Seismologists find seismic signals from Antarctic ice stream

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Washington, June 5 : Seismologists have found seismic signals from a giant river of ice in Antarctica, each one equivalent to a magnitude seven earthquake. The signals have been found by Douglas A. Wiens from Washington University, and colleagues at Pennsylvania State University and Newcastle University in the United Kingdom.

They combined seismological and global positioning system (GPS) analyses to reveal the two bursts of seismic waves from an ice stream in Antarctica every day, each one equivalent to a magnitude seven earthquake.

The ice stream is essentially a giant glacier 60 miles wide and one-half mile thick.

The data shows that the river of ice moves about 18 inches within ten minutes, remains still for 12 hours, then moves another eighteen inches. Each time it moves, it gives off seismic waves that are recorded at seismographs all around Antarctica, and even as far away as Australia.

Seismic waves from what are loosely called "glacial earthquakes," mainly near Greenland, were originally reported in 2003, and the numbers have been increasing in recent years.

The new results show that at least some of the glacial earthquakes are produced by sudden sliding of large ice sheets.

GPS instruments placed directly on the ice stream can detect where slipping motion begins and where it stops.

Scientists describe the motion as "stick-slip", which is the classic motion of earthquakes, occurring when the area around a fault moves slowly but the fault is stuck, remaining stationary until the stress builds up and the fault finally slips.

The data shows that the slip always starts from the same spot on the bed of the ice stream, what glaciologists call a "sticky" spot, which has more friction than the surrounding part of the bed.

According to Wiens, by some measures, the seismic impact is equivalent to a very large earthquake, but it doesn't feel like it because the movement is much slower than a real earthquake.

"The data look an awful lot like an earthquake, but the slip lasts for 10 minutes, while on the other hand an earthquake of this size would last for just ten seconds," he said.

"I guess you could call it an earthquake at glacial speed. This is very strange behavior, and we need to understand more about it," he added.

Wiens said that it is important to understand the physics behind what is controlling the speed of the ice streams.

"We need to understand what controls the speed of the ice streams, because that will affect how fast the ice in Antarctica will go away and sea level will rise as global warming melts the West Antarctic Ice Sheet," he said.

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