Abstract
<jats:p>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 and 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.</jats:p>