FINAL TECHNICAL REPORT U.S. Geological Survey External Grant

FINAL TECHNICAL REPORT

U.S. Geological Survey External Grant Award Numbers G19AP00072 and G19AP00073

Grant Period: July 1, 2019 to December 31, 2020

DETAILED MAPPING OF THE EAST AND WEST BEAR LAKE FAULT ZONES, UTAH AND IDAHO, AND THE OQUIRRH, SOUTHERN OQUIRRH MOUNTAINS, TOPLIFF HILLS, AND RUSH VALLEY FAULT ZONES, UTAH—USING NEW HIGH- RESOLUTION LIDAR DATA TO REDUCE EARTHQUAKE RISK

Submitted by

Adam I. Hiscock1, Zachery M. Lifton2, Greg N. McDonald1, and Emily J. Kleber1

March 31, 2021

1Utah Geological Survey, 1594 W. North Temple, Salt Lake City, Utah 84114-6100, http://geology.utah.gov/ AIH: [email protected], 801-537-3388 GNM: [email protected], 801-537-3383 EJK: [email protected], 801-538-4770

2Idaho Geological Survey, Idaho Water Center, Suite 201, 322 E. Front Street, Boise, Idaho 83702, http://www.idahogeology.org ZML: [email protected], 208-364-4099

Although this product represents the work of professional scientists, the Utah Department of Natural Resources, Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use. The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by users of this product.

This project was funded by the Utah Geological Survey, Idaho Geological Survey, and the U.S. Geological Survey, National Earthquake Hazards Reduction Program, through USGS External Grants award numbers G19AP00072 and G19AP00073. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

CONTENTS ABSTRACT 3 INTRODUCTION 3 GEOLOGIC SETTING 6 East Bear Lake Fault and West Bear Lake Fault 6 Oquirrh Fault Zone 8 Southern Oquirrh Mountains Fault Zone 10 Topliff Hills Fault Zone 12 Central and Western Rush Valley Faults 12 DATA SOURCES 13 Lidar Elevation Data 13 Aerial Photography 14 Previous Geologic Mapping 14 East Bear Lake and West Bear Lake Faults 14 Blackfoot Volcanic Field Faults 15 Oquirrh Fault Zone, Southern Oquirrh Mountains Fault Zone, Topliff Hills Fault Zone, and Rush Valley Faults 15 FAULT MAPPING 15 Fault Interpretations 15 East Bear Lake and West Bear Lake Faults 15 Blackfoot Volcanic Field Faults 15 Oquirrh Fault Zone 16 Southern Oquirrh Mountains Fault Zone 17 Central and West Rush Valley Faults 18 Topliff Hills Fault Zone 19 Fault Traces 19 Special-Study Zone Delineation 20 POTENTIAL PALEOSEISMIC INVESTIGATION SITES 22 East Bear Lake Fault 22 Oquirrh Fault Zone 23 Southern Oquirrh Mountains Fault Zone 23 Central and Western Rush Valley Faults 24 Topliff Hills Fault Zone 24 CONCLUSIONS 24 REFERENCES 26

FIGURES

Figure 1 – Areas mapped in this study in northern Utah and southern Idaho Figure 2 – Map showing the Soda Springs, Idaho, earthquake sequence from September 2, 2017, to March 15, 2021 Figure 3 – The East and West Bear Lake faults in Utah and Idaho, and the Blackfoot Volcanic Field faults and Gem Valley faults in Idaho. Figure 4 – The Oquirrh fault zone, Southern Oquirrh Mountains fault zone, Topliff Hills fault zone, and Rush Valley faults in Utah Figure 5 – Comparison between aerial photography and lidar hillshade images Figure 6 – Examples of special circumstances used when creating surface-fault-rupture special- study zones

TABLES

Table 1 – Potential paleoseismic sites along the East and West Bear Lake fault zones, the Oquirrh fault zone, Southern Oquirrh Mountains fault zone, Topliff Hills fault zone, South Mountain marginal fault, Clover fault zone, Sheeprock fault zone, and Gem Valley fault.

Fault mapping available through the UGS Utah Geologic Hazards Portal –

https://geology.utah.gov/apps/hazards/

ABSTRACT

The Utah Geological Survey (UGS) and the Idaho Geological Survey (IGS) mapped Quaternary-active faults in southeastern Idaho and northern Utah using recently collected airborne high-resolution topographic data in addition to available aerial photography and field reconnaissance. Specifically, the UGS and IGS mapped the East Bear Lake fault zone and West Bear Lake fault zone in northeastern Utah and southeastern Idaho, and the UGS mapped the Oquirrh fault zone, the Southern Oquirrh Mountains fault zone, the Topliff Hills fault zone, and the central and western Rush Valley faults in north-central Utah. High-resolution topographic data derived from airborne light detection and ranging (lidar) elevation data has allowed for detailed mapping of fault traces along these fault zones. Previously, the surface location and extent of fault traces associated with these fault zones were not well understood in many areas, owing to limited aerial photography coverage, heavy vegetation near range fronts, and the difficulty in recognizing moderate (<3 feet [<1 m]) displacements in the field or on aerial photographs. Previous geologic mapping, paleoseismic investigations, historical aerial photography, and field investigations were also used to identify and map surface fault traces and infer fault locations. In Utah, special-study areas were delineated around mapped faults to facilitate understanding of the surface-rupture hazard and associated risk. Defining these special- study zones encourages the creation and implementation of municipal and county geologic- hazard ordinances dealing with hazardous faults in Utah. We identified 51 potential paleoseismic investigation sites where fault scarps appear relatively pristine, are located in geologically favorable settings, and where additional earthquake timing data would be beneficial to earthquake research of the faults mapped in this study. More accurate mapping and characterization of these faults helps to mitigate earthquake risk in southeastern Idaho and northern Utah by developing surface-fault-rupture hazard maps and refining fault segmentation models and fault activity levels for use in regional earthquake-hazard assessments.

INTRODUCTION

The Utah Geological Survey (UGS) and Idaho Geological Survey (IGS) performed detailed fault-trace mapping for fault zones in northeastern Utah, southeastern Idaho, and north- central Utah (figure 1). Our investigation included: 1) mapping surface traces of northern Utah and southeastern Idaho faults at 1:10,000-scale using currently available high-resolution lidar data, aerial photography, and field reconnaissance, 2) identifying potential paleoseismic trenching sites for future investigation, 3) defining special study zones for fault traces in Utah for land-use planning, management, and local government ordinances and publishing in a feature- class layer in the UGS Utah Geologic Hazards Portal, 4) publishing new fault trace geometries and attributes to the UGS Utah Geologic Hazards Portal and the IGS Miocene and Younger Faults in Idaho database, and 5) presenting investigation results to professional groups, local governments, and the public in Utah and Idaho.

Northern Utah and southeastern Idaho are experiencing rapid growth in urban and rural areas. The extent of scarps along traces of the East Bear Lake fault (EBLF), West Bear Lake fault (WBLF), the Oquirrh fault zone (OFZ), the Southern Oquirrh Mountains fault zone (SOMFZ), the Topliff Hills fault zone (THFZ), and the central and western Rush Valley faults are not well understood in many areas owing to limited aerial photography for the area, difficulty in recognizing small to moderate (<3 feet [<1 m high]) scarps in the field or on stereo- paired aerial photographs, and dense vegetation near range fronts in some areas. Accurately mapping and characterizing these active fault traces are essential to mitigating earthquake risk in Bear Lake Valley (Idaho/Utah), Tooele Valley (Utah), and Rush Valley (Utah), to update the USGS National Seismic Hazard Maps, and to refine fault segmentation models and fault activity levels for use in regional earthquake-hazard assessments.

In September 2017, a magnitude (M) 5.3 earthquake initiated the robust Soda Springs earthquake sequence, which included more than 2000 Figure 1. Location of the areas referenced in this study in measured events (figure 2). The figures 2, 3, and 4. Faults mapped in this study shown as earthquake epicenters are in a heavy red lines, other faults shown in light grey (from Utah tight cluster east of the town of Geologic Hazards Portal, 2021, and Idaho Miocene– Soda Springs, Idaho, directly Quaternary Fault Map, 2021). Basemap from ESRI. under the mapped trace of the EBLF (figure 3). Most of the earthquakes have focal mechanisms suggesting movement on a normal fault, although some earthquakes have strike-slip focal mechanisms. More than 90% of the earthquakes occurred at depths less than 6 miles (10 km). The exact source fault is not clearly

4 known, but it is likely that the earthquakes occurred on a steeply dipping EBLF or a shallowly dipping WBLF. While southeastern Idaho is known to be seismically active, this earthquake sequence significantly increased awareness of seismic hazards in the region.

Figure 2. The Soda Springs, Idaho, earthquake sequence. Earthquake locations and magnitude show as circles from September 2, 2017, to March 15, 2021 (magnitude scale in top right corner). Earthquake data from U.S. Geological Survey earthquake catalog. Faults mapped in this study shown as heavy black lines.

Despite southeastern Idaho’s sparse population, seismic hazards potentially threaten critical industries and infrastructure. The phosphate mining industry has been active in southeastern Idaho since the early 1900s, and the area now has many active mines and two active processing plants in Caribou County. In 2015, the IGS estimated that the raw phosphate ore in the area was worth over $200 million. The phosphate mining industry currently employs approximately 1500 people with direct jobs and likely generates many additional indirect jobs. Tourism is another important and expanding economic driver in the greater Bear Lake area. An estimated 500,000 people annually visit the otherwise rural area. The mapped EBLF and WBLF cross or parallel several state and federal highways that serve as major freight-hauling routes, including U.S. Highways 30 and 89, and Idaho State Highway 36 (figure 1). Approximately 2.5 miles (4 km) south of Montpelier, Idaho, the EBLF is crossed by the Williams Northwest Pipeline interstate high pressure natural gas transmission pipeline.

Tooele County in northern Utah is a rapidly growing area with an estimated population growth of 48% from 2015 to 2040 (Tooele County General Plan, http://www.co.tooele.ut.us/Building/tcgeneralplan.htm). Of that growth, 86% of it is expected to occur in communities in Tooele Valley, which is bordered on the east by the OFZ (figure 4). Time is of the essence to map fault traces before development destroys surface faulting evidence, or development occurs in areas that likely have high surface-fault-rupture hazards, such as in Tooele Valley where several newly mapped scarps of the OFZ (from this study) have already been obscured by new housing development.

