Characteristics of landslides in forests and grasslands triggered by the 2016 Kumamoto earthquake

We examined the characteristics of landslides triggered by the 2016 Kumamoto earthquake (Mw = 7.0: focal depth=10.0 km) in forests and grasslands within two affected watersheds (Tokosegawa: 6.9 km(2) and Nigorigawa: 6.1 km(2)) in southwestern Japan. We identified 190 landslides using aerial photographs and analyzed their sizes by geographic information system (GIS). Field investigations were conducted to obtain landslide depth, volume and residual sediment for 38 selected landslides (21 in forests and 17 in grasslands). The minimum area of detected landslides in grasslands (400 m(2)) was smaller than in forests (1000 m(2)), probably because of reduced detectability of landslides under tree cover. The ratio of total area occupied by landslides for a given range of slope gradient in the watersheds increased from 3.2% on gentle grassland slopes (10-15 degrees) to 15.5% on steep (>45 degrees) slopes, whereas the maximum landslide-area ratio in forest sites (7.4%) occurred on relatively gentle slopes (25-30 degrees). Estimated landslide volume ranged from 27 to 9622 m(3), based on mean depth of each landslide measured around individual landslide scars. Moreover, the volumetric ratio of landslide deposit volume to total landslide volume exceeded 100% for 48% of the landslides within forests and 35% of the landslides within grasslands. Our findings show that land cover had extensive and recognizable effects on the characteristics of landslides and resulting in-channel sediment accumulations. Resetting sediment dynamics after earthquakes associated with different land cover distributions needs to be considered within watersheds. (c) 2019 John Wiley & Sons, Ltd.