The Nankai Trough is the surface expression of the subduction zone between the Philippine Sea and Amur plates. Honshu itself is formed from the island arc developed over the subducting plate. The megathrust boundary extends about 700 km from the southern end of Kyūshū to the triple junction with the Okhotsk Plate near Mount Fuji. At its southwestern end, there is another triple junction, where the overriding plate becomes the Okinawa Plate.