Swarms of small earthquakes sometimes precede a volcanic eruption at such rapid succession that they create a signal called harmonic tremor, which resembles sounds made by various musical instruments, though at frequencies much lower to humans.
A new analysis of an eruption sequence at Alaska's Redoubt Volcano in March 2009 shows 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.
Alicia Hotovec-Ellis, a University of Washington doctoral student in Earth and space sciences, said that the frequency of this tremor is unusually high for a volcano, and it's not easily explained by many of the accepted theories.
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 the Earth's crust.
But Hotovec-Ellis believes that 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.
Hotovec-Ellis said that because there is less time between each earthquake, there is not enough time to build up enough pressure for a bigger one and after the the frequency glides up to a ridiculously high frequency, it pauses and then it explodes.
She is the lead author of a forthcoming paper in the Journal of Volcanology and Geothermal Research that describes the research. Co-authors are John Vidale of the UW and Stephanie Prejean and Joan Gomberg of the U.S. Geological Survey.
Scientists at the USGS Alaska Volcano Observatory have dubbed the highest-frequency harmonic tremor at Redoubt Volcano "the screams" because they reach such high pitch 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 Soufriere Hills volcano on the Caribbean island of Montserrat.
The findings are set to be published in the Journal of Volcanology and Geothermal Research.