Characteristics of landslides triggered by the 2018 Hokkaido Eastern Iburi earthquake, Northern Japan

On 6 September 2018, a Mj 6.7 (Mw 6.6) earthquake with a focal depth of approximately 37 km and maximum seismic intensity of 7.0 on the Japan Meteorological Agency (JMA) scale (corresponding to approximately X on the Modified Mercalli Intensity (MMI) scale) struck the central and eastern Iburi regions of Hokkaido, Northern Japan, 1 day after the Typhoon Jebi passed through the region. Thousands of landslides were triggered and significant losses resulted from the earthquake sequence. A detailed landslide inventory map, including 5625 coseismic landslides, was delineated on the basis of 3307 published landslide sites. Most of the coseismic landslides are translational landslides of small to medium scale with high mobility and long run-out distance. Thirty-six people were killed by the landslides despite the afflicted area being sparsely populated. It is found that all the 5625 landslides spread in an elliptic area extending NNW/SSE, running approximately parallel to the strike of (active) faults in this region. The preferred aspect of the landslide-affected area is southerly, running nearly perpendicular to the NNW/SSE striking (active) faults. Most coseismic landslides are distributed in regions with seismic intensity of 7.0 to 8.0 (MMI Scale), with peak ground acceleration (PGA) of 0.4 to 0.7 g. Most of the coseismic landslides occurred at the elevation between 100 and 250 m, and the slope angle between 15 degrees and 35 degrees. Miocene sedimentary rock is the predominant bedrock type identified in the landslide area. Slope failures were triggered in stratified pyroclastic fall deposits, in the combination of strong seismic ground motion and intense antecedent precipitation. Sliding zone liquefaction phenomena were confirmed in the field investigation. The relationship between the old landslides (slope failures occurred prior to the Iburi earthquake) and the coseismic landslides is also discussed in this study.