• aftershocks;
  • anomalous zone;
  • interlocking;
  • megathrust event;
  • plate coupling

[1] The 2011 Tohoku-Oki earthquake (Mw9.0) ruptured a large portion of the plate boundary where the coupling was considered weak and represented by sparsely distributed small asperities. A typical asperity break produced an event of around Mw7.5 accompanied by a subsequent expansion of aftershock activity beyond the main rupture zone. Unlike previous large earthquakes, the Mw9.0 earthquake sequence does not show much spatial expansion of aftershock activity, but the areas of slip deficit in the main rupture zone are progressively filled in by aftershocks. We show a clear image of segmentation of physical properties in the subducted slab that has been revealed by joint tomography of P and S wave arrivals. The mapped variations separate the zones of dominant high-frequency radiation down-dip and dominant low-frequency radiation up-dip on the mainshock fault. These changes indicate significant variations in the effective plate coupling that interlocked the source areas of the past distinct earthquakes.