Seismicity in and Around West Texas
Contribution No. 297 of the Marine Science Institute

David B. Dumas
The University of Texas Marine Science Institute
Galveston Geophysics Laboratory
Galveston, Texas 77550

Tectonic Shelf Margins - Fig 1 - 174k GIF

Seismic Data - Fig 3 - 215k GIF

In 1975 the Geophysics Laboratory of the Marine Science Institute, University of Texas at Galveston, installed a permanent seismograph station (1 Hz vertical seismometer) at the McDonald Observatory (MOT) in West Texas. Four additional remote stations were installed in March 1977 to form a permanent seismlc network (fig. 1). Each station is operating on solar-battery power, and the seismic signal is telemetered to MOT by radio where all recording is accomplished (fig. 2), At first, all seismic signals plus WWV time code were recorded continuously on an eight-channel Brush recorder. Later, coincidence triggering logic was installed, enabling us to record events at a higher paper speed (10 mm/sec) than is feasible with continuous recordings (fig. 5).

A hypocenter-crustal model inversion program was written in order to locate regional and local events (S-P < 30 sec at MOT) (table 1). Because of the scarcity of data along with the poor spatial distribution of events, only a rough trial crustal model was obtained. Despite the lack of data we have sufficient data to show that two models are necessary for hypocenter locations in West Texas. The boundary line between the two models is a line striking north-northwest just east of the Davis Mountains. Although the models are just crude approximations, the epicenter locations I believe are reasonably accurate (fig. 3). With the acquisition of more data I hope to obtain a more refined crustal model. Using the refined crustal model all hypocenters will be recalculated. The revised crustal model will probably have little effect on epicenters that have already been located; however, I do expect changes in the origin times and depths (fig. 4).

The main areas of seismic activitiy are located the vicinity of Kermit, Texas the area just west of Van Horn, and in Mexico. The majority of the hypocenter locations in the Kermit area were provided by Randy Keller of The University of Texas at El Paso (personal communication) and Al Rogers of the U.S. Geological Survey (in preparation). These events appear to be rather diffused and uncorrelated with any surface or subsurface structures [Editor's note: This is the Mescalero Fault]. Rogers (in preparation) believes these events are associated with secondary oil recovery. The largest recent event in this area occurred on January 25, 1974 and had a local magnitude (ml) of 3.9 (Rogers).

Seismic activity near Van Horn is concentrated in a narrow zone striking northwesterly. This zone may constitute the possible continuation of the Rim Rock Fault, although the fault is not shown on any structural map north of the Carrizo Mountains. The largest event in this area had a magnitude of 2.6 ml.

Seismic activity in Mexico is concentrated along a narrow zone striking approximately N35ºE. This is nearly perpendicular to the regional structural trend. These events all occurred within a period of 36 hours on 13 February, 1978. There appeared to be two aftershock sequences. The first sequence was associated with a magnitude 3.6 ml event and the second with a magnitude 3.0 ml event. All located events are shown in figure 3.