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Erupting volcano will roar at subsonic level


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POSTED: Sunday, May 24, 2009

An erupting volcano sounds like the roar of a big jet engine taking off but people can't hear it because it's at such a low frequency, says Milton Garces, University of Hawaii Infrasound Laboratory director.

To the human ear, it sounds like "thumps, like rhythm," he said.

Garces and David Fee, laboratory field system engineer and analyst, participated in a study of volcanic sounds led by Robin Matoza, graduate student at Scripps Institution of Oceanography in La Jolla, Calif. Garces funded and directs Matoza's research.

The scientists said the sounds produced by the volcanoes can indicate how much ash is in the eruption and how much of threat the ash could pose to planes.

They hope eventually to be able to give the Volcanic Ash Advisory Center in Washington D.C. detailed information, such as the size or flow rate of a volcanic jet, to put into ash-dispersal forecasting models, Matoza said.

"We hypothesized that these very large natural volcanic jets were making very low frequency jet noise," Matoza said in a news release.

Infrasonic sound is lower in frequency than 20 cycles per second, below the limit of human hearing. Matoza's team speeded up the sound to record volcanic noise from two highly active volcanoes — Mount St. Helens in Washington and Tungurahua in Ecuador.

"We're trying to use that as an acoustic fingerprint of large eruptions that might eject ashes into the atmosphere," Garces said in a telephone interview from the Big Island.

Findings were published by Matoza and co-author Michael Hedlin, director of Scripps' Laboratory for Atmospheric Acoustics, in the April 18 journal "Geophysical Research Letters."

Garces joined the Hawaii Institute of Geophysics and Planetology in 1997 after infrasound studies in Alaska and Japan and founded the Infrasound Laboratory in Kona in 1999. It was the first "listening" station in an international system to monitor clandestine nuclear tests. About 40 stations now are in the network, he said.

The Infrasound Laboratory has a station about four miles from Kilauea and another on the Kona side doing real-time acoustic monitoring of eruptions. "If and when Mauna Loa erupts we'll be able to pick that up very clearly, Garces said. He said the laboratory picked up three volcanoes that erupted the past year in Alaska.

It also has monitoring stations at Mount St. Helens, Panama, Ecuador and Southern Colombia.

The Infrasound researchers don't forecast eruptions, Garces said. "What we do is provide the most up-to-date information to people who make decisions and provide notification. If we had a station within 40 kilometers (about 24 miles) from a volcano, we could provide notification in five minutes."