Overlapped Postseismic Deformation Caused by Afterslip and Viscoelastic Relaxation Following the 2015 Mw 7.8 Gorkha (Nepal) Earthquake

Abstract

AbstractStudying transient postseismic deformation is one of the key approaches to probe fault kinematics and rheological behavior of rocks surrounding tectonic faults. Previous studies have shown that transient postseismic deformation is dominantly induced by afterslip and postseismic viscoelastic relaxation. However, effects of these two mechanisms are generally overlapped in space and time, making the analysis of the observations challenging. Here, we investigate the overlapping effects of afterslip and viscoelastic relaxation following the 2015 Gorkha earthquake based on the first 4‐years’ Global Positioning System observations and finite element models. Separated mechanisms either fail to explain the observations or lead to unreasonable model parameters. A combined model that incorporates stress‐driven afterslip and viscoelastic relaxation can overcome the limitations of separated mechanisms. Our results indicate that only short‐term (<1 year) postseismic deformation in near‐to middle‐fields is caused mainly by afterslip, whereas that in far‐field or long‐term are dominated by the viscoelastic relaxation effect. The stress‐driven afterslip is mainly confined to a narrow zone adjacent to the coseismic rupture asperities, in contrast to the widespread deep afterslip as suggested by previous studies. The total seismic moment released by the afterslip is ∼5% of that released by the Mw 7.8 main shock. The optimal viscosity of the lower crust beneath the south Tibet is estimated to be 2.5 × 1018 Pa∙s, which is at least one order lower than that beneath Indian plate, suggesting a significant lateral variation across the Himalayan fault. Validations by using independent InSAR observations further confirm the reliability of the combined model.