Regional Resource Area Mapping in Nevada Using the USArray Seismic Network

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1 Regional Resource Area Mapping in Nevada Using the USArray Seismic Network Glenn Biasi, Ileana Tibuleac, and Leiph Preston Nevada Seismological Laboratory, University of Nevada Reno, Reno, NV Sandia National Laboratory, Albuquerque, NM Keywords: seismic imaging, surface wave tomography, regional seismic velocity structure Abstract The Earthscope Transportable Array (TA) seismic network is a significant new development for regional seismic velocity modeling and potential geothermal resource development. While very sparse compared to exploration scale applications, this network nevertheless affords regional modelers with unprecedented resolution and uniformity of coverage. The network is funded by the National Science Foundation through a major earth sciences initiative called Earthscope ( The network is presently in Nevada and the Great Basin, so data are accumulating from which regional velocity models can be constructed. Regional velocity models contribute to geothermal resource assessment in the Great Basin because seismic properties in the crust and shallow mantle correlate with geothermally consequential parameters such as rock temperature, composition and structure. In this paper we provide a first assessment of the actual ray coverage available for modeling structure in Nevada and the adjoining Great Basin and evaluate it in terms of prospects for resolution of crustal features and properties. While it is a work in progress, body-wave ray coverage should be sufficient for resolution of the crust and Moho at a scale of km or better for the central portion of the region. Ray coverage and likely resolution from surface-wave studies is somewhat less because of signal-to-noise competition with naturally occurring background sources. Introduction: Seismic waves at local and regional distances directly probe the physical and thermal state of the crust and shallow mantle. However, because of its scale, a systematic investigation of the Great Basin with dedicated instrumentation would be difficult and expensive. Despite that, just such an experiment is under way, funded the National Science Foundation. Drawing on the analogy of a research telescope as a shared community asset, the USArray seismic component of EarthScope will eventually cover the entire United States in a project to explore the geologic history and deep interior of the continent. The network includes a moveable -station seismic network called the Transportable Array (TA). These stations are presently installed, mostly in Washington, Oregon, Idaho, Utah, and Nevada (Figure ). It is the highest density seismic array of its size ever deployed in the U.S. Network stations are deployed on a grid with approximately 7 km station spacing. Each TA station will be in place for approximately two years, after which it will be relocated farther east. Data are freely available in real time and from an on-line data archive. Figure about here. Previous Seismic Studies in the Great Basin: The contrast of TA coverage with that of previous studies in the Great Basin is substantial. Broad-scale (~ km or more) measurements

2 of crustal thicknesses and upper mantle velocities have been reported based on receiver functions, Pn phases, and surface waves [e.g., Hearn et al., 99; Hearn and Rosca, 99; Priestly et al., 98; Ozalaybey et al., 997]. These measurements indicate an average crustal thickness of ~3 km for the state, with a range from ~5 km under the Battle Mountain Heat Flow High [Stauber and Boore, 978; Priestly et al., 98] to ~38 km under east central NV [Ozalaybey et al., 997; Gilbert and Sheehan, ]. Upper mantle P-wave velocities from these studies average ~7.8±. km/s, and show intriguing associations with faults and regions of high shear strain [Biasi and Humphreys, 99; Humphreys and Dueker, 99; Biasi, 5]. Surface wave studies [e.g., Priestly and Brune, 98] indicate that at least some parts of the state may possess an upper mantle high shear velocity lid ~3 km thick. More detailed studies have focused on specific areas within the state: along ~ N latitude with the PASSCAL and COCORP reflection/refraction lines [e.g., Knuepfer et al., 987; Benz et al., 99; Hauser et al., 987], the Ruby Mountains and the Nevada Test Site. These studies report generally flat or gently dipping Moho and locally, highly reflective lower crust interpreted as the brittle-ductile transition. Crustal velocities derived from these studies broadly show low-velocity basins (<5 km/s) and higher velocity ranges (5-6 km/s) down to ~ km depth. Below this depth the structure was imaged as nearly -D with velocities increasing to upper 6 km/s range at the bottom of the crust and jumping to upper 7 km/s below the Moho. These models, however, are very smooth and were constructed from stitching together -D models and forward modeling. Louie et al. [], Louie et al., [6], and Heimgartner et al., [6] have constructed fully - D velocity models of the crust and upper mantle along refraction lines west and southwest from central Nevada, but the resolution of the models are on the order of 5 km or more. This review shows the potential benefit afforded by the uniform coverage of high-quality stations in the USArray. Body waves and surface waves sample crustal velocity in different ways. The TA network offers station occupation time and coverage to allow inversion of either one. An innovation in our study is to recognize that of inverting both types of waves could make the TA data of particular value to the geothermal community. Body waves travel faster and deeper in the crust to 5 km for stations spaced at 7 km, and sample the shallow crust primarily in a limited cone near the stations. Unfortunately this leaves most of the volume between the stations unsampled except in the mid- to lower crust. At event-station spacing greater than about km, rays refract in the upper Moho, and pass under the crust entirely. By contrast, surface waves are dispersive, meaning that their velocity depends on frequency and integrated crustal shear and compressional wave velocities. At long periods the velocity at which energy travels, or group velocity, is a crustal thickness indicator. On proposing this study we could only project likely coverage and expectations. We can report now on the ray and waveform coverage as it has emerged to date. Body wave and surface wave inversions will rely on somewhat different data so they are discussed separately below. Body Wave Data Coverage: Body waves for smaller events tend to propagate farther at high signal-to-noise ratios than do surface waves. Figure illustrates the increase in measurement density available for geothermal research through the Transportable Array. Lines on this figure are seismic paths from events in the NSL catalog from January, 6 through April 8 to active stations of the TA and southern Nevada networks. The starting period at //6

3 approximately coincides with the first TA station deployments in Nevada. Two sizes of events are shown. The longer, red lines show M 3 events to a distance of.5 degrees (~7 km). These events normally have useable P-waves and some S-waves, but their surface-wave energies are too small to carry to such distances. The purple lines are events with magnitudes of at least.5 and are linked to TA stations within. degrees (~35 km). These are likely to yield good crustal body wave arrivals and based on testing, many surface waves at least to periods of a few seconds. Figure about here. Line density is a rough proxy for resolution. For most of the central Great Basin resolving blocks of km or smaller is clearly feasible. Surface Wave Coverage: Because surface waves do not have the impulsive onsets characteristic of body waves, specialized filtering and identification tools are required. Also, in the earth surface wave energy in periods longer than a few seconds competes with the natural microseismic background. Thus a greater minimum magnitude is adopted in assessing what coverage is available. Figure 3 shows waveforms and detection panels for two stations in northwest Nevada that recorded the April 6, 8 Mogul M.7 (Mw 5.) earthquake. (Figure 3 and here). In the upper panels waveforms are seen to have good energy at the relatively short period of second. Experiments with other events indicate that earthquakes as small as M.5 provide interpretable surface wave arrivals at these periods. Figure shows the Mogul earthquake recording near Wells, Nevada, after filtering it to isolate a range of frequency bands. For this Mw5. event, signal-to-noise ratios are good in all bands. Detections on these waveforms indicate that energy away from the actual back-azimuth can be significant, perhaps as an indicator of structure. To estimate coverage and resolution we show (Figure 5) the coverage of events of M> from the UNR catalog to stations less than degrees distance (~5 km). Coverage will be significantly enhanced by inclusion of events from greater distances in the surrounding region. Considered together resolution of km or smaller blocks for most bands of interest is expected. Figure 5 about here. Conclusions: The Transportable Array component of USArray provides a new and powerful compliment for regional seismic images of Nevada and the Great Basin. This array is presently gathering data, and the actual catalog of events indicates ray coverage for both body and surface wave tomographic images will set a new standard for resolution. Velocity measurements provide direct indications of anomaly location and indirect estimates of parameters such as temperature and composition, opening the door to a range of potentially significant applications of regional seismic data to geothermal resource evaluation in the Great Basin. 3

4 References Benz, H.M., R.B. Smith, and W.D. Mooney (99), Crustal structure of the northwestern Basin and Range province from the 986 program for array seismic studies of the continental lithosphere seismic experiment, Journal of Geophysical Research, 95 (B3),,83-,8. Biasi, G. (5) Mantle lithospheric clues to Walker Lane evolution, Seismological Research Letters, 76, 5. Biasi, G.P. and E.D. Humphreys (99), P-wave image of the upper mantle structure of central California and southern Nevada, Geophysical Research Letters, 9 (), 6-6. Catchings, R.D. and W.D. Mooney (99), Basin and Range crustal and upper mantle structure, northwest to central Nevada, Journal of Geophysical Research, 96 (B), Gilbert, H.J. and A.F. Sheehan, (). Images of crustal variations in the intermountain west, Journal of Geophysical Research, 9, B336, March,. Hauser, E., C. Potter, T.A. Hauge, S. Burgess, S. Burtch, J. Mutschler, R. Allmendinger, L. Brown, S. Kaufman, and J. Oliver (987), Crustal structure of eastern Nevada from COCORP deep seismic reflection data, Geological Society of America Bulletin, 99 (), Hearn, T., N. Beghoul, and M. Barazangi (99), Tomography of the western United States from regional arrival times, Journal of Geophysical Research, 96 (B), 6,369-6,38. Hearn, T.M. and A.C. Rosca (99), Pn tomography beneath the southern Great Basin, Geophysical Research Letters, (), Heimgartner, M., J.N. Louie, J.B. Scott, W. Thelen, C.T. Lopez, and M. Coolbaugh (6), The crustal thickness of the Great Basin: using seismic refraction to assess regional geothermal potential, Geothermal Resources Council Transactions, 3 (number), pages. Hole, J. and B. Zelt (995), 3-D finite-difference reflection traveltimes, Geophysical Journal International, (), 7-3. Humphreys, E.D. and K.G. Dueker (99), Western U.S. upper mantle structure, Journal of Geophysical Research, 99, Knuepfer, P.L.K., P.J. Lemiszki, T.A. Hauge, L.D. Brown, S. Kaufman, and J.E. Oliver (987), Crustal structure of the Basin and Range-Sierra Nevada transition from COCORP deep seismic-reflection profiling, Geological Society of America Bulletin, 98 (), Louie, J.N., M. Heimgartner, A. Pancha, W. Thelen, J. Scott, and C. Lopez (6), A matter of scale: understanding Nevada's sedimentary basins for seismic hazard assessment, Seismological Research Letters, 77 (), 95. Louie, J.N., W. Thelen, S.B. Smith, J.B. Scott, M. Clark, and S. Pullammanappallil (), The northern Walker Lane refraction experiment: Pn arrivals and the northern Sierra Nevada root, Tectonophysics, 388 (-), Ozalaybey, S., M.K. Savage, A.F. Sheehan, J.N. Louie, and J.N. Brune (997), Shear-wave velocity structure in the northern Basin and Range province from combined analysis of receiver functions and surface waves, Bulletin of the Seismological Society of America, 87 (), Priestly, K., J.A. Orcutt, and J.N. Brune (98), Higher-mode surface waves and structure of the Great Basin of Nevada and western Utah, Journal of Geophysical Research, 85 (B),

