Finite - Difference Modeling of Two Aftershocks of the 1994

Recent evidence for nonlinear behavior of strong motions generated by the Northridge, California, earthquake relies explicitly on the analysis of aftershock data. Thus, correcting these observations for propagational and source effects becomes a crucial step in these studies. Here, we demonstrate that the dipping structure near the edges of the San Fernando Valley can strongly alter the local site responses, especially for shallow events. We model two aftershocks with similar epicenters, but with shallow (4 km) and deep (16 km) hypocentral depths. Waveforms from 12 portable instruments deployed across the basin and recording these events were processed and studied in the 0.34-Hz bandpass. For the shallow event, distinctive features are a broad direct S phase and large-amplitude surface waves at the basin stations, and high-frequency S phases at stations in the Santa Monica Mountains. The deep event is less strongly affected by the basin but displays strong evidence of the source radiation pattern. An array analysis indicates 2D propagation for most of the strong arrivals. Some of these features are explained by a simple 2D basin model containing a strong shallow contrast, a factor of 2 jump in seismic velocity at a depth of 1 km within the basin, and a moderate deeper gradient.