the BRFZ.

Surface-Faulting Recurrence: The interevent interval between events Y and Z is 2250 (+690/-1050) yrs. The timing for event Z is 2370±1050 yr B.P., indicating that the elapsed time since the MRE exceeds the interevent interval between events Y and Z.

The Working Group recognizes the likelihood of a young age for the BRFZ, but also notes that in many places the BRFZ trends across a stable landscape that likely is hundreds of thousands of years old. This raises the possibility of an alternative fault-behavior model for the BRFZ, one of large infrequent earthquakes or clusters of earthquakes similar to the Pitaycachi fault in the southern Basin and Range Province, which produced a M>7.2 earthquake in Sonora, Mexico in 1887. The Pitaycachi fault is believed to have a recurrence interval on the order of 100 kyr (Bull and Pearthree, 1988). A long-recurrence interval fault likewise would leave little contemporary geomorphic evidence of its past history.

Unable to resolve which of the two possible fault-behavior models applies to the BRFZ based on available paleoseismic information, the Working Group’s preferred recurrence-interval estimate is reported as a broad range, rather than as a central value with approximate 2-sigma confidence limits, to better reflect the high level of uncertainty regarding surface-faulting recurrence on the BRFZ. 1-100 kyr Vertical Slip Rate: The vertical slip rates reported by West (1994) for BRFZ scarps are open ended because the time intervals used in his calculations extend to the present. Recalculating vertical slip rates for the BRFZ using the interevent interval between events Y and Z, and net vertical-displacement values determined from scarps, results in vertical slip rates for the Y-Z interevent interval that range from 0.5 to 1.9 mm/yr. If a single earthquake produced one particularly large scarp, one slip rate is as high as 2.3 mm/yr. However, because the scarp is likely the result of multiple earthquakes, the actual slip rate is probably lower.

Unable to determine from presently available paleoseismic data which of the two possible fault behavior models applies to the BRFZ, the confidence limits for the Working Group’s preferred vertical slip-rate estimate are intentionally broad to reflect large uncertainty.

0.05-1.5-2.5 mm/yr Summary: West (1994) believes the BRFZ is a young fault, likely representing normal-slip reactivation of a pre-existing thrust fault. However, the trace of the BRFZ crosses a landscape largely hundreds of thousands of years old, yet the geomorphic expression of the fault zone consists of scarps only a few meters to tens of meters high (no associated mountain range), indicating the possibility of large infrequent earthquakes or clusters of earthquakes separated by long periods of scarp erosion and burial. Trenching reveals good evidence for only two surface-faulting earthquakes, both of which occurred since the middle Holocene. Given the current short paleoseismic record available for the BRFZ, it is not known if the interevent interval between events Y and Z is representative of future surface-faulting recurrence on the BRFZ.

Additional References: Bull, W.B., and Pearthree, P.A., 1988, Frequency and size of Quaternary surface ruptures of the Pitaycachi fault, northern Sonora, Mexico: Bulletin of the Seismological Society of America, v. 78, p. 965-978. deJong, A.F.M., Becker, B., and Mook, W.G., 1986, High-precision calibration of the radiocarbon time scale, 3930-3230 BC, in Stuiver, M., and Kra, R.S., editors, Radiocarbon calibration issue – Proceedings of the 12th International Radiocarbon Conference, Trondheim, Norway: Radiocarbon, v. 28, no. 2B, p. 939-942. Linick, T.W., Long, A., Damon, P.E., and Ferguson, C.W., 1986, High-precision radiocarbon dating of bristlecone pine from 6554 to 5350 BC, in Stuiver, M., and Kra, R.S., editors, Radiocarbon calibration issue – Proceedings of the 12th International Radiocarbon Conference, Trondheim, Norway: Radiocarbon, v. 28, no. 2B, p. 943-953. Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone – A summary of recent investigations, interpretations, and conclusions, in Gori, P.L., and Hays, W.H., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-A, p. A1-A71. Pearson, G.W., and Stuiver, M., 1986, High-precision calibration of the radiocarbon time scale, 500-2500 BC, in Stu- iver, M., and Kra, R.S., editors, Radiocarbon calibration issue – Proceedings of the 12th International Radiocarbon Conference, Trondheim, Norway: Radiocarbon, v. 28, no. 2B, p. 839-862. Consensus preferred recurrence-interval and vertical slip-rate estimates 85

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Morgan fault zone (MFZ), central section, Morgan County, Utah

Paleoseismic Data Source Documents: Sullivan, J.T., Nelson, A.R., LaForge, R.C., Wood, C.K., and Hansen, R.A., 1988b, Central Utah regional seismotectonic study for USBR dams in the Wasatch Mountains: Denver, U.S. Bureau of Reclamation Seismotectonic Report 88-5, 269 p., scale 1:250,000. Sullivan, J.T., and Nelson, A.R., 1992, Late Quaternary displacement on the Morgan fault, a back valley fault in the Wasatch Range of northeastern Utah, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front: U.S. Geological Survey Professional Paper 1500-I, 19 p.

Age of Youngest Faulting: Holocene

Discussion: The MFZ is a range-front normal fault that extends for 22 km at the base of a bedrock escarpment along the eastern side of Morgan Valley, a back valley of the Wasatch Range. Sullivan and others (1988b) divided the MFZ into three linear sections based on the morphology of the bedrock escarpment in the fault footwall. No fault scarps are recognized on unconsolidated deposits along the three fault sections. The northern section is 13 km long and consists of a main western fault trace and an older eastern fault trace. The central section is 7 km long and consists of a main fault trace and an antithetic fault trace inferred to the west. The southern section consists of a single, short (2-km-long), northwest- trending fault trace. Only the central section has been trenched, so it is unknown if the three sections are independently seismogenic.

Sullivan and Nelson (1992) state that the central section of the fault shows evidence of Holocene movement, whereas the northern and southern sections show evidence only for late Quaternary movement, although scarp morphology is similar for all three sections. The central section of the MFZ consists of a north-trending, range-front main fault trace and an inferred northeast-trending antithetic fault trace to the west. Although trenching showed that early Holocene colluvium is faulted, scarps in unconsolidated deposits are not preserved along the fault. Sullivan and Nelson (1992) attributed this to the steepness (20-25°) of escarpment slopes and the inferred small amount of surface displacement (0.5-1.0 m) during earthquakes.

Earthquake Timing: Sullivan and others (1988b) excavated five trenches at the southern end of the central section of the MFZ. All five trenches were at or near the break in slope at the base of the footwall escarpment and two of the trenches exposed the main trace of the MFZ. Evidence for the MRE consists of an escarpment-derived colluvial wedge. Sullivan and Nel- son (1992) report a net vertical displacement of about 1 m for the MRE and cumulative net vertical displacement of approximately 4 m. Stratigraphic relations and 14C ages on pre-MRE peat (8320±100 14C yr B.P. [~9300 cal yr B.P.]) and wood (9105±270 14C yr B.P. [~10,250 cal yr B.P.]) provide maximum limits on timing of the MRE. It was not possible to establish minimum limits.

Sullivan and Nelson (1992) interpreted a massive, moderately indurated sandy silt unit in one of the trenches as a complex, escarpment-derived colluvial deposit that represents an unknown number of small (0.5 to 1 m) surface-faulting earthquakes that occurred in middle through late Pleistocene time; however, the timing of individual earthquakes is unknown.

Based on presently available paleoseismic information, the Working Group’s consensus fault chronology for the central section of the MFZ is:

Z <8320±100 14C yr B.P. [~9300 cal yr B.P.] Y-? middle through late Pleistocene, individual earthquake timing unknown

Surface-Faulting Recurrence: Sullivan and Nelson (1992) state that if 0.5 m is the average net displacement per earthquake, then 4 m of dis- 86 Utah Geological Survey placement represents eight earthquakes. If average per-earthquake displacement is 1.0 m, then the 4 m represents four surface-faulting earthquakes. Assuming that the displacement occurred over the past 200 to 400 kyr (based on soil developed on faulted deposits at several locations along the fault), the average middle to late Quaternary vertical slip rate would be 25 to 50 kyr for eight earthquakes, and 50 to 100 kyr for four earthquakes.

Because the timing of individual surface-faulting earthquakes is poorly constrained, the Working Group’s recurrence-interval estimate is reported as a broad range rather than as a central value with approximate 2-sigma confidence limits to reflect high uncertainty. The Working Group’s preferred recurrence-interval estimate for the MFZ is:

25-100 kyr Vertical Slip Rate: Sullivan and Nelson (1992) report a minimum average long-term vertical slip rate of 0.01 to 0.02 mm/yr based on 4 m of displacement in deposits that are 200 to 400 kyr old.

Because the age of the displaced deposits is poorly constrained (±200 kyr), the confidence limits for the Working Group’s vertical slip-rate estimate are intentionally broad to reflect the uncertainty associated with possible variations in slip through time. The Working Group’s preferred vertical slip-rate estimate and confidence limits for the central section of the MFZ are:

0.01-0.02-0.04 mm/yr Summary: The MFZ is a very low slip rate fault that has experienced a one-meter displacement surface-faulting earthquake during the Holocene. The timing and size of earlier earthquakes are unknown, but are estimated to include four to eight small earthquakes each having 0.5 to 1.0 m of net vertical displacement over the past 200 to 400 kyr. Available paleoseismic data for the MFZ permit only broad characterization of the fault’s paleoseismic parameters. Consensus preferred recurrence-interval and vertical slip-rate estimates 87

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: James Peak fault (JPF), Cache County, Utah

Paleoseismic Data Source Documents: Sullivan, J.T., Nelson, A.R., LaForge, R.C., Wood, C.K., and Hansen, R.A., 1988b, Central Utah regional seismotectonic study for USBR dams in the Wasatch Mountains: Denver, U.S. Bureau of Reclamation Seismotectonic Report 88-5, 269 p., scale 1:250,000. Nelson, A.R., and Sullivan, J.T., 1992, Late Quaternary history of the James Peak fault, southernmost Cache Valley, north-central Utah, in Gori, P.L., and Hays, W. W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-J, p. J1-J13.

Age of Youngest Faulting: Late Pleistocene

Discussion: The JPF is a short (7 km), northeast-trending, range-front normal fault along the northern flank of James Peak at the south end of Cache Valley. Cache Valley is a north-trending intermontane graben (bounded by high-angle normal faults on the east and west) between the Bear River and Wasatch Ranges. Faulting on the JPF displaces Bull Lake outwash deposits (~140 ka). The short fault length suggests that surface faulting may have extended northward, rupturing the southern section of the East Cache fault zone (ECFZ). Faceted spurs at the base of James Peak suggest recurrent Qua- ternary displacements, though the spurs are smaller, less continuous, and less steep than those along the nearby ECFZ and Wasatch fault zone.

Earthquake Timing: Sullivan and others (1988b) excavated a trench across a 7-m-high scarp formed on a Bull Lake glacial outwash fan. The trench exposed sandy, quartzite-derived outwash on which was developed a reddish, clayey soil horizon overlain by a bouldery wedge of silty fault colluvium. Silty colluvial units with thick cambic and argillic B horizons overlie the colluvial wedge. The contrasting lithologies of the outwash, colluvial wedge, and overlying colluvium; cumulative displacement across the scarp (4.2 m); and the short fault length suggest that the scarp was produced by two surface-faulting earthquakes rather than by a single, large earthquake. All age estimates are based on soil-profile development on Quaternary deposits of different ages. Nelson and Sul- livan (1992) estimated that both the soil formed on the pre-faulting glacial outwash and the soil formed on the post- earthquake fault colluvium required 30 to 70 kyr to develop. Therefore, the two postulated surface-faulting earthquakes happened after 110-70 ka (140 ka minus 30,000 to 70,000 years) and before 30-70 ka (period of post-earthquake soil formation). Where in that broad time interval the earthquakes actually occurred is unknown, except that no soil was found on the possible older wedge, indicating a relatively short period of time between earthquakes. Additionally, soil development indicates no surface-faulting earthquakes have occurred on the JPF since at least 30 ka and possibly not since 70 ka.

Surface-Faulting Recurrence: Because the soils developed on the outwash and on the colluvium overlying the fault-derived colluvial wedges provide only broad maximum and minimum constraints on the timing of surface faulting, the true interval of time between surface-faulting earthquakes is unknown. Both the soil formed on the glacial outwash beneath the fault-scarp colluvium and the soil formed on top of the colluvial wedges are estimated to have required 30-70 kyr to form. Thus, the first earthquake could have occurred as early as 110 ka or as late as 70 ka. Considering that the soil on top of the wedges could be as young as 30 ka, a time interval of 80 kyr is available in which the two earthquakes could have occurred. Assuming that 40 kyr separates the two earthquakes, the average recurrence interval is at least 50 kyr (50 kyr + 40 kyr + 50 kyr = 140 kyr). However, the absence of a soil formed on the first colluvial wedge argues for a short interval between the two postulated earthquakes, indicating that surface-faulting recurrence on the JPF is non-uniform. Because individual earthquake timing is unknown, the confidence limits for the Working Group’s recurrence-interval estimate for the JPF are intentionally broad to reflect uncertainty associated with possible large variations in recurrence through time. The Working Group’s preferred recurrence-interval estimate and confidence limits for the JPF are: 10-50-100 kyr 88 Utah Geological Survey

Vertical Slip Rate: Based on an estimated 4.2 m of net vertical displacement in ~140 kyr, Nelson and Sullivan (1992) estimate an average late Quaternary slip rate of 0.03 mm/yr for the JPF.

Nelson and Sullivan (1992) speculated that the JPF might be a southern extension of the ECFZ, and A.R. Nelson (U.S. Geological Survey, verbal communication to UQFPWG, 2003) now considers this likely to be the case. Follow- ing a review of available paleoseismic data for the JPF, the Working Group concurs with that assessment and recommends that the JPF be considered part of the ECFZ. Whether it is an extension of the currently defined southern section of the ECFZ (McCalpin, 1994), or is a fourth independent section awaits additional paleoseismic investigation on the southern section of the ECFZ to determine the timing of surface-faulting earthquakes there. Based on geomorphic relations, McCalpin (1994) reports a long-term vertical slip rate for the southern section of the ECFZ as high as 0.07 mm/yr, depending on the ages assumed for displaced deposits. The Working Group recommends using 0.07 mm/yr as the upper bound of possible vertical slip rates for the JPF until further study of the southern section of the ECFZ demonstrates otherwise. The Working Group’s preferred vertical slip-rate estimate and confidence limits for the JPF, pending further paleoseismic study of the southern section of the ECFZ, are:

0.01-0.03-0.07 mm/yr Summary: The JPF is a very short, very low slip rate fault located close to the southern end of the ECFZ. Both the number and timing of individual surface-faulting earthquakes are poorly constrained. Based on presently available paleoseismic evidence and its close physical proximity, the Working Group recommends that the JPF be considered part of the ECFZ to the north. Whether the JPF is a part of the southern section of the ECFZ (McCalpin, 1994) or a separate fourth section awaits additional paleoseismic investigation.

Additional References: McCalpin, J.P., 1994, Neotectonic deformation along the East Cache fault zone, Cache County, Utah: Utah Geological Survey Special Study 83, 37 p. Consensus preferred recurrence-interval and vertical slip-rate estimates 89

UTAH QUATERNARY FAULT PARAMETER WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Towanta Flat graben (TFG), Duchesne County, Utah

Paleoseismic Data Source Documents: Martin, R.A., Jr., Nelson, A.R., Weisser, R.R., and Sullivan, J.T., 1985, Seismotectonic study for Taskeech Dam and Reservoir site, Upalco Unit and Upper Stillwater Dam and Reservoir site, Bonneville Unit, Central Utah Project, Utah: Denver, U.S. Bureau of Reclamation Seismotectonic Report 85-2, 95 p. Nelson, A.R., and Weisser, R.R., 1985, Quaternary faulting on Towanta Flat, northwestern Uinta Basin, Utah, in Picard, M.D., editor, Geology and energy resources, Uinta Basin of Utah: Utah Geological Association Publication 12, p. 147-158.

