Introduction

The Northern San Francisco Bay Area (hereafter ``North Bay'') is sliced by three major right-lateral strike-slip faults, the northern San Andreas Fault (SAF), the Rodgers Creek Fault (RCF) and the Green Valley Fault (GVF). The RCF represents the North Bay continuation of the Hayward fault zone and the GVF is the northern extension of the Concord fault. North of the juncture with the San Gregorio fault, geodetic and geologic data suggest a SAF slip rate of 20-25 $mm.yr^{-1}$ (d'Alessio et al, 2007, Lisowski et al., 1991). Geodetically determined slip rates range from $20.2\pm1.4$ $mm.yr^{-1}$ (d'Alessio et al., 2007) to $23\pm3$ $mm.yr^{-1}$ (Freymueller et al., 1999). The remainder of the 40 $mm.yr^{-1}$ of Pacific plate to Sierra Nevada Great Valley microplate motion is primarily accomodated by the RCF and the GVF.
Earthquake cycle deformation is commonly modeled assuming lateraly homogeneous elastic properties in the Earth's crust. First-order variations in rock elastic strength both across and within fault zones can, however, strongly impact inferences of fault slip parameters and earthquake rupture characteristics. Near Point Reyes, the SAF separates two different geologic terranes. On the east side of the fault is the Franciscan Complex, made of a mixture of Mesozoic oceanic crustal rocks and sediments, which were accreted onto the North American continent during subduction of the Farallon plate. On the west side of the SAF is the Salinian terrane, which is composed of Cretaceous granitic and metamorphic rocks, overlain by Tertiary sedimentary rocks and Quaternary fluvial terrasses. Prescott and Yu, 1986, Lisowski et al, 1991 describe an asymmetric pattern along a geodetically measured surface velocity profile across to SAF at Point Reyes, which can be explained by higher rigidities to the SW of the fault. Le Pichon et al., 2005, describes also an asymmetric pattern further north along the SAF, at Point Arena, but not at Point Reyes. Chen & Freymueller, 2002, rely on near-fault strain rates determined from trilateration and GPS measurements to infer a $2$-$km$-wide near-fault compliant zone (with 50% reduced rigidity) near Bodega Bay and Tomales Bay. Here we use densily spaced GPS velocities across the SAF to evaluate changes in elastic properties and within the SAF zone.

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