As spectacle, it's hard to fault Nihon chinbotsu. The Japanese people have benefited from their intimate relationship with the sea, and the concept of the film implies that an entire world and way of life at risk - thanks to its volcanic heritage. From the standpoint of reality it's rather silly to have a drama wherein the entirety of Japan vanishes under the waves; why just Japan?
So, presume instead that we have movie reality, fueled by spectacle (and popcorn), and some may find this quite affecting. Compared to adventures with Japanese radioactive monsters, this comes off as more mature and better paced. The emotional element is underplayed, and it really works. (It loses a bit with the overblown theme song at the climax, however.)
Is Nihon chinbotsu credible? Probably not, but the thrilling eruptions, and the relaxed pacing make for a more pleasing entertainment that one night expect.