HR: 0800h
AN: G11A-0772    [Abstracts]
TI: Stream Channel Offset and Preliminary Slip Rate on the San Andreas Fault, at the Van Matre Ranch Site, in the Carrizo Plain, California
AU: * Noriega-Carlos, G R
EM: gnoriega@uci.edu
AF: University of California, Irvine, Environmental Health, Science and Policy , Irvine, CA 92697-7070 United States
AU: Grant, L B
EM: lgrant@uci.edu
AF: University of California, Irvine, Environmental Health, Science and Policy , Irvine, CA 92697-7070 United States
AU: Arrowsmith, R
EM: ramon.arrowsmith@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404 United States
AU: Young, J J
EM: jeri.young@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404 United States
AB: To understand the spatial and temporal variation in fault slip it is important to improve the spatial coverage of slip and slip rate measurements along major active faults. A set of well-preserved channels are offset across the San Andreas fault at the Van Matre Ranch (VMR) site (35.154N, 119.700W) in the Elkhorn Hills area of the Carrizo Plain. The fault zone and offset channels at VMR were exposed by excavation in 1993 and 2004. This study included one fault-perpendicular and 5 fault-parallel trenches that exposed the buried thalwegs of several offset channels. Seventeen samples were collected from channel margin deposits for $^{14}$C dating and survey data was taken for accurate offset measurement of the buried thalwegs and geomorphic channels. The geomorphic history of the site is well manifested in the excavations with clear evidence for initial incision of the channels into Plio-Pleistocene fan units that were typically heavily bioturbated. The channels then back filled and the stratified channel sediments grade laterally into clayey silts. The buried thalweg of the currently active channel is offset 24.8 m, while the geomorphic offset is 27.6 m (qualitatively defined conservative uncertainties on offsets are $\pm$ 1m). The thalweg of the first beheaded channel is offset 48.8 m with a geomorphic offset of 51.8 m. The geomorphic offset of the second beheaded channel ranges from 71.9 to 79.0 m. There are no ages associated with these channels. The median dates of samples from the clayey silts in the currently active channel margin range between A.D. 1221 and 1108, implying a 34.7 mm/yr slip rate. The significance of the samples ages is dependent upon interpretation of the sediments in which they were collected. They were collected from clayey silts which are either colluvium, washed down from adjacent hill slopes, or autochthonous alteration of the channel deposits by pedogenic processes (largely burrowing). If the samples were derived from colluvial processes, the ages of the samples would provide a maximum slip rate. However, if the samples were derived from older channel sediments, then they would indicate a slip rate minimum. This preliminary slip rate is consistent with the measured slip rate at Wallace Creek, approximately 18 km to the northwest where Sieh and Jahns documented a late Holocene slip rate of approx. 33.9 $\pm$ 2.9 mm/yr, and with the regionally assumed 35-mm/yr rate derived from decadal time-scale geodetic measurements.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 7200 SEISMOLOGY
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting