THE SEISMIC CODA, SITE EFFECTS, AND SCATTERING IN ALLUVIAL BASINS STUDIED USING AFTERSHOCKS OF THE 1986 NORTH PALM-SPRINGS, CALIFORNIA, EARTHQUAKE AS SOURCE ARRAYS

We use single-station recordings of widely distributed earthquake sour ces to identify heterogeneities in the Earth's crust that scatter ener gy into the early part of the observed S-wave codas. We assume that ra y theory accurately describes the propagation of body waves from the e arthquake sources to the scatterers, but we assume no propagation mech anism between the scatterers and the seismic station. A grid of hypoth etical scatterers is assumed to cover the Earth's surface, and the str ength of each scatterer is determined by an iterative inversion method . Our data are recordings of aftershocks of the 1986 North Palm Spring s earthquake recorded at a seismic station in the Coachella Valley, SM P, and at a station in the Morongo Valley, SMC. The technique is appli ed separately to each horizontal component of motion at each station. We identify a particular spot at the northwest edge of the Quaternary alluvium in the Coachella Valley that scatters waves into the S coda o bserved at SMP 8 km distant in the valley. The geologic structure that causes the strong scattering seems to be the basin-edge structure rat her than the mountainous terrain 3 km farther west. The early coda at SMC is dominated by scattering within a few kilometers of SMC, either in the basin itself or in the Morongo Valley fault zone. This study sh ows that laterally propagating waves are sometimes observable in the e arly coda, casting doubt on the use of 1-D techniques for estimating s ite responses. The northwest edge of the Coachella Valley seems to be an especially strong scatterer of incident waves. If other basins have similar loci of strong scattering, perhaps these places can be identi fied using microearthquake seismograms and used to predict strong grou nd motions in the basins.