Imaging Extensional Versus Strike-Slip Tectonics in the Northern Walker Lane

John Louie, Graham Kent, and Kenneth D. Smith Nevada Seismological Laboratory, University of Nevada, Reno Patricia Cashman and James Trexler Dept. of Geological Sciences and Engineering, University of Nevada, Reno

PROJECT DESCRIPTION

1 TABLE OF CONTENTS Introduction and Intellectual Merit ...... 1 Proposed Work ...... 6 Seismic-reflection surveying...... 6 Microearthquake (MEQ) recording...... 11 Expected Results and Broader Impacts ...... 11 Compliance with EAR Data Policy...... 14 Results of Previous NSF support in the last 5 years: G. M. Kent...... 14 Results of Previous NSF support in the last 5 years: J. H. Trexler and P. Cashman...... 15

Introduction and Intellectual Merit The Walker Lane, a zone of complex faulting in the western Great Basin adjacent to the Sierra Nevada, currently accommodates 20%-25% of the Pacific-North American plate relative mo- tion (e.g., Dixon et al., 1995; Thatcher et al., 1999), and may be an incipient intracontinental trans- form fault (Faulds et al., 2005a, b; Faulds and Henry, 2006). Accommodation of the plate boundary shear changed with the opening of the Gulf of California about 6 Ma (Oskin and Stock, 2003). The Walker Lane has been divided into structural domains characterized by (1) east-northeast-striking faults with left-lateral displacement; and (2) northwest-striking faults with right-lateral displacement (Stewart, 1988) (Fig. 1). The Walker Lane (as traditionally mapped) intersects the Sierran frontal system of normal faults in the vicinity of Reno, Nevada. The Neogene sedimentary basins of north- west Nevada and northeast California record the development of intra-plate deformation along the eastern side of the Sierra Nevada (e.g., with geometries as estimated geophysically by Saltus and Jachens, 1995). The earthquake hazards presented to the people and the economy of the Reno–Tahoe–Car- son City urban area crucially depend on fault length, segmentation, and event mechanism. Among the swarm of faults cutting the region (fig. 1), which present the greatest hazard? Strike-slip faults spanning several basins may be expected to rupture in longer segments and larger events than geo- metrically complex normal faults that are contained to the boundaries of one basin (Wesnousky, 2008). Our lack of knowledge of the region’s tectonic framework, and how faults may or may not link up to break in large events (up to Richter magnitude 7.5), call into question any attempts to as- sess the earthquake hazard or risk. The Genoa fault (Pease, 1979) and the Mount Rose fault zone (Sawyer, 1999) form the Sier- ra range-front fault between Carson Valley and Reno (fig. 1). These two north-striking faults are poorly characterized with respect to their components of right-lateral versus normal displacement history and future potential. The Genoa fault is a normal fault that is thought to have a minor com- ponent of dextral motion (Pease, 1979; Surpless, 1999). With its curved trace, tilt fanning in the Neogene section, and largest displacement in the middle section, the Genoa fault itself must have almost exclusively dip slip. Total vertical offset, based on gravity, is on the order of 3.2 to 3.7 km and has been on-going since at least Pliocene time, based on tilt fanning of the 2 Ma - 7 Ma Neo- gene section (fig. 2; Cashman et al., 2009). A 700-m-long seismic reflection survey hinted at the depths and dip of major stratigraphic transitions in the center of the basin (fig. 3), but was too limit- ed in extent to fit the reflections into any structural framework. Trenching studies document two late Holocene, large-displacement events on the Genoa fault (Ramelli et al., 1999).

2 Figure 1: (left) Regional location map taken from Cashman et al. (2009), showing the Neogene basins along the Sierra Nevada –Basin and Range transition zone. Map modified from Cashman and Fontaine (2000). Light gray is the Walker Lane proper; dark gray is outcrop areas of Neogene sedimentary rocks. Buried Neogene sedimentary rocks more extensive than these surface exposures. “Northern Walker Lane”, as used herein, contains the Pyramid Lake domain (characterized by northwest-striking dextral faults) and Carson domain (characterized by northeast-striking sinistral faults), and part of the Walker Lake domain (characterized by northwest-striking dextral faults), of Stewart (1988); and is extended here to include the basins east of Lake Tahoe. (right) Google map of the northern Walker Lane showing faults from the USGS Qfaults database (USGS et al., 2006; fault age keyed to line color with warmer colors more recent), the locations of the TRK and MNZA reflection profiles of Frary et al. (2009; 2010) as white lines, the paleoseismic trench and high-resolution profile presented by Kell-Hills et al. (2010b) at the yellow pin by MRFL, and the two reflection survey routes proposed here across the South Reno basin and Carson Valley, in pink.

