A new analysis of an eruption from more than three years ago shows a harmonic noise at a frequency too low for humans to hear that preceded the eruptions, according to a University of Washington (UW) news release.

The study suggests the noise is made by a rapid succession of small earthquakes just before a volcano erupts. The quakes create a noise called harmonic tremor and they are too low for any human to hear.

"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, a UW doctoral student in Earth and space sciences.

The study analyzed Alaska's Mount Redoubt eruption in March 2009 and should give scientists a better understanding of what happens during explosion cycles.

Hotovec-Ellis, lead author of the study, said harmonic tremors are produced by magma being forced into the heart of the mountain through small spaces. The thick magma sticks to the conduit through which it is being pressured until the pressure is great enough to push it higher. She believes this is what causes the small earthquakes.

"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. "After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes."

The study was published on July 14 in the Journal Volcanology and Geothermal Research and was co-authored by UW's John Vidale and Stephanie Prejean and Joan Gomberg of the U.S Geological Survey.

The harmonic tremor occurs at a frequency of about 30 hertz. A human's range only starts at 20 hertz. Hotovec-Ellis said one might be able to hear the harmonic tremor by putting their ear to the ground, but advised against it because the sound is followed by an explosion.

Hotovec-Ellis co-authored a separate paper in another journal published on the same day, in which the team analyzed the rapid succession of the quakes. The study showed five quakes in succession, then a pause before the sixth.

She said: "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."