In the 2011 Tohoku-Oki earthquake, the rupture in the subduction megathrust reached the trench axis and triggered a large tsunami. The shallow portion of the subduction megathrust fault was regarded as an aseismic stable zone. The frictional properties along the shallow subduction plate boundary are an important foundation for understanding the cause of the dynamic fault rupture in the earthquake near the trench. The critical taper model of a sedimentary wedge best describes the first-order mechanics of a subduction zone wedge. The tapered wedge geometry (slope angle α and basal dip angle β) is responsible for the strength of a shallow megathrust. However, to apply the critical taper model for the investigation of spatial heterogeneity, we need to improve handling β, since β is derived from the subsurface structure and its value depends on the number of accurately depth-converted seismic profiles. Here, the effect of décollement dip angle β in the critical taper model of a sedimentary wedge is examined. The effect is negligible for a high pore fluid pressure ratio, allowing the frictional variation to be obtained with only bathymetry data. We applied the model to the Japan Trench. The frictional variation indicates that a smaller frictional area corresponds to an area with a larger coseismic shallow rupture during the 2011 earthquake than those of the southern and northern areas. The method can be applied to other trenches to predict seismic potential.