of 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,
according to Penn State geoscientists.
one has ever seen anything with such regularity,” said Lucas K. Zoet,
recent Penn State Ph. D. recipient, now a postdoctoral fellow at Iowa
State University. “An earthquake every 25 minutes for a year.”
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. They found that the local earthquakes on the
David Glacier, about 20,000 identified, were predominantly the same and
occurred every 25 minutes give or take five minutes.
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
“Our leading idea is that part of the bedrock is poking through the ductile tilllayer beneath the glacier,” said Zoet.
The researchers have determined that the asperity?or hill?is about a halfmile in diameter.
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.
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
the tide comes in it pushes back on the ice, making the time between
slips slightly longer,” said Sridhar Anandakrishnan, professor of
geoscience. “When the tide goes out, the time between slips decreases.”
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 orlengthen the stick slip cycle are delayed.
was something we didn’t expect to see,” said Richard B. Alley, Evan
Pugh Professor of Geosciences. “Seeing it is making us reevaluate the
also noted that these glacial earthquakes, besides helping
glaciologists understand the way ice moves, can provide a simple model
for the stick slipearthquakes that occur between landmasses.
have not completely explained how ice sheets flow unless we can
reproduce this effect,” said Alley. “We can use this as a probe and look
into the physics so we better understand how glaciers move.”
2002, this area of the David glacier flowed smoothly, but then for
nearly ayear the 20-minute earthquake intervals occurred and then
stopped. Something occurred at the base of the ice to start and then
stop these earthquakes.
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,”
said Zoet. “The glacier is experiencing earthquakes again, although at a
different rate. It would be nice to study that.”
the seismographic instruments that were on the glacier in 2002 no
longer exist, and information is coming from only one source at the
The National Science Foundation supported this work.
Source: Penn State University