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New evidence for the existence and internal structure of a thin layer of magma at mid-crustal depths near Socorro, New Mexico

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

Abstract

The characteristics of a strong mid-crustal reflector in the Rio Grande rift southwest of Socorro, New Mexico, were studied using high-quality digital seismograms of microearthquakes. The data are from a tight cluster of hypocenters located almost directly beneath the recording station ( 2 km). Cross-correlation of the direct and reflected microearthquake S phases yields coefficients which are positive and large in absolute value (average = 0.79). Because these two phases leave the focal region 180° out of phase for this swarm, reflection must occur at a discontinuity where a 180° phase shift occurs. Waveforms for the P-wave reflections are not identical from event to event within the swarm. The character of the P reflection indicates a magma body with rapid lateral changes in internal structure. The spectral characteristics of the reflected P phase are suggestive of constructive interference.

The near-normal incidence geometry of both the reflected and direct P phase implies an essentially common path through the upper crust and near surface Phanerozoic rocks for these two phases. The earthquakes examined in this study are quite small (M_L < 0); hence, the initial portion of the direct P phase may be used as the response function for the path through the upper crust during modeling of the reflected P phase. Synthetic seismograms, which include attenuation effects and interbed multiples, were calculated and compared to the observed waveforms.

The results of modeling the stacked spectra of the P_zP phase indicate a thin (70 m) layer of nonrigid, low-velocity material underlain by a second, thin (60 m) layer of slightly higher velocity material. This second layer may be an earlier, partially solidified intrusion or a crystalline mush at the base of a single magma chamber. The total thickness of the magma body is estimated to be < 150 m.

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