Monitors get weird vibes from Antarctic

From Victoria, British Columbia, at a meeting of the Seismological Society of America

In August 2000, seismometers on islands in the South Pacific began picking up unusual signals coming from regions even farther south. During the next 5 months, 13 separate groups of pressure waves traveled through the ocean depths for thousands of miles before they smacked into the islands and were converted into detectable ground motions.

In most cases, the energy received by the instruments was concentrated at a few frequencies, says Emile A. Okal, a seismologist at Northwestern University in Evanston, Ill. That suggests the source of the vibrations had a distinct shape. Sometimes the islands’ shaking lasted no more than 2 minutes, but on other occasions it went on for hours. Scientists previously had detected similar vibrations from undersea volcanic activity, but these events displayed a puzzling characteristic: Unlike any volcano, the source of the pressure waves seemed to move across Antarctica’s Ross Sea, heading northwest at a rate of several kilometers per day.

Satellite observations of the Ross Sea during that time solved the puzzle handily.

The data enabled researchers to precisely pin the source of most of the events to a large, drifting iceberg dubbed B-15B. The space images show that this 135-kilometer-by-40-km chunk, a piece of a megaberg that calved from the Ross Ice Shelf a few months earlier (SN: 5/12/01, p. 298: Big Bergs Ahoy!), had been intermittently colliding with other large bergs as winds and currents propelled it across the ocean.

Exactly what caused the vibrations–which took place only occasionally during the iceberg’s drift–isn’t known, says Okal. Maybe bumping against other large bergs caused the pressure waves. Perhaps they resulted from the oscillation of water in a large crack or fissure in the iceberg, or maybe B-15B scraped along the ocean bottom or against an undersea mountain as it drifted northwest. The ocean floor in this area isn’t well mapped, Okal notes.

Whatever the phenomenon’s cause, he says, scientists should now recognize that icebergs can be a source of noise for seismometers and underwater acoustic sensors.

A worldwide network of such instruments plays an important role in monitoring the Comprehensive Nuclear Test-Ban Treaty (SN: 7/14/01, p. 25: The Silence of the Bams).