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11 Project Description Collaborative Research: UNR & UNLV: Lithospheric Architecture at a Magma Intrusion Event Observed 30 km Below Lake Tahoe, California and Nevada J. Louie, G. Biasi, R. Anooshehpoor, K. Smith U. of Nevada, Reno C. M. Snelson, U. of Nevada, Las Vegas C. Wilson, Lamont-Doherty Earth Observatory Motivation for This EarthScope FlexArray Project The recent detection of magma movement deep below Lake Tahoe by K. D. Smith et al. (2004) presents one of the only modern observations of active injection of magma into continental crust. Further study of this eature would help to answer fundamental questions relating to magma migration, melt extraction, and crustal assimilation of mantle magmas. Additionlly, Study of this unusual event has raised many questions about the structure and nature of the lower crust, most not well answered. We do not possess even baseline constraints on the configuration of the lithosphere in the northern Sierra Nevada. Nor do we have many constraints on the nature of the Sierra Nevada–Great Basin boundary zone (SNGBBZ). We must be able to describe the configuration of this boundary in at least a fundamental way before we can address questions such as whether the 2003 magmatic event was rare and isolated, or represents more pervasive lithospheric processes. Over the last 20 million years, Basin and Range extension has gradually encroached into the eastern edge of the stable Sierra block. This progression has been marked by the creation of large volcanic centers along the eastern edge of the Sierra (e.g. Coso and Long Valley). Clearly, the transition from the weak crust of the Basin and Range to the relatively cold, strong Sierra block provides an ideal conduit to channel mantle melts into the crust as has been repeatedly demonstrated over the last 5 million years culminating in the massive Long Valley eruption nearly 800,000 years ago. Extensional encroachment occurred more rapidly in the southern Sierra than in the northern Sierra as evidenced by the dramatic topography of the southern Sierra. The 2003 magmatic injections may be an indication that extensional strain has begun to modify the lower crustal structure of the Northern Sierra and has opened new pathways for melt migration in the crust. Unfortunately, we have only sparse information about the state f the crust and upper mantle in the northern Sierra Nevada, making it impossible to place the magmatic event of 2003 in a broader, tectonic context. This limits what we can infer about magmatic processes that produced the swarm. The need for focused work on the northern Sierra Nevada is clear when the region it is compared to the state of knowledge of the southern Sierra Nevada. Wernicke et al. (1996) summarized the results of the multi-year, multidisciplinary southern Sierra Nevada seismic experiments. Careful work such as by Jones et al. (1994) revealed the surprising lack of a deep crustal root below the topographically highest part of the range. New tectonic models for the southern Sierra Nevada resulted quickly, such as the delamination hypothesis of Ducea and Saleeby (1998). In contrast to the well-studied structure of the southern Sierra Nevada (Jones et al., 1994; Fleidner et al., 1996; Wernicke et al., 1996; Jones and Phinney, 1998; Boyd et al., 2004; etc.), information on the northern Sierra Nevada is relatively sparse *. Extrapolation of information from the southern to northern Sierra Nevada may not be supportable. Compilations such as by Braile et al. (1989) show the dearth of seismic constraints on the northern Sierra Nevada region, with Eaton (1963) and Pakiser and Brune (1980) being the only experiments there until * Note: A search of GeoRef for 1980-2004 papers with keywords = (southern Sierra Nevada) or (northern Sierra Nevada) and (crustal structure) yielded 7 citations for southern but 0 for northern. 21 2002. The 2002 refraction work of Louie et al. (2004) was a reconnaissance experiment lacking any blasts or on-line refraction sources in the Sierra Nevada. Although suggestive that a nonintuitively thicker (>50 km) crustal root may underlie the topographically muted northern Sierra Nevada, the 2002 survey could not constrain the upper crust without local shots. This project is partly intended to remedy the shortcomings of the reconnaissance results of Louie et al. (2004). With magma injection events observed below the northern Sierra, it is now imperative to collect baseline geophysical data there. In characterizing the transition between the California Peninsular Ranges and the Gulf extensional province, Lewis et al. (2001) found tremendous dips on the Moho discontinuity, in places exceeding 20°. With the suggestion of a >50 km root for the northern Sierra Nevada by Louie et al. (2004), such radical lithospheric discontinuities may be a possibility for the SNGBBZ there. Yet the lack of baseline seismic characterization is even more severe in considering this fundamental tectonic boundary. The refraction results of Spieth et al. (1981) are too far west to address the transition, and the results of Catchings and Mooney (1991) and Gilbert and Sheehan (2004) are too far east. A COCORP survey crossed the northern Sierra Nevada, as reported in, for example, Nelson et al. (1986), Klemperer et al. (1986), Knuepfer et al. (1987), and Klemperer (1987). However, this COCORP line was 50 km north of Lake Tahoe where the SNGBBZ is complicated by interaction with right-lateral faulting of the Walker Lane. Thus we cannot determine whether the magma injection below Lake Tahoe was interior to a geotectonic Sierra Nevada block or a feature of the transition boundary zone itself. The