Much of the information about the peak's formation during these years comes from local postmaster Masao Mimatsu, who kept detailed measurements of its progress.

At the beginning of 1944, the hypocenter started to move toward the eastern part of the base of Mount Usu and this resulted in a protuberance emerging from the overlying ground.

The surge managed to reach the northern bank of Lake Tōya, burning forests and houses in the process.

By the end of October 1944, Masao Mimatsu recorded a total of seven craters, seventeen major eruptions and countless minor ones.

Furthermore, centimeters of volcanic ash from the eruption had been found deposited up to one kilometer (0.6 mile) away from the volcano, while the protuberance of the ground continued to grow.