THE title of glacial seismologist is, relatively speaking, a new one in science. It took until the early 1950s for scientists at Baffin Island in Canada to report “icequakes”, the sudden movement of ice and frozen, saturated earth. The field has grown considerably as both interest and instrumentation have progressed, and scientists are now accustomed to listening intently to the groaning and creaking of ice sheets in frozen regions around the globe. But the pursuit has been considered inherently separate from seismology as we normally think of it; the slippage of ice sheets and that of slabs of the Earth’s crust arise from different causes, follow different dynamics and, for those who go to the trouble to listen, make different sounds. While a large earthquake can trigger tremors in distant, tectonically active regions, earthquakes and icequakes have been considered unconnected events. But this week saw the publication, in a paper in Nature Geoscience, of the first evidence linking the two.

Zhigang Peng, a seismologist at the Georgia Institute of Technology, in Atlanta, and colleagues noticed that glacial calving—the falling of large chunks of ice from the end of a glacier into the sea—can be triggered by earthquakes that originate thousands of kilometres away. The tsunamis created by such quakes tug on icesheets, inciting ruptures and fractures.

But Dr Peng wondered whether remote earthquakes could trigger icequakes farther inland in Antarctica. To be sure, seismic activity is not uncommon in Antarctica; ice formation, too, triggers icequakes of smaller intensity. But the team was looking for something more substantial. They chose to examine data around the time of the 2010 earthquake in Chile—among the strongest on record, and not too distant from Antarctica. They suspected that only so-called surface waves, which travel along the Earth’s surface rather than through its bulk, could trigger icequakes. Such waves travel at a known speed and have characteristic frequencies, so the team knew what to look for and when.





Seismic data from station AGO1, near the South Pole, shifted to audible frequencies. The first rumble represents waves from the Chile earthquake; later, higher-pitched ripples are icequakes (Courtesy Z. Peng)



On analysing the data from 42 Antarctic seismographs, they found 12 clear signals that marked the occurrence of icequakes within six hours of the Chilean quake, leaving little doubt as to the cause. Relatively speaking, the icequakes were minuscule. But as Kate Allstadt, an icequake specialist at the University of Washington, puts it, “things we think of as strong, like glaciers, can still react to them if the conditions are right". Dr Allstadt has found that the annual load of snow on Washington’s Mt Ranier is enough to trigger thousands of icequakes each year.

Understanding these delicate mechanisms as they play out in Antarctica is critical because the vast majority of the planet's ice is trapped there. The movement and eventual loss of glaciers at the Earth’s poles will have a tremendous global impact; the collapse of the Western Antarctic Ice Sheet, which has arguably already begun, will cause sea levels to rise by several metres (albeit a few hundred years later). The icequakes discovered by Dr Peng and his colleagues are of course too tiny to cause such a collapse, but they might quicken the pace. It is worth keeping watch.