Did a 4.9 earthquake trigger Nepal's deadly floods? ISRO satellite study offers clues

A preliminary satellite assessment by India’s National Remote Sensing Centre (NRSC), a unit of the Indian Space Research Organisation (ISRO), has offered a possible sequence of events behind the devastating flash floods in Nepal’s Rasuwa region.

The assessment suggests that a magnitude 4.9 earthquake in Tibet may have triggered the collapse of a cirque glacier, setting off an ice-rock avalanche and eventually sending a powerful surge of water and debris downstream.

Satellite images help trace disaster

NRSC scientists in Hyderabad examined satellite images taken before and after the disaster to map the affected area and understand the changes in the mountainous terrain.

They compared imagery from the European Sentinel-2 satellite captured on August 24 with post-disaster images from ISRO’s Resourcesat-2A taken on August 26. An older Resourcesat-2A image from April 4 was also studied to identify changes in the landscape.

The comparison showed major alterations along the river corridor, including extensive flooding and debris deposits. Such satellite data can be particularly useful in mountainous areas where damaged roads and difficult terrain make it hard for response teams to reach affected locations.

How the flood may have unfolded

According to the preliminary assessment, the chain of events began at high altitude in Tibet. A 4.9-magnitude earthquake may have destabilised a cirque glacier, causing a large mass of ice and rock to collapse.

The resulting avalanche is believed to have moved down a steep valley and temporarily blocked a river channel. Water then accumulated behind the blockage before the barrier gave way, releasing water, mud, rocks and other debris downstream.

The resulting flood surge travelled through the Trishuli river system and caused extensive inundation in Nepal.

A cirque glacier forms in a bowl-shaped depression on a mountain, where snow and ice accumulate. Such glaciers can also receive ice and snow from avalanches higher up the slopes. Their collapse can therefore send both ice and rock rapidly downslope.

Earthquake recorded in Rasuwa

India’s National Center for Seismology recorded a magnitude 4.9 earthquake at a depth of 10 km at 8.22 am IST. The reported coordinates, 28.076 latitude and 86.494 longitude, correspond to the Rasuwa region.

The Himalayan belt is highly earthquake-prone, adding another layer of risk to an already fragile mountain environment.

However, the NRSC assessment remains preliminary. It points to an earthquake-triggered glacier collapse as a possible explanation for the disaster rather than establishing it as the definitive cause.

A wider Himalayan warning

The event highlights how hazards in the Himalayas can overlap. An earthquake or other geological disturbance can destabilise ice and rock, which can then block rivers and produce sudden flooding when the blockage fails.

The assessment also demonstrates the role of satellite technology in disaster response. By comparing images from different dates, scientists can quickly identify damaged areas, changes in river channels and the spread of debris when access on the ground is severely restricted.

Experts familiar with the region have suggested that the disaster’s source, more than 200 km from the Indian border, could mean the most severe effects remain concentrated around the affected Nepal-Tibet river corridors. However, ISRO has not formally stated what the downstream impact on India may be.

As rescue and recovery operations continue, satellite observations are expected to remain useful for tracking changes in the affected region and identifying possible secondary hazards.