Phase changes and the frequency- magnitude distribution in the upper plane of the deep seismic zone beneath Tohoku, Japan.

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 mantel wedge is thought to be caused by the advection of free water from the oceanic crust. The fact that the b value drops to less than 0.7 in the upper seismic plane from 40- to 70-km depth reflects the low pore pressures left in the subducting oceanic crust after the advection of this free water. From 70 to 90 km an increase in b to greater than 1.0 is attributed to the passing of the hydrous oceanic crust through a dehydration front.-from Authors