Ridgecrest aftershocks at Coso suppressed by thermal destressing

Abstract

Geothermal and volcanic areas are prone to earthquake triggering. The Coso geothermal field in California lies just north of the surface ruptures driven by the 2019 Ridgecrest earthquake (moment magnitude M_w = 7.1), in an area where changes in coseismic stress should have triggered aftershocks. However, no aftershocks were observed there. Here we show that 30 years of geothermal heat production at Coso depleted shear stresses within the geothermal reservoir. Thermal contraction of the reservoir initially induced substantial seismicity, as observed in the Coso geothermal reservoir, but subsequently depleted the stress available to drive the aftershocks during the Ridgecrest sequence. This destressing changed the faulting style of the reservoir and impeded aftershock triggering. Although unlikely to have been the case for the Ridgecrest earthquake, such a destressed zone could, in principle, impede the propagation of a large earthquake.