Asymmetric motion along the San Franciso Bay Area faults. Implication for the magnitude of future seismic events

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 and North-America plates. A large part of the strain (75 %) is accommodated along structures lying in a 50 km wide land strip. At least two major seismic events (Mw>6.5) 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. The BARD network is a permanent GPS network comprising 40 GPS sites, installed since 1994 in Northern California. Originally started as a collaborative effort of different Bay Area institutions, since the establishment of the Plate Boundary Observatory it has focused on real-time data acquisition from stations operated by UC Berkeley, with plans for expansion in collaboration with USGS/Menlo Park. The BARD network is streaming data to the Berkeley Seismological Laboratory in real-time (sampling rates of 1s and 15s, depending on the site). All sites are transmitting data using Frame Relay technology which makes them safer in case of earthquake occurrence. Data are archived at the Northern California Earthquake Data Center (NCEDC, http://www.ncedc.org) and are freely available. The BARD network is currently able to provide high accuracy (error<1mm/yr) velocities in Northern California. We use the BARD data to show that the motion across the San Andreas fault may be asymmetric. Therefore, the common assumption that the deformation is symmetric across the fault could lead to a biased location of the region of maximum strain in the San Francisco Bay Area. The new location of the maximum static strain based on asymmetry influences estimates of the response of the Hayward Fault to deformation associated with the San Andreas fault. We also present preliminary velocities for PBO sites located in the San Francisco Bay Area and discuss them in the light of a BARD reference frame.