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Description: The deformation transient that follows large subduction zone earthquakes is thought to originate from the interaction of viscoelastic flow in the asthenospheric mantle and slip on the megathrust that are both accelerated by the sudden coseismic stress change. The surface deformation following the 2011 Mw 9.0 Tohoku earthquake provides some of the most comprehensive constraints on surface deformation following mega-quakes. Assuming that the flow of mantle rocks is Newtonian, the low viscosity required to explain surface deformation was attributed to a weak lithosphere-asthenosphere boundary, but these findings are at odds with well-established results from mineral physics. Here, we show that combining insight from laboratory solid-state creep and friction experiments can successfully explain the spatial distribution of surface deformation in the first few years after the Tohoku earthquake. The transient reduction of effective viscosity resulting from power-law (nonlinear) stress-strain-rate interactions in the asthenosphere explains the peculiar reversal of trench-perpendicular displacements revealed by seafloor geodesy, while the rapid slip acceleration on the megathrust accounts for surface displacements on land and offshore outside the rupture area. The low-velocity zone of the lithosphere-asthenosphere boundary has been previously associated with a permanent low-viscosity structure. In contrast, our results suggest that a rapid mantle flow takes place in the lithosphere-asthenosphere boundary with temporarily decreased viscosity in response to large coseismic stress, presumably due to the activation of power-law creep during the postseismic period.