William here. I’ll post over the next few days a bit more information about the Futagawa fault (布田川断層) and the geology of Kumamoto in general as recent events may have piqued interest in this area. This is an active fault zone that extends from Uto peninsula to the western slope of the Aso somma with a length of about 64km. It is a right-lateral strike-slip fault, which means that it creates raised areas on the southeast side - hence, Kumamoto Airport - and also the Tateno gap (立野火口瀬, たてのかこうせ）, without which the Aso caldera would be a large lake. Today, I’d like to introduce some Japanese mythology about how ancient people devised an explanation for the gap’s existence: the legend of Takeiwatasunomikoto (健磐龍命 - don’t even try to deal with this kanji).
He was said to be the grandson of the legendary first emperor of Japan, Jinmu, and chose Aso as his home due to its location in the center of Kyushu. After marrying the daughter of a local god, Asotsuhime, and settling in Takamori, he pondered how to turn the caldera, which at the time was a lake, into a lush agricultural area, and while pondering and wandering with his wife, she gave birth at the base of a mountain now called Ubuyama (産山, “birth mountain”). He then proceeded a bit further and struck the caldera outer rim with such force that it created Tateno gap. Fatherhood might do that to one.
As the water flowed out of the caldera, he noticed that it was not being replenished, and upon investigation, found that a giant catfish was blocking the flow. (Note that catfish are harbingers of earthquakes in Japan.)
This is where things get a bit difficult to believe.
Takeiwatasunomikoto tied the catfish by its nostrils with vines to some boulders at a spot in Ichinomiya now known as Hanaguriiwa (鼻ぐり岩, “nose-tying boulders”); the struggling catfish churned the river water, which is why the river is now known as Kurokawa (黒川, “black river”); and several deer came to see the water flowing from the caldera, which is why the waterfall is known as Sushikaryuketaki (数鹿流ヶ滝, “several deer waterfall”).
The degree of credence you choose to attribute to these stories is entirely up to you. What is interesting, though, is the folklore behind the caldera, the absence of a lake, and earthquakes.