Fault Imaging with High-Resolution Seismic Reflection for Earthquake Hazard and Geothermal Resource Assessment in Reno, Nevada

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL.???, XXXX, DOI: /, Fault Imaging with High-Resolution Seismic Reflection for Earthquake Hazard and Geothermal Resource Assessment in Reno, Nevada R. N. Frary, 1 J. N. Louie, 1 W. J. Stephenson, 2 J. K. Odum, 2 S. Pullammanappallil 3, and L. M. Liberty 4 R. N. Frary, Nevada Seismological Laboratory, University of Nevada, Reno/0174 Reno, NV 89557, USA. (rfrary0615@gmail.com) L. M. Liberty, Center for the Geophysical Investigation of the Shallow Subsurface, MG-206, Boise State University, 1910 University Dr, Boise, ID 83725, USA. J. N. Louie, Nevada Seismological Laboratory, University of Nevada, Reno/0174 Reno, NV 89557, USA. J. K. Odum, United States Geological Survey, MS 966, Box 25046, Denver, CO 80225, USA. S. Pullammanappallil, Optim, 200 S Virginia St, Ste. 560, Reno, NV, 89501, USA. W. J. Stephenson, United States Geological Survey, MS 966, Box 25046, Denver, CO 80225, USA. 1 Nevada Seismological Laboratory,

2 X - 2 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA Abstract. The Truckee Meadows basin is situated adjacent to the Sierra Nevada microplate, on the western boundary of the Walker Lane. Being in the transition zone between a range-front normal fault on the west and northweststriking right-lateral strike slip faults to the east, there is no absence of faulting in this basin. The Reno-Sparks metropolitan area is located in this basin, and with a significant population living here, it is important to know where these faults are. High-resolution seismic reflection surveys are used for the imaging of these faults along the Truckee River, across which only one fault was previously mapped, and in southern Reno, where a swarm of short faults has been mapped. The reflection profiles constrain the geometries of these faults, and suggest additional faults not seen before. Used in conjunction with depth to bedrock calculations and gravity measurements, the seismic reflec- University of Nevada, Reno, Reno, Nevada, USA. 2 United States Geological Survey, Golden, Colorado, USA. 3 Optim, Reno, Nevada, USA. 4 Center for the Geophysical Investigation of the Shallow Subsurface, Boise State University, Boise, Idaho, USA.

3 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X tion surveys provide definitive locations of faults, as well as their orientations. Offsets on these faults indicate how active they are, and this in turn has implications for seismic hazard in the area. In addition to seismic hazard, the faults imaged here tell us something about the conduits for geothermal fluid resources in Reno.

4 X - 4 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA 1. Introduction Geologists have studied fault activity in western Nevada for decades. There is a series of Quaternary fault mapping studies in Nevada [e.g., Bingler, 1974; Bell, 1984; depolo, 2008], a history of earthquakes around Lake Tahoe [e.g., Schweickert et al., 2000] and north through Carson City and Reno [e.g., depolo et al., 1997], and a more recent history of fault-controlled geothermal resources in western Nevada [e.g., Faulds et al., 2004]. But there still remains several unanswered questions about the Reno area and the basin it lies within. It has long been thought that the Truckee Meadows basin is a graben structure, with an east-dipping normal fault on the west side, coincident with the Carson Range and the Sierra Nevada frontal fault system, and a west-dipping normal fault on the eastern side of the basin, coincident with the Virginia Range [e.g., Stewart, 1971]. However, it has been more recently proposed that this basin is instead a half-graben structure, more similar to that of the Carson and Washoe Valleys to the south [e.g., Trexler et al., 2000; Cashman et al., 2009]. Because the Truckee Meadows basin is located in the transition zone between the Sierra Nevada and the Walker Lane [e.g., Stewart, 1988], it is likely this basin takes on the characteristics of the northern Walker Lane basins. Pyramid Lake and Honey Lake are both half-graben structures, with a combination of dextral-slip and normal faults in and around the basins [e.g., Henry et al., 2007; Eisses et al., 2012]. The planning scenario for an earthquake in the Reno Carson City corridor does imply that the Truckee Meadows basin is a half-graben structure, but there is only a single east-dipping fault rupture in the vicinity of Reno that appears in this scenario [depolo

5 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X et al., 1995]. Given the abundance of additional faults mapped in south Reno and in northwest Reno [e.g., depolo, 2008] and the seismicity in west Reno recently with the Mogul earthquake swarm in 2008 [Smith et al., 2008; Anderson et al., 2009], there must be additional fault ruptures to add to earthquake planning scenarios. To address these issues, high-resolution seismic reflection surveys were carried out in 48 Reno. First, three longer profiles were acquired with a minivibrator source, and then five smaller profiles were collected with the use of a weight-drop source. These sources were used specifically for their compatibility with urban environments. In this paper, we present the results of most of these surveys; the remaining two are presented in Stephenson et al. [2012]. 2. Geologic and Tectonic Setting The Reno-Sparks metropolitan area is in the Truckee Meadows basin; this basin is situated in the transition zone between the Sierra Nevada microplate to the west and the Walker Lane to the east. The structural domain of the Walker Lane encompasses much of eastern California and western Nevada [e.g., Stewart, 1988]. This area, along with the Eastern California Shear Zone to the south, accommodates at least 1 / 6 of the motion between the North American and Pacific plates [e.g., Faulds et al., 2005; Hammond et al., 2011]. Due to the northwest motion of the Pacific plate, the Walker Lane is subject to significant amounts of dextral shear and extension along this boundary, resulting in faulting of all types. The northern Walker Lane is characterized by basins with a combination of dextral strike-slip and normal faults that strike north to northwest. The Pyramid Lake basin in western Nevada exhibits this with the presence of a northwest-striking dextral Pyramid

6 X - 6 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA Lake fault zone trending along the axis of the lake itself, which transfers its strain to the west-dipping normal fault of the Lake Range on the east side of the lake farther north [Briggs and Wesnousky, 2004; Eisses et al., 2012]. The Honey Lake basin in northeastern California also exhibits this with the northwest-striking dextral Honey Lake Fault, which transfers its strain to the east-dipping normal Diamond Mountains Fault on the western side of the lake itself farther north into the basin [Henry et al., 2007]. If the Truckee Meadows basin follows this trend, then there ought to be a basin-bounding fault, in addition to a dextral-slip fault southeast of the basin. But there is no strike-slip fault like this near the Truckee Meadows basin. So this basin cannot be categorized as typical of the northern Walker Lane. Carson and Washoe Valleys, south of Reno, are half-graben structures. The Carson Valley is bounded on the west side by the east-dipping normal Genoa fault and is tilted to the west [e.g., Cashman et al., 2009, and references therein]. There is a similar basinbounding fault in Washoe Valley [Cashman et al., 2012]. The extension of the Genoa fault system to the north as the Mount Rose fault system [e.g., depolo et al., 1995; Ramelli et al., 1999] implies that there is a major east-dipping basin-bounding fault on the west side of the Truckee Meadows. However, it seems the Truckee Meadows has the opposite asymmetry and instead there is a west-dipping normal fault somewhere east of the Carson Range [Cashman et al., 2012]. This is especially evident with the fanning of dips in the 84 diatomite section in west Reno. From west to east, going up in the section, the dip decreases from about 24 to less than 7. This fanning of dips implies tilting toward the east, with deposition concurrent with an active west-dipping normal fault somewhere east of this section [Widmer et al., 2007; Trexler et al., 2012; Cashman et al., 2012].

