Paleoseismic investigations of the San Bernardino segment of the San Andreas Fault at Plunge Creek, San Bernardino, California

Cross cutting relations exposed in a trench across the San Bernardino Mountains segment of the San Andreas fault near Plunge Creek suggest that the most recent earthquake on this portion of the fault may have occurred between 360 and 580 years ago. Calibrated radiocarbon dates on detrital charcoal appear to constrain a faulting event between AD 1420–1640 (2-sigma). This trench (T8) reveals an earthquake horizon that is capped by strata in which the youngest detrital charcoal samples have calibrated date ranges of about AD 1440–1640. The lack of clear stratigraphy immediately above the remaining fault strands prevents us from determining whether or not these strands extend higher in the stratigraphic section. Even if some fault strands ruptured more recently, they are overlain by an unfaulted layer that is probably older than about AD 1720. However, none of the dates provides firm younger bounds on the age of any horizon because the detrital charcoal samples could overestimate the ages of the layers from which they were collected. It is possible that younger earthquakes may have occurred if the ruptures were entirely limited to fault strands within the stratigraphically massive area at the north end of the trench.; These results may indicate that the 1812 earthquake documented in the Wrightwood and Pitman Canyon areas did not rupture as far southeast as Plunge Creek. If true, this suggests that at least the southeastern part of the San Bernardino segment of the San Andreas fault has not ruptured for at least 360 years and may be overdue for a large magnitude earthquake.