A massive landslide in Glacier Bay National Park more than a week ago is exciting scientists around the world for the way it was the detected, the images of the slide and the sheer magnitude of it. It’s also near a similar slide that occurred in 2012 on Mount Lituya.
Scientists from NASA, the US Geological Survey, and elsewhere are calling the slide at Mount La Perouse the biggest in the world in the last four years and possibly the largest ever recorded in Alaska
A group of scientists first detected the slide after it happened on Feb. 16.
Colin Stark is a Research Professor with the Lamont Doherty Earth Observatory at Columbia University. Stark and his colleagues developed a technique, funded by the National Science Foundation, to identify massive landslides using global seismic information. Stark said when the La Perouse landslide showed up among their data, they knew something unusual had taken place.
“Last Sunday I was watching the catalogue generated every day, and I spotted an event that we detected but that wasn’t detected by the USGS or by an agency in Germany or the Alaska Earthquake Information Center. And when that happens it’s kind of a big, red flag,” Stark said. It sort of says ‘Ooh, this could be an anomalous event.’”
Stark and his colleagues try to identify long period waves in their analysis. Short period waves are the kinds that normally occur in earthquakes. But longer waves can show a slower surface movement that might indicate a landslide.
Stark enlisted help from NASA to get an earth observation satellite to pass over the area. He also put the word out to other scientists, hoping as the news spread, access to satellite imagery would also grow.
Word eventually reached Haines pilot Drake Olson who had located the Lituya slide. He heard the approximated location of this slide to be around Bradley Glacier. He was hesitant to go on a landslide goose chase, but he took the skies and on the southern end of the Fairweather range, he spotted the obvious change in landscape.
“There was a cloud layer in Glacier Bay and I was going ‘You know, I got a lot of work to do, I shouldn’t be going off to do this. This is like a needle in a haystack,’” Olson said. “I was generally perusing everything and looked out and there it was. It stuck out like a sore thumb.”
Scientists estimate the break away from the mountain had started at just under 9,000 feet. Olson landed near the terminus at about 3,700 feet. The debris field is estimated to be almost five miles long with an estimated 68 million metric tons of debris. For comparison, the Lituya slide released less than a third of that.
Olson skiied the area to take photos. At least until he noticed the mountain and debris was still emitted small movements and sounds.
“I put my skis on, I did a quick analysis of the snowpack. And then I hiked all around on the thing. When I got up to the ice fall, wouldn’t you know the thing had a pretty good release,” Olson said. “A big thundering rumble and I hightailed it out of there.”
Stark says its unlikely Olson was in danger. Most of the activity took place at the mountain, when it broke away. The debris field is so large because it was moving across ice and spread out, he said.
“He had nothing to work about at that point. Most of that was sliding over ice. Only the very initial part of the slide was very, very steep,” Stark said. “And that’s what makes the seismic forces we detect, not the sliding over ice.”
Stark said most massive slide events like these happen in Alaska than anywhere else in the world. And more happened then even scientists realize, he said. That’s because most go undetected and unseen. But the new technique used to find this slide will hopefully help notify scientists when other massive landslides occur. Stark said he can even use the analysis to go back through data to identify slides that have occurred in years previous. Doing that, he’s already identified massive slide that he says took place in 1997 near Mount McKinnely.
As for the cause of these slide events, Stark said that’s harder to determine. He says most slides like Lituya and La Perouse happen on south facing slopes, and often in the summer. That gives him a small clue into possible underlying causes.
“One thing I’m hesitant about disseminating too much, but a lot of the events in Alaska have been south facing warm summer events,” Stark said. “So there’s a hint, but only a hint that there may be some sort of rock permafrost thawing related, slowing, steadily losing permafrost ice and that’s affecting the stability of the slopes.”
Seismologists with the Alaska Earthquake Center said once they heard about the landslide they were able to go back and look for long period waves and found the event in their data. It was the equivalent of a 2.5 magnitude earthquake, they said.