Imaging Extensional Versus Strike-Slip Tectonicsin the Northern Walker Lane

John Louie, Graham Kent, and Kenneth D. SmithNevada Seismological Laboratory, University of Nevada, Reno

Patricia Cashman and James TrexlerDept. of Geological Sciences and Engineering, University of Nevada, Reno

PROJECT DESCRIPTION

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

Introduction and Intellectual MeritThe 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 motion (e.g., Dixon et al., 1995; Thatcher et al., 1999), and may be an incipient intracontinental transform 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 tradition-ally mapped) intersects the Sierran frontal system of normal faults in the vicinity of Reno, Nevada. The Neogene sedimentary basins of northwest Nevada and northeast California record the development of intra-plate de-formation 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–Carson 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 span-ning several basins may be expected to rupture in longer segments and larger events than geometrically 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 assess the earthquake hazard or risk.

The Genoa fault (Pease, 1979) and the Mount Rose fault zone (Sawyer, 1999) form the Sierra 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 his-tory and future potential. The Genoa fault is a normal fault that is thought to have a minor component of dextral motion (Pease, 1979; Surpless, 1999). With its curved trace, tilt fanning in the Neogene section, and largest dis-placement in the middle section, the Genoa fault itself must have almost ex-clusively dip slip. Total vertical offset, based on gravity, is on the order of

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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 Neogene 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 limited 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).

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

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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 eval-uated 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 un-known 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 un-known. Since these traces could bring rupture from the Genoa – Mt. Rose system into downtown Reno, it is important to clarify the nature of their ac-tivity.

Figure 2: Gravity profile and geology model along the proposed Carson Val-ley reflection line (fig. 1). The two-dimensional modeling assumed basement rocks as Mesozoic plutonic and metamorphic units averaging 2.75 g/cc, up-per 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 pro-gressive tilting along the basin-bounding fault through deposition (approx. 7 Ma - 2.5 Ma). Taken from Cashman et al. (2009).

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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) con-cluded 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 paleo-seismic trench, and new seismic-reflection surveys show active tectonics that are hard to fit into this accepted framework for the Walker Lane re-gion. 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, presented by Kell-Hills et al. (2010b), followed paleo-seismic trenching of the fault (at the MRFL on fig. 1) in Sept. 2009 by A. Sarmiento and S. Wesnousky. This discovery, proved by trench observation coupled with a Nov. 2009 high-resolution seismic study, demands new thinking about the tectonics of the Genoa fault system as it approaches Reno from the south.

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 vertical exaggeration for the veloci-ties found by NMO analysis. The interpretation (right) identifies three west-dipping reflections as deep as 1000 m. Their dips increase with depth and age, consistent with the fanning of dips seen in surface exposures (Fig. 2). At the left are the depth, stacking velocity and dip interpreted for each re-flection. 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).

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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 Ter-tiary-to-Quaternary sedimentary basins associated 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 his-tories.

This investigation of faulting history will test hypotheses of westward propagation of Basin and Range extension (e.g., Dilles and Gans, 1995; Sur-pless 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 accom-modation 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 addi-tional 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 current 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 strati-graphic and fault imaging despite urban noise. The Reno-area basin had previously been characterized from geothermal-well and gravity data by Ab-bott 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.

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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 ex-hibiting negative-moveout reflections, and a section of our imaging to date that may be showing fault-plane reflections. The negative-moveout reflec-tions 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 transtensional pull-apart basin along the Garlock fault. On the MNZA line, the sidewall reflections mi-grate 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 mo-tion. 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 hammer 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.

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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-dipping normal faulting accompanied by eastward tilting in the Quaternary and older sec-tion. 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-related tilting in Carson Valley (Cashman et al., 2009). However, such a significant and re-cent west-dipping 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.

