HR: 1340h
AN: T13A-1336    [Abstracts]
TI: Paleoseismology and Tectonic Geomorphology: Results From the Parkfield, CA Segment of the San Andreas Fault
AU: * Tok\'{e}, N A
EM: nathan.toke@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404
AU: Arrowsmith, J R
EM: ramon.arrowsmith@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404
AU: Crosby, C J
EM: chris.crosby@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404
AU: Young, J J
EM: jeri.young@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404
AB: The Parkfield segment of the San Andreas Fault (SAF) is the transition zone between the creep-dominated segment of the SAF to the northwest and the locked segment to the southeast. Geodetic studies indicate ~10 mm/year of the SAF 35 mm/year slip-rate is accommodated by creep at shallow depths of the SAF near Carr Hill, suggesting the remaining slip may occur in moderate to large earthquakes. Foreshock intensities of the 1857 M7.8 Fort Tejon earthquake were similar to 20th century Parkfield events, suggesting that the 1857 event may have nucleated in the Parkfield area. Paleoseismic investigations near Parkfield have achieved limited success in exposing useful fault zone stratigraphy. Our goals were to investigate the style and timing of late Pleistocene and Holocene faulting along this segment of the SAF. Specifically, did the 1857 or similar large prehistoric earthquakes rupture through the Parkfield segment? In addition, is it possible to distinguish deformation caused by these large magnitude earthquakes from deformation associated with the moderate 1966-style earthquakes and aseismic creep? Geomorphic mapping from Middle Mountain to Carr Hill revealed numerous tectonic landforms that define the fault trace and indicated possible excavation sites, including the sites used for this study. Two fault-perpendicular excavations were cut in a late Pleistocene fluvial terrace of Little Cholame Creek just north-northeast of Carr Hill. A thirty-meter excavation across an elongate sag pond, containing an apparently right-laterally offset fluvial channel and several small springs, revealed four fault zones. The outer fault zones were parallel with the regional trend of the SAF (313ø), while the inner fault zones trended obliquely at ~350ø. The three easternmost fault zones show apparent dip-slip deformation including truncation and down warping of sag and terrace deposits. The westernmost fault zone was characterized by upward splaying sub-vertical clay shear bands, which juxtaposed the youngest sag deposits against heavily bioturbated units. The orientation and styles of deformation are consistent with local extension due to a small right step in the SAF trace. Apparently, the two central, obliquely oriented, fault zones transfer slip across the step. A second ~35 m excavation across a ~2m pressure ridge, just southeast of the sag excavation, revealed two fault zones. The primary fault zone was characterized by upward splaying clay shear bands, most of which terminated within an undifferentiated organic unit associated with a spring line midway up the scarp. Higher on the scarp a zone of dip-slip faulting, characterized by apparent normal offset and stratigraphically lower terminations, is older than the primary fault zone. Strong indicators of coseismic ground rupture such as filled fissures were not observed. However, the lack of such features does not preclude the possibility of large earthquakes in Parkfield. Several upward terminating fault strands provide possible evidence for seismic events. It is likely that much of the deformation in these exposures (the down-warped sands in particular) is due to aseismic creep. Forthcoming radiocarbon ages will provide information about deformation rates and the development of the fault related structures exposed here.
UR: http://activetectonics.la.asu.edu/Parkfield/
DE: 9350 North America
DE: 7221 Paleoseismology
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting