Reconnaissance Study on the Rainfall-Induced Failure of the Izu-Oshima Volcanic Mountain Slope

A heavy rainfall occurred in October 2013 in a volcanic island of Izu Oshima. Induced by a typhoon, this rainfall produced more than 800 mm of precipitation in one night and a huge slope failure occurred in a slope that had been stable for many centuries. The debris flow claimed 39 lives in mid night. This tragedy was investigated and the present paper addresses some of the lessons. The volcanic slope consists of layers of different materials and their behavior is complicated. Of particular interest is the piping holes that suggest significant destabilization of the slope due to extra pore water pressure from the underground hydrological environment. By considering this effect by simple analyses, the roles played by lava layer was understood. After the disaster there remains many ridges whose stability in future was of public concern. The present study interpreted the collected bore hole data to suggest that those ridges are relatively stable because of the underground lava topography. Finally, ideas for future early warning and evacuation were discussed.