[J02-2] Recent large earthquakes II

[J02-2-03] The 2015 Nepal earthquake: Evidence for a horizontal underthrusting of India beneath the Himalaya

^○Qi Wang¹, Xuejun Qiao² (1.China University of Geoscience, Wuhan, 2.Institute of Seismology, China Earthquake Administration)

The 2016 Gorkha Nepal earthquake (Mw=7.9) is a fresh and bitter reminder of seismic potential induced by underthrusting the India subcontinent beneath the Himalaya and thus provide a rare opportunity to illuminate puzzles regarding the continental subduction and seismicity, for which we knew little as yet. Himalayan earthquakes are assumed to suddenly release elastic strain energy built up on the basal decollement, a mega-thrust fault along which the Indian plate is descending. However their growth to achieve greatness is impeded usually by complex structures on the megathrust. Here, by analysis of surface displacements observed mostly in southern Tibet with GPS geodesy, we show that coseismic slip of > 1 m in 2016 has propagated northward for 130-160 km at the base of the Himalaya, extending substantially into below southern Tibet. Our modeling reveals a shallowly-dipping plate interface that runs over a mid-crustal ramp fault assumed below the topographic front of the high Himalaya and stretches almost horizontally 15-25 km above a deep seismic reflector imaged beneath southern Tibet ( known as the Main Himalayan Thrust), therefore distinguishing a mid-crustal shear zone of Indian upper crust in between which underlies a tapered accretionary wedge. Our finding sheds insight into two puzzling features in the Himalaya: why mid-crustal seismicity is absent for the continental subduction, and convergent deformation is primarily localized at its frontal thrust. In addition, the planar surface of the megathrust and its seismogenic downdip width of at least 200 km highlight a potential for generating giant earthquakes ( Mw~9 ) . We conclude that any model adopting a mid-crustal ramp- decollement geometry as the plate interface for India underthrusting beneath the Himalaya and Tibet should be modified.