Modeling a volcano’s hum

A form of volcanic seismicity known as harmonic tremor—sustained vibrations in the infrasound range of 0.5–5 Hz—often precedes a major eruption as magma courses through underground cracks and conduits. A new analysis of a series of eruptions from Alaska’s Redoubt Volcano (shown here) in March 2009 examines how the initially low-frequency rumble rose over several minutes from 1 Hz to audible frequencies near 30 Hz—the highest yet recorded from a volcano—before ending abruptly 30–60 seconds prior to erupting. Ksenia Dmitrieva (Stanford University), Alicia Hotovec-Ellis (University of Washington), and their colleagues modeled the unusual frequency pattern of that seismicity and found it consistent with a frictional-faulting mechanism. Recent work this year by Hotovec-Ellis and others linked the harmonic tremor at Redoubt to a sequence of small earthquakes, each slipping just a millimeter on a fault several kilometers beneath the volcanic vent. The source of the slipping remains ambiguous. But in the new work, the researchers postulate that as thick magma moves upward through a narrow conduit, it becomes obstructed or sticks to the rocky walls until the pressure builds up enough to overcome friction, a process that repeats at ever-higher frequencies. The onset of seismic quiescence then occurs when the rate at which stress accumulates on the fault is high enough—about 20 MPa/s, according to their calculations—that the stick–slip behavior evolves into stable sliding. (K. Dmitrieva et al., Nat. Geosci. 6, 652, 2013, doi:10.1038/ngeo1879 .)