Ice harmonies

Vibrations originating from an iceberg were recorded seismographically at the Antarctic Neumayer Station by scientists of the Alfred Wegener Wegener Institute for Polar and Marine Research and ‘Fielax’, a private business. The recorded vibrations produce harmonic sounds with up to 30 overtones. However, the sounds are not audible to the human ear because of the tones’ low register. The data might facilitate a better understanding of the processes in volcanoes where vibration patterns are similar.

Results might help vulcanologists
The scientists are analysing the results of their measurements in a study just published in the scientific journal Science. Initially, volcanic activity was thought to cause the low frequency vibrations; so-called ‘tremors’. However, comparisons of seismic soundings revealed movement of the source of the vibrations. By means of satellite imagery, a giant iceberg covering an area of 30 by 50 kilometres, was identified as the cause.

The researchers suspect that water flowing within the iceberg’s system of crevasses and tunnels, is stimulating elastic vibrations, similar to those of an organ pipe. “Understanding these recordings that are so comparable to volcanic tremors might in turn also help volcanologists to explain the causes of volcanic tremors”, surmises Christian Müller from Fielax GmbH. “In contrast to complex volcanic systems, icebergs have a simpler structure.”

13 hours tremor
The most spectacular of a total of eleven events was recorded on July 22, 2000 and lasted for 16 hours. It was triggered by two brief earthquakes, which could be localised and were the result of a collision of an iceberg identified as B-09A with the continental slope. Subsequently, a two-hour sequence of seismic signals with highly variable frequencies was followed by an one-hour seismic pause. The subsequent harmonic tremor lasted 13 hours. The seismic sounds were caused either by continuing collisions of the iceberg scraping alongside the continental slope, or by incursions within the iceberg.

As early as1987, this particular iceberg had fractured from Ross Ice Shelf. On its way around Antarctica it had been beached twice for several years, before, in 2000, it drifted westward past the Alfred Wegener Institute’s Neumayer Station. In addition to the harmonic features of the tremors recorded from B-09A, their intensity was particularly notable. The vibrations were detected seismically over a distance of 800 kilometres and their strength is comparable to volcanic tremors by Mount St Helens, for instance, or by Hawaiian volcanoes.

The article „Singing icebergs“ will be published November 25 in “Science” (Vol. 310, issue 5752).

Your contact person is: Dr. Christian Müller.

Bremerhaven, November 24, 2005.

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