Redistributed Seismicity at the Coso Geothermal Field Investigated Through Stress Changes from Fluid Production and Migration

Abstract

The Coso geothermal field of southern California hosts one of the major geothermal power plants and was launched with energy production in 1986. Through looking at the relocated seismicity data from SCEC, we observed that low-magnitude seismic events (magnitude less than 4) are unevenly distributed across three distinct zones: the nearfield (less than 3 km), midfield (3-6 km), and far-field (greater than 6 km) from the Coso geothermal plant. These zones demonstrate noticeable changes in earthquake locations before and during geothermal production. Since the production was launched, the midfield region of the main flank has shown a significant drop in seismicity rate compared to surrounding areas before production. Between 1986 and 2019, far-field earthquakes clustered in the eastern and western parts of the greater Coso area, while pre-production events during 1981-1986 were more evenly distributed along the NW-SE and SW-NW trending structures within the geothermal field. InSAR time series data reveal surface subsidence (rate up to 14 mm/year) over the field since 2014, which is then reconciled with finite-element-based poroelastic simulations. The simulations estimate stress evolution and illustrate how the spatiotemporal distribution of seismicity is linked to stress changes perturbed by fluid migration during geothermal operations. Before the 2019 Ridgecrest earthquake (2014-2019), approximately 70% of co-production earthquake events from 2014 to 2019 occurs in zones of increased Coulomb stress with greater than 99% confidence. Meanwhile, the midfield zone, characterized by reduced seismicity, coincides with a region of decreasing pore-fluid pressure. This is also consistent with aftershock distributions and surface subsidence (rate up to 10 mm/year) that are continuously observed after the Ridgecrest earthquake. These findings provide a physical explanation for how decades of geothermal operations at Coso have altered the stress field and contributed to the evolving seismic patterns, offering valuable insights for assessing seismic hazards in other geothermal regions.