Tag Archives: ice quake

This glacier produces half a million ice quakes a year

Somewhere in the Arctic, in the interior of the Greenland ice sheets, there lies a glacier like no other. This glacier quakes once every minute, more frequently than ever observed. Geologists now believe that studying these ice quakes could help them better understand how ice melts and reacts to rising temperatures and better model ice flow.

Researchers work in Greenland to instill seismic activity detectors, buried into glacier sediment.
Credit:Timothy Bartholomaus

It’s only natural that as temperatures continue to rise across the globe, ice starts to melt. But while we know that very well, we’re still struggling to quantify exactly how and when that is happening. As glaciers melt, they also slide and budge, and this glacier motion could help us understand the short-term factors that contribute to the larger scale motion of glaciers, according to Timothy Bartholomaus, a glaciologist at the University of Texas.

“To make better predictions of how ice flow is going to occur in the future, we have to have that mechanistic understanding of how ice flows now,” Bartholomaus said.

He and other researchers from the University of Texas Institute for Geophysics and the University of Kansas have detected over one million icequakes produced by a single Greenland glacier named Kangerlussuup Sermia, or KS for short. Their findings were reported at the fall meeting of the American Geophysics Union (AGU). They explain that while many glaciers quake, none has been observed to do so at this frequency.

“This is not something that has been seen at other glaciers,” Bartholomaus said. “Other glaciers produce seismic signals, but not nearly this frequently.”

KS itself spans over 100 kilometers from the interior of the Greenland Ice Sheet to the Western coast. In order to study it seismically, the team placed three soup-can sized seismometers close to it in 2013, and have been recording data ever since. They believe the seismic activity can be correlated somehow with the amount of water coming out of the glacier.

“A hundred years ago seismologists knew not to put seismometers by rivers because they’re noisy and make it hard to study earthquakes. But now scientists are turning that logic on its head and saying maybe we can learn more about rivers through seismometers,” says Bartholomaus in a previous report. “We can do the same thing with glaciers. And now, for the first time, track subglacial discharge by looking at tremor.”

The seismometer sensor is buried underground, with surface – supplied batteries, GPS, and solar panels to record timing of icequakes.
Credit:Timothy Bartholomaus

Of course, tectonic earthquakes are fundamentally different from icequakes in a number of aspects; tectonic quakes are generally much stronger, and don’t happen as often and as regularly as icequakes do.

“You can imagine that glaciers are flowing down under the influence of gravity, and there are various factors that are holding them back, so they don’t just zip along at Mach speeds,” he said.

Estimating the rate at which ice melts is vital for future predictions, especially as it’s not just about ice melting directly. Some basins are held in place only by small chunks of ice, just like wine in a bottle is held by a cork. So even slight warming and melting could cause significant sea level rise. This melting rate could be the difference between Miami being a sunny vacation destination or an urban swamp.

“We’ve known for decades the glaciers are melting and sea level is rising. But there is still uncertainty regarding what controls how fast it’s happening,” says Bartholomaus. “Is sea level rise going to remain constant or will the rate double? We’re trying to bat down that uncertainty.”

Chile earthquake triggered icequakes in Antarctica

JAMEY STUTZ, OHIO STATE UNIVERSITY

Chile is one of the most seismically active countries in the world. In 2010, it was struck by a powerful 8.8 earthquake which produced temblors throughout the entire country, as well as in Peru and Argentina. But a new study concluded that its effects were felt even further, in Antarctica, where several seismic stations recorded “icequakes,” probably due to fracturing of the ice as the planet’s crust shook.

It’s been documented for a while that big earthquakes can affect Antarctica’s ice sheets both directly, and through generated tsunamis. Tsunamis can propagate across very long distances, pushing and shoving big chunks of ice on the frozen continent. But seismic waves can also chip away at Antarctica’s ice sheet, and that mechanism is not yet entirely understood.  Zhigang Peng, a geophysicist at the Georgia Institute of Technology in Atlanta figured it out almost by accident, while he was studying the effects of the Chile earthquake in South America.

His team was searching for the effects of surface waves – Love and Rayleigh waves.

 

Even though they generate very different ground movement, both Love and Rayleigh wave often generate powerful microtemblors as they travel across the surface. So, while Peng was searching for more recordings of the earthquake, he also analyzed data from Antarctica stations, and he started to observe an interesting pattern.

“We started to find tiny seismic signals that we believe are associated with ice cracking.”

It’s the first time that ice cracks have been thoroughly analyzed following a remote earthquake; the first thing which geophysicists noticed was that only Rayleigh waves (ground roll waves) generate ice quakes. After studying seismic data at 42 Antarctic stations from within 6 hours of the Maule temblor, the team found that 12 of the stations registered “clear evidence” of Rayleigh waves generated by the Chile earthquake passing through the crust beneath the ice sheet, in the form of small icequakes. Because both type of waves generate significant ground movement, but only Rayleigh waves generate ice quakes, they suspect that ice quakes are fundamentally different from earthquakes.

The study suggests a “coupling with the ground that seems to be important,” says Jeremy Bassis, a geophysicist at the University of Michigan, Ann Arbor, who was not involved in the work, but was among the first to link tsunamis with ice shelf cracking.

“I think the big picture of this is that we keep on finding out that these relatively small environmental perturbations generated far away—the ice seems to actually feel them,” Bassis says. By the time they get to the ice sheet, the signals are tiny, but they still can cause the ice to break and change a little bit. “Ten years ago, I don’t think anybody would have thought that.”

Peng admits that his results, while interesting, don’t yet paint a clear picture of what is happening in Antarctica. It’s still not clear if this is a common phenomenon, or if the earthquake (among the biggest ones on record) had some very specific circumstances.

“At this point we cannot say definitively that large events play an important role in accelerating or changing ice behaviors there,” he says.