Ramelli et al. (1994) explored selected Mt. Rose fault traces and evaluated the potential for past ruptures, also proposing some of the rupture to be dextral strike slip. This fault system, on the south end of the Reno-area basin, is enormously complex with a large number of short segments of unknown age and activity (fig. 1). A similar swarm of short, straight fault traces can also be seen on the eastern side of Carson Valley in fig. 1. Whether these faults might generate normal or strike-slip events is unknown. Since these traces could bring rupture from the Genoa – Mt. Rose system into downtown Reno, it is important to clarify the nature of their activity.

3 Figure 2: Gravity profile and geology model along the proposed Carson Valley reflection line (fig. 1). The two-dimensional modeling assumed basement rocks as Mesozoic plutonic and metamorphic units averaging 2.75 g/cc, upper basin fill as Quaternary and Neogene sediments averaging 2.3 g/cc, and deeper basin fill as sediments averaging 2.4 g/cc. Note that the fanning of dips in the Neogene section (angles shown above cross-section) record progressive tilting along the bas- in-bounding fault through deposition (approx. 7 Ma - 2.5 Ma). Taken from Cashman et al. (2009).

Active seismicity fails to confirm the preference for normal-slip on the Genoa – Mt. Rose fault system. Most of the recorded seismicity has been shown to be strike slip. Studies of Tahoe- area tectonics (Schweickert et al., 2004) and the Double Springs Flat earthquake (Ichinose et al., 1999) concluded that the major normal faults showed little seismicity, with strike-slip faulting in the step-overs between the normal-fault systems dominating the activity along the eastern Sierra. Recent observations from the Reno, Nevada area from a new paleoseismic trench, and new seismic-reflection surveys show active tectonics that are hard to fit into this accepted framework for the Walker Lane region. The new trenching and shallow seismic imaging of the obvious range-front fault in the Mount Rose zone shows a low-angle normal fault (fig. 4). This surprising result, pre- sented by Kell-Hills et al. (2010b), followed paleoseismic trenching of the fault (at the MRFL on fig. 1) in Sept. 2009 by A. Sarmiento and S. Wesnousky. This discovery, proved by trench observa- tion coupled with a Nov. 2009 high-resolution seismic study, demands new thinking about the tec- tonics of the Genoa fault system as it approaches Reno from the south.

4 Figure 3: Short reflection profile recorded near the middle of the proposed Carson Valley profile (fig. 1), interpreted at right. This unmigrated stacked section is plotted at approximately 1:1 verti- cal exaggeration for the velocities found by NMO analysis. The interpretation (right) identifies three west-dipping reflections as deep as 1000 m. Their dips increase with depth and age, consis- tent with the fanning of dips seen in surface exposures (Fig. 2). At the left are the depth, stacking velocity and dip interpreted for each reflection. Loss of fold near the ends of this very short (720 m) profile allows interpretation only within its center. Taken from Cashman et al. (2009).

The observations of Cashman et al. (2009) and Kell-Hills et al. (2010) point to the need for seismic-reflection imaging of the two principal Tertiary-to-Quaternary sedimentary basins associat- ed with the Genoa fault and the Mount Rose fault zone, to characterize the displacement histories of these faults. We propose to conduct two full-scale seismic-reflection studies of the basins along two west-east routes about 7 km long each (fig. 1). The surveys will reveal the internal stratigraphy and deformation of these two basins, and allow separation of Tertiary from Quaternary displacement histories. This investigation of faulting history will test hypotheses of westward propagation of Basin and Range extension (e.g., Dilles and Gans, 1995; Surpless et al., 2002), northward propagation of dextral slip related to changes in the plate boundary configuration (e.g., Faulds et al., 2005a), as well as specific questions about fault geometry and displacement related to accommodation zones between major fault systems. Most of the region’s Tertiary basin fill is hidden below Quaternary deposits. Seismic imaging of faulting and stratigraphy within the basins is the only practical way to acquire additional tests of this hypothesis. The basins likely record deformation history since the mid-Tertiary, and should show the shift in style from mid-Tertiary east-west extension to the cur- rent trans-tension, as proposed by Faulds et al. (2005a). To investigate faulting in the northernmost of the basins along the Genoa – Mt. Rose fault system, a collaboration between UNR, Boise State, the USGS, and nees@UTexas performed minivibe and hammer surveys of the basin below the Reno urban area under USGS sponsorship (white lines at MNZA and TRK on fig. 1, right). Frary et al. (2009; 2010) and Louie et al. (2009) showed that the June 2009 minivibe surveys achieved clear stratigraphic and fault imaging despite urban noise. The Reno-area basin had previously been characterized from geothermal-well and gravity data by Abbott and Louie (2000) and Widmer et al. (2007), demonstrating that no more than 200 m of Quaternary sediments overlie up to 1.5 km of Tertiary basin fill. 5 Figure 4: velocity optimization and prestack depth migration of high-resolution reflection survey along paleoseismic trench across the Mount Rose fault (at MRFL in fig. 1), presented by Kell-Hills et al. (2010b). The low-angle normal fault observed in the trench (shown in yellow) can be traced to 40 m depth where it cuts the slip surface of an older landslide (shown in red).

Direct fault imaging has been essential in the Reno-area basin, to see faults that do not offset basement significantly. Fig. 5 from Kell-Hills et al. (2010a) shows one of hundreds of raw shot records from the MNZA line exhibiting negative-moveout reflections, and a section of our imaging to date that may be showing fault-plane reflections. The negative-moveout reflections in the shot records appear to be sidewall bounces of direct energy off the shallow fault zone (one of the Mt. Rose strands). Such strong sidewall reflections have been observed rarely, such as in COCORP shot records across the San Andreas fault at Parkfield and across the Garlock fault, in California. Louie et al. (1988) imaged the San Andreas reflections as steeply dipping structure bounding the active fault zone, to 3 km depth. Louie and Qin (1991) imaged the steeply dipping walls of a transtension- al pull-apart basin along the Garlock fault. On the MNZA line, the sidewall reflections migrate into east-dipping structures at either 70° or 30° dip (Kell-Hills et al., 2010a). Additional analysis will tell which dip is more likely for this fault, and thus whether it is likely to have much of a strike-slip component of motion. A strike-slip component is unlikely for a shallow-dipping normal fault. We conducted further work with L. Liberty of Boise State in March 2010, doing high-resolution ham- mer reflection surveys in the area of the fault sidewall reflections along the MNZA line. Preliminary results (not shown) confirm the locations of possible faults seen in June 2009, and the depth of the volcanic flows at the bottom of the sediments. Figure 6 shows the result of first-arrival picking, velocity optimization, and prestack depth migration by Kell-Hills et al. (2010a) on a section of the TRK lines, in downtown Reno (fig. 1). Clearly imaged interruptions of east-dipping stratigraphy, right up to the surface, suggest west-dip- ping normal faulting accompanied by eastward tilting in the Quaternary and older section. This is consistent with surface exposures and gravity models west of Reno (Bell and Garside, 1987; Trexler et al., 2000; Abbott and Louie, 2000; Trexler and Cashman, 2007), but is the opposite of fault-relat- ed tilting in Carson Valley (Cashman et al., 2009). However, such a significant and recent west-dip- ping normal fault does not fit well with traditional thinking that the Genoa – Mt. Rose system is the range-bounding fault on the east side of the Sierra Nevada. Reversals in the sense of subsidence from east to west along this fault system point to more complex tectonics.

6 Fig. 5: (left) Correlated shot record from the east end of the MNZA survey (fig. 1), showing possible linear sidewall reflections originating at mapped faults, similar to those see by Louie et al. (1988) from the San Andreas fault at Parkfield, Calif. (right) Preliminary velocity optimization and prestack depth migration (PSDM) results from this part of MNZA. The vertical scale is depth in meters in this 1:1 section. East is to the right. From Kell-Hills et al. (2010a). The basin floor of Tertiary volcanics is cut and displaced by a fault in the northern segment of the Genoa – Mt. Rose system. Advanced imaging has located two possible downdip paths for the fault, at either 30° and 70° dip, leaving open the question of whether this important fault has predominantly dextral strike slip and is steeply dipping, or predominantly normal slip at shallow dip. Presented by Kell-Hills et al. (2010a).

