By Chris Mooney
25 June 2015
(Washington Post) – If Greenland goes, it is becoming clear that it won’t go quietly.
Scientists have already documented entire meltwater lakes vanishing in a matter of hours atop the vast Greenland ice sheet, as huge crevasses open beneath them. And now, they’ve cast light on the mechanisms behind another dramatic geophysical effect brought on by the rumbling and melting of this mass of often mile-thick ice: earthquakes.
In a new paper [pdf] in the journal Science, a team of researchers from Swansea University in the UK, the Lamont-Doherty Earth Observatory at Columbia University, and several other institutions explain how the loss of Greenland’s ice can generate glacial earthquakes. In brief: When vast icebergs break off at the end of tidal glaciers, they tumble in the water and jam the glaciers themselves backwards. The result is a seismic event detectable across the Earth.
“These are all around magnitude 4.6 to 5.2, they’re all pretty close to magnitude 5,” says Meredith Nettles of the Lamont-Doherty Earth Observatory at Columbia University, a co-author of the study. “Which is a pretty big earthquake.”
Granted, these earthquakes aren’t caused by faults – they’re caused by massive movements of ice and how those impact the ground beneath. Compared with the early 1990s, Nettles says, scientists are now measuring seven times as many of these glacial earthquakes coming from Greenland — the rate has shot up as the ice sheet has begun to lose more mass from the calving of icebergs at the front end of glaciers.
To understand the dynamics behind how these glacial earthquakes are happening, the researchers put GPS instruments atop Greenland’s fast moving Helheim Glacier, which is located in the southeast part of Greenland, across the Denmark strait from Iceland. They also monitored the glacier’s calving front, where it meets the water, by camera, and used global seismic data to track earthquake occurrences.
To get a better sense of what they discovered, you first have to wrap your mind around how big these calving icebergs actually are. The amount of ice mass that breaks off in large iceberg calvings from Helheim Glacier, explains Nettles, is around a gigaton, or a billion metric tons. “If you took the whole National Mall, and covered it up with ice, to a height about four times as high as the [Washington] monument,” says Nettles, you’d have about a gigaton of ice. “All the way down from the Capitol steps to the Lincoln Memorial.”
Measured in space rather than mass, a big iceberg breaking off Helheim can be 4 kilometers in length — or over two miles. So maybe it is no surprise that a body this large can shake the Earth when it moves — and especially when it throws its weight against another solid object, as occurs during iceberg calving. [more]
ABSTRACT: Nearly half of Greenland’s mass loss occurs through iceberg calving, but the physical mechanisms operating during calving are poorly known and in situ observations are sparse. We show that calving at Greenland’s Helheim Glacier causes a minutes-long reversal of the glacier’s horizontal flow and a downward deflection of its terminus. The reverse motion results from the horizontal force caused by iceberg capsize and acceleration away from the glacier front. The downward motion results from a hydrodynamic pressure drop behind the capsizing berg, which also causes an upward force on the solid Earth. These forces are the source of glacial earthquakes, globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in Greenland and Antarctica.