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Basin Structure Influences on the Teleseismic Wave Propagation in the Santa Clara Valley, California

David Dolenc, Douglas Dreger, Shawn Larsen

Research

It is well known that basins can generate strong ground motions. In order to recognize the areas of greatest damage from the future earthquakes it is important to know the characteristic properties of the basins such as their geometry and seismic velocities of the basin materials. Influence of the Santa Clara Valley basin on the propagation of the teleseismic P-waves was studied in order to evaluate the possibility of detecting and constraining the basin structure.
During the Santa Clara Valley Seismic Experiment (SCVSE; USGS/UCB/PASSCAL, 6/98-12/98) several teleseismic events were recorded with the array of 41 seismic stations (Figure 21.1). Amplitudes and the residual travel times for the teleseismic P-wave arrivals recorded on the SCVSE stations were measured (Figure 21.2). At the same time a model of the Santa Clara Valley basin (Stidham, 1999) and a 3D finite-difference code (Larsen and Schultz, 1995) were used to simulate propagation of a plane wave through the basin. For each event the orientation of the incoming plane wave was taken into the account. Calculations were repeated using a homogeneous model to evaluate the influence of the model size and the reflections from the model boundaries on the results.
Analysis of the recorded data and of the calculated synthetics show that in both cases the amplitudes and residual travel times for the first arrivals outline the alluvial basins (Figure 21.2). Amplitude and the duration of the P-wave coda are also much larger for the stations located above the basins. Presented results are consistent for all the events studied.
Further work will include the analysis of the P-wave coda duration and its frequency composition to constrain the basin structure.

Acknowledgements

This research was supported by the USGS grants 99HQGR0057 and 00HQGR0048.

References

Larsen, S. and C. A. Schultz, ELAS3D: 2D/3D elastic finite-difference wave propagation code, Technical Report No. UCRL-MA-121792, 19 pp., 1995.

Stidham, C., Three-dimensional crustal structure influences on wave propagation and generation of strong ground motion in the greater San Francisco Bay region, Ph.D. Thesis, University of California, Berkeley, November 1999.


  
Figure 21.1: Locations of the teleseismic events and of the Santa Clara Valley.
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Figure 21.2: Amplification of the P-waves recorded on the SCVSE stations relative to station 186 for the event 7 (A). Residual travel times for the event 7, recorded on the SCVSE stations. Arrival times relative to station 186 were first calculated by crosscorrelation. Residual travel times were obtained by removing the planar wave from the data (B). Amplitude amplifications and travel time residuals for the synthetics calculated for the event 7 (C, D). Crosses: SCVSE stations. Red lines: contours of the alluvial thickness at the surface, at 1 km and at 2 km depth for the model used in the calculations.
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Next: Historical Earthquake Re-analysis Project Up: Ongoing Research Projects Previous: Influence of Near-source Velocity

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