Three-dimensional numerical modeling has been performed to account for possible earthquake migration phenomena along a northwestern Pacific subduction slab, on the assumption that the lithosphere is a linear visco-elastic body, but the asthenosphere and the mesosphere respond to the applied stress as non-linear Newtonian materials obeying the power law of creep. On the basis of a three-dimensional thermal structure of the northwestern Pacific region, the preliminarily computed stress regimes show that: 1) to induce a stick-slip type trench even, prefailures at a depth of about 40 km in the front of the locked zone between two lithospheric plates are required; 2) the compositional density contrast due to phase change at typical depths is prerequisite of stress concentration to cause a dislocation type deep-focus earthquake; and 3) the co-seismic change in gravitational potential is a controlling factor for earthquake migration downward along the subduction slab.