5 Priestly, K.F. and J.N. Brune (98), Shear wave structure of the southern volcanic plateau of Oregon and Idaho and the northern Great Basin of Nevada from surface wave dispersion, Journal of Geophysical Research, 87 (B), Stauber, D.A. and D.M. Boore (978), Crustal thickness in northern Nevada from seismic refraction profiles, Bulletin of the Seismological Society of America, 68 (), Tibuleac, I.M. and J. M. Britton (6), An automated short-period surface-wave detection algorithm, Bulletin of the Seismological Society of America, 96, Acknowledgements: This work was supported by the Great Basin Center for Geothermal Energy, U.S. Department of Energy, DE-FG36-ID3. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy s National Nuclear Security Administration under Contract DE-AC-9AL85. 5

6 Figure Captions Figure. Station coverage. Red triangles are the USArray Transportable Array stations operating as of March 8. The permanent seismic network (green circles) cannot match the TA for coverage or data quality, but they do contribute the body-wave ray coverage in western and southern Nevada. Figure. Ray coverage to TA stations for events M 3 and distance < 7 km (red) and M.5 and distance < 35 km (purple). Station symbols as from Figure. See text for other details. Figure 3. Example of Rg detection at two stations in Reno vicinity. Upper plots show waveforms recorded on the East (), North () and Vertical (3) components at each station. Lower plots show values of the detection function as a function of time and back azimuth. Detection is declared at a back azimuth corresponding to the maxim of the detection function as shown in Tibuleac and Britton, 6. The true back-azimuth is shown as a green line in the detection function plots. Figure. Waveforms filtered using zero-phase Butterworth filters centered of periods shown in each plot. Rayleigh phases of 3 km/sec velocity arrive at a time lag of approximately seconds. Figure 5. Ray coverage to TA stations likely to yield surface wave detections. Lines connect sources of M earthquakes in the Nevada Seismological Laboratory database within degrees distance (~5 km) of a TA station. Larger events from California and Oregon will contribute to this coverage, but are not shown. 6

7 TA Station Coverage N N 36 N W 6 W W Figure

8 TA Ray Coverage, Likely Body Waves N 36 N W 6 W Figure

9 x 6 PAH Period sec x 7 WCN Period sec Period Period Back azimuth (deg) 3 BAZ 5.86 (deg) AT 7 (sec) 3 Time (sec) from the origin time BAZ (deg) AT (sec) 5 5 Time (sec) from the origin time 8 6 Figure 3

10 5 Period from 3.7 to 7.3 sec x Period from 5. to.3 sec x?-5 5 5? Period from 6.8 to 3. sec x Period from 7.5 to.7 sec ?-5?- 5 5 Period from 8.3 to 6. sec x Period from 9. to 7.7 sec x. E component. N component 3. Z component? NA Period from.3 to. sec x 3 3? Period from 3.6 to 6.5 sec x?- 5 5 Period from 5.8 to 3. sec 5 3 3?- 5 x 3 5?-5 Period from 6. to.8 sec x? Period from.5 to 8.8 sec x 5 5 Time (sec) from the origin time 5 3? Time (sec) from the origin time Figure

11 TA Ray Coverage, M to Degrees N 36 N W 6 W Figure 5