GPS measured rates of deformation in the northern San Francisco Bay Region, California, 1990–1993

Abstract

A 100-km-long, 13-station profile extending across the San Andreas fault system north of San Francisco Bay was measured 7 times between March 1990 and January 1993 with the Global Positioning System (GPS). The data have been processed using the Bernese Version 3.2 software. Data from a continental-scale fiducial network were included in the solutions to aid orbit improvement and provide a consistent reference frame. We find 33 ± 2 mm/yr of fault-parallel (N33°W) shear evenly distributed southwest and northeast of the Rodgers Creek fault and a near linear velocity gradient across the profile. The profile spans most of the zone of active deformation associated with the San Andreas fault system. Shear is negligible at the east end of the profile near the Great Valley. Additional shear of a few millimeters per year is likely beyond Point Reyes Head, the west end of the profile. We observe no systematic convergence upon the fault. The GPS measured velocities are similar to those derived previously from trilateration. The velocity change across the GPS profile (31–35 mm/yr) plus that west of the profile (0–3 mm/yr) and that observed with VLBI east of the Sierra Nevada Mountains (∼10–12 mm/yr) accounts for the North American-Pacific plate rate (46–47 mm/yr).