SUBSIDENCE AT THE GEYSERS GEOTHERMAL-FIELD, N CALIFORNIA FROM A COMPARISON OF GPS AND LEVELING SURVEYS

Between 1994 and 1996 three GPS surveys were conducted in The Geysers region of northern California. Our aim was to constrain models of the stresses and strains induced by geothermal power production in that re gion. Each survey spanned The Geysers geothermal field and consisted o f typically 40 monuments. These monuments had been previously employed in a series of first order leveling surveys during the 1970's. This e arlier study had determined that The Geysers region was subsiding, wit h a maximum rate of 0.048 +/- 0.0055 m/yr between 1973 and 1977. In or der to be able to directly compare the leveling and GPS surveys we tra nsform them to the same reference frame using the GEOID 96 geoid model . For the period 1977-1996 we determine a maximum susbsidence rate of 0.047 +/- 0.002 m/yr. We then model this subsidence using a series of point sources of contraction and find their optimal configuration by a pplying the random cost method and the F-test. The minimum volume stra in that we find consistent with this subsidence is approximately 5 x 1 0(-4). Such a strain cannot be explained by a thermoelastic mechanism, but does seem to be consistent with poroelastic deformation and a qua si-static reservoir bulk modulus, K, of less than or equal to 3.6 x 10 (9) Pa.