Volcanoes produce a wide variety of seismic signatures of which harmonic tremor is easily the most famous. Research into another popular signature, Drumbeats (aka repeaters or multiplets), has been in vogue since seismometers at Mount Saint Helens started recording them by the thousands last decade.
In a recent paper in Nature Geoscience, Dmitrieva, K. et al. introduce us to a new seismic signature that is the fusion of drumbeats into a harmonic tremor characterized by a pronounced increase in pitch before an explosion. Dubbed “The Scream” by seismologists at the Alaska Volcano Observatory, it was first (?) observed during eruptions at Redoubt Volcano. At Redoubt drumbeats occur in such rapid succession that they blurr into harmonic tremor which then “glides” up in frequency as if the volcano is screaming out load (listen here). Then, as if the volcano is catching its breath, the screaming abruptly ends, silence ensues and then tens of seconds later Redoubt goes stratospheric. The Scream brings the old saying among volcano seismology graduate students “Love a seismologist, feel the harmonic tremor” to another level.
Is this period of silence between the Scream and the eruption just the calm before the storm? In an interview with Science Friday, Alicia Hotovec-Ellis, a Doctoral Student at the University of Washington and co-author, says that “[They] decided this is where the earthquakes are coming so fast they cannot keep up. So the fault that is moving- it is still sliding…We call this stable aseismic sliding”, implying that the activity is so vigorous and stressing rates so high that the activity transitions from stick-slip to sliding.
This new observation, however, might not be volcano monitoring’s smoking gun. In her interview with Science Friday, Alicia goes on to say that “The screaming, however, and these kind of accelerating earthquakes, they come only perhaps the last few hours or minutes before an explosion.” That is to say, by the time you hear her scream, the eruption is already underway. That leaves very little time for a prediction based on this signature to be of much use to volcano dwellers and airplane pilots. And it is also still entirely possible that this gliding happens without necessarily heralding an explosion. At many volcanoes such as Reventador in Ecuador, harmonic tremor is associated with ongoing passive, effusive activity. There is no doubt that seismologists will be taking another look at the helicorder plots there and elsewhere for the Scream.
Regardless of The Scream’s utility to volcanic prediction, it just might be the missing piece of information a bright seismologist needs to unravel another mystery or see the problem in a new, revealing way.
As Alicia says, each volcano has its own personality. Redoubt just happens to be a screamer.
Read and Listen to more:
- Listen to an interview with Alicia on Science Friday
- Listen to the Scream at Why did Alaska volcano ‘scream’ prior to 2009 eruption? The pops and crackles are individual earthquakes.
- Read about it at the BBC: Volcanic ‘scream’ precedes explosive eruptions
- Read about it at Nature: Frictional-faulting model for harmonic tremor before Redoubt Volcano eruptions
The peer-reviewed paper:
Dmitrieva, K., Hotovec-Ellis, A.J., Prejean, S., Dunham1, E.M. (2013) Frictional-faulting model for harmonic tremor before Redoubt Volcano eruptions, Nature Geoscience DOI:10.1038/NGEO1879