HR: 0800h
AN: S31A-1024    [Abstracts]
TI: Subsurface Study of Faults Near Point Arena and the San Andreas Fault Zone, Northern California, Using High-Resolution Reflection and Refraction Profiling
AU: * Grijalva, K A
EM: kgrijalva@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: * Grijalva, K A
EM: kgrijalva@usgs.gov
AF: Dept. of Geophysics, Stanford University, Stanford, CA 94309 United States
AU: Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Goldman, M R
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Steedman, C E
EM: steedman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Prentice, C S
EM: cprentice@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AB: We acquired seismic reflection and refraction data along a 600-m-long profile located about 6 km southwest of the San Andreas fault (SAF), near Point Arena, California to determine possible relations between faulting and synclinal folding in the Miocene Point Arena Formation. Seismic sources were generated by a combination of 118 BETSY Seisgun and sledge-hammer shots. Seisgun shot points were spaced every 50 m, and sledge-hammer shot points were spaced every 5 m between the seisgun shots. Data were recorded in a linear array of two Geometrics Strataview RX 60 seismographs, with each seismograph having 60 active channels. Geophones were spaced at 5-m intervals and were laterally offset 1 m from the shot points. We recorded 2 s of data at a sampling rate of 0.5 ms. The seismic profile crossed two known faults and a prominent, asymmetric syncline that trends northwest-southeast, subparallel to the SAF. The two faults extend into unconformable, late Quaternary marine terrace deposits and may still be active. The syncline has gently dipping beds (13 to 22 degrees) on the southwest side, and strata dip on the northeast side 25 to 56 degrees. Because the seismic profile was oriented NE-SW, approximately normal to the syncline axis and fault trend, we observe similar features on the seismic images. Seismic velocities range from about 800 m/s near the surface to about 2200 m/s at a depth of 40 m. We observe two low-velocity gradients along the profile that we interpret to be the northwest-southeast-trending syncline and a thrust fault, both of which have linear trends visible in aerial photographs. The seismic reflection image also reveals sub-horizontal reflectors apparently offset by faulting and the prominent syncline. The seismic reflection image shows at least four faults, two more than previously mapped. These faults, and their recency of activity, represent a significant change in understanding of the Quaternary tectonic setting of the region.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 6982 Tomography and imaging
DE: 0905 Continental structures (8109, 8110)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting