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 alongâ€”until one was recently shown to pack a powerful one-two punch, generating seismic waves twice a day.
The seismic signals fromÂ Antarctica's 60-mile-wide Whillans Ice Stream are as strong as those of a magnitude-7Â 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.