HR: 14:55h
AN: S23C-06 INVITED    [Abstracts]
TI: Before the 1868 Earthquake: An 1800-year geologic history of surface ruptures along the southern Hayward fault
AU: * Lienkaemper, J J
EM: jlienk@usgs.gov
AF: U.S. Geological Survey, MS 977 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Williams, P L
EM: plw3@earthlink.net
AF: Dept. Geological Sciences, San Diego State University, San Diego, CA 92182, United States
AB: We summarize* geologic evidence of the occurrence and timing of paleoearthquakes on the southern Hayward fault as interpreted from trenches excavated within a sag pond at the Tyson's Lagoon site in Fremont, northern California. We use the information to estimate the mean value (μ) and aperiodicity (σ/μ) of the fault's recurrence interval (RI), two fundamental parameters for estimation of regional seismic hazard. In addition to evidence for the 1868 Hayward earthquake, we present evidence for at least ten paleoruptures since about AD 170. We document these events with evidence for ground rupture, such as the presence of blocky colluvium at the base of the main trace fault scarp, and with corroborating evidence such as simultaneous liquefaction or increase in deformation immediately below event horizons. The mean RI is 170 ± 82 yr (1σ, standard deviation of the sample), aperiodicity is 0.48, and individual intervals may be expected to range from 30 yr to 370 yr (at a 95.4% confidence level). The mean RI is consistent with the recurrence model of Working Group on California Earthquake Probabilities (2003) (mean, 161 years; range: 99 yr (2.5%); 283 yr (97.5%)). We note that the mean RI for the five most recent events may have been only 138 ± 58 yr (1σ). Hypothesis tests for the shorter RI do not demonstrate that any recent acceleration has occurred compared to the earlier period or the entire 1800-yr record, principally because of inherent uncertainties in the event ages. *Based on work published in the December 2007 issue of Bulletin of the Seismological Society of America
DE: 7221 Paleoseismology (8036)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting