Volcanic earthquakes produce a “seismic scream” just before eruption

The Redoubt Volcano, after a 1990 eruption melted most of a glacier off its southern face.

USGS

Volcanic activity is intimately associated with seismic activity. You simply can't force molten or semi-molten rock through a mountain without cracking a few faults in the process. If we were ever able to understand how to read the seismic activity correctly, it could provide valuable advanced warning about impending eruptions.

A 2009 eruption of Alaska's Redoubt Volcano may not get us much closer to an advanced warning, but it provides a detailed glimpse of the last moments before an explosive eruption. Shortly before the eruption, small faults within the volcano were breaking so frequently that they merged into what's being called a "seismic scream." Then, within a few minutes of the eruption, the scream got cut off as the last resistance gave way.

Redoubt is a stratovolcano, built from material that melted as the Pacific plate subducted beneath Alaska. Like some more famous examples such as Mount St. Helens, it alternates between slow eruptions of extremely viscous rock and sudden, explosive ones. The 2009 eruption was accompanied by a number of small explosions (small at least in the sense that the mountain was still there afterwards); the researchers focused on the seismic activity that lead up to these explosions.