On 6th September 2018, a large earthquake (the 2018 Hokkaido Eastern Iburi Earthquake) with the magnitude of 6.7 struck the eastern Iburi region, Hokkaido and triggered numerous landslides. The landslides were distributed densely over hilly areas (400 km2) in Atsuma, Abira, and Mukawa Town, the total area of which reached 13.4 km2. All landslides were shallow landslides except a large-scale deep-seated one generated in midstream area of the Hidaka-Horonai River. The area is covered by thick pyroclastic fall deposits derived from the Tarumae Volcano etc., and the strong seismic shock triggered shallow landsliding of them. Some shallow landslides occurred at a relatively gentle slope (<15°). Shallow landslides moving along a valley topography tended to travel longer than those moving along a non-valley topography. In catchments where shallow landslides densely occurred, the ratio of landslide area to the total catchment area was very large compared with past earthquake events, suggesting that the catchments became highly devastated. Based on these survey results, we propose viewpoints for preventing secondary disasters in the damaged area and for promoting countermeasures against and studies on earthquake-induced landslide disasters in volcanic regions.
In 1926 a volcanic mudflow triggered by the eruption of Mt. Tokachi, named "Taisho mudflow", ran through the Furano River in Hokkaido and caused destructive damage along the course. Several disaster maps presenting the path have been produced since then, while additional information of the arrival time and the levels of damage on the map was considered helpful to establish evacuation plans for the future eruption. In this study, the travelling time of the flow was estimated by interviewing locals who experienced the disaster (Nanri et al., 2004). The damage level was also classified into three groups, High (Ⅰ) : 90% of houses were completely destroyed and 30% of residents were killed, Moderate (Ⅱ) : 30-90% of houses were completely destroyed and 10-30% of residents were killed, and Low (Ⅲ) : houses were partly destroyed and less than 10% of residents were killed. Estimating fluid dynamic force produced by the flow, more than 103 KN/m for the level Ⅰ, 102-103 KN/m for the level Ⅱ, and 101-102 KN/m for the level Ⅲ were required during the travel respectively. Applying the relationship between average river bed slope and the force given, the distribution of the damage level was estimated and demonstrated through the path. In summary, it took the flow 20 minutes to travel from the crater through the valley to 0.5 km downstream of the outlet after the eruption, giving a high level of damage. Subsequently, another 10 minutes were required to travel over a flood plain as damaging the area to a moderate degree. Finally, after 40 minutes from the eruption the flow arrived at the Kami-Furano Township, as yielding a low degree of damage.