Regional landslide susceptibility analysis following the 2015 Nepal Earthquake

A magnitude 7.8 earthquake struck Nepal on April 25, 2015 triggering several thousands of landslides and causing widespread damages to mountain villages and the evacuation of thousands of people. This contribution describes landslide susceptibility analysis performed in the Dhading (1885 km2), Sindhupalchok (2488 km2), Rasuwa (1522 km2) and Nuwakot (1194 km2) districts. Three landslide inventories have been prepared covering most of the area affected by coseismic landslides in Nepal. The first one is a coseismic and post-seismic landslide inventory based on multi-temporal images (Google Earth, Google Crisis maps, Bing maps), and helicopter-based video. The inventory includes more than 15,000 landslides. The second one is a pre-event shallow landslide inventory showing landslides already active before the occurrence of the earthquake. This inventory includes more than 2500 events. For these two inventories, the most abundant landslide types are debris flows, shallow translational slides, and rockfalls. The third inventory includes almost 20,000 deep-seated landslides, mostly rock avalanches, slumps, rockslides and deep-seated gravitational slope deformations (DSGSD). Starting from these inventories, a multivariate statistical analysis of geo-environmental variables with respect to landslide occurrence was performed, aimed at recognizing the most significant controlling factors, such as lithology, slope gradient, and the presence of older deep-seated landslides. This analysis was complemented by field activities carried out in October 2015. During the survey, local knowledge has been systematically exploited through interviews with local people that have experienced the earthquake and the coseismic landslides. This helped us to recognize fractures and active deformations and to reconstruct a correct chronicle of landslide events.