The Futagawa fault runs from Shimabara in Nagasaki up to Aso. Its northern section is divergent - pushing apart - and this explains the Tateno Gap, through which Shirakawa flows (see photo below). Without this gap, the Aso caldera would be a lake.
Hydrologists had created a hydroelectic power plant which jammed up during the quakes, causing pent-up water to flood the lower valley. This destroyed all transportation links and many houses (photo below).
Recovery of the area is proceeding at a snail’s pace, Kumanichi reports. Only 30% of its former residents (about 100 of some 350 households) have returned, with the remainder still in temporary housing or having abandoned the area completely. Concerns regarding safety were most cited. Full restoration is not expected until 2020. Kumanichi: https://this.kiji.is/429776937625551969?c=92619697908483575 - William