Abstract
Abstract. The main shock of the 2015 Gorkha Earthquake in Nepal induced numerous avalanches, rockfalls, and landslides in Himalayan mountain regions. A major village in the Langtang Valley was destroyed and numerous people were victims of a catastrophic avalanche event, which consisted of snow, ice, rock, and blast wind. Understanding the hazard process mainly depends on limited witness accounts, interviews, and an in situ survey after a monsoon season. To record the immediate situation and to understand the deposition process, we performed an assessment by means of satellite-based observations carried out no later than 2 weeks after the event. The avalanche-induced sediment deposition was delineated with the calculation of decreasing coherence and visual interpretation of amplitude images acquired from the Phased Array-type L-band Synthetic Aperture Radar-2 (PALSAR-2). These outline areas are highly consistent with that delineated from a high-resolution optical image of WorldView-3 (WV-3). The delineated sediment areas were estimated as 0.63 km2 (PALSAR-2 coherence calculation), 0.73 km2 (PALSAR-2 visual interpretation), and 0.88 km2 (WV-3). In the WV-3 image, surface features were classified into 10 groups. Our analysis suggests that the avalanche event contained a sequence of (1) a fast splashing body with an air blast, (2) a huge, flowing muddy mass, (3) less mass flowing from another source, (4) a smaller amount of splashing and flowing mass, and (5) splashing mass without flowing on the east and west sides. By means of satellite-derived pre- and post-event digital surface models, differences in the surface altitudes of the collapse events estimated the total volume of the sediments as 5.51 ± 0.09 × 106 m3, the largest mass of which are distributed along the river floor and a tributary water stream. These findings contribute to detailed numerical simulation of the avalanche sequences and source identification; furthermore, altitude measurements after ice and snow melting would reveal a contained volume of melting ice and snow.
Highlights
A great earthquake of 7.8 Mw, namely the 2015 Gorkha Earthquake, occurred in the district of Lamjung, central Nepal, on 25 April 2015 (Ge et al, 2015; Parameswaran et al, 2015), causing more than 9000 deaths and injuring 23 000 people (Roy et al, 2015)
The brightness of these images corresponds to the amplitude of the microwave signal reflected to the PALSAR-2 antenna
Comparison of the pre-event PALSAR-2 backscatter amplitude image (Fig. 5b) and an ALOS PRISM/AVNIR-2 pansharpened image taken on 12 October 2008 (Fig. 5c) showed bright points and a valley-shaped feature corresponds to buildings and water streams, respectively
Summary
A great earthquake of 7.8 Mw, namely the 2015 Gorkha Earthquake, occurred in the district of Lamjung, central Nepal, on 25 April 2015 (Ge et al, 2015; Parameswaran et al, 2015), causing more than 9000 deaths and injuring 23 000 people (Roy et al, 2015). Damage in urban areas especially affected stone and brick masonry structures (Goda et al, 2015), whereas numerous landslides were induced in rural and mountain areas (ICIMOD, 2015a; Kargel et al, 2015). The most catastrophic collapse on the mountainside was reported in the Langtang Valley, located 70 km north of Kathmandu (ICIMOD, 2015b; Kargel et al, 2015). Landslides, avalanches, and a sudden air pressure wave travelled from a south-facing steep slope to the bottom of a U-shaped valley as described in detail in Sect. The fallen materials, i.e. a mass of boulders, snow, and ice, covered the valley bottom, affecting almost all the buildings in Langtang Village. On the opposite side of the valley, trees were blown down and had lost their leaves from the sudden air pressure wave
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