Landslide distribution resulting from the 2015 Gorkha, Nepal earthquake sequence

Abstract:

Thousands of landslides (predominantly rock slides and rock falls) were triggered as a result of the 2015 Gorkha, Nepal earthquake sequence. Given the steep, high relief of the epicentral zones and the widely distributed population of Nepal in these regions, hundreds of fatalities occurred as a direct result of landsliding. Further, roads, hydroelectric plants, and other critical infrastructure were subjected to considerable secondary hazards including highly weakened slopes and inundation from lake impoundments that formed upslope of valley-blocking landslide dams. As part of a humanitarian and scientific mission to Nepal supported by the U.S. Agency for International Development, Office of Foreign Disaster Assistance, we conducted landslide assessments throughout earthquake-affected areas (report available at http://dx.doi.org/10.3133/ofr20151142) and, in the process, developed a sense for the overall landslide distribution resulting from the earthquakes. Whereas landslides were abundant near the major earthquake epicenters, few landslides were observed in many steep areas of the country where effects would normally have been expected. For example, although avalanches and ice and rock falls occurred near Mt. Everest, located approximately 220 km from the April 25 epicenter, we noted few landslides in a similar area of steep terrain located 40 km closer to the epicenter. Similarly, although we noted entire mountainsides covered by landslides within 20 km of the mainshock epicenter, we observed many other mountainsides within this same region lacking any indication of ground disturbance. Observations of shattered ridgetops with ridge parallel fractures at several locations indicate that topographically-amplified ground shaking occurred in some areas. This, along with the complex geology and the asymmetric directionality of rupture, could help explain the landslide distribution and thus where hazards are most likely from similar future earthquakes in central Nepal.