PALEOSEISMIC STUDIES ON THE SAN ANDREAS FAULT IN TRENCH 9 AT THE PLUNGE CREEK SITE, SAN BERNARDINO, SOUTHERN CALIFORNIA

A five-meter deep trench was excavated across the south branch of the San Bernardino segment of the San Andreas fault in order to determine the ages of prehistoric earthquakes. The trench exposed interbedded fluvial channel sediments and debris flow deposits derived from Oak Creek. The only earthquake horizon was found at approximately four meters depth. Three charcoal samples from above the earthquake horizon have been radiocarbon dated so far and are stratigraphically consistent. Additional samples will be submitted. A sample from 1.7 meters depth has an age of 152 ± 28 radiocarbon years BP (2-sigma, calibrated age: five ranges within AD 1667-1950). A sample from 2.5 meters depth has a radiocarbon age of 384 ± 29 years BP (2-sigma, calibrated age: AD 1441-1523 or AD 1564-1628). A sample from 3.3 meters depth has an age of 410 ± 40 radiocarbon years BP (2-sigma, calibrated age: AD 1426-1524 or 1562-1629). Thus the earthquake horizon is older than AD 1628. This is consistent with the age of the youngest visible earthquake horizon in three other trenches at this site. Based on the dates available so far, there is no evidence for an earthquake in 1812, even though an earthquake of this age has been documented at other paleoseismic sites to the northwest. Although the ages of the charcoal samples could be older than the depositional ages of the layers, 16 dates from four trenches at this site consistently indicate an age of at least 350 years for the strata that cap the youngest earthquake horizon that is visible in any of the trenches. Younger earthquakes can not be conclusively ruled out, however, because additional fault strands may lie beyond the north end of trench 9 and may be present but not visible within locally massive areas in the other trenches. Work is in progress to address the possibility that younger earthquakes may have occurred on other fault strands.