Short-range distance measurements along the San Andreas fault system in central California, 1975 to 1979

Abstract

Periodic measurements of fault-crossing networks with a side length of 1 to 3 km are being made to monitor deformation across fault zones in California. The distance measurements are made with a Hewlett-Packard 3800 or 3808 electronic distance meter and have a maximum standard deviation of 5 mm. Deformation measured within networks that span the San Juan Bautista-Cholame segment of the San Andreas fault in central California yields slip rates similar to those measured across a 100- to 300-m-wide zone by repeated alinement array surveys. Fault slip rates increase from near 0 to 32 mm/yr between San Juan Bautista and Bitterwater Valley in step-like increments. From Bitterwater to Slack Canyon slip rates vary between 26 and 32 mm/yr. Slip rates decrease southwestward of Slack Canyon to 3 mm/yr at Cholame. In contrast, Geodolite measurements of deformation across a 20-km-wide zone are consistent from San Juan Bautista to Slack Canyon and imply a 32 ± 2 mm/yr slip rate. Deformation across the Calaveras fault accounts for the difference between Geodolite and near-fault slip rates between San Juan Bautista and Bear Valley, although the zone of deformation is wider than 2.5 km just south of Hollister. At Bear Valley, measurements of a short-range network crossing the Paicines fault imply a slip rate of 10 ± 3 mm/yr during the period 1976 to 1979. From Slack Canyon to Cholame, Geodolite measurements show a constant decrease in the rate of shallow slip.