Extensional tectonics at The Geysers Geothermal Area, California

Comparisons between the yearly volume of steam withdrawn from The Geysers, California, geothermal reservoir and the number of earthquakes indicate that most of the seismicity is induced. To establish the inducing mechanism, an estimate of the stress field orientation was calculated from inversion of 210 fault plane solutions. Below 1 km depth The Geysers is undergoing uniaxial extension with the least principal stress σ3 oriented near horizontal at approximately 105° azimuth. The consistency between the local and regional stress fields indicates that the regional tectonic stress is much larger than the stresses induced locally through geothermal activities. The stress data are equivocal as to whether earthquakes are induced due to conversion of aseismic to seismic slip or by increased shear stresses arising from reservoir contraction. The observed tendency for downward growth of seismicity may arise from a preferential fracture orientation that is near vertical in conjunction with a stress state where the gradient of σ1 exceeds that of σ3. Evidence of extension at The Geysers and in the surrounding Clear Lake region indicates a direct relationship to the presence of the Quaternary Clear Lake Volcanics.

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