Dynamic glaciers identified as cause of Southeast Alaska’s summer ice quakes

Jason Amundson took this picture of Speel Glacier, located approximately 6 miles south of Wright Glacier and Mt. Ogden, while investigating the source of ice quakes in 2020. (Photo courtesy of Jason Amundson/UAS)

Within a set of glaciers and mountains near Juneau, there’s seismic activity almost every day in the summer. They’re called ice quakes. They’re not as widely understood as earthquakes, but researchers are monitoring them closely.

This groaning or rumble sound is sped up about three times. This is a big iceberg in Antarctica either scraping against the ocean bottom or another iceberg.

It’s called an ice quake. It’s one of many types, and it’s probably quite similar to what has been happening almost every day for the last few weeks in the mountains and glaciers near Mount Ogden, about 40 miles directly east of Juneau.

Southeast Alaska made headlines in some blogs and online news sites recently, with people connecting them to the area’s recent heat wave. But they aren’t anything new — scientists have been studying them for a long time.

Wright Glacier
Photo taken 1905 of Wright Glacier, looking southeast. The start of the glacier and Mt. Ogden on the U.S.-Canada border are likely out of view on the left side of the picture. (Nelles, D.H.. 1905. Wright Glacier: From the Glacier Photograph Collection. Boulder, Colorado USA: National Snow and Ice Data)

Seismologist Natalia Ruppert of the Alaska Earthquake Center said there were 360 ice quakes in the area last year, including some significant shakers.

“They were up to magnitude three. And magnitude three is a quite significant signal, that some of those ice quakes were reported to be felt in Juneau,” Ruppert said. “So those were quite large.”

This year the ice quakes started in May, with a big spike in activity starting four days before late June’s heatwave in Southeast Alaska. Ruppert said there have already been a hundred ice quakes recorded so far in 2021.

Last year, glaciologist Jason Amundson of the University of Alaska Southeast actually flew out to investigate one of the bigger ice quakes near Wright Glacier and Mount Ogden on the United States-Canada border.

“The glaciers in that area are pretty small,” Amundson said. “To produce an earthquake like that, that could be detected regionally, you would need to have a pretty big event.”

AEC 070821
Dots show location of ice quakes, largely of magnitude 2.9 or lower, that were detected in two weeks preceding July 8, 2021. Far right cluster of yellow dots are located at the start of Wright Glacier. Mt. Ogden is on the northeast side of the glacier just north of the blue dot. (Screen capture from Alaska Earthquake Center website)

For those of you who might be new to ice quakes or glaciology, let’s get a few things out of the way first.

Scientists can’t see or feel the quakes themselves, so they use seismometers. That’s the same instrument they use to measure and locate earthquakes.

“Ice is always moving. Ice is always deforming and cracking. And every time it moves, deforms or cracks, it creates some energy that propagates in the form of seismic waves,” Ruppert said. “And our seismic sensors are able to record that energy.”

Ice quakes actually may be caused by many different things, like a glacier scraping against the bedrock or a crevasse opening up.

In Greenland and Antarctica, giant icebergs slowly breaking away from tidewater glaciers create vibrations that can be detected around the world.

In Antarctica, scientists detected seismic activity which revealed how a giant river of ice called the Whillans Ice Stream lurches ahead into the ocean every 12 hours, as the tide rises and falls.

So how do scientists know the difference between an earthquake and an ice quake when they’re looking at the data? It turns out, that data looks and even sounds different.

When a big event occurred last year east of Juneau, Ruppert noticed that it didn’t resemble a typical earthquake.

You know those squiggly and jumpy lines on a seismogram?

If you were to convert those squiggles to a sound file, the magnitude 9 earthquake in Japan in 2011 would sound like this:

Ice quakes, like this calving iceberg in Greenland observed by Amundson and sped up 25 times, don’t have a lot of high-frequencies. And they start more gradually.

Glacial seismologist Rick Aster of Colorado State University said they’ve actually known about ice quakes around Juneau, Wright Glacier and Mount Ogden for decades.

As the glacial ice flows down the valleys, Aster said friction and obstructions can slow it down. But then temperatures rise every spring and summer, and snowmelt and rainfall increases.

“The seasonal influx of water into the glacier can make it more likely to slip,” Aster said. “And in this case, the annual cycle of melting water and water getting under the glacier drives the seasonality to the occurrence of these ice quakes as the glacier slips at its base.”

Aster said there are even practical applications for seismometers to detect ice quakes created by glacial outburst floods.

“This is happening in Iceland now, for example, where they have enormous glacier floods and the seismometers can detect when the water is coming out of the ice cap before it appears on the surface,” Aster said.

That was actually done very close to Juneau’s Mendenhall Glacier a few years ago. A temporary seismometer was able to detect the ground shaking during a jökulhlaup — a glacial outburst flood — because of the force of all that water gushing through cracks in the glacier.

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