Age of Youngest Faulting: Middle Quaternary

Discussion: Nine short, northeast-striking fault scarps are present on Towanta Flat about 6 km south of the Uinta Mountains in the northern part of the Uinta Basin. The scarps bound a 5-km-long graben that varies in width from 170 to 610 m. Scarp heights range from 5-15 m. Nelson and Weisser (1985) found no significant net vertical displacement across the graben, although they determined the average throw across individual scarps to be 2.1-2.6 m per earthquake. The lack of net vertical displacement across the graben, together with an orientation that differs from planes defined by micro- seismicity, the limited extent of the scarps, and an average recurrence interval that has been exceeded by the elapsed time since the MRE, led Martin and others (1985) to suggest that the faults comprising the TFG may not have a seismogenic origin, and may not be capable of significant future surface-faulting earthquakes. Black and others (2003) cat- egorized the TFG as a “Suspected” fault in the Quaternary Fault and Fold Database and Map of Utah.

Earthquake Timing: The U.S. Bureau of Reclamation (Martin and others, 1985; Nelson and Weisser, 1985) excavated three trenches on the TFG. Two trenches crossed aerial-photo lineaments in a glacial meltwater channel in the southeastern part of Towan- ta Flat. The other trench crossed a 5-m-high scarp bounding the graben on the north near its western end. The trench across the scarp revealed stratigraphic and structural relations that indicate at least three surface-faulting earthquakes. Based on soil-profile development of paleosols formed on colluvial wedges, Nelson and Weisser (1985) believe that the three earthquakes occurred within the past 250-500 kyr, with no scarps mapped in deposits younger than Bull Lake age (130-150 ka), indicating no surface displacement in late Quaternary time. The two trenches in the glacial meltwater channel exposed unfaulted Bull Lake deposits.

Based upon presently available paleoseismic information, the Working Group’s consensus surface–faulting chronology for the TFG is:

Z, Y, X >130 ka, <250-500 ka

Surface-Faulting Recurrence: Martin and others (1985) reported a mean recurrence of 25 to 90 kyr for surface displacement between 250-500 ka and 130-150 ka, with no displacement since 130-150 ka.

Due to uncertainty regarding the seismogenic capability of the TFG, the long elapsed time since the MRE, and the unknown lengths of the intervals between surface-faulting earthquakes, the confidence limits for the Working Group’s preferred recurrence-interval estimate for the TFG are intentionally broad to reflect high uncertainty regarding this enigmatic structure. The Working Group’s preferred recurrence-interval estimate and confidence limits for the TFG are:

25-50-200 kyr Vertical Slip Rate: Martin and others (1985) estimate maximum vertical slip rates across individual TFG scarps range from 0.02 to 0.04 mm/yr. Piety and Vetter (1999) indicate the maximum vertical slip rate for the TFG faults is ≤0.09 mm/yr. However, 90 Utah Geological Survey

Nelson and Weisser (1985) found no net vertical displacement across the graben as a whole, and question the seismogenic capability of the TFG. Because there is no net vertical displacement across the TFG, the Working Group makes no vertical slip-rate estimate for the TFG.

Summary: The TFG is an enigmatic structure: a short, narrow graben exhibiting no cumulative net vertical displacement and located far from any other active or potentially active faults. The timing of individual surface-faulting earthquakes is unknown, with three earthquakes occurring within a broad time window between about 500 and 130 ka, and no earthquakes since 130-150 ka. Martin and others (1985) and Nelson and Weisser (1985) questioned whether this low-slip- rate fault system is seismogenic. At present, that question remains unanswered; however, the current elapsed time since the MRE exceeds the estimated middle Pleistocene average recurrence interval by tens of thousands of years, indicating either long-term quiescence of the fault system or cessation of surface-faulting activity.

Additional References: Black, B.D., Hecker, S., Hylland, M.D., Christenson, G.E., and McDonald, G.N., 2003, Quaternary fault and fold database and map of Utah: Utah Geological Survey Map 193DM, scale 1:50,000, compact disk. Piety, L.A., and Vetter, U.R., 1999, Seismotectonic report for Flaming Gorge Dam, Colorado River Storage Project, northeastern Utah: Denver, Bureau of Reclamation Seismotectonic Report 98-2, 78 p. Consensus preferred recurrence-interval and vertical slip-rate estimates 91

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Bald Mountain fault (BMF), Wasatch County, Utah

Paleoseismic Data Source Documents: Sullivan, J.T., Martin, R.A., and Foley, L.L., 1988a, Seismotectonic study for Jordanelle Dam, Bonneville Unit, Central Utah Project, Utah: Denver, U.S. Bureau of Reclamation Seismotectonic Report 88-6, 76 p., scale 1:24,000. Sullivan, J.T., Nelson, A.R., LaForge, R.C., Wood, C.K., and Hansen, R.A., 1988b, Central Utah regional seismotectonic study for USBR dams in the Wasatch Mountains: Denver, U.S. Bureau of Reclamation Seismotectonic Report 88-5, 269 p., scale 1:250,000.

Age of Youngest Faulting: Middle Quaternary

Discussion: The BMF is a very short (2 km), northeast-trending normal fault on the east side of Bald Mountain, west of Jor- danelle Reservoir and close to Jordanelle Dam. Tertiary volcanic rocks, primarily highly erodible tuff, dominate the geology of the area. The fault escarpment is more eroded, but appears similar to those in other back valleys east of the Wasatch Range. A steep-sided trough (imaged by seismic refraction) beneath the Provo River Valley to the south may be fault bounded, but trenching studies for Jordanelle Dam determined that Quaternary (probably >130 ka) deposits are unfaulted.

Earthquake Timing: Sullivan and others (1988a) excavated three trenches in surficial deposits across the inferred trace of the BMF, as projected from adjacent bedrock and borehole control (no scarps were recognized on unconsolidated deposits), about 1.5 km northwest of Jordanelle Dam. Late Quaternary faulting was not recognized in the trenches. Sullivan and others (1988a, 1988b) estimated the age of the MRE as >130 ka based on soil-profile development on an unfaulted, escarpment-derived colluvial apron and associated basin-fill deposits, and by the morphology of the base of the escarpment along the fault.

Surface-Faulting Recurrence: Unknown

Vertical Slip Rate: Unknown

Summary: Based on a review of available paleoseismic data, the Working Group is unable to make recurrence-interval or vertical slip-rate estimates for the BMF. 92 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Strawberry fault (SF), Wasatch County, Utah

Paleoseismic Data Source Documents: Nelson, A.R., and Martin, R.A., Jr., 1982, Seismotectonic study for Soldier Creek Dam, Central Utah Project: Denver, U.S. Bureau of Reclamation Seismotectonic Report 82-1, 115 p., scale 1:250,000. Nelson, A.R., and VanArsdale, R.B., 1986, Recurrent late Quaternary movement on the Strawberry normal fault, Basin and Range - Colorado Plateau transition zone, Utah: Neotectonics, v. 1, p. 7-37.

Age of Youngest Faulting: Early to middle Holocene

Discussion: The SF is an approximately 43-km-long, down-to-the-west normal fault characterized by a zone of north- to northwest-trending faulting along the eastern and northern side of Strawberry Valley near the western edge of the Uinta Basin. Strawberry Valley is the easternmost of several back valleys of the Wasatch Range, a north-south series of discontinuous valleys in the Wasatch Hinterlands east of the Wasatch Range. The SF forms a single bedrock escarpment, 100 to 230 m high, from its southern end northward to Trout Creek. Continuing north from Trout Creek, the SF forms multiple scarps on alluvium and bedrock across a zone 5 km wide. At Co-op Creek, a 200-m-high escarpment juxtaposing Quaternary alluvial-fan sediments against Tertiary bedrock marks the main fault, while subsidiary scarps about 1.3 km to the west formed on the Co-op Creek alluvial fan are up to 3 km long and as much as 7 m high. Due to back-tilting and graben formation, stratigraphic displacement on these scarps is much greater than net vertical displacement. Latest Pleistocene or Holocene deformation is also indicated by the asymmetry of stream channels (evidence for tectonic tilting) and the presence of nickpoints in small stream channels above the scarps.

