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

DEPARTMENT OF GEOSCIENCE NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY, SOCORRO, NEW MEXICO 87801

Abstract

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 S_xP and S_xS 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 S_xP to S_xS amplitudes in conjunction with plane-wave reflection theory indicate a zone of very low rigidity beneath the discontinuity. Large S_xS 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.