Distribution Characteristics of Mass Movements in the Upper Bhote Koshi Watershed Before and After the Gorkha Earthquake and Their Susceptibility Evaluation

Abstract

Open image in new window The 2015 Gorkha earthquake (Mw 7.8) triggered about 4000 mass movements in the mountainous areas of Nepal. We investigated the mass movements in the upper Bhote Koshi river valley, which is one of the most severely affected regions in central Nepal. In that valley, there were about 775 landslips before the earthquake and about 1360 new failures were initiated by the quake. The mass movement investigation was carried out based on detailed fieldwork and satellite image interpretation. These mass movements were classified into rock falls, rockslides, debris flows, colluvial slides and their distribution is strongly controlled by their position on the landscape. A majority of rock falls were located at higher elevations with very steep slopes consisting of hard rocks, such as quartzites, schists, and gneisses; rockslides occurred mainly on moderate to steep slopes; while most of the debris and colluvial slides developed on moderately steep slopes. Though a few large landslips were reactivated by the quake, most of such pre-earthquake landslides were insensitive to seismic shaking. The mass movements before the earthquake and those triggered by the earthquake were used for the preparation of landslide susceptibility maps of the region before and after the earthquake using GIS and an Evidential Belief Function model. Subsequently, the two susceptibility maps were combined to illustrate the locations of maximum landslide probability. The combined susceptibility map can be utilized for future land-use planning and infrastructural development in the region.