Abstract

Detailed mapping of the San Andreas fault zone, where it crosses Cajon Creek in Southern California, has revealed a number of late Quaternary deposits and geomorphological features offset by the fault. Radiocarbon dating of these alluvial and swamp deposits has provided 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 yr. The similarity of the four values, which span different intervals of time from 5,900 to 14,400 yr ago, suggests that the slip rate has been constant during this period. The sum of the Holocene slip rate on the San Andreas (∼24.5 mm/yr) and the Quaternary rate on the San Jacinto (∼10 mm/yr) faults southeast of their junction is the same as the Holocene slip rate on the San Andreas farther northwest (∼34 mm/yr). Although the slip rate confirms that the San Andreas fault is accumulating slip faster than any other fault of the plate boundary, a large fraction of the plate boundary's rate of slip (∼20 of 56 mm/yr) cannot be accounted for on major faults in southern California.

An excavation has provided evidence for at least 2 earthquakes, and perhaps as many as 4, that caused rupture on the fault between 1290 and 1805 A.D.; it has provided tentative evidence for 6 earthquakes in about the past 1,000 yr. Both lines of evidence suggest an average recurrence interval for large earthquakes of ∼1½ to 2 centuries. Combined with the historic record, this investigation indicates that the last major earthquake at Cajon Creek was probably near the beginning of the 18th century.

Models consistent with the record at Cajon Creek and data from other localities along the San Andreas fault have been constructed to estimate the timing and rupture length for future earthquakes on the San Andreas fault.

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