Abstract

We have documented the stratigraphy and structure of several trenches across the San Bernardino strand of the San Andreas fault at the Plunge Creek site, near San Bernardino, southern California. The most recent faulting event exposed in the trenches (event W) appears to have occurred between about A.D. 1440 and A.D. 1660, if the radiocarbon dates are taken at face value. Two of the trenches reveal suggestive evidence for an older faulting event (event R), which postdates A.D. ∼1220.

Because the age control at Plunge Creek is based on radiocarbon dating of detrital-charcoal samples, we must consider all of the radiocarbon ages as maximum estimates of the depositional ages for the layers from which the samples were collected. Thus, event W is not strictly constrained to predate A.D. 1660. We use ecological arguments to infer that the detrital-charcoal samples at the Plunge Creek site probably overestimate the depositional ages of the sedimentary layers by about 1 ± 1 fire-cycle (i.e., by about 70 ± 70 years). An independent estimate, based on extrapolation of sedimentation rates to the ground surface, suggests a similar value (0–95 years) for the lag time between the calibrated radiocarbon date of a sample and the depositional date of the layer from which it was collected. After applying an estimated correction (70 years) for the inherited ages of the detrital-charcoal samples, the date of event W is most likely between A.D. 1510 and A.D. 1730, with a preferred date of about A.D. 1630. The preferred date for event R is about A.D. 1450.

Given the range of allowable dates, event W probably correlates with either the second-youngest earthquake (A.D. ∼1690) or the third youngest earthquake (A.D. ∼1600) documented at the nearest paleoseismic site to the northwest (Pitman Canyon). Comparison with the paleoseismic record at other sites along the southern San Andreas fault suggests a rupture length of 85–190 km for event W, implying a magnitude of M 7.3–7.7. Event R probably correlates with the fourth-youngest event at Pitman Canyon (∼A.D. 1450). Prehistoric earthquakes within several decades of this date have been documented all along the ∼450-km length of the southern San Andreas fault, suggesting (though by no means requiring) that event R could potentially have been a very large earthquake (M ∼8.2). If our interpretation is correct, surface rupture from the youngest earthquake at Pitman Canyon (A.D. 1812) apparently died out between Pitman Canyon and Plunge Creek. Previous estimates of the probability of future earthquakes have assumed that the entire length of the San Bernardino strand slipped during the 1812 earthquake. Our results suggest that the southeastern half of this section of the fault did not slip in 1812 and that strain has been accumulating on this portion of the fault for at least the past 3 centuries. Depending on the slip rate, accumulated strain sufficient to generate 4.2–7.5 m of right-lateral slip may currently be stored on the southeastern half of the San Bernardino strand of the San Andreas fault.

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