Hydrologic and Topographic Changes in Long Valley Caldera, California, Induced by Geothermal Development 1985-1992

Abstract

Long Valley caldera in east-central California has been the site of accelerated geothermalexploration and development and crustal unrest in the form of seismicity and ground deformation since 1980. These factors have contributed to changes observed in the hydrothermal system, including changes in the flow rate and temperature of thermal springsandfumaroles, and changes in pressures and temperatures in wells. The concurrent pattern of ground deformation involves uplift of the resurgent dome and surrounding moat areas in response to magmatic intrusion, and relative subsidence in the area of geothermal development on the south edge of the resurgent dome. Declines in reservoir pressure and temperature in the geothermal well field appear to be responsible for much of the relative subsidence, as well as for decreases in flow rates of thermal springs out to distances of about km and increases in steam discharge from fumaroles within the well field. In spite of these hydrologic changes, the geothermal reservoir appears to have reached a new, relatively stable condition for the current power output of about 40 MW.