Not far from Anchorage, Alaska, there is a volcano that screams. Then falls silent. Then erupts and throws volcanic ash 10 kilometers into the sky. Then starts all over and does it again, and again.
This strange and repeating seismic behavior was discovered in the 2009 eruptions of Redoubt Volcano by Alicia Hotovec-Ellis of the University of Washington. But now she and her colleagues have a model to explain what might cause all the screaming and then the silence before the volcanic storm.
To get an idea what the screaming sounds like – converted from seismic waves, sped up 60 times into audible sound waves – listen to this file which is a real recording of Redoubt Volcano. What you hear is a drum beat of quakes that get faster and faster until they blur together in a rising sound and then go quiet. It's the blurred rising sound that has been called the scream.
"You are getting 20 to 30 earthquakes a second," said Stanford University seismologist Eric Dunham explaining how the screams are really harmonic seismic waves smearing together and becoming one big seismic sound. "Then you get 30 seconds of silence. Part of what we did was to try and figure out why."
Dunham, Hotovec-Ellis and their colleagues published their model today in the journal Nature Geoscience. The model shows the rapid slipping of lots of small faults that have been identified underneath the volcano. The cause of the slipping is that they are near the conduits of molten, gassed-up rock that is super-pressurized and about to blow.
"It has to be close to the conduit because the pressure is incredibly high," Dunham said of the calculated forces required to create both the scream and the silence.
The closest analogy Dunham has for what's happening is scratching fingernails on a slate chalkboard. By scratching slowly, the repeated slipping and catching of the nails creates a lot of noise. But if you press really hard and move your hand really fast, the horrible noise disappears as your fingers begin to glide over the board.
Similarly, in the high pressure just before the volcano erupts might be enough to speed up the quaking faults so that they slide more quickly and quietly, instead of slipping and halting.
"The next time Redoubt erupts people will be looking for these screams," said Mike West, director of the Alaska Earthquake Information Center at the University of Alaska. And if the pattern reappears again, or is detected at another volcano, it could be helpful in providing a bit more warning before an eruption.
Since the primary danger of most of Alaska's volcanoes is to commercial aircraft, said West, any added warning time is valuable.