Proposed Work Seismic-reflection surveying– Optim Inc., a seismic acquisition firm with an established record of basin and fault imaging in Nevada will perform 14.8 line-kilometers of innovative 2D sur- veying. The UNR group, together with Honjas and Pullammanappallil at Optim, have been directly imaging faults in the western Great Basin for more than a dozen years. Honjas et al. (1997) per- formed 2d velocity optimization from tens of thousands of first-arrival picks on about a thousand prestack reflection records in a grid of seismic lines covering the northern Dixie Valley geothermal field. The direct imaging showed the existence of basinward step-faults, hidden from the surface by younger alluvial deposits. Chavez-Perez et al. (1998) applied the same analysis to existing CO- CORP data across Death Valley, imaging the Black Mountains range-front fault and quantifying its heave. Abbott et al. (2001) conducted a reflection survey in southern Dixie Valley, directly imaging the shallow-dipping 1954 M7.2 rupture below up to 1 km of alluvium using the same combination of velocity optimization and depth migration. Louie and Pullammanappallil (2007) and Louie et al. (2008) reviewed that work and added the 2003 shallow imaging of a low-angle normal fault in the west Ruby Mountains. Louie et al. (2007) elucidated these, plus much additional Optim work, for the Nevada geothermal industry. Figure 7, an example of commercial Optim analysis taken from Louie et al. (2007), shows how powerful the combination of a industry-scale reflection survey with a high-quality 1st-arrival pick set, velocity optimization, and PSDM can be. At this Nevada geothermal prospect, only the range-front fault is manifested at the surface. The other normal faults are blind and hidden by younger alluvium. Optim achieved clear, direct fault-plane reflection images of these faults despite their not significantly offsetting the basin floor, nor being associated with significant lateral velocity changes. The interior stratigraphy of the alluvial fans is also imaged. The faults are located with the most possible confidence for seismic data: within one lateral wavelength, rather than within a much 7 larger Fresnel zone that would have to be allowed if the image was of lower quality. In most geo- thermal prospects in central Nevada, the geothermal resource is controlled by these hidden faults, and it is crucial to locate and drill them.

Figure 6: PSDM of part of the TRK1 reflection line in downtown Reno, with layered Quaternary and Tertiary stratigraphy cut and tilted by possible west-dipping normal faults, from Kell-Hills et al. (2010a). The west dip of the basin fill is consistent with the west-dipping Neogene section exposed along the east flank of the Carson Range, west of Reno; but it opposes the east dips found throughout the Genoa – Mt. Rose fault systems, aligned with these faults to the south.

In the first project year Optim will survey a 7.2-km transect across the south Reno basin to characterize the history of the Mount Rose fault zone, and an 7.6-km transect across Carson Valley to investigate the Genoa fault (pink lines on fig. 1, right). These routes cross the full width of both basins where Tertiary sediments and volcanics are obscured below Quaternary sediments. This cov- erage will allow the most active basin-bounding faults to be imaged, as well as the hidden Tertiary stratigraphy and structure. Both surveys will deploy at least two industry-sized heavy vibrators. Each will have a stan- dard 60,000-lb (27-ton) hold-down weight with a 4-ton reaction mass, for vertical forcing. As well, we will use a new design of wireless, portable 3-component (3C) geophone recorders. Geophones will extend along each profile at 110-ft (33.5-m) intervals. All 220+ 3C geophones along a line will record every vibrator location. This procedure assures the longest offsets and deepest-penetrating 1st-arrival information and velocity constraint. This full-offset recording technique is used by Optim for geothermal exploration, as in fig. 7, with excellent results. A full suite of field tests will be run at the beginning of each line before the walk-through, to refine the parameters beyond the typical values given here. If data quality begins to deteriorate along a line, we will run further tests before proceeding.

