Japan is known for volcanoes and hot springs, and having a critical position between different tectonic plates. The conditions within the mantle should add to that story. Division between large convection rolls is expected to be located under Japan, affecting the smaller convection rolls of the asthenosphere.
One convection roll is highlighted here with red, because a large number of volcanoes happens to be located within that area. This volcanic roll, swaying towards NE, is found at the depth of around 120-265 kilometers. The rolls underneath, swaying towards NW, are at the depth of 265-410 km. Below 410 km, the large convection rolls of lower mantle are found. As the large scale lower mantle division line is found under the highlighted roll of the asthenosphere, the importance of that roll becomes more understandable. To fully understand the geology of Japan, both the tectonic drift of the converging plates and the relevant mantle currents have to be considered, to see how these factors interact with each other.
Note the location of Fuji, is directly above the division lines of two layers (and the division line of the lower mantle as well).