Continued seismic monitoring of the Geysers, California geothermal area

ABSTRACT Probable effects of geothermal development on seismicity at the Geysers are shown by the spatial coherence of decreases in gravity and pressure with maximum geodetic deformation and seismic moment sum along a line through the most developed area of the geothermal field. Increases in the mean number of events per day and in the magnitude of largest annual event correlate with increases in steam production. The two largest earthquakes in the steam field occurred near the two injection wells most distant from production wells, and large events (M/sub c greater than or equal to 2.5) occurred most frequently during months of peak injection. Spatial seismic clusters in proximity to injection wells have occurred soon after injection began. Preliminary data also indicate an increase in seismicity in a previously aseismic area near plant 15 following the beginning of power production at that plant in 1979.