7 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X The 1990s planning scenario for an earthquake in the Reno Carson City corridor suggests the major fault through the Truckee Meadows basin is an east-dipping fault that is part of the Sierra Nevada Carson Range frontal fault system [depolo et al., 1995]. The scenario only examines rupture on this fault for a large-magnitude earthquake. This is a simplified scenario, but there are most certainly additional faults in the basin that are likely to rupture. The Dixie Valley Fairview Peak earthquake sequence in 1957 in central Nevada ruptured along several different faults within the 100-km vicinity [Slemmons, 1957], and Wesnousky [2005] has shown that earthquakes in the Walker Lane rupture along multiple faults in most cases. The faults in the Truckee Meadows basin that could rupture in a major earthquake here are many and varied. The Truckee Meadows basin has undergone a variety of deformation over its history. The basin was dominated initially by volcanism, with an extensional tectonic episode occurring ca Ma [Silberman et al., 1979; Henry and Perkins, 2001; Trexler et al., 2012]. This extension resulted in a shift in volcanism as well as a variety of north- to northwest-trending normal faults. These are consistent with those that are predominant in the Basin and Range Province to the east. With the continuous movement of the Pacific plate relative to the North American plate, the strain is partitioned into additional faults in the Walker Lane. The northwest-striking faults accommodate right-lateral deformation, and the northeast-striking faults accommodate left-lateral deformation due to rotation of fault-bounded blocks [Stewart, 1988; Cashman and Fontaine, 2000]. These faults are all fairly young; an upper bound on the age is about 2.67 Ma, from biostratigraphy and tephrochronology [Kelly and Secord, 2009; Trexler et al., 2000, 2012].

8 X - 8 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA Young faults are indicative of recent earthquake activity. Movement on these faults occurs to accommodate stress changes. There have been at least fifteen M5.0 and greater earthquakes in western Nevada in the last 150 years [depolo et al., 1997; Ramelli et al., 1999]. There were two large (about M6.5) earthquakes on the Olinghouse fault zone east of Reno in 1860 and 1869 [Sanders and Slemmons, 1996], and a M5.4 earthquake in Truckee, California, west of Reno, in 1966 [Kachadoorian et al., 1967]. More recently, there were earthquakes at Double Spring Flat (maximum-magnitude 5.9) from 1994 to 1996, 75 km south of Reno [Ichinose et al., 1998]. In 2008, an earthquake swarm occurred in the Reno suburb of Mogul, west of downtown Reno. This swarm lasted from February through August, with the maximum magnitude of 5.0 occurring in late April [Dhar et al., 2008; Smith et al., 2008]. It was shortly after the 2008 swarm that the United States Geological Survey (USGS) decided that the Reno Carson City Lake Tahoe urban corridor was one of the three largest and most vulnerable areas in the Intermountain West [Gallagher et al., 2010]. Reno-Sparks is a growing metropolitan area; it has seen an increase in population of about 75,000 people (about 21.7%) since 2000 [United States Census Bureau, 2012]. The USGS and the Nevada Bureau of Mines and Geology (NBMG) have mapped several faults throughout Reno and the Truckee Meadows basin [Bingler, 1974; Bell, 1984; US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008], but in many cases, homes and business have been built near or on top of them. This study will help to better locate these inferred faults, and to potentially image additional faults in proximity to downtown Reno, where many are covered by urban development.

9 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X Methods Locations for 2D seismic lines (Figure??) were chosen based on discussions with local geologists and the USGS Fault and Fold Database [US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008; Louie, 2009]. The first line is along the Truckee River, west to east, through the middle of the Truckee Meadows basin [Stephenson et al., 2012]. There is only one fault mapped crossing the river (Figure??), so the potential to image additional, or continuations of other, faults is high. The east-west orientation of this profile also increased the chances of imaging the north-, northwest-, and northeasttrending faults in the basin. The second mini vibroseis line is in south Reno, where there are several previously-mapped faults [e.g., depolo, 2008]. Imaging these faults will allow for confirmation of their existence, evaluation of their importance, and determination 142 of their orientation(s). Both of these lines are in urban areas, so time, spacing, and acquisition were very important. The profile in southern Reno is along the straightest west-east road that crossed the majority of the faults, which was necessary to maintain appropriate geometries. An accelerated weight drop was used as the source for shorter lines in southern Reno for a higher-resolution study. The locations of all profiles in south Reno are shown in Figure 2. A weight-drop profile, called here M1, started at 2940 m line distance on the minivib profile (Figure 2) and went east to 3385 m. Another weight-drop profile, called here M2, started at 2470 m line distance and went west to 2145 m. Two north-south weight-drop profiles were collected as well. One, line PS, crossed the minivib profile at line distance 2470 m. The other, line WW, crossed the minivib profile at line distance 3145 m. By collecting seismic data in profiles with multiple orientations, the attempt was

10 X - 10 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA made to image the faults in a pseudo-3d manner. This allows for interpretation of the fault orientations in multiple places along the faults Methods for Minivib Profiles line-kilometers of 2D seismic reflection data along three profiles in Reno were collected using a minivibrator source. This paper will focus on the minivibrator profile in south Reno ( ML in Figure??), and profiles done with the accelerated weight-drop source throughout Reno. The minivib I vibrator source ( Thumper ) from nees@utexas was used, as it provided the capability to image about a kilometer depth, and could be permitted for the streets of Reno. A 15- to 100-Hz linear sweep was used for acquisition. 144 channels of data were recorded per shot point, using a 5-m shot- and receiver-station interval. A single, vertical-component 8-Hz geophone was set at each receiver station. Maximum source-receiver offset was 720 m, and maximum fold was 72, which is relatively high for this type of data. At each vibration station, two records were vertically stacked. Due to the high levels of urban noise, signal-to-noise ratio was fairly low. Data processing was conducted with typical oil-industry procedures. Significant effort was made in removal of surface waves and other noise events generated by the traffic on the streets nearby. Processing steps like eigenvector filtering and adaptive deconvolution were helpful for this purpose. A 60-Hz notch filter was introduced for the removal of overhead 171 power line noise. A tapered bandpass filter of Hz was applied as well The low-cut ramp removed most of the low-frequency surface-wave noise, but some high- frequency linear and random noise events are still evident. Although a fairly high level of noise exists throughout the data as a result of the urban environment, maintaining the

