April 18, 2008 -- The grinding together of vast Antarctic icebergs in the Ross Sea creates a growling subsonic song that seismologists compare to the ringing of a wine glass.
The newly discovered iceberg song has been detected traveling through the ground and oceans.
The discovery began when seismologists at ocean listening stations as far north as French Polynesia stumbled onto an unexplained, high-frequency tremor signal that lasted anywhere from two minutes to three hours.
"They range from harmonic to chaotic," said geophysicist Rick Aster of the New Mexico Institute of Mining and Technology. Aster spoke about the discovery Wednesday at the meeting of the Seismological Society of America in Santa Fe, New Mexico.
By combining the data with satellite images, the researchers quickly found the apparent source: a king-sized iceberg called C16.
If the iceberg was in fact making the sounds, the seismologists had three questions: 1) Is the entire iceberg vibrating to make the sound, like a violin? 2) Is the sound caused by fluids surging through cracks or tunnels in the iceberg, like air blown in a flute? 3) Is it something else entirely?
To find out, a team led by seismologist Emil Okal of Northwestern University undertook what he and colleagues called Project Southberg, to plant some seismometers on the iceberg and listen in.
The signals from the iceberg seismometers were much stronger, Aster said. Some had remarkably clear harmonics. Applying the same techniques used to locate earthquakes epicenters, the researchers located the spot where the sounds was being made -- right where the Jamaica-sized iceberg B15 was grinding against C16.
"We got very lucky," said Aster. "One station was within two kilometers (1.2 miles) of the source."
The clearer signals and the source location made it clear that the icebergs were being played like fiddles -- by rapid slipping and sticking in the collision zone where the icebergs were sawing back and forth.
"It's tens of thousands of small ice quakes," Aster said. That iceberg song, then, is similar to what a violin bow does on a string. The sound propagates through the water, from the water to the seafloor, and onward through the ground, Aster explained.
Okal later compared the vibrations to those of a finger "ringing" a wine glass -- also a slip and stick process -- and found them to be very similar.
"It's a spectacular signal," agreed seismologist Greg Berozaof Stanford University, who was among the audience for the presentation.
Related Links:
Larry O'Hanlon's blog: Earth Impacts