Creep on the San Andreas fault

Abstract

Progressive destruction of buildings and other works of man at the W. A. Taylor Winery near Hollister, California, indicates that one side of a segment of the San Andreas fault is creeping relative to the other. Three different types of measurements all yield an annual rate of creep of approximately one-half inch per year. Steinbrugge and Zacher: Measurements of the separations of pairs of reference marks adjacent to the line of creep have been repeated periodically since 1956, and damage to structures provides a good measure of the total creep since 1948. Reports of damage to older buildings on the same site suggest that the creep may have been going on at about the present rate for fifty or more years. Tocher: Creep recorders designed to measure continuously the differential lateral movement of adjacent sections of the concrete floor have been installed in the main winery building. The creep rate so determined has been about one-half inch per year (with right-lateral sense) for the past two years. Creep accumulates largely in spasms of rather short duration (on the order of a week) separated by intervals of weeks or months during which little or no creep takes place. Ninety-two per cent of the movement in a recent 371-day period accumulated in four spasms of total duration 34 days. Three of these spasms began at times when no local earthquakes were recorded on near-by seismographs; the fourth began with a sudden right-lateral fault movement of 3 mm. at the time of a sharp local earthquake (Richter magnitude 5.0) on January 20, 1960 (GCT). Whitten and Claire: Resurveys over monumented points established near the winery also yield a rate of slippage or creep along the fault line of one-half inch per year. A new method for analyzing the data obtained by retriangulating over monumented points at wide intervals (10 to 20 years) is presented and applied to two triangulation networks which cross the San Andreas fault in central California. Results from a net near Hollister show an average creep rate of about one-half inch per year; results from a net near Cholame (about 75 miles southeast of Hollister) show an average creep rate of about one-tenth inch per year. The results also give an angular value which represents the deformation in the crust adjacent to the fault line.