seismic refraction, tomography, and teleseismic imaging experiments proposed here for the Lake Tahoe area are targeted to understand the structures and context of the 2003 deep magmatic event. Is the shallow mantle unusual near north Lake Tahoe, or are similar conditions prevalent throughout the eastern Sierra Nevada? What are the dimensions of the melt body and how does it facilitate melt extraction form the mantle? Is this body a single reservoir or several small reservoirs? Are they a feature of the Sierra Nevada block, or of the SNGBBZ transition that might give insight into the development of other young volcanic centers along the Easter\n Sierra? Is there a deeper mantle source for the magma, and if so, what is its distribution? If lithospheric anomalies are limited to just the area of the 2003 injection, then it may have been an isolated and rare event or potentially associated with focused extensional strain within the north-south structural transition expressed in the surface faults at Lake Tahoe. The Lake Tahoe Figure 1. Map showing the epicentral locations area is a region is of relatively high extensional of the deep (~30 km) earthquake swarm in 2003. strain (Smith et al., 2004). If these anomalies A perspective view on the locations of the deep are part of a broader pattern, though, magma earthquakes and an inferred “plane” to model the injection of the lower crust could be a typical intrusion is overlain in lower right. feature of the boundary between the Sierras and the extensional Great Basin, and perhaps of the boundaries of extensional provinces in general. Background and Rationale Geodetic data indicate that the Sierra Nevada block is moving about 13 mm/yr N40-45W relative to stable North America. This motion accounts for about 20-25% of the western North American plate motion budget and is generally oblique to north-striking down-to-the-east active normal faults along the SNGBBZ in northern Nevada and eastern California near the latitude of Lake Tahoe. This motion drives the earthquake hazard of the region. In view of this hazard, the Nevada Seismological Laboratory 31 (NSL) has operated seismic stations in the Lake Tahoe area for over 30 years. As the SNGBBZ boundary has evolved westward since 15 Ma (Dilles and Gans, 1995) in a process that incorporates Sierra granites into the Basin Range Province (Surpless et al., 2002), the normal fault systems at this boundary have accommodated the eastward collapse of the Sierra block and development of Basin and Range physiography. Along the Sierra Nevada front, north-striking normal faults have developed in a leftstepping geometry from about the latitude of Long Valley, extending northward through the northern end of Lake Tahoe. The Lake Tahoe basin represents the westward, and youngest, expression of the SBGBBZ having developed over the past 3 Ma. To the east the Walker Lane Belt (Stewart, 1988) primarily accommodates dextral plate-boundary shear including extensional faulting in overall transtensional tectonics. In other words, whereas the eastern front of the Sierra Nevada is the focus of down-to-the-east normal faulting reflecting the westward evolution of the Basin and Range into the Sierra Nevada block, strike-slip faulting, more representative of plate boundary shear, characterizes the Walker Lane Belt. Figure 2: The geodetic displacements at Slide Mountain, Nevada, before, during, and after the earthquake swarm in the deep crust of late 2003. The normal and strike-slip regimes in the upper crust along the Sierra Nevada front operate under regionally consistent E-W directed extension (T-axis from focal mechanisms; Ichinose et al., 2001), with the P-axis rotating locally to reflect either normal or strike-slip faulting. Along the SNGBBZ at the latitude to of Lake Tahoe, Quaternary moment release is dominated by the normal fault systems, with strike-slip faulting apparently playing a role in slip transfer between these primary normal-fault systems (Schweickert et al., 2004). Large volume Tertiary ash-flow tuff sequences that initiated around 30 Ma extend throughout northern Nevada and eastern California near Lake Tahoe. Volcanism progressed throughout the Miocene and by about 3 Ma had evolved to arc-related andesitic magmas and then waned. The most recent mapped volcanics comprise a number of 1-2 Ma basaltic volcanic sources in the north Lake Tahoe area, and additional source areas including the rhyolitic domes of the Steamboat Hill geothermal area in metropolitan Reno east of the lake (Henry and Faulds, 2004). There is an ongoing debate concerning the 41 most recent age of volcanism in the north Lake Tahoe area (Sylvestor 2004, personal communications); and age dating is underway for some suspected young basalts at Tahoe. Only the Long Valley caldera and Lassen Peak volcanic areas show volcanics younger than 1 Ma along the eastern front of the Sierra. Evidence for magma injection in the SNGBBZ at lower crustal depth beneath north Lake Tahoe in late 2003 has recently been published in Science (Smith et al., 2004). A sequence of 1600 earthquakes (Figure 1) from August 2003 through February 2004 define a planar structure at a depth of roughly 30 km below the lake that was coeval with a 10 mm permanent displacement observed on a nearby GPS instrument at Slide Mountain, Nevada (Figure 2). The observations were modeled as a 40 km2, 1-m thick, lower-crustal dike injection event; and therefore the process accommodated about 1 m of extension in the lower crust. The structure is oriented approximately N30ºW and dips 50º to the northeast. Observed EW extension in the Lake Tahoe region would support tensile failure for a structure with this geometry. In addition, the