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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 Subduction zone accretionary prisms are commonly modeled as elastic structures where permanent deformation is accommodated by faulting and folding of otherwise elastic materials, yet accretionary prisms may exhibit other deformation styles over relatively short time scales. In this study, we use 6.5‐year (2014–2021) Sentinel‐1 interferometric synthetic aperture radar (InSAR) time‐series of post‐seismic deformation in the Makran accretionary prism of southeast Pakistan to characterize non‐linear viscoelastic deformation within an active accretionary prism on short timescales (months to years). We constructed a series of 3‐D finite‐element models of the Makran subduction zone, including an accretionary prism, and constrained the elastic thickness of the upper wedge and the flow‐law parameters (power‐law exponent, activation enthalpy, and pre‐exponential constant) of the lower wedge through forward model fits to the InSAR time‐series. Our results show that the prism is elastically thin (8–12 km) and the non‐linear viscoelastic relaxation of the deep portions of the prism alone can sufficiently explain the post‐seismic surface deformation. Our best fitting flow‐law parameters (n = 3.76 ± 0.39,Q = 82.2 ± 37.73 kJ mol⁻¹, andA = 10^{−3.36±4.69}) are consistent with triggering of low temperature dislocation creep within fluid‐saturated siliciclastic rocks. We believe that the fluids necessary for this weakening originate from sedimentary underplating and/or the presence the hydrocarbons. The presence of power‐law rheology within the lower wedge impacts the estimated plate coupling and the stress state in the subduction system, with respect to the conventional elastic wedge model, and hence should to be considered in future earthquake cycle models. 

Abstract ICON observations were used to investigate local time (LT) and latitudinal variations of thermospheric meridional winds in the middle‐high thermosphere (160–300 km) during quiet times in 2020 June and December. At middle‐low latitudes (10°S–40°N), meridional winds were predominantly equatorward in the summer hemisphere while mostly poleward in the winter hemisphere. The meridional winds showed that the diurnal variation was dominant between ∼20°N and ∼40°N, but the semi‐diurnal variation played a leading role at lower latitudes (below ∼20°N) during solstice months. Thermosphere‐Ionosphere Electrodynamics General Circulation Model reproduced the ICON observed meridional wind variations qualitatively. A model diagnostic analysis shows that the pressure gradient force dominated the semi‐diurnal variation of the winds, while the Coriolis force played a leading role in the diurnal variation in June. In December, LT variations of meridional winds were primarily driven by pressure gradient and ion drag forces. During both months, the vertical viscosity was important, tending to balance the effects of pressure gradients. Additionally, semi‐diurnal variations of low‐latitude meridional winds in June were more affected by upward propagating tides than those in December.