Finite-difference modeling of two aftershocks of the 1994 Northridge, California, earthquake

Recent evidence for nonlinear behavior of strong motions generated by the N orthridge, California, earthquake relies explicitly on the analysis of afte rshock data. Thus, correcting these observations for propagational and sour ce effects becomes a crucial step in these studies. Here, we demonstrate th at the dipping structure near the edges of the San Fernando Valley can stro ngly alter the local site responses, especially for shallow events. We model two aftershocks with similar epicenters, but with shallow (4 km) a nd deep (16 km) hypocentral depths. Waveforms from 12 portable instruments deployed across the basin and recording these events were processed and stu died in the 0.3-4-Hz bandpass. For the shallow event, distinctive features are a broad direct S phase and large-amplitude surface waves at the basin s tations, and high-frequency S phases at stations in the Santa Monica Mounta ins. The deep event is less strongly affected by the basin but displays str ong evidence of the source radiation pattern. An array analysis indicates 2 D propagation for most of the strong arrivals. Some of these features are e xplained 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.