For this UGS/IGS/USGS-funded project, we have produced updated surface fault trace mapping of the EBLF, WBLF, OFZ, SOMFZ, THFZ, and the central and western Rush Valley faults, principally using high-resolution airborne lidar-derived imagery as well as aerial photos, previous geologic mapping, and field investigations. The mapping shows surface fault geometries at 1:10,000 scale or greater, approximate age categories determined from previous paleoseismic investigations, geologic mapping, and geomorphic relationships, and special-study zones for faults within Utah (Lund and others, 2020). Age categories in the Utah Geologic Hazards Portal (https://geology.utah.gov/apps/hazards/) are based on Lund and others (2020) and Western States Seismic Policy Council (WSSPC, 2018) and will ensure a seamless integration of fault geometries and attributes into the USGS Quaternary Fault and Fold Database of the United States. Additionally, these surface-fault-rupture hazard maps are available through the UGS Utah Geologic Hazards Portal (https://geology.utah.gov/apps/hazards/). Surface-fault-rupture special-study zones can be implemented in geologic hazard ordinances (i.e., building setbacks, critical infrastructure avoidance, etc.) by local governments to reduce risk from surface faulting hazard (Lund and others, 2020). As part of this investigation, we also identified potential paleoseismic trenching sites (table 1).

GEOLOGIC SETTING

East Bear Lake Fault and West Bear Lake Fault

The Bear Lake and surrounding faults are part of the structural transition zone between extension from the Basin and Range Province to the west, and the uplifting Middle Rocky Mountains Province to the east and north (Stokes, 1977, 1986). Bear Lake Valley lies between the Bear River Range to the west, the Preuss Range to the northeast, and the Bear Lake Plateau to the southeast. The Idaho/Utah state boundary bisects the valley and Bear Lake near the midpoint of the lake. The valley can be subdivided into three parts: the southern end that contains Bear Lake (18 miles [30 km] long, 9 miles [15 km] wide), the central part with Dingle Swamp and Mud Lake (9 miles [15 km] wide), and the northern end, which narrows from Georgetown to Soda Springs, Idaho. Fault traces of the EBLF continue as far north as Blackfoot Reservoir, Idaho.

The Bear Lake Valley is bounded on the east by the EBLF and on the west by the WBLF (figure 3). Each fault is separated into three sections, based on the fault geometry (McCalpin, 2003). Scarps typically range in height from 7 to 20 feet (2–6 m) with a maximum scarp height

of >100 feet (>30 m) on the southern section of the EBLF. Seismic reflection and sidescan sonar of the lake floor and sub- bottom stratigraphy suggest a main, west-dipping listric fault on the east side of the valley and a secondary, east-dipping normal fault on the west side of the valley, with several faults cutting the youngest lake sediments and modern lake floor (Colman, 2006).

Evidence for active faulting in Bear Lake Valley was first recognized by Mansfield and Girty (1927), who noted a normal fault on the east side of the valley and concluded that it was recently active based on the freshness of scarps. Additional geomorphic evidence of active faulting, such as truncated alluvial fans, offset deltas, displaced shorelines, discontinuous scarplets, and aligned hot springs, was also described by Willard (1959), Williams and others (1962), McClurg (1970), and Kaliser (1972).

Although the west- dipping East Bear Lake fault (EBLF) is considered the primary structure in the valley (Mansfield and Girty, 1927; Colman, 2006), there is also evidence for late Quaternary Figure 3. Fault traces identified in the Bear Lake region movement on the dominantly including the East and West Bear Lake faults, Blackfoot east-dipping West Bear Lake fault Volcanic Field faults, and Gem Valley faults as heavy black (WBLF). Roberston (1978) lines. Other faults shown in light grey (from the UGS Utah inferred the presence of an east- Geologic Hazards Portal, 2021, and IGS Idaho Miocene– dipping normal fault on the west Quaternary Fault Map, 2021). Basemap from ESRI.

side of the valley. Seismic reflection and sonar surveys of Bear Lake confirmed that the valley is an east-tilted half-graben with a flexural margin on the west side of the valley (Colman, 2006). Laabs and Kaufman (2003) used lake paleoshoreline highstand observations to conclude that both the EBLF and WBLF are Holocene active. Evans and others (2003) reinterpreted historical records and suggested that the 1884 Mw 6.3 earthquake likely occurred on the WBLF.

The EBLF and WBLF both offset late Pleistocene and Holocene deposits. Colman (2006) concluded from seismic reflection data that faults cut the youngest sediments in the modern lake floor. McCalpin (2003) excavated two paleoseismic trenches on each fault and found evidence for five to seven faulting events since 39 ka on the southern section of the EBLF and at least two faulting events since ~13 ka on the southern section of the WBLF. These data yielded broadly constrained long-term slip rates of 0.6 mm/yr on the EBLF and 0.5 mm/yr on the WBLF. Reheis and others (2009) used water-level history of Bear Lake to suggest that Pleistocene slip rates on the EBLF increased from north to south, and that the slip rate on the southern section of the EBLF has decreased over the last 50 ky.

North of Soda Springs, Idaho faulting continues north-northwest across the Blackfoot Volcanic Field (BVF, figure 3). The BVF is composed of Quaternary basaltic lava flows, basaltic cinder cones, and rhyolite domes filling in the valley now occupied by Blackfoot Reservoir and the adjacent Gem and Willow Creek Valleys (Fiesinger and others, 1982; Polun, 2011; Welhan and others, 2011; McCurry and others, 2015). BVF basalts have similar composition and lithospheric origin (Pickett, 2004; Ford, 2005). BVF basaltic lava flows are not precisely dated, but began erupting in the early Pleistocene (Polun, 2011, and references therein). The youngest rhyolite domes in the BVF (China Hat, China Cap, and North Cone) have been dated at 57±8 ka (Heumann, 2004). Faulting is expressed across the BVF as many distributed, discontinuous normal fault scarps and fissures. We refer to these faults as the Blackfoot Volcanic Field faults (BVFF). Although the BVFF appear to be a northern continuation of the EBLF, their character is different, and it is not clear that they share the same tectonic origin.

Oquirrh Fault Zone

Tooele Valley occupies a structural basin bounded to the east by the OFZ, which extends along the western base of the northern Oquirrh Mountains (figure 4). The Quaternary geology of the valley is dominated by the lake cycles of late Pleistocene Lake Bonneville. Numerous surficial deposits and lake shorelines from both transgressive and regressive lake cycles of Lake Bonneville are found along most of the trace of the OFZ.

The OFZ is divided into three sections: the northern section, central section, and southern section (figure 4, Olig and others, 1996). The northern section is 3 miles (4.8 km) long and consists mainly of north- to south-trending fault scarps in lacustrine and alluvial deposits along the range front of the northern Oquirrh Mountains. The central section of the OFZ near the town of Erda, Utah, diverges from the range front and trends southwest along the Erda salient. The scarps here cut predominantly undifferentiated lacustrine and reworked alluvial-fan deposits (Clark and others, 2017) overlain on the bedrock salient. The salient ends approximately at the

Figure 4 (left). Fault traces identified in Tooele Valley and Rush Valley including the Oquirrh fault zone (OFZ), Southern Oquirrh Mountains fault zone (SOMFZ), Topliff Hills fault zone, and Rush Valley faults (SMMF - South Mountain Marginal fault, SJSFZ - Saint Johns Station fault zone, CFZ - Clover fault zone, VHFZ - Vernon Hills fault zone, SFZ - Sheeprock fault zone). Fault sections of the OFZ labeled: NS - North Section, CS - Central Section, SS - Southern Section. Fault sections of the SOMFZ labeled: SCF - Soldier Canyon fault, LOKF - Lakes of Killarney fault, WMF - West Mercur fault, WEHF - West Eagle Hill fault. All mapped faults from this study shown as heavy black lines; other Quaternary faults shown as dark grey lines (from the UGS Utah Geologic Hazards Portal, 2021). 2010 U.S. Census data are approximate population density per census block (Utah Automated Geographic Reference Center, 2010). White circles indicate communities in Tooele and Rush Valleys.

North Oquirrh thrust fault (Clark and others, 2017), where the OFZ trends back to the southeast and returns to the range front, marking the beginning of the southern section. The southern section climbs in elevation and is completely above the Lake Bonneville highstand shoreline where it enters the northern Oquirrh Mountains just to the northeast of Settlement Canyon.

The northern end of the OFZ terminates at the southern shoreline of the Great Salt Lake, and at the southern end of the Great Salt Lake fault zone (GSLFZ). In some literature, the OFZ and GSLFZ are combined to form the Oquirrh-Great Salt Lake fault zone (e.g., Wong and others, 2016). Helm (1994) theorized that the northern end of the OFZ is controlled by a Precambrian transverse crustal structure, which also influences the Salt Lake City-Weber segment boundary on the Wasatch fault zone to the east, as well as the northern terminus of the Stansbury fault zone to the west. Helm (1994) also postulates that the southern end of the OFZ, and boundary with the SOMFZ, is controlled by another Precambrian transverse zone, which lines up with the Salt Lake City-Provo segment boundary on the Wasatch fault zone, as well as a segment boundary on the Stansbury fault zone to the west (Helm, 1994).

The northern and central sections of the OFZ have been active in the Holocene based on paleoseismic trenching performed at the Pole Canyon (northern section) and Big Canyon (central section) trench sites (Olig and others, 1996). Olig and others (1996) found evidence for two surface-faulting earthquakes in the past 27 ka, calculated a recurrence interval of 13.3 to 22.1 kya, and estimated a slip rate of 0.1 to 0.2 mm/yr for the OFZ. A scarp profiling study on the central section of the OFZ (Barnhard, 1988; Barhard and Dodge, 1988) found evidence of a late Pleistocene earthquake comparable to what Olig and others (1996) found in the Pole Canyon trench. No paleoseismic work has been conducted on the southern section; however, Olig and others (1996) inferred that due to the large displacements documented in their trenches on the northern section, the northern, central, and southern segments probably form a single rupture segment. More recently, work on the Great Salt Lake fault zone (GSLFZ) north of the OFZ (Dinter and Pechmann, 2000) raises questions about its potential seismogenic relationship to the OFZ, and the possibility of multi-segment ruptures including the OFZ and GSLFZ (Olig and others, 2001; Wong and others, 2016).

