Redoubt Volcano
An aerial view shows steam rising from Redoubt Volcano, located 100 miles southeast of Anchorage, Alaska, January 31, 2009. Reuters

The Redoubt volcano, located in the Aleutian Range about 110 miles south-southwest of Anchorage, Alaska, erupted in March 2009, and an analysis of its eruption sequence has revealed that the volcano screamed at a high pitch before it blew its top and flung volcanic ash about six miles into the sky.

Previous research has shown that a number of tiny earthquakes occur before a volcanic eruption in rapid succession to create a type of harmonic tremor at low frequencies, which cannot be detected by human ears. However, the Redoubt volcano was a different case, where researchers found the harmonic tremor gradually reached frequencies high enough to be heard by humans, and then stopped abruptly just before six of its multiple eruptions.

“The frequency of this tremor is unusually high for a volcano, and it’s not easily explained by many of the accepted theories,” Alicia Hotovec-Ellis of the University of Washington, the lead author of a forthcoming paper in the Journal of Volcanology and Geothermal Research, said in a statement on Sunday.

Hotovec-Ellis collaborated with other researchers for another paper, published on July 14 in Nature Geoscience, which focuses on the increase of harmonic tremor frequency at Redoubt in 2009. According to the second paper, the tremors occurred about two kilometers beneath the volcanic crater, near the magma chamber that feeds into the base of the volcano's funnel.

Some volcanoes generally make a sound when magma resonates while pushing up through thin cracks in the Earth’s crust. But, in Redoubt’s case, the harmonic tremor occurred when magma was forced through a narrow channel under great pressure into the heart of the mountain.

Before many of the 2009 Redoubt eruptions, many small earthquakes between magnitudes of 0.5 to 1.5 were detected, which got smaller as the final moment of explosion approached.

“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.”

Hotovec-Ellis documented the rising frequency starting at about 1 hertz and gliding upward to about 30 hertz. In humans, the audible frequency range starts at about 20 hertz, but a person lying on the ground directly above the magma conduit might be able to hear the harmonic tremor when it reaches its highest point, she said.

Researchers believe that the new analysis of Redoubt’s eruptions will give more insights into a volcano’s pressurization right before an explosion. It could also be helpful in providing warnings before a volcanic eruption.

Here are two recordings of the seismic activity, dubbed “the screams,” created by Hotovec-Ellis: