Low‐Viscosity Zones Beneath the Coso Volcanic Field Revealed by Postseismic Deformations Following the 2019 Ridgecrest Earthquake

Abstract

Abstract The rheology of materials near fault zones controls the deformation of the fault, thus playing an important role in the rupture propagation of large earthquakes. Here, we model the postseismic deformation in the first 4 years following the 2019 Ridgecrest earthquake, and probe the laterally variable lower‐crustal viscosity and fault afterslip, using a combined model incorporating afterslip and viscoelastic relaxation. Modeling results reveal that laterally variable low‐viscosity zones (1017∼1018 Pa·s) beneath the Coso Volcanic Field are required to explain the observations well. These low‐viscosity materials may reduce the width of the brittle part of the fault, thus affecting the northwestward propagation of the 2019 Ridgecrest earthquake rupture and the spatial pattern of nearby seismicity. Numerical simulations reveal that the postseismic viscoelastic relaxation of the event will cause contractional strain in the upper‐crustal magma reservoirs, which may counteract the extensional strain caused by the coseismic rupture.