Induced seismicity in The Geysers Geothermal Area, California

A simultaneous inversion for hypocenters, velocities, and station delays was performed from the P arrival times of 55 microearthquakes and four explosions distributed throughout a 90- by 100-km region centered at The Geysers geothermal area in northern California. The resulting one-dimensional layered model shows velocity increasing with depth from 4.4 km/s at the surface to 5.9 km/s at 8 km and deeper. We relocated 7215 earthquakes which occurred between May 1975 and February 1982, using the new velocity model. These relocations show a significant spatial clustering near production wells in the steam field that was not apparent in earlier studies. We infer from the spatial and temporal pattern of seismicity that the seismicity at The Geysers is induced by geothermal production activities. With the expansion of power production by approximately 70% in 1979–1980, seismic activity has developed near the new production areas. Seismicity has also expanded northwestward of the present production area, although there are no active steam wells in this area. Statistical cross correlations between the number of earthquakes per month and both steam withdrawal per month and injection volume per month were calculated. For the wells studied, we find no consistent pattern of correlation between injection and seismicity. Likewise, we find little or no correlation between steam withdrawal and seismicity for steam wells in production longer than 7 years. Two mechanisms remain plausible for inducing seismicity at The Geysers. Volumetric contraction due to mass withdrawal may perturb the stress field and cause faulting in reservoir rock already near failure due to the regional stress field. Alternatively, aseismic deformation due to regional tectonic stress may be converted to stick-slip(seismic) deformation due to an increase of coefficient of friction along fault surfaces, such as could occur from exsolution of dissolved silica into fractures. Neither mechanism would be likely to show strong correlation between production and seismicity rates. However, we expect seismicity to continue in The Geysers and for seismicity to appear in areas where new production occurs.

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