HR: 14:35h
AN: T12E-04 INVITED     [PDF]
TI: A 2000-Year Preliminary Record of Large Earthquakes on the Southern Hayward Fault
AU: * Lienkaemper, J J
EM: jlienk@usgs.gov
AF: USGS MS 977, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Dawson, T E
AF: USGS MS 977, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Williams, P L
AF: P. Williams & Assoc., P.O. Box 1492, West Tisbury, MA 02575 United States
AU: Personius, S F
AF: USGS, Denver Federal Center, Denver, CO 80225 United States
AU: Seitz, G G
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, L-397, Livermore, CA 94551 United States
AU: Schwartz, D P
AF: USGS MS 977, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: 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 are conducting a multiyear paleoseismic investigation to better understand the Hayward fault's past earthquake behavior. Our site is near the south end of Tyson's Lagoon, a sag pond formed in a right step of the fault in Fremont. Because the Hayward fault creeps at the surface, we identified paleoearthquakes primarily using features which we judge to be unique to ground ruptures or the result of strong-ground motion, such as the presence of fault-scarp colluvial deposits and liquefaction. We correlate the most recent event evidence to the historical 1868 M 6.9 earthquake, which caused liquefaction in the pond. We recognize nine additional paleoruptures since about AD 115 (+/- 135 yr) and two earlier events as yet undated. Event ages were estimated by chronological modeling, which incorporates historical and stratigraphic information as well as radiocarbon and pollen data. The preliminary mean recurrence interval (RI) for these ten events is 195 +/- 15 yr. This long-term (AD 115-1868) RI is somewhat greater than a previously determined RI of 130 +/- 40 yr for the period AD 1470-1868. Our event sequence includes event evidence from fault traces on both sides of the pond by tracing key stratigraphic units across. Our continuing work at this site focuses on carefully examining the possibility of missing events due to a hiatus in sedimentation to verify that the current record of paleoearthquakes is complete over the 2000-yr period. More age data is still needed to refine and complete the chronologic model to characterize the aperiodicity of the recurrence interval for the southern Hayward fault.
DE: 7221 Paleoseismology
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting