Tilts, strains, and ground-water levels near an earth fissure in the Mimbres Basin, New Mexico

Abstract

Vertical deflection profiles derived from microradian-sensitivity tiltmeter data collected over a 1 yr period suggest that an earth fissure in the subsiding Mimbres Basin is located near the inflection point of an evolving surficial fold of infinitesimal amplitude. Water levels beneath the study site also fell steadily during the period of observation. Calculated curvature profiles, which reflect the sign and relative magnitude of fiber strain along the ground surface, further imply that the fissure was within a zone of contraction throughout the study period. Observed fissure-perpendicular tilts and the location of the fissure at an inflection point rather than along an anticlinal crest are consistent with a mode II fracture (shearing mode crack or fault) rather than a mode I fracture (opening mode crack or joint). Tilting parallel to the fissure further suggests a component of mode III (tearing-mode crack) displacement during the study period. Thus, the three-dimensional displacement, strain, and stress fields around the fissure were considerably more complicated than those suggested by previous fissure monitoring studies and suggest that the fissure may have been formed in compression as a shear crack rather than in tension as an extensional crack. On the basis of the occurrence of right steps in the trace of the fissure, we speculate that our data may reflect contraction across one of the steps as a consequence of left-lateral slip along the fissure.