August 24, 2012

Ice Sheet Quakes Help Explain Ice Movement And Earth Quakes

Brett Smith for — Your Universe Online

Glaciologists from Pennsylvania State University and Washington University said their recent study on the David Glacier that appeared in Nature Geoscience not only provides a window into the behavior of glaciers–it also could be used as a simple model for the mechanism behind slip-stick earthquakes like those that occur in Haiti or along the San Andreas Fault.

From 2002 to 2003, the research team recorded over 20,000 seismic events on the glacier in eastern Antarctica that took place about every 25 minutes.

"No one has ever seen anything with such regularity," said Lucas Zoet, a recent Penn State PhD. recipient. "An earthquake every 25 minutes for a year."

Based on these recordings, they were able to notice “a remarkable similarity of the waveforms “¦ (that) indicates that they share the same source location and source mechanisms," according to the team´s report.

The source of these events was an asperity, or mound of earth measuring a half-mile in diameter that juts up and into the glacier, the report said. This asperity “repeatedly ruptures in response to steady loading from the overlying ice,” however the force driving the ice and its resulting seismic repercussions is counterbalanced “by stress from the tide at the glacier front."

The hill lying underneath the glacier caused a slip-stick phenomenon that is comparable to events that unfold along the San Andreas fault. Ice buildup pushes the glacier along the Antarctic bedrock until it becomes stuck at the asperity, much like two landmasses slide alongside each other until they hit a snag. The two objects remain trapped in place until a seismic event knocks them loose, resulting in the radiation of shock waves throughout both bodies that emanates from the point of the snag or asperity.

The researchers also found another force acting on the David Glacier and influencing its behavior. As the glacier grows, it begins to jut out over the ice and as it does–the tide of the Southern Ocean starts to have an effect on the glacier´s movement.

"When the tide comes in it pushes back on the ice, making the time between slips slightly longer," said co-author Sridhar Anandakrishnan, professor of geosciences at Penn State.

"When the tide goes out, the time between slips decreases,” he added.

The effects of the tide most prominent at the ground line, researchers said. Since this is quite a distance from the location of the asperity, the effects that shorten or lengthen the stick slip cycle are delayed.

"This was something we didn't expect to see," said co-author Richard B. Alley, Evan Pugh Professor of Geosciences. "Seeing it is making us reevaluate the basics."

There has been a temporary break in seismic activity since 2003, but that activity has resumed. Unfortunately, the Penn State and Washington University team is unable to properly record these events because the instruments they used in 2002 are no longer around.

"The glacier is experiencing earthquakes again, although at a different rate,” Zoet said. “It would be nice to study that.”