EFFECTS OF GEOTHERMAL DEVELOPMENT ON DEFORMATION IN THE LONG VALLEY-CALDERA, EASTERN CALIFORNIA, 1985-1994

Long Valley caldera in east central California has been the site of cr ustal unrest in the form of seismicity and ground deformation in respo nse to magmatic inflation since 1980. Uplift of the resurgent dome has totaled similar to 0.6 m over the 1975-1992 period. Within this regio n of uplift, and near the southwestern edge of the resurgent dome, is the Casa Diablo area, which experienced relative subsidence of 0.17 m between 1985 and 1992. Geothermal fluid production began at Casa Diabl o in 1985 to supply a binary electric power plant; currently, three su ch plants generate a total of about 40 MW of electricity. The plants a re supplied by 170 degrees C water pumped from depths near 150 m; all the produced water is reinjected at cooler temperatures at depths near 600 m. Analyses of data from (1) regional leveling lines and a networ k of bench marks in the geothermal well field area, (2) tilt observati ons from an L-shaped array of bench marks, and (3) calculations relati ng pressure and temperature changes to subsidence indicate that relati ve subsidence at Casa Diablo results mainly from reductions in pressur e in the shallow production reservoir and overlying formations and red uctions in temperature in the underlying injection reservoir. Secondar y effects on deformation are caused by offsets along the Taylor-Bryant fault bounding the west side of the well field.