Surface Ruptures in the Northwestern Part of the Inner Aso Caldera

Abstract

AbstractIn the northwestern part of the inner Aso caldera, distinctive fault rupture zones appeared at the time of the mainshock of the Kumamoto earthquake sequence, separated from the northern end of the Futagawa fault. They extend NE-SW for totally about 8 km in the seismically active Futae-toge zone of normal faulting (Sudo and Ikebe 2001) and the certain aftershocks of the earthquake were distributed in the fault zone. They consist of several parallel and sub-parallel sharp scarps by normal faulting as observed after the 1987 Edgecumbe earthquake in New Zealand. Along the fault traces, paddy fields were displaced vertically up to 2 m along with up to ~30-cm right-lateral horizontal displacement in places. The rupture zones develop on the paddy fields generally dipping less than 1.0 degrees on the fluvial surfaces from the central cones of the Aso volcano. It is noteworthy that the fault ruptures are distributed in the area with seismic intensity less than the JMA scale 5+. From the characteristics of the surface fault ruptures, we conclude that the ruptures were triggered by normal faulting with right-lateral slip during the mainshock, but not simple lateral spreading caused by shaking.KeywordsNormal fault rupturesAso calderaDrone photogrammetry