Abstract

The geologic part of a seismic safety investigation of San Andreas Dam focused on a two mile reach of the fault between the dam and Crystal Springs Reservoir. Historical documents and detailed mapping revealed that the 1906 faulting caused 7 to 9 ft of right shear in the left (eastern) abutment and occurred on nearly straight traces that experienced a maximum of 9 ft of slip, 90 per cent of which was concentrated within a zone 100 ft wide.

Radiocarbon dates on detrital charcoal from two sites of ponded alluvium established that the 1906 traces have been active for at least the last 3,000 yr. Deflected stream channels further indicate that the 1906 traces have been the locus of right slip for the last 6500 yr and perhaps for all of Holocene time. Detailed geomorphic and stratigraphic analyses of one site showed that five 1906 magnitude slip events have occurred within the last 1130 ± 160 radiocarbon years from which a minimum strain rate of 1.2 cm/yr and an average recurrence frequence of 224 ± 25 yr can be calculated.

Trenches excavated across the 1906 traces of the San Andreas fault revealed that the zone of active faulting is marked by a slow upwelling of highly sheared rock materials. This apparent vertical movement of fault gouge, when coupled with right shear and differential stream erosion, has produced most of the distinctive landforms of the rift valley.

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