Holocene rate of slip and tentative recurrence interval for large earthquakes on the San Andreas fault, Cajon Pass, southern California

Abstract

Abstract Detailed mapping of the San Andreas fault zone where it crosses Cajon Creek, southern California, has revealed a number of late Quaternary deposits and geomorphological features offset by the fault. Radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the San Andreas fault. Four independent determinations of the slip rate on the San Andreas fault yield an average rate of 24.5 ± 3.5 mm/yr for the past 14,400 years. The similarity of the four values, which span different intervals of time from 5900 to 14,400 years ago, suggest that the slip rate has been constant during this period. The slip rate confirms that the San Andreas fault is accumulating slip faster than any other fault in the plate boundary, throughout California. Also, the sum of the slip rates on the San Andreas (24.5 mm/yr) and the San Jacinto (10 mm/yr) faults south of their junction is the same as the San Andreas north of their junction (35 mm/yr). An excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 A.D., and tentative evidence for six earthquakes in about the last 1000 years. Both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries. Combined with the historic record, this investigation indicates that the last major earthquake at Cajon Creek was probably at the beginning or middle of the 18th century. While the record at Cajon Creek may suggest that the next major earthquake on the San Andreas fault is overdue, the data can be interpreted to indicate that the recurrence interval for earthquakes on the San Andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century.