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Seismic response to fluid injection and production in two Salton Trough geothermal fields, southern California


California hosts both the largest geothermal resource capacity and highest seismicity rate in the nation. With plans to increase geothermal output, and proven earthquake triggering in the vicinity of geothermal power plants worldwide, it is important to determine the local and regional effects of geothermal power production. This study examines the link between fluid injection and seismicity at the Salton Sea and Brawley geothermal fields in southern California by attempting to answer three motivating questions: 1) Does fluid injection at the geothermal field change local seismicity in a measurable way? 2) Are aftershocks triggered at the same rate inside and outside of the field? 3) How do the triggered aftershocks interact with regional fault networks, specifically, could these aftershocks trigger a societally significant event on the southern San Andreas or Imperial faults?

Injection and production data consist of monthly logs from 1980 to 2012 for wells at both Salton Sea geothermal fields (California Department of Conservation, 2012). Seismic data for the same time span comes from the relocated Hauksson, Yang, and Shearer earthquake catalog for southern California (2012), which is augmented by the Southern California Seismic Network earthquake catalog (2012). We employ an epidemic-type aftershock sequence model to predict background seismicity and aftershock rates in the geothermal fields, and use empirical earthquake relationships to measure aftershock productivity. Background seismicity rate closely tracks net production volume at the Salton Sea geothermal field for early and late stages of field development and we find that earthquakes inside the field trigger aftershocks at a higher rate than ordinary earthquakes. Finally, we calculate small but finite probabilities that the aftershocks of events in the geothermal fields will trigger significant earthquakes on the large, regional faults, of order 10e-5 to 10e-8 per year.

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