Introduction

The San Andreas Fault system stretches from the southern California border 1,100 km northeastwards right up to the Mendocino triple junction offshore northern California. For much of its length, the fault is locked, displaying no significant offset between large seismic events. The parts of the fault that ruptured during the 1857 $M_{w}$ 7.9 Fort Tejon earthquake and the 1906 $M_{w}$ 7.9 San Francisco earthquake are examples of portions of the fault that are locked. In between these two rupture zones lies the 170 km-long creeping segment, from now on abbreviated CSAF. Various types of surface measurement in the last three decades or so have amply demonstrated that creep occurs along this section, with estimated creep rates up to 34 mm/year (Burford and Harsh, 1980; Lisowski and Prescott, 1981; Schulz, 1982; Schulz, 1989; Titus et al., 2005). Since the discovery of creep at the Cienega Winery by Tocher in 1960 (Tocher, 1960), the CSAF has essentially become the world's type locality for fault creep: no other fault section is known to creep along such a great length, nor at such a high rate. Several other faults in the San Andreas Fault system have well-documented creep, for example the Calaveras Fault (e.g. Rogers and Nason, 1971; Johanson and Bürgmann, 2005) and the Hayward Fault (e.g. Savage and Lisowski, 1993; Simpson et al., 2001), but the rates are less than 10 mm/year. Why some faults creep while others are locked is not known. It is, however, important to study this question. Collectively, the creeping faults in the San Francisco Bay region constitute a major part of the San Andreas Fault system; if we are to know the system well enough to predict earthquakes, then we need to understand the mechanics of creep. In this project we use Interferometric Synthetic Aperture Radar (InSAR) measurements covering almost a decade to record spatial variations in creep rate along the CSAF. We then invert these surface data for shallow creep rates and deep slip rates on the fault.

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