Concealed Quaternary strike-slip fault resolved with airborne lidar and seismic reflection: The Grizzly Valley fault system, northern Walker Lane, California

Authors


Corresponding author: R. D. Gold, Geologic Hazards Science Center, U.S. Geological Survey, MS 966, PO Box 25046, Denver, CO 80225, USA. (rgold@usgs.gov)

Abstract

[1] The Grizzly Valley fault system (GVFS) strikes northwestward across Sierra Valley, a low-relief basin situated within a network of active dextral slip faults in the northern Walker Lane, California. Quaternary motion along the Grizzly Valley fault system has not been previously documented. We used high-resolution (0.25 m) airborne lidar data in combination with high-resolution, P wave, seismic reflection imaging to evaluate Quaternary deformation associated with the GVFS. We identified suspected tectonic lineaments using the lidar data and collected seismic reflection data along six profiles across the lineaments. The seismic reflection images reveal a deformed basal marker that we interpret to be the top of Tertiary volcanic rocks overlain by a 120–450 m thick suite of subhorizontal reflectors that we interpret to be Plio-Pleistocene lacustrine deposits. Three profiles image features that we interpret to be the principal active trace of the GVFS, which is a steeply dipping fault zone that vertically offsets the volcanic rocks and the lacustrine basin fill. These data suggest that the GVFS may have been active in latest Quaternary time because (1) the lidar data show subtle surficial geomorphic features that are typical of youthful faulting, including a topographic lineament marked by an ~1 m high ridge composed of discontinuous, left-stepping lobes, and (2) the seismic profiles demonstrate shallow faulting of the lacustrine strata that coincides with the left-stepping ridge. This investigation illustrates the potential for unidentified, low-rate, strike-slip faults in basins and emphasizes the value of high-resolution topographic data and subsurface imaging as a means of identifying these structures.

1 Introduction

[2] In many settings, it may be difficult to recognize Quaternary strike-slip faults with low (<1 mm/yr) slip rates because their traces may not coincide with classic strike-slip geomorphic features such as scarps, shutter ridges, linear valleys, troughs, sidehill benches, and offset and beheaded streams. This is especially true for faults that cut poorly consolidated basin sediments in low-relief regions. In these settings, active faults can remain unidentified, despite their potential for producing damaging earthquakes. For example, the 2010 Mw 7.1 Darfield earthquake in New Zealand occurred on a previously unknown fault and produced deformation over a 30–300 m wide zone in a post–Last Glacial Alluvial Outwash surface [Barrell et al., 2011; Elliott et al., 2012; Quigley et al., 2011]. Similarly, surface slip along the previously unmapped Indiviso fault during the 2010 Mw 7.2 El Mayor-Cucapah earthquake was distributed in unconsolidated, water-saturated deltaic sediments and resulted in a diffuse, kilometer-scale warping of the surface [e.g., Oskin et al., 2012]. Recognition of faults in these challenging settings is critical for seismic hazard analysis and mitigation.

[3] In this investigation, we evaluate contemporary deformation and seismic hazards on the Grizzly Valley fault system (GVFS) in Sierra Valley, a shallow, ancient lacustrine basin in the Sierra Nevada of northeastern California (Figure 1). The GVFS is situated east of the Sierra Nevada and west of the Great Basin in the northern Walker Lane (NWL) and is one of a suite of regional northwest trending faults that include the Pyramid Lake, Honey Lake, Warm Springs Valley, and Mohawk Valley fault systems. Geodetic transects indicate that the NWL accommodates 7 mm/yr of dextral shear at this latitude [Dixon et al., 2000; Hammond and Thatcher, 2007; Hammond et al., 2011; Thatcher, 2003], which represents ~15% of the relative plate motion between the Pacific and North American plates [DeMets and Dixon, 1999]. The geodetic strain budget is significantly more than the 1.4–3.3 mm/yr cumulative geologic slip rate measured across the NWL fault systems [Gold et al., 2013; Sawyer et al., 2005; Turner et al., 2008; Wills and Borchardt, 1993]. Previous mapping has documented faulting of Tertiary volcanic rocks and Cretaceous-Jurassic granite on the GVFS but no clear evidence of faulted Quaternary deposits [Grose, 2000b, 2000c, 2000d, 2000e]. If we can document Quaternary movement on the GVFS, then it may help explain some of the discrepancy in deformation rates between the geologic and geodetic data. Furthermore, confirmation of Quaternary movement on the GVFS may allow it to be characterized as a seismic source in the National Seismic Hazard maps [Petersen et al., 2008]. Demonstrating the potential for the GVFS to generate strong ground shaking would have significant implications for seismic hazard assessments in Reno, Nevada, and Truckee, California, and for dams and other critical infrastructure in the region.

Figure 1.

(a) Study area within the tectonic framework of western North America. Faults (black lines) simplified from the U.S. Geological Survey Quaternary fault and fold database [2006]. (b) Northern Walker Lane study area and regional strike-slip and normal faults (bold and thin black lines, respectively), which are based on the U.S. Geological Survey Quaternary fault and fold database [2006], the California Department of Water Resources [1963], Saucedo and Wagner [1992], Hunter et al. [2011], and our mapping using lidar data and field observations. Sierra Valley is the brown polygon. Abbreviations: MVFS, Mohawk Valley fault system; GVFS, Grizzly Valley fault system; HSFS, Hot Springs fault system; HLFS, Honey Lake fault system; WSVF, Warm Springs Valley fault; PLF, Pyramid Lake fault. Arrows indicate relative direction of lateral fault movement.

[4] To overcome challenges associated with evaluating Quaternary movement across this low slip rate structure within a low-relief basin, we conducted geomorphic mapping using high-resolution airborne Light Detection and Ranging (lidar) data, in combination with high-resolution seismic reflection imaging. As we explain below, results from this investigation suggest that the GVFS is an active Quaternary structure with probable dextral motion.

2 Geologic Framework

[5] The GVFS is composed of multiple fault strands that extend northward ~60 km from Sardine Valley, across Sierra Valley, and along the eastern margin of Lake Davis (Figure 1b) [Grose, 2000a, 2000b, 2000c, 2000d, 2000e; Saucedo and Wagner, 1992]. This system is named for Grizzly Valley Creek, which was dammed to form Lake Davis. To the north and south of Sierra Valley, oblique dextral and normal displacements on the fault system offset Jurassic and Cretaceous granite and Tertiary volcanic rocks [Grose, 2000b, 2000c, 2000d, 2000e]. Where the fault system crosses Sierra Valley, inferred fault traces were mapped on the basis of broad drainage patterns and tonal vegetation lineaments [California Department of Water Resources, 1963]. In Sierra Valley, the deformation zone is ~15 km wide. Western fault traces are mapped as the Hot Springs fault system (HSFS) and coincide with Marble Hot Springs, whereas the eastern traces are mapped as the Grizzly Valley fault system (GVFS) (Figure 2). Here we focus on only the eastern traces.

Figure 2.

(a) Detailed map of northeastern Sierra Valley, California, showing the area of lidar coverage (white polygons), seismic reflection profiles (yellow lines), faults and lineaments mapped through this study (red and black lines, respectively), and selected roads (gray lines). Fault traces from previous studies indicated by thin dashed-dotted black lines [California Department of Water Resources, 1963; Saucedo and Wagner, 1992]. Abbreviations follow Figure 1. (b) Active stream areas, tonal lineaments, and regions of low and high topography. This drainage pattern is interpreted to have resulted from Quaternary faulting. Base map is National Agriculture Imagery Program (NAIP) imagery (1 m pixel size, acquired 2010). (c) Mapped stream channels overlain on digital elevation model (DEM) color ramp (derived from National Elevation Dataset [NED], 1/3 arc second). Little Last Chance Creek is diverted to the northwest where it flows into the escarpment coincident with the eastern/primary trace of the GVFS. Streams mapped from lidar data, aerial photographs, and digital raster graphics.

[6] Where the HSFS and GVFS cross Sierra Valley, the basin surface ranges in elevation from 1485 to 1490 m above sea level (asl). Sierra Valley was formerly occupied in the Pleistocene by pluvial Lake Beckwourth that filled the valley to a maximum elevation of 1585 m asl [Ramelli et al., 1999]. Preliminary studies suggest that Lake Beckwourth may have formed in response to downstream damming of the Middle Fork of the Feather River by glaciers and landslides [Ramelli et al., 1999; Redwine and Adams, 2010]. The Middle Fork of the Feather River presently drains the valley.

[7] Sierra Valley contains three principal rock types: basement composed of Cretaceous granodiorite associated with the Sierran batholith (in Saucedo and Wagner [1992] from Evernden and Kistler [1970]), overlying Oligocene and Miocene andesite, tuff, and basalt [Grose, 2000c, 2000e; Saucedo and Wagner, 1992, and references therein], and Plio-Pleistocene lacustrine and other Quaternary deposits (Figure 3). In Sierra Valley, the volcanic rocks are 300–500 m thick [Saucedo and Wagner, 1992]. The Plio-Pleistocene lacustrine deposits consist of thinly bedded gravel, sand, silt, and clay that have a maximum thickness of ~500 m [California Department of Water Resources, 1963; Saucedo and Wagner, 1992]. We do not know of any ages directly obtained from the shallow lacustrine section; however, both a moderately well preserved shoreline at 1550 m asl and a weakly preserved shoreline at 1585 m asl on the margins of Sierra Valley suggest that a lake may have filled the valley in middle to late Pleistocene time, perhaps as recently at 150 ka [Ramelli et al., 1999].

Figure 3.

Schematic diagram showing generalized stratigraphy for Sierra Valley, CA, based on previous geologic studies [California Department of Water Resources, 1963; Grose, 2000b, 2000c, 2000d, 2000e; Saucedo and Wagner, 1992] and selected drill holes in the valley.

