Most volcanoes rumble before they erupt, but Washington and Alaska researchers say a big recent eruption was preceded not by a rumble, but a scream.
Alaska’s Mount Redoubt blew its top several times in 2009. Leading up to many of the explosions were a series of little earthquakes—not uncommon for an active volcano. But these quakes began to accelerate, one after another, like a drumbeat building to a climax.
The quickening quakes built to a fever pitch that seismologists describe as a scream. The shrieking volcano then suddenly fell silent before violently exploding.
“I think that this is one of the more startling things I've seen the earth do in my time that I've been studying … volcanoes and earthquakes. Seeing such a dramatic change in seismicity in those moments prior to the eruption in itself is really exciting,” said Stephanie Prejean, a research geophysicist with the U.S. Geological Survey Alaska Volcano Observatory.
Prejean sent the data to University of Washington researchers. Doctoral student Alicia Hotovec-Ellis found several more examples among Redoubt's cluster of eruptions. She said these screams were higher-pitched than any scientists had recorded, meaning the quakes came fast and furious.
“So we were kind of surprised by how high the pressure had to be in order to force 30 small earthquakes per second. So we need to figure out why it's so high,” she said.
The distinctive pattern of ramping-up earthquakes could give a more precise warning of when an eruption is coming, though it's not clear how applicable these rare findings will be to other volcanoes. In any case, better understanding of the seismic activity underneath trigger-happy volcanoes could be key to improving the forecasts.
The seismic waves are generally too slow to hear, but they can be sped up into the audible range. To hear what a volcano’s scream sounds like, click the listen link above. Or find the audio hereand here.