Southern Oquirrh Mountains Fault Zone

The SOMFZ is the basin-bounding fault zone at the range front of the southern Oquirrh Mountains in northeastern Rush Valley (figure 4). The fault zone is divided into four sections, from north to south, named by Gilluly (1932) and refined by Wu and Bruhn (1994): the Soldier Canyon fault, the Lakes of Killarney fault, the West Mercur fault, and the West Eagle Hill fault (figure 4). At the northern extent of the SOMFZ, the Soldier Canyon fault is mapped as a bedrock fault in Mississippian limestone that shows no evidence of Quaternary displacement north of Soldier Creek. To the south at the mouth of Soldier Creek Canyon, discontinuous fault scarps displace Holocene and older alluvial-fan deposits (Clark and others, 2012). The Lakes of Killarney fault extends from 1 mile (1.6 km) south of Soldier Creek to Silverado Canyon. The West Mercur fault, also called the Mercur fault, is a ~10 mile (~16 km) northwest-southeast- trending fault cutting Quaternary fans from Ophir Canyon to West Mercur. The Mercur fault starts at the bedrock-fan interface at Ophir Canyon for ~3 miles (~5 km) before trending away from the range front where it cuts Quaternary fan deposits. The West Eagle Hill fault starts in bedrock north of Mercur Canyon and continues for ~9 miles (~15 km) along the southern Oquirrh Mountains range front, through Fivemile Pass, and terminates in Quaternary alluvial fans west of Thorpe Hills. Several paleoseismic investigations have been conducted on the SOMFZ (Everitt and Kaliser 1980; Barnhard and Dodge, 1988) and specifically on the West Mercur fault (Olig and others, 2001).

The Quaternary geology along the SOMFZ is dominated by late Quaternary sedimentation in alluvial fans initiating from the southern Oquirrh Mountains as most of the fault is above the highstand elevation of Lake Bonneville (Clark and others, 2012). Gravity data indicate that the bedrock basement in Rush Valley is deeper at the southern end of the valley, and shallows to the north (Wu and Bruhn, 1994). At the southern extent of the Mercur fault, near Utah Highway 73, Holocene fault scarps intermingle with Bonneville highstand shoreline deposits (Clark and others, 2012).

The Soldier Canyon fault is primarily a bedrock fault that cuts from 1 mile (1.6 km) north to 1 mile (1.6 km) south across Soldier Canyon wash (Clark and others, 2012). This fault is discontinuous and shows approximately 3200 to 4000 feet (1000–1200 m) of offset in the upper- Mississippian Great Blue Limestone mapped in the southern Oquirrh Mountains (Wu and Bruhn, 1994). Evidence from bedrock observations indicates two large Quaternary-active rupturing events occurred prior to the highstand of the Bonneville lake cycle (Wu and Bruhn, 1994). The Lakes of Killarney fault is a bedrock fault with a bedrock-alluvial contact at its northern extent. Cosmogenic radiocarbon dates from a bedrock scarp on the Lakes of Killarney fault indicate a mid-Holocene surface rupture of ~4400 yr B.P. (Handwerger and others, 1999). Scarp profiling and paleoseismic timing from trenching on the West Mercur fault indicate that the Lakes of Killarney and West Mercur fault have either ruptured coseismically or have caused triggered events on these neighboring faults (Wu and Bruhn, 1994; Handwerger and others, 1999; Olig and others, 2001).

The West Mercur fault is the longest section of the SOMFZ and has the best geomorphic expression in Quaternary deposits and the highest scarps. Fault scarps near the mouth of Mercur Canyon, at over 30 feet (10 m) of vertical offset, are the highest of any along the SOMFZ and scarp heights decrease to the south and north (Wu and Bruhn, 1994). Data from trenching across three scarps of the West Mercur fault north of Mercur Creek indicate evidence for five to seven surface-fault-rupturing earthquakes occurring since ~92 ka, suggesting a recurrence interval of 12 to 25 kyr (Olig and others, 2001). As indicated from the last four to six seismic cycles, vertical slip per event estimations of 4.6 to 7.2 feet (1.3 to 2.2 m) yielded vertical slip rates of 0.09 to 0.14 mm/yr on the West Mercur fault (Olig and others, 2001). Comparing paleoseismic data from the OFZ (Olig and others, 1996), Olig and others (2001) cite an overlap of the most recent events on the OFZ and SOMFZ as well as similar vertical slip rates, slip-per-event, and along-strike displacement patterns as possible indications for coseismic rupture of the two fault zones. Wu and Bruhn (1994) suggested that the Soldier Canyon, Lakes of Killarney, and West Mercur faults could rupture coseismically. Olig and others (2001) agree that the Mercur fault can rupture coseismically with the other faults of the SOMFZ, but it may not rupture with all four of the faults in the fault zone.

About 3 miles (5 km) west of the SOMFZ are several discontinuous, east-dipping faults cutting older alluvial-fan deposits (Clark and others, 2012; Kirby, 2012). The scarp heights increase to the south where the footwall deposits are mapped as very old (Quaternary, possibly Tertiary) alluvial-fan surfaces. No data exist for these faults beyond the geologic mapping, but we infer they may be associated with, and antithetic to, the SOMFZ given their location in the valley and general strike that parallels the SOMFZ.

Topliff Hills Fault Zone

The THFZ is a poorly understood fault system along the southeastern margin of Rush Valley (figure 4). The southern part of the THFZ is mostly defined by a range front escarpment at the base of the East Tintic Mountains. Along the northern part of the THFZ, discontinuous fault scarps have been mapped on mostly older alluvial-fan deposits, but ages of faulted deposits are not well constrained. In some cases, previous mapping of well-located faults has conflicting interpretations regarding the age of displaced surfaces. Everitt and Kaliser (1980) mapped a faulted alluvial fan as younger than Lake Bonneville, whereas Barnhard and Dodge (1988) show the same faulted alluvial fan as shoreline etched, and thus older than Lake Bonneville.

Preliminary, unpublished work by Utah Valley University faculty and students (Ward and others, 2019) shows evidence of six surface-faulting earthquakes since about 70–40 ka based on interpretations from two trenches at a site near the central part of the THFZ. Their trenches exposed faulting on a 30-foot-high (9 m) scarp cutting a pre-Bonneville alluvial-fan surface and on an adjacent, subtle 1.6-foot-high (0.5 m) scarp displacing an inset, post-Bonneville alluvial deposit. Their trench interpretations and age-dating analyses indicate the fault has experienced three post-Lake Bonneville earthquakes with average displacements of about 7 feet (2 m) per event. These preliminary results contrast with the geomorphic expression of the entire THFZ that exhibits more subdued, weathered, and discontinuous fault traces best defined on older, pre- Bonneville deposits. Additional paleoseismic data on the THFZ are needed to better constrain the timing and recurrence of the fault zone, especially for the most recent events.

Central and Western Rush Valley Faults

Several fault zones exist in the central and western side of Rush Valley. These include the (from north to south, figure 4): South Mountain Marginal faults (SMMF), Saint Johns Station fault zone (SJSFZ), Clover fault zone (CFZ), Sheeprock fault zone (SFZ), and Vernon Hills fault zone (VHFZ). These fault zones are poorly understood, and no paleoseismic data exist for any of them.

Late Quaternary deposits in Rush Valley and Tooele Valley indicate similar and different lake levels during pluvial lake cycles in the Bonneville basin. Like Tooele Valley, Rush Valley has highstand shoreline deposits from Lake Bonneville (5240 feet, [1597 m]), which overtopped the Stockton Bar at 4960 feet (1512 m) elevation. However, Rush Valley does not have deposits from the older Cutler Dam and Little Valley lake cycles (Olig and others, 2001). Lacustrine deposits and shorelines associated with the Lake Bonneville regression are present in Rush Valley at elevations that do not coincide with prominent shorelines elsewhere, notably the Provo (4860 feet, 1480 m) (Burr and Currey, 1988, 1992; Kirby, 2013a). Given the lake in Rush Valley was cut off from Lake Bonneville at levels below the Stockton Bar threshold (Scott and others, 1983; Currey and Oviatt, 1985; Machete and others, 1992; Clark and others, 2012), at least two unique lake levels in Rush Valley formed shoreline features. These include the more prominent Lake Shambip shoreline (5050 feet [1539 m]) and the more discontinuous Lake Smelter shoreline (about 4990 feet [1520 m]). The late Holocene Rush Lake, at the northern end of Rush Valley, still exists seasonally during exceptionally wet years (Gilbert, 1890; Kirby, 2013a).

The SJSFZ lies nearly in the middle of Rush Valley and consists of a series of both east- and west-dipping faults, forming several prominent horst and graben structures (Kirby, 2013a). Fault scarps offset late Pleistocene mixed alluvial and lacustrine deposits from between the Lake Shambip level and the Lake Bonneville highstand (Kirby, 2013a). Scarps of the SJSFZ cut the Tertiary Salt Lake Formation and are considered to be late Quaternary (Olig and others, 2001), although previous studies have reported SJSFZ cutting lake Bonneville deposits (Everitt and Kaliser, 1980).

The SMMF and CFZ form a north-south-trending belt of faults along the western margin of Rush Valley (Barhard and Dodge, 1988; Kirby, 2013a). The SMMF is a zone of distributed faulting running from near the southern margin of South Mountain to near the town of Clover. To the north, the fault trace is above the Lake Bonneville highstand and cuts primarily undifferentiated pre-Bonneville alluvial-fan deposits (Clark and others, 2012). As the SMMF nears the northern end of the CFZ, both the SMMF and CFZ offset late Pleistocene fan deposits between the Lake Bonneville highstand and Lake Shambip shorelines (Barnhard and Dodge, 1988). Olig and others (2001) suggest a possible structural relationship between the SJSFZ, CFZ, and SOMFZ due to their close proximity in the center and topographic bottom of Rush Valley.

The VHFZ and SFZ lie near the southern margin of Rush Valley. The VHFZ includes west- and east-facing scarps along both sides of Vernon Hills, which lie east of the town of Vernon and approximately at the midpoint of Rush Valley. Fault scarps offset Pennsylvanian-age bedrock and late Pleistocene-age alluvial-fan deposits (Kirby, 2013b).

The SFZ offsets alluvial-fan deposits along the range front of the Sheeprock Mountains. Fault scarps offset late Pleistocene and Holocene alluvial-fan deposits, forming large 9- to 20- foot high (3–7 m) scarps (Kirby, 2010b). These scarps show evidence of at least one Holocene surface faulting event (Everitt and Kaliser, 1980), but no paleoseismic data exist for the SFZ.

