July 15, 2013
Volcanoes Like To Scream Out Before They Blow Their Lids
Brett Smith for redOrbit.com - Your Universe Online
Scientists often detect a series of small earthquakes just before a volcanic eruption and a new study published in the Journal of Volcanology and Geothermal Research has shown that these tremors can happen in such a rapid succession -- that they create sound vibrations that rise in pitch until the volcano erupts.
The study authors came to their conclusion by looking at the March 2009 eruption of Alaska's Redoubt Volcano, which was preceded by a series sound signals of significantly higher frequencies, referred to as a harmonic tremor.
"The frequency of this tremor is unusually high for a volcano, and it's not easily explained by many of the accepted theories," said co-author Alicia Hotovec-Ellis, a University of Washington doctoral student in earth and space sciences.
Some volcanoes emit sound as liquid-hot magma rises through thin cracks in the Earth's crust, generating resonant frequencies. However, the study researchers said harmonic tremors occur as magma is forced into the center of the mountain. Inside the volcano, thick magma sticks to the rock surface until the pressure forces it higher and then the process is repeated.
The forcing of magma through the volcano causes the tiny earthquakes, ranging in magnitude from about 0.5 to 1.5, according to Hotovec-Ellis. The tremors become smaller, but happen more frequently, eventually blending 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. "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," Hotovec-Ellis added.
During the Redoubt event, the team observed the tremor frequency begin at 1 cycle per second, or 1 hertz. The frequency gradually rose to about 30 hertz. Human ears can detect frequencies starting at about 20 hertz. The scientists said a person with his/her ear to the ground directly above the magma conduit might be able to hear the harmonic tremor at its highest point, which would then be followed by an explosion. Some Alaskan scientists dubbed the highest-frequency harmonic tremor during the 2009 eruption "the screams" because they reach such a high pitch.
Hotovec-Ellis created two audio recordings of the seismic activity. A 10-second recording made from about 10 minutes of seismic activity sped up 60 times and a one-minute recording made from about an hour of activity that included over 1,600 small earthquakes that foreshadowed the first explosion.
"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.
The Redoubt volcano is famous among volcanologists for having the first eruption ever predicted by seismic events, in 1989, using a method developed by Bernard Chouet.