EARTHQUAKE SITE RESPONSE IN SANTA-CRUZ, CALIFORNIA

Aftershocks of the 1989 Loma Prieta, California, earthquake are used t o estimate site response in a 12-km(2) area centered on downtown Santa Cruz. A total of 258 S-wave records from 36 aftershocks recorded at 3 3 sites are used in a linear inversion for site-response spectra. The inversion scheme takes advantage of the redundancy of the large data s et for which several aftershocks are recorded at each site. The scheme decomposes the observed spectra into source, path, and site terms. Th e path term is specified before the inversion. The undetermined degree of freedom in the decomposition into source and site spectra is remov ed by specifying the site-response factor to be approximately 1.0 at t wo sites on crystalline bedrock. The S-wave site responses correlate w ell with the surficial geology and observed damage pattern of the main shock. The site-response spectra of the floodplain sites, which includ e the heavily damaged downtown area, exhibit significant peaks. The la rgest peaks are between 1 and 4 Hz. Five floodplain sites have amplifi cation factors of 10 or greater. Most of the floodplain site-response spectra also have a smaller secondary peak between 6 and 8 Hz. Residen tial areas built on marine terraces above the flood-plain experienced much less severe damage. Site-response spectra for these areas also ha ve their largest peaks between 1 and 4 Hz, but the amplification is ge nerally below 6, Several of these sites also have a secondary peak bet ween 6 and 8 Hz. The response peaks seen at nearly all sites between 1 and 4 Hz are probably caused by the natural resonance of the sediment ary rock column. The higher amplifications at floodplain sites may be caused by surface waves generated at the basin margins. The secondary peak between 6 and 8 Hz at many sites may be a harmonic of the 1- to 4 -Hz peaks. We used waveforms from a seven-station approximately linear array located on the floodplain to calculate the apparent velocity an d azimuth of propagation of coherent arrivals within moving windows of the S-wave codas. The initial windows give results that are consisten t with direct S-wave arrivals. The apparent velocities are high (great er than 4.0 km/sec), and azimuths are from the source. Waves arriving later than 2 sec after the direct S waves have apparent velocities of less than 1 km/sec, indicating that they are surface waves, and arrive from divergent azimuths. This analysis indicates that after the direc t S-wave arrival, long-duration shaking comes from surface waves that are generated at the basin margin and reverberate in the floodplain se diments.