DATA SOURCES

Lidar Elevation Data

High-resolution (0.5-meter) USGS Quality Level 1 (Heidemann, 2018) lidar data of the Bear Lake area were acquired by the State of Utah and its partners in 2016 (Utah Automated Geographic Reference Center, 2016) for fault mapping, urban planning, and other purposes. This initial dataset did not fully cover the EBLF along the eastern margin of Bear Lake. In 2018, additional high-resolution (0.5-meter) USGS Quality Level 1 lidar data were collected along the eastern margin of Bear Lake, filling in the missing areas from the 2016 data collection (Utah Automated Geographic Reference Center, 2018). Also in 2018, high-resolution (1-meter) USGS Quality Level 2 lidar data of the Tooele and Rush Valleys were acquired by the State of Utah and its partners for fault mapping, urban planning, and other purposes (Utah Automated Geographic Reference Center, 2018). For mapping in Idaho, we used high-resolution lidar datasets publicly available from the Idaho Lidar Consortium (https://www.idaholidar.org/): 2017 Bear Lake (Quality Level 1), 2017 Blackfoot-Portneuf (Quality Level 1), and 2018 Franklin and Bear Lake Counties (Quality Level 1). Lidar derivative products that were useful for identifying and refining surficial fault traces include slope-shade images, various hill shade images with different

13 light directions and altitudes (figure 5), and contour lines. Global Mapper (v.18) software was used to generate these images, as well as to generate topographic profiles perpendicular to scarps to investigate fault-scarp morphologies.

Figure 5. Comparison between aerial photography and lidar hill shade images of the Oquirrh fault zone in Tooele Valley, Utah. The fault trace is faintly visible in the aerial photo on the left, but far more visible on the hill shade images. The right image shows the mapped fault traces based on the hill shade image.

Aerial Photography

Historical aerial photography from the UGS Aerial Imagery Collection (https://geodata.geology.utah.gov/imagery/) was used to map in urban areas where surface fault traces have been obscured by modern development.

Previous Geologic Mapping

East Bear Lake and West Bear Lake Faults

We used geologic mapping by Lewis and others (2012) as a basis for broadly understanding the geology of Bear Lake Valley. Dover (1997) mapped the southern part of Bear Lake Valley with a focus on bedrock at 1:100,000 scale. Valenti (1982) mapped the Laketown 7.5-minute quadrangle with an emphasis on bedrock at the very southern end of Bear Lake Valley. Coogan (1997) mapped the Bear Lake South 7.5-minute quadrangle that includes detailed mapping of Quaternary deposits and fault traces for the southern section of the EBLF. Elsewhere, no 7.5-minute quadrangle geologic maps have been published for the EBLF and WBLF, but we did review unpublished mapping of the area by the IGS. Geologic mapping and descriptions by Robertson (1978), McCalpin (2003), Colman (2006) and Laabs and others (2007) were helpful in our interpretations and fault mapping.

Blackfoot Volcanic Field Faults

We used previous mapping of normal fault scarps within the Blackfoot Volcanic Field by Polun (2011) as a reference and guide for our mapping and interpretation.

Oquirrh Fault Zone, Southern Oquirrh Mountains Fault Zone, Topliff Hills Fault Zone, and Rush Valley Faults

Previous surficial and bedrock geologic mapping was useful for this project. The Tooele, Rush Valley, and Lyndell 30 x 60-minute geologic quadrangle maps (Moore and Sorensen, 1979; Pampeyan, E.H., 1989; Clark and others, 2012, 2017) were used for the regional geology and as a check on our fault-trace mapping. Additionally, we reviewed mapping from several 7.5- minute quadrangles (Disbrow, 1961; Barnhard and Dodge, 1988; Solomon, 1993; Kirby, 2010a, 2010b, 2012, 2013a, 2013b, 2013c). Mapping and descriptions from previous paleoseismic and fault studies were also used (Wu and Bruhn, 1994; Solomon, 1996; Olig and others, 2001). Geologic mapping and abstracts from Dr. Susanne Jänecke at Utah State University were also used as a guide and reference for our surficial fault mapping (Jänecke and Evans, 2017; Jänecke and others, 2020).

FAULT MAPPING

Fault Interpretations

East Bear Lake and West Bear Lake Faults

East and West Bear Lake fault scarps are well exposed throughout Bear Lake Valley. Scarps immediately adjacent to Bear Lake have limited exposure because there is little onshore space between the lake and the steep valley walls. Many additional scarps may be submerged. Where the fault crosses alluvial fans or deltas, the surface expression tends to become more diffuse and the surface trace divides into multiple strands. North of Bear Lake, fault scarps are more widely distributed across the valley floor. The WBLF scarps north of Bear Lake, in particular, are distributed further into the valley floor. However, most of the offset occurred at the valley margins. A bedrock high south of Soda Springs, Idaho, marks the northern end of Bear Lake Valley, and acts as a transition zone. This transition zone separates two distinct patterns of faulting: south of the transition zone, faulting is expressed as two parallel fault zones on either side of the valley to the south; north of the transition zone, faulting is broadly distributed across the valley floor.

Blackfoot Volcanic Field Faults

The BVFF are expressed as numerous discontinuous scarps and fissures preserved in basalt bedrock across the valley. Some scarps on the eastern margin of the BVF record significant displacement (>~165 feet [>~50 m]). North of Soda Springs, Idaho, the BVFF scarps are preserved almost exclusively in Quaternary volcanic rocks. Faulting across the BVF is not confined to discrete zones, so it is difficult to determine if it is a continuation of the EBLF,

WBLF, or neither. Faults in adjacent Gem Valley also displace related basalt flows but are confined to a much narrower discrete linear zone along the eastern side of Gem Valley. Fissures are expressed at the surface as either open bedrock fissures, or as fissures buried under loess and expressed as linear depressions or chains of depressions (“dimples”). Cross-cutting relationships suggest that the BVFF scarps formed during and after eruptions of basalt flows and rhyolite domes, thus it is not clear if these faults have a volcanic or tectonic origin. The origin of these faults has important implications for seismic hazards because they may or may not be significant seismic sources.

Oquirrh Fault Zone

The OFZ extends south about 19 miles (30 km) from the northern end at Great Salt Lake, to the southern end in Settlement Canyon in the Oquirrh Mountains. The northern terminus is thought to connect into the GSLFZ; however, this study found no direct connection to faults mapped by Dinter and Pechmann (2000) at the southern edge of Great Salt Lake. However, several of our mapped fault traces do trend in approximately the same strike as several traces of the GSLFZ, so it is possible there may be a connection between the OFZ and the GSLFZ. It is possible that scarps near the northern end of the OFZ have been destroyed by wave action and wind erosion from the GSL.

Newly mapped fault traces on the floor of Tooele Valley trend north-northeast for approximately 5.5 miles (9 km). Scarps along these traces are very subtle, ranging from approximately 1.5 to 3 feet (0.5–1.0 m) in height, and offset regressive Lake Bonneville beach bar deposits along their length. The relationship between these faults and the main trace of the OFZ may be similar to the relationship between the West Valley fault zone and the Wasatch fault zone in the Salt Lake Valley. A fault located near the mouth of Silcox Canyon to the south, near the boundary with the SOMFZ, lies almost directly on trend with these valley floor faults, suggesting a possible structural connection. This fault is mapped as being older than the faults in the floor of Tooele Valley due to the age of the deposits cut.

The southern end of the OFZ extends into the Oquirrh Mountains near Settlement Canyon for about 3.75 miles (6 km), before making an abrupt westward bend and cutting across a mountain ridgeline before terminating just north of Soldier Canyon. This westward bend approximately lines up with the axis of South Mountain and the South Mountain salient, the small range defining the southern end of Tooele Valley and northern end of Rush Valley. The nature of faulting in this area suggests the possibility of this salient acting as a structural boundary to faulting between the OFZ and SOMFZ, similar to segment boundaries on the Wasatch fault zone to the east. This segment boundary has been modeled previously in probabilistic seismic hazard assessments on the Wasatch Front (Youngs and others, 1987; Wong and others, 1995; Wong and others, 2016). This study has not identified evidence of surface rupture that would indicate whether the OFZ and SOMFZ behave as a single or segmented fault system. However, it is our opinion that the existing convention of treating the OFZ and SOMFZ as separate segments of a larger fault zone extending from Great Salt Lake to Furner Pass is reflected in our new fault mapping (Olig and others, 2001; Wong and others, 2016).

Southern Oquirrh Mountains Fault Zone

The SOMFZ consists of four previously identified fault strands: the Soldier Canyon fault, the Lakes of Killarney fault, the West Mercur fault, and the West Eagle Hill fault. For this study, we used the same naming conventions as previous studies (Gilully, 1932; Wu and Bruhn, 1994; Olig and others, 2001). Starting from north to south, we will discuss the differences between previous mapping and our lidar-based mapping. One important characteristic previously interpreted for the SOMFZ that is reinforced by our lidar mapping is the role that bedrock faults near the range front play in Quaternary faulting. The surface expression of some faults in Quaternary deposits clearly continues into bedrock faults. The clarity of this relationship is not always visible in lidar mapping on the Wasatch Front (McDonald and others, 2020).

The Soldier Canyon fault is mostly a bedrock fault that is best shown by a north-south- trending Quaternary scarp in Mississippian Great Blue Limestone mapped in the drainage containing the Jacob City trail, ~1 mile (1.7 km) south of the mouth of Soldier Canyon. Additional Holocene scarps cut younger fan material deposited on top of the Soldier Canyon fan, increasing the density of mapped faults from what was previously mapped as a single scarp through older fan units. We also mapped a short (1500 feet [500 m]) late Quaternary-age scarp in an older fan unit to the north of Soldier Creek that was previously unidentified. Bedrock faults associated with the Soldier Canyon faults should be considered Quaternary active.

The Lakes of Killarney fault is mapped as a bedrock-alluvial and bedrock fault for ~5.5 miles (~9 km) from south of Soldier Canyon to south of Ophir Canyon. Our lidar mapping showed a longer extent of the fault from the range front south of Soldier Canyon into bedrock, then at the alluvial-bedrock contact north of and then through Ophir Canyon. We put this fault into the late Quaternary age category based on interpretations from fault scarp profiling that indicate these scarps formed before the Bonneville shoreline (Wu and Bruhn, 1994). Although previously mapped as a bedrock fault (Clark and others, 2012) and not included in the Utah Geologic Hazards Portal, our new mapping indicates that it is Quaternary active.

The West Mercur fault is the most studied fault of the SOMFZ. Our lidar fault mapping mostly followed previous mapping and improved the extent of faults cutting alluvial fans, especially north of Mercur Creek, and in fans coming from McFait Canyon and Sunshine Canyon. Olig and others (1999, 2001) did extensive detailed fault mapping associated with their paleoseismic trenching project. Our lidar mapping did not indicate the same detail as they reported. For example, in several places, Olig and others (2001) map parallel fault strands of two or even three faults on all strands of the West Mercur fault. This level of detail was not always visible in the lidar mapping; however, the essential shape and occurrence of the three strands of the West Mercur fault is preserved. While this interpretive difference is important to highlight, the delineation of special-study zones along the West Mercur fault mostly covers the interpretive differences in fault trace mapping between our study and Olig and others (2001).

