## 5 Discussion
### Possible Causes of Landslide and GLOF  
The occurrence of GLOF may be affected by many factors, and the triggering mechanisms are intricate. [PERSON] et al. (2013) summarized the causes and mechanisms of moraine-dammed lake failures, identifying five dynamic factors and three long-term factors. The former includes earthquakes, slope movements into the lake, floods from upstream lakes, blockage of underground outflow channels, and intense rainfall or snowmelt. The latter can be considered as spontaneous \"dam self-destruction\" resulting from melting buried ice, hydrostatic pressure, and temporal effects [44; 45; 46].  
The relationship between the circular area of landslides affected by earthquakes and the magnitude (\(M>4\)) can be expressed as [47]:  
\[\text{log}_{10}\text{ A}=\text{M}-3.46\ (\pm 0.47) \tag{1}\]  
Among them, A represents the affected area (km\({}^{2}\)), and M denotes the earthquake magnitude. We utilize the following formula to calculate the maximum critical distance \(R\), in kilometers, of landslides affected by earthquakes:  
\[R=\sqrt{\frac{10^{(M-2.99)}}{\pi}} \tag{2}\]  
Figure 12: Slope map of the study area calculated by NASADEM HGT v001, where the dashed blue line represents the pre-disaster lake contour outlined using Landsat 8 images acquired on 25 June 2023, and the solid red line delineates the extent of the landslide.  
By substituting \(M=5\) into Formula (2), the maximum critical distance is calculated to be approximately 5.7 km. According to seismic records from the United States Geological Survey (USGS) ([[http://earthquake.usgs.gov/earthquakes](http://earthquake.usgs.gov/earthquakes)]([http://earthquake.usgs.gov/earthquakes](http://earthquake.usgs.gov/earthquakes)) accessed on 13 April 2024), no earthquakes of magnitude 5.0 or higher occurred within a 100 km radius of South Lhonak Lake from January 2023 to October 2023. Thus, it is unlikely that the South Lhonak Lake outburst studied in this article was caused by an earthquake. Furthermore, [PERSON] et al. (2021) [28] identified the cliffs around South Lhonak Lake most likely to be prone to avalanches based on mass movement potential. No avalanches were found through optical imaging in this area before the GLOF, essentially ruling out the possibility of the moraine-dammed lake failure being due to avalanches.  
Numerous studies have highlighted rainfall as one of the primary triggers of landslides in many regions worldwide [48], due to its significant impact on slope stability [49; 50]. The analysis in Section 5.1.1 also indicates a rapid increase in the slope's deformation rate around the moraine-dammed lake as the rainy season commences. Sikkim received 101 mm of rainfall in the first five days of October 2023. Between 3-4 October alone, the rainfall in the state escalated to five times the normal level. On 4 October, daily rainfall across Sikkim reached 115-205 mm, coinciding with the occurrence of the moraine-dammed lake failure. Observation of Landsat 8/9 images from just before and after the GLOF revealed no significant landslides in the moraines on 29 September. However, on 7 October, a notable landslide in the moraine was visible. Therefore, during the period of sustained heavy rainfall from 29 September to 7 October, a large-scale landslide occurred in the moraine area, characterized by significant slope and cumulative deformation. Consequently, we attribute heavy rainfall as the primary probable cause of the landslide in this study. Simultaneously, the GLOF disaster occurred in South Lhonak Lake on 4 October 2023. In summary, we speculate that the landslide in the moraine induced by heavy rainfall may be one of the possible reasons for the eventual breach of the terminal moraine dam. Under such climate conditions, glaciers will persist in retreating and melting. Consequently, over time, the glacial lake area may revert to its original volume after the outburst, posing a potential hazard as a glacial lake susceptible to external drivers, potentially leading to recurrent GLOF events.