Abstract

Teleseismic P-wave data collected at the San Andreas Observatory (SAO) seismic station show strong frequency-dependent, nonrectilinear particle motions consisting of off-azimuth horizontal displacements that are sometimes larger than the vertical displacements. The azimuthal dependence of the horizontal components amplitude is directly related to the symmetry of the fault zone. A 3D finite-difference program (Frankel and Vidale, 1992) is used to examine the response of the San Andreas fault-zone velocity structure to incident teleseismic P waves. Synthetic seismograms are formed using a 3D representation of an assumed 2D fault-zone structure. The fault-zone velocity model produces large-amplitude phases that contain combinations of refracted and internally reflected P-to-S converted phases, diffractions, and possible P-to-Rayleigh converted phases. The largest-amplitude phases observed in the data are nondispersive and have complicated particle motions confined to the vertical plane perpendicular to the fault zone. The data are fit best by synthetic receiver functions for field points similar to the location of SAO relative to the San Andreas fault zone. Since the velocity model used in the computations is nonunique, sensitivity studies were performed by modifying the structure. Synthetics match the timing and nature of the observed phases at SAO showing that assumption of plane-layered structure is invalid in the area of this station.

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