The en echelon pattern of faulting north of Strawberry Reservoir suggests that the main SF is segmented, although similarities in escarpment morphologies suggest a similar movement history along the entire fault.

Earthquake Timing: Nelson and Martin (1982) excavated two trenches across a 7-m-high fault scarp formed on the Co-op Creek alluvial fan west of the main trace of the SF. The trenched scarp is one of four scarps on the alluvial fan. The two trenches were about 1 km apart, and both exposed faulted alluvial-fan deposits. Investigators did not recognize scarps on unconsolidated deposits along the main fault trace at the base of the bedrock escarpment to the east.

Based on stratigraphic relations in the two trenches, Nelson and VanArsdale (1986) interpret two to three surface- faulting earthquakes on the subsidiary fault strand in the past 15 to 30 kyr. Deposit age estimates are based on soil-profile development and indicate that the MRE occurred during the early to mid-Holocene, with a minimum possible constraint on timing of 1.5 ka based on a 14C age on organic material filling an animal burrow. Older earthquake(s) can only be constrained as older than the MRE and younger than 15 to 30 ka. The relation between surface-faulting earthquakes on the subsidiary fault strand and the main SF is unknown.

Surface-Faulting Recurrence: Nelson and VanArsdale (1986) reported a mean recurrence of 5 to 15 kyr based on the estimated number of earthquakes (two or three) on a single alluvial-fan scarp over the past 15 to 30 kyr. The relation between the displacement histories of the fan scarps and the main fault trace are unknown. Because the trenched scarp is only one of four scarps on the fan, the number of surface-faulting earthquakes, displacement per earthquake, and total displacement across the SF as a whole are unknown.

Because paleoseismic-trenching data available for the SF poorly constrain earthquake timing and are restricted to a single alluvial scarp west of the main fault trace, the confidence limits for the Working Group’s preferred recurrence- interval estimate for the SF are intentionally broad to reflect large uncertainty. The Working Group’s preferred recurrence-interval estimate and confidence limits for the SF are:

5-15-25 kyr Consensus preferred recurrence-interval and vertical slip-rate estimates 93

Vertical Slip Rate: A latest Pleistocene and Holocene vertical slip rate calculated from the estimated net vertical displacement across the 7-m-high subsidiary scarp (1-2 m per earthquake) is 0.04-0.17 mm/yr. However, Nelson and VanArsdale (1986) state that whereas the trenched scarps formed on the Co-op Creek alluvial fan may represent most of the displacement within the fault zone during the earthquakes that produced them, additional displacement probably occurred on the main fault during at least some of these earthquakes. They believe that scarps in unconsolidated deposits at the base of the bedrock escarpment along the main fault are less likely to be preserved than are scarps on the gently sloping alluvial fan 1.3 km west of the escarpment. For this reason, the investigators did not assume that the displacement represented by the alluvial-fan scarps was the total displacement across the fault zone during the earthquakes that produced the scarps. A minimum late Quaternary (~70-90 ka) vertical slip rate for the SF of 0.03-0.06 mm/yr is provided by 14C and amino acid dating of samples collected from flood-plain drill cores in the fault footwall near the main escarpment along the southern section of the fault (Nelson and VanArsdale, 1986).

Because paleoseismic-trenching data available for the SF poorly constrain earthquake timing and are restricted to a single alluvial scarp west of the main fault trace, the confidence limits for the Working Group’s preferred vertical slip- rate estimate for the SF are intentionally broad to reflect large uncertainty. The Working Group’s preferred vertical slip- rate estimate and confidence limits for the SF are:

0.03-0.1-0.3 mm/yr Summary: The SF is a low-slip-rate normal fault, the easternmost of the Wasatch back-valley faults of the Wasatch Range. The main trace of the fault shows no evidence of displaced Quaternary deposits, but four scarps formed on the Co-op Creek alluvial fan about 1.3 km west of the main fault trace provide evidence for latest Pleistocene/Holocene surface faulting. All available paleoseismic trenching data for the SF come from one of those subsidiary fault scarps formed on alluvium, and the relation between slip and surface-faulting recurrence on that fault strand to the main SF is unknown. 94 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Hansel Valley fault (HVF), Box Elder County, Utah

Paleoseismic Data Source Documents: McCalpin, J.P., 1985, Quaternary fault history and earthquake potential of the Hansel Valley area, north-central Utah: Final Technical Report to the U.S. Geological Survey, contract no. 14-08-001-21899, 37 p. McCalpin, J.P., Robison, R.M., and Garr, J.D., 1992, Neotectonics of the Hansel Valley-Pocatello Valley corridor, northern Utah and southern Idaho, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-G, p. G1–G18.

Age of Youngest Faulting: Historic

Discussion: The HVF is a 22-km-long, east-dipping normal fault in southwestern Hansel Valley characterized by northeast- trending scarps several kilometers east of the Hansel Mountains in northwestern Utah. The HVF is the site of the 1934 ML 6.6 Hansel Valley earthquake, Utah’s only historical surface-faulting earthquake. The northern half of the fault is a single continuous trace, whereas the southern half is a wide zone of several short, en echelon fault traces. The most recent prehistoric earthquake on the fault is estimated to have produced a net vertical displacement of 2.2-2.5 m. The 1934 earthquake ruptured only the southern few kilometers of the HVF and produced a maximum vertical displacement of 0.5 m where the southern part of the fault intersects the mudflats at the north end of Great Salt Lake (Walter, 1934; dePolo and others, 1989).

Earthquake Timing: McCalpin (1985) logged a gully exposure near the northern end of the HVF. Stratigraphy, sedimentology, and ostra- code assemblages, along with five TL ages and a 14C age on a gastropod shell, provide a framework within which McCalpin and others (1992) interpreted surface faulting within a context of pluvial lake cycles. McCalpin and others (1992) argued for multiple earthquakes between about 140 and 72 ka, no earthquakes between 72 and 58 ka, at least one earthquake (but possibly more) between 58 and 26 ka (nearer the latter), an earthquake around 15 to 14 ka, and possibly another earthquake at 13 ka. The exposed part of the gully wall did not include colluvial wedges or other evidence of individual surface-faulting earthquakes, so it was not possible to determine the timing or displacement of individual surface-faulting earthquakes. Post-Bonneville alluvium truncates all exposed faults in the gully wall, indicating no Holocene surface faulting, including the 1934 earthquake, has occurred on this part of the HVF.

Surface-Faulting Recurrence: Available paleoseismic data (McCalpin and others, 1992) implies widely varying lengths of interevent intervals between surface-faulting earthquakes on the HVF. Multiple earthquakes of unknown timing between 140 and 72 ka give no clear indication of average recurrence, but no earthquakes between 72 and 58 ka creates a 14-kyr interval. One or more earthquakes between 58 and 26 ka produces a recurrence interval potentially as long as 32 kyr, or possibly several shorter intervals. An earthquake at 14-15 ka, possibly followed by another at about 13 ka, would give an interevent interval of only 1-2 kyr, followed by an interval of 13 kyr preceding the 1934 earthquake.

Based on the limited information available, the confidence limits for the Working Group’s preferred recurrence- interval estimate for the HVF are intentionally broad to reflect large uncertainty associated with possible large variations in recurrence through time. The Working Group’s preferred recurrence-interval estimate and confidence limits for the HVF are:

15-25-50 kyr Vertical Slip Rate: Neither McCalpin (1985) nor McCalpin and others (1992) report a slip rate for the HVF.

Black and others (2003) estimated a vertical slip rate of 0.14-0.22 mm/yr, based on a 10-16 kyr recurrence and prehistoric displacement of 2.2-2.6 m. The vertical slip rate based on 0.5 m of displacement in the 1934 earthquake and a Consensus preferred recurrence-interval and vertical slip-rate estimates 95 recurrence of 13 kyr would be much lower. J.P. McCalpin (GEO-HAZ Consulting, verbal communication to Working Group, 2003) re-evaluated his paleoseismic data for the HVF based on an estimated 1 to 4 m of displacement since ~17 ka. The Working Group adopts McCalpin’s late Pleistocene/Holocene slip rate and confidence limits as their preferred vertical slip-rate estimate for the HVF:

0.06-0.1-0.2 mm/yr Summary: Both the number and timing of surface-faulting earthquakes on the HVF are unknown. The fault exhibits an irregular pattern of surface faulting with interevent intervals ranging from possibly as little as 1-2 kyr to more than 30 kyr, indicating that interevent intervals between surface-faulting earthquakes on the HVF have been highly variable through time. Although there is circumstantial evidence to indicate that the time between some surface-faulting earthquakes may be as short as 1-2 kyr (likely triggered by the Bonneville flood if the earthquake did in fact happen [J.P. McCalpin, GEO- HAZ Consulting, verbal communication to Working Group, 2004]), the Working Group believes that much longer interevent intervals are a more likely norm for the HVF.

