Wide-angle seismic-reflection/refraction data obtained in the vicinity of the epicenter of the 1989 Loma Prieta earthquake provide evidence for a slab of oceanic crust extending from the deep-sea floor to the San Andreas fault. During marine seismic-reflection profiling of the north-central California margin in May 1990, two onshore recording arrays were deployed transverse to and straddling the San Andreas fault in the Santa Cruz Mountains and the Diablo Range. Wide-angle reflections recorded by these arrays define the top and bottom of oceanic crust as far eastward as the San Andreas fault. The wide-angle refraction data obtained by the arrays provide evidence that the oceanic crust, apparently connected to and inferred to represent Pacific plate, is overlain by a seaward-tapering wedge of rocks with velocities of 6.0 to 6.6±0.1 km/s. We interpret this wedge as composed of slivers of San Simeon and Salinian (and, possibly, other) rocks. The wide-angle data are consistent with a landward dip on the oceanic crust of 8°-11°. The velocity model inferred from these data suggests that the hypocenter of the earthquake was located near the base of the interpreted wedge of Salinian rocks beneath the continental margin but above the oceanic crust. On the basis of this geometry, we infer that the top of oceanic crust represents a detachment surface accommodating oceanward movement of North America. This oceanward movement mirrors the eastward-directed tectonic wedging observed beneath the Diablo Range and the western part of the Great Valley. An observed concave-upward geometry of the Moho in the vicinity of the San Andreas fault should strongly focus wide-angle reflections from the Moho that travel along the fault from earthquake sources on or near the fault.