SOUTHERN EXTENSION OF THE HAYWARD FAULT AND ITS RELATIONSHIP TO THE CALAVERAS AND SILVER CREEK FAULTS, NORTHERN CALIFORNIA

Geophysical interpretations, constrained by geologic, seismicity, and seismic profiling data provide insights into the three-dimensional structure of the Hayward, Calaveras, and Silver Creek Faults and intervening basins, and play an important role in the assessment of their seismic behavior. Because gravity and magnetic anomalies reflect lateral changes in the density and magnetic properties of subsurface rocks they can be used to infer subsurface geologic structure. In the eastern San Francisco Bay Area, gravity lows reflect moderately deep Neogene sedimentary basins, including the Evergreen basin in the Santa Clara Valley, whereas gravity highs reflect more dense Mesozoic rocks of the Franciscan and Great Valley Complexes. Magnetic highs primarily reflect moderately magnetic mafic and ultramafic rocks in this region such as serpentinite exposed at Oak Hill and along the Silver Creek and Calaveras Faults and a gabbro body located northeast of the Silver Creek Fault near Metcalf Canyon.

One area of particular importance, especially in terms of seismic hazard, is the stepover region between the Hayward and Calaveras Faults. Geophysical modeling, seismicity data, and seismic-reflection data suggest that the generally steep northeast dip of the Hayward Fault progressively shallows to the southeast. Although there is no apparent through-going structure at the surface, hypocenter data suggest that the Hayward Fault ultimately connects with the Calaveras Fault in a simple way at seismogenic depths. The northeast dip of the Hayward Fault also accounts for the apparent off-fault seismicity that follows along the Mission seismicity trend connecting the two faults. At the surface, the southern extension of the Hayward Fault is expressed as a series of en echelon reverse and oblique faults of modest seismicity. The left-stepping connection between the Hayward and Calaveras Faults spans a zone about 4-km wide and about 25-km long. Geophysical models across this step-over region indicate that the Evergreen basin is about 5-km thick; that serpentinite, gabbroic, and forearc Great Valley Sequence rocks lie above basin-filling gravels; and that gabbro, exposed at the south end of the Hayward-Calaveras stepover occurs only as a thin near-surface feature.