Abstract, talk to be presented at: Joint Conference of Seismological Society of America/Earthquake Engineering Research Institute, to be held April 18-22, 2006, San Francisco.

A 1650-Year Record of Large Earthquakes on the Southern Hayward Fault

Lienkaemper, J. J. (jlienk@usgs.gov; USGS MS 977, 345 Middlefield Rd, Menlo Park, CA 94025)

Williams, P. L. (Williams Assoc., P.O. Box 1492, West Tisbury, MA 02575)

The Hayward fault, a major branch of the right-lateral San Andreas fault system, traverses the densely populated eastern San Francisco Bay region, California. We conducted a six-year paleoseismic investigation to better understand the Hayward fault's past earthquake behavior. Our site is in Fremont near the south end of Tyson's Lagoon, which is a sag pond formed in a right step of the fault. Because the Hayward fault creeps at the surface, we identified paleoearthquakes primarily using features that we judge to be unique to ground ruptures or the result of strong-ground motion, such as fault-scarp colluvial deposits, fissure fills and evidence of liquefaction. We correlate the most recent event evidence to the historical 1868 M 6.9 earthquake, which caused liquefaction in the pond. We recognize ten additional paleoruptures since about AD 350 (+200/-160 yr). Event ages were estimated by Bayesian chronological modeling using the program Oxcal, which incorporates historical and stratigraphic information as well as radiocarbon and pollen data. The mean recurrence interval (RI) for these 11 events is 151 +/- 23 yr (2s of mean RI). The sample standard deviation of the RI is ±72 yr (2s). This long-term (AD 350-1868) RI is similar to a previously determined RI of 130 +/- 40 yr for the period AD 1470-1868. Our event sequence supported by redundant event evidence from several trenches across fault traces on both sides of the pond, correlated by tracing key stratigraphic units across the pond. Our preliminary estimate of aperiodicity or coefficient of variation (COV) in the recurrence interval for the southern Hayward fault is approximately 0.24. The current regional earthquake probability model, Working Group 2002, assumes much larger COV values for the Hayward and other major faults, so future models are likely to significantly increase the earthquake probabilities for the Hayward fault and for the region as a whole.