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

Abstract

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 at 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. Manuscript received 27 August 2001.