Seismic and hydrological triggers for a complex cascading geohazard of the Tianmo Gully in the southeastern Tibetan Plateau

On 11th July 2018, a destructive rock-ice avalanche and subsequent glacier debris flow occurred in the Tianmo Gully in the southeastern Tibetan Plateau (SETP). However, the source area and triggering factors of this cascading geohazard event remained unclear. In this study, we combined satellite remote sensing, meteorological observations, numerical modeling, and post-event field investigation to comprehensively analyze its evolution processes and potential triggers. The remote sensing observations of terrain and landform changes suggest that the initial avalanche occurred on the southeastern flank of the glacier, releasing approximately 2.77 × 106 m3 of lithic and ice material. From our analysis, we suggest that the complex evolution process of this cascading geohazard event could be manifested as earthquake and hydrological triggers → an initial rock-ice avalanche → glacial debris flow → triggered landslides → landslide dam → dammed lake. Our results suggest that the 2017 Ms. 6.9 Nyingchi earthquake, the unusually high meltwater from snow and ice during the abnormally warm and dry summer in 2018, and the short-duration intense rainfall (22 mm on 10 July) recorded one day before the event are the three main factors for the catastrophic event. These factors caused the rock-ice avalanche in the source zone and subsequently cascaded glacial debris flow and shallow landslides. This study highlights the urgent need for regular monitoring of high-risk glaciers, including anomalous changes in temperature and precipitation, and accelerated movements of glaciers due to earthquakes, especially in the SETP, where climate warming will be expected to intensify occurrences of such cascading geohazard events in the future.