Big quakes can free grounded icebergs

From San Francisco, at a meeting of the American Geophysical Union

COOL GEAR. Instruments installed atop iceberg B15A enable scientists to track its movements. J. Landis/NSF

Sensors installed on an immense iceberg stuck in shallow water off Antarctica have relayed data suggesting that the ground motions spawned by large, distant earthquakes can set such grounded icebergs afloat again.

In mid-March 2000, an iceberg nearly the size of Connecticut broke away from Antarctica’s Ross ice shelf (SN: 4/1/00, p. 215: Available to subscribers at Titanic iceberg sets sail from Antarctica). After this 300-meter-thick behemoth—dubbed B15 by scientists—broke in two, winds and ocean currents drove the larger fragment, B15A, aground about 300 kilometers to the west of its birthplace (SN: 5/12/01, p. 298: Big Bergs Ahoy!).

Global positioning system (GPS) instruments placed on the ice mass soon after it became stuck show that it constantly jostles very slightly back and forth, probably as a result of the sloshing of ocean tides, says Emile A. Okal of Northwestern University in Evanston, Ill. But beginning on June 23, 2001, B15A skittered westward for several days before becoming stuck again about 5 km away and resuming its small, back-and-forth movements.

Okal and his colleagues suspect that the ground motions spreading from a magnitude 8.4 quake, which occurred off the coast of Peru on June 23, triggered B15A’s sudden jaunt down coast. The slow, rolling ground waves that spread along Earth’s surface from that temblor caused the seafloor beneath the grounded iceberg to oscillate up and down about 2 centimeters every 20 seconds or so.

That frequency happened to be roughly equivalent to the natural bobbing frequency of the iceberg—a characteristic that depends primarily on the thickness of the ice mass, Okal notes. Therefore, he says, it’s likely that the thump upward from each cycle of the temblor-induced ground motion reinforced the berg’s bobbing, resulting in a resonance that jolted B15A free of the seafloor.

Tsunamis spreading from the earthquake source off Peru probably had little to do with B15A’s 2001 excursion, says Okal. That’s because the GPS instruments indicate that the iceberg began moving down coast before such tsunamis would have arrived.

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