INVERSION OF LOCAL EARTHQUAKE DATA FOR SITE RESPONSE IN SAN-DIEGO, CALIFORNIA

Portable seismic stations were deployed in the San Diego area to inves tigate local site effects on weak ground motion. We analyzed horizonta l S-wave recordings from 161 local and near-regional earthquakes, usin g a least-squares matrix factorization (LSMF) inversion algorithm to s eparate the site effects from the combined effects of the source and p ath. Recording sites included stations underlain by Holocene fill, all uvium, bay sediments and artificial fill, Quaternary and Tertiary form ational deposits, and crystalline rock exhibiting various degrees of w eathering. Spectral ratios of S-wave coda were also used to estimate s ite response at two of the study sites. The coda analysis results were consistent with the results obtained from the inversion analysis. Res ults indicate that severe amplifications are present at some sites und erlain by holocene deposits but demonstrate that surface geology alone provide an inadequate foundation for microzonation. Site-response amp lifications up to a factor of 15, relative to a crystalline rock refer ence site, were observed at one site underlain by artificial fill and bay sediments. Yet a second site underlain by bay sediments exhibited a site response nearly 75% lower than the other bay sediment site, con sistent with the site response observed at sites underlain by more ind urated deposits. The site response observed at the Quaternary and Tert iary sedimentary sites was fairly consistent with the response of the younger alluvial sites, which may be attributable to the relatively sh allow burial of the formational deposits in San Diego. The site respon ses at all five crystalline rock sites included in the study were very similar at frequencies below 4 Hz, and four of the five were similar up to 10 Hz, becoming more distinct at higher frequency. Seismic P-wav e and S-wave refraction data collected at the crystalline rock sites d emonstrate that deviations in response at higher frequencies are attri butable to the presence of weathered horizons in the near surface at t hese sites.