With the dense, 660+ channels of 2D-3C data on each line, P-velocity optimization analyses by the Seismo Lab group will extend to recover S-velocity and Poisson’s ratio sections on both lines, and use the velocity sections to derive prestack depth-migrated cross-section images of P-P, S-S, as well as P-S and S-P converted-wave reflections. Processing and imaging will be done in the first and second project years by PIs Louie and Kent, and the graduate student at UNR, with consul- tations to Pullammanappallil at Optim. The data-analysis procedures will include:

8 a. Picking of vertical and horizontal first arrivals from the correlated and summed 3C vibrator records. Following Pullammanappallil and Louie (1994), this will be the essential data set al- lowing complete characterization of lateral velocity variations in the upper portion of the basin, where they are strongest. It is anticipated that 300,000 traces will need to be picked for their first arrivals. The graduate student supported by this project will do most of this work over a pe- riod of 2-4 months, using the most convenient of the ProMAX, SPW, OpendTect, or JRG seis- mic software platforms that we have available. b. Development of 2d optimized velocity models from the P and S 1st-arrival picks of the prestack data. This task will be completed in collaboration with subcontractor Optim using their propri- etary SeisOpt® technology, an extensively tested and industry-vetted implementation of the simulated-annealing travel time optimization of Pullammanappallil and Louie (1994), making use of finite-difference travel times (e.g., Vidale, 1988). This nonlinear optimization assures the best possible representation of lateral velocity discontinuities. No assumptions are made about the form or smoothness of the discontinuities. If insufficient depth coverage of the optimized velocities from the 1st-arrival time picks results, we will conduct additional optimizations, in- cluding for instance prestack reflection coherency in the objective function (Pullammanappallil and Louie, 1997). c. Preprocessing and filtering of the prestack records will be performed by the PIs and the gradu- ate student in collaboration with contractor Optim. Given the exurban setting of the surveys, and the use of a single geophone per recording channel rather than a geophone group array, the prestack data records will likely be dominated by strong, low-velocity surface waves. Data qual- ity will thus profit from application of the Hale and Claerbout (1983) Butterworth dip filter, as did the direct fault imaging of Kanbur et al. (2000) at the Upheaval Dome, Utah impact struc- ture. d. Prestack depth migration (PSDM) of all the prestack reflection records through the optimized velocities will bring the steeply dipping fault-plane reflections into focus at their true locations. The use of finite-difference travel times (e.g., Vidale, 1988) and properly assessed lateral-veloc- ity variations from Optim’s SeisOpt® results will allow the proper placement of reflection depth points along even strongly curved raypaths. Optim and the PIs and graduate student at UNR will collaborate on this step as well. We will include migration operator antialiasing control (Lumley et al., 1994), and possibly Bayesian signal/noise separation (Harlan et al., 1984). These en- hancements were employed in PSDM work by Kanbur et al. (2000) and Louie et al. (2002), as well as by Optim for the numerous geothermal imaging examples shown in Louie et al. (2007), such as in figure 7. e. The PIs and graduate student will interpret the PSDM images for quality and accuracy, com- paring for instance basin depths derived from Abbott and Louie (2000) in south Reno and from Cashman et al. (2009) in Carson Valley against the major impedance contrasts shown by the PSDM. Additional processing will be done as necessary. We will identify and interpret direct fault images within the sections (as suggested in fig. 5), making sure fault interpretations are in- ternally consistent with models of basin structure. f. As in Louie et al. (2002), we will produce multicomponent images of P-P, S-S, P-S, and S-P re- flections from each of the three components of recorded seismograms. Three components of data times four components of images will yield a total of twelve component cross-sectional im- ages for each survey line. Comparison of a reflector’s response on each of the component im- ages will allow an assessment of physical properties at the reflective interface. For example, both a wedge of porous sediment caught between hard volcanic flows, and an isolated hard vol- canic flow within sediment will produce strong S-S reflections. The sediment wedge will also produce strong P-P reflections, while an isolated flow may not. We hope to correlate multicom- 9 ponent reflector response against geological information such as the location of the boundary between Tertiary volcanics and hard Tertiary sediments, and the boundary between Tertiary and Quaternary sediments. As well, the multicomponent images will help us distinguish between fractured fault zones and dipping sedimentary boundaries. Such distinctions are hard to make in the single-component images recorded by the USGS, as in fig. 5 (right). g. PIs Cashman and Trexler will interpret the seismic sections produced by Louie, Kent, and the graduate student to produce a comprehensive geologic interpretation of each line, accounting for all known geophysical and geological data sets, including the MEQ study at the south Reno line. Trexler will analyze the internal stratigraphy of the Neogene and Quaternary basin fill, cor- relating units in the seismic records with known (and locally dated) facies in surface exposures and