HR: 0830h
AN: T11D-0425    [PDF]
TI: Geophysical Anomalies and Seismicity Suggest a Connection Between the Hayward and Calaveras Faults, Eastern San Francisco Bay Area, Northern California
AU: * Ponce, D A
EM: ponce@usgs.gov
AF: U.S. Geological Survey, MS989 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Phelps, G A
AF: U.S. Geological Survey, MS989 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Graymer, R W
AF: U.S. Geological Survey, MS989 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Jachens, R C
AF: U.S. Geological Survey, MS989 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Simpson, R W
AF: U.S. Geological Survey, MS989 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Wentworth, C M
AF: U.S. Geological Survey, MS989 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: Gravity, magnetic, and seismicity data of the eastern San Francisco Bay Area are used to reveal the three-dimensional subsurface geologic structure of the eastern San Francisco Bay Area and its relationship to ongoing seismicity. Combined, these data suggest a connection between the Hayward and Calaveras Faults. Gravity and magnetic modeling of a tabular gabbro body near San Leandro and relocated, double-difference seismicity data along the Hayward Fault (Ellsworth et al., 2000) suggest that the Hayward Fault dips to the northeast. Further southeast, double-difference seismicity data indicate that the fault dip becomes shallower, possibly connecting the creeping surface trace of the Hayward Fault with the diverging Mission seismicity trend at depth as suggested by Manaker and Michael (2003). In the stepover region, the southern extension of the Hayward Fault is parallel to the active central Calaveras Fault for about 25 km and the 4-km wide area in between is characterized by en echelon reverse (oblique?) faults. At depths below about 5 km, seismicity appears to be continuous, connecting the Hayward fault to the left-stepping central Calaveras Fault along the Mission seismicity trend. Geophysical interpretation of offset magnetic rock units also suggests that the northern Calaveras Fault has at most a few tens of kilometers of total offset and that most slip may be transferred from the southern Calaveras Fault, with a total offset of about 175 km, along the central Calaveras, Silver Creek, Hayward, and other faults west of the northern Calaveras Fault, consistent with present seismicity. Cross-sectional and 3D visualizations of these data are used to illustrate the proposed geometry of the connection between the Hayward and Calaveras Faults.
DE: 1219 Local gravity anomalies and crustal structure
DE: 1517 Magnetic anomaly modeling
DE: 7299 General or miscellaneous
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting