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Reconstruction of a channel offset in 1857(?) by the San Andreas fault near Cholame, California

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MS 977, MENLO PARK, CALIFORNIA 94025

Abstract

The U.S. Geological Survey has formally forecast the next moderate (M6) Parkfield earthquake for 1988 ±5 yr. Sieh and Jahns (1984) forecast that a M7 earthquake is likely to accompany the next Parkfield earthquake. The latter forecast rests on interpretation of 3 to 4 m of 1857 slip on eight gullies near Cholame and slip-loading rate of 3.5 cm/yr since 1857. But Lienkaemper (1987) interprets 1857 slip here as 4 to 8 m. These eight gullies yield larger slip when one includes slip on secondary faults (at three gullies), and one picks gully centerlines judging the post-1857 alteration differently (at five sites). Our goal in this paper is to show our interpretation of slip on one gully deemed the least ambiguous of these eight (as they appear in large-scale, 1966 aerial photographs). We interpret slip as 2-m larger at this key site, suggesting a much lower likelihood of the next Parkfield earthquake triggering a larger Cholame earthquake. Our detailed studies of many sites showing 1857(?) slip are continuing and will be published later.

Using 1966 aerial photos, we infer the shape of this key gully before fault offset by eliminating effects of later natural and artificial processes of alteration. On 1986 photos right-lateral offset seems 3.5 ± 0.5 m, but on 1966 photos we measure 5.7 ± 0.7 m. Differencing precise 1966 and 1986 topographic maps shows that little incision occurred, but soil deposited up to a 1-m thickness on the left bank (probably drifted downslope from agricultural disking in the adjacent field). This soil deposit forced the channel bottom 2-m away from the left bank (upstream from fault only), causing offset in 1986 to appear 2-m smaller than in 1966. This 2-m alteration presumably occurred before Sieh (1978) interpreted 3.3-m slip here.

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