Earth-fissure movements associated with fluctuations in ground-water levels near the Picacho mountains, south-central Arizona, 1980-84

Abstract

The Picacho earth fissure transects subsiding alluvial sediments near the eastern periphery of the Picacho basin in southcentral Arizona. The basin has undergone land subsidence of as much as 3.8 m since the 1930's due to compaction of the aquifer system in response to ground-water-level declines that have exceeded 100 m. The fissure extends generally north-south for 15 km and exhibits horizontal tensile failure as well as up to 0.6 m of normal dip-slip movement at the land surface, with the west side of the fissure downthrown. The fissure was observed as early as 1927, following an earthquake, and is the longest earth fissure in Arizona. Vertical and horizontal displacements have been monitored along a line normal to the fissure. The survey line extends from a bedrock outcrop in the Picacho Mountains on the east, past an observation well near the fissure, to a point 1422 in to the west. From May 1980 to May 1984, the western, downthrown side of the fissure subsided 167 ±1.8 mm and moved 18 ±1.5 mm westward into the basin. Concurrently, the eastern, relatively upthrown side subsided 147 ±1.8 mm and moved 14 ±1.5 mm westward. Dislocation modeling of deformation along the survey line near the fissure suggests that dip-slip movement has occurred along a vertical fault surface that extends from the land surface to a depth of about 300 m. Slip was 9 mm from May to December 1980 and 9 mm from March to November 1981. Continuous measurements were made of horizontal movement across the fissure using a buried invar-wire horizontal extensometer, while water-level fluctuations were continuously monitored in four piezometers nested in two observation wells. The range of horizontal movement was 4.620 mm, and the range of water-level fluctuation in the nearest piezometer in the deeper alluvium was 9.05 m. The maximum annual opening of the fissure during the study period was 3.740 mm from March to October 1981, while the water-level declined 7.59 m. The fissure closed 1.033 mm from October 1981 to March 1982 while the water level recovered 6.94 m. Opening and closing of the fissure were smooth and were correlated with water-level decline and recovery, respectively, in the nearby piezometers. Pearson correlation coefficients between the water-level fluctuations in the deeper piezometers and horizontal movement ranged from 0.913 to 0.925. The correlogram of water-level decline as ordinate, versus horizontal strain as abscissa, exhibits hysteresis loops for annual cycles of water-level fluctuation as well as near-vertical excursions for shorter cycles of pumping and recovery.