HR: 0800h
AN: T21A-0457    [Abstracts]
TI: The San Andreas Fault at SAFOD: Seismic Imaging and Borehole Comparisons
AU: * Catchings, R D
EM: catching@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd MS 977, Menlo Park, CA 94025 United States
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd MS 977, Menlo Park, CA 94025 United States
AU: Goldman, M R
EM: goldman@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd MS 977, Menlo Park, CA 94025 United States
AB: We acquired three high-resolution (2.5-m CDP spacing) seismic reflection and refraction profiles across the SAFOD site in 1998 and 2003. In 1998, we acquired a 5-km-long, high-resolution seismic reflection and refraction profile from ~2 km southwest of the SAFOD drill site to about 1.5 km northeast of the surface trace of the San Andreas fault. In 2003, we acquired two additional high-resolution seismic reflection and refraction profiles that were centered on the SAFOD drill site. All three profiles provide velocity images to maximum depths of 800 m and reflection images to maximum depths of 5 km, which correlate with structures and lithologies observed in the SAFOD borehole. Our seismic images show that the San Andreas fault is located within an approximately 1.7-km-wide, southwest-dipping low-velocity zone at about 1 km depth. On the basis of velocity data, we previously interpreted traces of the San Andreas fault to lie within a wedge of sediments (Catchings et al., 2002; BSSA), a result that was confirmed by SAFOD borehole data. Small repeating earthquakes that were the principal target events for SAFOD drilling reside within the wedge of sediments and are probably related to low-strength sedimentary rocks. However, our reflection images show that additional strands of the San Andreas fault extend into more competent rock, well northeast of the SAFOD target faults. The relatively wide fault zone, inferred from our observations of numerous fault strands on seismic reflection images, suggests that SAFOD sampled only part of the San Andreas fault zone, which is wider and more structurally complex than previous believed.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Tectonophysics [T]
MN: Fall Meeting 2005