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Seismic-reflection evidence that the Hayward fault extends into the lower crust of the San Francisco Bay area, California

U.S. Geological Survey, MS 999, Menlo Park, California 94025tparsons{at}usgs.gov

Abstract

This article presents deep seismic-reflection data from an experiment across San Francisco Peninsula in 1995 using large (125 to 500 kg) explosive sources. Shot gathers show a mostly nonreflective upper crust in both the Franciscan and Salinian terranes (juxtaposed across the San Andreas fault), an onset of weak lower-crustal reflectivity beginning at about 6-sec two-way travel time (TWTT) and bright southwest-dipping reflections between 11 and 13 sec TWTT. Previous studies have shown that the Moho in this area is no deeper than 25 km (8 to 9 sec TWTT). Three-dimensional reflection travel-time modeling of the 11 to 13 sec events from the shot gathers indicates that the bright events may be explained by reflectors 15 to 20 km into the upper mantle, northeast of the San Andreas fault. However, upper mantle reflections from these depths were not observed on marine-reflection profiles collected in San Francisco Bay, nor were they reported from a refraction prifile on San Francisco Peninsula. The most consistent interpretation of these events from 2D raytracing and 3D travel-time modeling is that they are out-of-plane reflections from a high-angle (dipping 70° to the southwest) impedance contrast in the lower crust that corresponds with the surface trace of the Hayward fault. These results suggest that the Hayward fault truncates the horizontal detachment fault suggested to be active beneath San Francisco Bay.

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