HR: 0800h
AN: S21A-0239    [Abstracts]
TI: Segmentation of the Calaveras-Hayward Fault System Based on 3-D Geometry and Geology at Large-Earthquake Depth
AU: * Graymer, R W
EM: rgraymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025, United States
AU: Simpson, R W
EM: simpson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025, United States
AU: Jachens, R C
EM: jachens@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025, United States
AU: Ponce, D A
EM: ponce@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025, United States
AU: Phelps, G A
EM: gphelps@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025, United States
AU: Watt, J T
EM: jwatt@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025, United States
AU: Wentworth, C M
EM: cwent@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025, United States
AB: For the purpose of estimating seismic hazard, the Calaveras and Hayward Faults have been considered as separate structures and analyzed and segmented based largely on their surface-trace geometry and the extent of the 1868 Hayward Fault earthquake. Recent relocations of earthquakes and 3-D geologic mapping have shown, however, that at depths associated with large earthquakes (>5 km) the fault geology and geometry is quite different than that at the surface. Using deep fault geometry inferred from these studies we treat the Hayward and Calaveras Faults as a single system and divide the system into segments that differ from the previously accepted segments as follows:
1. The Hayward Fault connects directly to the central Calaveras Fault at depth, as opposed to the 5 km wide restraining stepover zone of multiple imbricate oblique right-lateral reverse faults at the surface east of Fremont and San Jose (between about 37.25°-37.6°N).
2. The segment boundary between the Hayward, central Calaveras, and northern Calaveras is based on their Y- shaped intersection at depth near 37.40°N, 121.76°W (Cherry Flat Reservoir), about 8 km south of the previously accepted central-northern Calaveras Fault segment boundary.
3. The central Calaveras Fault is divided near 37.14°N, 121.56°W (southern end of Anderson Lake) into two subsegments based on a large discontinuity at depth seen in relocated seismicity.
4. The Hayward Fault is divided near 37.85°N, 122.23°W (Lake Temescal) into two segments based on a large contrast in fault face geology. This segmentation is similar to that based on the extent of 1868 fault rupture, but is now related to an underlying geologic cause. The direct connection of the Hayward and central Calaveras Faults at depth suggests that earthquakes larger than those previously modeled should be considered (~M6.9 for the southern Hayward, ~M7.2 for the southern Hayward plus northern central Calaveras). A NEHRP study by Witter and others (2003; NEHRP 03HQGR0098) suggested evidence for large surface ruptures on the northern central Calaveras, but that work is not peer-reviewed and there is little or no other paleoseismic or geodetic data from the stepover zone or northern central Calaveras Fault (all commonly cited data are from the southern central Calaveras Fault), so the sparse surface data neither demands nor precludes our interpretation. The additional segmentation of the central Calaveras Fault proposed here may explain the observation that this segment seems to generate characteristic moderate (~M6.0-6.5) earthquakes rather than the larger ~M6.9 earthquakes that could be generated by rupture of the previously defined longer central Calaveras segment. Better information regarding fault plane geometry and 3-D distribution of rock properties adjacent to the faults at seismogenic depths should help us revise proposed segmentation models of other faults for seismic hazard analyses.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8002 Continental neotectonics (8107)
DE: 8038 Regional crustal structure
SC: Seismology [S]
MN: 2007 Fall Meeting