While scientists could only guess as to what triggered the vibrations, there was no question about what happened next: The seismic scream was followed by 30 seconds of silence, and then a series of volcanic eruptions — 20 over a two-week period — that launched an enormous plume of ash resembling a mushroom cloud.

In a study published recently in Nature Geoscience, researchers examined data from the 2009 eruption and concluded that the sustained vibrations were caused by numerous so-called stick-slip movements on faults more than a mile beneath the volcano.

While researchers said it was beyond the scope of their study to determine the cause of the quake, they theorized that it was the result of pressure building in blocked magma conduits underground.

“Blockage of conduit flow increases magma pressure, driving increasingly rapid deformation until the obstruction is breached and an explosion commences,” wrote lead study author Ksenia Dmitrieva, a graduate student in the department of geophysics at Stanford University, and colleagues.

High-frequency harmonic tremors have been recorded before other volcanic eruptions, as well as in the collision of icebergs. Usually, these tremors cannot be heard by people. In the case of the Redoubt eruption, however, the shaking was just barely audible as a hum, according to study authors.

“Although this is a rare and extreme example, if you ever happen to be near an active volcano and feel the ground humming, you might want to take cover,” wrote study co-author Alicia Hotovec-Ellis, a graduate student in the Department of Earth and Space Sciences at the University of Washington, in Seattle.

Click here to read more and listen to a recording of the scream.