Mt. Fuji in Japan is a symbol of all volcanoes, and is very strategically located on one of the six main up-welling lines of the World.
The ridge south of Japan follows the line accurately.
This shows that the trench system spans 3° from east to west, beginning with a local down-welling of upper mantle at 120 km depth, and the seafloor seems to be engulfed by the main division line of lower mantle, resulting in shaping the western edge of the ridge, besides Mt. Fuji.
The local upper mantle down-welling line is directly above the large-scale lower mantle up-welling line, resulting in a contraversy between large-scale tectonic drift and local trend.