Who knew? From the NY Times science section: At Mount St. Helens, the Big Eruption Is of Data, Not Lava. One of the big surprises of post-1980 volcanic research is the discovery that rising steam around a volcano can cause earthquakes that have a particular resonance frequency, signaling building pressure and possible eruption:
When an earthquake fault slips, breaking rocks, the seismograph reading is a messy, patternless jumble of squiggle. But around St. Helens, the seismic signal often contained a single characteristic frequency, almost as if the earth were singing a particular note.
Indeed, steam rising up through rock cracks resonates “almost like an organ pipe,” Dr. Chouet said. Such resonant earthquakes, particularly if nothing is occurring at the surface, indicates pressures are building, he said.