Assembling a Nevada 3D Velocity Model: Earthquake-Wave Propagation in the Basin & Range, and Seismic Shaking Predictions for

Assembling a Nevada 3D Velocity Model: Earthquake-Wave Propagation in the Basin & Range, and Seismic Shaking Predictions for Las Vegas, Figure 2 Figure 2.
Maps comparing M7.5 Furnace Creek fault scenarios affecting Las Vegas. The upper image is a basin thickness map for the region of computation; middle is a snapshot map of E3D wave propagation at 0.3 Hz through the assembled model. Below are maximum ground-motion (PGV) maps, on the left for rupture away from Las Vegas, and on the right for rupture toward the city.
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The development of an open-source 3d modeling environment allows seismologists, explorationists, engineers, and students to predict wave propagation through geologically complex regions. The environment combines geologic and geotechnical data sets with gridding, modeling, and output specifications into portal packs for execution on standalone workstations, clusters, and supercomputing grids. A tutorial interface helps the user scale the grid to the facilities available, from small test runs to efforts requiring major resources. The ability to configure computations at a range of scales and model complexity is intended to promote wide use of advanced seismic modeling. Geologic models can include many basins in addition to the target urban basin, and detailed geotechnical information where available. To predict earthquake shaking in Nevada urban areas, the 3d model assembles several data sets at a wide variety of scales, from regional geologic maps to shallow shear-velocity measurements from microtremor transects having 0.3-km spacing (fig. 1). For Las Vegas the principal earthquake hazard is from the Furnace Creek fault system, capable of M7.5 events. Peak ground velocity (PGV) results from finite-difference wave modeling at 0.3 Hz show no obvious correlation between amplification and basin depth or dip of the basin floor. Animations of shaking show the expected strong trapping and long shaking durations within basins, as well as diffusion and scattering of energy between the many basins in the region. The two Furnace Creek scenarios tested, involving rupture away from and toward Las Vegas, produced unexpectedly different PGV in the city (fig. 2). Rupture directivity toward the city may amplify shaking by a factor of fifteen at some locations. Despite affecting only the very shallow- est zone of models (<30 m), the Vs30 geotechnical shear-velocity shows clear correlation to 0.3-Hz PGV predictions in basins. Increasing basin thicknesses to 1.3 km correlate with increased PGV, but the basin effect at 0.3 Hz saturates for basin thicknesses greater than 1.3 km; deeper parts of the basin show variance and uncertainty of a factor of two in predicted PGV.
</p><p>References
</p><p>Louie, John N., 2008, Assembling a Nevada 3-d velocity model: earthquake-wave propagation in the Basin & Range, and seismic shaking predictions for Las Vegas: SEG Expanded Abstracts, 27, 2166-2170.
</p><p>Acknowledgements: Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award numbers 08HQGR0015 and 08HQGR0046; by Lawrence Livermore National Laboratory under LDRD Test Readiness funds; and by a Fulbright Senior Scholar award for work in New Zealand. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. Instruments used in the field program were provided by the PASSCAL facility of the Incorporated Research Institutions for Seismology (IRIS) through the PASSCAL Instrument Center at New Mexico Tech. Data collected during this experiment will be available through the IRIS Data Management Center. The facilities of the IRIS Consortium are supported by the National Science Foundation under Cooperative Agreement EAR-0552316 and by the Department of Energy National Nuclear Security Administration.</p>

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