HR: 17:00h
AN: NS24A-05    [Abstracts]
TI: Near-surface geophysical surveying of the Green Valley fault, Fairfield CA
AU: * Craig, M
EM: mitchell.craig@csueastbay.edu
AF: Calif. State Univ. East Bay, Dept. of Earth & Environ. Sci., Hayward, CA 94542,
AU: Lienkaemper, J
EM: jlienk@usgs.org
AF: U. S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025,
AU: Crook, N
EM: ncrook@pangea.stanford.edu
AF: Stanford Univ., Geophysics Dept., Stanford, CA 94305,
AU: Dieter, B
EM: bdieter@mlml.calstate.edu
AF: Moss Landing Marine Labs, 8272 Moss Landing Rd., Moss Landing, CA 95039,
AU: Narvaez, R
EM: orwaves@isp.com
AF: Calif. State Univ. East Bay, Dept. of Earth & Environ. Sci., Hayward, CA 94542,
AU: Nase, S
EM: snase@horizon.csueastbay.edu
AB: We conducted seismic and ground-penetrating (GPR) surveys of the strike-slip Green Valley fault zone with the goals of imaging buried stream channels offset by the fault and determining the long-term slip rate of the fault. Recording geometry and line locations were selected based on results from previous geophysical surveys and lithologic logs from a 5 m deep pit excavated the previous year. We recorded a grid of nine fault-parallel lines, 90 m long and spaced 5 m apart, using both seismic refraction and GPR. Stratigraphy was resolved in the upper 2-3 m of the GPR profiles and may be confidently correlated between lines. Ground-truth information was provided by the 5 m deep pit and two new 3 m deep trenches. In addition GPR lines were recorded at the trench locations prior to trenching. We conducted particle-size analyses of sedimentary units exposed in the pit. Sediments in the upper 5 m have a bimodal particle size distribution with distinct sandy and silty fractions at 3φ and 7-8φ. In general, the silty fraction is dominant, but two sandy layers a few tens of cm thick were observed in both the pit and trenches and were evidently imaged in the GPR profiles. GPR penetration at the site of the new survey is limited by a highly-conductive clay layer at a depth of 2-3 m below the surface. Seismic refraction surveys provide velocity information to depths of 6-8 m, where a sharp increase in velocity occurs. While previous seismic refraction surveys have helped to constrain the location of the Green Valley fault, the new data with closer line spacing may prove more useful for defining paleodrainage geometry. Auger boreholes and cone-penetration testing (CPT) provide stratigraphic information to 15 m depth that will help constrain velocity models derived from the seismic refraction surveys.
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0925 Magnetic and electrical methods (5109)
DE: 0935 Seismic methods (3025, 7294)
DE: 7221 Paleoseismology (8036)
DE: 8107 Continental neotectonics (8002)
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting