Abstract

Intermediate-period seismograms recorded at Pasadena of earthquakes occurring along a profile to Imperial Valley are studied in terms of source phenomena versus path effects. Some of the events have known source parameters, determined by teleseismic or near-field studies, and are used as master events in a forward modeling exercise to derive the Green's functions (displacements at Pasadena due to a pure strike-slip or dip-slip mechanism) that describe the propagation effects along the profile. Both timing and waveforms of records are matched by synthetics calculated from two-dimensional velocity models. The best two-dimensional section begins at Imperial Valley with a thin crust containing the basin structure and thickens towards Pasadena. The detailed nature of the transition zone at the base of the crust controls the early arriving shorter periods (strong motions), while the edge of the basin controls the scattered longer-period surface waves. From the waveform characteristics alone, one can easily distinguish shallow events in the basin from deep events as well as the amount of strike-slip versus dip-slip motions. Those events rupturing the sediments, such as the 1979 Imperial Valley earthquake, can be recognized easily by a late arriving scattered Love wave which has been delayed by the very slow path across the shallow valley structure.

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