Wide rivers of ice, called ice streams, flow through relatively slow-moving polar ice sheets, en route to the sea. Glaciologists had assumed that ice streams just creep steadily along—until one was recently shown to pack a powerful one-two punch, generating seismic waves twice a day.

The seismic signals from Antarctica's 60-mile-wide Whillans Ice Stream are as strong as those of a magnitude-7 earthquake, which could cause major damage in a developed area. But, whereas an earthquake of magnitude 7 might last 10 seconds, the Whillans signals continue for ten minutes or longer. They resemble earthquakes at glacial speed, says Douglas A. Wiens of Washington University in St. Louis.

[Because of the relatively long time over which the slip takes place, scientists standing right on the slipping ice stream feel nothing. In contrast, most rock earthquakes, which can take place in as little as a few seconds, are felt intensely by people in the area.]

Wiens, with three colleagues, discovered the signals after analyzing recordings from seismographs located 600 miles from the ice stream.

To pinpoint the signals' cause, they embedded GPS antennae on and near Whillans. It's the ice stream advancing abruptly, by about 18 inches, that causes the signals, the team discovered. In turn, the advances are caused by a combination of ocean tides—which lift and lower floating ice at the stream's outlet—and pressure from ice upstream. One of the daily slips is triggered by high tide, with a second following five to 12 hours later.

The findings were detailed in the journal Nature.