The West Eagle Hill fault is the range front fault on the very southern edge of the southern Oquirrh Mountains. Of all the west-dipping faults of the SOMFZ included in the Utah Geologic Hazards Portal at the time of this project, we added the most lidar-based fault mapping to the West Eagle Hill fault. We extended the fault from the range front extent of the West Eagle

Hill fault north of the mouth of Mercur Canyon, 1.25 miles (2 km) into Mississippian Limestone bedrock. Lidar observations improved the mapped extent of the West Eagle Hill fault in Quaternary fan deposits between Mitchell and Sunshine Canyons, including a ~2800-foot-long (~1 km) graben. We also mapped a moderately constrained Quaternary-age range front fault in this same area. Our mapping extended the southern end of the West Eagle Hill fault by inferring it through Fivemile Pass to connect with fault scarps on the northwestern side of Thorpe Hills.

We excluded one previously mapped fault from our mapping due to lack of evidence in high-resolution topographic elevation data. A ~1.25-mile (~2 km) approximately north-south- trending down-to-the-west fault was previously mapped on the Tooele Army Depot (Kirby, 2012). In the high-resolution topographic elevation data, this feature had a rough and inconsistent texture compared to what we see in faulted alluvial fans. Upon field review, we observed that this feature was a thin, wind-blown sand deposit, possibly related to the sub- perpendicular lake berm deposit at its southern extent. This feature was likely identified using stereo-paired aerial imagery in the past.

Central and West Rush Valley Faults

The central and western Rush Valley area contains the SMMF, SJSFZ, CFZ, SFZ, and VHFZ. These fault zones run south from the southwestern margin of South Mountain, approximately 31 miles (50 km) to the southern end of Rush Valley near the Sheeprock Mountains (figure 4). Most of these fault zones consist of short, discontinuous fault traces cutting older pre-Bonneville alluvial fans 0.6–1.25 miles (1–2 km) from the range front of the Stansbury, Onaqui, and Sheeprock Mountains. Scarp heights vary from small (3–7 feet [1–2 m]) to large (16+ feet [5+ m]).

Newly identified fault traces have expanded mapping of the SMMF to the north, nearly to the southern edge of South Mountain. Near the mouth of East Hickman Canyon, we identified a broad zone of largely discontinuous scarps, with multiple prominent horst and graben structures. Scarp heights in this area range from 10–13 feet (3–4 m) to 33+ feet (10+ m) and appear older geomorphically (late Quaternary age).

The CFZ consists of a series of east-dipping faults cutting mostly pre-Bonneville alluvial- fan materials. Although Kirby (2013c) suggested several of these faults may cut Holocene-age fans, we feel these scarps are not Holocene in age based on their geomorphic appearance. These younger Holocene fans likely reflect alluvial deposits draping older fault scarps along parts of the CFZ.

Our new mapping of the northwestern part of the SFZ has revealed a much more complex zone of young, Holocene-age faulting than previously mapped. We identified a complex zone of north- to northeast-striking and east- and west-dipping scarps. Several prominent graben structures exist in this zone. Unfortunately, we were not able to continue our mapping farther to the south due to the lack of high-resolution topographic data for the remainder of the SFZ.

Topliff Hills Fault Zone

The THFZ extends south-southwest about 15.5 miles (25 km) from the northern end of the Topliff Hills at Fivemile Pass to the southwestern end of the East Tintic Mountains. In the southern part of the THFZ, we mapped mostly well-defined, often discontinuous scarps cutting older alluvial-fan deposits. Along the northern part of the THFZ, we mapped discontinuous faults trending away from bedrock with less well-defined scarps often stepping and splaying along strike. For the length of the THFZ, we mapped mostly well-defined scarps on older alluvial-fan deposits, but the ages of faulted deposits are not well constrained. Our fault trace mapping confirms well-defined scarps are developed on alluvial deposits that are pre-Bonneville in age (Barnhard and Dodge, 1988); however, the relative ages of these Quaternary deposits are unknown. We also mapped subtle (<3 feet [<1-m] high) faults on younger, inset and hanging- wall post-Bonneville alluvium locally, but some of the scarps may reflect alluvial deposits draping older scarps.

Fault Traces

Fault traces were mapped according to standards and experience of the UGS and IGS mappers and authors of each map. Each mapper employed several different techniques to best represent fault scarps indicative of previous surface-fault rupture or deformation over time. High-resolution topographic products derived from the lidar data proved to be the most useful tool when mapping; however, it was not exclusively used. In areas of urban development, pre- development stereo-paired images were used to identify and map fault traces. These photos were particularly useful in identifying fault traces that have been obscured by development. Additional derivative lidar products such as slope-angle maps, slope-aspect maps, and topographic contours were used to discern fault scarps. Topographic contours were particularly useful when trying to discern a fault scarp from a paleo-shoreline, which are very prevalent throughout northern Utah and southern Idaho.

Fault activity classifications in the UGS Utah Geologic Hazards Portal and the USGS Quaternary Fault and Fold Database of the United States reflect the best available timing information for the most recent surface-rupturing earthquake on a specific fault trace, as well as lidar data, previous geologic mapping, and geomorphic relationships to determine these classifications. Each mapped fault trace was assigned a fault activity classification based on Lund and others (2020) and Western States Seismic Policy Council (WSSPC, 2018) guidelines. These classifications are as follows:

● Latest Pleistocene to Holocene – a fault whose movement in the past 15,000 years before present has been large enough to break the ground surface. ● Late Quaternary – a fault whose movement in the past 130,000 years before present has been large enough to break the ground surface. ● Middle Quaternary – a fault whose movement in the past 750,000 years before present has been large enough to break the ground surface. ● Quaternary – a fault whose movement in the past 2,600,000 years before present has been large enough to break the ground surface.

Special-Study Zone Delineation

We delineated surface-fault-rupture special-study zones along the Utah parts of the EBLF and WBLF, and along the OFZ, SOMFZ, THFZ, and central and western Rush Valley faults in accordance with Utah State Code 79-3-202(f) that define areas where additional investigation is warranted to evaluate the risk from surface faulting prior to residential, business, and infrastructure development. Together with the fault trace mapping, these special-study zones are critical to the creation and implementation of municipal and county geologic-hazard ordinances associated with hazardous faults and understanding surface-rupturing hazard and associated risk (Lund and others, 2020).

We categorized Quaternary faults as “well defined,” “moderately defined,” or “buried or inferred” fault traces. We considered a fault well defined if its trace is clearly detectable by a trained geologist as a physical feature on the ground surface (Bryant and Hart, 2007). Additionally, lineaments that we were unable to conclusively determine were fault-related were mapped just as “lineaments.” For well-defined faults, the special-study-area zones extend 500 feet (152 m) on the downthrown side and 250 feet (76 m) on the upthrown side of each fault. For moderately defined and buried or inferred faults, the special-study zones extend 1000 feet (305 m) on each side of the suspected trace of the fault. The special-study area dimensions are based on the Guidelines for Evaluating Surface-Fault-Rupture Hazards in Utah (Lund and others, 2020).

Several criteria were established for distinct circumstances pertaining to fault-related special-study zones. For traces of buried or inferred faults less than 1000 feet (305 m) long that lie between and on-trend with well-constrained faults, the well-constrained fault special-study- area zone was used (figure 6A). For buried or inferred faults greater than 1000 feet (305 m) long, the special study area includes 1000 feet (305 m) on both sides of the fault. For inferred faults at the end of a mapped fault trace that are longer than 1000 feet (305 m), we used an inferred fault special-study-zone area (figure 6B). Where two or more well-constrained faults are antithetic to, and within 250 feet (76 m) of each other, the buffer zone created for the primary fault supersedes zones for any secondary faults. For example, a 500-foot (152 m) downthrown side special-study area on a main fault trace may extend beyond the 250-foot (76 m) upthrown side special study area associated with an antithetic fault, and therefore be used for the special study zone. In areas where a buffer “window” exists (a space between the buffer zones of two sub-parallel fault traces), we include the window in the buffer zone if its width is less than the greater of the two surrounding buffers (figure 6C).

Figure 6. Examples of special circumstances used when creating surface-fault-rupture special-study zones.

POTENTIAL PALEOSEISMIC INVESTIGATION SITES

We analyzed each fault segment of the EBLF, WBLF, OFZ, SOMFZ, THFZ, and central and western Rush Valley faults for potential paleoseismic investigation sites as part of our fault- trace mapping. Sites were selected based on: (1) presence of a normal fault scarp, (2) scarp height that is reasonable for paleoseismic investigation (roughly 1.6–33 ft [0.5–10 m]), (3) scarp cutting young deposits (late Pleistocene to Holocene), and (4) mostly undisturbed. A list of 51 identified potential paleoseismic site locations is shown in table 1. Below are descriptions of specific site selection considerations for the EBLF, WBLF, OFZ, SOMFZ, THFZ, and central and western Rush Valley faults.

The UGS works to maintain a relationship with local geotechnical engineering firms and consultants who conduct trenching studies for clients on faults. The UGS is often invited to visit consultant trenches for a few hours to observe and document faulting. While not as useful as a full paleoseismic research investigation, these site visits still provide useful information in areas we will most likely never be able to conduct a full research-level investigation.

East Bear Lake Fault

We identified several potential paleoseismic investigation sites along the EBLF. Along the southern section adjacent to Bear Lake, we identified seven potential paleoseismic investigation sites (table 1). Several good potential trench sites exist beyond the northern side of the mouth of the North Eden Creek drainage where McCalpin (2003) excavated trenches across two scarps. North of Bear Lake on the central section of the EBLF, we identified at least five potential trench investigation sites where well-defined, relatively undisturbed, single-trace fault scarps are present on young alluvial deposits. Just south of the city of Montpelier, Idaho, a several-meter-high, mostly single-strand intrabasin scarp extends for several kilometers. We identified two potential trench sites on the northern section of the EBLF, where at least two sites have relatively undisturbed, single-trace scarps on young alluvial deposits. Further north, fault scarps become less well defined and visible scarps are limited to those on bedrock or very old alluvial deposits.

West Bear Lake Fault

We did not identify specific potential paleoseismic investigation sites for the WBLF. Potential paleoseismic sites on the WBLF are limited to the intrabasin faults on the central section northwest of Bear Lake. This part of the fault consists of an up to 1.8-mile-wide (3 km) zone of distributed, roughly parallel, north-south-trending faults that dip to both the east and west. Thus, a comprehensive paleoseismic investigation on this part of the fault would require a transect of several trenches to ensure the complete surface-faulting earthquake record is captured. The zone of faulting also includes scarps that are either likely stream channels or that have been enhanced and/or modified from stream erosion, and fault scarps that are distributed across a broad zone. Shallow groundwater and fine-grained lacustrine deposits are also logistical factors limiting depths and extent of potential trench excavations for this part of Bear Lake Valley. McCalpin (2003) excavated two east-dipping scarps on this part of the WBLF near the town of Bloomington. Both trenches encountered shallow groundwater and one exposed warped, rather than faulted, stratigraphy.

We identified no scarps cutting young geologic deposits on the southern section of the WBLF; most of the southern section is likely submerged by Bear Lake. We also identified no potential trench investigation sites on the northern section of the WBLF. Fault scarps on this section are not well defined and we found no scarps displacing young surficial deposits.

Oquirrh Fault Zone

The OFZ on the eastern side of Tooele Valley is largely undeveloped. However, rapid growth in the area is encroaching on the fault zone. We identified 14 potential trench sites (table 1) along the length of the OFZ. Several good sites exist on the northern part of the OFZ, where the scarp trends away from the range front and cuts several alluvial fans below the Provo shoreline of Lake Bonneville. Olig and others (1996) performed a paleoseismic investigation on this section of the OFZ. Farther south, scarps associated with the range front faults become less amenable to trenching with often high, footwall bedrock scarps. We identified three potential investigation sites (table 1) along this southeastern section of the OFZ.

Approximately 3 miles (5 km) west of the main range front trace of the OFZ, we identified several potential investigation sites on newly mapped fault traces (Jänecke and others, 2020; this study) on the southeastern flank of Tooele Valley. These scarps are much smaller in height (2–4 feet [0.5–1 m]) and more subtle than scarps along the main trace but they offset Lake Bonneville deposits and appear geomorphically fresh. One challenge we have observed in trenching low-height scarps is that some scarps form from warping rather than discrete offset from faulting. Nonetheless, understanding the nature of faulting on these new traces is important, as they are being rapidly developed over. In the time since we started this project, several of these scarps have already been obscured by new residential development.

Several sites exist on the southern part of the OFZ (OFZ-9 and OFZ-10, table 1). These sites should be seen as a priority for future paleoseismic investigation, due to the fact that no paleoseismic data exist for the southern part of the OFZ. The large (16+ feet [5+ m]) scarps at both these sites cut pre-Bonneville alluvial-fan deposits above the Bonneville highstand shoreline, but are relatively close to the range front, increasing the potential for encountering shallow bedrock in a trench.

Southern Oquirrh Mountains Fault Zone

We identified six potential paleoseismic trenching sites on the SOMFZ (table 1). The northernmost site is on the Soldier Canyon fault in alluvial-fan deposits just south of the mouth of Soldier Canyon. At this site, several fault splays cut at least two different pre-Holocene alluvial-fan surfaces. A detailed field analysis of the site is needed to better map surficial deposits and delineate fault traces prior to a trenching investigation. Farther south, we identified four sites on the West Eagle Hill fault. The northernmost site is ~1.25 miles (~2 km) south of the mouth of Mercur Canyon and ~1600 feet (~500 m) north of McFait Canyon. The site includes a southwest-facing 7- to 13-foot-high (2–4 m) scarp cutting a late to middle Pleistocene alluvial fan. Another potential site is at the mouth of McFait Canyon, with a similarly sized scarp. The other potential trenching site is ~1000 feet (300 m) south of Prospect Creek wash within an approximately 2800-foot-long (1 km) graben that is cutting late to middle Pleistocene alluvial- fan deposits. The west-facing scarp at this site is 10–13 feet (3–4 m) high, and the east-facing

scarp is less than 1.6 feet (0.5 m) high. These composite scarps appear to represent most-recent- event (MRE) faulting adjacent to older, multi-event scarps. One site was identified on the West Mercur fault. At this site, there appears to be a subtle, less than 1.6-foot (0.5-m) west-facing scarp on young fan deposits, which continues into a good scarp on a late to middle Pleistocene alluvial-fan surface (7–10 feet [2–3 m] high). One site was identified on a presumed antithetic fault of the SOMFZ in the valley floor that is cutting late to middle Pleistocene alluvial-fan deposits. This uphill-facing, down-to-the-east scarp is less than 7 feet (2 m) high, and locally may have some alluvial modification at its base.

Central and Western Rush Valley Faults

Most of Rush Valley is very sparsely populated, with only several very small population centers (Vernon, Clover, figure 3). The fault scarps in the region are almost entirely undisturbed, making for plentiful paleoseismic trenching sites. However, most of the scarps cut older surficial deposits, making it more difficult to date and study past earthquakes. We identified eight potential trench sites in this area (table 1). The two sites identified on the northern SFZ (table 1, SRF-1 and SRF-2) have the highest potential to yield good earthquake timing data, due to the interpreted Holocene-age scarps at these locations. However multiple trenches may be needed to capture all potential paleo-earthquakes, due to the complex nature of faulting at these locations. Our list of potential paleoseismic investigation sites is preliminary and given the relatively undisturbed nature of scarps in this region, other sites may exist.

Topliff Hills Fault Zone

The THFZ is mostly undeveloped and its fault scarps are relatively undisturbed. We identified seven potential trench sites along the length of the THFZ (table 1). We selected sites where scarps appeared relatively undisturbed with well-defined faults cutting relatively young surficial deposits. We visited several locations in the field to observe the nature of the displaced deposit and the site accessibility for heavy equipment. Scarps at several sites show a composite scarp cutting two or more ages of alluvium, making for a potentially good paleoseismic investigation. Our list of potential paleoseismic investigation sites is preliminary and given the relatively undisturbed nature of scarps along the THFZ, other sites may exist.

CONCLUSIONS

This report presents the motivation, process, and products funded by USGS External Grant Award Numbers G19AP00072 and G19AP00073 conducted by the UGS and IGS. We summarize new detailed mapping of the EBLF, WBLF, OFZ, SOMFZ, THFZ, and central and western Rush Valley faults in northern Utah and southern Idaho. This mapping was completed using high-resolution airborne lidar-derived products, historical aerial photos, previous geologic mapping, and field investigations. The motivation for this work was timely due to the availability of the high-resolution lidar data, and the increasing population growth and development in these areas.

Special-study zones were defined in Utah based on the certainty of the fault trace mapping, and fault geometry. The special-study area dimensions are based on the Guidelines for Evaluating Surface-Fault-Rupture Hazards in Utah (Lund and others, 2020). These special-study

zones are delineated in Utah to assist in land-use planning and regulation for local governments. Paleoseismic sites were identified along the EBLF, WBLF, OFZ, SOMFZ, THFZ, and central and western Rush Valley faults in northern Utah and southern Idaho to foster future paleoseismic research in areas that are being rapidly developed or lacking good earthquake timing and recurrence information. We identified 51 potential sites with varying geologic conditions deemed potentially suitable for paleoseismic investigation (table 1). The 51 identified potential paleoseismic sites should not be considered a complete list of all sites on the mapped faults, as additional sites likely exist. We focused on identifying sites where the fault scarps are sparse given the nature of the fault and in areas where development and ongoing disturbance have obscured fault scarps. This dataset is designed to assist the UGS, IGS, and other potential investigators in determining future sites for paleoseismic study.

The results of this work will be implemented in the form of a peer-reviewed UGS Report of Investigation (ROI) publication, a digital database publication from IGS, and final publication of fault mapping in the UGS Utah Geologic Hazards Portal (https://geology.utah.gov/apps/hazards/) and the IGS fault database. Once the final publications are complete, the UGS and IGS will contact local governments to present them with the fault mapping and help in developing local ordinances based on special-study zones. These maps will serve as a critical tool to helping communities assess their earthquake risk and become more resilient to earthquake effects and geologic hazards.

REFERENCES

Barnhard, T.P., 1988, Fault-scarp studies of the Oquirrh Mountains, Utah, 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. 52–54.

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, 1 sheet, scale 1:250,000.

Bryant, W.A., and Hart, E.W., 2007, Fault-rupture hazard zones in California—Alquist-Priolo earthquake fault zoning act with index to earthquake fault zones maps: California Geological Survey Special Publication 42, 38 p., Online://ftp.consrv.ca.gov/.

Burr, T.N., and Currey, D.R., 1988, The Stockton bar, 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. 66– 73.

Burr, T.N., and Currey, D.R., 1992, Hydrographic modeling at the Stockton Bar, in Wilson, J.R., editor, Field guide to geologic excursions in Utah and adjacent areas of Nevada, Idaho, and Wyoming, for Geological Society of America Rocky Mountain Section Meeting, Ogden, Utah: Utah Geological and Mineral Survey Miscellaneous Publication 92-3, p. 207–219.

Clark, D.M., Kirby, S.M., and Oviatt, C.G., 2012, Interim geologic map of the Rush Valley 30'x60' quadrangle in Tooele, Utah, and Salt Lake Counties, Utah: Utah Geological Survey Open-File Report 593, 65 p., 2 plates, scale 1:62,500.

Clark, D.M., Oviatt, C.G., and Dinter, D.A., 2017, Interim geologic map of the Tooele Valley 30' x 60' quadrangle in Tooele, Salt Lake and Davis Counties, Utah: Utah Geological Survey Open-File Report 669, 43 p., 2 plates, scale 1:62,500.

Colman, S.M., 2006, Acoustic stratigraphy of Bear Lake, Utah-Idaho—Late Quaternary sedimentation patterns in a simple half-graben: Sedimentary Geology, v. 185, p. 113– 125.

Coogan, J.C., 1997, Geologic map of the Bear Lake South quadrangle, Rich County, Utah: Utah Geological Survey Miscellaneous Publication 97-1, 4 plates, scale 1:24,000.

Currey, D.R., and Oviatt, C.G., 1985, Durations, average rates, and probable causes of Lake Bonneville expansion, still-stands, and contractions during the last deep-lake cycle, 32,00 to 10,000 yrs ago, in Kay P.A., and Diaz, H.F., editors, Problems of and prospects for predicting Great Salt LakelLevels: Proceedings of a NOAA Conference held March 26–

28, 1985, Salt Lake City, Center for Public Affairs and Administration, University of Utah, p. 9–24.

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, Annual Summary, v. 42, USGS External Grant award no. 98HQGR1013, 6 p.

