A series of about 300 small earthquakes with magnitudes up to 2.5 (ML) were observed near the Albuquerque Volcanoes, New Mexico, between September 28, 1978, and April 30, 1979.
A first approximation to a crustal model for the active area was derived using a delay time analysis of arrival times from local explosions. The thickness of the sedimentary section near the volcanoes was estimated to be about 5 km. The P-wave velocity in the sedimentary section above the refractor is estimated to be 3.5 km/sec. Earthquakes located by a small network of field seismographs were used to calculate station corrections for the 13-element U.S. Geological Survey (USGS) seismic array monitoring the Albuquerque Basin. Hypocenters for 93 well-recorded earthquakes were determined using this array with the station corrections.
The earthquake sequence was generated within a small source region about 2.5 km west of the fissures from which the Albuquerque Volcanoes erupted. The events have an average focal depth of 9.5 km and are interpreted, from a composite focal mechanism, to have occurred along a north-south—striking, high-angle, down-to-the-west normal fault. The active fault, when projected to the surface, is in good agreement with the position and strike of both the volcanoes' fissure and the County Dump fault, directly to the south of the volcanoes.