COSEISMIC LANDSLIDE HAZARDS AND GEOMORPHIC CONSEQUENCES OF THE MW 7.8 GORKHA EARTHQUAKE, NEPAL

Coseismic landsliding is often widespread in mountainous areas during large, shallow earthquakes and represents a significant hazard during and in the aftermath of such events. Material transported and weakened during large, shallow earthquakes has been shown to impact geomorphic and geochemical cycles for years following an event. Thousands of landslides were triggered by the April 25th 2015 M_w 7.8 Gorkha Earthquake that struck central Nepal and contributed to the > 9,000 fatalities, >23,000 injuries, and billions of dollars of economic loss caused by this event. Hillslope debris liberated and soil and regolith horizons weakened during this earthquake present an ongoing hazard for the people of Nepal and will effect geomorphic and geochemical processes for months-to-years as this unstable material is mobilized during the wet season and beyond. We produced a series of coseismic landslide hazard maps in the hours-to-days following the Gorkha Earthquake using a modified Newmark analysis, USGS ShakeMap peak ground acceleration (PGA) models and three different assumed regionally-uniform rock strength scenarios (weak, moderate and strong). These rapid-response maps were used to help focus early scientific investigation that in-turn aided decision making for rapid response and recovery efforts. Using a case-study of the Mw 7.9 Wenchuan earthquake we show how our coseismic landslide model can be used to invert earthquake-triggered landslide maps and models of PGA to quantify hillslope-scale rock strength. Determining rock strength at these geomorphically-relevant spatial scales is notoriously challenging, yet essential for improved landslide hazard assessment. We present preliminary results of our efforts to produce similar maps of near-surface rock strength for Central Nepal, which includes compiling a detailed inventory of landslides triggered by the Gorkha Earthquake. We discuss how our analysis can be used to better understand both coseismic and rainfall triggered landslide hazards in Nepal. We conclude by briefly outline our ongoing efforts to quantify and monitor the longer-term geomorphic and geochemical impacts of the Gorkha Earthquake to better understand the role of large, shallow earthquakes in the evolution of high-relief, tectonically active landscapes.