Robert S. Balch, Hans E. Hartse, Allan R. Sanford, and Kuo-wan Lin
A new map of the geographic extent of the Socorro mid-crustal magma body
Bulletin of the Seismological Society of America (February 1997), 87(1):174-182

Abstract:
For 35 years, strong reflected phases from the sill-like, mid-crustal Socorro magma body (SMB) have been observed on microearthquake seismograms recorded in the central Rio Grande rift, New Mexico. In 1979, the lateral extent of the SMB was estimated at 1700 km ² by mapping reflection points for 220 observed S _z S arrivals. We have remapped the magma body using 697 P _z P, 2169 S _z P, and 2589 S _z S reflections observed on seismograms recorded between 1975 and 1995 by New Mexico Tech seismic networks. These data indicate that the SMB covers an area greater than 3400 km ² . By comparing the distribution of observed reflection points with the distribution of all possible reflection points, limits can be set for the northern and southern boundaries of the SMB. The approximately 80 km north-south extent of the SMB falls just inside a approximately 100-km north-south range of enhanced seismicity near Socorro. This area, the Socorro seismic anomaly (SSA), covers approximately 5000 km ² and has the same elliptical shape as the outline of the SMB based on observed reflection points. The close spatial relation between the SSA and the mid-crustal magma body along their northern and southern boundaries suggests that the observed seismicity can be used to place limits on the eastern and western boundaries of the magma body. Using the distribution of earthquakes within the SSA over the last 34 years, the maximum east-west extent of the SMB is <60 km, our reflection based value is approximately 50 km. Our data indicates that the upper surface of the magma body displays no regional dip. Considering timing errors, maximum possible relief on the magma body surface is + or -0.50 km.