Additional References: Black, B.D., Hecker, S., Hylland, M.D., Christenson, G.E., and McDonald, G.N., 2003, Quaternary fault and fold database and map of Utah: Utah Geological Survey Map 193DM, scale 1:50,000, compact disk. dePolo, C.M., Clark, D.G., Slemmons, D.B., and Aymard, W.H., 1989, Historical Basin and Range Province surface faulting and fault segmentation, in Schwartz, D.P., and Sibson, R.H., editors, Fault segmentation and controls of rupture initiation and termination - Proceedings of Conference XLV: U.S. Geological Survey Open-File Report 89-315, p. 131-162. Walter, H.G., 1934, Hansel Valley, Utah, earthquake: The Compass of Sigma Gamma Epsilon, v. 14, no. 4, p. 178-181. 96 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETER WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Hogsback fault (HF), unnamed south section, Summit County, Utah

Paleoseismic Data Source Documents: West, M.W., 1994, Seismotectonics of north central Utah and southwestern Wyoming: Utah Geological Survey Special Study 82, 93 p.

Age of Youngest Faulting: Late Quaternary?

Discussion: The HF is expressed as linear drainage alignments, lineaments, and subdued west-facing scarps on Pleistocene terrace and pediment surfaces. The amount of east-directed tilt of terrace surfaces increases with increasing age of the surfaces, suggesting recurrent movement. The southern section of the fault (Elizabeth Ridge scarps) lies in Utah and is expressed as southwest-trending scarps, one of which is uphill facing and down to the south; the other two are downhill facing and down to the north. These scarps have apparent displacements of about 1.5-2.5 m and are subparallel to the North Flank thrust fault. The east scarp displaces the Oligocene Bishop Conglomerate on the Gilbert Peak erosion surface. Trenching revealed no direct evidence for faulting; however, West (1994) believes that geomorphic evidence is more in line with a tectonic rather than an erosional origin for the scarps. The subdued expression of the scarps (maximum scarp angles ~5°) suggests that they are substantially older than similar discordant scarps at the south end of the nearby Bear River fault zone (BRFZ).

Earthquake Timing: West (1994) excavated a trench across a down-to-the-south, uphill-facing, 2.5-m-high scarp about 1 km north of Elizabeth Pass in Utah. No datable material was recovered nor did the trench expose clear evidence of faulting.

Surface-Faulting Recurrence: The West (1994) investigation resulted in no data concerning the timing, number, or recurrence of surface-faulting earthquakes on the HF. West (1994) suggested that the recurrence interval for the nearby BRFZ, because of analogous tectonic setting, may approximate the recurrence interval for the HF as a whole, and recommends a recurrence interval for the HF of a few thousand years. However, no evidence exists of similar short recurrence intervals during the Holocene for the HF (Black and others, 2003). Based on available paleoseismic data, the Working Group is unable to make a recurrence-interval estimate for the HF.

Vertical Slip Rate: Poorly constrained vertical slip-rate estimates for the HF to the north range from 0.33-1.5 mm/yr (West, 1994). These vertical slip rates are based on a variety of possible ages for Bigelow Bench (150-600 ka), which is displaced as much as 200 m across the HF in Wyoming. The highest estimate infers a slip rate similar to the BRFZ (West, 1994). The 200-m offset of the Bigelow Bench surface results in a vertical slip rate of 0.33-1.33 mm/yr. Based on scarp-profile data, West (1994) reports a maximum displacement at his trench site of 1.5-2.47 m in the Oligocene Bishop Con- glomerate, suggesting a very low slip rate, especially considering that the trench did not expose evidence of faulting.

Black and others (2003) assigned the HF to the vertical slip-rate category of 0.2-1.0 mm/yr as a whole based on a belief that the Bigelow Bench surface is substantially older than 150 ka. However, the relation between the large 200- m-high escarpment along the HF in Wyoming and the small approximately 2-m-high scarp in Utah on a possible Oligocene-age surface is unclear, and the Working Group believes assigning such a high vertical slip rate to the Utah portion of the HF is not warranted. Based on available paleoseismic data, the Working Group is unable to make a vertical slip-rate estimate for the HF.

Summary: No definitive paleoseismic information resulted from trenching the HF in Utah, and no paleoseismic-trenching data are available for the fault in Wyoming. On the basis of geomorphic relations, West (1994) believes that the initiation of surface rupture on the HF could be as young as about 150 ka and that slip rates could be as high as 0.33-1.5 mm/yr Consensus preferred recurrence-interval and vertical slip-rate estimates 97 depending on the age of displaced surfaces farther north along the fault in Wyoming. However, in the absence of sub- stantive paleoseismic data and given the large uncertainty associated with both the age and amount of displacement of critical geomorphic surfaces, the Working Group believes assigning a slip rate to the HF comparable to that of the Wasatch fault zone is highly questionable, and particularly so for the small, subdued scarps along the southern section of the HF in Utah.

The Working Group considers the available paleoseismic data insufficient to make recurrence-interval and vertical slip-rate estimates for the HF in Utah, other than to note that existing evidence supports a very low slip rate for the southern section of the HF.

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: North Promontory fault (NPF), Box Elder County, Utah

Age of Youngest Faulting: Latest Quaternary

Discussion: The NPF is a 27-km-long Basin and Range normal fault bounding eastern Hansel Valley in northern Utah. Hansel Valley is in an aggregation of low, north-trending ranges and narrow valleys in northern Utah between Curlew Valley on the west and the Malad River Valley on the east. Scarps appear in only two locations; elsewhere the fault is covered by Holocene talus. At the northern location, a 13-m-high scarp (8 m net vertical displacement?) displaces a delta graded to the Bonneville shoreline. At the southern site, a branch fault diverges southwesterly from the range front and creates a scarp 12.9 m high (9.5 m net vertical displacement) on a pre-Bonneville alluvial fan. Although the fault scarps appear unbeveled, McCalpin and others (1992) believe both scarps resulted from more than one surface-faulting earthquake.

The southern part of the NPF (expressed as a prominent range front) does not displace upper Pleistocene deposits and likely last moved in the early to middle Pleistocene (Miller and Schneyer, 1990). A subsidiary fault 100 m east of the north end of the NPF is exposed in a road cut along Interstate 84 and shows evidence for a single surface-faulting earthquake.

Earthquake Timing: Paleoseismic data available for the NPF come chiefly from a geomorphic study of the two fault scarps and from reconnaissance geologic mapping. The road-cut exposure of the subsidiary fault was logged as part of the NPF study.

McCalpin and others (1992) concluded that the two scarps present along the main NPF are probably the result of multiple surface-faulting earthquakes, but because each scarp is limited to a single geomorphic surface and neither dis- plays evidence of multiple crests or bevels, reliable evidence of recurrent movement is lacking. The subsidiary fault exposed in the road cut near the north end of the main fault shows evidence for a single surface-faulting earthquake in the past ~100 kyr. McCalpin and others (1992) believe that this earthquake is young (<15 ka), and that it produced 2.6 m of displacement.

Lacking trench data from the main NPF, information on earthquake timing is poorly constrained. McCalpin and others (1992) believe the faulting is latest Pleistocene or early Holocene (?) based on the 13-m-high scarp formed on a delta graded to the Bonneville shoreline. Slope-angle versus scarp-height relations suggest that the northern scarp is roughly contemporaneous with the Bonneville shoreline, whereas the splay scarp to the south is older than the shoreline. Based on stratigraphic relations and soil-profile development, McCalpin and others (1992) believe the surface-faulting earthquake on the subsidiary fault occurred <15 ka, but one earthquake in ~100 kyr does not match the evidence for probable multiple late Pleistocene and early Holocene (?) surface faulting on the main NPF just 100 m to the west.

