Abstract
Abstract Before dawn on 6 September 2018, a powerful earthquake with a Japan Meteorological Agency magnitude ( M j ) of 6.7 hit central Hokkaido, causing more than 6000 landslides. As human damage, 36 of the 44 fatalities from the earthquake were from earthslides in Atsuma Town. Most slope movements were shallow earthslides of mantle-bedded tephra and soil layers, but some were deep rockslides involving basement rocks such as shale and mudstone of Miocene. Although the shallow earthslides were easily distinguishable in photos from satellites or airplane, the rockslides were more difficult to identify owing to their vegetation. Based on the quick interpretation of a high-resolution (0.5 m horizontal resolution) shaded relief map made from digital elevation model data by airborne laser survey and supplemental field surveys, we effectively identified 259 rockslides (196 certain ones and 63 possible ones) in Atsuma and Mukawa towns under newly invented identification criteria based on the scarp depth and positional relation between scarp and ridge topography. It was revealed that most rockslides were distributed within 10 km from the epicentre, while earthslides were distributed until 20 km north of the epicentre, and they seemed to be controlled by the thickness of mantle-bedded tephra and the soil layer. We also identified many traces of past rockslides and earthslides. The results show the possibilities for effective measurement of slope by clarifying the landslide distribution both this earthquake and past ones using high-resolution digital elevation model data.
Highlights
At 03:07:59.3 JST (18:07:59.3 UTC) on 6 September 2018 (18:07:59.3 UTC on 5 September), a powerful earthquake hit the Eastern Iburi district of Hokkaido Island in Northern Japan
The earthquake caused more than 6000 landslides in hilly areas of Atsuma, Abira and Mukawa towns (Kita 2018; GSI Japan 2018; Yamagishi and Yamazaki 2018; Figs 2 and 3) and 36 people died as a result of shallow earthslides in Atsuma Town
We considered the landslide topography to be a rockslide if either criterion was clearly met, because the earthslides are thought not to cross the ridge topography of a hill owing to weak consolidation and the thinness of the tephra layer, and not to have a thickness of more than 3 m in the studied area (Figs 4 and 8a, c)
Summary
At 03:07:59.3 JST (18:07:59.3 UTC) on 6 September 2018 (18:07:59.3 UTC on 5 September), a powerful earthquake hit the Eastern Iburi district of Hokkaido Island in Northern Japan. The basement rock around the epicentre mainly consists of Neogene sedimentary rocks that are conglomerate, sandstone and mudstone of the Fureoi Formation of Middle to Late Miocene, sandstone, hard shale and mudstone of the Karumai Formation of Late Miocene, and conglomerate, sandstone, hard shale and siltstone of the Moebetsu (Nina) Formation of Late Miocene to Early Pliocene (Fig. 1c; Matsuno and Ishida 1960; Kase et al 2018) These are foreland basins fill deposits formed by a collisional event between the Northeast Japan Arc and Kuril Arc (Fig. 1c, d; Kawakami and Kawamura 2003; Kase et al 2018). Pumice layers are Spfa-1 (34– 31 ka; Machida and Arai 2003) from the Shikotsu caldera, En-a (17–15 ka; Machida and Arai 2003)
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