Tidal triggering of earthquakes prior to the 2011 Tohoku‐Oki earthquake (Mw 9.1)

I observe highly significant tidal triggering of earthquakes prior to the 2011 M w 9.1 Tohoku‐Oki earthquake. Strong statistical correlations between tidally‐induced stresses and earthquake occurrence times are identified in the northern part of the Tohoku‐Oki source region, where the mainshock rupture initiated, in the several to ten years before the Tohoku‐Oki earthquake. The tidal phase distribution of earthquakes in this period exhibits a peak where the shear stress is at its maximum to promote failure. On the other hand, no significant tidal correlation is found after the Tohoku‐Oki mainshock. These observations suggest that tidal triggering occurs over a decade‐long period preceding the Tohoku‐Oki earthquake, and the initial rupture site of this event is already critically stressed in this precursory stage.