HR: 1320h
AN: S43A-0984    [Abstracts]
TI: THE PREDICTABILITY OF SITE RESPONSE BASED ON THE DATA FROM YOKOHAMA HIGH-DENSITY STRONG MOTION ARRAY
AU: * Tsuda, K
EM: kenichi@crustal.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara, Institute for Crustal Studies, University of California SAnta Barbara, Santa Barbara, CA 93106 United States
AU: Archuleta, R
EM: ralph@crustal.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara, Institute for Crustal Studies, University of California SAnta Barbara, Santa Barbara, CA 93106 United States
AU: Koketsu, K
EM: koketsu@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: By determining the site response at 150 stations of the Yokohama high-density accelerometer array we can analyze the spatial variation of site response over distances from 100 m to 20 km. Using accelerograms at 9 borehole stations we have isolated the frequency dependence in the source, path and site response. We use a nonlinear heat bath algorithm to solve for seismic moment, corner frequency, frequency dependent Q and frequency dependent borehole response (Mo, fc, QofŸx, Sb(f)) by iteratively comparing a computed spectrum with the observed spectrum (Tsuda et al., 2004). We found Q(f) = 285f0.06 for 24 deep events (depth > 40km) and Q(f) = 74f0.28 for 4 shallow events (depth < 25 km). The seismic moments are generally within the factor of two of values determined by NIED. Seismic moments are roughly proportional to fc-3 with a nearly constant stress drop of 100 bars for the deep events. Knowing Mo, fc, and Q(f), we compute the site response for the surface stations as the difference between the predicted and observed amplitude spectrum. A final site response is found by averaging the difference for all of the recorded events. The surface site responses for 0.5-1.0 Hz correlate with the averaged S-wave velocity for the upper 30m. Having obtained site response for 150 stations we have examined the predictability of site response by analyzing the frequency and spatial dependence variation of the site response among the different sites. We have considered both site-to-site and event-to-event variation. In our analysis we examine the ratio of the site response between two stations: a ratio that is frequency dependent. We divided the 150 stations into 7 groups based on the averaged S-wave velocity for upper 30m. The ratio is calculated for any two sites within 5 km of each other. The results showed that the variance of the site response is greater for different frequencies as compared to the spatial separation. We also analyzed event-to-event variation. Event-to-event variation based on shallow vs deep focal depths is small compared to site-to-site variation.
DE: 7223 Seismic hazard assessment and prediction
DE: 7203 Body wave propagation
DE: 7212 Earthquake ground motions and engineering
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting