2021 Volume 13 Issue 4 Pages 76-83
More than 6000 landslides over 400 km2 were triggered by the Eastern Iburi Earthquake that occurred on September 6, 2018, in Iburi Sub-prefecture, Hokkaido. A large amount of sediment was transported downstream in gentle and hilly landscapes (< 500 m in elevation) by the landslides. The landslides occurred within two major land cover types : forested areas (FA) and logged areas (LA). Here, the characteristics of landslides within these different land cover types were evaluated based on GIS analysis and field investigation. A total of 1440 landslide scars were identified in an area of 18.9 km2 consisting of 87% FA and 13% LA. The ranges of the slope gradient within the two land cover types were identical, from 30 to 40°. The mean landslide area in LA at 2306 m2 (standard deviation [SD] : 1675 m2) tended to be greater than that in FA (mean : 1762 m2 ; SD : 1749 m2). Based on the field investigation, the mean depth of landslides in FA (1.5 m) and LA (1.4 m) were similar, while the mean estimated volume of the landslides was 3610 m3 in FA and 6359 m3 in LA. The mean runout distance of landslides in LA was longer at 231 m (SD : 168 m) compared to that in FA at 146 m (SD : 140 m). The short runout distance in FA was possibly associated with a reduction in sediment momentum through the presence of wood pieces in FA. The findings of our study suggest that vegetation cover and the resultant differences in landslide size and runout distance are important factors for evaluating potential earthquake-related disasters, and future planning in forest management.
