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

Abstract

AbstractThe 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.