Pre-Glacial Lake Outburst Flood moraine deformation at South Lhonak Lake, Sikkim, from optical satellite feature-tracking

Glacial Lake Outburst Floods (GLOFs) represent a threat to communities living downstream of rapidly expanding glacial lakes, the hazard from which is exacerbated by ongoing climatic warming and global glacier mass loss. Glacial retreat also exposes unstable and unconsolidated moraine slopes that border glacial lakes, which can trigger GLOFs through mass-movements into the lake. However, few studies investigate the detailed links between multi-year moraine destabilisation mechanisms and eventual failure in these environments. In this study, we explore the pre-collapse deformation of the frozen lateral moraine of South Lhonak Lake, Sikkim, India, that collapsed into the lake and triggered the October 2023 GLOF.

We investigate the deformation using feature tracking of Sentinel-2 optical satellite imagery – a methodology better adapted for monitoring very rapid moraine deformation (>metres per year) than more commonly-used InSAR, particularly for N-S oriented displacements. The results confirm the presence of a dynamic frozen moraine complex in and around the 2023 collapse zone. Two zones of movement are identified, a fast-moving (~10m yr^-1), western Zone ‘A’ and - from 2020 onwards - an emergent eastern Zone ‘B’ (~5m yr^-1). Coupling of these two zones of moraine movement drives dynamic reorganisation of the entire deforming zone of the moraine complex, triggering a two-year acceleration and reorientation of flow direction in Zone A, followed by an abrupt slowdown in 2022. Our results indicate that emergent zones of landslide motion can alter the wider deformation pattern of adjacent moraine slopes, potentially driven by a reduction in slope shear strength following removal of lateral support. The co-occurrence of this movement and the eventual failure zone lead us to interpret that the observed movements are the precursory motion of the October 2023 permafrost landslide, although the results cannot forecast the exact timing or geometry of the collapse. Whilst glacier retreat undoubtedly facilitated the GLOF through growth of the lake and exposure of the unstable moraine, we find no instantaneous acceleration of the landslide velocities following glacial debuttressing. We highlight the possibility of using open-access remote-sensing data to assess mass-movement trajectories around glacial lakes to better inform GLOF hazard assessment and mitigation efforts.