11 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X fold common midpoint (CMP) coverage significantly improved final reflection images, as there was more subsurface coverage, reducing some of the noise through CMP stacking Methods for Weight-Drop Profiles After looking at the minivib data and interpretations, a few places to collect additional data for higher-resolution and more accurate images of the shallower subsurface were chosen (Figure 2). 3.5 additional line-kilometers of 2D seismic reflection data along five profiles were collected using a weight-drop source. The accelerated hammer source from Boise State University (BSU) provided the capability to image about 300 m depth, and it was fairly easy to mobilize for shorter profiles. Two to four drops of the 80-kg weight were recorded at each source station. Both source- and receiver-station spacing were 3 m for the north-south profiles, and 1 m for the east-west profiles, with source points halfway between receiver stations. 120 channels of data were acquired per shot point, using a 186 single, vertical-component 8-Hz geophone at each receiver station. Maximum source receiver offset was 360 m, and maximum fold was 60. The signal-to-noise ratio was even lower with this source, but recording multiple unstacked records addresses this. Sourcegenerated noise from the drop of the weight dominates the data. The basic processing sequence includes residual statics and diversity stacking to attenuate the coherent noise [??] Comparison of Methods Examples of shot records acquired by the two different methods used in this study show the higher frequencies obtained using the accelerated weight drop versus the minivib method (Figure 3). Prominent reflections are observed at about and 0.2 seconds

12 X - 12 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA on the minivib record. Reflections above about 0.15 seconds (about 10 m depth) are not resolvable on the minivib records, due to source and receiver spacing as well as the nature of the energy source. Attenuation of signal also prevents the interpretation of reflections below about 0.5 seconds, aside from some interesting arrivals that are discussed in a later section. The prominent reflections in the weight drop record are at about 0.13 to seconds. The shot record below about 0.25 seconds is dominated by the surface wave noise cone. There are a few additional probable reflections in this record down to about 0.35 seconds, but below that nothing is interpretable. Both of the shot records in Figure 3 are at the intersection of the ML and WW profiles, and therefore represent nearly identical stratigraphic sections. Some reflections seen in both records at about the same time are most likely the same unit. The reflections near the top of the hammer shot record represent features above the reflections seen in the minivib shot record. This is precisely what was expected with the combination of both methods here. 4. Data Analysis 4.1. Velocity Analysis Two different velocity analyses were performed on the minivib profiles. The first, de- scribed in detail in Yilmaz [2001], is an iterative process of determining velocities using 211 constant velocity stacks. This maximizes reflection coherency across CMP gathers by choosing different velocities that are accurate at different depths in the stack. Incorpo- rating the multiple sweeps per source station into the CMP gather allows the multiplicity to assist the accuracy of the constant velocity stacking analysis.

13 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X The other analysis is a Monte Carlo-based estimation process to arrive at an optimized velocity model. This is derived from first arrivals picked off raw shot gathers. A detailed velocity image of the subsurface is obtained by performing a nonlinear optimization on 218 these first arrivals [Pullammanappallil and Louie, 1994]. The optimization employs a 219 simulated annealing algorithm to invert first arrivals for subsurface velocity structure; 220 this is done with the commercial package SeisOpt R. SeisOpt is only suited for two-dimensional array geometries. Because the profiles are dictated by the locations and orientations of the streets, they have bends in them. This is overcome by projecting the source locations to a straight line, while maintaining the true offset of the source-receiver pairs. As a result of this projection, there is some lateral smearing in the optimized velocity model. The two to four source activations at most minivib or hammer source stations do not provide enough multiplicity to overcome much of the urban noise. Summing multiple sweeps of the minivibrator, in particular, did not clarify the first arrivals. It was difficult to pick first arrivals on these records, so there is a shallow limit to the depth of the velocity optimization results Migrations 230 Both velocity models were used to perform prestack depth migrations on the minivib 231 data. The migration with the first velocity model (from constant velocity stacks) is presented in Figure 4b. In order to perform the prestack migration with the optimized velocity model (from first arrivals), the velocities were extended down to a depth of 1.0 km. This was done by finding the maximum constrained velocity value and substituting that for every velocity value below the depth of the maximum velocity, similar to what was done in Pullammanappallil

14 X - 14 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA 237 and Louie [1994], Chávez-Pérez et al. [1998], and Abbott et al. [2001]. The resulting 238 velocity model section was used in the migration algorithm. This particular prestack Kirchhoff summation algorithm is used to image faults as seismic reflectors directly (Figure 4c). It has been used to image the San Andreas fault zone and modified to account for ray bending through lateral velocity variations [Louie et al., 1988; Louie and Qin, 1991; Louie et al., 2011]. The prestack depth migration with the stacking velocity model shows good stratigraphic imaging (Figure 4b). The migration with the optimized velocity model shows much clearer structure at depth, as well as clearer discontinuities (Figure 4c). These migrations complement each other well. Stacking velocities are useful for average velocity analysis at mid-range depths, while the first arrivals are useful for defining lateral velocity heterogeneity at shallow depths. The stacking velocities are more sensitive to depth, with no real indication of velocity variation in the near-surface, while the optimized velocities take into account the shallow velocity variation. The strong reflector is more continuous in Figure 4b than in Figure 4c because of this difference in the velocity models. Typically, a depth migration is useful when velocities vary significantly over a cable length, whereas when velocity contrast is reasonably small, it is not necessary [Hill and Rüger, 2008]. However, a prestack depth migration with an optimized velocity model assures that structures appear at their true depths [Chávez-Pérez et al., 1998; Louie et al., 2011]. The reflectors seen in the depth migrations (Figure 4b and c) are not as continuous as they are in the time migration (Figure 4a). These discontinuities indicate probable faults. There are slight data losses in the depth migration from constant velocity stacks, especially at about 700 m, 1200 m, and 1700 m line distance. These data losses could be

15 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X due to stratigraphic discontinuity, but these discontinuities are much more obvious in the depth migration with the optimized velocity model than in the depth migration using the constant velocity stacks. The weight-drop profiles were post-stack migrated. Due to the shallow depth of investigation (about 300 m) of the weight-drop source, a single velocity was used for migration. This was a constant 1100 m / s. This simple migration worked well for each of the weightdrop profiles. 5. Interpretations Each of the reflection profiles acquired has something to offer in terms of seismic stratigraphy and structure. The ML, M1, and M2 profiles provide images of several previouslymapped faults, as well as additional faults that were not seen before. The crossline profiles, PS and WW, also provide images of both known and unknown faults. The weight-drop profile along the Truckee River in downtown Reno exhibits a fault that had not been noted before East-West Profiles There were a total of three lines were acquired along the same profile: ML, M1, and M2. The higher resolution in the weight-drop profiles allows for better interpretation of the parts of the minivib profile where they overlap (Figure??). Reflectors in the weightdrop profiles correspond to reflectors in the minivib profile, only with slightly different amplitudes, due to source and survey acquisition differences, and processing differences. The strongest reflector in the section is the orange line in Figure??. The depth of this line correlates well with depth to bedrock estimates from gravity measurements by Abbott

