Probing afterslip and viscoelastic relaxation following the 2015 Mw 7.8 Gorkha earthquake based on the 3-D Finite Element Model

Abstract We analyzed GPS coordinate time series from 34 continuous GPS stations in Nepal and 5 continuous GPS stations in South Tibet of China, and extracted the first 4.8 years postseismic displacements after the 2015 Mw7.8 Gorkha earthquake. With the longer duration GPS observations, we found that postseismic displacements mainly exhibit the southward and uplift movement at the epcientral area. To study the postseismic afterslip and viscoelastic relaxation, we then built 3-D spherical finite element model (FEM) with heterogeneous material properties and surface topography across the Himalayan range, accounting for the strong variations of material properties and surface elevation along central Himalayan arc. The sophisticated FEM is more realistic and perform better than the flat layered earth model. On the basis of it, we reveal that the predicted viscoelastic relaxation of cm level is opposite to the observed postseismic deformation; the postseismic deformation with viscoelastic relaxation deducted is well explained by afterslip downdip of coseismic rupture, which indicates the afterslip is still dominant during 4.8 years postseismic deformation after the 2015 Mw7.8 Gorkha earthquake; The lack of slip on a shallow portion and western segment of the MHT during and after the 2015 Gorkha earthquake implies continued seismic hazard in the future.