PALEOSEISMIC STUDIES OF THE SAN ANDREAS FAULT AT THE PLUNGE CREEK SITE, NEAR SAN BERNARDINO, CALIFORNIA

We have examined and documented several trenches across the south branch of the San Andreas fault at the Plunge Creek site, near San Bernardino, southern California. A total of 52 detrital charcoal samples from the trenches have been dated. In all of the trenches, no faulted sediments that are clearly younger than AD 1650 have been identified. Trenches 4, 7, 8 and 9 all reveal an earthquake horizon that is capped by strata in which the youngest detrital charcoal samples have calibrated date-ranges of about AD 1440-1650. Sixteen, detrital charcoal samples from the layers that most closely overly this earthquake-horizon in the four trenches have been radiocarbon-dated. Two of these sixteen samples have calibrated date-ranges that are older than and do not overlap with the date-ranges for the other fourteen samples. These samples are interpreted as having been about 200 ± 150 years old at the time they were deposited. Of the remaining fourteen samples, only one has any calibrated date-ranges that are younger than about AD 1650. For that sample, date-ranges as young as AD 1763-1802 and AD 1938-1946 are allowed by the calibration curve, but these date-ranges represent only 4.8% and 0.7%, respectively, of the region within the 2-sigma error bounds and are much less likely than the older date-ranges that are allowed for this sample. Although all sixteen of these samples could be older than the layers from which they were collected, the simplest interpretation is that the an earthquake ruptured the south branch of the San Andreas fault at the Plunge Creek site sometime between about AD 1440 and AD 1650. No evidence for earthquakes younger than this has been found at the Plunge Creek site, but the possibility of younger earthquakes can not be entirely ruled out at this time because fault strands that extend to higher stratigraphic levels could be present but not visible within a few localized massive areas in each trench.