Tilting of Surficial Strata and Groundwater Level Fluctuations in the Subsiding Mimbres Basin, New Mexico

Tilts and water levels near an earth fissure in the Mimbres Basin of southern New Mexico were monitored between Guinea and September 1992, using a network of borehole tiltmeters and piezometers fitted with water level transducers. The objective of this study was to compare observed tilts near the fissure with the deformation patterns predicted by existing models of plane strain draping and differential compaction. Short-tenn tilt and water level records exhibit diurnal and semi-diurnal cycles superimposed on long-term trends, which can be explained using a regression model that includes earth tide, barometric, and annual irrigation harmonics, plus a linear monotonic trend. Daily water level maxima generally correspond to daily tilt maxima, and suggest that the fissure closes as water level rises. Long-term records (tens to hundreds of days) show complicated patterns of oblique tilt both toward and away from the fissure. These patterns are inconsistent with a model of simple plane strain stretching perpendicular to the fissure, and are not easily correlated with long-term changes in water level beneath the fissure. A model of an elastic plate subjected to complicated loading suggests that highly variable tilts may be a consequence of shallow stratigraphic and/or structural irregularities.