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

Abstract

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 km2 and Nigorigawa: 6.1 km2) 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 m2) was smaller than in forests (1000 m2), 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°) to 15.5% on steep (>45°) slopes, whereas the maximum landslide‐area ratio in forest sites (7.4%) occurred on relatively gentle slopes (25–30°). Estimated landslide volume ranged from 27 to 9622 m3, 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. © 2019 John Wiley & Sons, Ltd.

Land Cover and Characteristics of Landslides Induced by the 2018 M_w 6.7 Eastern Iburi Earthquake, Hokkaido

“…[2018] showed that more than 50% of 13,097 landslides in the 2015 Mw 7.8 Gorkha earthquake in Nepal had areas ranging from 10 3 to 10 6 m 2 . These landslides occurred in areas of high relief, but the sizes of the landslides of our study were similar to the landslides reported by Koyanagi et al . [2020] in the 2016 Mw 7.0 Kumamoto earthquake, Japan (range : 34-8835 m 2 ; mean : 1511 m 2 ), possibly because of similar local relief.…”

“…The landslide density observed in our study area, 48 landslides/km 2 , was higher than that reported in previous studies. For example, Koyanagi et al . [2020] showed that the landslide density in the 6.9 km 2 Tokosegawa watershed in Kumamoto was 12.4 landslides/km 2 .…”

“…The maximum landslide density in a single sub-catchment in our study was 218 landslides/ km 2 , whereas other studies have reported 1.36 landslides/km 2 for the 2008 Mw 7.9 Wenchuan earthquake, China [Dai et al ., 2011] and 30 landsides/ km 2 for the 2004 Mw 6.6 Niigata Chuetsu Earthquake, Japan [Kieffer et al ., 2006]. The high density of landslides in our study area is possibly related to the poorly consolidated volcanic soil deposits and liquefaction by shaking on hillslopes, particularly at slope gradients of 30-40° [Koyanagi et al ., 2020 ;Ling and Chigira, 2020].…”

“…A high proportion of landslides occurred on hillslopes with gradients of 30-40°in the 2013 Mw 5.9 Minxian earthquake in China [Tian et al ., 2016]. In Japan, Koyanagi et al . [2020] demonstrated that landslides occurred on upwardly convex landforms in the 6.9 km 2 Tokosegawa watershed of the Mount Aso volcano region in the 2016 Mw 7.0 Kumamoto earthquake.The mobility of landslides is an important factor in their characterization.…”

“…In general, both the horizontal and vertical extents of tree root systems play an important role in stabilizing hillslopes by anchoring and binding [Schmidt et al ., 2001 ;Cohen and Schwarz, 2017]. In the Kumamoto earthquake in Japan, Koyanagi et al . [2020] found that the landslide density of steep hillslopes (> 40°) in grassland tended to be high compared to the number of landslides observed on forested slopes due to the difference in root reinforcement.…”