Long runout geological disaster initiated by the ridge-top rockslide in a strong earthquake area: A case study of the Xinmo landslide in Maoxian County, Sichuan Province

In recent years, a typical type of catastrophic ridge-top (or high-position) rockslide often occur in the strong earthquakes such as the Wenchuan earthquake. It exits out from the upper part of the steep slope and forms a volley fall with impact and crushing effect and dynamic erosion effect, causing the slide body to disintegrate and fragment, which transforms into rapid and long run-out avalanche debris or debris flow, and entraining the lower part of rock and soil mass, so that the volume increased significantly. The Xinmo landslide is this typical, it occurred at Maoxian County, Sichuan Province on June 24, 2017. The elevation of the crown of the Xinmo landslide was about 3450 m and the front edge was about 2250m. The height difference of landslide was 1200m, and the horizontal distance was about 2800 m. Its volume was up to 16.37 million m³. The landslide buried the Xinmo Village, leading to the death of 83 people. The Xinmo landslide was located on the western wing of the Jiaochang arc-shaped tectonics. Its parent rocks were the medium to thick layered metamorphic sandstone intercalated with slate in the Middle Triassic. The region was not only the epicenter area of the Diexi earthquake with magnitude 7.5 in 1933 (the intensity of the earthquake was X) but also the strong earthquake-affected area of the Wenchuan Ms8.0 earthquake in 2008 (the intensity was IX). The mountains, especially the ridge-top rockmass, were fractured/cracked due to the strong earthquakes. There were multiple groups of discontinuous structural planes in the sliding source zone, and hence the thick blocky rock mass was cracked into fragmented blocks, and the bugling area was formed at the elevation varying from 3150 to 3450 meter. In particular, there were two sets of anti-dip large joints in the sliding source area, indicating a typical failure mechanism "locked-section". Rockslide with a volume of 3.9 million m³ exited and continuously accumulated at the back of previous residual landslide. The "overload effect" triggered the slope instability under the exit and transferred into long runout channeled avalanche debris. Because the terrain was wide and the slope angle gradually decreased, avalanche debris converted to diffused one and then to scattered accumulation. The Xinmo landslide presents a typical disaster mode of the rapid and long runout initialed due to rockslide at ridge-top in strong earthquake area. A new method should be established to recognize this type of landslides. Wherever there are large-scale rockslides in steep ridge-top region, the "dynamic erosion effect" and the "overloading effect" on the previous accumulation and the talus of slope due to impact processes should be considered. Especially in the place where there is abundant groundwater along the gully, the possibility of a rapid and long runout rockslide-avalanche debris will increase. Therefore, in conducting the investigation of geological disaster, the town, village or other populated areas should be zoned as risk area on the previous landslide accumulation of slope below the exit of the rockslide at the ridge-top. The authors emphasize that, in the strong earthquake mountainous regions, the static balance method for the landslide stability should be considered, and the dynamic research on the landslide runout processes and the disaster mode should be strengthened.