AFTERSHOCKS OF RIDGECREST EARTHQUAKES SUPPRESSED BY THERMAL DE-STRESSING IN COSO GEOTHERMAL FIELD

The M_w6.4 and 7.1 2019 Ridgecrest earthquakes triggered an intense sequence of aftershocks which extended well North of the Coso volcanic area. The seismicity there coincides with a lobe of increased Coulomb stress which lies at the northern tip of the rupture. A striking feature of the distribution of aftershocks is the lack of aftershocks within the Coso geothermal field area, which lies just North of their surface ruptures. Here we show through numerical simulations that geothermal heat production, which started 30 years ago at Coso, can significantly deplete shear stresses within the geothermal reservoir and therefore can impede dynamic earthquake triggering. The simulation is designed based on the public information on the Coso geothermal field, including stress state, flow rate and heat extractions. The simulation result exhibits a cumulated surface subsidence of ~30cm over 30 years over an area. The peak subsidence rate is ~1.5cm/s, which is reasonable first order estimate comparing to the InSAR measurement. The Coulomb stress increases as much as 13MPa due to the thermal contraction of the reservoir. Reservoir shear stress calculated at a depth of 1.5 km become significantly decreased due to that thermal contraction decreased the maximum and the minimum stresses, which are both horizontal, shifting the Mohr circle toward the failure envelope. As a result, our model predicts that the shear stress calculated at the depth of 1.5 km on a fault parallel to the Ridgecrest ruptures in the area of production, drops from an initial value of ~10MPa to less than 2 MPa after 30 years of operation. Geothermal operations at Coso induced intense seismicity and probably also significant aseismic deformation, which eventually released most of the shear stress, that was initially available to drive earthquakes. It could thus be argued that it not only reduced aftershocks productivity but possibly also impeded the northward propagation of the main rupture during the Ridgecrest earthquake sequence.