The model is constrained by the wedge geometry (surface slope α and basal dip angle β), the coefficient of friction of the wedge material μw (0.45), and the pore fluid pressure ratio in the wedge λ (see refs. 15,16,50 for details). The open circle indicates the ideal state of basal erosion, which is given by the intersection of the compressively critical strength envelope with the straight line that represents all principal solutions for which the effective strength of the megathrust is equal to the effective strength of the wedge (μ′ = μw(1 − λ)). Basal erosion occurs during coseismic strengthening of the shallow megathrust beneath the outer wedge and constrains the dynamic strength of the fault during great earthquakes16. The average interseismic strength of the megathrust (solid circle) used for the calculation of the megathrust shear force is taken to be lower by 0.01 than the dynamic strength (open circle) of the megathrust during coseismic strengthening of the shallow megathrust.