Lawrence H. Jaksha

Reconnaissance seismic refraction-reflection surveys in southwestern New Mexico

The U.S. Geological Survey conducted seismic refraction-reflection studies in the Datil-Mogollon volcanic field, southwestern New Mexico and southeastern Arizona, during 1976–1980. Explosions originating in the White Sands Missile Range (New Mexico), Tyrone (New Mexico), Morenci (Arizona), and Miami (Arizona) were used as energy sources. The observations can be satisfied with a simple three-layer crustal model, although there is evidence for complexities in both of the lower layers. The surface layer varies between about 2 and 4 km thick and is composed of volcanic rocks that have an average P-wave velocity of 4.0 km/s. The basement rocks are about 21 km thick and have a P-wave velocity of 6.0 km/s. There is evidence for a distinct lower crust beneath the volcanic field. This layer, about 9 km thick, has a P-wave velocity of about 6.5 km/s. The Pn velocity is 8.0 km/s, the total crustal thickness is about 33 km, and the average P-wave velocity in the entire crust is about 6.1 km/s. There is no evidence for substantial dip along any of the refractors beneath the profile.

Microearthquakes near the Albuquerque Volcanoes, New Mexico

A series of about 300 small earthquakes with magnitudes up to 2.5 (M L ) 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 volcanoes9 fissure and the County Dump fault, directly to the south of the volcanoes.