Introduction

The San Andreas fault system in northern California includes three sub-parallel right-lateral faults: the San Andreas, Ma'acama, and Bartlett Springs. This northernmost segment is the youngest portion of the fault system, forming in the wake of the northwestwardly propagating Mendocino triple junction where the Pacific, North America, and Gorda (southern Juan de Fuca) plates meet. The Pacific plate moves about 35-40 mm/yr relative to central California across a broad $\sim $100-km zone in northern California. Additional deformation in eastern California and the Basin and Range province contribute to the total relative Pacific-North America motion of $\sim $50 mm/yr. The San Andreas fault itself has been essentially aseismic and accumulating strain since it last ruptured in the great 1906 San Francisco earthquake, and no major earthquakes have occurred during the historical record on the more seismically active Ma'acama, and Bartlett Springs faults, which are northern extensions of the Hayward-Rodgers Creek and Calaveras-Concord-Green Valley faults in the San Francisco Bay area. Our earlier geodetic studies showed that the inferred slip rate on the Ma'acama fault ( $13.9^{+4.1}_{-2.8}$ mm/yr) implied an accumulated slip deficit large enough to generate a magnitude 7 earthquake, posing a significant seismic hazard (Freymueller et al., 1999).

Figure 13.38: Velocities of sites in the Coast Ranges relative to North America, with 95% confidence regions assuming white-noise process only. Included are sites from this study plus sites from the BARD continuous network and the USGS North Bay profile. Arrows with ellipses, observed velocities. Arrows without ellipses, velocities predicted from angular velocity-backslip block model assuming block boundaries (heavy black lines). We assume Pacific, North America, and Sierran-Great Valley blocks, plus 2 small blocks between the San Andreas, Ma'acama, and Bartlett Springs faults. The most significant misfits, such as near Cape Mendocino, can be reduced by refining the fault geometry. Additional sites in the southern portion of the network (circles), have been recently remeasured.
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Since 2003, we have been resurveying sites measured in our previous studies plus about 40 additional sites, originally surveyed by Caltrans and NGS (Murray, 2004). The additional sites are located along the San Andreas fault system just north the San Francisco Bay and will improve monitoring along the Rodgers Creek and Green Valley faults. Most of the monuments were last observed in 1993-1995, so the new observations significantly improve the velocity estimates and models of average interseismic strain accumulation, including possible spatial variations along the fault system. Additional sites in the region between the southern and northern portions of the network are planned for resurveying in the future. The entire network spans the region from Pt. Reyes to Cape Mendocino. Together with planned PBO stations, it forms a primary monitoring network for future observations to detect temporal variations in deformation.

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