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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

The 3D velocity structure of the Santa Clara Valley (SCV) was further investigated by modeling the teleseismic P-waves recorded by the 41 seismic stations of the SCV Seismic Experiment (USGS/UCB/PASSCAL, 6/98-12/98). In addition to the UC Berkeley (Stidham et al., 1999; Stidham, 1999) velocity model, the USGS ver. 2 (Jachens, 2000) velocity model was also used. The 3D finite-difference code E3D (Larsen and Schultz, 1995) was used to simulate the propagation of plane waves through each model.
The measured amplitudes and the residual travel times for the P-wave arrivals as well as duration of the P-wave coda for the recorded data outline the basin structure. The general pattern can be reproduced using the synthetics produced by either of the two velocity models. To better quantify differences between the two models and the data, the region was divided into three parts (Figure  20.1) and the differences between the synthetics and the data were analyzed for each part individually.
The results for travel time residuals as a function of distance across the SCV are presented in Figure 20.2. The long wavelength signal across the basins can be observed in data and in synthetics. Overall variations are larger for the UCB model than for the USGS ver. 2 model and their range better matches the observations. Due to computational limitations the slowest velocities in the USGS ver. 2 model were increased. Use of slower velocities would result in larger overall variations for the USGS ver. 2 model as well. USGS ver. 2 model better describes the shape of the observed data. It seems that the basins are wider as well as further apart than currently modeled in the UCB model.
Our future work will include more detailed comparison of the observed and simulated waveforms as well as analysis of the recorded microtremors which suggest continuous excitation of the SCV basins.

Acknowledgements

This research was supported by the USGS grants 99HQGR0057 and 00HQGR0048. The Hellman Faculty Fund is acknowledged for partial support.

References

Jachens, R., Personal communications, 2000.

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., M. Antolik, D. Dreger, S. Larsen, and B. Romanowicz, Three-dimensional structure influences on the strong-motion wavefield of the 1989 Loma Prieta earthquake, Bull. Seism. Soc. Am., 89, 1184-1202, 1999.

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 20.1: Map of the Santa Clara Valley region, located south of the San Francisco Bay. Seismic stations are grouped into sections A, B, and C. The polygons within the SCV region denote the contours of the alluvium basins from the UCB model at 1 km, and 2 km depth. The dashed box presents the size of the velocity model used in the E3D simulations.
\begin{figure}\begin{center}
\epsfig{file=dolenc01_1_1.ps, width=8.6cm}\end{center}\end{figure}

Figure 20.2: Observed and simulated travel time residuals as a function of distance across the SCV. Results for seven teleseismic events were averaged. Mean value and standard deviation is presented for every station. Sections A, B, and C correspond to sections shown in Figure 20.1.
\begin{figure*}\begin{center}
\epsfig{file=dolenc01_1_2.ps, width=14cm}\end{center}\end{figure*}


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