New Mexico Geological Society Annual Spring Meeting — Abstracts

During May and July 1983 two distinct microearthquake swarms occurred beneath Socorro Mountain in central New Mexico. In excess of 700 events (35 with magnitudes greater than 0.0) were detected at the closest station. HYP071 locations of the 60 strongest events indicate the hypocenter for both swarms were confined to a small volume of crust with an average focal depth of 8.8 km. Hypocentral migration appears to be less than 1 km. Digital data (at 100 sps) was acquired during most of the May swarm and all of the July swarm. The most striking feature of the digital data is the duplication of the P-phase up to but not beyond a magnitude of 1.2. The P-phase duplication is consistent throughout even though the S-phase is not well represented by a single wave-form.

A wide range in the S-phase to P-phase amplitude ratio (0.20 < S/P < 6.37) precludes the simple compositing of first motion data for fault plane solutions. This complexity further indicates that the earthquakes probably did not occur on a single straight planar fault. Rupture appears to have occurred on a rapidly curving fault or on closely spaced separate faults with different orientations.

The time between P-wave onset and first zero crossing (T_1/2) was measured and is the same (0.36±0.003 sec) up to a local magnitude of 1.2. Constant T_1/2 with changing seismic moment indicates the events with magnitudes less than 1. 2 are merely the impluse response of the path between source and receiver. This implies the waveforms of the smaller events may be treated as empirical Green's functions for the medium (Frankel and Kanamori, 1983). This fact then allows simple calculations of rupture duration, source radius and hence stress drop to be made for events with magnitudes greater than 1. 2.