|August 13, 2014|
Chile earthquake triggered icequakes in Antarctica
|In 2010, a powerful magnitude-8.8 earthquake struck off the coast of central Chile, rocking much of the country and producing tremor as far away as Argentina and Peru. But a new study suggests its effects were felt even farther away---in Antarctica. In the wake of the Maule temblor, the scientists found, several seismic stations on the frozen continent registered "icequakes," probably due to fracturing of the ice as the planet's crust shook.|
Earthquakes are already known to affect Antarctica's ice shelves, thanks to the tsunamis they can spawn. Tsunami waves can propagate for great distances across the ocean; if the waves reach Antarctica's ice shelves---the floating platforms of ice surrounding the continent---they can push and pull on the ice, promoting fractures and ultimately helping large chunks of ice break off, or calve.
But whether earthquake seismic waves, traveling through the ground, can chip away at Antarctica's ice sheet---the ice piled on top of the continent---remained an unanswered question. Zhigang Peng, a geophysicist at the Georgia Institute of Technology in Atlanta, found the answer by accident while studying effects of the Chile quake in South America. His team was looking for surface waves---shallow seismic waves that travel along the planet's crust rather than going deeper into the mantle. Surface waves come in two basic types: Love waves, which shake the ground from side to side; and Rayleigh waves, which move in a rolling motion, compressing and expanding the ground as they travel. Both types of surface waves can in turn trigger numerous microearthquakes, called tremor.
Peng didn't initially intend to look at signals from Antarctic seismic stations, but data from a few somehow sneaked onto their research list. And when his team looked for the surface wave signals at those stations, "we found something very interesting," Peng says. "We started to find tiny seismic signals that we believe are associated with ice cracking."
Scientists had never seen seismic evidence that a far-away earthquake can register in Antarctica's ice---in part because they lacked data. Since the International Polar Year from 2007 to 2009, however, large-scale, international polar monitoring networks such as POLENET and AGAP have brought dozens of year-round GPS and seismic stations to the continent. Peng and his colleagues reached out to the principal investigators of those projects, requesting the data "to make our story complete."
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. The quake's Love waves, however, produced no effect in Antarctica, the researchers report online today in Nature Geoscience. (To hear what the triggered icequakes sound like at two seismic stations, visit here and here.)
Because both Rayleigh and Love waves produce tremor but only Rayleigh waves seemed to produce icequakes, "that makes us suspect that the mechanisms of icequakes might be different from regular earthquakes," Peng says. Probably, he says, the ice is more susceptible to changes in the ground's volume produced by the Rayleigh waves as they compress and expand the crust. As those waves roll by, the ice above closest to the surface cracks and fractures, producing new crevasses that register on the seismic stations as icequakes.
There was no clear geographic pattern to which stations registered icequakes. Most that did were in West Antarctica, closer to the Chile source, but many stations in West Antarctica didn't register any seismic disturbances. Peng suspects that icequakes can occur only in places where the ice sheet is already somewhat weakened.
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. Bassis and other researchers have linked tsunamis with ice shelf cracking, but Bassis says the new study is the first to show seismic waves affecting Antarctic ice on land. "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."
Whether that translates to any significant impact on the overall ice sheet, however, is unclear. The Maule earthquake was the sixth largest ever recorded, and it was relatively close to Antarctica. "This might be the perfect storm of an event, that hit the ice shelf like a gong," Bassis says.
Peng agrees that the overall impact of such earthquakes on the ice sheet isn't clear. "At this point we cannot say definitively that large events play an important role in accelerating or changing ice behaviors there," he says. He and his colleagues examined the HOWD seismic station, which had the strongest icequake signal, more closely to determine whether other large quakes, such as the 2011 Tohoku-Oki quake in Japan, also produced icequakes there. It seems that only the Chile event produced icequakes at HOWD, but the team is planning a more comprehensive exam of large events from over the past 4 to 5 years at all the Antarctic stations. They're also planning to combine the seismic work with GPS data from stations on the ice surface to determine whether ice movement is accelerated following large earthquakes, as was suggested by previous work at the Whillans Ice Stream in West Antarctica. "It's an ongoing study," Peng says. "We haven't finished the story yet."