The endlow creep of a glacier may seem uneventful, but now researchers have shown that when an iceberg breaks off into the ocean the glacier sometimes moves quickly backward and downward, causing the heretofore poorly understood phenomenon known as “glacial earthquakes”.
Scientists studied the Helheim Glacier in Greenland over a 55 day period in 2013. Recording 10 glacial earthquakes, some of which registered a magnitude of 5.0, they saw the glacier retreat by about 1 mile (1.5 kilometers) following the shaking events.
When a large chunk of ice would break off, or “calve,” from a glacier and drops like a giant cannonball into the cold water, the scientists discovered something. It could force the glacier not only to stop inching forward, but also to push it backward.
Co-author Meredith Nettles, a professor of earth science at Columbia University, said, talking to NPR:
“We’re talking about something that is a gigaton of ice. That’s sort of the size of an ice cube you would have if you filled up Central Park in New York City to the top of the Empire State Building.
Imagine that you could go and just push on the front of the glacier with your thumb, really hard, so hard that you could reverse the direction that the front of the glacier is moving, and then you let it go. And that backward and then forward motion is actually recorded in the GPS data from the front of the glacier.”
The glacier moves backward for a few minutes before springing forward again and moving as normal, Nettles said.
The research team also simulated a glacier in their lab, with a plastic iceberg in a tank of water. They fitted the front of the make-believe glacier with force and pressure sensors, to monitor it as the simulated iceberg tipped over and floated off.
“And what they see matches very closely what we see with the GPS data and the seismic data,” Nettle says. “That allows us to actually build a better model for how the earthquake source works.”
Reverse glacier motion during iceberg calving and the cause of glacial earthquakes
T. Murray, M. Nettles, N. Selmes, L. M. Cathles, J. C. Burton, T. D. James, S. Edwards, I. Martin, T. O’Farrell, R. Aspey, I. Rutt, and T. Baugé
Science, Published online 25 June 2015 [DOI:10.1126/science.aab0460]