Exploding with Sound

What does Mount St. Helens have in common with man-made aircraft?

Their engines — whether natural or man-made — rumble in a similar way, according to a new study led by researchers at Scripps Institution of Oceanography at UC San Diego. These new research findings offer scientists a more useful probe of the inner workings of volcanic eruptions.

The study, led by Robin Matoza, a graduate student at Scripps Oceanography, was published in the April 18 issue of the journal Geophysical Research Letters, a publication of the American Geophysical Union (AGU).  Matoza measured low-frequency sound from Mount St. Helens in Washington State and Tungurahua volcano in Ecuador, both highly active volcanoes close to large population centers.

“We hypothesized that these very large natural volcanic jets were making very low frequency jet noise,” said Matoza, who conducts research in the Scripps Laboratory for Atmospheric Acoustics.

The researchers speeded up the infrasound recordings – sound that is lower in frequency than the limit of human hearing – taken from two volcanoes and uncovered a noise very similar to typical jet engines.

Using an array of microbarometers – instruments similar to weather barometers but sensitive to smaller changes in atmospheric pressure — and low-frequency infrasonic microphones, the research team tested the hypothesis, revealing the physics of how the large-amplitude signals from eruptions are produced. Matoza and colleagues recorded these very large-amplitude infrasonic signals during the times when ash-laden gas was being ejected from the volcano. The study concluded that these large-scale volcanic jets are producing sound in a similar way to noise made by man-made jets, generated by the turbulent flow of air out of a jet engine.

“We can draw on this area of research to speed up our own study of volcanoes for both basic research interests, to provide a deeper understanding of eruptions, and for practical purposes, to determine which eruptions are likely ash-free and therefore less of a threat and which are loaded with ash,” said Michael Hedlin, director of the Scripps Laboratory for Atmospheric Acoustics and a co-author of the paper.

Large-amplitude infrasonic signals from volcanic eruptions are currently used in a prototype real-time warning system that informs the Volcanic Ash Advisory Center (VAAC) network operated by NOAA. Researchers hope this new information can improve hazard mitigation and inform pilots and the aviation industry.

“The more quantitative we can get about how the sound is produced, the more information we can provide to the VAAC,” said Matoza. “Eventually it could be possible to provide detailed information such as the size or flow rate of the volcanic jet to put into ash-dispersal forecasting models.”

“Clearly, such eruptions are off-limits to close scrutiny but listening to frequencies made during the eruption opens up possibilities for understanding flow velocities of ash and volcanic bombs being ejected from volcanoes as well as densities and pressures of the ejecta material itself," said David Hilton, a Scripps scientists studying the geochemistry of volcanoes throughout the world.

-- Annie Reisewitz