Surface-Faulting Recurrence: McCalpin and others (1992) assumed that three to four earthquakes, each exhibiting 2.0-2.5 m of net vertical displacement in the past 15 kyr, would be required to construct the northern NPF scarp. On that basis, the recurrence interval for surface-rupturing earthquakes would be 3.75 to 5.0 kyr. However, McCalpin and others (1992) believe that interval is too short in comparison with nearby faults, particularly the much more active-appearing Wasatch fault zone.

The southern NPF scarp displaces a pre-Bonneville alluvial fan of otherwise uncertain age. The fan may correlate Consensus preferred recurrence-interval and vertical slip-rate estimates 99 with either isotope stage 4 (58-72 ka) or stage 6 (140 ka). Depending on the age assumed for the fan, McCalpin and others (1992) report that recurrence intervals of 8.6-10.8 kyr and 25-31.3 kyr are possible.

Given these uncertainties, McCalpin and others (1992) state that all that can be said with confidence is that the NPF has sustained surface rupture at least once since Bonneville time (<18 ka) and several times since either oxygen isotope stage 4 or 6 time. Given the limited information available on earthquake timing, the Working Group is unable to make a recurrence-interval estimate for the NPF.

Vertical Slip Rate: None reported by McCalpin (1985) or McCalpin and others (1992).

J.P. McCalpin (GEO-HAZ Consulting, verbal communication to Working Group, 2003) re-evaluated his paleoseismic data for the NPF based on an estimated 8 m of displacement since ~17 ka. The Working Group adopts McCalpin’s revised late Pleistocene/Holocene vertical slip rate and confidence limits as their preferred vertical slip-rate estimate for the NPF: 0.1-0.2-0.5 mm/yr

Summary: Results of the McCalpin and others (1992) study show that the NPF is a low-slip-rate fault typical of many similar normal-slip faults in the Basin and Range Province. The fault was not trenched and is not well exposed in stream cuts. Given the lack of information on the number and timing of surface-faulting earthquakes on the NPF, the Working Group is unable to make a recurrence-interval estimate for the NPF.

Additional References: Miller, D.M., and Schneyer, J.E., 1990, Geologic map of the Sunset Pass quadrangle, Box Elder County, Utah: Utah Geological and Mineral Survey Open-File Report 201, 32 p., scale 1:24,000. 100 Utah Geological Survey

UTAH QUATERNARY FAULT PARAMETER WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Sugarville area faults (SAFs), Millard County, Utah.

Paleoseismic Data Source Documents: Dames and Moore, 1978, Phase II - preliminary geotechnical studies, proposed power plant, lower Sevier River area, Utah: Los Angeles, unpublished consultant's report for Intermountain Power Project, job nos. 10629-00206 and 10629- 003-06, 45 p., scale 1:24,000.

Age of Youngest Faulting: Holocene (?)

Discussion: The SAFs comprise a short (<5 km), northeast-trending zone of Quaternary normal faults or fractures in the northern Sevier Desert north and west of Delta, Utah. Lake Bonneville deposits dominate the surficial geology of the area. Lineaments and subtle relief in lake deposits characterize the SAFs. Parallel tonal lineaments 10 km to the north of the zone may be related faults, but are not mapped. Trenching revealed underlying faults, but their relation to deeper structures is unknown. A minimum throw of 3.8 m across one of the faults, combined with the short apparent rupture length, suggests that numerous small-displacement earthquakes occurred along the fault zone (Dames and Moore, 1978).

Earthquake Timing: Dames and Moore (1978) investigated the SAFs as part of a preliminary geologic-hazards site evaluation for a coal- fired electrical generating plant. They excavated eight trenches and logged five across two suspected fault-related lineaments (the northwestern and southeastern fault zones). Liquefaction features, including injection dikes and distorted bedding, were present in one of the trenches across the southeastern fault. Trenches across the northwestern fault zone revealed stratigraphic evidence for surface faulting as well as complex faulting relations. Dames and Moore (1978) based their interpretation of surface faulting on differential displacement of geologic horizons of different ages – older horizons are displaced more than younger horizons. They established the age of faulting by broad correlation with Lake Bonneville stratigraphy.

Dames and Moore (1978) assumed a minimum of 3.8 m of cumulative vertical displacement on one fault based on maximum trench depth and the lack of correlative stratigraphic units across faults in the trenches; actual net vertical displacement is not known. Given the short length of the fault zone (4.3 km), they concluded that the 3.8 m of displacement must have resulted from multiple earthquakes of likely 30 to 60 cm each. However, the trenches did not expose evidence of individual surface-faulting earthquakes.

Surface-Faulting Recurrence: None reported. Not all faults were trenched and evidence for individual earthquakes was not exposed in the trenches, so neither the number nor timing of surface-faulting earthquakes could be determined.

Vertical Slip Rate: None reported; no reliable net vertical-displacement measurements resulted from the SAF investigation.

Summary: Based upon a review of available paleoseismic information, the Working Group concludes that the data are insufficient to make recurrence-interval or vertical slip-rate estimates for the SAFs. However, the Working Group concurs with Black and others (2003) that the rate of slip on the SAFs is very low and probably <0.02 mm/yr.

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Washington fault zone, northern section (WaFZ), Washington County, Utah and Mohave County, Arizona

Paleoseismic Data Source Documents: Earth Sciences Associates, 1982, Phase I report, seismic safety investigation of eight SCS dams in southwestern Utah: Palo Alto, California, unpublished consultant's report for U.S. Soil Conservation Service, 2 volumes, variously paginated.

Age of Youngest Faulting: Latest Quaternary (<15 ka)

Discussion: The WaFZ extends for about 43 km (trace length) in a general north-south direction in northwestern Arizona and southwestern Utah. Pearthree (1998) subdivided the WaFZ into three sections: northern, Mokaac, and Sullivan Draw based on structural and geomorphic relations. Earth Sciences Associates (1982) excavated trenches across lineaments of uncertain origin at three flood-control dams along the northern section in Utah. Relative ages of Quaternary deposits were estimated from soil development and stratigraphy; no radiometric dating was performed. The trenches at two of the dam sites revealed no evidence of faulting. At the third dam (Gypsum Wash Dam), Earth Sciences Associates (1982) excavated five trenches. Three trenches crossed lineaments about 45 m west of the main fault trace, and revealed a wide zone of high-angle shears that form a series of horsts and grabens with a net down-to-the-west displacement. Younger unfaulted alluvial-fan deposits overlie older faulted alluvium estimated to be 5-10 ka. The other two trenches crossed the main fault trace and revealed bedrock in fault contact with late Pleistocene (?) alluvial-fan deposits, which showed a minimum of 1.2 m of displacement in the past 10-25 kyr. Younger alluvial-fan deposits, estimated no older than 1- 1.5 ka, showed 5 cm of vertical displacement. Earth Sciences Associates (1982) state that this displacement could be the result of one of several possible non-tectonic processes, including gypsum dissolution.

Earthquake Timing: None reported; individual surface-faulting earthquakes could not be identified; the Earth Sciences Associates (1982) study documented displacement only.

Surface-Faulting Recurrence: None reported; individual surface-faulting earthquakes not recognized or dated.

Vertical Slip Rate: None reported. However, Earth Sciences Associates (1982) state that up to 5 cm of displacement has occurred in the past 1.5 kyr and a minimum of 1.2 m in the past 10-25 kyr. Those displacements and deposit ages result in slip rates of 0.003 mm/yr for the past 1.5 kyr and a minimum slip rate of 0.05-0.12 mm/yr for the past 10 to 25 kyr. Several kilometers to the north, a subsidiary strand of the WaFZ displaces the Washington basalt flow approximately 4.5 m. Best and others (1980) determined a K-Ar age of 1.7±0.1 Ma for the Washington flow. Assuming the 4.5-m displacement represents a close approximation of net vertical displacement, the long-term (early Quaternary) slip rate for the subsidiary fault is 0.003 mm/yr.

