For the 2011 off the Pacific coast of Tohoku, Japan, Earthquake, we have investigated the spatio-temporal changes in seismicity from the Mw 7.3 foreshock, March 9, 2011, 11:45, to the Mw 9.0 mainshock, March 11, 14:46 (Japan Standard Time). We found that seismic activities slowly migrated from the source area of the foreshock, which presumably reflected the propagation of the after-slip. The mainshock rupture was initiated when the migration reached the hypocentral location of the mainshock. We also found that the migration slowed down as it expanded, where the migration distance was well fitted by a certain curve proportional to the square root of the duration, suggesting that the propagation was limited by diffusion with a diffusion coefficient of about 104 m2 s−1. This slow slip mechanism differs from nucleation-related pre-slip traditionally applied in earthquake predictions. The obtained value of the diffusion coefficient is of the same order of magnitude as that reported for the migration of a deep non-volcanic tremor. These results appear to be compatible with a conceptual model having strongly coupled patches which, although being separated by decoupled stable regions on this plate-interface, are not mechanically isolated and which became interactive when they broke.