Disbrow, A.E., 1961, Geology of the Boulter Peak quadrangle, Utah: U.S. Geological Survey map GQ-141, 1 plate, scale 1:24,000.

Dover, J.H., 2007, Geologic map of the Logan 30' x 60' quadrangle, Cache and Rich Counties, Utah, and Linoln and Uinta Counties, Wyoming: Utah Geological Survey Miscellaneous Publication MP 06-8 DM, digitized from U.S. Geological Survey Miscellaneous Investigation Series Map I-2210 (1995), 3 plates, scale 1:100,000.

Evans, J.P., Martindale, D.C., and Kendrick, R.D., 2003, Geologic setting of the 1884 Bear Lake, Idaho earthquake—Rupture in the hanging wall of a Basin and Range normal fault revealed by historical and geological analyses: Bulletin of the Seismological Society of America, v. 93, no. 4, p. 1621–1632.

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.

Fiesinger, D.W., Perkins, W.D., and Puchy, B.J., 1982, Mineralogy and petrology of Tertiary- Quaternary volcanic rocks in Caribou County, Idaho, in Bonnichsen, B., and Breckenridge, R.M., editors, Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 465–488.

Ford, M.T., 2005, The petrogenesis of Quaternary rhyolite domes in the bimodal Blackfoot Volcanic Field: Pocatello, Idaho State University, M.S. thesis 133 p.

Gilbert, G.K., 1890, Lake Bonneville: U.S. Geological Survey Monograph I, 438 p.

Gilluly, J., 1932, Geology and ore deposits of the Stockton and Fairfield quadrangles, Utah: U.S. Geological Survey Professional Paper 173, 171 p.

Handwerger, D.A., CerIing, T.E., and Bruhn, R.L., 1999, Cosmogenic 14C in carbonate rocks: Geomorphology, v. 27, p. 13–24.

Heidemann, H.K., 2018, Lidar base specification (ver. 1.3, February 2018): U.S. Geological Survey Techniques and Methods, book 11, chap. B4, 101 p., Online, http://dx.doi.org/10.3133/tm11B4.

Helm, J.M., 1994, Structure and tectonic geomorphology of the Stansbury fault zone, Tooele County, Utah, and the effect of crustal structure on Cenozoic faulting patterns: Salt Lake City, University of Utah, M.S. thesis, 128 p.

Heumann, A., 2004, Timescales of evolved magma generation at Blackfoot Lava Field , Southeast Idaho, USA: Abstract for International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) General Assembly, Volcanism and its impacts on society, Pucon, Chile, Online, http://www.iavcei.org/documents/pucon04/pucon04abs.html.

Idaho Geological Survey, 2021, Idaho Miocene-Quaternary fault map: Online, https://www.idahogeology.org/sites/default/files/2018-files/IGS_FAULTS_v11-15- 06.kmz

Jänecke S.U., and Evans, J.P., 2017, Revised structure and correlation of the East Great Salt Lake, North Promontory, Rozel and Hansel Valley fault zones revealed by the 2015-2016 lowstand of Great Salt Lake, in Lund, W.R., Emerman, S.H.; Wang, W., Zanazzi, A., editors, Geology and resources of the Wasatch — back to front: Utah Geological Association Publication 46, p. 295–360.

Jänecke, S.U., Ellis, N.M., Oaks Jr., R.Q., Lee, C.M., and Evans, J.P., 2020, LIDAR reveals >150 km of new Holocene ruptures and faults in NE Basin-and-Range province, N Utah and SE Idaho: Geological Society of America Abstracts with Programs, v. 52, no. 6, https://gsa.confex.com/gsa/2020AM/meetingapp.cgi/Paper/358128.

Kaliser, B.N., 1972, Environmental geology of the Bear Lake area, Utah and Idaho: Utah Geological and Mineral Survey Bulletin 28, 56 p.

Kirby, S.M., 2010a, Geologic map of the Lofgreen quadrangle, Tooele County, Utah: Utah Geological Survey Open-File Report 563, 2 plates, 22 p., scale 1:24,000.

Kirby, S.M., 2010b, Geologic map of the Vernon quadrangle, Tooele County, Utah: Utah Geological Survey Open-File Report 564, 2 plates, 22 p., scale 1:24,000.

Kirby, S.M., 2012, Geologic map of the Ophir quadrangle, Tooele County, Utah: Utah Geological Survey Map 257M, 2 plates, 16 p., scale 1:24,000.

Kirby, S.M., 2013a, Geologic map of the Saint John quadrangle, Tooele County, Utah: Utah Geological Survey Map 264DM, 2 plates, 12 p., scale 1:24,000.

Kirby, S.M., 2013b, Geologic map of the Faust quadrangle, Tooele County, Utah: Utah Geological Survey Map 265DM, 2 plates, 12 p., scale 1:24,000.

Kirby, S.M., 2013c, Geologic map of the Vernon NE quadrangle, Tooele County, Utah: Utah Geological Survey Map 266DM, 2 plates, 12 p., scale 1:24,000.

Laabs, B.J.C., and Kaufman, D.S., 2003, Quaternary highstands in Bear Lake Valley, Utah and Idaho: Geological Society of America Bulletin, v. 115, no. 3, 16 p.

Laabs, B.J.C., Munroe, J.S., Rosenbaum, J.G., Refsnider, K.A., Mickelson, D.M., Singer, B.S., and Caffee, M.W., 2007, Chronology of the Last Glacial Maximum in the upper Bear River basin, Utah: Arctic, Antarctic, and Alpine Research, v. 39, p. 537–548.

Lewis, R.S., Link, P.K., Stanford, L.R., and Long, S.P., 2012, Geologic map of Idaho: Idaho Geological Survey Map 9 digital data, scale 1:750,000.

Lund, W.R., Christenson, G.E., Batatian, L.D., and Nelson, C.V., 2020, Guidelines for evaluating surface-fault-rupture hazards in Utah, in Bowman, S.D., and Lund, W.R., editors, Guidelines for investigating geologic hazards and preparing engineering-geology reports, with a suggested approach to geologic-hazard ordinances in Utah: Utah Geological Survey Circular 122, 24 p.

Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, Paleoseismology of the Wasatch fault zone-A summary of recent investigations, conclusions, and interpretations, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazard and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500A, p. A1–A72.

Mansfield, G.R., and Girty, G.H., 1927, Geography, geology, and mineral resources of part of southeastern Idaho with descriptions of Carboniferous and Triassic fossils: U.S. Geological Survey, Professional Paper 152, scale 1:62,500.

McCalpin, J.P., 2003, Paleoseismology of Utah, volume 12—Neotectonics of Bear Lake Valley, Utah and Idaho—A preliminary assessment: Utah Geological Survey Miscellaneous Publications 03-4, 50 p.

McClurg, L.W., 1970, Source rocks and sediments in the drainage area of North Eden Creek, Bear Lake Plateau, Utah and Idaho: Utah State University Department of Geology, M.S. Thesis, scale 1:24,000.

McCurry, M., Pearson, D.M., Welhan, J., Nawotniak, S.K., and Fisher, M., 2015, Origin and potential geothermal significance of China Hat and other late Pleistocene topaz rhyolite lava domes of the Blackfoot Volcanic Field, SE Idaho: GRC Transactions, v. 39.

McDonald, G.N., Kleber, E.J., Hiscock, A.I., Bennett, S.E.K., and Bowman, S.D., 2020, Fault trace mapping and surface-fault-rupture special study zone delineation of the Wasatch fault zone, Utah and Idaho: Utah Geological Survey Report of Investigation 280, 23 p., https://doi.org/10.34191/RI-280.

Moore, W.J., and Sorensen, M.L., 1979, Geologic map of the Tooele 1° x 2° quadrangle, Utah: U.S. Geological Survey Miscellaneous Investigations Map I-1132, scale 1:250,000.

Olig, S.S., Gorton, A.E., and Chadwell, L., 1999, Paleoseismic investigation of the Mercur fault and its implications to seismic hazard along the Wasatch Front urban corridor: Technical report to U.S. Geological Survey, Reston, Virginia, under Contract 1434-HQ-97-GR- 03154, June 1999.

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: Technical report to U.S. Geological Survey, Reston, Virginia, under Contract 98HQGR1036, January 25, 2001, variously paginated.

Olig, S.S., Lund, W.R., Black, B.D., and Mayes, B.H., 1996, Paleoseismology of Utah, volume 6—Paleoseismic investigation of the Oquirrh fault zone, Tooele County, Utah, in Lund, W.R., editor, The Oquirrh fault zone, Tooele County, Utah—Surficial geology and paleoseismicity: Utah Geological Survey Special Study 88, p. 18–64, 1 plate, scale 1:24,000.

Pampeyan, E.H., 1989, Geologic map of the Lynndyl 30' x 60' quadrangle, west-central Utah: U.S. Geological Survey Map I-1830, 12 p., 2 plates, scale 1:100,000.

Pickett, K.E., 2004, Physical volcanology, petrography, and geochemistry of basalts in the bimodal Blackfoot Volcanic Field, southeastern Idaho: Pocatello, Idaho State University, M.S. thesis, 92 p.

Polun, S.G., 2011, Kinematic analysis of late Pleistocene faulting in the Blackfoot Lava Field, Caribou County, Idaho: Pocatello, Idaho State University, M.S. thesis, 86 p.

Reheis, M.C., Laabs, B.J.C., and Kaufman, D.S., 2009, Geology and geomorphology of the Bear Lake Valley and upper Bear River, Utah and Idaho: Geological Society of America Special Paper 450, p. 15–48, doi: 10.1130/2009.2450(02).

Robertson, G.C., III, 1978, Surficial deposits and geologic history, northern Bear Lake Valley, Idaho: Logan, Utah State University, M.S. thesis, 162 p.

Scott, W.E., McCoy, W.D., Shroba, RR., and Rubin, M., 1983, Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, Western United States: Quaternary Research, v. 20, p. 261–285.

Solomon, B.J., 1993, Quaternary geologic maps of Tooele Valley and the West Desert hazardous industry area, Tooele County, Utah: Utah Geological Survey Open-File Report 296, 19 plates, 48 p., scale 1:24,000.

Solomon, B.J., 1996, Surficial geology of the Oquirrh fault zone, Tooele County, Utah, in Lund, W.R., editor, The Oquirrh fault zone, Tooele County, Utah—Surficial geology and paleoseismicity: Utah Geological Survey Special Study 88, p. 1–17, 1 plate, scale 1:24,000.

Stokes, W.L., 1977, Physiographic subdivisions of Utah: Utah Geological and Mineral Survey Map 43, scale 1:2,400,000.

