Thwaites Glacier in Antarctica is Now Causing Earthquakes

Thwaites Glacier is one of Antarctica’s largest contributors to sea level rise from Antarctica.  Its rate of loss has doubled in the past three decades, earning it the moniker “doomsday glacier.” Understanding why it’s retreating so quickly has been a challenge, but glaciologists have recently discovered that the glacier is now generating its own seismic activity when it calves (breaks off icebergs into the ocean), which could help in unlocking the physical keys to this process. The findings were published early this year in Geophysical Research Letters.  

Combing through seismograph readings collected in West Antarctica during a large calving event at Thwaites on February 8th 2014, a team of researchers found evidence of two low frequency earthquakes, each about 10-30 seconds long. Their hunch—that the quakes came from the calving—was confirmed when they matched the seismograph readings with satellite images taken on the same day. 

Thwaites Glacier
The rate of ice loss from Thwaites Glacier has doubled in the last thirty years. (Source: NASA)

They also discovered high frequency blips of seismic activity that chirped on and off in the week preceding the event. Glaciologist and lead author of the study, Paul Winberry, explained to GlacierHub that in these short bursts they were actually “hearing all these little cracks start to propagate.” It was the sound of countless cracks forming and popping apart, heralding the large break about to come. 

“Frequency” refers to the behavior of shockwaves that reverberate out from the source of the earthquake. Waves repeat their motion as they travel in a peak-valley-peak-valley pattern. Waves that do this rapidly are called high-frequency and those that do it slowly are called low frequency. High frequency waves are detectable over short distances; low frequency waves over long distances.       

Thwaites is the only known glacier in Antarctica to exhibit seismic behavior, whereas glaciers in Greenland have been recorded causing earthquakes for some time. This difference can be explained by the fact that the majority of Greenland’s icebergs capsize when they break off into the water. The result is a more boisterous form of calving that produces detectable earthquakes. Why Greenland’s icebergs capsize and Antarctica’s do not has to do with the physical makeup of each landmass’s ice sheets and where they start to float on the water.

Greenland glaciers flow down the island’s mountainous sides and break into icebergs when they hit the water. This behavior is common where a glacier’s terminus is close to where it starts to float—also known as the grounding line. Antarctic glaciers flow outwards horizontally, and continue on into the water as huge floating shelves that stretch miles out to sea. 

“Basically when [Greenland glaciers] start to go afloat, they form icebergs as opposed to Antarctica, where in most places they go afloat they don’t break off instantaneously but they form these big long ice shelves—floating extensions,” said Winberry. “It’s completely different.”

The other key component of capsizing is the physical shape. Greenland’s icebergs are top-heavy. “They’re taller than they are wide. They’re not stable, so when they break off they want to flip over,” said Winberry. 

Tim Bartholomaus, a glaciologist from the University of Idaho who has studied Greenland’s glaciers told GlacierHub that the capsizing icebergs bang into the front of the glacier as they’re flipping over and that generates the earthquake. “As they’re rotating en masse, they’re putting their shoulder against the back of the terminus and giving it an enormous push as they’re rotating.” 

Icebergs near the terminus of Thwaites Glacier. If it were to collapse it could raise global sea levels by ten feet. (Source: NASA)

These collisions don’t normally occur during calving in Antarctica because the ice sheets are far bigger, already floating on the water, and terminate far from the grounding line. “Those icebergs break off and form New England or Delaware-sized chunks. And when that happens they kind of slowly drift away,” said Winberry. That Thwaites is now generating detectable seismic earthquakes means one thing: its icebergs are likely capsizing because its terminus is now close to the grounding line. 

“The fact that Thwaites is now doing this slab capsize style of calving, that means that it is breaking off right at the point where the glacier is hitting the ocean,” said Bartholomaus. 

The capsize calving at Thwaites on February 8th 2014 sent low frequency waves traveling—and shaking—through the ice and land underneath for hundreds of miles. It generated enough energy to show up on seismometers over 900 miles away as a magnitude 3.0 earthquake.  

Over the last three decades, the Thwaites glacier has lost about 600 billion tons of ice. Some scientists fear that with an increased rate of 50 billion tons of ice lost a year in recent times, runaway instability of the glacier may already be underway. Total collapse of the glacier would raise global sea levels by 10 feet. Thwaites’ newfound seismic activity suggests that its retreat has now reached land. 

“It’s lost all of its floating ice,” Winberry told GlacierHub. “The floating extension has basically disappeared. So to understand the future retreat of the glacier, we need to understand this different style of calving behavior.” 

While that may be concerning, it also gives scientists a new tool for better understanding the process of calving at Thwaites. So far, glaciologists have relied heavily on satellite imagery for studying large scale calving events in Antarctica, but satellites usually only take one picture a day or every two days.  “A lot happens between those two days. In these calving events, the flipping of these icebergs and actual breaking apart can happen over minutes to hours,” said Winberry. Being able to “listen” to them unfold in near real time adds a whole new element. 

