Overriding Plate Deformation Controls Inferences of Interseismic Coupling Along the Himalayan Megathrust

The seismic hazard along the Himalayan arc remains a focus of discussion due to its huge potential societal impact. Taking advantage of modern, dense geodetic observations, several attempts have been made to provide a clear picture of the present-day rate of strain accumulation caused by interseismic coupling on the Main Himalayan Thrust (MHT). However, there are differing opinions regarding the interpretation of spatial variation in interseismic coupling along some parts of the arc. Critically, the resolution of heterogeneity of coupling is limited due to sparse geodetic observations in some areas. In the present work, we use an updated compilation of all available Global Navigation Satellite System (GNSS) data along the Himalayan arc and a suite of kinematic block models to account for deformation within the overriding plate to characterize the status of interseismic plate coupling. Our results show that the MHT is highly coupled (>0.8) along its entire length and the coupling distribution is nearly binary along-dip. This translates to a high seismic hazard in the densely populated Indo-Gangetic plains with a seismic moment accumulation of one Mw 8.7 megathrust earthquake per 100 years. Our results suggest that previously inferred low coupling zones along the MHT are possible manifestations of block modeling artifacts, where the fault segments in the overriding plate control the megathrust slip distribution more strongly than structures within the Indian lithosphere. This result highlights a strong need for better characterization of deforming structures in the overriding plate within the Himalayas and southern Tibetan plateau.