Correction: An earlier version of this article incorrectly identified the scientific scale used to measure the magnitude of the quake and aftershocks. The U.S. Geological Survey uses the moment magnitude scale to describe earthquakes; it no longer uses the Richter scale. This version has been corrected.

Helicopter teams began evacuating critically injured climbers at Mount Everest’s base camp Sunday morning, but the effort came to an abrupt halt when a significant new aftershock triggered more avalanches and fears of additional casualties at the world’s highest peak.

Dozens of climbers and Sherpas, their Nepali guides, remain trapped on the side of the mountain at two camps that sit above where the avalanche fell, climbers said in tweets and other social-
media posts. Ropes and other equipment left in place to help them descend had been swept away in Saturday’s avalanche.

Daniel Mazur, a climber trapped at Camp 1, tweeted Sunday “Aftershock @ 1 pm! Horrible here in camp 1 Avalanches on 3 sides. C1 [Camp 1] a tiny island. We worry about the icefall team below. Alive?”

Col. Rohan Anand, a spokesman for the Indian army, which had a mountaineering team training on Everest at the time of the disaster, said the rescue effort also has been hampered by communications difficulties and weather. The aftershock occurred around 1 p.m. Nepal time Sunday and registered 6.7 on the moment magnitude scale, the U.S. Geological Survey said.

The army said that 19 people had died at the Everest base camp Saturday after an enormous sweep of ice, rocks and snow tumbled toward the camp in an avalanche triggered by Nepal’s deadly earthquake, which has killed more than 3,600 in the country so far. The army had rescued 61 climbers, mostly foreign tourists.

A woman who was injured receives treatment at the Mount Everest base camp. (Azim Afif/European Pressphoto Agency)

The wind that accompanied the avalanche “completely pulverized and blew the camp away,” American climber Jon Kedrowski, who was at the base camp, wrote on his blog Sunday. “Many of the injuries were similar to ones you might see in the Midwest when a tornado hits, with contusions and lacerations from flying debris. Head injuries, broken legs, internal injuries, impalements also happened to people. Some people were picked up and tossed across the glacier for a hundred yards.”

He continued: “People in tents were wrapped up in them, lifted by the force of the blast and then slammed down onto rocks, glacial moraine and ice on the glacier.”

Rescue helicopters had begun to land at the base camp — which is used by hundreds of climbers as the starting point for Everest ascents during peak climbing season — in the morning, after the weather cleared and the sun peeped through the clouds. This gave rescuers an opportunity to ferry about 50 of the most critically wounded — climbers and Sherpas — to safety.

Xinhua News Agency reported that more than 400 mountaineers on the north side of Mount Everest were safe, quoting the sports administration of Tibet. There was an avalanche near the north side of the North Col, but it didn’t hit any of them.

But the continued seismic activities halted rescue operations.

A Danish climber, Carsten Lillelund Pedersen, wrote in a Facebook message exchange with The Washington Post that the injured have been evacuated but that the dead remain.

“It’s very tragic, we have many climbers and Sherpas stuck higher up in camp 1 and 2. . . . And they are getting desperate,” he wrote in the message.

A large aftershock sent more avalanches Mount Everest on Sunday.

Another climber, Alex Gavan, tweeted that base camp had grown quiet, taking on the look of an aftermath of a nuclear blast, with “great desolation” and “high uncertainty” among those who remained.

One section was especially hard hit, a Dutch climber named Eric Arnold wrote on his blog. “There was hardly anything left. I see very personal stuff, a log book, shampoo, slippers, reading glasses, everything.”

The jittery survivors rushed out of the dining tent every time they felt a shock Sunday, Arnold wrote. “Fear has got the better of us.”

At any point during peak climbing season, more than 1,500 people can inhabit Everest base camp, including climbers, Sherpa guides, porters and other staff, said Eric Johnson, a Montana emergency physician who sits on the board of Everest ER, which runs a clinic there. It’s difficult to know how many climbers are trapped on the mountain — or how many may have perished during the avalanche near or in its perilous Khumbu icefall, Johnson said. Sixteen Sherpas were killed in an avalanche at Everest last year.

Dan Richards, the chief executive of the travel risk and crisis management firm Global Rescue in Boston, said that his company has about three dozen mountaineering clients who had been near or around Everest at the time of avalanche. Six are unaccounted for, and eight remained trapped on the mountain in the camps above where the avalanche started.

“They’re up there. They’re well supplied, and they’re safe, but they’re not able to descend,” Richards said. His firm hopes to be able to rescue the trapped climbers via helicopter as soon as circumstances on the ground permit, he said.

Mrigakshi Shukla in New Delhi contributed to this report.

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