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 afterward); the researchers focused on the seismic activity that led up to these explosions.

Most of the earthquakes associated with the eruption were small (between magnitude 0.5 and 1.5) and centered a few kilometers below the volcanic vent. There was plenty of activity of this sort seen during the eruption, but something unusual happened before the largest explosion: "These small earthquakes occurred in such rapid succession—up to 30 events per second—that distinct seismic wave arrivals blurred into continuous, high-frequency tremor." This continuous tremor is what is being called the "seismic scream."

The earthquakes themselves might be enough to make you nervous, but something even more unnerving happened after a few minutes of screaming: things suddenly went quiet. For somewhere between 30 seconds to a minute, the low magnitude quakes stopped, although sometimes larger ones would happen. And then, the explosion hit.

The authors extrapolated the scales of known earthquakes down to something of this magnitude and came up with the rupture of small faults, about 20m long, that are only sliding a millimeter with each event. To understand the forces involved, the authors built a model of the internal faulting at the volcano and got it to reproduce the behavior seen on the seismograph.

As they gradually increased the pressure on the faults, the authors' model responded with more frequent earthquakes, with the frequency slowly ramping up through frequent earthquakes before reaching the seismic scream phase at stress rates of about five MegaPascals a second. At that point, the faults move steadily but alternate with sticking and sudden slips. As the stress rate reaches 20MP/s, the sticking stops, and the fault goes into a smooth glide.

The authors consider that value, 20MP/s, unexpectedly high and don't seem to want to go into what might require that much force to shift. "It is beyond the scope of this study to rigorously evaluate potential [magma] conduit processes responsible for such extreme loading conditions." But then they go ahead and do so anyway, suggesting that the magma is forcing an obstructing piece of rock against the walls of the conduit to the surface. All told, the obstructions seem to move only about five meters, after which the way is clear for the material to explode to the surface.

Nature Geoscience, 2013. DOI: 10.1038/NGEO1879 (About DOIs).