HR: 1340h
AN: S13B-0190    [Abstracts]
TI: Further Resolution of Past Earthquake Surface Ruptures at the Carrizo Wash Site, Superstition Mountain Strand of the San Jacinto Fault, Imperial Valley, Southern California.
AU: * Verdugo, D
EM: Danie143@aol.com
AF: Dept. of Geological Sciences, San Diego State University, 5500 Campanile Drive, MC-1020, San Diego, CA 92182 United States
AU: Ragona, D E
EM: dragona@ucsd.edu
AF: IGPP, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Rockwell, T K
EM: trockwel@geology.sdsu.edu
AF: Dept. of Geological Sciences, San Diego State University, 5500 Campanile Drive, MC-1020, San Diego, CA 92182 United States
AB: We present results from new trench exposures in Carrizo Wash along the northernmost part of the Superstition Mountain strand (SMF) of the San Jacinto Fault, southern California. Prior paleoseismic work by Ragona (2003) recovered evidence for 7 earthquake events, 3 of which occurred in the past 1000 years. The most recent event produced only 15 cm of slip and is only recognized along the Coyote Creek Fault (CCF). The other two recognized events cumulatively produced nearly 6 m of slip. However, an erosional unconformity in the initial excavations removed evidence for at least one event, based on problematic correlations to nearby sites resulting from mismatches in both the number of recognized lake units as well as the relative stratigraphic position of events with respect to the lakes. The new trenches, south of the effects of the erosional channel, contain evidence for an additional event and an additional delta-lake sequence not present in the original exposures. The new event likely correlates to the penultimate event at the Northern Shoreline site (Gurrola and Rockwell, 1996). Thus, the observed 6 m of slip was mostly accommodated by three events, suggesting about 2 m of slip per event for large SMF ruptures. Our new observations also agree well with data from four other nearby trench sites along the CCF and SMF, and suggest that 1) the SMF has ruptured in only three large events in the past 1100 years, 2) at least two of these events are likely recognized along the CCF, indicating that the step-over between SMF and CCF is soft in large events, 3) most CCF ruptures do not propagate onto the SMF, indicating that the step-over is hard for small displacements, such as 1968-type events (30-50 cm of slip). These observations support the concept of segmentation but indicate that a perceived segment boundary may be transparent if slip exceeds a threshold value.
DE: 7221 Paleoseismology (8036)
DE: 8036 Paleoseismology (7221)
SC: Seismology [S]
MN: Fall Meeting 2005