Physics-Based Shake Map Simulation for the 2008 Wells, Nevada Earthquake

Physics-Based Shake Map Simulation for the 2008 Wells, Nevada Earthquake Shaded-relief map of basin-floor topography in northeastern Nevada (Saltus and Jachens, 1995) surrounding the Feb. 21, 2008 Wells earthquake, 225 km on a side (the basin dataset used ends artificially at 42°N). Locations of USArray Transportable Array (TA) stations recording the event are indicated. Warmer colors correspond to greater shaking computed for the event by the E3D code of Larsen et al. (2001), including the effects of geologic basins and predicted geotechnical velocities. Solid yellow indicates peak ground velocities (PGV) above 5 cm/s. This computation assumes an eastward directivity of the earthquake rupture. Shaking is high along the many basins, and high levels of shaking channel along the basins and can tunnel across ranges between basins, with examples indicated. In the inset graph this shaking prediction (orange bars) matches the recorded PGV (red) well, while other directions of directivity (blue) do not match.
The standard USGS ShakeMap for the Feb. 21, 2008 M6.0 Wells, Nevada earthquake shows many “bulls-eye” anomalies. The simple ShakeMap algorithm tried to average between high shaking in the populated basins reported by citizens to the USGS Community Internet Intensity Map (CIIM), and low shaking measured at several USArray Transportable Array (TA) stations on bedrock. Starting with the complex basin-thickness map of Saltus and Jachens [1995], we built a 3d seismic-velocity model that also included a projection of shallow geotechnical velocities. Various models for the directivity of rupture of 15x15-km normal-fault planes fed into computations predicting shaking across the region, employing the physics-based E3D code of Larsen et al. [2001]. The simulated peak-ground-velocity (PGV) maps show a high degree of channeling along the many basins in the region, with adjacent basin and bedrock areas only 2 km apart predicted to experience levels of shaking differing by a factor of ten. There is also evidence that strong basin shaking “tunnels” across narrow parts of bedrock ranges separating basins. The physics-based shake map is highly heterogeneous in this Basin and Range region (see figure) and contains prominent features that a standard, statistical USGS ShakeMap cannot predict. The physics-based simulation produces a good match against PGV values recorded at nearby TA stations, when an eastward rupture directivity is used at the source. The match is highly sensitive to the rupture directivity.
</p><p>Larsen, S., Wiley, R., Roberts, P., and House, L., 2001, Next-generation numerical modeling: incorporating elasticity, anisotropy and attenuation: Society of Exploration Geophysicists Annual International Meeting, Expanded Abstracts, 1218-1221.
</p><p>Saltus, R. W., and Jachens, R. C., 1995, Gravity and basin-depth maps of the Basin and Range Province, Western United States: U.S. Geological Survey, Geophysical Investigations Map, Report: GP-1012, 1 sheet.
</p><p>Acknowledgements: Research partially supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award numbers 08HQGR0046 and 08HQGR0015. 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.</p>


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