SPACE GEODETIC MEASUREMENT OF CRUSTAL DEFORMATION IN CENTRAL AND SOUTHERN CALIFORNIA, 1984-1992

We estimate the velocity field in central and southern California usin g Global Positioning System (GPS) observations from 1986 to 1992 and v ery long baseline interferometry (VLBI) observations from 1984 to 1991 . OUT core network includes 12 GPS sites spaced approximately 50 km ap art, mostly in the western Transverse Ranges and the coastal Borderlan ds. The precision and accuracy of the relative horizontal velocities e stimated for these core stations are adequately described by a 95% con fidence ellipse with a semiminor axis of approximately 2 mm/yr oriente d roughly north-south, and a semimajor axis of approximately 3 mm/yr o riented east-west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncert ainty is larger. These uncertainties are calibrated by analyzing the s catter in three types of comparisons: (1) multiple measurements of rel ative position (''repeatability''), (2) independent velocity estimates from separate analyses of the GPS and VLBI data, and (3) rates of cha nge in baseline length estimated from the joint GPS+VLBI solution and from a comparison of GPS with trilateration. The dominant tectonic sig nature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model from the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eas tern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 +/- 1, 5 +/- 1, 5 +/- 1, and 2 +/- 1 mm/yr, respectively. North of the Big Bend, some compression normal to the trace of the San Andre as fault can be resolved on both sides of the fault. The rates of rota tion about vertical axes in the residual geodetic velocity field diffe r by up to a factor of 2 from those inferred from paleomagnetic declin ations. Our estimates indicate that the ''San Andreas discrepancy'' ca n be resolved to within the 3 mm/yr uncertainties by accounting for de formation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of 1-2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties . South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the offshore islands and the mainland.