volcanic dike in map view is situated within 4 mapped locations of 1-2 Ma basalts in the north Lake Tahoe area (Henry and Faulds, 2004). Remarkably, the dike injection event apparently triggered an increase in shallow seismicity (< 15 km) throughout the northern Lake Tahoe area that included an M 4.2 earthquake in June 2004 at about 10 km epicentral distance from the deep crustal sequence. This inferred magmatic event represents a unique opportunity, possibly not to be repeated anytime soon, to study a coherent magmatic intrusion in the northern Sierra Nevada. Thus it is reasonable to combine the detailed analysis of the deep crustal context of the 2003 event with a broader look at the structure of the northern Sierra Nevada at the latitude of Lake Tahoe, as proposed here. We also propose to apply state-of-the-art imaging methods to the zone of the deep earthquake cluster (and presumed magmatic intrusion) and to the problem of defining deep fault zones of the Tahoe Basin. The observation of magma injection with the SNGBBZ also brings up fundamental questions regarding the evolution and westward encroachment of the Basin and Range province that could contribute to our understanding of volcanic hazards and public safety. Broadly, this proposal will address two scales, one identified with the SNGBBZ faulting and magmatism in the Tahoe Basin, and one more regional in scope that will provide the larger context. The proposed work will address: 1. Is the dike related to the SNGBBZ frontal fault system or another of the Tahoe Basin faults? Association with the frontal fault zone would require a fairly high angle in the mid-crust and a more listric geometry eastward to join the surface expression of the frontal fault system. 2. Can we image lower crustal magma similar to those interpreted from the COCORP Sierra Valley line (Klemperer et al., 1986; Knuepfer et al., 1987)? Are lower crustal magmas pervasive? 3. Can we specifically image a deeper magma source for the 2003 magma injection event under Lake Tahoe? What is the depth of this source? 4. What is the nature of the boundary between the Sierra Nevada and the Great Basin? The presence of volcanism in the physiographic Sierra Nevada suggests that the boundary is offset between the crust and mantle as it is in the southern Sierra, but this remains to be demonstrated. 5. What is the structure of the Moho and crustal thickness through the SNGBBZ? 6. Are there other regions of the lower crust and upper mantle that suggest potential future magma movement at depth? Passive Tomography Experiments and Analysis 51 Objectives: 1) Image the crustal and upper mantle structure in the Lake Tahoe area, including the region of the inferred deep crustal magmatic injection in 2003; 2) Develop a velocity model for the structure under the northern Sierra Nevada and Lake Tahoe that can be used to guide the teleseismic wave-equation imaging at depth and to improve hypocentral locations for deep seismicity in this area; 3) Identify shallow mantle regions with similar physical properties and presumably similar conditions for magmatic activity; and 4) Link the study area to the broader context of subduction and evolution of the Sierra Nevada. Motivation: The crustal structure across the Sierra Nevada at the latitude of Lake Tahoe is poorly known. Figure 2.5a provides the larger context of the northern Sierra Nevada and the study area. P-wave velocity variations are shown for California and western Nevada at a depth of 50-70 km using the combined California and Nevada permanent seismic arrays. Known effects of crustal thickening (Oppenheimer and Eaton, 1984) were removed before inversion to improve resolution of the mantle contribution. The western half of the Sierra Nevada range and eastern half of the Great Valley are high velocity relative to model average. At these depths a silicic root would be lower velocity, so it appears rather to be cold and perhaps eclogitic in composition. Its extent along strike indicates that it formed by subduction-related processes, either with the Mesozoic construction of the arc or by modification during Cenozoic subduction. Figure 2.5b shows volcanism since 13 Ma on an enlarged portion of the regional image. Volcanism clearly concentrates east of the Sierra Nevada, but extensive volcanism north and west of Lake Tahoe, which lies well within the physiographic Sierra, suggest that the mantle definition of the SNGBBZ is 15-30 km to the west. The detailed context for volcanism in Tahoe Basin and SNGBBZ is essentially unknown. The recent magmatic event makes clear that at some level the system is active, but work proposed here will be required before it can be known whether it was simply a local anomaly, whether it is part of a system with similar source conditions, or whether it is the general state of the upper mantle beneath the eastern Sierra Nevada. The active source portion of the proposed experiment discussed below is designed to evaluate whether master faults of the Tahoe region or perhaps other fine-scale features work to localize magmatic events. 