Heterogeneous lower crust rheology beneath the Himalaya

We used the first 5-year GPS observations to study the main postseismic processes following the 2015 Mw 7.8 Gorkha (Nepal) earthquake, including viscoelastic relaxation and afterslip, based on a three-dimensional (3D) finite element model (FEM). We considered a reliable geometry of the underthrusting Indian plate according to various geophysical images. We found that the models with a uniform elastic Indian lower crust fail to fit the vertical observations. A heterogeneous Indian lower crust with the transition from elastic (high-viscosity) to low-viscosity approximately under the Main Central Thrust (MCT) is required to reproduce the observed postseismic uplift between China-Nepal border and Peiku Lake, indicating the weakness of the Indian lower crust from the Lesser to High Himalaya. The afterslip simulated using a weak shear zone takes place in the adjacent area downdip of the rupture zone. The preferred model suggested that viscosities of the Tibetan lower crust, weakened Indian lower crust and shear zone are 3 × 10¹⁸, 10¹⁹ and 4 × 10¹⁸ Pa s, respectively. The viscosity of the underthrusting Indian upper mantle was roughly estimated to be greater than 10²¹ Pa s. The model results show that the near-field postseismic deformation is dominated by both afterslip and viscoelastic relaxation of the weakened Indian lower crust, not only afterslip as suggested by previous studies.

Comparison of the first 5-year GPS observations with model predictions

Effects of the weakened location of the Indian lower crust to postseismic surface deformation

Research members: Jian Zhang, Dr. Yan Hu