Earth sometimes shivers beneath thick blankets of ice.

New analyses of old seismic data have unveiled a previously unrecognized type of earthquake--quakes created by brief surges of massive glaciers.

When fault zones slip, they emit most of their stored energy as high-frequency ground motions, so that's the type of vibrations that scientists typically monitor to detect earthquakes, says Goran Ekstrom, a geophysicist at Harvard University. However, by using a data-processing technique that considers only low-frequency ground vibrations, he and his colleagues have discerned dozens of quakes of about magnitude 5 that aren't associated with any known fault zone. Instead, Ekstrom notes, they originated beneath large glaciers.

Between 1993 and 2002, seismometers detected 120 subglacial quakes along the east and west coasts of Greenland, 6 in Antarctica, and 1 in Alaska. The Antarctic quakes don't show a seasonal pattern, but most of those in Greenland occurred during July, August, and September. That's when sunshine can melt large volumes of glacial ice, says Ekstrom. If some of that fluid makes its way beneath a glacier, it could lubricate the ice stream and enable it to lurch forward.

The subglacial quake in Alaska occurred in September 1999, and seismometer data suggest that the magnitude 5.0 event lasted 30 to 60 seconds. The energy released corresponds to 10 cubic kilometers of ice surging 13 meters, the researchers say.

Seismologists haven't focused on earthquakes' low-frequency ground motions because those aren't the ones that cause damage. Also, researchers haven't directly observed subglacial quakes because the regions where they occur are so remote, says Ekstrom.