Tremor vibrations, or seismic screams, observed at Redoubt Volcano, Alaska, just before eruptions in 2009 could have been generated by stick-slip movements on faults a few kilometres beneath the volcanic vent, reports a study published in Nature Geoscience this week. The tremor was observed to increase in frequency and then halt suddenly before each eruption, but until now the cause was unclear.

Ksenia Dmitrieva and colleagues use a numerical model to show that the volcanic tremor - sustained vibrations in the range 0.5 to 5 Hz - observed at Redoubt Volcano may have been caused by stick-slip movements along faults that generate small earthquakes. The quakes occur more frequently as stresses within the volcano increase, generating a blur of continuous tremor, or seismic scream. The authors report that, eventually, inertia effects stabilize movements on the faults, causing the fault to slip stably and without creating seismic waves, silencing the screams. This distinctive pattern, observed before eruptions, could aid in future volcanic hazard mitigation strategies.