“That is going to help us unravel the physics of how these icebergs actually form, which is what we need to know to produce better predictions of future retreat of this glacier” said Winberry. 

Read More on GlacierHub:

Video of the Week: Animation Shows Frequency of Antarctic Calving Events

A Catastrophic Glacier Collapse and Mudflow in Salkantay, Peru

Roundup: A New Glacier Surge Study, Three Decades of Caucasus Glacier-Debris Change, and Mining Expansion in Greenland

Using Seismic Waves to Measure Ice Melt? Sounds Good

A recent study in the journal Science Advances proposes a novel methodology to track melting ice sheets and the glaciers associated with them: rather than viewing the ice from above with airplanes and satellites, a team from MIT and Princeton is monitoring it from below. The new technique makes it possible to gather information about ice melt in real time by listening to the seismic activity of the Earth’s crust. Due to the continuous sound of ocean waves crashing on Greenland’s shore, there are near-constant seismic vibrations in the bedrock that can reveal a great deal of information about the overlaying ice. This method, which was originally developed to track volcanic and fault line activity, may be able to provide more  accurate data on exactly where and when melting is occurring, the authors report.

The Incorporated Research Institutions for Seismology IRIS) install a seismic station in Southwestern Greenland. Photo provided by Dr. Chris Harig
The Incorporated Research Institutions for Seismology IRIS) install a seismic station in Southwestern Greenland. Photo provided by Dr. Chris Harig

How can seismic data communicate information about glaciers? The researchers predicted that the great mass of the ice weighing down on the rock below would compress the Earth’s crust and change its density—possibly enough to have a measurable effect on the seismic waves passing through it. By listening to the speed of the seismic waves moving through the ground, the team was able to determine the density of the rock and calculate the amount of ice lying above. According to the study, the speed of the seismic waves depends on the crust’s porosity, or the amount of small spaces and cracks that are not solid rock. When the crust is compressed by heavy ice masses, the area of open spaces in the rock decrease and waves travel more quickly through the material.

However, when ice melts and there is less weight on the bedrock, more spaces in the crust open up and the velocity of the seismic waves is significantly slower. This newly tested method shows immense promise, and incorporating seismic data from other Greenland stations is the next step.

We think if the seismic station density were increased we would be able to observe these changes in greater spatial detail, and be able to make a map of the changes instead of averaging them over a large region,” study author Dr. Chris Harig of Princeton University explained in an email to GlacierHub.

In addition to calculating the amount of ice melt, this new method may be able to pinpoint the location of the melting. While findings are preliminary, the study indicates that the seismic data from 2013 picked up differences in melting between the main Greenland ice sheet and the Jakobshavn Glacier, widely considered the fastest moving glacier in the world.

Satellite image of the Jakobshavn Glacier. NASA/USGS image courtesy of the Science Visualization Studio, at Goddard Space Flight Center
Satellite image of the Jakobshavn Glacier. NASA/USGS image courtesy of the Science Visualization Studio, at Goddard Space Flight Center

If you do look at the station pairs individually, the stations near Jakobshavn Glacier show a bit more signal in 2013 than the rest. This could be due to the fact that Jakobshavn is a place of massive changes, and still had large changes in 2013,” Harig commented in an email.

This outlet glacier has shown significant melting since the 2013 data was collected—in 2015, a massive 12.4 square kilometer area of ice calved into the ocean, possibly the largest calving event in the glacier’s history. If seismic activity can pick up on the different rates of melting between Greenland’s glaciers and the main ice sheet, it may be possible to predict which glaciers are most fragile and likely to have calving events.

While testing is needed at more seismic stations, Harig seems optimistic about the potential applications of the new method.

I was surprised how well the results turned out in the end. We are measuring very small changes in the seismic velocity to compare them to the ice sheet mass. So it attests to the high data quality from these stations, how well the processing techniques worked, and the very large signal we have from the Greenland ice sheet as it gains and losses ice,” he said.

The new technique may be able to fill the gap that remote monitoring methods cannot: measuring ice melt on small, short-term time scales. The researchers state that monitoring methods like NASA’S Gravity Recovery and Climate Experiment satellites (GRACE) have collected valuable data on the long-term changes in ice sheet and glacier changes, but the resolution is not high enough to pick up on shorter inter-seasonal shifts in ice melt. On a seasonal scale, seismic waves may be better equipped to measure the melting ice, and the method introduced the exciting possibility of measuring melt in real time— impossible with the current monitoring mechanisms used today. By combining the wide scope of satellite data with the precision of this new seismic methodology, sea level rise projections will be more accurate and allow the global community to better adapt to the impacts of a warming climate.