16 X - 16 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA and Louie [2000], Widmer et al. [2007], and Cashman et al. [2012]. The average of these estimates [Dhar et al., 2010] is presented as a green dashed line in Figure??. Because of the high amplitude of this feature and its correspondence to the bedrock estimates, this reflector is interpreted to be the top of the Tertiary volcanic section (Ta3 in Figure 2), with Neogene and Quaternary alluvial sediments making up the upper section of the reflection profile. This is consistent with the stratigraphy of this basin described by others [e.g., Trexler et al., 2000, 2012]. The key to the ML profile is the fault that aligns with the west side of a significant high gravity anomaly that splits the Truckee Meadows basin into two sub-basins, with a structural saddle with an axis striking approximately north-northeast [Widmer et al., 2007; Cashman et al., 2012]. Within the western sub-basin, there is another local gravity minimum centered north of the Manzanita Lane profile, almost directly north of line distance 1825 m. The eastern edge of the anomaly is directly north of line distance 2675 m and seems to correspond to the fault indicated with the arrow in Figure 2. This fault splays southward into three different faults, two of which cross the ML profile (D and E in Figure 2). Both of these faults are seen in the depth sections (Figure??) at about 2405 m and 2620 m line distance, respectively. In addition to these faults, we have imaged the farthest western of these fault splays at about 2320 m, which is about where it should cross this profile if it were to continue striking southward (J in Figure 7). All three of these faults dip westward, at 72, 60, and 68, from west to east. The westward dips are consistent with the location of the gravity low to the west. The fault at about 2320 m (J) has more offset than the other two splays of this fault, which suggests that it is the main strand of the fault; the other two are interpreted to be minor splays.

17 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X There are also normal faults in this section to the east along the profile. These are consistent with the previously-mapped faults [US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008; Odum et al., 2011]. Three of these faults (G, H, and K in Figures 2 and 7) are interpreted at about line distances 2935 m (dipping 75 east), 3130 m (dipping 72 east), and 3400 m (dipping 67 east). They seem to correspond with the edge of the eastern sub-basin, east of Highway 395, seen in gravity maps [Widmer et al., 2007; Cashman et al., 2012]. Similar to the west-dipping faults just west, these seem to be splays of a fault; this one is south of the profile. The fault at 2935 m (G) has more offset than the others, also implying it is the main strand of this fault. However, there are two other inferred fault traces at line distances 2790 m and 3655 m (F and I in Figure 2) that were not interpreted on this profile in either depth migration. The strong reflector is continuous at 2790 m, so it is possible the vertical offset on this fault splay (F) is so small it is not resolvable [Widess, 1973; Zeng, 2009]. Given the predominant wavelengths in the depth migrations, a vertical offset of 5-10 m would not be visible. The fault at 3655 m line distance (I) is too close to the east end of the profile; there is not enough fold here to interpret anything with certainty. There are three other inferred fault traces crossed by the western side of the ML profile [US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008]. Two of these, B and C in Figure 2, correspond to discontinuities in the strong reflection off the top of the volcanics at line distances 1470 m and 1575 m, respectively (see Figure??). However, it is difficult to determine the direction of dip and the extent/depth of the faults. The inferred fault at line distance 375 m (A in Figure 2) is unclear as well, most likely due to the shallow depth of bedrock on this part of the profile, and therefore

18 X - 18 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA increased scattering of energy. But the fault does exist, dipping to the west, based on field observation Sidewall Reflections Figure 3 shows some unusual arrivals at about 1 s and 1.5 s at the far offset in the ML minivib shot record. These are evident in most shot records from line distance 3085 m through to the eastern end of the line. The amplitudes of these events are comparable to those of direct arrivals, but the events are not hyperbolic, like typical reflections. Similar events were observed by Robinson [1945] on the Gulf Coast and by Louie et al. [1988] near the San Andreas fault. These arrivals are hypothesized to be refraction waves horizontally reflected from a steeply-dipping fault interface. The same interpretation is made here, but because the arrivals are not hyperbolic, they are interpreted to be reflections off the fault 337 interface at or near the surface. There are faults in this area, and from the multiple such reflected refraction waves indicated on Figure 6, multiple different steeply-dipping faults can be interpreted. This is verified by calculating velocities of each of these arrivals. The first arrival, or P-wave refraction, is about 1960 m / s. The next arrival (denoted S 341 in Figure 6) has a velocity of about 1215 m / s. Assuming a Poisson solid, this arrival is an S-wave refraction, and it follows the P-wave refraction. The reflected refractions described above have a velocity of about 1215 m / s as well. Therefore, they too are S-wave refractions, being horizontally reflected. Tracking the locations of the arrivals in different shot records allows for determination of the location of the fault reflector. The arrival at 1 s at the far offset in Figure 3 moves down in the records as the line distance increases to the east. Therefore, the source is moving farther away from the fault as it moves east. Finding where this reflection crosses

19 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X the direct arrival will indicate where this fault projects onto the surface. This reflection is the green dashed line in Figure 6a, and its intersection with the direct arrival projects to about line distance 2910 m at the surface. This corresponds well to the fault that was previously mapped and interpreted in this paper at about line distance 2935 m [G; US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008]. The arrival at 1.5 s at the far offset in Figure 3 actually moves up in the records as the source moves east, so the source is moving closer to the fault. Unfortunately, the ML profile did not extend far enough east to find a similar reflection/direct arrival intersection, but extrapolation can give an estimate of the surface projection (orange dashed line in Figure 6b). This results in a fault at about 4400 m line distance, and its location is indicated L in Figure 7. There is another reflection (yellow dashed line in Figure 6) that projects to about 2600 m line distance. This corresponds well with the inferred fault E about line distance 2620 m [Figure??; US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008]. In inspection of the M1 and M2 weight-drop shot records, no unusual arrivals like those described above were recognized. This may be due to the short length and shorter offset of the profiles. The fault at line distance 2935 m (G) corresponds to the location of the first shot point of the M2 profile, and therefore it is not expected to be seen as one of these peculiar events. The M1 profile is much shorter, and there is not enough acquisition offset to expect to see any such events North-South Crossline Profiles The faults creating the sidewall reflections strike about north-south, so sidewall reflec- tions are not expected in the north-south PS and WW weight-drop profiles. Because the

20 X - 20 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA fault strikes vary, at least two faults are expected in the PS profile (the western northsouth profile in Figure 2), and at least one is expected in the WW profile (the eastern north-south profile in Figure 2). Both of these migrated profiles are presented in Figure??. Because of the orientation of these profiles, the faults should be dipping to the north in apparent dip. The WW weight-drop profile (Figure??a) exhibits some interesting reflections. This profile is in close proximity of two different faults, and approximately parallels the strike of both of these (G and H in Figure 2). From about line distance 225 m to 750 m, the profile is parallel to G, and the profile is parallel to H from about line distance 600 m to the northern end of the line. Because of the fault dips in the minivib profile, these should also be steeply-dipping faults. Although no sidewall reflections were seen in the weight-drop shot records, the unusual character of the reflectors in the migrated section allows interpretation of the reflections as sideswipe [e.g., McCarthy et al., 1988; Kent et al., 1997]. A line reflector can produce much stronger and more continuous sideswipe reflections than can a steeply-dipping fault plane. Such structures generate strong diffractions at a large range of angles, instead of the limited specular reflections that a steeply dipping fault plane can produce by itself. In this hypothesis, the depth axis in Figure??a could be interpreted as distance horizontally along the surface west of the profile, and in particular, to the steeply-dipping faults. One fault interface is interpreted as the strong reflector near the top of the section from the southern end of the line to about 875 m line distance. From the southern end to about 750 m (Point 1 in Figure??a), the fault trace is about 25 m west of the WW profile. At about 750 m, it begins to dip, but is still a continuous reflector. This indicates that the

