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Nepal avalanche carried half the force of an A-bomb

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The avalanche that buried the popular hiking village of Langtang in Nepal this past spring struck with about half the force of an atomic bomb, according to a study published this week in the journal Science.

A strong earthquake on April 25 touched off the avalanche, sending stone, earth and snow cascading down onto the village, which was crowded with backpackers, guides and their families. About 350 people were killed, many of them foreign tourists.

The impact was equivalent to the detonation of 7.6 kilotons of TNT, the study estimated; the bomb dropped on Hiroshima in 1945 was equivalent to about 16 kilotons.

Some survivors of the disaster in Langtang have described seeing boulders the size of cars demolishing buildings or being borne aloft by a torrent of wind and snow. When the cloud settled, only one structure in the village remained standing, a house that had been tucked into the mountainside under a rocky overhang.

International search teams sifted through the debris for weeks afterwards, looking for bodies or other remains, but nearly 100 people are still listed as missing.

The new study, the work of 64 scientists, concludes that a mass of ice and snow six and a half feet thick fell off a cliff and “became airborne,” plummeting to the ground, possibly as far as 4,000 feet, at a final speed of about 200 feet per second.

The air blasts that accompanied the avalanche exceeded 200 miles an hour, as powerful as the deadliest tornadoes on record — enough to blow stone houses from their foundations. Among the forensic evidence available to scientists was a small forest that had been flattened by the blast of air as far as 1,300 feet up the opposite slope of the valley, the study said.

“Their limbs were removed, their leaves were removed, their branches were removed — in many cases, the bark was removed,” said Jeffrey S. Kargel, a glaciologist at the University of Arizona who was the lead author of the study. “It really resembles the Hiroshima blast zone, and no wonder — the energy is of the same magnitude. It’s really horrifying.”

The study’s principal conclusion is that overall, the damage from the earthquake could have been much worse than it was. Despite the quake’s great strength — its magnitude was 7.8 — it caused fewer landslides than some experts had expected, and it did not cause any of Nepal’s glacial lakes to burst out and sweep away bridges and villages with torrents of debris-filled water.

This may have been because the quake shook the ground at a low frequency, producing “comparatively gentle swaying, not the rapid ratcheting that the high-frequency causes,” Mr Kargel said.

Joseph M. Shea, a contributing author of the study, said he had often done fieldwork in the Langtang Valley before the quake, and that he had been powerfully affected by the data about the destruction there.

“It must have been a massive wind,” said Mr Shea, a glacier hydrologist at the International Center for Integrated Mountain Development in Kathmandu, the Nepalese capital. “It would have been horrifying for the people there. The wind would have been the first thing that basically hit them.”

“Langtang village was just in the wrong place,” he added.

Roads leading to Langtang were so badly damaged that search teams could not reach it by land for four months after the earthquake, and human remains were buried under heavy snow and rubble, said Pravin Pokharel, deputy superintendent of police in the Rasuwa District. The authorities succeeded in recovering the remains of 21 people in mid-October.–Courtesy of The New York Times

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