Four years of very long baseline inteferometry (VLBI) measurements involving fixed and mobile stations in the western United States are analyzed using weighted least squares. Baseline solution time series are used to estimate rates of change of the vector baselines between pairs of VLBI stations. Horizontal vector motions are determined for 18 sites with respect to a site in the Mojave desert. Six sites west of and two sites just to the east of the San Andreas fault show northwestward motion closely parallel to the San Andreas fault. Their motions range from 18 to 43 mm/yr and are significant at confidence levels greater than 99%. Motions in California relative to interior North America plate sites are used to estimate an integrated contribution from Basin and Range spreading to the total relative Pacific plate motion as 9--10 mm/yr. A maximum relative velocity of 50--51 mm/yr is found between the interior North American sites and the western California sites, in close agreement with the NUVEL-1 plate motion model. ¿ American Geophysical Union 1987