We use over 6 years of mobile very long baseline interferometry (VLBI) baseline measurements between 12 sites in the western United States to infer their velocities relative to the North American plate. We find these velocities to be generally consistent with those determined from geologic data and contemporaneous satellite laser ranging measurements in the same region. The discrepancy between the largest velocities determined from the VLBI measurements of 40--48 mm yr-1 and the relative plate velocity of 50--56 mm yr-1 predicted from plate motion models is found to be consistent with a broadened distribution of interseismic strain from cyclic activity on the San Andreas and Subsidiary faults. The VLBI data are best explained by a cumulative rate of strike-slip motion near the plate boundary of approximately 48 mm yr-1, although exclusion of competing values of 56 and 41 mm yr-1 is based upon very few data. The rates of offshore fault slip inferred from this study range from ~15 mm yr-1 in central California to negligible amounts in the San Francisco region. Finite element calculations of multiple fault strain distributions show good agreement with systematic variations in the distribution of shear strain along the San Andreas system, as revealed by previous geodetic measurements. ¿ American Geophysical Union 1987