Offset Shotholes in West Texas: Evidence for Neotectonics, Stress Relief, or Blasting Phenomenon?

Duplicate Measurements After Nine Years Support a Non-tectonic Origin for Displacement of Shot Holes in West Texas

“…Evidence of active or recent faulting is important in the environmental and regulatory context because state and federal location standards for many types of waste management facilities explicitly address faults and standoff distances from active faults (e.g., 30 TAC 335.204;40 CFR 264.18). Meinardus et al (1993) concluded that active faulting was unlikely, that the offset shot holes were caused by blasting effects, and that the structural style and fracture patterns exposed in the roadcut did not support the Reidel shear system postulated by Bolden (1991). Meinardus et al (1993) made two sets of displacement measurements over 9 months in 1992 and concluded that there was no measurable displacement across the planes of interest during this time frame.…”

“…Meinardus et al (1993) concluded that active faulting was unlikely, that the offset shot holes were caused by blasting effects, and that the structural style and fracture patterns exposed in the roadcut did not support the Reidel shear system postulated by Bolden (1991). Meinardus et al (1993) made two sets of displacement measurements over 9 months in 1992 and concluded that there was no measurable displacement across the planes of interest during this time frame. The permanent benchmarks established in the rock offer the opportunity for subsequent displacement measurements over a much longer time period.…”

“…The previous investigation in Meinardus et al (1993) comprised three tasks: (1) structural mapping and surveying of the exposures in the roadcut; (2) fracture attitude measurements to evaluate the evidence for the structural style of wrench faulting and a Reidel shear system as postulated by Bolden (1991); and (3) measuring and monitoring displacement of the exposed limestone blocks. The limestone blocks consist of the lower Cretaceous Santa Elena Formation, a medium to massive bedded limestone with thin argillaceus deposits commonly present on the bedding planes.…”

“…The interpretation by Bolden (1991) implied an annualized total displacement of about 2.5 cm/year. The purpose of the geologic evaluation by Meinardus et al (1993) was to determine whether active faulting was occurring and whether the structural style invoked by Bolden (1991) as the cause of the active faulting was present. Evidence of active or recent faulting is important in the environmental and regulatory context because state and federal location standards for many types of waste management facilities explicitly address faults and standoff distances from active faults (e.g., 30 TAC 335.204;40 CFR 264.18).…”

“…A compendium of west Texas earthquakes with magnitude .3 is available from the University of Texas Institute of Geophysics at www.ig.utexas.edu. Meinardus et al (1993) believed that the displacement of the shot holes was caused by blasting effects when the roadcut was excavated. The roadcut was one of the first in west Texas to be excavated using the pre-split blasting technique.…”