More than 700 landslides and avalanches were detected after the M7.0 Alaska earthquake

ANCHORAGE, Alaska (KTUU) – No known damage or injuries have been reported following the 7.0 magnitude earthquake that struck Dec. 6 about 56 miles north of Yakutat.

Scientists call it the Hubbard Glacier Earthquake because it occurred under that glacier in the St. Elias Mountains in Canada, north of the Alaska border.

There is some interesting geology surrounding the earthquake and its effects, including the number of landslides and avalanches it triggered, its location in the Fairweather Fault System, how shallow the earthquake was, and its aftershocks.

Two people who experienced the earthquake from different parts of Alaska shared their stories as well.

Reclaim the moments

Joe Valle, who was in Yakutat when the quake struck, said the tremors started gently and then remained strong throughout the remainder of the quake, which he described as long and persistent.

“At first I thought maybe there was something going on at the house. Maybe the kids were being active or something, but then it kept happening,” Vale said. “In the time it took me to get up and walk into the other room, we started to feel some kind of big shaking.”

He said things usually start turning during earthquakes, but that wasn't his experience with the Hubbard Glacier quake.

“This time it was like swaying from side to side in a rather aggressive way. And the decorations on the Christmas tree were crazy,” Vale recalls. “We have a large collection of Lego pieces at home, and this set has been circulating.”

Kathleen Penner, who was at her home near Haines, about 200 miles from the epicenter, did not feel much shaking when the quake struck.

“I could see my chandelier, and it started swaying back and forth, and then, I had a bunch of bottles on my windowsill, and they were all shaking, and the couch, of course, started rocking back and forth, and [I] He pretty much knew it was an earthquake, Benner recalls.

Shallow earthquake effects

Scientists say the earthquake was very superficial.

The USGS says it's only about six miles deep, and an earthquake is considered shallow if the depth is less than about 44 miles.

The Alaska Earthquake Center said that because the quake was superficial, “ground shaking was very strong within a 50-mile radius of the epicenter.”

“When the fault is closer to the surface, the shaking has somewhat less time to attenuate because the fault is so close,” explained Kate Olstadt, a research geophysicist at the USGS. “So when you have a deeper earthquake, the shaking has to travel through a greater depth of rock, and that makes the shaking weaker when it reaches the surface.”

The “strong aftershock sequence” of the earthquake

“It appears to be generating a very strong series of aftershocks,” Alaska Earthquake State seismologist Michael West said within an hour of the quake hitting.

Hundreds of aftershocks were recorded in the week following the earthquake.

For shallow earthquakes like the Hubbard Glacier quake, Austin Holland, director of operations for the Alaska Earthquake Center, said scientists generally expect very large aftershocks.

As of December 12, he noted, there had been more than a hundred aftershocks of magnitude 4.0 or higher and a few of magnitude 5.0 or greater.

“There's a lot of modification involved, especially in complex fault systems like this,” Holland said. “You can get these very strong sequences of aftershocks where there's a lot of rebalancing of pressures inside the Earth.”

He said they are trying to track all the aftershocks, but it is difficult to count them when there are so many.

“There's still one earthquake after another. It's interesting,” Holland said. “It's so many aftershocks that a lot of them are basically happening on top of each other, and it becomes very difficult to even process the earthquakes themselves because the signals become entangled.”

However, he says the rate of aftershocks is decreasing.

“It will take months before it stabilizes to a somewhat lower level,” he said.

Hundreds of landslides have been detected

Hundreds of landslides and avalanches are believed to have been caused by the tremors.

More than 700 landslides and avalanches detected after M7.0 Hubbard Glacier earthquake (Yukon Geological Survey)

“The USGS has identified about 700 different sites that have characteristics consistent with some type of landslide or avalanche,” Alstadt said.

The USGS used remote sensing technology that uses radar to see landslides and avalanches. Because it was cloudy, a lot of snow fell, and daylight hours were limited, Alstadt said.

“A lot of them are probably avalanches because they were having a snowstorm at the time of the earthquake, and afterward as well, and there were a lot of aftershocks,” she said.

