The Himalayan flash floods that have killed at least 160 people and left some 1,500 missing across Nepal and Tibet were not triggered by an earthquake, contrary to initial reports, but by a glacier collapse so violent it produced its own seismic signal, the United States Geological Survey said.
The agency initially recorded a magnitude 4.4 earthquake along the Nepal-China border, north of Kathmandu, at about 8.37am local time on Wednesday, before revising its assessment in a statement issued that evening.
It said the seismic signal had come from "a glacial collapse and debris flow" rather than a tectonic quake, and that the collapse alone had caused the catastrophic flooding downstream. Once reclassified, the event registered at magnitude 5.2, a measure of the force of the ice and rock striking the valley floor rather than of any movement underground.
An initial study of satellite imagery from Planet Labs showed a large section of the lower end of a glacier breaking away at an altitude of about 5,200 metres (17,000ft) and falling roughly 1,200 metres (3,900ft) onto the valley floor, gathering rock and debris as it went.
The resulting avalanche struck the Lhende River in Tibet, about 20km (12 miles) northeast of the Rasuwagadhi border crossing with Nepal, temporarily damming the water before the barrier gave way and released a powerful flood downstream.
The surge fed into the Bhote Koshi River and crossed into Nepal at around 9am local time, then continued into the Trishuli River, where water levels rose by as much as nine metres in just half an hour.
Officials were caught off guard because there had been no rain, according to local news reports, and some residents ignored early warnings from neighbours precisely because the danger did not resemble the monsoon flooding they were used to.
Nepal's Rasuwa district, immediately south of the border, bore the worst of the damage, with the flood also reaching Nuwakot and Dhading further downstream. At least 19 bridges and around 40km (25 miles) of roads have been destroyed, and damaged hydropower infrastructure has left some areas without electricity.
Adrian McCallum, a glaciologist and polar engineer at the University of the Sunshine Coast, said the disaster had the hallmarks of a glacial lake outburst flood, in which meltwater builds up behind a natural dam of rock and ice before breaking through.
"As glaciers melt and recede, ice turns to water, the water is stored behind natural moraine dams, but when those dams breach, tremendous amounts of water can be released down a valley, taking out everything in its path," he said.
Ruth Gamble, an environmental historian and director of La Trobe Asia at La Trobe University, said the flood behaved "like an inland tsunami," travelling nearly 170km down the Bhote Koshi valley into central Nepal, along a route also used by tourists travelling between Kathmandu and Lhasa.
Scientists cautioned it was too early to say how far climate change contributed to the collapse itself. Glaciers across the wider Hindu Kush Himalaya region, which spans Afghanistan, Pakistan, India, China, Nepal, Bhutan, Myanmar and Bangladesh, lost ice 65 per cent faster between 2011 and 2020 than in the previous decade, and Nepal's own glaciers have lost close to a third of their ice volume over roughly three decades.
Satellite observations showed unusually warm conditions and reduced snow cover around the glacier before it collapsed, exposing more bare ice, though researchers have not yet established a direct link to this week's disaster.
Nader Naderpajouh, head of the school of project management at the University of Sydney, said the disaster could prompt the introduction of early-warning systems similar to those developed after the 2004 Indian Ocean tsunami, since a build-up of water behind a glacial dam can sometimes be detected before it bursts.
"Such communications and preparations can save lives and reduce the impact at scale," he said.
