Researchers Decode the Screams of Alaska's Redoubt Volcano Eruption

An analysis of Alaska's Redoubt Volcano eruption of 2009 has given scientists a unique insight into volcanoes' screams before eruptions.

These screams are called harmonic tremors that resemble the sound of musical instruments. These sound signals are , however, at frequencies lower than what humans can hear.

The new study of the Redoubt Volcano showed that its harmonic tremors were at a very high frequency and they stopped about six months before eruptions.

The Redoubt volcano erupted more than 20 times in a period of about two weeks in March, 2009, reports New Scientist. Local seismic stations near the volcano detected many tremors ranging from a magnitude of 0.5 to 1.5 just before the eruptions. The tremors then began appearing about 30 times per second followed by 30 seconds of complete silence before a final eruption.

"The frequency of this tremor is unusually high for a volcano, and it's not easily explained by many of the accepted theories," said Alicia Hotovec-Ellis,  University of Washington doctoral student in Earth and space sciences. 

How these earthquakes and harmonic vibrations occur before a volcano isn't clear yet. Some seismologists believe that the sound is created when the hot magma pushes against the tiny cracks in the earth.

Hotovec-Ellis and team believes that the earthquake along with harmonic vibrations at the Redoubt Volcano happened when the magma was forced through a narrow conduit in the volcano. The magma stuck to the walls of the conduit (the pipe that lets the magma reach the peak of the volcano) until there was enough pressure to move it up again. This sticking and slipping occurs many times with a higher amount of pressure giving rise to smaller quakes that occur so fast that they blend together "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 said in a news release. "After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes."

Hotovec-Ellis recent research has been published in the Journal of Volcanology and Geothermal Research. Another paper on the subject, of which she is a co-author, is published in the journal Nature Geoscience.

Seismologists analyzing the harmonic tremors of the Redoubt Volcano have created two recordings of the seismic activity which can be heard here.