Dates of the Two Most Recent Surface Ruptures on the Southernmost San Andreas Fault Recalculated by Precise Dating of Lake Cahuilla Dry Periods

摘要

The past two southernmost San Andreas fault (SAF) ruptures occurred when ancient Lake Cahuilla was full, based on faulted lake sediment relationships and extensive liquefaction at sites near the shoreline. The times of the past two southern SAF ruptures have been reevaluated with new radiocarbon data on in situ stumps that grew between the past three Lake Cahuilla highstands, which, when taken in combination with historical accounts and modeling of the time to fill and desiccate the lake, provide more precise and accurate ages for the past two SAF earthquakes. The C-14 dates on inner and outer rings combined with historical observations show that the dry period prior to the last lake ended after 1706 C.E., leaving a narrow window of less than 25 yrs to fill and begin desiccating the most recent lake, and that the penultimate lake began dropping from a highstand around 1640 C.E. or earlier. Our analysis shows that the most recent earthquake occurred about 1726 +/- 7 C.E., whereas the timing of the penultimate event is slightly older at 1577 +/- 67 C.E. (both at 2 sigma). These new dates, when combined with previous age estimates of earlier southern SAF events, suggest more regular recurrence of surface-rupturing events, with an average interval of about 180 yrs, but leave the open interval at nearly 300 yrs.