21 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X fault turns and continues west of the WW profile, and the last place it is seen is about 60 m away at 875 m line distance before the reflector disappears. This is consistent with the previous mapping [US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008] showing that this fault is about 63 m west horizontally from the WW profile at this location. It is likely that the same fault is the cause of the discontinuities from 850 m to 975 m line distance, and we are now imaging it obliquely, because it is so far off the section and/or is no longer parallel to the profile. This dip is northerly, consistent with the same fault (H) that appears in the ML minivib profile at 3130 m (Figure??). This profile approximately parallels another fault, and this fault is also present as a 403 sideswipe reflection in the section. This is the same fault (G) seen in the ML profile at 2935 m (Figure??). In this WW profile, at about 600 m line distance (Point 2 in Figure??a), it is about 250 m west, again using the depth axis. It ramps up the section to 175 m west at 1275 m line distance. Again, this is consistent with previous mapping [US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008], indicating this fault is about 180 m from the WW profile at the end of the line, and about 245 m from it further south. This fault reflection is not seen south of line distance 600 m, probably because it is so far away from the profile. In addition to the two known faults seen in sideswipe in this profile, there is another fault plane reflection. This new sideswipe fault (Point 3 in Figure??a) is more or less coincident with the reflector that is the top of the volcanic section, from the southern end of the line to about 975 m line distance. Then the fault plane cuts right through the volcanic reflector, and it is picked up north again at about 1050 m line distance. This is where it is indicative of the top of the volcanic section, as the seismic facies change from

22 X - 22 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA layered and continuous above to discontinuous and longer wavelengths below. Therefore interpretation of both stratigraphy on the vertical scale and fault plane reflections on the coincident, horizontal scale is possible. The stratigraphy continues through the fault reflections uninterrupted both here at 125 m and at 50 m. The fault plane reflection dips at 975 m line distance from about 140 m to about 155 m at 1125 m line distance, which indicates the fault is moving farther away from the profile. From here, the interpretations are multiple. This fault (M in Figure 7) may continue into the same fault that is reflected at 175 m, or it could be where the upper fault reflection becomes parallel to the profile again. Unlike the WW profile, the PS profile (Figure??b) does not display the same continuity of stratigraphy through fault planes. Here, interpretation is only of the vertical scale as depth, and not as coincident distance west or east. This profile does, though, reveal the faults expected. The fault at about line distance 381 m is the same fault that appears in the ML profile at about line distance 2620 m (E). At this point, this fault strikes almost perpendicular to the profile, so the dip in the section of 50 is approximately northwest. At 981 m line distance, another fault is indicated. This is the major through-going fault that was referred to above, with the arrow in Figure 2, and is J in Figure 7. This is the north-northwest-dipping fault that horsetails into the two faults on the ML profile at about 2320 m (D) and 2405 m (E) line distance. The fault is not perpendicular to the profile here, so it is only the apparent dip that is 45 north Truckee River Weight-Drop Profile In addition to the four weight-drop profiles in southern Reno, a 900-m profile, BP, was acquired along the bike path next to the Truckee River in downtown Reno. The minivib

23 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X profile along the Truckee River [Stephenson et al., 2012] has a gap due to inaccessibility; 440 the Thumper source could not fit along the bike path in this particular area. BSU s accelerated hammer source, on the other hand, could fit. This BP weight-drop profile was collected to bridge this gap, and includes some overlap with the minivib profiles (see inset of Figure??). Stephenson et al. [2012] places a fault with large down to the west 444 offset of the top of the Tertiary volcanics (about 90 m) in this area. Any rupture of this west-dipping fault would temporarily dam the east-flowing Truckee River, ponding sediment in the hanging wall to the west and upstream. The footwall, to the east of the fault and downstream, does not receive the ponded sediment. Figure?? shows the depth migration of the BP weight-drop profile. This profile reveals a fault in the same location as Stephenson et al. [2012], about line distance 350 m. There is a significant change in reflection character from the west side of the fault to the east side. The west side depicts finely-layered and continuous sediments, probably sand layers deposited at a higher rate of sedimentation, and the east side is longer wavelength and more discontinuous, probably older alluvium and bouldery outwash deposits, with a very low recent sedimentation rate. The fault produces a great change in depositional environment, so correlation of any reflections from one side of the fault to the other is impossible. A key observation from Figure?? is the shallowest, youngest reflections in the section, at a depth of only 10 m, are faulted. This observation indicates that this is the most recent, and most active, fault in Reno. The dip on this fault hypothesized by Stephenson et al. [2012] is confirmed. The fault dips in the same west direction and this is consistent with the fault that trends south through Reno and is the same fault (J)imaged on the west side of the basin divide in the ML and PS profiles (Figure 7). The fault dips 53 west

24 X - 24 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA in section, and this is consistent with the gravity maps of the area [Widmer et al., 2007; Cashman et al., 2012]. The gravity slopes from high to low from east to west along this part of the Truckee River. The gravity gradient at the river is shallower than the gravity gradient farther south, which corroborates the shallower dip of this fault here than to the south. 6. Implications In this study, several previously-mapped faults were imaged in south Reno [US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008]. In addition, faults that have not been previously mapped were imaged. These are indicated by the arrows in Figure 7. Table 1 shows these faults, and which of them were imaged, and where, along the ML, PS, and WW profiles. Fault A, at 375 m along the ML profile, was not directly imaged, nor is there an interpretable discontinuity where this fault trace is expected (Figure??). Faults B and C, at 1470 m and 1575 m along the ML profile, were also not directly imaged, but there are indications of discontinuities where the fault traces are expected (Figure??). The rest of the faults on the east side of the line were directly imaged. The fault sideswipe reflections (Faults G and H) in the WW weight-drop profile are also denoted in Table 1, including the reflection that did not correspond to any other previously-mapped faults (Fault M). Each of these faults is a candidate for rupture during an earthquake in the Reno Carson City area, and therefore they ought to be added to the earthquake planning scenario of depolo et al. [1995]. The fault along the ML profile with the most offset occurs at 2320 m (J; Figure??), indicating this is the most active of these faults. This is also the fault that can be traced to the north as the major through-going north-south fault that continues north through the

