Before the explosion — volcano’s warning tremors explained
New Haven, Ct. – No matter their size or shape, explosive volcanoes produce tremors at similar frequencies for minutes, days or weeks before they erupt. In the Feb. 24 issue of the journal Nature, researchers at Yale University and the University of British Columbia (UBC) describe a model that explains this strange phenomenon – and may help forecast deadly eruptions.
When such volcanoes erupt they can shoot hot ash up to 40 kilometers into the atmosphere and cause devastating destruction when the ash column collapses and spreads as “pyroclastic flows.” Prior to most of these explosive eruptions the volcanoes shake slightly but measurably, and the shaking becomes more dramatic during the eruption itself. This tremor is one of the primary precursors and warnings used by volcanologists for forecasting an eruption.
“Tremor is very mysterious, most notably because it shakes at pretty much the same frequency in almost every explosive volcano, whether it’s in Alaska, the Caribbean, New Zealand, or Central America,” said David Bercovici, professor and chair of the Department of Geology and Geophysics at Yale, and co-author of the research. “That it’s so universal is very weird because volcanoes are so different in size and character. It would be like blowing on five different musical wind instruments and having them all sound the same.”
For minutes to weeks before eruptions, tremors in nearly all volcanoes stay in a narrow band of frequencies from about 0.5 to 2 HZ. Just before and during the eruption, the frequency climbs to a higher pitch, and the range spreads out to between 0.5 and 7 HZ. This similarity in tremors has been hard to explain because each volcano differs in many variables such as physical structure, magma composition or gas content.
The mathematical model developed by Bercovici and his colleague Mark Jellinek at UBC suggests these similarities can be explained by “magma wagging” – or the rattling that occurs from the interaction of rising magma and the foamy jacket of gas that surrounds it. The factors that control this rattling or wagging vary little between volcanoes, which explains why the same tremors occur in very different volcanoes.
“Explosive eruptions are some of the most spectacular and destructive phenomena in nature, and tremor is both a warning of the event and a vital clue about what is going on in the belly of the beast,” Bercovici said. “This model will provide a much-needed framework for understanding the physics of tremors, and this can only help with the prediction and forecasting of destructive eruptions.”
Here’s the UBC Press release (thanks to WUWT reader “clipe”):
Oscillating “plug” of magma causes tremors that forecast volcanic eruptions: UBC research
University of British Columbia geophysicists are offering a new explanation for seismic tremors accompanying volcanic eruptions that could advance forecasting of explosive eruptions such as recent events at Mount Pinatubo in the Philippines, Chaiten Volcano in Chile, and Mount St. Helens in Washington State.
All explosive volcanic eruptions are preceded and accompanied by tremors that last from hours to weeks, and a remarkably consistent range of tremor frequencies has been observed by scientists before and during volcanic eruptions around the world.
However, the underlying mechanism for these long-lived volcanic earthquakes has never been determined. Most proposed explanations are dependent upon the shape of the volcanic conduit – the ‘vent’ or ‘pipe’ through which lava passes through – or the gas content of the erupting magma, characteristics that vary greatly from volcano to volcano and are impossible to determine during or after volcanic activity.
Published this week in the journal Nature, the new model developed by UBC researchers is based on physical properties that most experts agree are common to all explosive volcanic systems, and applies to all shapes and sizes of volcanoes.
“All volcanoes feature a viscous column of dense magma surrounded by a compressible and permeable sheath of magma, composed mostly of stretched gas bubbles,” says lead author Mark Jellinek, an associate professor in the UBC Department of Earth and Ocean Sciences.
“In our model, we show that as the center ‘plug’ of dense magma rises, it simply oscillates, or ‘wags,’ against the cushion of gas bubbles, generating tremors at the observed frequencies.”
“Forecasters have traditionally seen tremors as an important – if somewhat mysterious – part of a complicated cocktail of observations indicative of an imminent explosive eruption,” says Jellinek, an expert in Geological Fluid Mechanics. “Our model shows that in systems that tend to erupt explosively, the emergence and evolution of the tremor signal before and during an eruption is based on physics that are uniform from one volcano to another.”
“The role of tremors in eruption forecasting has become tricky over the past decade, in part because understanding processes underlying their origin and evolution prior to eruption has been increasingly problematic,” says Jellinek. “Because our model is so universal, it may have significant predictive power for the onset of eruptions that are dangerous to humans.”
The research co-led by Prof. David Bercovici of Yale University and was supported by the Canadian Institute for Advanced Research, the Natural Sciences and Engineering Research Council of Canada, and the U.S. National Science Foundation.