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Antarctic scientists reported something of an earthshaking
discovery in the June 5 issue of the journal Nature.
Douglas Wiens, a professor of earth and planetary sciences
at Washington University in St. LouisExternal Non-U.S.
government site, and colleagues combined seismological
and GPS data to reveal that an ice stream in West Antarctica
releases two bursts of seismic waves every day, each one
equivalent to a magnitude 7 earthquake.
Although this study was restricted to a single ice stream,
the new findings document behavior that runs counter to
how scientists generally have perceived glacial motion,
according to Wiens.
“Glaciologists model the flow of glaciers using
the assumption that it’s basically a kind of creeping
… 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
Wiens, who led the research team.
Sridhar Anandakrishnan, one of the Penn State UniversityExternal
Non-U.S. government site investigators, added, “This
is a different mode of ice-stream movement that could
be really important for understanding how all ice streams
develop and evolve.”
Seismologists use the Richter magnitude scale to measure
the amount of seismic energy released by an earthquake.
An earthquake measured at between 7.0 and 7.9 on the scale
is considered “major.” For comparison, the earthquake
that struck central China and killed an estimated 70,000
people on May 12, 2008, reached at least 7.9 on the Richter
scale.
Using an array of 19 GPS sensors and seismic sensors,
deployed as part of the National Science Foundation-funded
TransAntarctic Mountains Seismic Experiment (TAMSEIS)External
Non-U.S. government site and the Global Seismic NetworkExternal
Non-U.S. government site, the researchers say they measured
what they call a “stick-slip” interaction on
the Whillans Ice Stream, essentially a giant glacier about
100 kilometers wide and nearly a kilometer thick. Seismographs
that were deployed between 2001-2003 about 800 kilometers
away from the ice stream first detected the signals.
The data show that the river of ice moves about a half-meter
within ten minutes, remains still for 12 hours, then moves
another half-meter. Each time it moves, it emits seismic
waves that are recorded at seismographs around Antarctica,
and even as far away as Australia, a distance of more
than 6,400 kilometers.
Seismic waves from “glacial earthquakes,” mainly
near Greenland, were originally reported in 2003, and
the numbers have been increasing in recent years, according
to the researchers. Some scientists think the seismic
waves occur as chunks of ice calve off a glacier and float
away, a very violent activity that could generate strong
seismic signals.
The new results show that at least some of the glacial
earthquakes are produced by this stick-slip phenomenon.
“At first we didn’t know where the waves were
coming from; but eventually we were able to narrow down
the source to the ice stream,” Wiens said.
“This stick-slip phenomenon may provide a clue about
what makes these ice streams move faster or slower,”
he added.
Part of the mystery may be located below the ice. Scientists
believe subglacial hydrology, the waterworks underneath
the ice sheets that remains largely unmapped and unknown,
may help lubricate the ice and speed glacial movement.
“A big puzzle is why this particular ice stream
shows this slip-stick behavior and others don’t,
and we don’t really understand why,” Wiens said.
“Our results show that the stick-slip motion originates
from a sticky region on the bed of the ice stream where
friction is higher, perhaps due to the absence of water.
Also, we know that this ice stream is slowing down. Perhaps
the sticky spot is responsible for both phenomena.”
Added Anandakrishnan, “What is apparent from these
results is that the conditions beneath the glaciers and
ice streams are critically important in the flow of those
masses of ice. Without a better understanding of the subglacial
environment, our ability to model and predict future behavior
of ice sheets and sea level will be hampered.”
Wiens said he plans to study seismic records of stick-slip
events going back several decades to see if there are
changes, and also to search for similar signals from other
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.”
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Antarctic
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