HR: 0800h
AN: S31A-1025    [Abstracts]
TI: High-Resolution Seismic Reflection and Refraction Imaging of a Thrust Fault Near Point Arena, Northern California
AU: * Agar, B
EM: briana.agar@colorado.edu
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: Catchings, R D
EM: catching@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Goldman, M R
EM: goldman@usgs.gov
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 high-resolution seismic reflection and refraction data along a 240-m-long profile located about 6 km southwest of the San Andreas fault (SAF), near Point Arena, California. The study was conducted to determine structural relations of a thrust fault, which has natural exposures in a sink hole and coastal bluffs, that moved strata of the Miocene Point Arena Formation over uplifted late Quaternary marine terrace deposits. Seismic sources were generated by 120 sledge-hammer `shots'. Shot points and geophones were spaced every 2 m, and geophones were laterally offset 1 m from the shot points. Data were recorded in a linear array of two Geometrics Strataview RX-60 seismographs; we recorded 2 s of data at a sampling rate of 0.5 ms. The seismic profile crossed the known thrust fault within 15 m of exposures in a sink hole and a bluff. Seismic P-wave velocities range from about 500 m/s near the surface to about 1500 m/s at 10 m depth. Below 10 m velocities are laterally highly variable. Near the northwest end of the profile, maximum velocities are about 2700 m/s at 25 m depth; at about 80 m to the southeast and 20 m depth, velocities are 1700 m/s, increasing to 1900 m/s at 15 m depth near the southeast end of the profile. A migrated seismic reflection image shows the source of the thrust fault to be in subhorizontal bedding, which matches relations seen in the natural bluff exposures. The reflection image also shows a prominent, steeply dipping fault near the middle of the profile, the location of which coincides with the low-velocity zone in the deeper section of the velocity profile. The steeply dipping fault near the middle of the profile does not appear to offset reflectors in the uppermost, late Quaternary terrace deposit section of the reflection image. Fault relations revealed in the seismic sections further our understanding of the three-dimensional fault structure west of the San Andreas fault.
DE: 7218 Lithosphere and upper mantle
DE: 7230 Seismicity and seismotectonics
DE: 6982 Tomography and imaging
DE: 0905 Continental structures (8109, 8110)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting