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Why did Alaska volcano 'scream' prior to 2009 eruption?

In the leadup to eruption in March 2009, Redoubt Volcano emitted what's known as a harmonic tremor, a tone that may signal pending eruptive activity. AVO photo

When Redoubt Volcano erupted in March 2009, the effects of the volcanic activity were obvious -- plumes reaching up to 62,000 feet spread ash around a good portion of southern Alaska, and even closed the airport in Anchorage for nearly a full day as ash sprinkled on the state’s largest city, more than 100 miles northeast.

But there was other, less obvious activity taking place inside Redoubt in the leadup to the first eruptions. Seismic activity had increased several months prior to eruption, and volcanologists were watching, measuring ... and listening.

And now, a new study has outlined a phenomenon known as “harmonic tremor,” in which a volcano emits an acoustic signal, influenced by activity inside the volcano and potentially triggered by increased seismic activity at the peak. Some media have dubbed it a “scream,” but it sounds perhaps more similar to a tea kettle whistling to signal that the water is ready to go.

Listen for yourself -- the 10-second recording below is actually about 10 minutes of activity, sped up 60 times until it’s audible to the human ear, followed by the beginnings of an eruption.

And though the spiking frequency of the volcano may call to mind a whistling teapot, U.S. Geological Survey geophysicist Dr. Stephanie Prejean has another interpretation of the sound.

“It’s often happening at much lower frequencies than humans can hear, and it can be thought of as volcanoes singing,” said Prejean, one of the authors of the new study. She added that musical instruments would be another apt analogy for the sound created by the inner workings of the volcano. Indeed, the study itself makes reference to the one-man band that was Redoubt in 2009 -- the lengthy period of heightened activity at Redoubt between March and July of that year was characterized in the study as featuring “periods of harmonic tremor and drumbeat earthquakes.”

Not every volcano experiences harmonic tremor -- though two other recently-active Southwest Alaska volcanoes, Pavlof and Veniaminof, both featured the phenomenon in their recent fits, Prejean said -- and it’s still not clear what exactly causes it. It may even vary from volcano to volcano.

One theory suggests a fluid-filled crack in the volcano “acts like a tube bell or a flute,” while others think it may have to do with gas-filled bubbles or vibration of the volcanic conduit itself. In the case of Redoubt in 2009, the study’s authors theorize that the harmonic tremor in that case was coincident with the seismic activity at the mountain.

“Basically the tremor is composed of a bunch of tiny little earthquakes that are happening closer and closer together in time,” Prejean said. “Basically, they’re relieving stress in the conduit.”

That would help explain the increasing pitch of the harmonic tremor, coinciding with the frequency or seismic activity at the volcano prior to eruption.

Oddly enough, the harmonic tremor went silent in the seconds leading up to eruption. Though the frequency had intensified over the “tens of minutes” leading up to the eruption, Prejean said, the silence lasted for only seconds before the eruption began in earnest. Because of that short window, the harmonic tremor may not be ideal for predicting exact eruption times, but provides valuable insight into what’s happening inside the volcano in the leadup to eruption.

“The more important thing is just that the earth gave us some new information about what’s happening in those last few minutes” prior to Redoubt’s eruption, Prejean said.

She added that harmonic tremor is being increasingly studied by volcanologists around the world in hopes of better understanding what’s happening inside of a volcano before it blows its top.

Contact Ben Anderson at ben(at)alaskadispatch.com