Weakness of the Indian Lower Crust Beneath the Himalaya Inferred From Postseismic Deformation of the 2015 Mw 7.8 Gorkha Earthquake.

The 2015 Mw 7.8 Gorkha (Nepal) earthquake induced prolonged postseismic deformation extending northward beyond the Yarlung Zangbo Suture, which provides unique opportunities to better understand the lithospheric rheology in Himalaya and southern Tibet. Here, we used the first 5‐year Global Positioning System observations to study the main postseismic processes following this event, including viscoelastic relaxation and afterslip, based on a three‐dimensional finite element model. We considered a realistic 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 displacements. A heterogeneous Indian lower crust with the transition from elastic (high‐viscosity) to low‐viscosity approximately under the Main Central Thrust 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 imply that the near‐field deformation is dominated by both afterslip and viscoelastic relaxation of the weakened Indian lower crust, not only afterslip as suggested by previous studies. Plain Language Summary: On 25 April 2015, an Mw 7.8 earthquake occurred in the central Himalayan area (near Gorkha). Global Positioning System (GPS) measurements show that the induced postseismic deformation extends northward beyond the Yarlung Zangbo Suture. In this paper, we considered the postseismic displacements within 5 years after this earthquake and studied the two main postseismic processes (afterslip and viscoelastic relaxation of the lower crust and upper mantle) using finite element modeling. We explored the parameters governing afterslip and viscoelastic relaxation of the lower crust and upper mantle beneath Himalaya aiming to fit GPS observations. We found that an Indian lower crust with the transition from elastic to low‐viscosity approximately under the Main Central Thrust is necessary to explain the observed uplift between China‐Nepal border and Peiku Lake. The postseismic deformation of the 2015 Gorkha earthquake provides independent evidence for the weakness of the Indian lower crust from Lesser to High Himalaya, which is of great importance to understand the uplift evolution of the Himalaya and Tibetan Plateau. Key Points: We study the postseismic viscoelastic relaxation and afterslip due to the 2015 Mw 7.8 Gorkha earthquake by finite element modelingA heterogeneous Indian lower crust is required to reproduce the observed postseismic uplift between China‐Nepal border and Peiku LakeThe modeling results reveal the weakness of the Indian lower crust from the Lesser to High Himalaya [ABSTRACT FROM AUTHOR]