A new map of the geographic extent of the Socorro mid-crustal magma body

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 km2 by mapping reflection points for 220 observed SzS arrivals. We have remapped the magma body using 697 PzP , 2169 SzP , and 2589 SzS 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 km2. 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 ∼80 km north-south extent of the SMB falls just inside a ∼100-km north-south range of enhanced seismicity near Socorro. This area, the Socorro seismic anomaly (SSA), covers ∼5000 km2 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 ∼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 ±0.50 km.