The existence of impulsive S to S-wave (Sz S) reflections off an extensive sillshaped magma body on many local microearthquake seismograms near Socorro, New Mexico, allowed a straight-forward least-squares inversion to determine an S-wave velocity crustal model. The data consisted of 163 microearthquakes recorded by a small aperture array during 316 recording days from May 1975 to January 1977.

Two least-squares inversion models were attempted. M1 was used to calculate the overall average S-wave velocity for approximately 19 km of the crust and depths to 36 separate, horizontal rectangular surfaces representing the upper surface of the magma body. Due to lack of resolution in the data, the next model was simplified. M2 had as its initial unknown parameters an average S-wave velocity and depth to the reflector. M2 was then used to calculate the velocity of a lower 9.2-km-thick crustal layer starting just above the magma body. The data was then separated into different groups to search for local heterogeneities and anisotropy in the lower half of the upper crust.

The best average S-wave velocity for the Rio Grande rift near Socorro was found to be 3.41 ± 0.03 km/sec. Depth to a horizontal magma body, based on a velocity of 3.41 km/sec was 19.2 ± 0.6 km. The data do allow a maximum northward dip of 2°, which places the magma body parallel to the Moho. The lower 9.2 km of the crust above the magma body appears to have a 3 per cent higher velocity than the upper layer and on the average, there are no differences between east-west and north-south S-wave velocities. One segment of the crust west of Socorro appears to have an anomalously low velocity.

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