Based on recent seismic studies in the northwestern Pacific lithosphere, the oceanic lithosphere is characterized by 1) seismic velocity reductions towards the trench, 2)high velocity and strong anisotropy uppermost mantle, 3)ridge-wards dipping reflectors immediately above the high velocity mantle. The character 1) is interpreted to be caused of hydration and/or serpentinization during subduction. 2) and 3) are interpreted to represent a formation process of the oceanic lithosphere at the trench. At this moment, we have been presenting a conceptual model showing that a high velocity mantle flow creats strong anisotropy, and that this flow may drag the base of the curst resulting Riedal shear in the lower crust.
