Analysis of triggering factors behind the October 2023 South Lhonak GLOF event in the Sikkim Himalaya using multiple remote sensing data

A catastrophic Glacial Lake Outburst Flood (GLOF) occurred from the South Lhonak Glacial Lake (SLGL) in the Sikkim Himalaya between October 3 and 4, 2023, highlighting the urgent need to understand its triggering mechanisms. Given the inaccessibility of the lake region, we employed multiple state-of-the-art remote sensing approaches, including optical image interpretation, interferometric coherence and geospatial analysis, to examine all plausible triggering mechanisms while systematically eliminating non-contributing factors. Our primary objective was to evaluate the potential triggers, with particular focus on a landslide originating from the left lateral moraine, which appears to have been the direct cause of the GLOF. The estimated landslide volume was ~13.1 ± 2.7 × 106 m3. Subsequent analysis aimed to determine the underlying cause of this landslide. Results indicate that increased inflow—comprising both surface runoff and subsurface seepage, into the left lateral moraine significantly elevated slope saturation, thereby compromising stability and triggering the failure. Other possible contributors were also investigated but found to have negligible or no influence on the event. These included hydrostatic pressure from the adjacent North Lhonak Glacier, a potential outburst from the North Lhonak Glacial Lake (NLGL), blockage of the SLGL outlet by landslide, seismic activity, and precipitation events.