Three‐dimensional velocity anomalies in the upper crust near Socorro, New Mexico

Abstract

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 different 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).