well logs. Cashman will interpret the Neogene-present deformation history of each basin, focusing on fault sense and slip history. Both of these can be constrained by known surface faults and by the tilt and offset histories recorded in Neogene and Quaternary sediments. This comprehensive interpretation will address the following questions: the sense, amount and age of offset on the basin-bounding and the intra-basin faults; the existence of major west-dipping fault systems along the east flanks of the south Reno and Carson Valley basins; the reason for the dip reversal between the west-dipping Neogene section in Carson Valley and the east-dipping Neo- gene section in the Reno basin; the existence of previously unrecognized faults in the subsur- face; the potential continuity of a major fault or faults between segments of the Genoa – Mt. Rose fault system; the presence and nature of any accommodation zones (Faulds and Varga, 1998) linking major fault segments; and the thicknesses and deformation of Tertiary, Quater- nary, and younger basin fill. h. The geologic interpretation will be iterated in the sense that all models produced will be checked for consistency with known geophysical and geological constraints. The MEQ mecha- nisms developed by Smith and geologic interpretations by Cashman and Trexler will feed back into Louie and Kent’s interpretation of the multicomponent seismic data, and possibly into the processing. All of these iterations will be conducted as an interdisciplinary collaboration. i. The PIs and the graduate student, starting with the geologic interpretation, will make an assess- ment of the reflection response of Tertiary versus Quaternary faults from the multicomponent images, after separating the two sets of faults in the interpretation. The PSDM and direct fault imaging gives us the rare opportunity to examine the physical properties of the buried faults. Some questions we hope to collect data on include: Are any of the faults simple lateral velocity boundaries? Are they low-velocity zones? Do they show a reflection signal consistent with in- creased porosity? Decreased porosity and work hardening? Are there systematic differences in physical properties between faults active in the Tertiary, but not later, and those currently ac- tive? j. The PIs and the graduate student, starting with the geologic interpretation, will also make an assessment of fault reflection response versus vertical fault offset, where total offset can be as- sessed from stratal offsets across the fault. We will look for any correlations between fault age, offset, depth, and other factors against reflection attributes such as amplitude, phase, and fre- quency. Multicomponent AVO (amplitude versus offset or incidence angle) studies of the fault reflections may be possible. Microearthquake (MEQ) recording– To assess and locate microearthquakes along the Sierra Nevada range-front fault between two urban geothermal areas, about a dozen L4-3C seismometers will be deployed over a small region for 18 months (fig. 8). This would qualify as an EarthScope/USArray Flexible Array deployment, with data telemetered in real time to the Nevada Seismo Lab, and then streamed to the IRIS-DMS. MEQ recording will be in concert with the pair of seismic reflection lines across the two Sierra Nevada range-front basins. Station geometry will be optimized to most effectively take advantage of the existing permanent network. MEQ recording 10 will help the reflection imaging identify and characterize faults and tectonic style affecting the Neo- gene basins. The N. Tahoe area extending into the southern Truckee Meadows produces regular small magnitude events (Ichinose et al., 1999). PI Smith and his technical staff will be responsible for the MEQ recording and interpreta- tion. UNR will contribute at least two stations, and 8 to 12 RAMP station setups will be requested from PASSCAL. UNR instruments may be deployed in advance of project funding. Fig. 8 shows existing stations as yellow and red dots, and seven blue dots, indicating the region across which the RAMP stations may be deployed. The two broadband stations in the area of the south Reno reflection line (red dots on fig. 8) are not expected to contribute significantly in contraining local MEQs. Station WCN may be relo- cated before the project begins (the site Considering BMHS on thick sediments, Figure 8 illustrates a lack of adequate controls for event as small as M0.0. Also, note the closest short period stations (yellow dots on fig. 8) are nearly 10 km away from the focus of the south Reno experiment, and are limited to analog data resolution (e.g., MPK; effectively 10-bit dynamic range). Therefore, we are limited in our ability to locate small events (M ~0.0) in the south Reno area. Also, Smith ran a small portable instrument deployment for a geothermal producer in 2001 that showed a number of small events (many shallow), some right below the proposed south Reno line. That short term experiment showed numerous small magnitude events in the south Reno area (termed the “UBOAT” experi- ment from 2000-2001; data have been submitted to the IRIS data center). There were also events lo- cated at depth just south of the range front, most likely in the Carson Range footwall block; and most likely an extension of N. Tahoe-area seismcity. The MEQ deployment will establish a com- pleteness threshold of magnitude 0.0, or better, producing the most comprehensive view of the mi- cro-seismicity, and earthquake focal mechanisms, in this important structural transition zone along the eastern Sierra. MEQ locations and mechanisms will also contribute to better understanding of the structural controls on local geothermal resources.

Expected Results and Broader Impacts The nature and activity of the faults along the Sierra Nevada range front are crucial inputs to an earthquake-hazard assessment of the Reno–Carson City and Lake Tahoe urban areas of Nevada and California. The fault imaging and MEQ results will allow probabilistic assessment of the hazard presented by each of the many fault strands crossed by the 14.8 km of survey line. Our results will show, as well, the presence of any previously unmapped faults.

11 Figure 8: Northern Nevada seismic network coverage in the project area. The yellow dots are short-period stations most sensitive to small earthquakes. Blue dots show the proposed mi- croearthquake (MEQ) deployment area.

The possible linking and simultaneous rupture of the Genoa and Mount Rose fault system provides the longest and most dangerous fault rupture threatening these urban areas, with event magnitudes of 7.5 a possibility. Since dePolo et al. (1996) the most hazardous event for the Reno urban basin has been thought to be such a long rupture into the city from the south. Fig. 9 shows a deterministic prediction of ground shaking for an M7.5 event on the Genoa – Mt. Rose Fault sys- tem, making the assumption that all the segments can link into one long rupture (Louie and Larsen, 2007). This scenario injects an unexpected amount of energy into the Reno-area basin, resulting in shaking velocities exceeding 15 cm/s.

12 The proposed reflection surveys and MEQ recording will thus provide critical hazard assess- ment, helping to protect the region’s urban population and economy. Evaluation of the faulting and tectonic history of the basins as proposed here will provide information on the most important earthquake scenarios affecting the region. Despite the importance of imaging these fault systems, survey efforts funded by the USGS will be restricted for the foreseeable future to the denser urban areas. USGS-NEHRP funding has been directed toward locating the faults within densely populated areas. Such restrictions will not allow investigation of fault history through the stratigraphy and structure of the south Reno and Carson Valley basins, which are not densely urban.

Figure 9: Development of an M7.5 Genoa – Mt. Rose earthquake scenario using deterministic physics-based methods, from Louie and Larsen (2007). (left) Basin-thickness map assembled from multiple sources including Abbott and Louie (2000) for Reno-area details, and Saltus and Jachens (1995) for the rest of western Nevada. Zero basin thickness is green; dark blue is about 1 km thick- ness; the Lake Tahoe basin has an unknown greater depth. (center) Snapshot of computed wave propagation at 32 s after rupture initiation at the south end of the Genoa fault zone, with strong di- rectivity of 2-sec-period waves into the Reno-area basin, top center. (right) Maximum ground-mo- tion map for the computed scenario, with a red circle at the epicenter and red arrow along the straight, northward dextral-normal oblique rupture 85 km long and 15 km wide. Computed ground motion of 5 cm/s or greater are yellow, to a maximum of 16 cm/s in the west Reno sub-basin. Note the computation predicts only 3 cm/s ground motion in the other basins along the rupture.

In addition, the project will support one graduate student and provide part-time research ex- perience for two or three undergraduate students. The graduate student will get experience with ini- tiating a project, reflection data collection, PSMD imaging, high-precision earthquake source-pa- rameter estimation, supervision of undergraduate workers, and presentation and publication of re- sults. All of the students will benefit from participating in the close collaboration between geolo- gists and geophysicists in the interpretation of the results, and from the experience of working with a diverse research team. These benefits will help to strengthen Nevada’s scientific and technical workforce.

