Stratigraphy and distribution pattern of volcanogenic sediments beneath downtown Mashiki, Kumamoto, SW Japan, seriously damaged by the 2016 Kumamoto Earthquake

Abstract

The 2016 Kumamoto Earthquake resulted in severe damage to houses and buildings, particularly in the central part of Mashiki Town, Kumamoto Prefecture, SW Japan. We examined drill core and standard penetration test (SPT) samples obtained from the severely damaged area to understand the influence of the underlying geology on damage caused by strong earthquake motion. The downtown Mashiki area is underlain by thick volcanogenic sediments derived from Aso volcano, comprising the Aso-3 pyroclastic flow, the Aso-4/3 interval, the Aso-4 pyroclastic flow, lahar, and loess deposits, in ascending order. The stratigraphic correlation of several boreholes in Mashiki Town reveals that the severely damaged area is characterized by a thicker fine-grained pumice bed of the Aso-4 pyroclastic flow and overlying weathered lahar deposits saturated with groundwater.