GPS velocities along the Northern San Andreas Fault

We collected GPS data in Bodega Bay and Tomales Bay, using 1996-2000 GPS measurements from Chen & Freymueller (2002) to calculate the velocities. We also used data from the Point Reyes profile, provided by the Bay Area Velocity Unification (BAVU), a compilation of the San Francisco bay area GPS velocities (d'Alessio et al., 2005). The data are processed using the GAMIT/GLOBK GPS analysis software. The site velocities are shown with respect to BARD continuous GPS station LUTZ in Figure 2.67 [*].
A first analysis with a simple screw dislocation model, based on three parallel faults (SAF, RCF and GVF) provides a $23\pm1$ $mm.yr^{-1}$ slip rate on the SAF, with a $14\pm2$ $km$ locking depth, while the whole system is accomodating $40$ $mm.yr^{-1}$ of fault parallel displacement (we find a $8\pm1$ $mm.yr^{-1}$ slip rate on the RCF and $9\pm1$ $mm.yr^{-1}$ on the GVF)(Figure 2.67 [*]). d'Alessio et al. (2007) show that the velocity of the Farallon islands with respect to the Pacific plate is about $2.9$ $mm.yr^{-1}$ consistent with our modelled velocity field. But the half-space model velocity for the Farallon Island station is $4$ to $5$ $mm.yr^{-1}$ faster than the actual measured velocity. We next consider asymmetric models with a rigidity contrast across the SAF, fitting the data with the equation 34.2 [*]. We find that the modeled velocity profile better matches the Farallon Islands velocity with a $0.41$ K ratio. Thus, we infer that the Salinian terrane has a rigidity $1.4$ times higher than the Franciscan complex to the east of the SAF. Our results suggest an $18$ $mm.yr^{-1}$ slip rate on the SAF, with a $10$ $km$ locking depth. There is a significant trade-off between the inferred slip rate on the SAF and the rigidity contrast across the fault, with smaller rigidity contrasts leading to higher inferred slip rates.
The two networks across the SAF located further north, one in Tomales Bay and one in Bodega Bay, allow us to consider if the SAF represents a low-rigidity fault zone. Our preferred model for the Tomales Bay profile is a classic dislocation, with a $21$ $mm.yr^{-1}$ slip rate on the SAF, with a $12$ $km$ locking depth. We did not explore the corresponding trade-off but as our data set doesn't extend far away on both side of the fault, even using the PS-SAR data from Funning et al., 2007, the determined parameters are not well constrained. In Bodega Bay, our preferred model is based on a deep CFZM, with a $28$ $mm.yr^{-1}$ slip rate on the SAF, with a $15$ $km$ locking depth. The compliant zone is 40% weaker than the surrounding medium. But a classic homogeneous model with a $24$ $mm.yr^{-1}$ slip rate and a $7$ $km$ locking depth on the SAF satisfies the near-field data as well, as shown by the first-order trade-off between locking depth and the compliant fault zone rigidity contrast we found in the previous section. We prefer a $15$ $km$ locking depth and consequently introducing this deep CFZM because of the microseismicity near the Point Reyes profile, assuming that there is no significant changes in the locking depth.

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