Compliance with EAR Data Policy All seismic data resulting from this project will be contributed to the IRIS Data Management System in accordance with PASSCAL guidelines. A real-time feed from the telemetered MEQ recording stations will be delivered to the DMS and made open for immediate free access. Event sets, mechanisms, and catalogues will be contributed within 2 years of recording. 13 Although not recorded with PASSCAL facilities, all seismic-reflection records developed by this project will be contributed to the IRIS-DMS as soon as practical after data acquisition, and not later than the end of the first project year. Initial delivery will be in the PIC KITCHEN format, or as requested by IRIS. Processed products such as time picks, velocity models, and the multicomponent images will also be contributed within 2 years of recording. Presentation and publication of imaging and MEQ results will be comprehensive, with all geophysical results posted in standard electronic seismic data-exchange formats on the Western Basin and Range Community Velocity Model website: http://crack.seismo.unr.edu/wbrcvm. At least two presentations on the results will be made each project year at international professional conferences such as AGU, GSA, SSA, SEG, or AAPG. Results will also be submitted to peer-re- viewed journals for publication.

Results of Previous NSF support in the last 5 years: G. M. Kent G. M. Kent, A. J. Harding and S. C. Singh (BIRPS co-funded), PIs: OCE-9633774, $997,428 (U.S. amount), 3/1/97-2/28/00, The anatomy of a ridge-axis discontinuity (ARAD) 3D seismic experi- ment; OCE-9911802, $294,297 (U.S. amount), 4/01/2000-3/31/03, Amplitude Variation with Offset Studies and Pre-stack Imaging of the ARAD 3D Reflectivity Volume. The ARAD 3D seismic experiment was conducted aboard the R/V Maurice Ewing during September-October of 1997 and centered on the 9°03’N overlapping spreading center (OSC) along the East Pacific Rise. Key elements of this survey included: (1) the first 3D reflection survey of a mid-ocean spreading center, and (2) a coincident 3D crustal tomography experiment. The 3D im- ages of crustal reflectivity and velocity have provided considerable insight into crustal structure and melt dynamics beneath this prototypical feature. The observed distribution of crustal magma accu- mulations beneath this overlapper appears to be inconsistent with either a simple, broadly symmetri- cal structure for the OSC, or with models which depict the limbs of the OSC as attenuated ends of magmatic systems fed largely by horizontal flow of melt from distant sources (Kent et al., 2000). 3D reflectivity images also reveal the presence of Moho reflections beneath the melt lens, suggest- ing the formation of “zero-age” Moho (Singh et al., 2006). Additionally, travel-time variations be- tween the melt lens and Moho reflections suggest the presence of considerable amounts of melt dis- tributed within the lower crust beneath the northern limb of the OSC. The amplitude variation with offset (AVO) pattern of magma chamber reflections shows a coincident region of higher melt frac- tion overlying this anomalous lower crustal region, supporting the conclusion of additional melt at depth. Of interest, this region of high melt fraction also corresponds to the location of a recently dis- covered hydrothermal field. Thus far, eight journal articles have been written describing results from the ARAD 3D dataset, including two Nature, three Geology, one GRL and two JGR papers. The latest manuscript was recently published in Nature (2006) attesting to the richness of this first 3D reflection dataset of a mid-ocean ridge. A subset of published articles include:

Singh, S. C., A. J. Harding, G. M. Kent, M. C. Sinha, V. Combier, S. Bazin, C. H. Tong, P. J. Bar- ton, R. W. Hobbs, R. S. White and J. A. Orcutt, 2006, Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise: Nature, 442, 287-290. Kent, G. M., Singh, S. C., Harding, A. J., Sinha, M. C., Orcutt, J. A., Barton, P. J., White, R. S., Bazin, S., Hobbs, R. W., Tong, C. H., and Pye, J. W., 2000, Evidence from three-dimension- al seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinu- ities: Nature, 406, 614-618.

Results of Previous NSF support in the last 5 years: J. H. Trexler and P. Cashman EAR0510915: “Distribution and Kinematics of Late Paleozoic Deformation from Southeastern Cal- ifornia to Northeast Nevada” was awarded to a collaborative proposal including Boise State Univer- sity and University of Nevada Las Vegas, to study the tectonic history of the upper Paleozoic rocks of eastern California, Nevada and Idaho. This investigation has funded 6 graduate students from the

14 University of Nevada, and resulted in 3 publications in peer-reviewed journals, 9 abstracts at nation- al and regional meetings, and two MS degrees completed, one near completion.

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