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Abstract Megathrusts at convergent plate boundaries generate the largest and some of the most hazardous earthquakes on Earth. However, their physical properties, including those influencing fault slip accumulation and release and earthquake‐related surface displacements, are still poorly constrained at critical depths. Here, we combine seismic imaging and geodetic modeling to investigate the structure and mechanical behavior of the Main Himalayan Thrust fault (MHT) in the center of the 2015 Mw 7.8 Gorkha rupture in Nepal. Our results from two independent observations consistently suggest the presence of a channel associated with the MHT with high compliance (shear modulus as low as ∼4 GPa) and strain anisotropy (stiffer in the vertical orientation than in the horizontal), likely arising from a weak subducting layer with north‐dipping foliation. Such mechanical heterogeneity significantly influences the quantification of short‐term fault kinematics and associated earthquake potential, with implications on across‐scale dynamics of plate boundaries in Himalaya and elsewhere. 

Abstract Earthquakes are known to occur beneath southern Tibet at depths up to ∼95 km. Whether these earthquakes occur within the lower crust thickened in the Himalayan collision or in the mantle is a matter of current debate. Here we compare vertical travel paths expressed as delay times between S and P arrivals for local events to delay times of P-to-S conversions from the Moho in receiver functions. The method removes most of the uncertainty introduced in standard analysis from using velocity models for depth location and migration. We show that deep seismicity in southern Tibet is unequivocally located beneath the Moho in the mantle. Deep seismicity in continental lithosphere occurs under normally ductile conditions and has therefore garnered interest in whether its occurrence is due to particularly cold temperatures or whether other factors are causing embrittlement of ductile material. Eclogitization in the subducting Indian crust has been proposed as a cause for the deep seismicity in this area. Our observation of seismicity in the mantle, falling below rather than within the crustal layer with proposed eclogitization, requires revisiting this concept and favors other embrittlement mechanisms that operate within mantle material.