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Geodetic Evidence for a Near-Fault Compliant Zone along the San Andreas Fault in the San Francisco Bay Area

Geophysical Institute
University of Alaska Fairbanks
Fairbanks, Alaska 99775-7320
jeff{at}giseis.alaska.edu; qzchen{at}giseis.alaska.edu

Global Positioning System measurements in 1996 and 1997 and Electronic Distance Measuring data from the 1970s and 1980s at sites in five small-aperture geodetic networks along the San Andreas fault in northern California were used to determine the near-fault strain rate. The tensor shear strain rate (referred to a coordinate system with the 1 axis parallel to the fault and the 2 axis normal to the fault) in the Bodega–Tomales, Lake San Andreas, and Black Mountain–Radio Facility networks (from north to south) are 0.339 ± 0.025, 0.366 ± 0.095, and 0.316 ± 0.060 µstrain/yr, respectively. The shear strain rate near the fault in the Black Mountain–Radio Facility and Lake San Andreas networks can be explained either by a 2D inhomogeneous model in which a low-rigidity compliant zone concentrates strain near the fault or by a very shallow locking depth of 8 km. Other evidence points to a locking depth greater than 10 km, so we prefer the first explanation. The contrast in rigidity between the fault zone and the surrounding rock appears to become stronger to the south, starting at approximately the northern extent of the Salinian block at Bodega Bay, suggesting that both the materials on either side of the fault and the cumulative fault offset play a role in the development of a compliant fault zone. Estimates of fault slip rates from far field geodetic data are only weakly sensitive to the presence of a compliant zone, but estimates of locking depths can be biased by approximately 10% toward shallower values if a compliant zone is present and unmodeled.

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