Seismic refraction profiles across and along the San Andreas fault zone near Palmadale, California, were used to establish propagation paths for several phases whose travel times are being monitored for earthquake precursors. The profiles were generated using an airgun submerged in Bouquet Reservoir, together with surface and borehole geophone receivers. Source signal stacking, coupled with the noise reduction provided by the downhole receivers, provided data of sufficient quality to measure the ray parameters of the phases. Using the ray parameters and surface travel-time curves, velocity structures parallel and transverse to the fault zone have been constructed. Two of the observed phases are confined to the upper portions of the crustal structure, which was found to differ across the fault. A third phase is consistent with a reflection from a depth of approximately 10 km. Two later arrivals are suggestive of reflections from depths of 21 and 27 km.

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