Published on July 15th, 2013 | by James Ayre
Volcanoes 'Scream' At Ever-Increasing Pitches Until They Erupt, Research Finds
Some volcanoes ‘scream’ at ever-higher pitches until erupting — the ‘scream’ being the harmonic tremors that often accompany the earthquakes which typically precede volcanic eruptions, according to new research from the University of Washington and the USGS.
The new findings are the result of a new analysis of the March 2009 eruption of Alaska’s Redoubt Volcano — the new analysis found that “the harmonic tremor glided to substantially higher frequencies and then stopped abruptly just before six of the eruptions, five of them coming in succession.”
“The frequency of this tremor is unusually high for a volcano, and it’s not easily explained by many of the accepted theories,” stated Alicia Hotovec-Ellis, a University of Washington doctoral student in Earth and space sciences. “Documenting the activity gives clues to a volcano’s pressurization right before an explosion. That could help refine models and allow scientists to better understand what happens during eruptive cycles in volcanoes like Redoubt.”
The University of Washington press release continues:
The source of the earthquakes and harmonic tremor isn’t known precisely. Some volcanoes emit sound when magma — a mixture of molten rock, suspended solids and gas bubbles — resonates as it pushes up through thin cracks in Earth’s crust.
But Hotovec-Ellis believes in this case the earthquakes and harmonic tremor happen as magma is forced through a narrow conduit under great pressure into the heart of the mountain. The thick magma sticks to the rock surface inside the conduit until the pressure is enough to move it higher, where it sticks until the pressure moves it again.
Each of these sudden movements results in a small earthquake, ranging in magnitude from about 0.5 to 1.5, she said. As the pressure builds, the quakes get smaller and happen in such rapid succession that they blend into a continuous harmonic tremor.
“Because there’s less time between each earthquake, there’s not enough time to build up enough pressure for a bigger one,” Hotovec-Ellis stated. “After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes. We think the pause is when even the earthquakes can’t keep up anymore and the two sides of the fault slide smoothly against each other.”
The researchers documented the rising tremor frequency, “starting at about 1 hertz (or cycle per second) and gliding upward to about 30 hertz. In humans, the audible frequency range starts at about 20 hertz, but a person lying on the ground directly above the magma conduit might be able to hear the harmonic tremor when it reaches its highest point (it is not an activity she would advise, since the tremor is closely followed by an explosion).”
The highest-frequency harmonic tremor at Redoubt Volcano was dubbed ‘the screams’ because they reach “such a high pitch when compared with a 1-to-5 hertz starting point. Hotovec-Ellis created two recordings of the seismic activity. A 10-second recording covers about 10 minutes of seismic sound and harmonic tremor, sped up 60 times. A one-minute recording condenses about an hour of activity that includes more than 1,600 small earthquakes that preceded the first explosion with harmonic tremor.”
“Upward-gliding tremor immediately before a volcanic explosion also has been documented at the Arenal Volcano in Costa Rica and Soufrière Hills volcano on the Caribbean island of Montserrat.
Redoubt is unique in that it is much clearer that that is what’s going on,” Hotovec-Ellis said. “I think the next step is understanding why the stresses are so high.”
A paper on the new findings will be published in the Journal of Volcanology and Geothermal Research. A second paper will be published in Nature Geoscience.