We're used to thinking of earthquakes as tremors that happen along fault lines when tectonic plates shift. They're dramatic events: a good-sized earthquake can bring buildings and bridges toppling down.
Those destructive energy bursts are high frequency ground vibrations, and that's the range of vibrations seismologists most actively study. Just recently, though, a geophysicist at Harvard University named Göran Ekström studied low-frequency vibrations instead, and discovered something interesting.
He found over a hundred quakes have happened in the last decade that aren't along any fault line. What are they near instead? Ice.
A glacier can be thought of as a very slow-moving stream. It's made of solid ice, but over long periods of time it can flow gradually over the land, sometimes carving deep trenches and bulldozing up rock and soil. Just because it takes centuries to rearrange things, though, doesn't mean a glacier isn't enormously powerful at any given moment.
By carefully measuring seismic signals, Ekström was able to spot numerous low-frequency quakes in Greenland that hadn't been identified. He also found that they mostly occurred during July, August, and September. Those warmer months melt lots of glacial ice.
Ekström suspects that liquid water is pooling underneath the glacial ice to the point where the thin veneer of water at the base of the glacier allows the whole mass to slip a little, the way your foot can skid on a slick road.
The quakes he recorded, he believes, are the vibrations set up by all that ice--in one case, six cubic miles of it--skidding as much as forty-two feet in under a minute. When that much ice scrapes, the earth itself shakes.