A first look at a landslide triggered by the 2016 Kumamoto earthquake near the Aso Volcanological Laboratory Zili Dai 1,2,3* , Fawu Wang 1 , Kun Song 2 & Akinori Iio 1 1 Department of Earth Science, Shimane University, 1060 Nishikawatsu-cho, Matsue, Shimane 690-8504, Japan 2 Hubei Key Laboratory of Disaster Prevention and Reduction, China Three Gorges University, Yichang 443002, China 3 State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China * Correspondence: zili.dai@riko.shimane-u.ac.jp Abstract: On 16 April 2016, a strong earthquake occurred in Kumamoto, Japan, resulting in 97 landslides in the vicinity of the Aso Volcanological Laboratory. This paper reports the initial photographs and preliminary field investigation of a typical earthquake-triggered landslide near the Aso Volcanological Laboratory. An unmanned aerial vehicle was used to obtain an aerial view of the landslide. On the basis of these photographs, the basic features of the landslide were characterized and the damage caused by the landslide was described. The first-hand information about the earthquake and the landslide presented here should provide the basis for further work. Received 31 August 2016; revised 15 January 2017; accepted 15 March 2017 The 2016 Kumamoto earthquake occurred at 01:25 on 16 April 2016 (local time) in Kumamoto City of Kumamoto Prefecture in Kyushu Region, Japan. The epicentre was located at 32.782°N, 130.726°E (Global Positioning System (GPS) coordinates), with a focal depth of about 10.0 km. According to the Japan Meteorological Agency, its magnitude was M S 7.3 (Mw 7.0 according to the US Geological Survey). A foreshock earthquake of magnitude 6.2 shook the same region at 21:26 on 14 April. Figure 1 shows the aerial view of the earthquake region and the location of the seismic shocks, fault zones and the landslide discussed here. The larger circles represent the two major earthquakes, and the smaller circles denote the aftershocks until 07:00 on 16 April determined by Hi-net National Research Institute for Earth Science and Disaster Prevention (NIED). The bold lines show the traces of active faults in this area. According to Okumura (2016), this earthquake was triggered by the reactivation of the Hinagu Fault and Futagawa Fault. Landslide features During the earthquake, the strongest ground motion recorded in Mashiki Town was about 1.18 g. The intense ground shaking directly damaged numerous houses, roads and bridges, resulting in at least 49 deaths and forcing the evacuation of 90 000 people from their homes. As a result of the intense ground shaking, at least 97 landslides were triggered in the Aso area, one of which is located to the NE of the Aso Volcanological Laboratory of Kyoto University at 32.888°N, 131.007°E (star in Fig. 1). Figure 2 shows the general aerial view of this landslide (base map is from Kokusai Kogyo Co., Ltd. 2016); its outline is marked by the dashed lie. The crown, main scarp, surface of the rupture, zone of depletion and accumulation, right and left flanks, and toe of the landslide can be easily identified, and are indicated in the figure. The runout distance of the landslide is about 65 m and its slide direction is N25.0°W. Figure 3 is an oblique aerial view from the toe of the landslide towards the source area, taken by an unmanned aerial vehicle (DJI Phantom 3 Advanced). The Aso Volcanological Laboratory of Kyoto University is located on the top of the slope, about 220 m to the SW. Tension cracks that are tens of centimetres wide were observed in the crown area of the landslide (see Fig. 4). Figure 5 shows a contour map of the landslide (base map is from Geospatial Information Authority of Japan). The source area of the landslide (dark grey area in the map) was not steep (about 23°) and the surrounding terrain was gentle. The elevation difference between the landslide toe and top was only 60 m. The total volume of the displaced mass is about 5.0 × 10 4 m 3 . On the displaced mass, the vegetation basically remained intact, which indicates the internal integrity of the displaced mass. As shown in Figure 6, this landslide moved along a relatively planar surface of rupture with little rotational movement and backward tilting indicating a translation slide. The landslide surface is about 110 m in length and 90 m in width, and its area is about 7600 m 2 . As shown in Figure 2, the runout distance of the landslide is about 65 m with an H/L value of 0.3. Some loose soil with relatively coarser particle size remained on the surface. Figure 7 shows a close-up view of the smooth rupture surface, comprising mainly clay and fine particles. The grain-size distribution of the soil on the surface of rupture is shown in Figure 8. This area is characterized by an active volcano and soft ground composed of weathered volcanic cohesive soil. The soils are problematic owing to their high water content and amorphous clay minerals (Hayashi et al. 2005; Aramaki et al. 2008). Figure 9 shows the right and left flanks of the landslide, which are marked D and E respectively in Figure 2. From the right flank (Fig. 9a), it can be observed that the depth of the rupture surface is shallow, about 6.5 m, with a depth-to-length ratio of 0.06. The soil of the slope shows layers parallel to the surface of rupture, which might be caused by the deposition of volcanic ash from the Aso Volcano nearby. The top soil layer is brown; its average thickness is about 2.0 m. In this layer, plant root systems are developed. The second soil layer is almost black. The soil particle size in this layer is finer and denser relative to the top layer. Slickensides caused by the movement are particularly evident in the lateral shear of the right flank (Fig. 10). Damage caused by the landslide The earthquake-triggered landslides in this region destroyed many electrical communication facilities, blocked a number of roads, and © 2017 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Photographic feature Quarterly Journal of Engineering Geology and Hydrogeology Published Online First https://doi.org/10.1144/qjegh2016-085