Mechanical Decoupling Across the Deep Megathrust Boundary

We examine stress and strength across the deep subduction plate boundary at ∼60 km depth offshore northeastern Japan, near the downdip seismic limit of the megathrust. High‐frequency (2–5 Hz) moment tensor inversion of ∼2,000 M 2–3.5 earthquakes, integrated with existing catalogs, increases the number of focal mechanisms by more than a factor of six, enabling detailed mapping of the stress field near the interface. The results reveal an abrupt, kilometer‐scale stress contrast across the interface: downdip‐compressional stress in the upper slab persists to within ∼2 km of the megathrust, whereas the immediately overlying plate is under nearly strike‐parallel compression, producing a ∼90° rotation of maximum compression over only a few kilometers. Because the principal stress axes remain nearly parallel or normal to the interface, this geometry implies very low shear traction on the deep megathrust: only ∼11% of the maximum shear stress in the underlying slab. In absolute terms, the estimated megathrust shear stress is only ∼108 MPa even assuming hydrostatic pore pressure within the slab (slab maximum shear stress, ∼0.98 GPa; μ ′ = 0.07), and decreases to ∼11 MPa for a weaker slab sustaining a maximum shear stress of 100 MPa (pore‐pressure‐to‐lithostatic‐pressure ratio, 0.93; μ ′ = 0.006). These results indicate that the deep megathrust is a weak, aseismically shearing fault zone that mechanically decouples the slab and overriding plate, allowing large unbending‐related stresses in the slab to coexist with strike‐parallel compression in the overriding plate.