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 (d'Alessio et al, 2007, Lisowski et al., 1991). Geodetically determined slip rates range from (d'Alessio et al., 2007) to (Freymueller et al., 1999). The remainder of the 40 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) and 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, also describes 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 --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.
The classic way to interpret a GPS-derived velocity profile across a strike-slip fault is, assuming that the movement is only horizontal, to use the screw dislocation model (Savage and Burford, 1973):
Where is the predicted fault-parallel velocity of a surface point at distance from the fault and is the far field velocity. is also the slip rate on the dislocation below the locking depth . This model assumes an infinite dislocation burried in a semi-infinite elastic medium. Next we consider laterally heterogeneous models that account for variation of elastic properties across and within the fault zone.
We first consider the model developed by Le Pichon et al., 2005, where the fault separates two elastic media, with different Young's modulus and . They consequently use a rigidity ratio, , in the following equations:
Where is again the velocity at a distance y from the fault, is the far field velocity, the locking depth, and
is the asymmetry ratio.
We also evaluate the deep Compliant Fault Zone Model (CFZM) developed in Chen & Freymueller, 2002, following Rybicki and Kasahara, 1977. A low rigidity fault zone is introduced between two elastic blocks (Figure 2.19).
This model (A. in Figure 2.19) is based on an infinitely deep weak fault zone. If we consider that the fault zone is weak because of damage caused by repeated earthquakes, this zone should not extend deeper than the locking depth. Therefore, we developed, using Finite Element Modeling (Chéry et al., 2001), a shallow CFZM (B. in Figure 2.19). Both models tend to localize the deformation close to the fault trace, but the shear is more localized in the shallow CFZM.
We tried to fit the computed velocity profiles obtained with both CFZMs with the classic screw dislocation model, to evaluate the trade-off between the rigidity ratio and the obtained best-fit locking depth. For both models, there is an inverse relationship between the rigidity ratio and the fitted locking depth (linear for the deep CZFM and curved for the shallow CZFM). As the difference between the CFZMs and the fitted classic models is smaller than the typical error obtained with geodetic data (typically ), we cannot distinguish between a shallow locking depth and a compliant fault zone, relying only on geodetic data. Thus it is important to have independent constraints on the locking depth, for instance, from the depth extent of microseismicity.
This project was funded by the USGS National Earthquake Hazard Reduction Programm (NEHRP). We would like to thank J. Freymueller for his help during the GPS survey. Thanks to R. Bürgmann for having me in his team during 6 months and to all the Berkeley Active Tectonics Group.
d'Alessio, M. A., Johanson, I. A., Bürgmann, R., Schmidt, D. A. and Murray, M. H., Slicing up the San Francisco Bay Area: Block kinematics and fault slip rates from GPS-derived surface velocities, J. Geophys. Res., 110, B0640, 2007.
Chen, Q. and Freymueller, J. T., Geodetic evidence for a near-fault compliant zone along the San Andreas Fault in the San Francisco Bay Area, J. Geophys. Res., 92, 2002.
Chéry, J., Zoback, M. and Hassani, R., Rheology, strain and stress of the San Andreas Dault in Central and Northern California: A 3-d thermomechanical modeling study, J. Geophys. Res., 106, , B08406, 2001.
Freymueller, J. T., Murray, M. H., Segall, P. and Castillo, D., Kinematics of the Pacific-North America plate boundary zone, Northern California, J. Geophys. Res., 104, 7419-7442, 1999.
Funning, G. J., Bürgmann, R., Ferretti, A. and Fumagalli, A. Creep on the Rodgers Creek Fault, northern San Francisco Bay Area, from 10 year ps-insar dataset. submitted, 2007.
Le Pichon, X., Kreemer, C. and Chamot-Rooke, N., Asymmetry in elastic properties and the evolution of large continental strike-slip faults, J. Geophys. Res., 110, B03405, 2005.
Lisowski, M., Savage, J. C. and Prescott, W. H., The Velocity Field Along the San Andreas Fault in Central and Southern California, J. Geophys. Res., 96, 8369-8389, 1991.
Prescott, W. H. and Yu, S. B., Geodetic measurement of horizontal deformation in the northern San Francisco Bay region, California, J. Geophys. Res., 91, 7475-7484, 1986
Savage, J. and Burford, R., Geodetic determination of relative plate motion in Central California, J. Geophys. Res., 1973
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