Gd. Simpson et al., Late holocene slip rate and earthquake history for the northern Calaveras fault at Welch Creek, eastern San Francisco Bay area, California, B SEIS S AM, 89(5), 1999, pp. 1250-1263
Paleoseismic trenching was performed to assess the slip rate and earthquake
history of the northern segment of the Calaveras fault at a site along Wel
ch Creek in the eastern San Francisco Bay area, California. At Welch Creek,
the northern Calaveras fault crosses a series of fluvial terraces and disp
laces the intervening terrace risers, We derive a late Holocene slip rate u
sing 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 isopac
h contours to measure the offset of a debris flow unit within the terrace s
ediments. The terrace back-edge is offset 39 +/- 1 m and is between 5 and 1
3 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 severa
l more events must have occurred that are not discernable in the stratigrap
hic or structural record. Based on the maximum amount of time between age-c
onstrained 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 ac
count for the 39 +/- 1 m of back-edge offset, we derive a recurrence estima
te of between 125 and 685 yr.