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Mount Fuji is located on a triple junction between three plates. It should explain it all, besides the vicinity to the Pacific Plate descending into the mantle. But compared to the convection cell model, the position is unique. Fuji is located directly on one of the division lines of the lower mantle as shown here:

Now we zoom in on Japan to show more details. It is interesting that the convection cells are aligned directly NS at 32°N, and Japan is just north of that parallel, so here is an example of how it works:

This line is a counterpart of the main division line under Iceland:

As has been pointed out in other posts, this line represents both the main convection division at 410 km depth, and the upper most division line of small cells at 120 km depth. The smaller cells are down-welling, and the two lines to each side of course up-welling.

Therefore, in Iceland, The Reykjanes Ridge is found west of this line, whereas Hekla is east of it. Still, many volcanic and geothermal sites are also on the main division line, with the controversy of having up-welling deep below, but down-welling in the upper layer.

This is of course in a way complicated, but it is easy to see what Japan and Iceland have in common according to this model of the convection rolls within Earth’s mantle.

Indonesia does extend exactly 30° along equator, just as everyone can see on a map (approximately from 99°E to 129°E). It fits to the analysis that underneath there is a convection roll in the lower mantle, exactly 30° wide. This is the big mystery of the equator line!

Looking at the details of Indonesia, some interesting forms show up. Two of the convection division lines look like this:

With these calculated lines put into place, let us look at how nature has shaped the tectonics around:

This is a place of symmetry, and square forms! A more detailed analysis can tell us much more.