Landslides triggered by the Gorkha earthquake in the Langtang valley, volumes and initiation processes

Pascal Lacroix

doi:10.1186/s40623-016-0423-3

Abstract

The Gorkha earthquake (Nepal, 2015, M w 7.9) triggered many landslides. The most catastrophic mass movement was a debris avalanche that buried several villages in the Langtang valley. In this study, questions are raised about its volume and initiation. I investigate the possibility of high-resolution digital surface models computed from tri-stereo SPOT6/7 images to resolve this issue. This high-resolution dataset enables me to derive an inventory of 160 landslides triggered by this earthquake. I analyze the source of errors and estimate the uncertainties in the landslide volumes. The vegetation prevents to correctly estimate the volumes of landslides that occured in vegetated areas. However, I evaluate the volume and thickness of 73 landslides developing in vegetated-free areas, showing a power law between their surface areas and volumes with exponent of 1.20. Accumulations and depletion volumes are also well constrained for larger landslides, and I find that the main debris avalanches accumulated 6.95 × 106 m3 of deposits in the valley with thicknesses reaching 60 m, and 9.66 × 106 m3 in the glaciated part above 5000 m asl. The large amount of sediments is explained by an initiation of the debris avalanche due to serac falls and snow avalanches from five separate places between 6800 and 7200 m asl over 3 km length.

Highlights

The Gorkha earthquake (Mw 7.9) of April 25, 2015, triggered many catastrophic landslides and avalanches
One can conjure up the ground motion intensity, an important parameter in the landslide triggering (Meunier et al 2007, 2008; Lacroix et al 2015), that was possibly smaller for the Gorkha earthquake than for previous large himalayan earthquakes, due to the steady rupture velocity (Grandin et al 2015; Galetzka et al 2015) or the deeper source compared with other recent himalayan earthquakes that broke the surface
The largest and most destructive landslide triggered by the Gorkha earthquake occured in the Langtang valley (Collins and Jibson 2015; Kargel et al 2015), where the shaking triggered a debris avalanche composed of ice, snow and soil, burying several villages, and killing at least 350 people (Kargel et al 2015)

Summary

Introduction

The Gorkha earthquake (Mw 7.9) of April 25, 2015, triggered many catastrophic landslides and avalanches. One led by the University of Arizona (Kargel et al 2015) and the other led by the British Geological Survey (BGS) and Durham University, mapped more than 4000 landslides in the weeks following the earthquake in a collaborative work that focused on the rapid evaluation of the earthquake aftermath.1 These inventories show landslides of relatively small size and few river dams, compared with other recent earthquaketriggered landslides of similar magnitude in Pakistan (Owen et al 2008) or in China (Gorum et al 2011), or after medieval earthquakes in the same area (Schwanghart et al 2015). This avalanche dammed the river for few days and destroyed a large part of the valley

Methods

Results

Conclusion