Tidal 'icequakes' are shaking Antarctica

Slow and cold - but very powerful. That's the quake that shakes the West Antarctic ice shelf twice a day.

The massive tremors, which according to some measurements are the equivalent of magnitude-7 earthquakes, are caused by the movement of ice over rough patches of rock.

In the past few years, researchers have noticed that glaciers around the world seem to produce seismic waves that can appear to observers like large earthquakes. The waves generated in Antarctica can be picked up as far away as Australia, but until now no-one has been able to determine their cause.

Some scientists have suggested that they could be the "birthing groans" of icebergs as they break off of ice shelves. But new research shows that in some cases at least they are in fact "icequakes" - massive shaking produced by streams of ice grinding over rough bedrock.

Daily shakes

Douglas Wiens of Washington University, St Louis, US, and colleagues took a closer look at the seismic waves produced in Antarctica.

Between 2001 and 2003, 43 seismographs were peppered across the continent and revealed twice daily seismic waves originating from the Whillans Ice Stream - a river of ice, some 600 metres thick.

"The data look an awful lot like an earthquake, but it doesn't feel like it because the movement is much slower than a real earthquake," says Wiens. "I guess you could call it an earthquake at glacial speed. This is very strange behaviour, and we need to understand more about it."

The team combined the seismic measurements with GPS measurements taken at 19 different stations on the ice stream. This allowed them to monitor in detail the movement of the ice stream as it slid towards the ocean.

Their measurements confirmed what had previously been suggested - that the seismic activity happened at the same time as the high tides. They also showed that it was caused by a sudden downstream slip of the ice. Every day, twice a day, the ice accelerates forward by between 20 and 50 centimetres over about 20 minutes. ...

•	Tidal 'icequakes' are shaking Antarctica Environment New Scientist (UK), Wednesday, June 4, 2008 Byline: Catherine Brahic, NewScientist.com news service