61 Caption, Figure 2.5. (a) P-wave tomographic image of California and Nevada at a depth of 50-70 km. Teleseismic delays to permanent network stations of California and Nevada plus a local temporary array in the southern Sierra Nevada (Biasi and Humphreys, 1992; Jones et al., 1994) were inverted in a model with a total depth of 650 km. Block size is 30x35 km (EWxNS), extrapolated after inversion to 10x10 km for plotting purposes. Outer red line encloses the region at this depth with crossing-ray constraint, which can be interpreted as the resolved map region. Holes in the coverage exist in both the southern and northern Sierra Nevada, generally corresponding to gaps between regional networks. LVC: Long Valley Caldera; MTJ: Mendocino Triple Junction; SGVA: Southern Great Valley Anomaly; TRA: Transverse Ranges Anomaly. Stratovolcanos labeled with white triangles. (b) Enlargement of (a) around the study area. The prominent upper mantle velocity high velocity region is offset to the west some 15 to 30 km from the eastern bound of the physiographic Sierra Nevada. A similar offset in the southern Sierras, evident in (a), has been extensively studied (Jones et al., 1998; Ducea and Saleeby, 1998). Volcanism since 13 Ma (C. Henry, pers comm.; circles: dated samples; stars: interpreted locations of volcanic centers) indicate a fundamental difference in mantle conditions is present in shallow mantle depths. Inversion block sizes and native station density are too coarse to resolve questions raised in this proposal. Proposed Work: Briefly, we propose an 18 month deployment of 40 broadband seismic stations (Figure 3). Data acquired during this experiment will be supplemented by network station coverage of the California and Nevada regional seismic networks. Where needed to fill in azimuth or ray-parameter coverage, recordings during the experiment will be supplemented by archive data of the permanent seismic networks. The proposed temporary array will entail relatively high density coverage in the central area to ensure adequate resolution at upper mantle depths.. The usefulness of high-density teleseismic recording is shown in the Vp images shown in Figure 3.1. Magma was erupted from five small centers (green triangles), four at about 1 Ma, and the fifth (LC) approximately 120 Ka. Melt extraction increases seismic velocities, of itself and as it removes water from the solid phases (esp. olivine) in the mantle (Karato and Jung, 1998; Hammond and Humphreys, 2000). The only depth beneath the volcanic centers showing the predicted velocity increase is about 50 km. Petrologic evidence shows the magma originated at about 53 km (the center of the layer shown in Fig. 3.1a, black oval in b). Of note as well, the lowest velocity regions in the 45-60 km depth slice (south and east of LSM) are and have been substantially amagmatic. For reasons developed elsewhere (Biasi, in prep.) this lowest velocity region is likely to be hydrated and subsolidus. Finally, the scale of the upper mantle structures involved to produce the Quaternary volcanism here imaged are so small that imaging with teleseismic shear waves could be difficult. At, say, five second periods, the wavelength is of order 20 km, and structures large enough to likely be imaged are probably just wet (Karato and Jung, 1998). The southern Nevada example in Figure 3.1 shows that low velocities are not certain beneath regions of recent volcanism, and that a nuanced interpretation that considers all available data may be necessary to unravel the northern Sierra Nevada teleseismic images. 71 Caption. Figure 3.1. (a) Direct image of a melt extraction region using P-wave tomography. Melt extraction to Quaternary volcanic centers of Crater Flat (green triangles) should cause a 1-2% increase in Vp in part by melt extraction and in part by dehydration of olivine. The one depth at which velocities increase is imaged in the 45-60 km depth layer. One year of portable and network station teleseismic data from southern Nevada was used to produce the image. Net station density is comparable to the proposed northern Sierra Nevada experiment – 5-8 km in the center of the imaging area. This model shows that 4.5x4.5 km blocks can be resolved to a model depth of 80 to 100 km. Note that lowest mantle Vp regions have not been volcanically active; these regions are likely to be sub-solidus and hydrated. LSM: Little Skull Mountain; ESF: Exploratory Studies Facility, Yucca Mountain. (b) North-south cross-section beneath the Crater Flat volcanic cones. The magmatic source is centered on ~50 km depth, consistent with petrologic estimates. The teleseismic array will consist of 40 Guralp 40-T broadband seismometers and RefTek 130 recorders to be taken from the pool of EarthScope flexible array instruments. They will cover an approximate 2-D 60 km by 60 km grid centered on north Lake Tahoe, with portable stations at about 10 km spacing (Figure 3). Figure 3 reflects realistic positions based on roads, federal land units, and wilderness boundaries. The arrangement of the array will permit us to build a 3-D tomographic model of the crust and upper mantle under the study region. NSL has 10 stations within the proposed footprint of the teleseismic array. Arrivals obtained from these stations during the experiment will provide the link between the data of the proposed experiment and the earthquakes previously recorded on the network. Resolution will depend on a variety of factors including the number of useable teleseisms that occur during the experiment; but we estimate that we will be able to resolve features on the order of about 5 km in size to shallow mantle depths beneath most of the array. To increase the areal coverage of our broadband array, we plan to also analyze teleseismic arrivals at stations of the surrounding CISN (California Integrated Seismic Network) and NSL networks. It is important to analyze a box considerably larger and deeper than the focused study area so that unmodeled structure from regions outside the analysis box can be mitigated (Evans and Achauer, 1993). In the proposed study area this will be especially important since the subducting Gorda slab extends to a depth of at least 400 km not far north of the project area. To reduce the teleseismic data to relative traveltime 81 delays we propose to use a method similar to that used by VanDecar (1991) and by VanDecar et al. (1995). In their methodology, teleseismic P and S waveforms are cross-correlated to give highly accurate and robust estimates of P and S delay times and errors. The teleseismic inversion solves for velocity perturbations to a background model, source-related delay (such as source mislocation and mistiming as well as unmodeled structure outside the box) and station corrections (due to small-scale unmodelable structure near the stations). The teleseismic tomographic inversion code itself is described in Dueker et al. (1993). . Controlled-Source Experiments and Analysis Objectives: 1) to test whether the magma injection has a detectable seismic signature. 2) if so, to determine the extent of the magma body compared to its size inferred from the earthquake swarm beneath north Lake Tahoe; 3) to locate and measure the transition from the Sierra Nevada to the Basin and Range at the latitude of 39N (Lake Tahoe). Motivation: Active source experimentation provides better spatial resolution at high frequencies than can be achieved with passive teleseismic sources alone. We expect that if the magma intrusion is to be imaged, higher frequencies of the active source experiment will have the greatest chance of success. High frequencies are also more likely to image faults and determine whether the 2003 intrusion was assisted by a deep fault structure of some sort. Regardless of the detailed success of the active source component with the 2003 magmatic event, it is extremely likely to succeed in recovering the first-order structure of the crust and the crust-mantle boundary. Neither of these is well known from previous studies. Defining the mid-crust to upper mantle of this region is essential in understanding the transitional boundary between the Sierra Nevada and Great Basin and its potential influence on magma emplacement and migration through the crust. Figure 4. Proposed layout of the controlled-source seismic experiment. Yellow dots represent the shot point locations and the black lines represent station locations. The densest shot point spacing is located over the anomaly at Lake Tahoe. 91 Crustal tomographic inversion of the active source data will provide the first realistic velocity structure at the latitude of Lake Tahoe. Proposed Work: To accomplish these objectives, we plan to survey two lines (lines a and b in Figure 4). The swarm of earthquakes reported by Smith et al. (2004) will be the center of our two lines. The swarm has a strike of N30ºW with a 50ºNE dip (Figure 1). Line a will trend perpendicular to the dike swarm whereas Line b will parallel the feature. This constitutes an effort to produce a 3D tomographic image of the feature. Figure 5. Ray paths of forward and back-scattered phases generated by an impinging teleseismic P wave front after Clouser and Langston (1995). Solid lines mark P ray paths with S rays marked by dashed lines. Uppercase letters denote downgoing phases with lowercase letters symbolizing upgoing phases. The free surface multiples returning to the surface as S (e.g. PpSs) will be contained in the receiver function. The idealized vertical and radial seismograms indicate the approximate arrival and amplitude relationships for some of the arrivals in the ray diagram above. of the controlled-source portion is similar Snelson. We plan to deploy RefTek 125 receivers ("Texans") every 500m along the lines and place shot points every 10 km near the source of the swarm and every 100 km outboard. The shots will range in size from 250 lbs to 3000 lbs along the lines. The density of receivers will provide a generalized image of the deep crust and upper mantle that can be used in a joint inversion with the earthquake hypocenters. The receivers for both lines will be laid out initially, and then all shots will be recorded by all receivers on both lines. We estimate that there will be 12 shot points and 1200-1500 receivers for the total controlled-source experiment. This will allow a limited 3D coverage similar to other experiments of this type. We anticipate that it will take a week to deploy the receivers and approximately two nights to shoot all of the shots. Thus, the controlled-source experiment will last about 10 days. It will take approximately one year to obtain the permits for the shots and receivers, and a few months to have the shot holes drilled. The bulk of the line is under the jurisdiction of the US Forest Service and/or the BLM, which will simplify the permitting process; but there will to be a significant effort devoted to the portion of the line in Lake Tahoe. The scope to that accomplished in the Las Vegas Basin in 2003 by C. The data from the controlled-source portion of the experiment will be processed initially as 2D data along the two lines and then as 3D data when all of the shots and receivers are taken together. The goal will be to produce a good image of the lateral extent of the swarm and the crust-mantle in the SNGBBZ transition. We plan to integrate these data with the earthquake data and wide-angle reflections to produce a joint inversion further constraining the location of magma body (Zelt and Smith, 1992; Hole and Zelt, 1995; Zelt and Barton, 1998). Although the well understood trade-off between reflector depth and the velocities above will still exist, the presence of direct P-wave arrivals in the inversion constrain the upper crustal velocities, thus reducing this trade-off. The explosion sources will also help constrain velocities within the mid-to-upper crust of our 3-D study area, providing much better earthquake locations and velocities in the lower crust than would be available from the earthquake data alone. Receiver Functions