BASIN STRUCTURE EFFECTS ON LONG-PERIOD STRONG MOTIONS IN THE SAN-FERNANDO VALLEY AND THE LOS-ANGELES BASIN FROM THE 1994 NORTHRIDGE EARTHQUAKE AND AN AFTERSHOCK

The strong ground motions recorded within the San Fernando Valley and in the Los Angeles Basin during the 17 January 1994 Northridge earthqu ake show complex waveforms and complicated patterns of peak accelerati on and velocity. This complexity persists even at long periods (1 to 1 0 sec). We investigate basin structure effects on long-period wave pro pagation in the San Fernando Valley and in the Los Angeles Basin by th e two-dimensional finite-difference modeling of the mainshock and one selected aftershock with M = 4.1. The aftershock ground motion records in the San Fernando Valley and in the Los Angeles Basin can be explai ned by basin structure effects, assuming a simple point source model. The structural effects and a source model composed of two subevents ca n explain the most prominent characteristics of the strong-motion wave forms observed during the mainshock at sites south of the epicenter, w hile such a combination is not enough to explain the large strong moti ons observed in the north. This result suggests that the rupture propa gation effects significantly amplified the ground motions at sites nor th of the epicenter.