Summary: The Earth Sciences Associates (1982) study did not develop information on the number, timing, or displacement of individual surface-faulting earthquakes. Therefore, the Working Group is unable to make recurrence-interval or vertical slip-rate estimates for the northern section of the WaFZ, other than to state that the long-term rate of slip is low and likely ≤ 0.1 mm/yr. However, the northern section of the WaFZ traverses one of Utah’s most rapidly urbanizing areas, and this development is taking place in the absence of a clear understanding of the seismic hazard presented by this fault. The Work- ing Group recommends that additional paleoseismic investigation of the WaFZ be conducted prior to significant additional development along its trace to adequately characterize the seismic hazard represented by this potentially active fault. 102 Utah Geological Survey

Additional References: Best, M.G., McKee, E.H., and Damon, P.E., 1980, Space-time-composition patterns of late Cenozoic mafic volcanism, southwestern Utah and adjoining areas: American Journal of Science, v. 280, p. 1035-1050. Pearthree, P.A., compiler, 1998, Quaternary fault data and map for Arizona: Arizona Geological Survey Open-File Report 98-24, scale 1:750,000, 122 p. Consensus preferred recurrence-interval and vertical slip-rate estimates 103

UTAH QUATERNARY FAULT PARAMETER WORKING GROUP CONSENSUS RECURRENCE-INTERVALAND VERTICAL SLIP-RATE ESTIMATES

Fault/Fault Section: Fish Springs fault (FSF), Juab County, Utah

Paleoseismic Data Source Documents: Bucknam, R.C., Crone, A.J., and Machette, M.N., 1989, Characteristics of active faults, in Jacobson, J.L., compiler, National Earthquake Hazards Reduction Program, summaries of technical reports volume XXVIII: U.S. Geological Survey Open-File Report 89-453, p. 117. U.S. Geological Survey unpublished data.

Age of Youngest Faulting: Late Holocene

Discussion: The FSF is a 20-km-long, range-front normal fault along the eastern base of the Fish Springs Range, a north-trending mountain range in the Basin and Range Province in western Utah. Unconsolidated deposits in the valley east of the range are mainly lake deposits and alluvium. Extreme youth for the MRE is suggested by a lack of scarp dissection and by sharply defined nickpoints in small washes within several tens of meters upstream from the scarps, but the scarps lack free faces and thus are likely hundreds to thousands of years old. Scarps related to the MRE are up to 6 m high (Ertec Western, Inc., 1981) and indicate approximately 0.5-3.5 m of net vertical displacement based on scarp profiles measured by Bucknam and Anderson (1979). Black and others (2003) show a 12.1 km straight-line rupture length and indicates two ages of faulting (a youthful northern half and an older southern half). An exposure of Holocene alluvium overlying older, more steeply dipping alluvium on the east side of Fish Springs Flat, across from the FSF, shows about 6.5° of pre-Holocene westward back-tilting (Oviatt, 1991).

Earthquake Timing: The U.S. Geological Survey (USGS) excavated three trenches across the FSF. One trench, near the northern end of the fault, exposed monoclinally folded Lake Bonneville sediments (Provo-aged and younger) but no fault ruptures (M.N. Machette, USGS, written communication to UGS, September 2001). The other two trenches, excavated across a prominent fault scarp about 12 km south of the northern trench, both exposed faulted sediment and scarp-derived colluvium indicative of a single surface-faulting earthquake (M.N. Machette, USGS, verbal communication to Working Group, 2003).

The northern FSF scarps appear distinctly younger than the nearby Drum Mountains fault scarps, dated at about 9 ka, and have a diffusion-based morphologic age of 3 ka (Hanks and others, 1984). Quantitative morphometric indices used by Sterr (1985) yielded a scarp age of 4.8 ka. Faulted post-Provo alluvial fans provide an upper limit for scarp age. The larger of the two southern trenches contained a soil A horizon buried by scarp-derived colluvium. Radiocarbon dating of bulk carbon from the soil A horizon provided a maximum limit of 2280±70 14C yr B.P. on surface-faulting timing, from which Bucknam and others (1989) concluded the MRE occurred at about 2 ka.

The number and timing of the surface-faulting earthquakes represented by the older appearing southern fault scarps are unknown.

Surface-Faulting Recurrence: None reported: trenching provided evidence for only a single surface-faulting earthquake.

Vertical Slip Rate: None reported: trenching provided evidence for only a single surface-faulting earthquake.

Summary: The FSF is a basin-and-range fault characterized by a young scarp formed over a portion of its length on lacustrine and alluvial deposits. Available paleoseismic information is insufficient for the Working Group to make recurrence- interval or vertical slip-rate estimates for the FSF. 104 Utah Geological Survey

Additional References: Black, B.D., Hecker, S., Hylland, M.D., Christenson, G.E., and McDonald, G.N., 2003, Quaternary fault and fold database and map of Utah: Utah Geological Survey Map 193DM, scale 1:50,000, compact disk. Bucknam, R.C., and Anderson, R.E., 1979, Estimation of fault-scarp ages from a scarp–height–slope–angle relationship: Geology, v. 7, no. 1, p. 11-14. Ertec Western, Inc., 1981, MX siting investigation, faults and lineaments in the MX siting region, Nevada and Utah: Long Beach, California, unpublished consultant's report no. E-TR-54 for U.S. Air Force, volume I, 77 p.; volume II, variously paginated, scale 1:250,000. Hanks, T.C., Bucknam, R.C., Lajoie, K.R., and Wallace, R.E., 1984, Modification of wave-cut and faulting-controlled landforms: Journal of Geophysical Research, v. 89, no. B7, p. 5771-5790. Oviatt, C.G., 1991, Quaternary geology of the Fish Springs Flat, Juab County, Utah: Utah Geological Survey Special Study 77, 16 p. Sterr, H.M., 1985, Rates of change and degradation of hill slopes formed in unconsolidated materials, a morphometric approach to dating Quaternary fault scarps in western Utah, USA: Zeitschrift für Geomorphologie, v. 29, no. 3, p. 315-333. Consensus preferred recurrence-interval and vertical slip-rate estimates 105

APPENDIX C EXAMPLES OF FAULT/FAULT SECTION SYNOPSIS FORM AND PALEOSEISMIC STUDY SUMMARY FORM

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP FAULT/FAULT SECTION SYNOPSIS FORM

Name and Location of Fault/Fault Section: Paleoseismic Data Source Documents: Geomorphic Expression: Evidence for Segmentation: Age of Youngest Faulting: Summary of Existing Recurrence Interval Information: Summary of Existing Slip-Rate Information: Comments: References: Map:

UTAH QUATERNARY FAULT PARAMETERS WORKING GROUP PALEOSEISMIC STUDY SUMMARY FORM

Site Name: Study Synopsis ID: Fault/Fault Section: Map Reference: Study: Type of Study/Commentary: Fault Parameter Data: Number of Surface-Faulting Events/How Identified: Age of Events/Datum Ages/Dating Techniques: Event Slip/Cumulative Slip: Published Recurrence Interval: Published Slip Rate: Sources of Uncertainty: Summary: 106 Utah Geological Survey

APPENDIX D

SOURCES OF UNCERTAINTY IN FAULT-ACTIVITY STUDIES

(Modified from Hecker and others, 1998)

UNCERTAINTIES IN EARTHQUAKE RECOGNITION • Uncertain correlation of earthquakes between fault strands • Uncertain recognition of discrete earthquakes (for example, no clear event horizon) • Earthquake recognition based on indirect stratigraphic/structural evidence • Number of earthquakes based on models or inferences • Number of earthquakes based on known cumulative net vertical slip divided by an inferred slip per event (for example, use of characteristic earthquake model)

