William here - Introducing the Futagawa fault had occurred to me before, but this evening provides a good opportunity. The fault is a rift zone, meaning the land is spreading apart, which extends from Amakusa - creating the Uto peninsula - through Mt. Aso. In fact, the Tateno gap, through which Shirakawa flows out of the Aso caldera, is a result of the fault; without it, the Aso caldera would be a lake. It created the plain which currently hosts the airport, and the valley between the airport and the mountains to the south is the rift zone. Rift faults are not capable of such powerful earthquakes as other types such as subductive or thrust. What you have experienced is likely the largest quake this fault is capable of.