Vector constraints on western U.S. deformation from space geodesy, neotectonics, and plate motions

The rate-of-slip vector on the San Andreas fault in central California estimated from geodetic and Holocene geological data (34 ± 3 mm/yr, N41°W ± 2°) is inconsistent with the prediction of rigid plate models such as RM2 (56 ± 3 mm/yr, N35°W ± 2°). This well-known “San Andreas discrepancy” is diagnostic of plate deformation distributed both east of the fault in the Basin and Range and west of the fault along the California continental margin. We construct constraints on the integrated deformation rates across these two regions consistent with (1) the kinematical boundary conditions imposed by the rigid plate model, (2) neotectonic and paleoseismic estimates of deformation rates, (3) ground-based geodetic measurements, and (4) rates of change observed by very long baseline interferometry along seven baselines to western U.S. sites. The space-geodetic data on Basin and Range extension taken over a 4-year interval are compatible with geological observations averaged over the Holocene; the best estimate of its integrated deformation rate, provided by the joint inversion of both data types, is 9.7 ± 2.1 mm/yr, N56°W ±10°, too small and in the wrong direction to account entirely for the San Andreas discrepancy. We therefore attribute much of it to deformation west of the Sierra Nevada-Great Valley block accommodated, for example, on faults such as the San Gregorio-Hosgri system of coastal California. The integral of this deformation, estimated by subtracting the Basin and Range contribution from the discrepancy vector, requires significant right-lateral shear parallel to the San Andreas (13 ± 5 mm/yr) and some compression perpendicular to it (9 ± 3 mm/yr).