Dynamic glaciers identified as the cause of summer glacial earthquakes in Southeast Alaska

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Jason Amundson took this photo of Speel Glacier, located about 6 miles south of Wright Glacier and Mt. Ogden, while investigating the source of the 2020 ice quakes (Image via Jason Amundson/UAS)

Within the range of glaciers and mountains near Juneau, there is seismic activity nearly every day in summer. They are called ice quakes. They are not as widely understood as earthquakes, but researchers are watching them closely.

https://media.ktoo.org/wp-content/uploads/2021/07/09icequake.mp3

The sound of the whine or rumble is accelerated about three times. This is a large iceberg in Antarctica that is either drifting off the ocean floor or another iceberg.

https://media.ktoo.org/wp-content/uploads/2021/07/HarmonicTremor2006_215_09_20excerpt.mp3

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

Southeast Alaska has made headlines on some blogs and online news sites recently, with people linking it to the region’s recent heat wave. But it is nothing new – scientists have studied it for a long time.

The photograph was taken in 1905 of the Wright Glacier, looking southeast. The beginning of the glacier and Mt. Ogden’s location on the US-Canada border is likely out of sight on the left side of the image. (Nelles, D. H. 1905. Wright Glacier: From the Glacier Photography Collection. Boulder, Colorado USA: National Snow and Ice Data)

Last year, seismologist Natalia Robert of the Alaska Seismological Center said the area experienced 360 ice tremors, including some ice tremors.

“They’re at three degrees. And the third force is a very important indication, that some of those ice quakes were felt in Juneau,” Robert said. “So that was too big.”

This year’s ice tremors began in May, with a significant spike in activity starting four days before a heat wave in late June in Southeast Alaska. Robert said that a hundred ice quakes have already been recorded so far in 2021.

Last year, glaciologist Jason Amundson of Southeast Alaska University investigated one of the largest glacial earthquakes near the Wright Glacier and Mount Ogden on the US-Canada border.

“The glaciers in that area are very small,” Amundson said. “To produce an earthquake like this, that can be detected regionally, you would need a very large event.”

The points show the location of glacial earthquakes, of magnitude 2.9 or less, that were detected in the two weeks prior to July 8, 2021. The far right set of yellow points is at the start of the Wright Glacier. Mountain. Ogden is on the northeast side of the glacier north of the Blue Point. (Screenshot from the Alaska Earthquake Center website)

For those of you who may be new to glacial earthquakes or glaciology, let’s get a few things out of the way first.

Scientists cannot see or feel earthquakes themselves, so they use seismographs. This is the same tool they use to measure and locate earthquakes.

“Ice is always moving. Ice is always warping and cracking. Every time it moves, deforms or cracks, it produces some energy that is spread out as seismic waves.” “And our seismic sensors are able to record this energy.”

Ice quakes may actually be caused by many different things, such as glaciers eroding bedrock or a fault opening.

In Greenland and Antarctica, giant icebergs are slowly moving away from tidal water glaciers and creating vibrations that can be detected all over the world.

In Antarctica, scientists detected seismic activity that revealed how a giant glacier called the Whillans Glacier flows forward 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 look at the data? It turns out that this data looks different and even looks different.

When a major event occurred last year east of Juneau, Robert noticed that it looked nothing like a typical earthquake.

Do you know those zigzag, straight lines on a seismometer?

If you were to convert these statues into an audio file, the magnitude 9 earthquake in Japan in 2011 would look like this:

Ice quakes, like this Greenland iceberg observed by Amundson and accelerated 25 times, don’t have many high frequencies. They start gradually.

Glaciologist Rick Astor of Colorado State University said they have already known about the glaciers around Juneau, Glacier Wright 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 precipitation increase.

“The seasonal influx of water into the glacier can make it more prone to slipping,” Aster said. “And in this case, the annual cycle of melt and water under the glacier drives seasonality to these glacier earthquakes when the glacier slides at its base.”

Aster said there are practical applications for seismometers to detect glacial earthquakes caused by glacial floods.

“This is happening right now in Iceland, for example, where there are massive floods in glaciers, and seismometers can detect when the water came out of the ice sheet before it came to the surface,” Aster said.

This was actually done near the Mindenhall glacier in Juneau a few years ago. The makeshift seismometer was able to detect the shaking of the ground during an glacier flood – due to the force of all that water flowing through the cracks in the glacier.

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