Heterogeneous Power‐Law Flow With Transient Creep in Southern California Following the 2010 El Mayor‐Cucapah Earthquake.

The rheology of the crust and mantle and the interaction of viscoelastic flow with seismic/aseismic slip on faults control the state of stress in the lithosphere over multiple seismic cycles. The rheological behavior of rocks is well constrained in a laboratory setting, but the in situ properties of the lithosphere and its lateral variations remain poorly known. Here, we access the lower‐crustal rheology in Southern California by exploiting 8 years of geodetic postseismic deformation following the 2010 El Mayor‐Cucapah earthquake. The data illuminate viscoelastic flow in the lower crust with lateral variations of effective viscosity correlated with the geological province. We show that a Burgers assembly with dashpots following a nonlinear constitutive law can approximate the temporal evolution of stress and strain rate, indicating the activation of nonlinear transient creep before steady‐state dislocation creep. The transient and background viscosities in the lower crust of the Salton Trough are on the order of ~1018 and ~1019 Pa s, respectively, about an order of magnitude lower than those in the surrounding regions. We highlight the importance of transient creep, nonlinear flow laws, and lateral variations of rheological properties to capture the entire history of postseismic relaxation following the El Mayor‐Cucapah earthquake. Key Points: Coupled viscoelastic flow and afterslip explain the 8 years of postseismic deformation following the El Mayor‐Cucapah earthquakeThe effective viscosity in the lower crust is heterogeneous over space and timeThe mechanical response of the lower crust includes nonlinear transient creep transitioning to steady‐state dislocation creep [ABSTRACT FROM AUTHOR]