The frequency-magnitude distribution (b value) in the upper plane of the deep seismic zone beneath Tohoku, Japan, shows significant variation with depth which we attribute to dewatering of the oceanic crust during the subduction process. High b value from the surface to 40 km in the upper seismic plane and overlying mantle wedge is thought to be caused by the advection of free water from the oceanic crust. The fact that the b value drops to 1.0 is attributed to the passing of the hydrous oceanic crust through a dehydration front. The resulting increase in pore pressure lowers the effective stress and increases the b value in the upper plane. Beneath the volcanic line, b drops back to <0.7, implying that the newly released water advects immediately into the asthenosphere, lowering the mantle melting temperature and producing the calc-alkaline volcanism on Tohoku. At depths greater than that beneath the volcanic line, the b value again rises, indicating a water source at 100--125 km which might be the further dehydration of talc, chlorite, and amphibole in the oceanic crust.