Rw. Graves, MODELING 3-DIMENSIONAL SITE RESPONSE EFFECTS IN THE MARINA DISTRICT BASIN, SAN-FRANCISCO, CALIFORNIA, Bulletin of the Seismological Society of America, 83(4), 1993, pp. 1042-1063
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.