Abstract Geodetic and seismic observations before the 2011 Mw 9.0 Tohoku‐oki earthquake indicate decadal‐scale acceleration of aseismic slip along the northern Japan Trench, but the mechanism remains debated. We show that this transient can be explained by creep‐front propagation that progressively erodes heterogeneously locked asperities. We invert GNSS velocity fields from 1998 to 2009 using a mechanically motivated model in which locked regions are confined to historical rupture areas while their effective sizes and process‐zone stressing rates evolve. The models fit both velocity fields and predict slip accelerations consistent with repeating‐earthquake estimates. Inferred asperities occupied about 66% of their corresponding rupture areas in 1998 and about 37% in 2009, indicating substantial expansion of aseismic creep over the decade before rupture. The inferred shrinkage rates are consistent with fracture‐mechanics predictions for plausible coseismic stress drops. These results suggest that megathrust locking can evolve substantially over decadal timescales.