Scientists have determined that the devastating flash floods in Nepal were caused by a glacial collapse rather than the 4.4-magnitude earthquake initially blamed for the disaster.
They said the incident bore similarities to the deadly flash flood that struck India’s Uttarakhand state in February 2021.
The United States Geological Survey said further examination of long-period seismic waves showed that the tremors were associated with a glacial collapse and debris flow rather than an earthquake.
The floods, which originated in Tibet on Wednesday, swept through Nepal’s Rasuwa and Dhading districts, leaving a trail of destruction. Hundreds of people were killed, while hundreds more remained unaccounted for following the disaster.
“This event was initially reported as a magnitude 4.4 earthquake. Additional analysis of nearby seismic stations, long period seismic waves, and satellite imagery led to the determination that the seismic event was instead a glacial collapse and debris flow, and that no earthquake had occurred,” the USGS said in an update to its note on the event.
The agency also revised the magnitude associated with the seismic event upward, placing it at 5.2 instead of the earlier 4.4 assessment.
“The USGS determined that the seismic event was a glacial collapse and debris flow that caused subsequent catastrophic impacts downstream,” it said.
Daniel Shugar, an associate professor of geomorphology at the University of Calgary in Canada, explained that a massive section of glacier had fallen from a cliff at an elevation of about 5,200 metres (around 17,000 feet) above sea level before plunging thousands of feet into the valley and triggering the flash floods.
The New York Times reported that the glacier collapse occurred approximately 64 km north of Kathmandu. The resulting flow of ice travelled northwest through the valley towards the Bhote Koshi River, which lies along the border between Nepal and the Chinese territory of Tibet.
Shugar examined satellite images captured on Wednesday alongside those taken a day earlier and observed significant changes in the glaciers around the affected area.
“Comparing satellite imagery from yesterday and today, the glaciers in the vicinity do seem to have lost a fair bit of snow in the last 24 hours,” Shugar was quoted as saying by the New Scientist magazine.
“My guess is that snow melt, and glacier ice melt as well, injected a lot of liquid water into the glacier, which percolated down to the bed and lubricated it. That was enough for it to collapse,” he said.
Simon Cook, a glaciologist at the University of Dundee in Scotland who participated in analysing the satellite imagery, told The New York Times: “This is a big chunk of ice that is stored at high elevation in the mountain, with a huge amount of potential energy.”
He explained that once such a large mass of ice breaks away, the energy generated by its descent can transform it into a highly destructive mixture. “That ice just gets pulverized into water,” he said.
Cook also told the NYT that the Nepal disaster resembled an incident in Uttarakhand in 2021, when a collapsing glacier triggered extensive flooding and killed more than 100 people.
The scientists, however, cautioned that other factors may also have played a role in triggering or intensifying the disaster.
