Scientists piecing together satellite imagery and seismological data said on Thursday that a high-altitude landslide coinciding with a glacial collapse likely caused the deadly flash flooding in Nepal and Tibet.
Glaciologists and geologists had already determined that an enormous chunk of a glacier had snapped off on Wednesday morning. On Thursday, a group of scientists ***yzing the events preceding the flooding said that newly available images, captured at a time with less cloud cover, show that an enormous chunk of the mountainous bedrock beneath that glacier had also given way.
“The rock that the glacier was sitting on collapsed,” said Kristen Cook, a geomorphologist at the Université Grenoble Alpes in France. The combined force of the collapse sent millions of metric tons of rock and ice into the valley below, she added.
“It was a much larger collapse than we were initially able to see in the satellite imagery,” she said.
The deluge of rock and ice, which pulverized into mud and water as it surged down the mountainside, was so powerful that the U.S. Geological Survey registered the landslide as a magnitude 5.2 seismic event.
That this event registered as an earthquake is not unusual. Whenever a huge amount of energy is transferred to the Earth’s crust — from an earthquake, an asteroid strike, or even a crowd of people jumping up and down at a concert — seismic waves are produced, which are detected by seismometers all over the planet.
This debris flow would have released the same amount of energy as 1,000 tons of TNT. “Perhaps more importantly than energy, we can directly compute the m*** from our force estimate. This comes out as nearly 400 million metric tons,” said Stephen Hicks, a seismologist at University College London.
To put it another way, this landslide was comparable to 1,000 Empire State Buildings crashing down into the river valley simultaneously.
Warming temperatures caused by climate change may have destabilized the rock face, scientists said, but they added that it was too early to determine a precise cause.
The landslide and glacial collapse took place on the north face of Langtang Lirung, a Nepalese mountain near the border with Tibet, at an elevation of around 17,000 feet, according to the scientists.
“Essentially, a chunk of the mountainside collapsed,” said Daniel Shugar, a geologist at the University of Calgary in Canada who is part of the group.
Scientists are slowly building a more detailed picture of the chain of events that led to Wednesday’s flooding. Torrents swept through the Himalayan valleys between Nepal and the Tibet region of China, devastating villages, killing hundreds and leaving more than 1,000 people missing.
According to Dr. Cook, the flow of rock and ice incorporated water and river sediment as it tumbled downstream along a network of rivers and tributaries, reaching dozens of miles from the site of the original landslide.
While the new imagery added to scientists’ understanding of the cause of the disaster, many questions remained. Among them is how much rock and ice was released by the landslide, and why the volumes of floodwater were so huge.
Dibas Shrestha, an expert in hydrology and meteorology at Tribhuvan University in Nepal, said in an email that he “couldn’t even imagine the volume of ice-water-rock mix flow that we have witnessed downstream.” Videos taken at the Gyirong border crossing between China and Nepal show a wall of water overcoming the area.
One possibility, scientists say, is that the landslide created upstream river dams that eventually burst, unleashing an enormous gusher of water.
Mohan Bahadur Chand, a Himalayan glaciologist and ***istant professor at Kathmandu University, said that one initial hypothesis is that acblockulated debris temporarily blocked the Lhende River, leading to a catastrophic overflow.
As higher-resolution satellite imagery becomes available in the coming days, Dr. Chand said it could become clearer how much of the glacier had broken off and what other sediment had been swept into the water. From there, it could be possible to calculate the volume of water that had swept across the region, he said.
Judson Jones contributed reporting.
