SEISMICITY AND THE MAJOR STRIKE-SLIP FAULTS BORDERING THE SANTA CLARA VALLEY, CALIFORNIA

The Santa Clara Valley lies between two major strike-slip faults: the San Andreas to the SW and the Calaveras-Hayward to the NE. The current geometry of these faults, branching as they do from the creeping section of the San Andreas in central California, is not sustainable in the long term without deformation within and around the branches and/or adjustments of the major faults themselves. Numerous secondary faults, strike-slip, oblique, and reverse, including the Sargent, Zayante, and Monte Vista, currently help to accommodate the necessary deformation. The geometry of one of the major fault systems, the Calaveras-Hayward, has also changed since strike-slip tectonics became the dominant mode of deformation in the region as the Mendocino Triple Junction moved northward past this latitude from ~12 to 8 Ma.

Large historical events on the San Andreas occurred in 1989 (M6.9), 1906 (M7.8), 1890 (M6.3), and 1838 (M6.8) and on the southern Hayward in 1868 (M6.8). Historical events on the Calaveras fault have been more moderate in size, including ones in 1984 (M6.2), 1979 (M5.9), and 1911 (M6.2). Most seismicity in the region (1984-2004) lies above ~12 km depth, with the exception of the 1989 Loma Prieta events, which reach nearly 20 km. The San Andreas fault surface is well defined by seismicity in some reaches, but only poorly in others. In contrast, seismicity on the central and southern Calaveras fault clearly defines a near vertical to steeply east dipping fault surface. This surface exhibits a more linear geometry at depth than the mapped Calaveras trace, suggesting that the upper several kilometers of the fault have been warped by deformation. The strike-slip Calaveras surface appears to be offset in places by intersections with active oblique and reverse faults. Such intersections might explain the linear alignments of repeating micro-earthquakes on the Calaveras and other faults. An apparent discontinuity in the orientation of the Calaveras surface at the location where the extrapolated Silver Creek fault surface would intersect could indicate activity on the Silver Creek fault in the not too distant past. These examples strongly suggest that the faults in and around the Santa Clara Valley are constantly evolving and reorganizing themselves.