SEISMIC WAVE-FORM MODELING IN THE LOS-ANGELES BASIN

The proximity of several recent earthquakes to the Los Angeles sedimen tary basin provides an opportunity to isolate the effects of the basin on wave propagation. The 4 October 1987 aftershock (M(L) 5.3) of the Whittler Narrows sequence and the 28 June 1991 Sierra Madre mainshock (M(L) = 5.8) are on a similar azimuth to stations overlying the deepes t part of the basin, A distinctive feature of records from basin stati ons recording the 4 October aftershock is the large amplitude of multi ple S, SS, etc, The multiples have up to twice the amplitude of the di rect S phase on the tangential component, At such a short range, less than 25 km, a horizontal seismic velocity gradient is needed to turn r ays rapidly enough for large-amplitude multiples to form. A forward mo deling approach is employed, using finite-difference numerical techniq ues that produce double-couple point-source solutions. A model based o n a recent geologic cross section constructed for the east edge of the Los Angeles Basin generates more phases than are seen in the seismic records. Simpler models, based on dipping layers with low shear veloci ties in the top few layers, fit the data better. The seismic velocity, depth, and dip of the layers are varied to fit the timing between the direct P, the direct S, and the first S multiple. The timing and ampl itude of the direct and first multiple S pulses are well modeled, thou gh the phase of the first multiple does not match the data. Including a steeply dipping west edge in the basin model has little effect on th e synthetic waveforms, except at distances near that basin edge. The a mplitude of SS is greatest in the deepest part of the basin, where it is two times larger than direct S. The coda duration increases from 8 sec to more than 20 sec from the NE to the SW. The Sierra Madre mainsh ock occurred about 25 km to the NE of the Whittler Narrows sequence. T he model for Whittler Narrows was extended to this distance, with a sh allow basin between Whittler and the Sierra Madre hypocenter to simula te the San Gabriel sedimentary basin. Phases generated by the edge of the deep basin continue to dominate the synthetic waveforms, but this model generates a lengthy coda, This study shows that specific phases with frequencies up to 1 Hz that have traveled through deep sedimentar y basins can be explained by two-dimensional seismic velocity models.