Slope failures/landslides over a wide area in the 2018 Hokkaido Eastern Iburi earthquake

The 2018 Hokkaido Eastern Iburi Earthquake caused a large number of slope failures/landslides over a wide area. In particular, the enormous damage due to slope failures was centered in the towns of Atsuma and Abira which are located to the north of the epicenter. In Atsuma, a seismic intensity (Japan Meteorological Agency) of seven was observed. This was the first time an earthquake of such a large seismic intensity was recorded in Hokkaido, Japan. The geology in this region was mainly formed by three kinds of pyroclastic fall deposits (fa) due to the erupting of Mt. Tarumae, Mt. Eniwa and Shikotsu Caldera. The most serious damage was generated in these tephra stratus. In addition, the large-scale sliding of rock slopes composed of mudstone occurred in the Hidaka-Horonai River Basin in the Horonai district of Atsuma, and its debris flowed into the river and caused the blockage of the river channel. This paper summarizes the earthquake-induced damage to natural slopes, and presents the physical features and the mechanical properties of the collapsed pyroclastic fall deposits distributed over this area.