The strong ground motions recorded within the San Fernando Valley and in the Los Angeles Basin during the 17 January 1994 Northridge earthquake show complex waveforms and complicated patterns of peak acceleration and velocity. This complexity persists even at long periods (1 to 10 sec). We investigate basin structure effects on long-period wave propagation in the San Fernando Valley and in the Los Angeles Basin by the 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 explained by basin structure effects, assuming a simple point source model. The structural effects and a source model composed of two subevents can explain the most prominent characteristics of the strong-motion waveforms observed during the mainshock at sites south of the epicenter, while such a combination is not enough to explain the large strong motions observed in the north. This result suggests that the rupture propagation effects significantly amplified the ground motions at sites north of the epicenter.