Dipping San Andreas and Hayward faults revealed beneath San Francisco Bay,California

The San Francisco Bay area is crossed by several right-lateral strike-slip faults of the San Andreas fault zone. Fault-plane reflections reveal that t wo of these faults, the San Andreas and Hayward, dip toward each other belo w seismogenic depths at 60 degrees and 70 degrees, respectively, and persis t to the base of the crust, Previously, a horizontal detachment linking the two faults in the lower crust beneath San Francisco Bay was proposed, The only near-vertical-incidence reflection data available prior to the most re cent experiment in 1997 were recorded parallel to the major fault structure s. When the new reflection data recorded orthogonal to the faults are compa red with the older data, the highest amplitude reflections show clear varia tions in moveout with recording azimuth, In addition, reflection times cons istently increase with distance from the faults. If the reflectors were hor izontal, reflection moveout would be independent of azimuth, and reflection times would be independent of distance from the faults, The best-fit solut ion from three-dimensional traveltime modeling is a pair of high-angle dipp ing surfaces. The close correspondence of these dipping structures with the San Andreas and Hayward faults leads us to conclude that they are the faul ts beneath seismogenic depths. If the faults retain their observed dips, th ey would converge into a single zone in the upper mantle similar to 45 km b eneath the surface, although we can only observe them in the crust.