Study of Glacial Earthquakes Shakes Up Idea of How Ice Streams Move
News story originally written on June 5, 2008
New research shows that a 7,000-square-mile region of the Whillians Ice Stream in West Antarctica moves more than two feet twice every day in an earthquake-like pattern equal to a Magnitude 7 earthquake. Seismologists use the magnitude scale to describe the seismic energy released by an earthquake. An earthquake that measures between 7.0 and 7.9 on the scale is considered "major," and can cause serious damage over large areas of the world.
In an earthquake, stress builds between two plates on the Earth's crust as the energy of their movement accumulates. Finally, one plate or the other moves, causing shudders and jolts at the Earth's surface.
A similar movement was observed in the Whillians Ice Stream by the research team. Using GPS sensors and seismic sensors, the researchers say they have measured what they are calling a "stick-slip" interaction on the massive ice stream. The "stick-slip" pattern in the ice sheet is very different that the way scientists usually think of ice streams as flowing at a constant speed.
"Glaciologists model the flow of glaciers using the assumption that it's basically a kind of creeping kind of motion. But recently we've been seeing seismic signals coming from a number of ice streams and glaciers, and no one's been able to interpret them," said Douglas Wiens, a professor of Earth and planetary sciences at Washington University in St. Louis, who led the research team.
The "stick-slip" events on the Whillians Ice Stream occur twice a day and seem to be related to the daily tidal action of the Ross Sea. During each slip, a 96 by 193 kilometer (60 by 120 mile) region of the the ice stream moves as much as .67 meters (2.2 feet). The ice is 609 meters (almost 2000 feet) thick. The slip takes place over about 25 minutes, so scientists standing right on the slipping ice stream don't feel anything. In contrast, most rock earthquakes, which can take place in as little as a few seconds, are felt strongly by people in the area.
The new findings don't say if these ice movements have anything to do with global warming. But they are significant, because they add another piece to the mosaic of scientific understanding of ice dynamics.