THE WRIGHTWOOD PALEOSEISMIC RECORD, DOUBLED

At the Wrightwood, California paleoseismic site, the combination of local patterns of paleoearthquake deformation and an excellent stratigraphic record preserve a long (~4,000 year) record of prehistoric earthquakes on the San Andreas fault. Previous evaluation of the event series indicates a 105-year average recurrence interval over 14 paleoearthquakes. This series shows a weakly time-predictable behavior that, nonetheless, can not be distinguished from a Poisson distribution. Because uncertainty in the recurrence behavior at this site is largely controlled by the length of the event series, we initiated this study to extend the event series and robustly characterize the recurrence behavior of the San Andreas fault in the San Gabriel mountains. Here we present event evidence for ~12 earthquakes that predate and lengthen the existing record. Strong evidence of paleoearthquakes is preserved as fissures and growth strata across structural features like meter-scale thrust faults, mini-grabens, and broad folds. Moderately compelling evidence is expressed by consistency of upward terminations at a depositional horizon. Preliminary carbon-14 dates of the older section indicate ~10 of these events occurred between 2,000 and 4,000 14C years BP. In order to characterize the peat layers in the older section, we present dates from the humic and humin fractions of 21 layers, and when possible, compare these to macrofossil ages. The older event series is compared with previous calculations of 30-year conditional probabilities given Poisson and lognormal models.