New Mexico Geological Society Annual Spring Meeting — Abstracts

The characteristics of a strong mid-crustal reflector in the Rio Grande rift near Socorro, New Mexico we r e studied using high quality, digitally recorded, microearthquake seismograms. Comparisons of the direct and reflected microearthquake S-phase indicates reflection from a discontinuity where a 180° phase shift occurs. First motioris of the P reflection support this conclusion. However, waveforms for the P-wave reflections are not identical from event to event within the swarm and are very dissimilar to the direct P-phase. Collectively, the character of the P and S reflections indicates a magma body with rapid lateral changes in internal structure. The duration of the reflected P-wave signal implies a maximum thickness of about 400 m for the magma body (if a full melt of basalt is assumed). The spectral characteristics of the reflected P-phase are suggestive of constructive interference. If this is the case, the basic characteristics of the waveform can be explained by a single layer of full-melt basalt as thin as 7 5 m. Prior to comparing spectral characteristics for different models of the mid-crustal magma body, a model for the upper crustal elastic properties of the epicentral area of the swarm was developed. Synthetic seismograms were calculated (including attenuation effects) using this model and compared to the direct p-phase. These modelS indicate significant near-surface gradients in velocity and especially in Q (seismic quality factor) a re present. Convolving the impulse response of the near surface model with results from the magma-body model allows a direct comparison with the reflected P-phase.