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Late holocene slip rate and earthquake history for the northern Calaveras fault at Welch Creek, eastern San Francisco Bay area, California

William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596

Abstract

Paleoseismic trenching was performed to assess the slip rate and earthquake history of the northern segment of the Calaveras fault at a site along Welch Creek in the eastern San Francisco Bay area, California. At Welch Creek, the northern Calaveras fault crosses a series of fluvial terraces and displaces the intervening terrace risers. We derive a late Holocene slip rate using two independent methods: (1) by measuring the offset of the back-edge (i.e., terrace angle) of one of the offset terraces and (2) by using isopach contours to measure the offset of a debris flow unit within the terrace sediments. The terrace back-edge is offset 39 ± 1 m and is between 5 and 13 ka old. The debris flow deposit is offset 27 ± 1 m; the deposit age is estimated to be between 4840 and 5325 cal yr B.P. These findings suggest a late Holocene slip rate of 6 ± 1 mm/yr for the northern Calaveras fault. We recognize as many as seven surface-rupturing earthquakes at Welch Creek, although the amount of terrace back-edge displacement suggests that several more events must have occurred that are not discernable in the stratigraphic or structural record. Based on the maximum amount of time between age-constrained paleoearthquakes that are preserved in the record at Welch Creek, we derive an estimate of the maximum recurrence interval of between 1375 and 3425 yr. Using the assumption that additional events are required to account for the 39 ± 1 m of back-edge offset, we derive a recurrence estimate of between 125 and 685 yr.

Footnotes

^* Present address: SHN Consulting Engineers & Geologists, 812 W. Wabash, Eureka, California, 95501.