The new analysis of Alaska's Redoubt volcano shows that the tremor glided to higher frequencies and then stopped abruptly less than a minute before eruption.
"The frequency of this tremor is unusually high for a volcano," explained Alicia Hotovec-Ellis, a doctoral student involved in the study, from the University of Washington.
"Because there's less time between each earthquake, there's not enough time to build up enough pressure for a bigger one. After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes."
The earthquake noise sounds like a scream before eruption when the seismometer data are speeded up sixty times to make them audible. The authors suggest a simple model of brittle fracture may explain their results, although the precise details of what is going on underneath volcanoes before they erupt remain unclear.