Dj. Wald et Rw. Graves, THE SEISMIC RESPONSE OF THE LOS-ANGELES BASIN, CALIFORNIA, Bulletin of the Seismological Society of America, 88(2), 1998, pp. 337-356
Using strong-motion data recorded in the Los Angeles region from the 1
992 (M-w 7.3) Landers earthquake, we have tested the accuracy of exist
ing three-dimensional (3D) velocity models on the simulation of long-p
eriod (greater than or equal to 2 sec) ground motions in the Los Angel
es basin and surrounding San Fernando and San Gabriel Valleys. First,
the overall pattern and degree of long-period excitation of the basins
were identified in the observations. Within the Los Angeles basin, th
e recorded amplitudes are about three to four times larger than at sit
es outside the basins; amplitudes within the San Fernando and San Gabr
iel Valleys are nearly a factor of 3 greater than surrounding bedrock
sites, Then, using a 3D finite-difference numerical modeling approach,
we analyzed how variations in 3D earth structure affect simulated wav
eforms, amplitudes, and the fit to the observed patterns of amplificat
ion. Significant differences exist in the 3D velocity models of southe
rn California that we tested (Magistrale et al,, 1996; Graves, 1996a;
Hauksson and Haase, 1997). Major differences in the models include the
velocity of the assumed background models; the depth of the Los Angel
es basin; and the depth, location, and geometry of smaller basins. The
largest disparities in the response of the models are seen for the Sa
n Fernando Valley and the deepest portion of the Los Angeles basin. Th
ese arise in large part from variations in the structure of the basins
, particularly the effective depth extent, which is mainly due to alte
rnative assumptions about the nature of the basin sediment fill. The g
eneral ground-motion characteristics are matched by the 3D model simul
ations, validating the use of 3D modeling with geologically based velo
city-structure models. However, significant shortcomings exist in the
overall patterns of amplification and the duration of the long-period
response. The successes and limitations of the models for reproducing
the recorded ground motions as discussed provide the basis and directi
on for necessary improvements to earth structure models, whether geolo
gically or tomographically derived. The differences in the response of
the earth models tested also translate to variable success in the abi
lity to successfully model the data and add uncertainty to estimates o
f the basin response given input ''scenario'' earthquake source models
.