HR: 0800h
AN: T41F-1282    [Abstracts]
TI: GPR Surveying for Paleochannels at the Green Valley Fault
AU: * Craig, M S
EM: craig@csuhayward.edu
AF: California State University, Hayward, Dept. of Geological Sciences 25800 Carlos Bee Blvd, Hayward, CA 94602 United States
AU: Heller, S J
EM: heller4@llnl.gov
AF: California State University, Hayward, Dept. of Geological Sciences 25800 Carlos Bee Blvd, Hayward, CA 94602 United States
AU: Kimball, M A
EM: mindy.kimball@us.army.mil
AF: California State University, Hayward, Dept. of Geological Sciences 25800 Carlos Bee Blvd, Hayward, CA 94602 United States
AU: Lienkaemper, J J
EM: jlienk@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: We conducted a ground-penetrating radar (GPR) survey on the Green Valley Fault, located on the NE margin of San Francisco Bay, in an effort locate paleochannels to better constrain the slip rate of the fault. We began this study with a detailed topographic survey to determine the most likely paths of paleochannels, and situated the GPR lines so as to cross them. The GPR survey consisted of nine lines recorded using 50 MHz antennas, providing a total footprint of 100 $\times$ 180 m. The GPR data indicate possible paleochannels 5-10 m wide, at 1-5 m depth. The top surface of GPR basement, Sonoma Volcanics, is the most continuous reflector in the data. It is clearly imaged on several of the GPR lines and can be easily correlated on adjacent lines. It outcrops at the surface near the western boundary of the survey area and dips eastward to reach depths of 20 m at the eastern boundary of the survey area. We also recorded a 70 m long seismic refraction line near the middle of the survey area using a 24-channel seismograph and a sledgehammer source, and derived a three-layer model from the data. At the location of the seismic line, the top layer is 3-5 m thick with $V_P \approx$ 300 m/s, the second layer extends to 20 m depth, with $V_P \approx$ 1700 m/s, and the deepest layer is Sonoma Volcanics, which extends beneath 20 m depth, with $V_P \approx$ 6000 m/s. The geophysical response of paleochannels is derived from the material contrast of sandy channel deposits vs silts and clays. We used a hand auger to sample to 5.3 m depth near the center of the survey area, and encountered mostly fine-grained sediments, clayey and sandy silts. We did not intersect any obvious channel sands. A modern stream that passes within a few 100s of m of the survey site provides an analog useful for estimating potential channel size and sediment types of paleochannels. The modern channel is approximately 5 m wide and contains a bedload of sand and gravel with cobbles and occasional small boulders. Sandy point bars are present. We expect similar coarse-grained sediments associated with paleochannels to be present in the survey area, and to be detectable with GPR and seismic methods. Additional seismic lines and auger boreholes should confirm whether or not channel-like features of the GPR data are truly paleochannels.
DE: 7221 Paleoseismology
DE: 8100 TECTONOPHYSICS
DE: 8107 Continental neotectonics
DE: 7200 SEISMOLOGY
DE: 0900 EXPLORATION GEOPHYSICS
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting