Hydrodyamic Forces to Blame for Glacial Earthquakes?
By Calla Cofield
Icebergs form at Ilulissat glacier, Greenland.
Photo courtesy of Jason Amundson, Emory University
APS March Meeting, Denver — Massive icebergs that break off from their parent glaciers might be responsible for earthquakes detectable from thousands of miles away, sometimes reaching 5 on the Richter scale. How this happens could depend on the way the icebergs slosh around in seawater and release energy against the glacier wall.
At the March Meeting, Justin Burton presented results from his laboratory at Emory University, where he and his colleagues are looking at iceberg calving events. Burton added evidence to the case that hydrodynamics is a crucial part of how the icebergs are able to create significant seismic events in their parent glaciers.
In the early 2000s, Douglas MacAyeal at the University of Chicago and colleagues made the connection between glacial calving and earthquakes. Several studies have shown that the events are well correlated, and scientists want to use the seismic data to keep track of calving activity and track details about the icebergs, like their size. But to do that, they need to work out the mechanisms causing the seismic activity.
Icebergs that break off from glaciers are often (but not always) gravitationally unstable: They are tall and skinny and occasionally break off so that the taller side is pointing down into the ocean (imagine a book-shaped ice cube trying to float with its cover vertical). An iceberg with this arrangement will flip 90-degrees (so the cover of the “book” goes from vertical to horizontal).
In a particular fjord on the west coast of Greenland where this phenomenon has been extensively studied, the largest of these icebergs may be up to about one kilometer tall, two kilometers wide, and 500 meters thick as they break off from the glacier. According to work published in 2012 in the Journal of Geophysical Research: Earth Surface, Burton et al. show that the iceberg takes a few minutes to make the 90 degree adjustment, but in that time it will release the energy equivalent to two Hiroshima nuclear bombs, or 40 kilotons of TNT.
There are multiple forces at work when an iceberg forms. As the bottom half swings up, the top half exerts a contact force against the glacier. But Burton and his collaborators have shown that the contact force on the glacier is larger when hydrodynamic forces are included in the calculation. This is due to various effects of the water on the system. For example, the iceberg’s rotation away from the glacier creates suction between the two surfaces, creating a force in the opposite direction from the contact force.
Other scenarios may be involved. A group at Harvard University, led by James R. Rice, showed that it is possible to replicate the seismic signals without hydrodynamic forces, but it requires that the iceberg push off of other icebergs, which must lie directly in front of it.
Burton is now investigating how the sea waves created by this violent rotation may also contribute to seismic activity. In work he is preparing for publication, Burton says his laboratory experiments suggest that the period of the waves created by a calving iceberg could match the period of the seismic data. While the calving of a large iceberg may last a few minutes, seismic events often continue long after that. This longer time period could be explained by the waves.
Minor seismic activity can be detected as a result of waves pounding on shores, so the idea is not unfounded. In addition, the setting is unique: The glacier is a straight cliff face, stretching roughly a kilometer down into the ocean. Waves striking the vertical face could potentially transfer their energy directly into the glacier, as opposed to a horizontally inclined shore, where the wave would be broken up and the energy would be more dispersed.
“I guess it’s just really surprising if an ocean wave like this can produce seismic waves that are detected globally,” said Jason Amudson, a glaciologist at the University of Alaska Southeast and an occasional collaborator with Burton. “On the other hand, this is a wave hitting a vertical face, so it’s a different type of problem than seismologists usually think about.”
If Burton’s hypothesis proves correct, it would be a situation where water waves cause seismic waves. “Which is weird,” he says, “because it’s usually the other way around.”
The scientists studying iceberg calving are primarily glaciologists and seismologists. For that reason, a discussion of the hydrodynamics involved in this process has had to come from outside sources, such as physicists like Burton.
“Nobody knew how to think about [the hydrodynamics] and it really took these experiments to figure out that they were important,” said Burton in an interview. “In fact we didn’t know that they were important until we just couldn’t model the data without turbulent forces in the water. So most people would agree that they’re important.”
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