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GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 14, PAGES 1839–1842, 1997

Subsidence at The Geysers Geothermal Field, N. California from a Comparison of GPS and Leveling Surveys

Antony Mossop

Department of Geophysics, Stanford University

Paul Segall

Department of Geophysics, Stanford University

Abstract

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 region. Each survey spanned The Geysers geothermal field and consisted of typically 40 monuments. These monuments had been previously employed in a series of first order leveling surveys during the 1970’s. This earlier study had determined that The Geysers region was subsiding, with a maximum rate of 0.048 ± 0.0055 m/yr between 1973 and 1977. In order to be able to directly compare the leveling and GPS surveys we transform 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 applying the random cost method and the F-test. The minimum volume strain that we find consistent with this subsidence is approximately 5 × 10⁻⁴. Such a strain cannot be explained by a thermoelastic mechanism, but does seem to be consistent with poroelastic deformation and a quasi-static reservoir bulk modulus, K, of ≤ 3.6 × 10⁹ Pa.

Received 11 March 1997; accepted 16 May 1997.

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Citation: Mossop, A., and P. Segall (1997), Subsidence at The Geysers Geothermal Field, N. California from a Comparison of GPS and Leveling Surveys, Geophys. Res. Lett., 24(14), 1839–1842.

Copyright 1997 by the American Geophysical Union.