THE SEISMIC RESPONSE OF THE LOS-ANGELES BASIN, CALIFORNIA

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 .