UNCERTAINTIES IN AGES • Uncertainties inherent with dating technique(s) • Age is near limits of range of dating technique • Assumption or model-based age estimates (for example, inferring climate-related deposition) • Uncertainties in interpolating age of feature from dated material (for example, using average sedimentation rates) • Uncertainties in correlating age of feature to some dated feature (for example, estimating soil age from a compar ison to a dated chronosequence) • Questionable origin of sample material used in dating (for example, origin/age of detrital charcoal) • Questionable sampling technique used to collect datable material (for example, channel sampling the entire thick ness of a paleosol A horizon) • Questionable environmental influences on sample material used in dating (for example, secondary contamination, open versus closed system) • Uncertainties arising from the small number of samples used in dating • Uncertainties arising from the small amount of material used in dating • Uncertainties arising from stratigraphic or geomorphic inconsistencies in age estimates (for example, out-of- sequence radiocarbon ages) • Uncertainties arising from use of relative ages (for example, age estimates based on degree of soil-profile development) • Uncertainties in a process rate calibrated using an independent dating method (for example, scarp diffusion rates, sedimentation rates) • Displaced feature may be older than faulting (for example, faulted pediment or basalt flow) • Upper limits of faulted stratigraphy uncertain (for example, applicable to seismic-reflection studies)

UNCERTAINTIES IN SLIP AMOUNT • Uncertain pre-faulting geometry of the feature (for example, alluvial-fan slope) • Uncertain pre-faulting elevation of the feature (for example, stream-terrace elevation) • Uncertain correlation of features across a fault • Feature partially buried (for example, buried pre-faulting surface on downthrown side of fault) • Feature partially eroded • Vertical slip interpreted from displacement-related deposition (for example, colluvial wedge thickness) • Uplift as a proxy for vertical component of slip (for example, stream incision rate) • Vertical separation assumed to approximate net vertical component of slip (for example, unknown effect of antithetic faults) • Cumulative uncertainties from summing displacements across multiple fault strands • Uncertain event portioning of cumulative slip (applicable to slip-per-event reporting) • Uncertain correlation of earthquake-specific displacements between fault strands (applicable to slipper-event reporting) • Measured slip represents secondary (or unknown) component of total slip (for example, horizontal component of oblique slip faulting) • Event slip inferred from empirical relation to fault length • Event-to-event slip assumed to be uniform (use of characteristic earthquake model) • Number of single-earthquake slip measurements may be too few to yield representative average (applicable to slip-per-event reporting) • Unclear whether apparent displacement is caused by faulting Consensus preferred recurrence-interval and vertical slip-rate estimates 107

UNCERTAINTIES RELATED TO REPORTING • Results not fully or poorly documented • Interpretations do not readily follow from data presented • Slip estimate uncertainties not made explicit • Basis for age estimates uncertain • Uncertainties insufficiently quantified (for example, reporting laboratory uncertainty associated with a numerical age, but not constraining the geologic uncertainty associated with the age)

UNCERTAINTIES IN ACCURACY AND APPLICABILITY OF PARAMETER ESTIMATE • Value reflects local variation in deformation along a fault (for example, localized effects of antithetic faulting) • Time period too long to represent contemporary conditions (for example, long-term slip rate) • Number of interevent intervals encompassed by time period may be greater or fewer than the number of earth quakes recognized (applicable to recurrence-interval determinations) • Number of interevent intervals may be too few to yield representative average (applicable to slip-per-event report ing) • Selected value is a compromise between disparate values determined for a site • Selected value conflicts with other data at a site • Actual value may be significantly less or greater than the estimate • Data come from different locations or features along a fault and may not be comparable • Identification of associated fault is uncertain (for example, uncertain geometry of fault[s] beneath surface fold) • Uncertain if a fault or fault-related feature (for example, fault inferred from a ground-water barrier) is tectonic in origin • Seismogenic capability of a fault uncertain (for example, fault inferred to rupture sympathetically; no net vertical slip across a feature) • Zone of deformation wider than area of study (for example, not all scarps trenched)

OTHER UNCERTAINTIES • Layers of assumptions from nested models • Slip-rate uncertainties (applicable to recurrence-interval determinations) • Slip-per-event uncertainties (applicable to recurrence-interval determinations)

Hecker, Suzanne, Kendrick, K.J., Ponti, D.J., and Hamilton, J.C., 1998, Fault map and database for Southern California – Long Beach 30′x60′ quadrangle: U.S. Geological Survey Open-File Report 98-129, 27 p., 3 appendices. 108 Utah Geological Survey

APPENDIX E

GLOSSARY AND LIST OF ABBREVIATIONS

TERM ABBREVIATIONS

AMRT Apparent mean residence time: describes 14C ages obtained on organic concentrates typically from the A horizons of soils, or any organic-rich bulk material such as colluvial-wedge matrix or tectonic crack-fill deposits. Because of the wide range of carbon ages in bulk organic samples, the interpretation and calibration of AMRT 14C ages are geologically complex and their total associated errors are larger than the errors on ages from charcoal (Machette and others, 1992).

14C Carbon-14: a heavy radioactive isotope of carbon having a mass number of 14 and a half-life of 5730±40 yrs. Carbon-14 is useful in dating materials that are typically less than about 50,000 years old that are involved in the Earth’s carbon cycle (Bates and Jackson, 1987).

14C yr B.P. Radiocarbon years before present: designates the age of a sample in 14C years prior to calibration to correct for the uneven production of 14C in the atmosphere over time. Present, by convention, is taken as A.D. 1950. cal yr B.P. Calendar years before present: designates 14C ages that have been calibrated to calendric years according to one of several available data sets used to correct 14C ages for the uneven production of 14C in the atmosphere over time. Present, by convention, is taken as A.D. 1950. ka Kilo-annum: thousand years before present; restricted by the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 1983) to designate an age measured from the present. Present, by convention, is taken as A.D. 1950. For example, “The age of the stream-terrace deposits is estimated to be 15-30 ka based on soil-profile development.” kyr Thousand years: refers to an interval of time without reference to the present. For example, “The interevent interval between the two most recent surface-faulting earthquakes on the fault is 3.6 kyr.”

MRE Most recent surface-faulting earthquake (event): the youngest surface-faulting earthquake in an earthquake chronology.

PE Penultimate surface-faulting earthquake (event): the second-oldest surface-faulting earthquake in an earthquake chronology.

TL Thermoluminescence: the property possessed by many crystalline substances of emitting light when heat- ed as energy stored as electron displacements in the crystal lattice is released upon heating. Quantifying the amount of heat released as emitted light to determine the time of burial of silt and other fine-grained materials is widely used as a dating technique in archeological and geologic studies. yr B.P. Years before present: designates the age of a sample determined by thermoluminescence techniques, thermoluminescence ages are not thought to require calendar calibration.

FAULT/FAULT SECTION ABBREVIATIONS

BMF Bald Mountain fault BRFZ Bear River fault zone EBLF Eastern Bear Lake fault ECFZ East Cache fault zone FSF Fish Springs fault GSLFZ Great Salt Lake fault zone PS Promontory section1 FIS Fremont Island segment2 AIS Antelope island segment HFZ Hurricane fault zone AJS Anderson Junction section Consensus preferred recurrence-interval and vertical slip-rate estimates 109

HF Hogsback fault HVF Hansel Valley fault JPF James Peak fault JVFZ Joes Valley fault zone EJVF East Joes Valley fault WJVF West Joes Valley fault MMF Middle Mountain fault (intragraben) BMFs Bald Mountain faults (intragraben) MFZ Morgan fault zone NPF North Promontory fault OFZ Oquirrh fault zone SAFs Sugarville area faults SF Strawberry fault SOMFZ Southern Oquirrh Mountains fault zone LKF Lakes of Kilarney fault MF Mercur fault SCF Soldier Canyon fault WEHF West Eagle Hill fault TFG Towanta Flat graben WFZ Wasatch fault zone BCS Brigham City segment WS Weber segment SLCS Salt Lake City segment PS Provo segment NS Nephi segment LS Levan segment WaFZ Washington fault zone WCFZ West Cache fault zone CF Clarkston fault JHF Junction Hills fault WF Wellsville fault WVFZ West Valley fault zone TF Taylorsville fault GF Granger fault

2 “Segment” refers to a portion of a fault, typically also defined on the basis of geomorphic or structural criteria, but for which historical surface ruptures or paleoseismic data show that the history of surface-faulting earthquakes is different from other adjacent portions of the fault, and that the segment therefore behaves in an independently seismogenic manner from the remainder of the fault.