Scientists have discovered massive, slow-motion "ice quakes" trembling twice a day through the West Antarctic Ice Sheet, an Alaska-sized swath of Antarctica. Detective work has traced the source of the shaking to the Whillans Ice Stream, a glacier 100 kilometers across and 1 kilometer thick, which flows from the ice sheet's interior.
It may seem strange that magnitude-7 quakes went unnoticed for so long--a temblor of similar size leveled entire towns and killed at least 15,000 in Turkey in 1999--but people standing on the Whillans Ice Stream never notice the shaking. "The reason that it doesn't rattle the whole continent is that it's a very slow event," says Sridhar Anandakrishnan, a glaciologist at Pennsylvania State University in State College, who made the discovery along with Douglas Wiens, a seismologist at Washington University in St. Louis, Missouri. Normal earthquakes release their energy over a few seconds, but the Whillans's shaking unfolds over 20 minutes.
Wiens first noticed the ice quakes 3 years ago as he analyzed data from 43 seismic sensors across Antarctica. Hoping for faint signals that would help him pinpoint previously unknown faults, he was instead greeted by booming vibrations from the same spot in West Antarctica. Wiens then looked at seismic data collected in 2004, the same year that Anandakrishnan had deployed 16 GPS sensors on the Whillans Ice Stream that tracked the ice's movement every 10 seconds. Most glaciers edge forward continuously, but the Whillans was already known to behave bizarrely: It sits still most of the time, then surges more than a half-meter forward twice per day as ocean tides lift and lower a slab of floating ice that extends from the end of the glacier onto the Ross Sea, just off the coast of Antarctica. By combining the GPS and seismic data, the researchers saw that the ice quakes and glacial surges synchronized perfectly in time, suggesting that the shaking was caused by the ice grinding over a rough spot in the rock below, they report 5 June in Nature.
Finding the causes of ice quakes--which also occur in Greenland--could lead to better understanding glacial movement and improved models of how glaciers will respond to climate change, says Robert Bindschadler, a glaciologist at NASA Goddard Space Flight Center in Greenbelt, Maryland, who first reported the twice-daily surges of the Whillans Ice Stream in 2003. "What has come from these discoveries is a realization that glaciers have other modes of behavior than we have thought of previously," says Göran Ekström, a seismologist with Columbia University.