Evidence for a Twelfth Large Earthquake on the Southern Hayward Fault in the Past 1900 Years

We present age and stratigraphic evidence for an additional paleoearthquake at the Tyson Lagoon site. The acquisition of 19 additional radiocarbon dates and the inclusion of this additional event has resolved a large age discrepancy in our earlier earthquake chronology. The age of event E10 was previously poorly constrained, thus increasing the uncertainty in the mean recurrence interval (RI), a critical factor in seismic hazard evaluation. Reinspection of many trench logs revealed substantial evidence suggesting that an additional earthquake occurred between E10 and E9 within unit u45. Strata in older u45 are faulted in the main fault zone and overlain by scarp colluviums in two locations. We conclude that an additional surface-rupturing event (E9.5) occurred between E9 and E10. Since 91 A. D. (+/- 40 yr, 1 sigma), 11 paleoearthquakes preceded the M 6: 8 earthquake in 1868, yielding a mean RI of 161 +/- 65 yr (1 sigma, standard deviation of recurrence intervals). However, the standard error of the mean (SEM) is well determined at +/- 10 yr. Since similar to 1300 A. D., the mean rate has increased slightly, but is indistinguishable from the overall rate within the uncertainties. Recurrence for the 12-event sequence seems fairly regular: the coefficient of variation is 0.40, and it yields a 30-yr earthquake probability of 29%. The apparent regularity in timing implied by this earthquake chronology lends support for the use of time-dependent renewal models rather than assuming a random process to forecast earthquakes, at least for the southern Hayward fault.