25 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X suspected graben of Virginia Lake, north toward downtown Reno [US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008], and crosses the Truckee River, as revealed in Figure??. This fault J is now referred to as the Virginia Lake Fault. It is the most active, most recent fault crossed by any of the minivib or weightdrop surveys. Stephenson et al. [2012] corroborate this observation with the minivib lines along the Truckee River. Typical vertical displacement on normal faults in western Nevada is about 0.6 to 2 meters [e.g., Cashman and Ellis, 1994; Wesnousky, 2005]. Because the shallowest, youngest reflections are faulted at only 10 m depth, there have been at least 5 earthquake events, possibly more, on this fault in recent time. The Virginia Lake Fault corroborates the local gravity low described above, and corroborates the larger western sub-basin in the gravity anomaly [Widmer et al., 2007; Cashman et al., 2012]. This sub-basin has been filling with sediment since the mid-tertiary, suggesting that the Virginia Lake Fault has been active for more than 10 million years [e.g., 497 Trexler et al., 2000; Cashman et al., 2012]. The eastern edge of this larger sub-basin stretches more or less from 2000 m north of I-80 to South McCarran Blvd, and follows the same trend as the fault. If the Virginia Lake Fault stretches the entire length of the gravity anomaly, it would continue north from the Truckee River through the University of Nevada, Reno campus (see Figure 7). The near-surface offset on the fault poses significant seismic hazard to the entire downtown area of Reno, as well as the University, and all the homes and businesses located near it. The combination of the gravity and seismic reflection confirms the existence of the westdipping normal fault through the center of the Truckee Meadows basin, and therefore the opposite asymmetry of the Carson and Washoe Valleys [Widmer et al., 2007; Trexler

26 X - 26 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA et al., 2012; Cashman et al., 2012]. The basin-bounding fault is the Virginia Lake Fault, which runs north-south through the center of the basin. This change in the understood geometry of the Truckee Meadows basin will improve the earthquake planning scenario for Reno and Carson City [depolo et al., 1995]. The Virginia Lake Fault also has important implications on its southern end. As noted above, this fault horsetails, or splays into multiple fault strands (D and E). It was also noted that the faults to the east (G, M, H, and K) look to be similar horsetailing faults that dip the opposite direction, toward the east. Horsetailing faults are a favorable setting for geothermal resources Faulds et al. [2006]. These are good conduits for fluid flow [Sibson, 1981], as are intersecting faults [Curewitz and Karson, 1997]. The south end of the Virginia Lake Fault horsetails into several different splays that dip to the west; in the same area, there are several splays of a fault that dip to the east. These splays overlap and intersect, and therefore are favorable for fluid flow and perhaps a geothermal system. In fact, the presence of the Moana geothermal system, in the vicinity of Faults J, D, E, G, H, M, and K, confirms this. In 1984, there were approximately 150 individual home wells in the Moana area, used to provide heat and hot water to residents in southwest Reno [Flynn et al., 1984]. This system now includes the Peppermill Casino, which has been using geothermal energy to heat the domestic hot water since 2007, and currently uses geothermal energy produced on the immediate property to heat the entire hotel, casino, 526 and spa [Jeffers-Peaua, 2009]. The many producing geothermal wells in this system suggest that these horsetailing and intersecting faults in southern Reno are significant in the local economy. 529 Acknowledgments.

27 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X We would like to thank the students and colleagues who helped collect the seismic reflection data: Dave Worley, Zeb Maharrey, Martin Messmer, Iris Tomlinson, Mike Conley, Mahesh Dhar, Betsy Littlefield, Amanda Hughes, Sumant Jha, Kristin Kohls, Amr Wakwak, Sue Konkol, Patricia Capistrant, Troy Christensen, Rob Ghiglieri, Jay Goldfarb, Jason Henderson, Molly Hunsaker, Fariha Islam, Miles Kreidler, Janice Kukuk, Amie Lamb, Ryan Loux, Josh Michaels, Todd Morine, Courtney Murphy, Nick Prina, Zach Reynolds, Garry Richardson, Katie Ryan, Samuel Sanders, Gretchen Schmauder, Nicole Shivers, Justin Skord, Ross Whitmore, Andrew Evans, Brady Flinchum, Ellen Hall- Patton, Megan Hanke, and Jessica Pence. We would also like to thank Robert Kent and Cecil Hoffpauir with nees@utexas for their hand in operating the Thumper and helping plant geophones. Nick Prina, now with Geologica, spent a summer picking first arrivals on quite a bit of this data. ProMAX R used under license at USGS Golden, Colorado, under donation at BSU, and under academic license at UNR. Additional seismic processing and plotting with Louie s JRG package downloaded from crack.seismo.unr.edu/jrg. Research partly supported by the USGS, Department of the Interior, under USGS award numbers G09AP00051 and G09AP 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. References Abbott, R. E., and J. N. Louie, Case History: Depth to bedrock using gravimetry in the Reno and Carson City, Nevada basins, Geophysics, 65, , 2000.

28 X - 28 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA Abbott, R. E., J. N. Louie, S. J. Caskey, and S. Pullammanappallil, Geophysical confirmation of low-angle normal slip on the historically active Dixie Valley fault, Nevada, Journal of Geophysical Research, 106 (B3), , Anderson, J. G., I. Tibuleac, A. Anooshehpoor, G. Biasi, K. Smith, and D. von Seggern, Exceptional ground motions recorded during the 26 April 2009 Mw 5.0 earthquake in Mogul, Nevada, Bulletin of the Seismological Society of America, 99 (6), , Bell, J. W., Quaternary fault map of Nevada: Reno sheet 1:250,000, Bingler, E. C., Reno Quadrangle, chap. Earthquake Hazards Map 1:200, Environmental Folio, Nevada Bureau of Mines and Geology, Briggs, R. W., and S. G. Wesnousky, Late Pleistocene fault slip rate, earthquake recurrence, and recency of slip along the Pyramid Lake fault zone, northern Walker Lane, United States, Journal of Geophysical Research, 109 (B08402), 1 16, Cashman, P., J. H. Trexler, M. C. Widmer, and J. Queen, Post-2.6 Ma tectonic and topographic evolution of the northeastern Sierra Nevada: The record in the Reno and Verdi basins, [in review], Cashman, P. H., and M. A. Ellis, Fault interaction may generate multiple slip vectors on a single fault surface, Geology, 22 (12), , Cashman, P. H., and S. A. Fontaine, Strain partitioning in the northern Walker Lane, western Nevada and northeastern California, Tectonophysics, 326, , Cashman, P. H., J. H. Trexler, T. W. Muntean, J. E. Faulds, J. N. Louie, and G. L. Oppliger, Neogene tectonic evolution of the Sierra Nevada-Basin and Range transition zone at the latitude of Carson City, Nevada, in Late Cenozoic Structure and Evolution

29 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X of the Great Basin-Sierra Nevada Transition, edited by J. S. Oldow and P. H. Cashman, pp , Geological Society of America, Chávez-Pérez, S., J. N. Louie, and S. K. Pullammanappallil, Seismic depth imaging of normal faulting in the southern Death Valley basin, Geophysics, 63 (1), , Curewitz, D., and J. A. Karson, Structural setting of hydrothermal outflow: Fracture permeability maintained by fault propagation and interaction, Journal of Volcanology and Geothermal Research, 79 (3-4), , depolo, C. M., Quaternary faults in Nevada, Nevada Bureau of Mines and Geology Map 167, depolo, C. M., G. L. Johnson, S. L. Jacobson, J. G. Rigby, J. G. Anderson, and T. J. Wythes, Planning scenario for a major earthquake: Reno-Carson City urban corridor, western Nevada, Open File Report 95-2, Nevada Bureau of Mines and Geology, Reno, Nevada, depolo, C. M., J. G. Anderson, D. M. depolo, and J. G. Price, Earthquake occurrence in the Reno-Carson City urban corridor, Seismological Research Letters, 68 (3), , Dhar, M. S., M. Thompson, A. Kell-Hills, J. N. Louie, K. D. Smith, J. Tirabassi, S. Tom, and T. Irwin, Educating a community impacted by an earthquake swarm: 106 volunteers host EarthScope flexible array records during the Mogul, Nevada sequence, Eos Transactions AGU Fall Meeting Supplement, 89 (53), PA13B 1342, Dhar, M. S., M. Thompson, A. Kell-Hills, J. N. Louie, K. D. Smith, and M. C. Widmer, Cross-constraints between station delays, gravity, and reflection for the Reno-area basin floor, Nevada, Seismological Research Letters, 81 (2), 381, 2010.

30 X - 30 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA Eisses, A., A. Kell, G. Kent, N. Driscoll, R. Baskin, K. Smith, R. Karlin, J. Louie, and S. Pullammanappallil, New constraints on slip-rates, recurrence intervals, and strain partitioning beneath Pyramid Lake, Nevada, [in review], Faulds, J. E., M. Coolbaugh, G. Blewitt, and C. D. Henry, Why is Nevada in hot water? structural controls and tectonic model of geothermal systems in the northwestern Great Basin, Geothermal Resources Council Transactions, pp , Faulds, J. E., C. D. Henry, and N. H. Hinz, Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific-North American plate boundary, Geology, 33 (6), , Faulds, J. E., M. F. Coolbaugh, G. S. Vice, and M. L. Edwards, Characterizing structural controls of geothermal fields in the northwestern great basin: A progress report, Geothermal Resources Council Transactions, 30, 69 76, Flynn, T., G. Ghusn, and D. T. Trexler, Geologic and hydrologic research on the Moana geothermal system Washoe County, Nevada, Geo-Heat Center Quarterly Bulletin, 8 (2), 3 6, Gallagher, P. D., W. C. Fugate, S. Suresh, J. Zients, J. P. Holdren, and M. McNutt, Annual report of the National Hazards Reduction Program (NEHRP) for fiscal year 2009, Annual report, NEHRP, Hammond, W. C., G. Blewitt, and C. Kreemer, Block modeling of crustal deformation of the northern Walker Lane and Basin and Range from GPS velocities, Journal of Geophysical Research, 116 (B04402), 1 28, Henry, C. D., and M. E. Perkins, Sierra Nevada-Basin and Range transition near Reno, Nevada: Two-stage development at 12 and 3 Ma, Geology, 29 (8), , 2001.

31 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X Henry, C. D., J. E. Faulds, and C. M. depolo, Geometry and timing of strike-slip and normal faults in the northern Walker Lane, northwestern Nevada and northeastern California: Strain partitioning or sequential extensional and strike-slip deformation?, Geological Society of America Special Papers, 434, 59 79, Hill, S. J., and A. Rüger, Illustrated seismic processing for interpreters: Essentials, potential and pitfalls, seg Introduction to Seismic Processing course notes, Ichinose, G. A., K. D. Smith, and J. G. Anderson, Moment tensor solutions of the 1994 to 1996 Double Spring Flat, Nevada earthquake sequence and implications for local tectonic models, Bulletin of the Seismological Society of America, 88 (6), , Jeffers-Peaua, J., The Peppermill s big gamble on green pays off, press release: Kachadoorian, R., R. F. Yerkes, and A. O. Waananen, Effects of the Truckee, California earthquake of September 12, 1966, Geological Survey Circular 537, United States Geological Survey, Washington, D.C., Kelly, T. S., and R. Secord, Biostratigraphy of the Hunter Creek Sandstone, Verdi Basin, Washoe County, Nevada, in Late Cenozoic Structure and Evolution of the Great Basin- Sierra Nevada Transition, edited by J. S. Oldow and P. H. Cashman, no. 447 in Special Paper, pp , Geological Society of America, Kent, G. M., R. S. Detrick, S. A. Swift, J. A. Collins, and I. I. Kim, Evidence from Hole 504B for the origin of dipping events in oceanic crustal reflection profiles as out-of-plane scattering from basement topography, Geology, 25 (2), , 1997.

32 X - 32 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA Louie, J. N., Workshop on Nevada 3-D seismic community velocity models, Final technical report, United States Geological Survey, NEHRP, External Research Program, earthquake.usgs.gov/research/external/reports/08hqgr0015.pdf, Louie, J. N., and J. Qin, Subsurface imaging of the Garlock Fault, Cantil Valley, California, Journal of Geophysical Research, 96 (B9), 14,461 14,479, Louie, J. N., R. W. Clayton, and R. J. LeBras, Three-dimensional imaging of steeply dipping structure near the San Andreas Fault, Parkfield, California, Geophysics, 53 (2), , Louie, J. N., S. Pullammanappallil, and W. Honjas, Advanced seismic imaging for geothermal development, in Proceedings of the New Zealand Geothermal Workshop 2011, 32, Auckland, New Zealand, McCarthy, J., J. C. Mutter, J. L. Morton, N. H. Sleep, and G. A. Thompson, Relic magma chamber structures preserved within the Mesozoic North Atlantic crust?, Geological Society of America Bulletin, 100 (9), , Odum, J. K., W. J. Stephenson, R. N. Frary, G. C. Schmauder, and J. N. Louie, Shallow 3.5 km long, shear-wave velocity profile in Reno, Nevada derived from ambient noise analysis of uncorrelated P-wave minivib data, Seismological Research Letters, 82 (2), 299, Pullammanappallil, S. K., and J. N. Louie, A generalized simulated-annealing optimization for inversion of first-arrival times, Bulletin of the Seismological Society of America, 84 (5), , Ramelli, A. R., J. W. Bell, C. M. depolo, and J. C. Yount, Large-magnitude, late Holocene earthquakes on the Genoa Fault, west-central Nevada and eastern California, Bulletin

33 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X of the Seismological Society of America, 89 (6), , Robinson, W. B., Refraction waves reflected from a fault zone, Geophysics, 10 (4), , Sanders, C. O., and D. B. Slemmons, Geomorphic evidence for Holocene earthquakes in the Olinghouse fault zone, western Nevada, Bulletin of the Seismological Society of America, 86 (6), , Schweickert, R. A., M. M. Lahren, R. Karlin, J. Howle, and K. Smith, Lake Tahoe active faults, landslides, and tsunamis, in Great Basin and Sierra Nevada: GSA Field Guide 2, edited by D. R. Lageson, S. G. Peters, and M. M. Lahren, pp. 1 22, Geological Society of America, Boulder, Colorado, Sibson, R. H., Earthquake Prediction: An International Review, Maurice Ewing, vol. 4, chap. Fluid flow accompanying faulting: Field evidence and models, pp , American Geophysical Union Monograph, Silberman, M. L., D. E. White, T. E. C. Keith, and R. D. Dockter, Duration of hydrothermal activity at Steamboat Springs, Nevada, from ages of spatially associated volcanic rocks, Professional Paper 458-D, United States Geological Survey, Washington, D.C., Slemmons, D. B., Geological effects of the Dixie Valley-Fairview Peak, Nevada, earthquakes of December 16, 1954, Bulletin of the Seismological Society of America, 47 (4), , Smith, K. D., D. H. von Seggern, D. depolo, J. G. Anderson, G. P. Biasi, and R. Anooshehpoor, Seismicity of the 2008 Mogul-Somersett west Reno, Nevada earthquake sequence, Eos Transactions AGU Fall Meeting Supplement, 89 (53), Abstract S53C 02, 2008.

