Statistical Studies of Induced and Triggered Seismicity at The Geysers, California
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This study considers the statistics of fluid-induced and remotely triggered seismicity at The Geysers geothermal field, California. Little seismicity was reported before steam extraction began in 1960. Beginning in 1980 the residual water associated with power generation was re-injected, producing induced seismicity. Beginning in 1997 large-scale injections of cold water began to enhance the generation of steam. This led to an increase in M < 1.2 earthquakes from approximately 5 per month to 20. Two excellent seismic networks generate two earthquake catalogs for the fluid-induced seismicity at The Geysers. Although this seismicity satisfies Gutenberg–Richter (GR) scaling to a good approximation, the scaling parameters differ. We propose a correction that eliminates this problem. We show that the seismicity at The Geysers is nearly independent of time for the period 2009–2014 and suggest that this supports our hypothesis that the seismic moment release is in near balance with the geodetic moment accumulation in the region. Our study demonstrates that aftershocks of the larger fluid-induced earthquakes also satisfy GR scaling as well as Omori’s law for their time dependence. Our results support the hypothesis that the earthquakes are caused by the reduction in friction on faults due to the injected fluids. Statistics of remotely triggered earthquakes and their associated aftershocks at The Geysers are also presented. The 8/24/14 M = 6.02 South Napa earthquake triggered an M ≈ 4.38 event as well as some 80 other M > 1.25 events. The GR and decay statistics are given. However, to separate aftershocks from remotely triggered earthquakes, an additional triggered sequence is studied. The M = 7.2 4/4/10 Baja earthquake triggered some 34 M > 1.25 earthquakes at The Geysers in the first hour including an M = 3.37 event. We conclude that the remotely triggered seismicity is dominated by local aftershocks of the larger remotely triggered earthquakes.
KeywordsFluid-induced seismicity remotely triggered seismicity The Geysers geothermal seismicity
Data products and metadata for this study were accessed through the Northern California Earthquake Data Center (NCEDC), doi: 10.7932/NCEDC. We thank Douglas Neuhauser of the Berkeley Seismological Laboratory for discussions about earthquake data catalogs.
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