Abstract

Recent excavation and new radiocarbon dates of sediments at Pallett Creek are the basis for new conclusions regarding the late Holocene history of the San Andreas fault. Systematic dissection of a 50-m-long, 15-m-wide, 5-m-deep volume of earth, centered on the fault, enables documentation in three dimensions of fault patterns, lateral offsets and vertical deformation associated with large earthquakes of the past. The excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857 A.D., with an average recurrence interval of about 145 years. Prehistoric slip events that occurred in 1720 ± 50, 1550 ± 70, 1350 ± 50, 1080 ± 65 and 845 ± 75 A.D. have lateral offsets that are comparable to those of the most recent great earthquake, in 1857. Thus, all of these events represent major and probably great earthquakes accompanied by lengthy fault ruptures and earthquakes of large magnitude. The lateral offsets of two other events, in 935 ± 85 and 845 ± 75 A.D., are an order of magnitude smaller and may be interpreted in several ways with regard to the size of these events.

The new data constrain the average recurrence interval for large earthquakes at this site to between 145 and 200 years, but suggest a monotonic decrease in individual intervals to below this range during the past 900 years. Based upon these data, the probability of a large earthquake with surficial fault rupture at this site is between 0.3 and 5% during 1983 and 7 to 64% by the year 2000.

Paleo-earthquake data from several sites along the southern half of the San Andreas fault provide a basis for discussing several scenarios for future great earthquakes in southern California. These data suggest that a repetition of the 1857 earthquake is unlikely. A more plausible event would involve rupture of the southernmost 300 kilometers of the fault.