[8] The GVFS was first mapped as a possible active Quaternary structure in a report by the California Department of Water Resources [1963], presumably based on the presence of regional hot springs, tonal vegetation lineaments, and drainage patterns. In this report, fault strands are depicted as steeply dipping normal faults that are concealed by the uppermost part of the lacustrine section (Figure 2a). This mapping is consistent with results from a local gravity survey in Sierra Valley that show a northwest trending pattern of gravity highs, coincident with the mapped eastern traces of the GVFS [Jackson et al., 1961]. Reed [1975] also shows Quaternary faults in Sierra Valley, noting NW trending tonal vegetation lineaments and disrupted drainage patterns. Saucedo and Wagner [1992] updated the regional mapping and simplified the traces of the GVFS (Figure 2a). More recent mapping by Grose [2000c, 2000d, 2000e] shows faulting of the granitic basement and volcanic rocks along the GVFS, including 410–850 m of apparent dextral offset in the 16 Ma Lovejoy basalts and ~350 m in a 23.4–26.8 Ma silicic tuff. However, Grose explicitly notes a lack of evidence for active faulting on these structures. The Fault Activity Map of California [Jennings and Bryant, 2010] does not show Quaternary displacement on the GVFS. Similarly, it is not included in the U.S. Geological Survey's Quaternary Fault and Fold Database (http://earthquake.usgs.gov/hazards/qfaults/), nor as a seismic source in the 2008 National Seismic Hazard Map [Petersen et al., 2008].

[9] In summary, previous mapping demonstrates that the GVFS and HSFS deform Tertiary units but the question of Quaternary movement on the GVFS remains unresolved. To address the issue of Quaternary movement on the fault system, we focus our study in Sierra Valley because of the wide distribution of Quaternary deposits.

3 Methods

3.1 Remotely Sensed Imagery and Lidar

[10] We analyzed aerial photographs and high-resolution (less than 1 m pixel) satellite imagery in Sierra Valley to identify lineaments that might have a tectonic origin (Figure 2). Following this analysis, we acquired 31 km2 of high-resolution (15–17 pulses per m2) airborne light distance and ranging (lidar) data along two zones of deformation associated with the GVFS (Figures 4-6). We generated a 0.25 m resolution bare-earth digital elevation model (DEM) from these data. The lidar data are publically available from OpenTopography, which includes a detailed project report with details on data acquisition, processing, and quality assessment (www.opentopography.org). We also provide uninterpreted versions of the lidar imagery, both as digital elevation model color ramp and hillshade images, in the supporting information.

Figure 4.

(a) DEM derived from airborne lidar along the eastern part of the Grizzly Valley fault system. (b) Detailed topographic map showing the left-stepping tectonic ridge, which is interpreted to be a fault-related feature. Projection of faulting observed in subsurface from seismic reflection profiles indicated. Faults, lineaments, and seismic reflection survey lines are indicated by red, black, and yellow lines, respectively. Dashed lines indicate approximate location. Lidar data are publically available from OpenTopography (www.opentopography.org). Uninterpreted versions of the lidar images are provided in the supporting information.

Figure 5.

Topographic evidence supporting the occurrence of Quaternary faulting on the Grizzly Valley fault system (GVFS). (a) Oblique view of the lidar data looking southeast along a tectonic ridge that is interpreted to be the geomorphic expression of Quaternary movement on the GVFS. The location of topographic profiles shown in Figure 2b are labeled A–A'. Yellow line is the location of reflection profile 2a. (b) Topographic profiles spaced 2 m across the tectonic ridge showing the steep southwest slope of the asymmetric, approximately 1 m high ridge. Topographic profiles are derived from DEM. (c) View to the north from crest of tectonic ridge and along northern one half of seismic reflection line 2a. Fence shows downward-sloping surface (white line marks top of green paint on fence posts). (d) Panoramic view looking south-southeast at tectonic ridge. Fence shows upwardly warped surface in vicinity of ridge (white line traces base of fence). Seismic reflection line 2a was located adjacent to the fence.

Figure 6.

DEM derived from airborne lidar along the western part of the Grizzly Valley fault system. Symbology follows Figure 4. Uninterpreted versions of the lidar images are provided in the auxiliary materials.

3.2 Shallow Seismic Reflection Survey

[11] We acquired high-resolution, P wave, seismic reflection data along six profiles that cross suspected tectonic lineaments that we identified from previous mapping and our analysis of optical imagery and lidar data (Figure 2a and Table 1). The profiles are 0.5–2.0 km long. For most of the profiles, we used a 230 kg accelerated weight drop source, but for line 2b, we used a 4.5 kg sledgehammer. Geophone spacing ranged from 2 to 5 m, and shots were colocated with the geophones. We deployed 8 Hz vertical-component geophones on each profile and recorded data on 120 to 168 of these sensors per source location. Conventional seismic data processing included datum statics, amplitude correction (automatic gain adjustment), band-pass filter, dip filter to mitigate coherent noise, adaptive spiking deconvolution, velocity analysis, and residual statics [see Yilmaz, 2001]. We applied a poststack, finite difference time migration prior to conversion to depth using an optimized stacking velocity function of each profile.

Table 1. Grizzly Valley Fault System Seismic Reflection Survey Parameters
Survey LineLine LengthReceiver SpacingShot SpacingSourceFault Detected?
(m)(m)(m)
Line 2a103733230 kg weight dropYes, GVFS (primary)
Line 2b529224.5 kg sledge hammerYes, GVFS (primary)
Line 5203355230 kg weight dropYes, GVFS (primary)
Line 759955230 kg weight dropNo
Line 8166055230 kg weight dropYes, GVFS (secondary)
Line 1260255230 kg weight dropNo
Total6461    

[12] We limit our interpretation to depths greater than 50 m, particularly on the 5 m spaced lines, because field records have limited aperture (very few traces) at the shallowest depths to accurately identify reflection phases versus direct arrival and refraction phases. Lines with tighter spacing can be interpreted to shallower depths with more confidence, but these tend to be noisier (e.g., line 2b).

3.3 Well Logs

[13] We used proprietary well logs in Sierra Valley to independently constrain the depth of critical stratigraphic contacts. Wells in the area of interest range from 13 to 485 m deep, and most of the deep wells (>100 m) were drilled for center-pivot irrigation systems. The quality of well logs varies, but drillers typically report three primary stratigraphic units: (1) unlithified sedimentary units including clay, silt, sand, and fine gravel; (2) volcanic rocks, especially basalt; and (3) granite. We interpret these units to correlate with the basin stratigraphy of Plio-Pleistocene lacustrine strata, Tertiary volcanic rocks, and granitic basement (Figure 3). As these well data are confidential, we can only generalize those records here.

4 Sierra Valley Geomorphology—Lidar Analysis

[14] The study area lies between two drainage systems that feed the Middle Fork of the Feather River: the Sierra Valley Channels to the west and Little Last Chance Creek to the east (Figure 2). A topographically high area 4–5 m above the floodplain abruptly deflects these drainages and causes them to flow parallel to one another until they converge to the northwest and then flow into the Middle Fork of the Feather River (Figure 2). The California Department of Water Resources [1963] and Reed [1975] previously interpreted this unusual drainage pattern to be evidence of Quaternary faulting.

[15] In addition to the deflected drainage pattern, our analysis of the imagery and lidar data revealed several northwest trending zones of aligned scarps and tonal lineaments that bound and cut across the topographically high area (Figure 2). Of these features, the easternmost lineament has a morphology that is most suggestive of recent tectonic activity (Figures 4 and 5). In detail, the lidar data show an ~4 km long, curving, and left-stepping uplift which we interpret to be a fault-related tectonic ridge (Figure 4b). Scarps associated with the ridge show a left-stepping pattern (Figure 4b), indicative of dextral strike-slip faulting [Sylvester, 1988]. The ridge is asymmetric in cross section with a steeper southwest facing slope (Figure 5b), and it rises approximately 1 m above the surrounding landscape. The steepest slopes correlate with minor closed depressions on the southwest side of the scarp, which could be the result of small steps along a strike-slip fault and/or eolian scouring (Figure 4b). Little Last Chance Creek is located 400 m northeast of the ridge and may have enhanced the ridge through erosional processes, but as we describe below, the ridge coincides with subsurface faulting and an antiformal structure in the underlying lacustrine sediments, which supports our interpretation that this feature has a tectonic origin.

[16] The tectonic ridge is most prominent west of Little Last Chance Creek (Figure 4). To the southeast in an area of numerous man-made canals, the ridge is no longer evident (Figure 4a); however, small escarpments and tonal lineaments persist southward for several kilometers. Seismic reflection line 5 crosses this series of subtle features and demonstrates the existence of an underlying fault.

[17] Additional northwest trending lineaments crossed by seismic reflection lines 8, 7, and 12b are characterized by tonal vegetation lineaments, aligned drainages, and subtle topographic discontinuities (Figure 6). As we describe below with the exception of line 8, the shallow seismic reflection profiles show that the subsurface lacustrine strata are undeformed and that these queried lineaments are not related to Quaternary faulting.

5 High-Resolution Seismic Reflection Profiles

[18] The six high-resolution, P wave seismic reflection profiles acquired across the suspected faults and lineaments (Table 1 and Figures 7-10) provide insights into the subsurface deformation associated with these structures. Lines 2a, 2b, and 5 cross the easternmost zone of mapped lineaments, corresponding to the primary trace of the Grizzly Valley fault. Lines 7, 8, and 12b cross lineaments to the west. In this section, we first describe the stratigraphic packages imaged in the seismic reflection survey and correlate reflections to the regional stratigraphic framework. We then evaluate the structural features imaged in each reflection profile.

Figure 7.

Seismic-reflection lines 2a and 2b. (a) Line 2a topographic profile from RTK GPS data showing expression of tectonic ridge, presented at 10x vertical exaggeration. (b) Uninterpreted and (c) interpreted line 2a. (c) Detailed topographic profile of line 2b, presented at 10x vertical exaggeration. (e) Uninterpreted and (f) interpreted line 2b. Interpretations discussed in text.

Figure 8.

Seismic-reflection line 5. (a) Line 5 topographic profile from RTK GPS data, presented at 10x vertical exaggeration. (b) Uninterpreted and (c) interpreted line 5. Interpretations discussed in text.

Figure 9.

Seismic-reflection line 8. (a) Line 8 topographic profile from RTK GPS data, presented at 10x vertical exaggeration. (b) Uninterpreted and (c) interpreted line 8. Interpretations discussed in text.

Figure 10.

Seismic-reflection lines 7 and 12b. (a) Line 7 topographic profile from RTK GPS data, presented at 10x vertical exaggeration. (b) Uninterpreted and (c) interpreted line 7. (d) Detailed topographic profile of line 12b, presented at 10x vertical exaggeration. (e) Uninterpreted and (f) interpreted line 12b. Interpretations discussed in text.

[19] The profiles image a strongly reflective, deformed basal marker that we interpret as the top of the Tertiary volcanic rocks (reflector “V,” Figures 7-10) and a 120–450 m thick suite of subhorizontal reflectors (reflectors L1–L9, Figures 7-10) that we interpret as lacustrine deposits composed of alternating pluvial sand, silt, and clay. These correlations are largely guided by previous mapping in the region and are consistent with the depth of major stratigraphic boundaries reported in local well logs. The top of the Tertiary volcanic rocks (V) is distinguished by the transition from overlying, subhorizontal reflectors to longer wavelength tilted, and discontinuous reflectors. We used the undeformed lacustrine section in lines 5 and 2a to correlate sets of reflectors based on their depth and wavelength/frequency. We correlated reflections within individual seismic reflection profiles (e.g., across fault zones) and between the seismic reflection profiles, but our correlations are not definitive because we lack age control to definitively link these spatially separated stratigraphic sections.

[20] The profiles across the easternmost and primary trace of the Grizzly Valley fault system show the top of the volcanic rocks to be 100–300 m below the surface (lines 2a, 2b, and 5). This contact dips gently to the west, as shown by correlating this reflection from the eastern set of profiles westward to line 8, where this contact is as much as 450 m deep. The lacustrine deposits onlap the volcanic rocks, a relationship that is clear at a horizontal distance of 1000–1500 m in line 5. These onlap relations indicate that paleotopography existed in the basin at the onset of lacustrine deposition. This paleotopography could be primary topography related to eruption of the volcanic rocks, it could be erosional, it could result from faulting and tectonic deformation, or it could be some combination of the aforementioned processes.

5.1 Line 2a

[21] Line 2a (Figure 7) shows ~70 m of southside-up vertical offset of the top of the volcanic rocks. The lacustrine deposits are warped upward in the center of the profile, which coincides with an area of faulting in the underlying volcanic rocks. As we discuss below, we interpret upward warping to have resulted from contraction associated with left steps across a dextral fault system. Furthermore, the lower part of the lacustrine section is clearly faulted. Faulting can be traced to within 55 m of the surface, and the fault zone projects upward to the location of the overlying ridge, which we interpret as a tectonic feature (Figure 4b). Across the entire section, line 2a does not show a significant net vertical offset of the lacustrine strata, although the basin deepens southward. The shallow lacustrine strata, within 75 m of the surface, are poorly resolved south of the fault zone, which, as we discuss below, may relate to differences in water saturation across the fault.

5.2 Line 2b

[22] We acquired data for line 2b (Figure 7) using a 4.5 kg sledgehammer source, which inputs a higher frequency signal to yield potentially higher resolution in the shallow subsurface. Unfortunately, this source also has a relatively low seismic energy signature, and therefore, the signal is not easily distinguished from ambient noise from wind and cultural activity. Profile 2b shows ~75 m of apparent southwestside-up vertical offset of the top of the volcanic rocks (V). The profile images a well-stratified lacustrine sequence on the northeast part, but the incoherent signals from the southwest part limits our confidence in correlation of the lacustrine strata across the interpreted fault zone. Despite these resolution issues, it is clear that the lacustrine strata are warped upward as they approach the GVFS, and this deformation extends upward to within 50 m of the surface. Based on our correlations of lacustrine reflectors L5–L9, it is also probable that deeper beds are progressively offset and that net uplift coincides with the modern midbasin surficial high. Projection of the fault zone to the surface coincides with the tectonic ridge imaged by lidar (Figure 4b). The shallow lacustrine strata southwest of the fault are poorly resolved relative to the strata northeast of the fault. As with line 2a, we suspect this variability may relate to differences in water saturation, with the upthrown southwest block less saturated than the downthrown northeast block.

5.3 Line 5

[23] Line 5, the longest in our survey at over 2 km length, provides key insights into the overall structure of the basin (Figure 8). Notably, the top of the volcanic rocks (V) defines a 350 m deep depression in the basin's prelacustrine topography at 1250 m horizontal distance on the profile. The lacustrine strata onlap and fill the depression over the volcanic rocks. At 600 m distance, contact (V) is faulted with about 80 m of up-to-the-west vertical offset. Details of the relationship between the lacustrine strata and faults are poorly resolved near the GVFS; however, the western termination of lacustrine strata within 50 m of the surface is interpreted to be a fault-related contact that probably extends to the surface. Projection of the fault zone to the surface coincides with subtle scarps in the lidar data and tonal lineaments on optical imagery (Figures 2 and 4). West of the fault, similar to line 2b, the lacustrine strata are poorly resolved in the upthrown (western) block and may be related to differences in water saturation across the fault. If the fault is a barrier to water, contrasting degrees of water saturation may explain the tonal lineaments (unsaturated to the west and saturated to the east of fault).

5.4 Line 8

[24] Line 8 (Figure 9) extends across a secondary strand of the GVFS, a westward dipping fault that vertically offsets the “V” contact 15–20 m in a normal sense. We can trace the upward extent of this fault to within 300 m of the surface in the well-stratified lacustrine sequence. Above 300 m depth, the lacustrine deposits are apparently warped. Details of the structure are difficult to resolve at depth shallower than ~150 m. We observe that the strata appear to be either warped or draped onto an underlying fault scarp to within 50 m of the surface. We note that no coherent, topographic escarpment coincides with this fault trace, so we interpret this fault to be a subsidiary trace of the GVFS.

5.5 Line 7

[25] Line 7 (Figure 10) shows a gently westward dipping sequence of undeformed lacustrine strata that conformably overlie the top of the volcanic rocks (V). The well-defined stratification across this profile precludes the presence of any significant faults.

5.6 Line 12b

[26] Line 12b (Figure 10) shows a westward dipping contact at the top of the volcanic rocks (V) that is onlapped and buried by gently westward dipping lacustrine strata. We see no evidence of significant faulting on this profile.

5.7 Summary

[27] Our analysis of the airborne lidar data combined with the seismic reflection data identifies a continuous and steeply dipping fault system that extends to the surface in Sierra Valley. We interpret the structures we see on the eastern set of profiles (lines 2a, 2b, and 5) as the primary trace of the GVFS and a single, buried structure on one western profile (line 8) as a subsidiary trace. Lines 2a, 2b, and 5, which cross the easternmost zone of mapped lineaments, consistently image a steeply dipping fault that vertically offsets the top of the Tertiary volcanic rocks with a southwest side up sense of throw. The sense of vertical offset of the Tertiary volcanic rocks is consistent with the location of the modern topographic high in the center of the basin. The lacustrine sequence is also faulted and upwardly warped across several splay faults.

[28] Another observation from the eastern set of profiles is that the lacustrine reflectors are poorly defined to the southwest of the principal trace of the GVFS. This variability could be the result of differences in either groundwater depth or saturation, but our review of well records did not reveal significant variations in water depths across the study area. Therefore, we suspect that this difference in reflectivity probably corresponds to a change in water saturation across the GVFS. To the northeast, where the reflectors are clearly defined, the ground surface is lower and is water saturated through much of the year. We suspect that lacustrine strata northeast of the GVFS are water saturated, and these sediments more efficiently transmit seismic reflection energy. In contrast, the ground surface southwest of the GVFS tends to be dry through the majority of the year. We suspect that these strata are not water saturated and, therefore, are less efficient in transmitting seismic energy. The abrupt change in hydrology coincident with the GVFS indicates that the fault may serve as a barrier to groundwater flow in this part of Sierra Valley.

6 Discussion

6.1 Grizzly Valley Fault System Kinematics

[29] Our seismic reflection data image the GVFS as a steeply dipping structure in the upper 500 m that has vertically offset the top of the Tertiary volcanic rocks at least 70–80 m with a southwest side up sense of throw. The direction of displacement of this contact is similar over a distance of at least 6.8 km between lines 2a and 5. These relationships are consistent with an episode of normal faulting that occurred in Sierra Valley sometime after 10 Ma following deposition of the volcanic rocks. Pronounced extension in the region associated with the Basin and Range and its encroachment into the northern Sierra Nevada has occurred as recently as 3 Ma [e.g., Henry and Perkins, 2001; Surpless et al., 2002].

[30] Our observation that the most prominent faults tend to splay upward in the lacustrine deposits and that the lacustrine sediments are commonly warped upward (lines 2a and 2b) suggests that the style of deformation may have changed between the initial episode of faulting that offset Tertiary volcanics in a normal sense and the younger faulting that extends through the Quaternary lacustrine sediments. The upward branching pattern of fault strands in the lacustrine sediments is consistent with a flower structure, typical of contractional steps in strike-slip faults [Roberts, 1983; Sylvester, 1988]. Our profiles, particularly lines 2a and 2b, show two subsurface strands that bracket an antiformal structure, coincident with the 1 m high, curving, and left-stepping topographic ridge. We interpret the antiformal structure and overlying ridge to have resulted from left steps between two subvertical, right-lateral, strike-slip fault strands [Sylvester, 1988; Tchalenko, 1970], which is consistent with regional dextral shear across the northern Walker Lane.

[31] In summary, we interpret two periods of faulting: an earlier episode of postvolcanic normal faulting that is likely associated with extension in the Basin and Range and a later episode of shear deformation in which the GVFS was reactivated with a dextral slip sense. The shear deformation likely began after 6–3 Ma, based on the time when dextral shearing began in the region [Faulds et al., 2005; Henry et al., 2007; Hinz et al., 2009; Trexler et al., 2009]. We note that the apparent normal faulting of the lacustrine sequence in line 8 may be evidence that extensional deformation may have persisted into the Plio-Pleistocene, consistent with regional transtension measured by modern GPS geodesy [Hammond et al., 2011].

6.2 Timing of Most Recent Faulting

[32] The presence of surficial features that are consistent with dextral faulting is evidence for Quaternary movement on the GVFS. We suspect that the youngest movement on the fault system is likely late Quaternary in age because of the preservation of the broad escarpment and left-stepping ridge, which we interpret to be fault-related features. If the GVFS had been inactive during Quaternary time, we expect that surface processes—including erosion from surface runoff and burial by eolian processes—would have entirely obscured or removed the GVFS lineament and scarps. We do not have absolute age control for the faulted lacustrine deposits. However, Ramelli et al. [1999] suggest that a pluvial lake filled the basin as recently as 150 ka. If correct, then this provides a maximum bound on the age of faulted deposits and the time of the most recent faulting.

[33] Absolute age control on the faulted deposits across the GVFS either from trenching or coring studies is required to constrain the timing of the most recent earthquake with certainty. The Mazama or Tsoyowata tephra (~7.6–7.8 ka) [Bacon, 1983], which was identified in a trenching study on the Mohawk Valley fault in the southwestern arm of Sierra Valley [Sawyer et al., 2005], could be a useful stratigraphic marker if it is present along the GVFS. Our preliminary analysis and comparison to regional studies lead us to suggest that the fault has been active in the latest Quaternary.

6.3 Implications for Seismic Hazard Analysis

[34] At its southern terminus, the GVFS is only about 25 km from Reno, Nevada, and Truckee, California. Furthermore, numerous man-made reservoirs and other critical infrastructure (e.g., interstate highway, rail lines, gas pipe lines, etc.) are within 50 km of the GVFS. Evidence indicating latest Quaternary motion on this structure suggests that it should be considered in regional seismic hazard assessments.

[35] We use the empirical relationships between moment magnitude and fault rupture area for strike-slip faults worldwide of Wells and Coppersmith [1994] and Hanks and Bakun [2002, 2008] to estimate a range of moment magnitudes from earthquakes on the Grizzly Valley fault system. We consider two rupture scenarios: scenario 1, a 60 km long, full fault length rupture from Lake Davis to Sardine Valley; and scenario 2, an approximately 25 km long rupture that extends across Sierra Valley. For both scenarios, we assume the fault is vertical and ruptures to a depth of 15 km, and we use the regression equation from strike-slip faults.

[36] For the 25 km long rupture, both Wells and Coppersmith [1994] and Hanks and Bakun [2002, 2008] yield an estimated magnitude of Mw 6.6. For a 60 km long rupture, the two equations yield an estimated magnitude of Mw 7.0. An earthquake of Mw 6.6–7.0 could cause significant ground shaking in the Reno-Truckee urban area.

6.4 Implications for Distributed Faulting and Comparing Results From Geologic and Geodetic Studies

[37] Our conclusion that the GVFS is a Quaternary active fault makes it relevant in comparisons of geodetic and geologic deformation rates across the northern Walker Lane. Geodetic transects across the region record a modern regional dextral strain rate of about 7 mm/yr (Figure 11). This rate is at least a factor of 2 higher than the sum of Holocene dextral geologic slip rates of 1.4–3.3 mm/yr across the region, which is derived by summing slip rates of 0.3–0.5 mm/yr along the southern Mohawk Valley fault (a minimum rate) [Sawyer et al., 2005], 1.7 ± 0.6 mm/yr or 1.1–2.6 mm/yr along the Honey Lake fault (Turner et al. [2008] and Wills and Borchardt [1993], respectively), and <0.2 mm/yr along the northern Warm Springs Valley fault [Gold et al., 2013]. Both the geodetic and geologic slip rates are subject to revision, given their respective uncertainties, but another potential explanation for the mismatch is that other faults are accommodating part of the regional dextral strain but are not included in the inventory [Foy et al., 2012]. The GVFS is a candidate fault system that might accommodate a portion of the regional shear strain. We note that our investigation focuses on a limited part of Sierra Valley (<7 km wide, perpendicular to fault strike), and we have not evaluated evidence of possible Quaternary movement on the Hot Springs fault system (HSFS) or other potential fault-related features in Sierra Valley. This leaves the possibility that other structures within Sierra Valley could accommodate additional regional strain.

Figure 11.

(a) Slopeshade image and (b) hillshade image showing the location of significant sedimentary basins in the northern Walker Lane where subtle and distributed strike-slip faults may be present but unidentified. Images derived from the NED (1/3 arc second). Low-relief basins in Figure 11a correspond to light (unshaded) areas in Figure 11a and are shown by brown polygons in Figure 11b. Regional faults are shown by black lines and are from references listed in the caption of Figure 1. Area outlined in red is regional GPS profile oriented 040° that records a modern dextral shear deformation of ~7 mm/yr [Hammond et al., 2011].

[38] If the GVFS or other structures in Sierra Valley accommodate a modest amount of regional dextral strain (e.g., ~1 mm/yr), then block models developed from geodetic data may be enhanced by including these fault systems as block boundaries. For example, the mismatch between the high geodetic and low geologic slip rate on the Mohawk Valley fault to the west may be the result of motion on the GVFS, which is not included in block models [Hammond et al., 2011]. Preliminary geologic studies suggest that the Mohawk Valley fault (~17 km southwest of this study area) has a minimum slip rate of 0.3–0.5 mm/yr [Sawyer et al., 2005], but this rate is on the order of 6 times lower than the block model slip rate of 2.9 ± 0.2 mm/yr proposed by Hammond et al. [2011]. Some of this apparent mismatch could be reduced by adding one or more block boundaries corresponding to active faults in Sierra Valley.

6.5 Implications for Low Slip Rate Strike-Slip Fault Studies

[39] Finally, we consider the implications of our results with respect to studies of hazardous faults with low slip rates, especially in low-relief settings. This class of active faults can go unidentified, as has been illustrated by historical surface ruptures, including the 2003 Mw 6.6 Bam, Iran, the 2010 Mw 7.1 Darfield, New Zealand, and the southeastern half of the 2010 Mw 7.2 Sierra El Mayor-Cucapah ruptures, which were characterized as previously unknown or “blind” faults along their surface traces where the rates of surficial processes were sufficient to remove or obscure fault-related geomorphology [e.g., Quigley et al., 2011; Talebian, 2004; Wei et al., 2011]. In Nevada, the 1932 Ms 7.2 Cedar Mountain, 1954 Ms 6.3, and 1954 Ms 7.0 Stillwater earthquakes included distributed strike-slip ruptures in valley alluvium along cryptic fault traces [DePolo et al., 1991]. The problem of obscure strike-slip faults in Nevada is not restricted to large ruptures: Recent moderate-sized strike-slip earthquakes near the Reno-Carson urban corridor have occurred along widely distributed or previously unrecognized active dextral fault systems, including the 1994 Mw 5.9 Double Spring Flat and the 2008 Mw 4.7 Reno-Mogul events [Amelung and Bell, 2003; Bell et al., 2012]. Recognition of these types of faults is critical for seismic hazard analysis and mitigation.

[40] The Walker Lane is a wide zone of transtension composed of moderate to low slip rate faults. The subtle surficial expression of the GVFS leads us to speculate that low slip rate strike-slip faults could be present beneath many sedimentary basins in the region but are unrecognized because of their subtle or absent tectonic geomorphology. In Figure 11, we show the widespread distribution of sedimentary basins in the northern Walker Lane, where basin fill could conceal the presence of low slip rate strike-slip faults. Basins such as these would be targets for future efforts to search for unidentified faults and better characterize the seismic hazard in the Reno-Carson City urban corridor. Such studies would benefit from high-resolution topographic data and subsurface seismic imaging. Recent studies have successfully identified previously unknown faults in basin settings using CHIRP in Lake Tahoe [Brothers et al., 2009; Dingler et al., 2009; Kent et al., 2005] and Pyramid Lake [Eisses, 2012; Eisses et al., 2011]. In subaerial settings, we anticipate that the increasing availability of high-resolution airborne lidar data coupled with seismic techniques for subsurface exploration will enhance searches for additional active structures with subtle geomorphology.

[41] This study illustrates the challenge of identifying active, low slip rate strike-slip faults in low-relief settings. Even when faults are identified, as in the case of the GVFS, placing constraints on the timing of most recent faulting events is similarly challenging because low strain rates translate to infrequent earthquakes in regions with sedimentary records that may not be conducive to classic paleoseismic trenching studies.

7 Conclusions

[42] Our investigation along the GVFS illustrates the challenge of identifying active Quaternary faults that have a low slip rate (<1 mm/yr) in low-relief settings. In the past half century, a number of geologic studies have yielded differing conclusions regarding Quaternary faulting on the GVFS [California Department of Water Resources, 1963; Grose, 2000c, 2000d, 2000e; Reed, 1975]. We have used high-resolution topographic data from airborne lidar and shallow seismic reflection imaging to demonstrate Quaternary deformation on the GVFS. The seismic reflection data image the GVFS as a steeply dipping structure that vertically offsets the top of Tertiary volcanic rocks 70–80 m, southwestside up. Faults in the basin-fill lacustrine sediments extend to within 50 m of the surface. They are likely not imaged at shallower depths due to resolution limitations of the seismic techniques as employed in this study. The seismic reflection profiles show that the faults splay upward, which is characteristic of a flower-structure geometry associated with strike-slip faults. Two of the profiles show that subsurface faulting coincides with a left-stepping topographic ridge that rises about 1 m above the surrounding terrain. The subsurface fault geometry and left-stepping pattern of surface geomorphology indicate recent right-lateral strike-slip motion on the GVFS, which is consistent with the regional dextral strain in the northern Walker Lane.

[43] We do not have constraints on the age of most recent faulting, but the topographic expression of faulting and the presence of pluvial shorelines estimated to be ~150 ka in Sierra Valley suggest latest Quaternary slip on the GVFS. Based on its mapped length and assuming a 15 km downdip fault width, the GVFS could be capable of producing an Mw 6.6–7.0 earthquake and therefore should be considered in regional seismic hazard analyses. Additionally, the GVFS may accommodate some of the regional dextral shear, and including it in the regional strain budget may reduce the discrepancy between geodetic and geologic deformation rates. This investigation shows that previously unidentified, low slip rate strike-slip faults in basins in the northern Walker Lane may be significant seismic sources that accommodate shear and affect regional hazard assessments. Furthermore, this study illustrates the challenge of identifying and understanding the earthquake records of low slip rate yet potentially hazardous faults.

Acknowledgments

[44] David Worley, James Allen, and Daniel Bowden provided assistance in the field. John McBride provided the weight drop source used for the seismic reflection profile work. We thank local landowners Scott Thompson, Darrin Damonte, Elmer Roberti, Jeff Carmichael, and Todd York, as well as Plumas County, for providing permission for us to conduct this study. The U.S. Geological Survey Earthquake Hazards Program supported this work. Reviews by Stephen Personius, two anonymous reviewers, and the Associate Editor significantly improved this paper.

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