HR: 0830h
AN: S51E-0093    [PDF]
TI: Long-Period Microtremor Observations in the Santa Clara Valley, California
AU: * Dolenc, D
EM: dolenc@seismo.berkeley.edu
AF: UC Berkeley Seismological Lab, 215 McCone Hall, UC Berkeley, Berkeley, CA 94720 United States
AU: Dreger, D
EM: dreger@seismo.berkeley.edu
AF: UC Berkeley Seismological Lab, 215 McCone Hall, UC Berkeley, Berkeley, CA 94720 United States
AB: In our previous work we investigated the 3D velocity structure of the Santa Clara Valley (SCV) by modeling the teleseismic P-waves recorded by the 41 seismic stations of the SCV Seismic Experiment (USGS/UCB/PASSCAL, 6/98-12/98). To complement these results, we now focused on the microseisms that were recorded during the same SCV seismic experiment. Microseisms are generated by the pressure variations on the sea floor due to the ocean waves and can be observed in the 0.1 to 5 Hz frequency range. The seismic noise level of several earthquake-free periods was first compared to the ocean wave heights recorded at the Santa Cruz weather buoy and showed that the two are correlated. Horizontal to vertical (H/V) spectral ratios of microtremor signals for a 5-day earthquake-free period were then calculated for each station. The H/V spectral ratios for the 5-minute segments at the beginning of each hour were first calculated and then averaged over the 5 days. The results showed that the dominant period of the H/V spectral ratios in the 0.1 to 1 Hz frequency range is stable with time and is a function of the USGS model basin depth. The longer periods can be observed for the stations above the two basins and the shorter periods for the stations in-between the basins. There was no correlation between the amplitudes of the H/V spectral ratios and the USGS model basin depths. We measured the peak ground velocity (PGV) and the recorded energy for the two local earthquakes recorded during the SCV seismic experiment (San Juan Bautista, M5.4, and Gilroy, M4.0). The PGV and the recorded energy are also a function of the station location. The results obtained with the local earthquakes as well as the results obtained with the previous teleseismic study validated the use of the microtremor H/V method in the prediction of the effects of the deep basin structure on long-period earthquake ground motions as well as provided the calibration for the H/V method. In addition we modeled the response of the sedimentary layers using the 1D structure from the USGS and the UCB model under each SCV seismic station to simulate the observations and to test if the results could be used to refine the basin model structure.
DE: 7200 SEISMOLOGY
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2003 Fall Meeting