MODELING 3-DIMENSIONAL SITE RESPONSE EFFECTS IN THE MARINA DISTRICT BASIN, SAN-FRANCISCO, CALIFORNIA

Loma Prieta aftershock data recorded in the Marina District of San Fra ncisco clearly show a large amplification in ground motion and a much longer duration of shaking relative to the response recorded at nearby rock sites for frequencies between 1 and 10 Hz. Some of the amplifica tion can be explained by the difference in impedance between the Marin a sites and the rock sites; however, 1D models underpredict the amplif ication in the range 1 to 3 Hz and cannot account for the increased du rations. The available geologic information within the Marina District indicates lateral variations in sediment thickness and composition. O ur previous experience with modeling wave propagation in these types o f basin environments has shown that these structures can greatly ampli fy and prolong the duration of the signals observed at sites within th e basin. In this study, the seismic response of the Marina basin is mo deled with a 3D finite-difference wave field simulation technique. Res ults from the numerical simulations at the basin sites indicate that t he large amplitude arrivals following the direct S wave are surface wa ves traveling at relatively slow apparent velocities. In general, the surface waves contain the strongest recorded motions and produce the l ong duration of shaking observed at these sites. In addition, spectral ground motions from the 3D basin models do fairly well in reproducing the observed spectral amplification near 1 Hz that is characteristic of the basin sites. The simulation results suggest that focusing effec ts due to the geometry of the basin structure itself have a significan t impact in determining the resonant periods at these basin sites.