Massive mountain rockslide near Juneau shakes the earth on Christmas Eve

Around Christmas, there were reports of an earthquake and major mountain rockslide in the wilderness near Juneau. But what came first? The earthquake or the slide?

Jamie Tait is a helicopter pilot based out of Atlin, British Columbia. The day after Christmas, he and his family were flying along the Taku River valley after checking on a cabin in the area.

“It’s pretty interesting because I’ve flown down that river for probably the better part of 35 years,” Tait said. “I noticed it right away before the rest of my family did and it’s pretty significant.”

One mountain along the river about 46 miles northeast of Juneau looked like part of its face had been sliced off.

“The area in the valley was all full of the debris and mud that have been pushed up when the mountainside collapsed,” Tait said.

Sonia Nagorski, assistant professor of geology at the University of Alaska Southeast, says some of that area’s mountain slopes are over 50 degrees. In other words, the slopes are so steep that they are almost like a cliff face.

“And so, it just crumbles and fails,” Nagorski said. “It’s pretty astounding that those rocks are sitting at the steep angle at all.”

Nagorski says there’s a lot of volcanic rock that used to be under the earth’s surface. It’s not as strong as other forms of rock. And, it’s more susceptible to weathering like rain, snow and ice combined with a lot of freezing and thawing.

“It’s easy to think that our mountains are solid and stable, but the Earth’s surface is constantly shifting, especially when there are tectonic changes and glacial changes,” Nagorski said.

“A lot of the changes can be quite sudden,” she said. “And land sliding is the primary method of erosion in mountains steeper than about 30 to 40 degrees, which are plentiful here.”

Nagorski says it’s not clear exactly how much of the nearly 4,000-foot mountain actually flaked off and fell away at 10:50 a.m. on Christmas Eve. Her best estimate is 20 million cubic feet of material. That’s enough to fill up half of the Houston Astrodome.

When all that rock fell, it was literally enough to shake the earth. Natalia Ruppert, a seismologist with the Alaska Earthquake Center, says if the shaking was caused by an earthquake, then it would’ve registered as a magnitude 2.8 or 2.9.

Ruppert is clear, however. The landslide itself was not triggered by any previous earthquake. She knows that because it’s very easy for seismologists to distinguish a sudden, abrupt start of an earthquake from a landslide’s gradual building intensity, sometimes over several minutes.

“It starts very small, and then gradually the amplitude of the signal picks up and then becomes larger and larger as the landslide mass kind of gains momentum and moves down the slope,” Ruppert said. “And then it plays out as all those rocks come to a final resting place.”

Ruppert says this wasn’t the first time or even the biggest event. Bigger landslides were detected around Southeast Alaska in 2005 and 2008.

“And then in subsequent years, and some of them are much, much larger than this landslide on the Taku River,” she said. “Some of those were so large that they were detected even globally.”

The Taku River is one of biggest salmon producing rivers in Southeast Alaska. Since it crosses an international boundary, both United States and Canada share the harvest.

David Harris, commercial fisheries area management biologist with the Alaska Department of Fish and Game, says the slide likely destroyed any salmon roe at the foot of the mountain. But he believes it won’t have a big impact on salmon runs overall.

“The channel is fairly braided. There’s two or three large channels already,” Harris said. “Plus, smaller connecting side sloughs and whatnot.”

“There’s plenty of room for the water to find a way through,” he said. “It’s not like a narrow valley that’s filled up with material.”