Stokes, W.L., 1986, Geology of Utah: Salt Lake City, University of Utah Museum of Natural History and Utah Geological and Mineral Survey, 280 p.

Utah Automated Geographic Reference Center, 2010, Populated block areas— approximation— Created using U.S. Census Bureau 2010 Census Data: Online, Utah Automated Geographic Reference Center, State Geographic Information Database, https://gis.utah.gov/data/demographic/census/#2010Census, accessed September February 24, 2021

Utah Automated Geographic Reference Center, 2018, Northern Utah lidar elevation data: Online, Utah Automated Geographic Reference Center, State Geographic Information Database, https://gis.utah.gov/data/elevation-and-terrain/2018-lidar-northern-utah/.

Utah Automated Geographic Reference Center, 2016, Bear Lake, Bear River, Cache Valley, and Upper Weber Valley lidar elevation data: Online, Utah Automated Geographic Reference Center, State Geographic Information Database, https://gis.utah.gov/data/elevation-and- terrain/2016-lidar-blbrcvuwv/.

Utah Geological Survey, 2021, Utah Geologic Hazards Portal: Online, https://geology.utah.gov/apps/hazards.

Valenti, 1982, Preliminary Geologic map of the Laketown quadrangle, Rich County, Utah: Utah Geological and Mineral Survey Map 58, 2 plates, scale 1:24,000.

Ward, S., Richards, R., Whitney, B., Toke, N.A., and Bunds, M.P., 2019, Preliminary results from the Topliff Hill paleoseismic site—Evidence for 5-7 Basin and Range normal faulting events in Tooele County, Utah, since the late Pleistocene: American Geophysical Union Fall Meeting Abstracts, v. 2019, p. T13H-0314.

Welhan, J.A., Breckenridge, R.M., Garwood, D.L., Kauffman, J.D., Lewis, R.S., and Gillerman, V.S., 2011, The Idaho Geological Survey’s activities in the National Geothermal Data Program, data compilation and heat-flow drilling: GRC Transactions, v. 35, pages?.

Western States Seismic Policy Council, 2018, Policy Recommendation 18-3—Definition of fault activity for the Basin and Range Province: Western States Seismic Policy Council, 4 p.

Willard, A.D., 1959, Surficial geology of the Bear Lake Valley, Utah: Logan, Utah State University M.S. thesis, 68 p.

Williams, J.S., Willard, A.D., and Parker, V., 1962, Recent geologic history of Bear Lake Valley, Utah and Idaho: American Journal of Science, v. 260, p. 24–36.

Wong, I., Lund, W., DuRoss, C., Thomas, P., Arabasz, W., Crone, A., Hylland, M., Luco, N., Olig, S., Pechmann, J., Personius, S., Peterson, M., Schwartz, D., Smith, R., and Bowman, S., 2016, Earthquake probabilities for the Wasatch Front region in Utah, Idaho, and Wyoming: Utah Geological Survey Miscellaneous Publication 16-3, variously paginated.

Wong, I., Olig, S., Green, R., Moriwaki, Y., Abrahamson, N., Baures, D., Silva, W., Somerville P., Davidson, D., Pilz, J., and Dunne, B., 1995, Seismic hazard evaluation of the Magna tailings impoundment: Utah Geological Association Publication 24, 1995 Symposium and Field Conference on Environmental and Engineering Geology of the Wasatch Front Region, p. 95–110.

Wu, D., and Bruhn, R.L., 1994, Geometry and kinematics of active normal faults, South Oquirrh Mountains, Utah: Journal of Structural Geology, v. 16-8, p. 1061–1075, doi: 10.1016/0191-8141(94)90052-3.

Youngs, R.R., Swan, F.H., Power, M.S., Schwartz, D.P., and Green, R.K., 1987, Probabilistic analysis of earthquake ground shaking hazard along the Wasatch front, 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 Open-File Report 87-58S, v. II, p. Ml– MIlO.MI10 or M10?

Table 1. Potential paleoseismic sites along the East and West Bear Lake fault zones, the Oquirrh fault zone, Southern Oquirrh Mountains fault zone, Topliff Hills fault zone, South Mountain Marginal fault, Clover fault zone, Sheeprock fault zone, and Gem Valley faults. The table identifies 51 sites and includes the potential site location as well as a cursory comment regarding good or poor qualities of the site for paleoseismic investigation.

Site UTM Zone 12N Number Fault Zone Comments Easting Northing East Bear Lake Scarp of older fan surface; multiple EQ EBLF-1 475549 4636027 fault zone events East Bear Lake Possible single event scarp; could EBLF-2 475912 4637285 fault zone potentially clean up culvert cut Decent multi-event scarp; would need 2 East Bear Lake EBLF-3 trenches to north; fan to west contains 476513 4638517 fault zone highly disturbed scarps East Bear Lake Decent scarp; relatively undisturbed; may EBLF-4 476582 4639757 fault zone have shallow BR in FW Decent scarp on inset terrace; scarp of East Bear Lake EBLF-5 fan-delta to west composite scarps to 476944 4641522 fault zone north East Bear Lake EBLF-6 Good scarps; main and antithetic 477640 4646186 fault zone Good scarp north of drainage; potential East Bear Lake scarps to west. Offset deposit; upper and EBLF-7 478683 4660787 fault zone lower surfaces appear to be correlative. Good access from paved road. East Bear Lake Moderately disturbed scarp; check north EBLF-8 475460 4681201 fault zone and south East Bear Lake EBLF-9 Check; scarp may be too disturbed 476040 4685719 fault zone East Bear Lake Composite scarp; inset alluvium faulted EBLF-10 476790 4694542 fault zone likely by MRE East Bear Lake Composite scarp; inset alluvium faulted EBLF-11 475304 4696866 fault zone likely by MRE Good scarp; single trace, appears East Bear Lake EBLF-12 relatively undisturbed; apparent down-to- 474353 4699350 fault zone east antithetic 1/2 mi down fan Good scarp on young alluvium. Displaced East Bear Lake ephemeral fluvial deposit. The age of the EBLF-13 467066 4711811 fault zone deposit is not known, but it appears to be young Oquirrh fault Scarp on older fan, potentially additional OFZ-1 394730 4502189 zone sites to N and S of here Good scarp, could be antithetic fault to Oquirrh fault OFZ-2 the NW, and double fault to E. Need a 394742 4501414 zone long trench Oquirrh fault Good scarp, near existing UGS/USGS OFZ-3 394184 4500838 zone trench sites

Site UTM Zone 12N Number Fault Zone Comments Easting Northing Oquirrh fault Another good scarp, but near a couple OFZ-4 393707 4500220 zone bends in fault Oquirrh fault Good NW facing young scarp. Fault splits OFZ-5 393465 4499604 zone just to W, but is one continuous fault here Oquirrh fault Good SW facing large scarp cutting up OFZ-6 394028 4494841 zone Bonneville shorelines Large scarp on fan, may have nearby Oquirrh fault OFZ-7 shallow bedrock, but probably the 398055 4488636 zone best/only trench site in area Oquirrh fault Large scarp on fan, strange geometry in OFZ-8 394864 4487607 zone fault here, but could be trenchable Oquirrh fault Scarp on older fan, may be shallow OFZ-9 388371 4482523 zone bedrock nearby Oquirrh fault Same as OQFZ-9, but less chance of OFZ-10 388185 4482174 zone shallow bedrock Oquirrh fault Site between Bonneville beach bars, TVF-1 388137 4491199 zone subtle fault Oquirrh fault Site along crest of Bonneville beach bar, TVF-2 388446 4491640 zone subtle fault Oquirrh fault Potential site on western strand of Tooele TVF-3 390002 4493152 zone Valley Faults Oquirrh fault Another potential site on eastern Tooele TVF-4 389402 4494717 zone Valley Fault Southern Oquirrh SOMFZ-1 Mountains fault Scarp(s) on older fan alluvium 388760 4474716 zone Southern Oquirrh SOMFZ-2 Mountains fault DTE scarp 384684 4468793 zone Southern Oquirrh Good scarp of older fan deposit; no SOMFZ-3 Mountains fault 394652 4460580 apparent scarp on Qaf1 deposit zone Southern Oquirrh SOMFZ-4 Mountains fault Similar to SOMFZ-3 394883 4460252 zone Southern Oquirrh Graben on older alluvial-fan deps; SOMFZ-5 Mountains fault 396398 4457818 apparently MRE scarps adj to older scarps zone Southern Oquirrh May be subtle scarp on Qafy; good scarp SOMFZ-6 Mountains fault 393736 4457166 of Qafo surface zone Topliff Hills THFZ-1 Single trace northern end of FZ 396902 4445462 fault zone Topliff Hills THFZ-2 Relatively undisturbed scarps; antithetic 397190 4441796 fault zone Topliff Hills THFZ-3 Good single scarp; good access 396843 4438955 fault zone

Site UTM Zone 12N Number Fault Zone Comments Easting Northing Topliff Hills Good main scarp with possible antithetic; THFZ-4 395460 4433586 fault zone near road Topliff Hills Scarp with likely correlative surfaces; THFZ-5 395216 4432342 fault zone may be antithetic Topliff Hills Good scarps here and to north; likely THFZ-6 394484 4429939 fault zone coeval undisturbed FW and HW Topliff Hills Good scarp older FW surface; southern THFZ-7 393850 4427731 fault zone end of THFZ South Mountain Scarp on old fan, may avoid broad zone SMMF-1 374177 4472942 marginal fault of faulting with this site South Mountain Scarp on old fan, no distributed zone SMMF-2 374309 4472242 marginal fault evident to E or W Huge older scarp, could trench here and South Mountain SMMF-3 SMMF-4 to potential capture graben 372972 4477656 marginal fault bounding fault South Mountain Antithetic fault, combine with SMMF-3 SMMF-4 373749 4477230 marginal fault to capture full faulting history Older scarp, just S of eastward step, may CFZ-1 Clover fault zone 372944 4466727 miss some faulting from W scarp CFZ-2 Clover fault zone Nice scarp, but on older fans 374225 4461609 Good site, but older fault scarp and older CFZ-3 Clover fault zone 374512 4461014 fan CFZ-4 Clover fault zone Good E facing scarp 374524 4460340 Potential site on Sheeprock fault, one of Sheeprock fault SRF-1 the better sites to avoid multiple fault 371670 4437534 zone strands Sheeprock fault Good young-looking scarp, with antithetic SRF-2 370623 4436107 zone fault to E, may need long trench Truncated alluvial fan that appears to be displaced by fault. We did not field check GVF-1 Gem Valley fault 432449 4739426 this site, so the characteristics of the deposit are not known