EARTH-FISSURE MOVEMENTS IN SOUTH-CENTRAL ARIZONA, U.S.A. (UNITED STATES)

Ground-water pumping has led to subsidence and many earth fissures in unconsolidated alluvial basins in Arizona. Earth fissures result from tensile failure; however, mechanisms producing the tensile forces are not well understood. Horizontal displacement measurements (opening and closing) of seven earth fissures were made semi-monthly during 1976 to 1982 in the lower Santa Cruz Basin and Avra Valley. Permanent and temporary short-base extensometers with a resolution of ±2.54 μm were developed and perfected which use dial gauges and transducers. Among different fissure movements, the greatest total was 41.44 mm, the greatest single opening was 31 mm, and exclusive of that, the greatest net opening was 16.54 mm. Fissures opened and closed repeatedly, exhibiting smooth movements over long periods of time, punctuated by sudden jumps. Generally, old and new earth fissures exhibited similar behavior. Earth fissures tend to close after long, dry periods and to open after heavy rainfalls. The earth fissure with the greatest movement was closest to the area of the greatest subsidence