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.
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.”
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.