“Some of them, especially some of the smaller ones, may be avalanches, but we know for sure that a few of the really big ones had some boulders in them.”

A lot of material flowed into the glaciers, making the slides easier to see, she said.

“The contrast was very different from before and after, so we can often see deposits of snow, rock and ice on these glaciers,” she said.

She said it was normal for many landslides and avalanches to occur due to an earthquake like this.

“Typically when their strength is greater, the fault is close to the surface, and in a mountainous area, we typically expect a significant amount of landslide activity,” she said.

“There have been quite a few earthquakes in the past in this part of the world that had similar behavior, such as the Denali earthquake in 2002, [which] It caused thousands of landslides. The Great Alaska Earthquake of 1964 triggered a large number of landslides, some similar in character to those triggered by the recent earthquake.

She added that of the more than 700 landslides and avalanches found, 21 were believed to be large.

“I think the longest length we measured was at least about four miles,” she said.

Radar technology makes it difficult to see the entire landslide and where it came from, so she said some may be longer, and more data will help determine that.

One of the largest landslides identified by the USGS was on the flanks of Mount Logan, Canada's highest mountain at 19,551 feet, she said. Quite a number of the landslides measured, including the tallest, were on the same massif as Mount King George, a famous landmark in Canada.

She said there likely would be no impact on humans from the landslide identified by the USGS near the epicenter.

“It's a very remote area and full of these huge glaciers. So, none of the areas I looked at seemed to be anywhere close to where there could be [an] impact. “You can have smaller landslides further away from the earthquake fault and from the epicenter,” she said.

She said she had not heard any reports from Yakutat or any nearby communities of landslides.

Interesting earthquake site

The earthquake was caused by a fault in the Fairweather Fault system, known for triggering a 1958 earthquake that triggered a landslide in Lituya Bay, Alaska, generating the longest tsunami in recorded history.

The Fairweather Fault System is a “complex fault zone” known for its ability to produce large earthquakes, such as the Hubbard Glacier earthquake, Holland said.

But scientists are still searching for exactly where the earthquake occurred.

“Although we currently call it a rupture in one of the northern strands of the moderate weather fault system, it is worth noting that in 2010, then-graduate student Julie Elliott at the University of Alaska Fairbanks named the section that ruptured today the Mosul Fault. The National Seismic Hazard Model picked up that name in 2023,” the Alaska Earthquake Center wrote about the quake on the day it happened.

“It's definitely not one location on a map. It's a big area of ​​rocks sliding over each other with great force,” Holland said.

“It's a complex geological area, and there's a lot going on,” Holland said. “These earthquakes certainly remind us that this is an active fault system capable of producing large crustal earthquakes, which can have very serious consequences.”

Hubbard Glacier Connection

The Hubbard Glacier Earthquake occurred under a glacier – go to figure.

It's certainly not the first time something like this has been recorded. The 2002 Denali earthquake was also under a glacier, Allstadt said.

But it's an interesting situation for scientists when this happens, especially in this field.

“We don't know the geometry of the faults in this area very well,” she said. “A lot of seismic geologists want to know: Did the fault rupture, where was it, and can we learn more about Alaska tectonics here?”

The Yukon Geological Survey made reconnaissance flights on Dec. 12 to look for a crevasse rupture on the surface of the glacier, which could look like a large crack or break in the ice, Olstadt said.

The Yukon Geological Survey found no evidence of surface rupture, “but I would say that doesn't rule out surface rupture because we have that fresh snow,” which could cover up the evidence, she said.

If there is a superficial rupture, it likely won't cause a dramatic change to the glacier when it comes to its future, Olstadt said.

“I'm sure with the glaciers, something will change, but I think the total compensation is fairly low,” she said.

“What could make a more dramatic change is the material from landslides,” she said. “If you have dark matter on a glacier, it depends [on] How dense it is, it can isolate the glacier or cause it to melt faster.

“We definitely try to learn as much information from each event as possible. So, to understand, ‘What does a large, shallow earthquake do to the glacier and its behavior?’ We don’t necessarily have all the answers to what that might look like for Hubbard Glacier,” Holland said.

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