Slip, Slide, Shake

GPS, seismic data yield clues about surging Antarctic glacier

Analyses of data gathered by Global Positioning System equipment atop one of Antarctica’s largest and most dynamic glaciers and by seismometers nearby suggest that friction in just one small area beneath the broad glacier regularly halts the ice’s lurching march to the sea.

Many of Antarctica’s largest ice streams — the megaglaciers that carry most of the ice draining off the continent — move at an irregular pace, first sticking in place, then surging ahead (SN: 3/31/07, p. 202). For instance, twice each day — at times related to the cycle of tides at the Antarctic coast — a large portion of the Whillans Ice Stream surges forward about 70 centimeters over the course of 25 minutes, says Matt King, a geophysicist at Newcastle University in England.

That may not sound impressive. But the total seismic energy triggered as this 200-kilometer-long, 100-kilometer-wide section of the ice stream scrapes across the underlying material is about the same as that of a magnitude-7 earthquake. Seismometers almost 1,000 kilometers away can detect the resulting ground motions, he notes.

Previous studies have suggested but not proved that surging glaciers can produce earthquakes (SN: 1/3/04, p. 14), because GPS data to record the movements of those glaciers wasn’t available. In this case, however, an array of GPS instruments atop the Whillans Ice Stream, each collecting data about 10 times each second, complemented the information gathered by seismometers in the region, he and his colleagues report in the June 5 Nature.

Each time the ice stream surged, its seaward motion began near a broad, low hump near the middle of its channel. Flow lines frozen into the ice suggest that the hump lies over a patch on the glacier bed where friction is significantly higher than under surrounding areas of the ice stream, the researchers say. And analyses of ice-penetrating radar images hint that this patch contains little if any subglacial water, which would tend to lubricate the ice stream’s flow. Together, the data indicate that the ice stream’s stick-slip behavior is largely controlled by friction in this one spot, roughly 10 kilometers wide. Resistance there prevents the glacier from flowing downhill until tidal and gravitational forces overcome the friction, the researchers speculate.

Whillans Ice Stream “is arguably the most unusual, dynamic, large glacier on Earth,” says Ted Scambos, a glaciologist at the National Snow and IceDataCenter in Boulder, Colo. The team’s research, he adds, “is a fascinating study … that shows how complex the Antarctic Ice Sheet is, and how even now we can be amazed by the interplay of some of the Earth’s most basic systems: oceans, ice and sediments.”

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