HR: 11:50h
AN: U42A-07    [Abstracts]
TI: A Composite Chronology of Earthquakes From the Bidart fan Paleoseismic Site, San Andreas Fault, California
AU: * Grant, L B
EM: lgrant@uci.edu
AF: University of California, Irvine, Dept. Environmental Health, Science & Policy, Irvine, CA 92697-7070
AU: Arrowsmith, J R
EM: ramon.arrowsmith@asu.edu
AF: Arizona State University, Dept. of Geological Sciences, Tempe, AZ 85287-1404
AU: Akciz, S
EM: sakciz@uci.edu
AF: University of California, Irvine, Dept. Environmental Health, Science & Policy, Irvine, CA 92697-7070
AB: Chronologies of earthquakes spanning at least ten ruptures at multiple sites are required for developing robust models of fault behavior and forecasts of future earthquakes. Such a long chronology can be obtained by placing multiple trenches across the San Andreas fault at the Bidart alluvial fan paleoseismic site in the Carrizo Plain to capture the spatio-temporal record of earthquakes created by the interplay of surface rupture and spatially varying deposition. Exposures from one trench reveal evidence of at least 6 and probably 7 earthquakes since 3000 BP. Evidence of 7 earthquakes since 2200 BP has been interpreted from exposures in 3 other trenches. Analysis of exposures from two new trenches is in progress. Excavations reveal alternating sequences of depositional preservation and gaps in the record of earthquakes. The "gaps" are massive featureless zones caused by bioturbation of the fan surface while that portion of the fan was depositionally inactive. When the depositional record of 4 trenches is combined, it yields a composite chronology of at least10 surface ruptures over the last 3000 years, for a minimum average recurrence interval of 300 years if the most recent event exposed in all trenches is assumed to be the 1857 Fort Tejon earthquake. So far, the uncertainty in dates of pre-1857 ruptures ranges from decades to millennia, and at least 5 of the 10 recognized earthquakes are obscured by depositional gaps at one of the trench sites. Therefore, synchroneity of ruptures at different trench sites is difficult to establish, and there is the possibility that the existing record contains more than 10 earthquakes and/or additional ruptures may have occurred that are not preserved by deposition.
DE: 7221 Paleoseismology (8036)
SC: Union [U]
MN: Fall Meeting 2005