Explosive eruptions larger than M7.0 occur 0.072 times per 1000 years in Japan. The frequency increases tenfold with decrease of M by 1 to an inflection point of M3.6. From the point the frequency increases less than twofold with decrease of M by 1.
There seems an upper limit of M near 9.0. The integration of three lines gives M7.0 for the total output of magma per 1000 years in Japan.