Postseismic Deformation Following the 2019 Mw 7.1 Ridgecrest Earthquake

Abstract:

The 2019 Ridgecrest earthquake sequence provides a new opportunity to study ongoing postseismic deformation following the largest earthquake in southern California in two decades. Prior to the Ridgecrest earthquakes, studies of the 1992 Landers and the 1999 Hector Mine earthquakes show that postseismic deformation is comprised of viscous mantle flow and on-fault afterslip, where mantle flow involves an early transient-weakening phase. Furthermore, the rheological structure of southern California can broadly be understood as a strong lower crust overlying a weaker upper mantle with a relatively low steady-state viscosity of <10¹⁹ Pa s that is effectively reduced by a factor of 3-30 during the transient phase following large earthquakes. In this study, we utilize 110 days of GPS solutions following the M _w 7.1 July 5 Ridgecrest earthquake to investigate the early transient phase of postseismic deformation in order to better understand both the mechanisms associated with transient rheology and implications for steady-state mantle viscosity. We generate candidate viscosity models by applying both biviscous Burgers and power-law rheology to a steady-state 3D viscosity model constructed using regional seismic velocity anomalies. We compute the postseismic deformation due to afterslip and coupled mantle flow for each candidate viscosity model. Our results suggest a transient viscosity is required to fit the data; either a combined Burgers and power-law rheology or purely Burgers rheology fit observed surface displacements. All models which match the data require a low effective postseismic viscosity on the order of 1x10¹⁷ to 5x10¹⁷ Pa s at 60 – 100 km depth. From the suite of postseismic viscosity models which match surface data, we back-calculate a permissible range of steady-state (pre-earthquake) effective viscosity depth profiles showing viscosity range of 1-8x10¹⁸ Pa s in the 60-100 km depth interval, which tends towards lower estimates inferred in previous studies.