Shun NEMOTO

Examination of seismic behavior of pyroclastic flow ground during the 2016 Kumamoto earthquake

Takaaki IKEDA

The Kumamoto earthquake struck in Kumamoto prefecture district in 2016. The 2016 Kumamoto earthquake is a series of earthquake. It includes two intense earthquakes which were Mw6.2 event on April 14 and Mw7.0 event on April 16. These earthquakes are called fore-shock and main-shock respectively. Seismic intensity of 7 was recorded twice in Masiki Town near source area. It was the first time that the seismic intensity 7 was observed twice in the same earthquake series. Earthquake damage in Kumamoto prefecture was enormous, with 208 people dead, and more than 180000 houses were damaged. Furthermore, damage of road bridge, ground subsidence and slope failure were caused along the active fault. More than 150 slop failures occurred and it caused isolate area by road blocking. The damaged area is covered with Aso pyroclastic flow deposits, which is thought to have caused the damage.
Heavy damage occurred in the lifeline including the water resource facility. Especially damage in the Akita water source area and Nuyamazu water source area, which located near the Kiyama River basin of Higashi-ku, Kumamoto City. In this area, many pump towers tilted and function stopped. Also we found ground subsidence. It was thought that the damage of the pump tower caused by earthquake induced ground subsidence. 
We need to consider ground subsidence in various ways. In this study, we try to evaluate of the surface ground behavior during the earthquake by numerical analysis approach. The analysis method is 1-dimensional earthquake response analysis of DYNEQ (Based on equivalent linear method). The ground model was modeled on the local geological survey results. We calculated the dynamic ground behavior during the foreshock and main shock. Comparing these results, we clarified the behavior during the Kumamoto earthquake.