and Imaging 101 Objectives: 1) determine crustal thickness across a major tectonic boundary from the Sierra Nevada to the Basin and Range; 2) constrain the location of significant reservoirs of partial melt in the crust beneath Lake Tahoe. Motivation: The support mechanism for the high elevation of the northern Basin and Range and the adjacent Sierra Nevada remains unknown primarily because little is known about the crustal thickness across this region. Most likely the difference in elevation between the Sierra Nevada and the Basin and Range is also related to the location of the magmatic intrusion beneath Lake Tahoe. Exploring these connections through crustal-scale, converted-wave images of the transition from the Sierra Nevada to the Basin and Range will elucidate important features of tectonic-magmatic interactions. Proposed Work: We propose to use teleseismic converted phases recorded by the three component passive array to map the crustal thickness and the location of partial melt within the crust beneath the array using receiver function analysis (Burdick and Langston, 1977; Langston, 1977; 1979; Phinney, 1964) as applied by Wilson et al., (2003). Receiver functions analysis exploits teleseismic P-to-S conversions to identify velocity discontinuities in the lithosphere by using the Pwave arrivals recorded on the vertical component to enhance converted arrivals recorded on the radial component (deconvolution). The amplitude of the converted phases varies as a function of the incidence angle of the impinging wave and the impedance contrast across the interface. The delay time between the direct P arrival and the converted phase are related to the depth to the conversion point and the Vp/Vs ratio above the interface. We intend to use teleseismic converted waves to determine the crustal thickness from the High Sierra across the Lake Tahoe area and into the Basin and Range to the east . Additionally, sufficiently large regions of partial melt and/or areas of sufficiently low velocity should also generate observable converted phases as demonstrated along the southern Sierra Nevada front at Long Valley Caldera (Wilson et al., 2003; Zandt et al., 2003; Steck and Prothero, 1988; 1994) and in the Coso geothermal area (Wilson et al., 2003). In some cases, converted phases from partial melt bodies have been effectively used to determine seismic velocities within the partial melt region through forward modeling of synthetic seismograms (Wilson et al., 2003; Zandt et al., 2003) In addition to basic receiver function analysis, we have designed the passive seismic network to enable the application of a new teleseismic depth migration scheme based on the shot-profile formulation of wave equation migration (Wilson et al., 2004; Shragge et al., 2005). The wave equation depth migration algorithm is well suited to areas with complex velocity variations (e.g. sub-salt or sub-melt imaging) and for multi mode imaging using forward-scattered converted phases and back-scattered, reflected and converted phases. 111 We will use a combination of the wavespeed models derived from the explosive and earthquake source travel times for three-dimensional depth imaging for all imaging modes available in the teleseismic wavefield. The images produced through depth migration will provide a broad scale view of crustal structure across the transition from the Sierra Nevada to the Basin and Range and will also help to locate crustal melt reservoirs beneath the study area. w Education and Outreach Nevada K-12 Network: The Nevada K-12 Seismic Network is a nationally unique program that seamlessly integrates data collected from low-cost seismographs located in a small number of Nevada schools (http://www.seismo.unr.edu/k12network) with existing Nevada regional seismic network operations and the data at the IRIS DMC. Basic hardware for the network has been funded through the Nevada Public Agency Insurance Pool (rural Nevada) and the Department of Energy (Las Vegas); it has been underutilized for education and outreach due to limited funding. This network is a component of the U.S. Educational Seismic Network (http://www.indiana.edu/~usesn). The Nevada K-12 network provides an established communications network (network in terms of real-time technologies as well as personal relationships) for public participation in EarthScope outreach activities on a number of levels. The technologies of the Nevada K-12 network integrate live data from diverse sources, and this can include USArray data and any data available through EarthScope. The incoming live feed creates a real-time national and global perspective of active tectonics for Nevada students. Students can view the waveform data, locate earthquakes, map locations, filter time-series data, import standard seismic data formats, and control displays, in a robust and professional, yet user-friendly, interactive data-management environment. From the scientific perspective, K-12 students on the network have already supplied important data. For example, the Nevada K-12 network stations at Virginia City High School, Douglas County High School, and Storey County High School have contributed waveform data for locating the deep crustal swarm of earthquakes beneath Lake Tahoe, an unprecedented sequence described in Science (Smith et al., 2004). We propose to: 1) conduct a workshop for rural Nevada K-12 network schools (one workshop per year to be conducted in Reno), 2) make site visits to rural schools for presentations on EarthScope to students, and 3) perform basic site maintenance and upgrades on existing K-12 network stations. The workshop will be based on the successful March 2004 workshop that was conducted in Las Vegas for 24 teachers and high-school students from that area. Training in use of the PC networking software and basic information on Nevada earthquakes, seismic hazard and the EarthScope program were provided. Basic interpretation of seismograms and concepts of real-time systems and data management were also presented. Each participant operated real-time seismic software during the one-day workshop. An evaluation form was distributed and compiled to assess the workshop results. IRIS supplied EarthScope materials that were distributed. NSL Hallway Displays: The NSL has recently undertaken, with the aid of non-NSF funding, to upgrade its hallway displays for the public. The NSL hosts nearly 2000 K-12 students per year on tours of our 121 seismological lab, raising their awareness of earthquake hazard and of seismology in general. The new displays will greatly enhance the experience with audio-visual clips of various aspects of earthquake safety and of seismological investigations. Prominent display screens will be able to present seismic data not only from the lab’s network but from the entire world. We propose to add data feeds from several more stations in the Lake Tahoe area to these displays. We will also emphasize our work on this EarthScope project in these tours by showing maps of the layout of the seismic experiments and showing data collected in these experiments. Educational Component: This project will help to achieve EarthScope educational goals in several ways. Firstly, a post-doc and two graduate students will be supported by this proposed project. In addition, many undergraduates will be used in the instrument deployment for the active seismic experiment. These students will be trained on seismological equipment, field deployment methodology, and rudiments of seismology. Their participation in an actual seismic experiment will expose them to careers in geoscience and to one important type of experimental work done by geoscience professionals. Time-Table A time table incorporating the basic elements of this proposal is shown here. In the first year, we will start the permitting process for the two field experiments. We estimate that this will take 3-6 months for the passive and a year for the controlled-source part. We will deploy the passive experiment in the first half of the first year, and run it for over a year (i.e., into the second half of the second year). The controlled-source experiment will be executed in the second year and will be concurrent with the passive recording time period. The final half-year of the project will be for data analysis and synthesis. Project Management The planning and execution of the controlled-source survey will be the responsibility of Catherine Snelson and John Louie. This component will consist of permitting, surveying, drilling, drill watching, loading of the shots, shooting, deploying, pick-up, and clean-up. The shooting will be overseen by Catherine Snelson and executed by a local blaster. The first year will be devoted to permitting the shots and receivers for this survey, and the second year will be for the execution of the survey and for the processing and interpretation of the data. The receiver deployment will be done by volunteers from UNLV and UNR, as well as from the local communities and institutions. We have been successful in previous years in getting a large number of volunteers to participate. 131 The planning and execution of the passive tomography experiment and associated analysis will be the responsibility of John Louie and Leiph Preston. Analysis of the data will be concentrated in the second year of the project. Rasool Anooshehpoor will be supervising the deployment and maintenance of the passive seismic array and data collection. He has extensive experience in deploying portable seismic stations, having done so for every moderate to strong earthquake in Nevada and adjacent eastern California since 1990. He has also been involved in the installation of the ANSS (Advanced National Seismic System) stations in the urban centers of northern and southern Nevada. This component will consist of permitting, obtaining instruments, deployment, data-gathering, and pick-up. Permits will be sought in the first few months of the project so that a spring-summer 2005 deployment can be achieved. We have already approached the USFS, in whose domain most of the stations will lay, concerning a “blanket” permit for the sites. The relocation of the deep-crustal earthquake swarm, and related shallow earthquakes, will be the responsibility of David von Seggern. This aspect of the work is naturally performed near the beginning of the project, as all necessary data is already available. Near the end of the project, the relocation effort will be revisited with improved velocity models for the Lake Tahoe area. The development of receiver functions will be the responsibility of Charles Wilson, who will also use the teleseismic event recordings from the experiment to create scattered wave images of the crust beneath the passive array. The E&O component of the proposal will be carried out mainly by Ken Smith. He has already established a leading-edge K-12 seismograph network in Nevada and is well versed in outreach activities to schools, and he has established many contacts that will enable the E&O component to be successfully implemented. Intra-project communication will be handled by email and 2-3 meetings per year, generally associated with annual meetings that we already attend (AGU, SSA). In addition, the PI’s Louie and Snelson are often at each other’s respective campuses, affording additional contact points. The data will be modeled by both UNR and UNLV and shared among the various investigators for integration purposes. Externally, we will organize a special session at AGU (and possibly GSA) to show the results to the larger community, present results at national meetings and at EarthScope meetings, and publish in peerreviewed journals. Data Handling and Submission Controlled-source and passive array data will be delivered immediately, as it becomes available, to the IRIS DMC; a data report will be delivered within 2 years of acquisition of the data. Since PASSCAL will handle submission of the data to the IRIS DMC, we do not have a significant requirement for graduate student time in this respect. Investigators at UNR are familiar with CSS3.0/Antelope databases through nearly five years of network operations based on Antelope, and they will be using these throughout the experiments. Results From Prior NSF Support Results From Related Previous NSF Support of John Louie: NSF EAR-00-01130: Evolution of the Sierra Nevada - Basin and Range boundary — tephrochronologic and gravity constraints on the record in Neogene basin deposits, 6/2000-5/2002 for $55,182 between 3 PIs, Louie 2 weeks. 141 Geophysical and tephrochronological investigations in the Gardnerville Basin, western Nevada show that the deepest part of the basin is more symmetric than previously thought, perhaps due to a west-dipping normal fault on the east side of the basin, active in Tertiary time. Small basins separated by accommodation zones developed above the Genoa normal fault during Miocene-Pliocene motion. Selected presentations: P. Cashman, J. Trexler, T. Muntean, J. Faulds, J. Louie, G. Oppliger, R. E. Abbott, and M. Clark, 2003, Neogene tectonic history of the Sierra Nevada - Basin and Range transition zone at the latitude of Carson City, Nevada: geological and geophysical evidence: presented at Geol. Soc. Amer. Ann. Mtg., Seattle, Nov. 2-5. W. Thelen, J. Louie, M. Clark, and J. Scott, 2003, Geophysics and paleoseismology: the signature of active faults in the Great Basin: poster presented at the XVI INQUA Congress, July 23-30, Reno, Nevada. Internet Dissemination: Links to presentations and data are at: www.seismo.unr.edu/ftp/pub/louie/gardnerville/ Contributions to Education and Human Resources: This project provided practical and research experience to undergraduate and graduate geology and geophysics majors, and funded the B.S., M.S., and Ph.D. thesis research of five students. NSF EAR-97-06255: Geophysical test of low-angle dip on the seismogenic Dixie Valley fault, Nevada, 9/1997-8/1999 for $91,313 between 3 PIs, Louie 2 weeks. We conducted a 3-km-long seismic-reflection survey in Dixie Valley, central Nevada, with 125 shot points. The reflection survey, along with shallow seismic refraction and reflection, gravity, and electromagnetic measurements, tested whether the 1954 Dixie Valley Earthquake (Ms=6.8) produced slip on a low-angle normal fault. Results show that earthquake slip took place along a fault plane of unusually low dip (25-30 degrees). This is the first large historical earthquake for which slip on a low-angle normal fault has been documented. Publication: Abbott et al., 2001. Internet Dissemination: Links to presentations, data, and publications are at: http://www.seismo.unr.edu/ftp/pub/louie/dixie/ Contributions to Education and Human Resources: This project provided practical and research experience to undergraduate and graduate students, helping several to gain employment, and funding Ph.D. thesis research. Results from Related Previous NSF Support of K. D. Smith: NSF EAR-99-03200--Processes of active deformation and slip transfer along the Sierra Nevada-Great Basin boundary zone in the Lake Tahoe basin, with R. A. Schweickert, M. M. Lahren, and R. Karlin. Awarded $221,000 for three years, 8/1999- 10/2002. The study integrated geological and earthquake observations of the faults, seismicity, and seismotectonics of the Lake Tahoe Basin. Fault studies were conducted supplementing new bathymetry data from the lake bottom to access the slip rates, fault extent and seismic hazard at Lake Tahoe. Microearthquake deployments were conducted to constrain the sense of motion in different tectonics regimes around the basin and to constrain the dip and activity rates on the Lake’s major normal faults. Several presentations were conducted by the investigators at GSA meetings as well as presentations to numerous local groups at Lake Tahoe outlining the structures, particularly on the lake bottom, and the seismicity of the lake area. A fault map was generated that is currently available from the Nevada Bureau of Mines and Geology. A paper describing the results of the study and the tectonic model (Schweickert et al., 2004) has been accepted for publication in Tectonophysics. Refs added 151 Oppenheimer, D.H., and J.P. Eaton (1984). Moho orientation beneath Central California from regional earthquake travel times, J. Geophys. Res. 89, 10267-10282. Biasi, G.P. and E.H. Humphreys, P-wave image of the upper mantle structure of central California and southern Nevada, Geophys. Res. Lett., 19, 1161-1164. Dueker, K., E. Humphreys, and G. Biasi (1993). Teleseismic imaging of the western Unites States upper mantle structure using the simultaneous iterative reconstruction technique, in Seismic Tomography: Theory and practice, H.M. Iyer and K. Hirahara, eds., Chapman and Hall, London. Hammond, W. and E. Humphreys (2000). Upper mantle seismic wave velocity: effects of realistic partial melt geometries, Journal of Geophysical Research, 105, 10975-10986. Karato, S. and H. Jung (1998). Water, partial melting, and the origin of the seismic low velocity and high attenuation zone in the upper mantle. Earth and Planetary Science Letters 157, 193207.