262 P-wave arrival times associated with 40 local events obtained from a movable array of up to eight short-period seismographs were used to determine hypocentral parameters and an accurate, three-dimensional seismic velocity model for the Rio Grande rift zone near Socorro, New Mexico. The initial attempt to model these data resulted in a representative half-space velocity of 5.85¿0.02 (1 s.d.) km/sec. No azimuthal velocity variation was found that was significantly differrent from the half-space solution. The area was then subdivided at depth (4km) and into blocks one-tenth of a degree on a side. The resulting model showed that a block in the lower layer approximately 15 km west of Socorro had an average velocity of 5.17¿0.11 (1 s.d.) km/sec (0.68 km/sec less than the half-space velocity). Other blocks had smaller, but still significant, anomalies relative to the half-space velocity. The preferred explanation for the anomalously low velocity is that this area represents a site of magmatic intrusion into the upper crust (4--10 km deep).