Geophysical Investigations of Ground Failure Related to Ground‐Water Withdrawal — Picacho Basin, Arizona

Precise geophysical surveys across ground failure zones related to ground‐water withdrawal at nine sites in Picacho basin in south‐central Arizona indicate that earth fissures in alluvium near exposed bedrock are spatially associated with local gravity and magnetic anomalies ranging from local highs to convex‐upward changes in slope. We interpret the gravity anomalies, which range from 0.1 to 1 mGal with half‐widths of 50 to 300 m (160 to 980 ft), to be caused by convex‐upward irregularities in the bedrock surface underlying the alluvial aquifer. Most irregularities are inferred to be at depths less than 250 m (820 ft). Bedrock irregularities were not detected beneath failures that are more than 2 km (1.2 mi) from bedrock outcrop. The association of earth fissures with zones of variable aquifer thickness suggests that differential compaction is occurring near these fissures. Theoretical estimates, based on the finite element method, of horizontal strains generated by localized differential compaction suggest that this mechanism is the dominant source of horizontal tension causing earth fissures in Picacho basin. Our analysis indicates that tensile strains at fissures at the times of their formation ranged from 0.1 to 0.4 percent. Prediction of the location of earth fissures near exposed bedrock in Picacho basin and in adjacent basins with similar geologic settings appears feasible by delineation of convex‐upward bedrock irregularities. Failures far from exposed bedrock, however, may not be as readily predicted on this basis. If the rheological properties and thicknesses of subsurface materials are known, prediction of the magnitude of water‐level decline required to induce sufficient differential compaction to cause failure at potential earth fissures appears feasible.