Introduction

The San Francisco Bay area is one of the tectonically most deformed areas in the world. This deformation is the result of relative motion of the Pacific (PAC) and North-America (NAM) plates. A large part of the strain ($75 \%$) is accommodated along structures lying in a 50 km wide land stripe. At least two major seismic events ($M_w\geq6.5$) are expected along the San Andreas (SAF) and Hayward faults (HAY) within the next decades. Triggering effects between the two seismic events may not be excluded (Lienkaemper et al., 1997). The last major event in the area occurred in 1989 (Loma Prieta event) (Segall and Lisowski 1990, Dietz and Ellsworth 1990). The velocity field after this large event was perturbed (Argus and Lyzenga 1994) and the microseismicity significantly increased since then in the southern portion of the San Francisco Bay Area (SFBA) (ANSS catalog). As the prediction of the next event is based on the estimation of the energy accumulated along active faults (San Andreas, Hayward, Rogers Creek, Calaveras, Green Valley fault, Greenville fault), it is important to quantify the strain amplitude during the interseismic cycle across these faults. Dedicated to monitoring the deformation of Northern California, the Bay Area Regional Deformation (BARD) network provides data with which we can investigate the spatial distribution of the strain accumulation, the motion along faults and any unexpected transient deformation that would be related to seismic events (acceleration along the fault, landslides, hydrogeological features). Our study area will be limited in this work to the San Francisco Bay Area (SFBA).

Here we test the potential asymmetry of the motion along the San Andreas and San Gregorio faults. The asymmetry of the motion is a unique opportunity to link the velocity models used in seismic tomography and moment tensor inversions with geodetic observations, by investigating the rigidity of the PAC oceanic crust. The asymmetry was already mentioned both by geodesists (Lisowski, M., 1991) and by seismologists Le Pichon et al., 2005) but never tested in the SFBA on a real geodetic dataset. We evaluate the velocity field in the light of this hypothesis. We discuss the implications of the new location of the strain peak from the perspective of a future event. Additionally we provide some elements on the geometry of tectonic features in the San Pablo Bay region.

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