Thousands of tiny quakes shake Antarctic ice at night

Washington: Scientists say they have recorded thousands of tiny 'ice quakes' in Antarctica at night, a phenomenon that may help track glacier melting and explain the breaking of large ice shelves.

Scientists at the University of Chicago in the US placed seismometers on the McMurdo Ice Shelf and recorded the 'ice quakes' that appear to be caused by pools of partially melted ice expanding and freezing at night. "In these areas we would record tens, hundreds, up to thousands of these per night," said Douglas MacAyeal, a professor at the University of Chicago. "It's possible that seismometers may be a practical way for us to remotely monitor glacier melting," MacAyeal said in a statement. Climate change is causing the Antarctic to melt, but glaciologists are still mapping how, where and why. 

There is much we still do not understand about the process -- as evidenced by the massive Larsen B ice shelf collapse in 2002, which took glaciologists by surprise -- and understanding these mechanisms is key to predicting the future for the ice. MacAyeal and the team were interested in the role of "quakes" on the floating ice shelves. However, they wondered how often the phenomenon was occurring in ice in the Antarctic, and what role it might play in the melting and breakup of ice. 

The team set up seismometers for 60 days during the melt season in two locations near seasonal meltwater lakes on the McMurdo Ice Shelf. One was drier; the other was slushier, with pools of melted water forming and refreezing. The wetter location, they found, was alive with seismic activity at night. "In these ponds, there is often a layer of ice on top of melted water below, like you see with a lake that is only frozen on top," MacAyeal said. 

"As the temperature cools at night, the ice on the top contracts, and the water below expands as it undergoes freezing. This warps the top lid, until it finally breaks with a snap," he said. The energy vibrates out into the surroundings, where it is picked up by seismometers. Some of the cracks re-heal, but some do not, MacAyeal said. It may explain why icebergs actually break off more frequently during colder times of the year. "Perhaps this is happening at longer, slower scales," he said.