When large chunks of ice break off of a glacier and plop with a giant splash into the chilly water, the result can be lots of thunderous shaking. These mysterious glacial quakes have increased seven-fold in Greenland in the past two decades, according to new research.
Now, scientists think they've figured out the cause of the rumbling phenomenon, at least in Greenland.
Scientists monitored the Helheim Glacier, a major outlet of the Greenland Ice Sheet, over 55 days from July to September 2013. They recorded 10 glacial earthquakes, some of which registered a magnitude of 5.0, and saw the glacier retreat by about 1 mile (1.5 kilometers) following the shaking events.
The scientists discovered that, when a big chunk of ice splits, or "calves," from a massive glacier and tips forward into the ocean, it could force the glacier not only to stop inching forward, but also to push it backward. The backward movement and the subsequent change in water pressure cause glacial earthquakes, which can trigger massive tsunami waves and thunderous rumbling. [See Photos of Greenland's Gorgeous Glaciers]
"It's like taking a really strong spring, pushing on the front of it and just making it compress," said study co-author Meredith Nettles, a professor of earth science at Columbia University's Lamont-Doherty Earth Observatory in New York City. The glacier moves backward for a few minutes before springing forward again and moving as normal, Nettles said.
Glaciers typically move about 95 to 100 feet (about 30 meters) per day (or about 0.35 millimeters per second), but when an iceberg calves off and causes an earthquake, the force can turn the glacier completely around and force the front edge to move in the opposite direction at a rate of 130 feet (40 meters) per day — about 0.46 mm per second — for a few minutes, Nettles said.
As a recently calved iceberg begins to topple over into the ocean, it displaces a lot of water, Nettles said. Simultaneously, water rushes in to fill the space between the iceberg and the remaining glacier. That water motion causes a low-pressure zone behind the iceberg (the one that just plunged into the water) strong enough to suck water up from the ocean floor. The upward force on the Earth and the force of the falling iceberg produce a measurable seismic wave, Nettles explained.
As the climate warms, such icy quakes will increase in frequency because calving rates rise when water temperatures and air temperatures rise, and they change depending on how fast the glacier is flowing, the scientists said.
Glacial earthquakes induced by calving occur seven times more frequently than they did in the early 1990s, according to Nettles. "Calving is such an important component of the mass loss in both Greenland and Antarctica — it's really important to try and understand how calving actually works," Nettles said. This fast pace "is very human in its timescale," she said, linking it to anthropogenic climate change.
Calved icebergs often weigh around 1 gigaton (1 billion tons) and hold enough water to fill Central Park up to the Empire State Building, Nettles said. "The mass loss of ice from Greenland is quite large," she continued. "It's something like 300 to 400 gigatons of ice per year." The size of the iceberg appears to determine the magnitude of the earthquake.
"The difficult thing about Greenland is, it's so important for sea level rise because [compared to other countries with massive ice sheets] it's quite far south," said study co-author Timothy James, a professor of geography at Swansea University in the United Kingdom. Watching a glacial earthquake unfold back in 2010 for a prior mission to study glacial earthquakes "was a really lucky experience," he said. "Every once in a while, you'd hear a crack and a bang," he added, "but by the time the sound actually got to you, you turned and didn't really see anything."
"We found that we were actually having to sit there very carefully, looking at it and going, 'Do you see anything moving? I think the front's getting higher.' It was just all kind of quite slow to look at, but the noise was absolutely chaotic. I think that was the most surprising thing," James said.
The researchers detailed their findings today (June 25) in the journal ScienceXpress.