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 WorkSeismic-reflection surveying– Optim Inc., a seismic acquisition firm

with an established record of basin and fault imaging in Nevada will per-form 14.8 line-kilometers of innovative 2D surveying. The UNR group, to-gether with Honjas and Pullammanappallil at Optim, have been directly

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imaging faults in the western Great Basin for more than a dozen years. Hon-jas et al. (1997) performed 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) ap-plied the same analysis to existing COCORP data across Death Valley, imag-ing 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 allu-vium using the same combination of velocity optimization and depth migra-tion. 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 reflec-tion 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 hid-den 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 in-terior stratigraphy of the alluvial fans is also imaged. The faults are located with the most possible confidence for seismic data: within one lateral wave-length, rather than within a much larger Fresnel zone that would have to be allowed if the image was of lower quality. In most geothermal prospects in central Nevada, the geothermal resource is controlled by these hidden faults, and it is crucial to locate and drill them.

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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 coverage 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 standard 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 wire-less, 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 con-straint. 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.

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Fig. 7: 2d seismic-reflection (wiggle traces) and velocity sections (colors) from a Nevada geothermal prospect, showing direct detection and imaging of hidden faults below the basin floor. The data were collected and processed with velocity optimization and prestack depth migration (PSDM) by Optim. From Louie et al. (2007).

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-ve-locity and Poisson’s ratio sections on both lines, and use the velocity sec-tions 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 consultations to Pullammanappallil at Optim. The data-analysis procedures will include:

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 allowing complete characteriza-tion 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 period of 2-4 months, using the most convenient of the ProMAX, SPW, OpendTect, or JRG seismic soft-ware 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 proprietary SeisOpt® technology,

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an extensively tested and industry-vetted implementation of the simu-lated-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 con-duct additional optimizations, including 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 graduate student in collaboration with contractor Optim. Given the exurban setting of the surveys, and the use of a single geo-phone per recording channel rather than a geophone group array, the prestack data records will likely be dominated by strong, low-velocity surface waves. Data quality will thus profit from application of the Hale and Claerbout (1983) Butterworth dip filter, as did the direct fault imag-ing of Kanbur et al. (2000) at the Upheaval Dome, Utah impact structure.

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-velocity variations from Optim’s SeisOpt® results will allow the proper place-ment of reflection depth points along even strongly curved raypaths. Op-tim 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 enhancements 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 fig-ure 7.

e. The PIs and graduate student will interpret the PSDM images for quality and accuracy, comparing for instance basin depths derived from Abbott and Louie (2000) in south Reno and from Cashman et al. (2009) in Car-son Valley against the major impedance contrasts shown by the PSDM. Additional processing will be done as necessary. We will identify and in-terpret direct fault images within the sections (as suggested in fig. 5), making sure fault interpretations are internally 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 reflections 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 images for each survey line. Comparison of a reflector’s response on each of the component images 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 volcanic flow

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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 multicomponent reflector response against geologi-cal information such as the location of the boundary between Tertiary volcanics and hard Tertiary sediments, and the boundary between Ter-tiary and Quaternary sediments. As well, the multicomponent images will help us distinguish between fractured fault zones and dipping sedimen-tary boundaries. Such distinctions are hard to make in the single-compo-nent 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 geo-logic 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 Quater-nary basin fill, correlating units in the seismic records with known (and locally dated) facies in surface exposures and well logs. Cashman will in-terpret 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 Neo-gene 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 Neogene section in the Reno basin; the existence of previously unrecognized faults in the subsurface; the potential continuity of a major fault or faults be-tween 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, Quaternary, 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 mechanisms 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 interpreta-tion, will make an assessment of the reflection response of Tertiary ver-sus 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-ve-locity zones? Do they show a reflection signal consistent with increased

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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 active?

j. The PIs and the graduate student, starting with the geologic interpreta-tion, will also make an assessment of fault reflection response versus ver-tical fault offset, where total offset can be assessed from stratal offsets across the fault. We will look for any correlations between fault age, off-set, depth, and other factors against reflection attributes such as ampli-tude, phase, and frequency. Multicomponent AVO (amplitude versus off-set or incidence angle) studies of the fault reflections may be possible.

