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
7.9 Fort Tejon earthquake and the 1906
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.
Berkeley Seismological Laboratory
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