Use of reflection phases on microearthquake seismograms to map an unusual discontinuity beneath the Rio Grande rift

Microearthquake seismograms recorded by stations located in or bordering the Rio Grande rift near Socorro, New Mexico, frequently have two sharp impulsive phases following direct S. These phases have been identified as SxP and SxS reflections from a sharp discontinuity that has a depth beneath Socorro of 18 km and dips northward at an angle near 6° for a distance of 30 km. Farther north, the dip steepens so that at a distance of 60 km from Socorro the depth is about 30 km. Ratios of SxP to SxS amplitudes in conjunction with plane-wave reflection theory indicate a zone of very low rigidity beneath the discontinuity. Large SxS amplitudes are believed to be the result of the large velocity contrast across the discontinuity and a fault mechanism that radiates more S-wave energy downward than outward from the focus.