Before a volcano erupts, rising magma will often trigger numerous small earthquakes (or "tremors") that scientists can monitor with a seismometer. But how do you know if those earthquakes will lead to an eruption or not? It turns out that these earthquakes can come in such rapid succession that they create a sound called a harmonic tremor, which may directly precede an eruption.
Alicia Hotovec-Ellis, doctoral student of the University of Washington, has been studying the Mount Redoubt Volcano, and found that during the 2009 eruption, the mountain gave off gliding harmonic tremor immediately before six consecutive explosions.
During the course of this harmonic tremor, the fundamental frequency climbed from less than 1 hertz to about 30 hertz in 10 minutes. After the harmonic tremor, about 10 to 60 seconds prior to the explosion, the mountain went quiet. For reference, humans can hear frequencies that range from about 20 to 20,000 hertz, so if you have acute hearing and were lying on the mountaintop, you might be able to hear the volcano "scream" right before it blows (mind you, doing so would be a really, really bad idea).
While harmonic tremors aren't unusual, apparently it is unusual for these tremors to occur at frequencies high enough to be audible. In fact, these high frequencies even approach or exceed the upper limits of existing volcanic tremor models. This activity could provide scientists with clues about the pressure buildup that occurs immediately before a volcanic eruption, and this new data could help refine scientific models that would help scientists better understand what happens during eruptive cycles in volcanoes.
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