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Borehole Response Studies at the Garner Valley Downhole Array, Southern California.

Luis Fabián Bonilla, Jamison H. Steidl, Jean-Christophe Gariel and Ralph J. Archuleta

Bulletin of the Seismological Society of America, Vol. 92, No. 8, pp. 3165-3179, December 2002

Document type > *Article de revue

Keywords > earthquakes

Research Unit > IRSN/DEI/SARG/BERSSIN

Authors > BONILLA Fabian, GARIEL Jean-Christophe

Publication Date > 26/12/2002

Summary

 

The Garner Valley Downhole Array (GVDA) consists of a set of seven downhole strong-motion instruments ranging from 0- to 500-m depth. One of the objectives of this experiment is to estimate site response and study wave propagation as the energy travels from the bedrock underneath the site up through the soil column. The GVDA velocity structure is studied by computing synthetic accelerograms for a small event located at an epicentral distance of 10 km. These synthetics simulate well the data recorded at the borehole stations. In addition, theoretical transfer func­tions are calculated using the obtained velocity model and compare well with the empirical transfer functions from 54 recorded events. It is also observed that the downgoing wave effect is predominant in the first 87 m and is strongly reduced al depth. Using the velocity structure at GVDA and the transfer function results, it has also been possible to develop a simple method to compute the incident wave field, which is needed in nonlinear site response for instance.

Recently there have been many comparative studies between horizontal-to-vertical (H/V) spectral ratios and traditional spectral ratios. Although many of these studies show that H/V spectral ratios can reproduce the shape of the site response curve, most show differences in the amplitude level. In the case of Garner Valley, where we have both surface and multiple borehole instruments, we find that this discrepancy in amplitude of the site response estimates is because the vertical component has significant site response associated with it due to S-to-P conversions that begin in the weathered granite boundary at 87-m depth.


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