D. C. Templeton, R. M. Nadeau, and R. Burgmann
The juncture between the San Andreas fault and the southern Calaveras fault is a complex area where subsurface seismicity does not always follow surface fault traces and where secondary faults, such as the Quien Sabe fault zone, may play an active role in accommodating deformation. In this study, we relocate seismicity at the juncture of these two major faults to determine the subsurface seismic structure. Earthquakes are relocated using the USGS double-difference hypocenter relocation program hypoDD on Northern California Seismic Network (NCSN) data archived between 1984 and 2001. We found that the seismicity on the San Andreas and southern Calaveras faults is offset to the west of their surfaces traces and that seismicity on the Calaveras fault is more diffuse and discontinuous than on the San Andreas fault. In the northern part of our area, the southern Calaveras fault is composed of several short, well-defined stepping segments but in the southern half its seismicity is more diffuse. The Quien Sabe fault zone is highly complex and composed of several short, parallel and intersecting fault planes that are much more seismically active than the neighboring Calaveras fault. We identify repeating earthquakes via waveform cross-correlation techniques to determine subsurface fault slip rates. The approach and assumptions of Nadeau and McEvilly (1999) were used to identify repeating earthquake sequences, associated recurrence interval times and slip rates. Initial slip rate estimates for the San Andreas, southern Calaveras and Quien Sabe fault zone are found to be between 12 and 24 mm/yr, 4 and 13 mm/yr, and 4 and 10 mm/yr, respectively. The simple fact that we see repeating earthquakes on these faults, indicate that they are slipping at depth even though there may be little surface expression of this slip. These calculated fault slip rates are compared to simple elastic models of slip rate distribution and complementary, independently determined geodetic and geologic data.