Using moment tensors from earthquakes between 1850 and 1995, we determine the horizontal velocity gradient tensor field associated with the seismic deformation in the western U.S. plate boundary zone. The velocity vectors obtained from the integration of the seismic strain rates across the entire plate boundary lie within 5¿ of the NUVEL-1A Pacific-North American plate motion direction. The magnitude of the earthquake-related velocity is 62% of the NUVEL-1A total Pacific-North American plate motion. If earthquake occurrence is a random, Poisson-like process, then the large formal errors associated with the model velocity estimates are an indication that the 144-year interval is too short to define the long-term seismic moment release rate to within an uncertainty less than the deficit between the observed seismic moment release rate and the total long-term moment release rate associated with both the seismic and aseismic deformation. To spatially resolve the deficit of the earthquake moment release in the last 144 years, we have also estimated the total long-term horizontal velocity gradient tensor field using seismic strain rate tensors and recent geodetic velocities, both with and without the NUVEL-1A Pacific-North American rigid plate motion constraint. We find a systematic difference between the Southern California Earthquake Center (SCEC) geodetic velocity map and NUVEL-1A North American reference frames with a pole at (38.8 ¿N, 124.2 ¿W, 0.33¿ m.y.-1). This difference is significant at the 99% confidence level and indicates that 4--7 mm/yr of NE-SW convergence is absorbed in offshore areas of southern California if both the SCEC velocity map and NUVEL-1A correctly describe the relative motion between the Pacific and North America plates. Deficit strain rates calculated from comparison of observed seismic strain rates over the last 144 years with long-term model strain rate tensors are largest in areas east of San Francisco Bay, in the Transverse Ranges, and along the southern San Andreas fault and San Jacinto fault. The deficit in each 100 by 50 km grid area within these regions is equivalent to an accumulation of strain in the last 144 years that would be released by an Mw>7 earthquake. Elsewhere the accumulated strain in the last 144 years is small, except in northern Mojave Desert, along the Garlock fault, and in the region southwest of the southern San Andreas-San Jacinto fault zone. Within each 100 by 50 km grid area in these regions the accumulated strain is equivalent to that released by an Mw=6.5--7.0 earthquake. ¿ 1998 American Geophysical Union