CRUSTAL VELOCITY-FIELD NEAR THE BIG BEND OF CALIFORNIA SAN-ANDREAS FAULT

We use geodetic data spanning the 1920-1992 interval to estimate the h orizontal velocity field near the big bend segment of California's San Andreas fault (SAF). More specifically, we estimate a horizontal velo city vector for each node of a two-dimensional grid that has a 15-min- by-15-min mesh and that extends between latitudes 34.0 degrees N and 3 6.0 degrees N and longitudes 117.5 degrees W and 120.5 degrees W. For this estimation process, we apply bilinear interpolation to transfer c rustal deformation information from geodetic sites to the grid nodes. The data include over a half century of triangulation measurements, ov er two decades of repeated electronic distance measurements, a decade of repeated very long baseline interferometry measurements, and severa l years of Global Positioning System measurements. Magnitudes for our estimated velocity vectors have formal standard errors ranging from 0. 7 to 6.8 mm/yr. Our derived velocity field shows that (1) relative mot ion associated with the SAF exceeds 30 mm/yr and is distributed on the Earth's surface across a band (>100 km wide) that is roughly centered on this fault; (2) when velocities are expressed relative to a fixed North America plate, the motion within our primary study region has a mean orientation of N44 degrees W +/- 2 degrees and the surface trace of the SAF is congruent in shape to nearby contours of constant speed yet this trace is oriented between 5 degrees and 10 degrees counterclo ckwise relative to these contours; and (3) large strain rates (shear r ates > 150 nrad/yr and/or areal dilatation rates < -150 nstr/yr) exist near the Garlock fault, near the White Wolf fault, and in the Ventura basin.