Several years ago Meredith Nettles, a seismologist from Colombia University, and two colleagues made a remarkable discovery: they identified a new kind of earthquake. These quakes were substantial – measuring magnitude five – but had been invisible because they did not show up on seismographs. (While orthodox tremors registered for a couple of seconds, these occurred rather more slowly, over a minute.)
The new earthquakes were traced almost exclusively to Greenland, where they were found to be specifically associated with large, fast-flowing outlet glaciers. There have been 200 of them in the last dozen years; in 2005 there were six times as many as in 1993.
Nettles nimbly explains the science as she heaves bags of equipment on to a helicopter, which will fly her to study Kangerdlugssuaq glacier. “It’s quite a dramatic increase, and that increase happened at the same time as we were seeing dramatic retreats in the location of the calving fronts of the glaciers, and an increase in their flow speed,” she says. “The earthquakes are very closely associated with large-scale ice loss events.”
In other words, the huge chunks of ice breaking off from the glaciers and entering the oceans are large enough to generate a seismic signal that is sent through the Earth. They are happening more regularly and, when they occur, it appears that the glacier speeds up even more.