Microearthquake (MEQ) recording– To assess and locate mi-croearthquakes along the Sierra Nevada range-front fault between two ur-ban 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 Earth-Scope/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 opti-mized to most effectively take advantage of the existing permanent net-work. MEQ recording will help the reflection imaging identify and charac-terize faults and tectonic style affecting the Neogene 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 interpretation. UNR will contribute at least two stations, and 8 to 12 RAMP station setups will be requested from PASSCAL. UNR instru-ments 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 con-training local MEQs. Station WCN may be relocated before the project be-gins (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 resolu-tion (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 pro-ducer 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” experiment from 2000-2001; data have been submitted to the IRIS data center). There were also events located at depth just south of the range front, most likely in the Carson Range footwall block; and most likely

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an extension of N. Tahoe-area seismcity. The MEQ deployment will establish a completeness threshold of magnitude 0.0, or better, producing the most comprehensive view of the micro-seismicity, and earthquake focal mecha-nisms, 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 ImpactsThe 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 sur-vey line. Our results will show, as well, the presence of any previously un-mapped faults.

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

The possible linking and simultaneous rupture of the Genoa and Mount Rose fault system provides the longest and most dangerous fault rup-ture threatening these urban areas, with event magnitudes of 7.5 a possibil-

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ity. 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 system, 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.

The proposed reflection surveys and MEQ recording will thus provide critical hazard assessment, 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 Ab-bott 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 thickness; the Lake Tahoe basin has an unknown greater depth. (center) Snapshot of computed wave propagation at 32 s after rupture initi-ation at the south end of the Genoa fault zone, with strong directivity of 2-sec-period waves into the Reno-area basin, top center. (right) Maximum ground-motion map for the computed scenario, with a red circle at the epi-center and red arrow along the straight, northward dextral-normal oblique

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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 experience for two or three undergraduate students. The graduate student will get experience with initiating a project, reflection data collection, PSMD imaging, high-precision earthquake source-parame-ter estimation, supervision of undergraduate workers, and presentation and publication of results. All of the students will benefit from participating in the close collaboration between geologists and geophysicists in the inter-pretation 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 PolicyAll 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 deliv-ered to the DMS and made open for immediate free access. Event sets, mechanisms, and catalogues will be contributed within 2 years of recording.

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 com-prehensive, with all geophysical results posted in standard electronic seis-mic data-exchange formats on the Western Basin and Range Community Ve-locity Model website: http://crack.seismo.unr.edu/wbrcvm. At least two pre-sentations 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-reviewed journals for publication.

Results of Previous NSF support in the last 5 years: G. M. KentG. 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 experiment; 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 Mau-rice Ewing during September-October of 1997 and centered on the 9°03’N overlapping spreading center (OSC) along the East Pacific Rise. Key ele-

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ments of this survey included: (1) the first 3D reflection survey of a mid-ocean spreading center, and (2) a coincident 3D crustal tomography experi-ment. The 3D images of crustal reflectivity and velocity have provided con-siderable insight into crustal structure and melt dynamics beneath this pro-totypical feature. The observed distribution of crustal magma accumulations beneath this overlapper appears to be inconsistent with either a simple, broadly symmetrical 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 reflectiv-ity images also reveal the presence of Moho reflections beneath the melt lens, suggesting the formation of “zero-age” Moho (Singh et al., 2006). Ad-ditionally, travel-time variations between the melt lens and Moho reflections suggest the presence of considerable amounts of melt distributed 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 fraction overlying this anomalous lower crustal re-gion, supporting the conclusion of additional melt at depth. Of interest, this region of high melt fraction also corresponds to the location of a recently discovered 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 re-cently published in Nature (2006) attesting to the richness of this first 3D reflection dataset of a mid-ocean ridge. A subset of published articles in-clude: 

Singh, S. C., A. J. Harding, G. M. Kent, M. C. Sinha, V. Combier, S. Bazin, C. H. Tong, P. J. Barton, 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-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities: Na-ture, 406, 614-618.

Results of Previous NSF support in the last 5 years: J. H. Trexler and P. CashmanEAR0510915: “Distribution and Kinematics of Late Paleozoic Deformation from Southeastern California to Northeast Nevada” was awarded to a col-laborative proposal including Boise State University 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 University of Nevada, and resulted in 3 publi-cations in peer-reviewed journals, 9 abstracts at national and regional meet-ings, and two MS degrees completed, one near completion.

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