Implications of Ground-Deformation Measurements Across Earth Fissures in Subsidence Areas in the Southwest

Geodetic monitoring of ground deformation was conducted across 14 earth fissures in areas of ground-water extraction in Arizona, California, and Nevada. The monitoring sites were in areas of documented ongoing ground-water level decline and subsidence caused by aquifer-system compaction. Durations of monitoring ranged from 2 to 13 years. Both changes in elevation and horizontal position, respectively, were monitored by first-order leveling and electronic distance measurements. In addition, horizontal deformation at closely spaced (~30 m) bench marks that spanned the fissures was monitored by chaining. All of the subsidence profiles spanning the earth fissures indicate that localized differential subsidence is occurring across the fissures. The maximum curvature (convex upwards) in each subsidence profile is at the fissure. Over time, the overall shape of the profile stays similar with maximum curvature remaining at the fissure. Horizontal displacements are largest near the fissure. In general, horizontal displacements are small to negligible away from the fissure. Variation of horizontal displacements in the profiles indicates tensile horizontal strains are at a maximum at the fissure. Maximum horizontal strain is coincident with the maximum curvature in the subsidence profiles. Monitoring of the closely spaced bench marks spanning the fissures indicates that horizontal tensile strain continues to accumulate after fissures have opened. Rates of opening range from 0.9 to 3.5 mm/yr. The pattern of ground deformation at the monitoring sites is consistent with the proposed mechanism that fissures form in response to localized bending of a surface layer over a differentially compacting aquifer system. Localized lateral changes in aquifer system thickness that are consistent with the ground deformation are confirmed by microgravity surveys at some of the monitor sites in Arizona.