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

Earthquakes Rattling Glaciers, Boosting Sea Level Rise

An iceberg from the Helheim Glacier in calm waters, Sermilik fjord, East Greenland. ©  Mads & Trine
An iceberg from the Helheim Glacier in calm waters, Sermilik fjord, East Greenland.
© Mads & Trine

Talk of earthquakes likely calls to mind giant fissures opening up along the earth’s crust, the trembling of rock, buildings crumbling to their knees and, depending on your age and cast of mind, the love of Superman for Lois Lane. But it does not likely conjure up images of giant tongues of sliding ice or the splash of calving icebergs. And yet it should.

Most earthquakes are generated by the friction produced by two bodies of rock rapidly sliding past each other on a fault in the Earth’s crust, but a different breed of earthquakes was discovered in 2003: glacier earthquakes.

Map showing 252 glacial earthquakes in Greenland for the period 1993–2008, detected and located using the surface-wave detection algorithm. (b) Map showing the improved locations of 184 glacial earthquakes for the period 1993–2005 analyzed in detail by Tsai Ekström (2007). ©  Glacial Earthquakes in Greenland and Antarctica, Meredith Nettles and Göran Ekström, Lamont-Doherty Earth Observatory of Columbia University
Map showing 252 glacial earthquakes in Greenland for the period 1993–2008, detected and located using the surface-wave detection algorithm. (b) Map showing the improved locations of 184 glacial earthquakes for the period 1993–2005 analyzed in detail by Tsai Ekström (2007). © Glacial Earthquakes in Greenland and Antarctica, Meredith Nettles and Göran Ekström, Lamont-Doherty Earth Observatory of Columbia University

These newly documented earthquakes are occurring in glaciated areas of Alaska, Antarctica and Greenland and are caused by the dumping of giant icebergs–equal in size to, say, 400,000 Olympic swimming pools–into the sea. They produce seismic signals equivalent to those found in magnitude 5 earthquakes, which can be felt thousands of kilometers away. And there are many more of them today than there were just a couple of decades ago: six to eight times more than in the early 1990s have been recorded at outlet glaciers along the coast of Greenland.

This sudden surge in glacier earthquakes is expected to set off a series of events that will result in faster sea level rise over the coming century than had previously been estimated, according to research conducted there by Dr. Meredith Nettles, Associate Professor of Earth and Environmental Sciences at Columbia University, and some of her colleagues, as a part of Project SERMI. In 2013, the Intergovernmental Panel on Climate Change (IPCC) revised estimates for the next century dramatically upward (from 11-17 inches by 2100 to 10-39 inches) when taking Dr. Nettles and her colleagues’ earthquake research into account for the first time. This upward revision reflects the fact that the earthquakes change the internal dynamics of the glaciers, causing them to flow more rapidly, and to shed more ice into the ocean.

Monitoring station on Helheim glacier. © SERMI
Monitoring station on Helheim glacier. © SERMI

Nettles gave a talk on glacier earthquakes last November at the American Museum of Natural History. In the summer of 2006, she and 11 other scientists from six institutions in the U.S., Denmark and Spain traveled to a small town in East Greenland to take seismic, GPS and time-lapse photography measurements of the Helheim Glacier. They wanted to examine the location, dynamics and frequency of glacier earthquakes and to develop a method for using seismic data to map changes in the ice. They also wanted to learn how these earthquakes shape the behavior of outlet glaciers, which cluster around coastlines and deposit ice and meltwater into the oceans.

After setting up camp in town, the scientists flew a helicopter out to the glacier, drilled holes 6 feet deep in the ice, and drove 9-foot poles into those holes to anchor their GPS, time-lapse and seismic equipment. From the data they collected, they learned that short-term acceleration of glacier ice flows—up to 25% increases in velocity—coincided with the earthquakes. They also found that the increase in glacier earthquakes corresponded to net retreat of the ice front in Greenland. In particular, the section of the Greenland coast with earthquake-producing glaciers expanded northward. And whereas in the 1990s, a few glaciers were causing earthquakes; by 2005, those glaciers were associated with more frequent earthquakes, and other glaciers began to have seismic activity as well.

Map showing locations of GPS stations (blue and yellow dots). Arrows show average velocities over this time period. Red dots represent locations of rock-based GPS reference sites. Dashed lines show the location of the calving front at the beginning (eastern line) and end (western line) of the network operation period. Inset shows location of Helheim glacier in southeast Greenland (black arrow) and locations of glacial earthquakes (white dots). © Glacial Earthquakes in Greenland and Antarctica, Meredith Nettles and Göran Ekström, Lamont-Doherty Earth Observatory of Columbia University
Map showing locations of GPS stations (blue and yellow dots). Arrows show average velocities over this time period. Red dots represent locations of rock-based GPS reference sites. Dashed lines show the location of the calving front at the beginning (eastern line) and end (western line) of the network operation period. Inset shows location of Helheim glacier in southeast Greenland (black arrow) and locations of glacial earthquakes (white dots).
© Glacial Earthquakes in Greenland and Antarctica, Meredith Nettles and Göran Ekström, Lamont-Doherty Earth Observatory of Columbia University

Future research should focus on ice-ocean interactions that promote or reduce glacier calving, said Nettles. And scientists still need to better understand the specific mechanisms of loss of ice at the calving front and the effects of loss of ice on flow speeds. Nettles’ current research examines the impact of tides on glacier calving. Preliminary analysis of the data suggests that glacier earthquakes are more likely to occur at low tide.

Nettles and her colleagues collected most of their seismic data and GPS observations of the glacial earthquakes through facilities run jointly by IRIS (Incorporated Research Institutions for Seismology) and the USGS (U.S. Geological Survey). Thanks to grants from the USGS and the National Science Foundation, that data is open sourced and available to the public.