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

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 × 10 6 m 3 of deposits in the valley with thicknesses reaching 60 m, and 9.66 × 10 6 m 3 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.

Anomalous winter-snow-amplified earthquake-induced disaster of the 2015 Langtang avalanche in Nepal

“…Another point of discussion is whether the entire slope became instable by the earthquake. Although the volume detached from the upper catchment (14.38 × 10 6 m 3 ) estimated by Lacroix (2016) seems consistent with the estimate in this study, this could be a mere coincidence if both gain and loss of mass on a typical Himalayan glacier in summer season (Ageta and Higuchi, 1984), the glacier dynamics (Cuffey and Paterson, 2010), and the winter snow are taken into account.…”

“…A set of pre-and post-event images of SPOT6/7 successfully quantified the mass movement in the Langtang valley due to the earthquake-induced avalanche (Lacroix, 2016). The main deposit covering the Langtang village was estimated to be 6.95 × 10 6 m 3 , which is consistent with the volume evaluated in this study from different data sources (6.81 ± 1.54 × 10 6 m 3 ).…”

“…At high elevations the imagery was of high quality (clear and with a lot of contrast) (Fig. 5b in Lacroix, 2016) such that the estimated detached volume (14.38 × 10 6 m 3 ) seemed to be reliable. It is problematic, however, that the pre-event image was taken on 21 April 2014, 1 year earlier than the event, so that snowfall on and melting of the glacier surface during the 2014 monsoon season were fully ignored in the differentiation of two DEMs.…”

“…Differences between two digital elevation models (DEMs) created from images on 21 April 2014 and 10 May 2015 showed distribution of not only the deposit but also destroyed forest on the opposite slope ( Fig. 5a in Lacroix, 2016) with errors not exceeding 3 m over the unaffected terrain. Nevertheless, the contributions and sequences of avalanches (snow and ice) and rockfalls are not yet fully understood, though some of the event was described in Kargel et al (2016).…”

“…Based on a mean thickness (≥ 2 m) and density (2200 kg m −3 ) of the deposited material, the released gravitational potential energy was estimated as ≥ 3.2 × 10 13 J, although the thickness was highly uncertain (Kargel et al, 2016). Utilizing a set of high-resolution SPOT6/7 images, Lacroix (2016) estimated a deposit volume of 6.95 × 10 6 m 3 over the village, with a larger amount of avalanche material deposited above 5000 m a.s.l. (9.66 × 10 6 m 3 ), and a total avalanche volume of 14.38 × 10 6 m 3 .…”