34 X - 34 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA Stephenson, W. J., R. N. Frary, J. N. Louie, and J. K. Odum, Extensional growth faulting beneath Reno, Nevada imaged by urban seismic profiling, [in review], Stewart, J. H., Basin and Range structure: A system of horsts and grabens produced by deep-seated extension, Geological Society of America Bulletin, 82 (4), , Stewart, J. H., Tectonics of the Walker Lane belt, western Great Basin: Mesozoic and Cenozoic deformation in a zone of shear, in The Geotectonic Development of California, edited by W. G. Ernst, pp , Prentice-Hall, Edgewood Cliffs, New Jersey, Stewart, J. H., and J. E. Carlson, Geological Map of Nevada 1:500,000, U.S. Geological Survey and Nevada Bureau of Mines and Geology, Trexler, J., P. Cashman, and M. Cosca, Constraints on the history and topography of the northeastern Sierra Nevada from a Neogene sedimentary basin in the Reno-Verdi area, western Nevada, [in press], Trexler, J. H., P. H. Cashman, C. D. Henry, T. Muntean, K. Schwartz, A. TenBrink, J. E. Faulds, M. Perkins, and T. Kelly, Neogene basins in western Nevada document the tectonic history of the Sierra Nevada-Basin and Range transition zone for the last 12 Ma, in Great Basin and Sierra Nevada: GSA Field Guide 2, edited by D. R. Lageson, S. G. Peters, and M. M. Lahren, pp , Geological Society of America, Boulder, Colorado, United States Census Bureau, State of Nevada QuickFacts, quickfacts.census.gov, US Geological Survey, and Nevada Bureau of Mines and Geology, compiled Fault and Fold Database of the United States, earthquake.usgs.gov/hazards/qfaults, Wesnousky, S. G., Active faulting in the Walker Lane, Tectonics, 24 (TC3009), 1 35, Widess, M. B., How thin is a thin bed?, The Leading Edge, 38 (6), , 1973.

35 Figure 1. Map of the Reno metropolitan area, showing generalized geology (Stewart and Carlson, 1978). Geologic units are: JTRsv- late Triassic to early Cretaceous sedimentary and volcanic deposits; Kgr- Cretaceous granitic rocks; Tri- Eocene to Miocene rhyolitic intrusive rocks; Ts3- late Eocene to late Miocene tuffaceous sedimentary rocks; Ta3- late to middle Miocene andesitic rocks; Tba- early Miocene to early Pliocene andesitic and basaltic flows; QTb- Miocene to Quaternary basalt flows; Qa- Quaternary alluvial deposits. The dark blue lines are recent faults (US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008) and the red lines T1, T2, and ML are the mini vibroseis seismic reflection profiles. Dashed purple box is the area of Figure 2.

36 Nevada 500 m N Tri Ta3 Kgr Figure area JTRsv Tba McCarran Blvd T1 T2 I-80 Truckee River Ts3 Qa ML US 395 QTb

37 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA X - 35 N 500 m Ts A ML B C PS M2 D E McCarran Blvd WW M1 F G H I Figure 2. Ta Qa Map of the Manzanita Lane area profiles. Area corresponds to purple outline in Figure??. Geologic units are: Ts3- late Eocene to late Miocene tuffaceous sedimentary rocks; Ta3- late to middle Miocene andesitic rocks; Qa- Quaternary alluvial deposits [Stewart and Carlson, 1978]. The dark blue lines are recent faults [US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008], red line ML is a mini vibroseis seismic reflection profile, green lines M1, M2, PS, and WW are the accelerated weight drop seismic reflection profiles. Letters A-I correspond to faults indicated in text. Arrow indicates the most prominent, continuous fault in this area Widmer, M. C., P. H. Cashman, F. C. Benedict, and J. H. Trexler, Neogene through quaternary stratigraphy and structure in a portion of the truckee meadows basin: A record of recent tectonic history, in Cordilleran Section - 103rd Annual Meeting (4-6 May 2007), Geological Society of America: Cordilleran Section, Yilmaz, Ö., Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data, 2nd ed., Society of Exploration Geophysicists, Tulsa, Oklahoma, Zeng, H., How thin is a thin bed? An alternative perspective, The Leading Edge, 28 (19), , 2009.

38 X - 36 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA 0.0 ML minivib Shot and receiver spacing 5 m WW weight drop Shot and receiver spacing 3 m Time, s Figure 3. Example shot records acquired by the two different methods in this study. Both records are at the intersection of the ML and WW profiles. Neither records has any processing applied. Both records are plotted at the same scale. D R A F T May 11, 2012, 4:50pm D R A F T

39 FRARY ET AL.: FAULT IMAGING IN RENO, NEVADA Line Distance, m X Time, ms a Line Distance, m Depth, m b Line Distance, m Depth, m c 800 Figure 4. Migrations of the Manzanita Lane minivib seismic re ection pro le. a: Post-stack nite-di erence time migration from constant velocity stacks velocity model. b: Prestack depth migration from constant velocity stacks velocity model. c: Prestack depth migration from a rst arrival optimized velocity model Line Distance, m S 3575 Figure 6. Line Distance, m S S 0.5 Time, s Time, s a S 2.0 b ML shot records exhibiting re ected refraction waves. a: Record from 3100 m line distance, b: Record from 3715 m line distance. Yellow, green, and orange dashed lines follow the re ections indicated in text. Shear waves denoted with yellow S. D R A F T May 11, 2012, 4:50pm D R A F T

40 Figure 5. Interpreted depth sections of Manzanita Lane minivib seismic reflection profiles, overlain on these are the weight-drop profiles acquired along the same line. The tops of the highresolution sections are set at the appropriate ground surface elevation in the minivib section. a: Migration from constant velocity stacks, b: Migration from first arrival optimized velocity model. Dashed green line is average of depth of bedrock from gravity (Abbott and Louie, 2000; Widmer et al., 2007; Cashman et al., 2012; Dhar et al., 2010). Red lines are interpreted faults. Orange line is strongest reflector, believed to be the top of Tertiary volcanics. Blue arrows are the inferred fault traces from the USGS Fault and Fold Database (US Geological Survey and Nevada Bureau of Mines and Geology, 1999; depolo, 2008).

41 Line Distance, m Depth, m a Line Distance, m Depth, m b