Updated 5 p.m. Friday
A huge landslide tumbled down a mountain in southeast Alaska near Glacier Bay early Sunday morning.
Sunday's slide appears to have come down the flanks of 10,728-foot Mount La Perouse at 5:24 a.m. local time and flowed in the east-northeast direction, said Colin Stark, a Columbia University geologist who helped develop a new system to detect major landslides around the world using satellite imagery. Calculations of mass suggest it sent about 68 million metric tons of debris down a mountain slope, though that is a "pretty rough estimate," he said in a telephone interview. "To put in concrete terms, the mass was equivalent to about 190 Empire State Buildings," he said.
The landslide was far bigger than the 2012 Lituya Mountain slide, measured at about 20 million metric tons. Stark and Columbia University colleague Goran Ekstrom also detected that slide with their newly developed monitoring system. An approximately 30 million metric ton slide occurred last August in Wrangell-St. Elias National Park.
Unlike the rain-triggered mudslides that closed roads on Prince of Wales Island in January and the slide that in October clogged a portion of the road that snakes through Denali National Park, Sunday's southeast Alaska slide apparently went unnoticed by people -- except for Stark and Ekstrom.
Waves emanating from landslides, which send rock and earth over the surface of the ground, are different from those emanating from earthquakes below the ground's surface, Stark said. While earthquakes create waves of varying frequencies, the waves from landslides are dominated by low frequencies, Stark said.
The Columbia scientists' monitoring system is different from that used by the U.S. Geological Survey and the Alaska Volcano Observatory, organizations that focus on below-ground seismic events and are likely to miss most movement on the surface, Stark said.
But the system developed by Stark and Ekstrom cannot detect landslides triggered by big earthquakes. In those cases, seismic waves from the earthquakes would be so overwhelming that they would swamp any signals from landslides, he said.
It is unclear, he said, whether landslides are more frequent in Alaska than they used to be or whether they are simply more frequently detected. Nevertheless, landslides happen fairly often in the state, he said.
"Landslides like this occur with the highest frequency in the world in Alaska," he said.
There are numerous potential triggers. In some cases, big subduction earthquakes -- like the Great Alaska Earthquake of 1964 -- can continue to trigger slides for years afterward as landforms settle, he said. Sometimes glacier and ice dynamics set off the slides, he said. And sometimes warming temperatures, occurring gradually or over shorter periods, are the cause, he said.
"There's a hint -- just a hint, at the most -- that some of these very big landslides might be triggered by a permafrost thaw," he said. That would be thaw of permafrost in rock, not in soil, he said. It may be a continuation of a trend in place since the end of the Little Ice Age, or possible accelerated by more recent climate warming.
Some of the big landslides -- such as last August's Wrangell slide -- have occurred on south-facing slopes, when sun is beating down in hot-weather periods, supporting the idea that ice within rocks is thawing, he pointed out. But there are plenty of other possible factors that unleash landslides, he said. "It's hard to establish a particular trigger," he said.
Stark said he hopes to have confirmation of the slide's location within days and enough information to reveal the mystery site within the week.
Contact Yereth Rosen at yereth(at)alaskadispatch.com