Abstract
Strong motion recordings of the 1989 Loma Prieta earthquake show a large, coherent SH wave displacement pulse across much of the San Francisco Bay area. This distinctive ground motion has been modelled using approximate, broadband synthetics for a double‐couple centroid source in a 3‐D crustal model for the region. The synthetics indicate that (1) horizontal bending and focusing of SH waves by the lateral crustal velocity contrast across the San Andreas fault and (2) the source radiation pattern, contribute significantly to the observed azimuthal variation of the SH pulse and hence to the regional variations of strong ground shaking in the period range of 1 to 8 sec. In addition, the scattering of surface‐reflected shear waves by deep basins under the Santa Clara Valley may contribute to increased coda amplitude and duration observed for sites around Oakland.The wave modelling also supports the earlier conclusion that an increase in intensity in portions of San Francisco and Oakland is in part the result of energy reflected from the base of the crust [Somerville and Yoshimura, 1990].
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