Nepal–Tibet: reconstructing the dynamics of a high-mountain hazard cascade

On 26 August 2026, a major rock-and-ice collapse occurred near the border between Nepal and Tibet. As it travelled through a confined valley, the moving mass eroded and entrained additional material before interacting with the drainage network and generating a sediment-laden flood. At the Institut de physique du globe de Paris (IPGP), research is under way to reconstruct the successive stages of this disaster by combining remote sensing, seismology, and numerical modelling.

Publication date: 11/09/2026

Research

From a collapse to a cascade of processes

The event of 26 August cannot be described simply as an avalanche or a flood. It involved a cascade of gravitational and hydro-sedimentary processes: an initial collapse involving both rock and ice evolved as it travelled downslope, eroding the valley floor and incorporating additional material. The moving mass then interacted with water flows, producing a particularly sediment-laden flood and propagating its effects far downstream.

The event is therefore better described as a mixed rock-and-ice avalanche followed by a gravitational and hydro-sedimentary cascade, or, more broadly, as a hazard cascade in a paraglacial environment.

Understanding these successive transformations is essential for assessing threatened areas. The volume initially detached is not always sufficient to explain the final magnitude of a disaster: erosion, sediment entrainment, fragmentation, ice melt and the incorporation of water can considerably increase both the mobility and volume of the moving mass.

Rapidly changing high-mountain environments

Glacier retreat, permafrost degradation and changes in water circulation are rapidly transforming high-mountain slopes. Glacier retreat can alter the stresses acting on rock walls, expose unstable terrain and release large quantities of sediment. Meltwater can also infiltrate fractures, modify internal pressures or abruptly reorganise drainage networks.

These changes do not mean that every rock-and-ice avalanche can be directly attributed to climate change. In some regions, however, they may create conditions conducive to more frequent destabilisation or to the emergence of process cascades in areas that were previously less active. The frequency, seasonality and nature of events may evolve differently depending on the processes involved, including rockfalls, ice avalanches, debris flows, glacial lake outburst floods and landslides.

Can such disasters be anticipated?

Some collapses are preceded by detectable warning signs: progressive acceleration of a slope or glacier, the opening of fractures, an increase in rockfall activity, local seismic activity or changes in water flow.

Unusual variations in discharge, repeated drainage events or rapid changes in a glacial lake may therefore indicate a reorganisation of the hydrological system. However, on their own, they do not provide evidence that a major collapse is imminent. A glacial flood may signal that the system is evolving, but it may also be a consequence of, or one stage within, the hazard cascade itself.

Anticipation therefore relies on combining several types of observation and monitoring how they evolve over time. Some events display clear precursors; others occur without sufficiently early warning signals, or in areas where no instruments are available to detect them. The challenge is to identify which observations are genuinely discriminating and to establish stronger links between the signals measured and the physical mechanisms at work.

Initial comparison between a SHALTOP simulation of the gravitational cascade, computed using a Pléiades digital elevation model, and the vertical ground motions recorded at three seismic stations. Work in progress; unpublished results.

Unravelling the mechanisms through a multidisciplinary approach

The research currently being conducted at IPGP aims to reconstruct the dynamics of the Nepal–Tibet event, from the initial failure through the avalanche and flood phases.

Remote sensing is being used to locate the source area, map surface changes, and constrain both the trajectory of the event and the extent of the deposits. A high-resolution digital elevation model derived from Pléiades satellite imagery provides the topography required to simulate propagation through the valley.

At the same time, long-period signals recorded by several seismic stations provide information on the event chronology. Inverting these data should make it possible to reconstruct the forces exerted on the ground by the moving mass and, in turn, to track its phases of acceleration, changes in direction and deceleration.

These observations are being compared with simulations performed using the SHALTOP numerical model. Several scenarios are being tested in order to assess:

the location and geometry of the initial failure;
the volume mobilised at the source;
the quantity of material eroded and entrained along the valley;
the role of ice and water in controlling flow mobility;
the timing of the transition between collapse, avalanche, debris-laden flow and flood;
the ability of the simulations to reproduce both the distance travelled and the chronology of the seismic signals.

The accompanying video presents an initial simulation performed using the Pléiades digital elevation model, compared with the vertical ground motions recorded at three seismic stations and their time-frequency content.

These results are still preliminary: they do not constitute either a validated reconstruction or published research. Their purpose is to progressively compare possible scenarios and determine which are compatible with the full set of available observations.

From Blatten to the Himalayas

This approach builds on recent work on the rock-and-ice avalanche that occurred at Blatten, Switzerland, in May 2025. For that event, comparison between the deposits, forces inferred from seismic data and numerical simulations showed that a substantial reduction in effective friction was required to explain the observed mobility.

The Nepal–Tibet event differs in its scale, geometry and transformation into a flood. It nevertheless raises similar questions: how do ice and water modify the mobility of a moving mass? What role does valley erosion play in amplifying the event? How can the different stages of a hazard cascade be distinguished within seismic signals?

Answering these questions should improve our understanding of these complex events and, in the longer term, help identify the observations that are most useful for monitoring them and anticipating future events.