Lessons from Recent Geo-Disasters in Hokkaido Under Earthquake

The 2018 Hokkaido Eastern Iburi Earthquake caused a large amount of slope failures and landslides over a wide area. In particular, the enormous damage due to slope failures was centered in Atsuma and Abira town which are to the north of the epicenter. In Atsuma town, a seismic intensity (Japan Meteorological Agency) of seven has been observed for the first time in Hokkaido, Japan. The geology in this region was mainly formed from three kinds of pyroclastic fall deposits (fa) erupted from Tarumae volcano, Eniwa volcano, and Shikotsu Caldera. Especially, the serious damage was generated in these tephra stratus. In addition, a large scale slide of rock slopes composed of mudstone also occurred in the Hidaka-Horonai river basin in Horonai district in Atsuma town, and its debris flowed into the river and caused the blockage of the river channel. This paper summarizes earthquake-induced damages on natural slopes and mechanical properties of Tarumae pyroclastic fall deposits.