24 August 2012
A’ndrea Elyse Messer
Analysis of the small, repeating earthquakes in an Antarctic ice sheet may not only lead to an understanding of glacial movement, but may also shed light on stick slip earthquakes like those on the San Andreas fault or in Haiti, say the geoscientists.
“No one has ever seen anything with such regularity,” says Lucas K. Zoet, who recently earned his Ph.D. at Penn State and is now a postdoctoral fellow at Iowa State University. “An earthquake every 25 minutes for a year.”
The researchers looked at seismic activity recorded during the Transantarctic Mountains Seismic Experiment from 2002 to 2003 on the David Glacier in Antarctica, coupled with data from the Global Seismic Network station Vanda.
The researchers note in the current Nature Geoscience that, “The remarkable similarity of the waveforms … indicates that they share the same source location and source mechanisms.” They suggest that, “the same subglacial asperity repeatedly ruptures in response to steady loading from the overlying ice, which is modulated by stress from the tide at the glacier front.”
“Our leading idea is that part of the bedrock is poking through the ductile till layer beneath the glacier,” says Zoet.
The researchers have determined that the asperity—or hill—is about a half mile in diameter.
The glacier, passing over the hill, creates a stick slip situation much like that on the San Andreas fault. The ice sticks on the hill and stress gradually builds until the energy behind the obstruction is high enough to move the ice forward. The ice moves in a step-by-step manner rather than smoothly.
But motion toward the sea is not the only thing acting on the ice streaming from David Glacier. Like most glaciers near oceans, the edge of the ice floats out over the water and the floating ice is subject to the action of tides.
“When the tide comes in it pushes back on the ice, making the time between slips slightly longer,” says Sridhar Anandakrishnan, professor of geoscience. “When the tide goes out, the time between slips decreases.”
However, the researchers note that the tides are acting at the ground line, a long way from the location of the asperity and therefore the effects that shorten or lengthen the stick slip cycle are delayed.
“This was something we didn’t expect to see,” says Richard B. Alley, professor of geosciences. “Seeing it is making us reevaluate the basics.”
He also notes that these glacial earthquakes, besides helping glaciologists understand the way ice moves, can provide a simple model for the stick slip earthquakes that occur between landmasses.
“We have not completely explained how ice sheets flow unless we can reproduce this effect,” says Alley. “We can use this as a probe and look into the physics so we better understand how glaciers move.”
Before 2002, this area of the David Glacier flowed smoothly, but then for nearly a year the 20-minute earthquake intervals occurred and then stopped. Something occurred at the base of the ice to start and then stop these earthquakes.
“The best idea we have is that during those 300 days, a dirty patch of ice was in contact with the mount, changing the way stress was transferred,” says Zoet. “The glacier is experiencing earthquakes again, although at a different rate. It would be nice to study that.”
Unfortunately, the seismographic instruments that were on the glacier in 2002 no longer exist, and information is coming from only one source at the moment.
The National Science Foundation supported this work.