Flash flood after the breaching of South Lhonak Lake (glacier lake outburst flood (GLOF)) along with prolonged rainfall event in the first week of October 2023 significantly triggered numerous landslides along the side of the river, roads, and vulnerable slopes within the upper catchment of Teesta basin, Sikkim. In this paper, we present documentation on the occurrences of series of landslides just after the flood and heavy rainfall event in upper Teesta basin and try to find out the relevant causes. Primarily, flash flood due to glacial lake outburst and extensive rainfall accelerated series of landslides; however, the landslides are controlled by some other secondary factors including soil and slope condition, lithological diversity, geological discontinuities, and anthropogenic influences. The InSAR coherence analysis significantly shows the lowering of coherence value (decorrelation) in the moraine dam area within pre and post GLOF event which indicates breaching of the dam. The result of the slope stability assessment showed that majority slopes along the river Teesta are highly vulnerable with less safety factor and low cohesiveness of soil materials. Furthermore, the northern part of the MCT (Main Central Thrust) is composed by high-grade metamorphic rocks with exposed structures (high lineament density) that provoke landslides. Consequently, the result also highlighted that most of the slides along the side of Teesta occurred due to high water discharge and elevated gauge height after the GLOF event. Here, NH-10 (National Highway 10) runs on the high grade vulnerable litho-units and in between Dikchu and Chungthang, NH-10 is passing at the close proximity of river Teesta. So, different pockets of North Sikkim along the NH-10 were engulfed by high river discharge and gauge height due to large scale destruction of Chungthang Hydroelectric Power Plant constructed across the river Teesta.

Understanding the destabilization mechanisms in bedrock and overburden layer slopes influenced by both rainfall and seismic activity is of significant engineering importance. A series of large-scale shaking table model tests was conducted to investigate the instability evolution in bedrock and overburden layer slopes after rainfall and seismic events. This study identifies and assesses degradation modes based on spatial deformation characteristics and slope surface displacement patterns. It integrates soil stress-strain behavior, permeability characteristics, seismic stress distribution, and slope deformation characteristics to explore the deformation mechanisms in bedrock and overburden layer slopes after rainfall and seismic events. The results indicate: (1) During rainfall, saturation significantly increases at the slope crest and toe, leading to notable strength degradation without significant overall deformation. However, during seismic activity, the slope crest initially experiences sliding failure, evolving into multi-stage sliding instability. (2) Macroscopic damage occurs suddenly, and the spatial strain distribution within the slope better identifies the evolution of plastic zone expansion, penetration, and instability. (3) The slope's instability evolution pattern, analyzed by residual displacement ratios, aligns well with the spatial strain evolution within the soil, showing greater sensitivity in identifying the slope's damage state compared to cumulative displacement. (4) Changes in moisture content affect soil mechanical properties, and post-rainfall infiltration field distribution affects the slope's overall mechanical behavior and the transmission and spatial distribution of seismic stress. Soil mechanical properties and dynamic stress spatial characteristics determine the slope's failure modes.