New model explains volcano tremors

Explosive volcanoes may have more in common than experts thought.

Researchers at Yale and the University of British Columbia in Vancouver have devised a new theoretical model for predicting a volcano’s harmonic tremor, an unexplained phenomenon that often precedes a violent volcano’s eruption. Their model, published in the Feb. 24 issue of the journal Nature, seems to show that a volcano’s size and structure may not be as important in tremors as the magma and the bubbles it contains. But experts interviewed said the model must be tested and developed further before it can be applied.

Professor David Bercovici, who is a co-author of the model, said the finding was striking because it can be applied to explosive volcanoes all over the world.

“It was like finding out that five totally different instruments all make the same noise,” he said.

Harmonic tremors have been documented for decades, but their origins remain mysterious, Bercovici said. He said this type of tremor can begin anywhere from one week to one hour before an eruption, and during that time its harmonic frequency increases steadily. The tremor has been attributed to moving magma, Johns Hopkins University graduate student Ryan Currier said. But beyond that, its causes have never been clear, he added.

One of the challenges in studying volcanoes is that researchers often must work backwards from an explosion, said Jenny Suckale, an M.I.T. graduate student completing her doctorate on volcanic simulations this spring.

Bercovici said he hopes the new model will help other volcanologists in their research by providing a framework for understanding what a volcano will do once harmonic tremors start.

“It’s a simple theory that explains this mysterious phenomenon,” said Bercovici, who is the chair of the geology and geophysics department. “That’s the best thing about it — it doesn’t require a giant computer. Sometimes science works that way.”

But Bercovici stressed that his model does not predict volcanic eruptions themselves.

“That’s silly, like [predicting] the weather or earthquakes,” he said.

Dr. Bruce Marsh, professor of earth and planetary science at Johns Hopkins University, said that by devising a model less sensitive to a volcano’s “geometry,” or basic structure, the model is pinning harmonic tremor on the properties of magma and its gas bubbles, rather than the shape of each volcano’s walls.

But both Marsh and Bercovici said that the model only applies largely to explosive systems, such as Mount St. Helens in Washington. Volcanoes that spill out lava more gradually, such as ones in Hawaii and Alaska, have a different kind of tremor that is not due to the same phenomenon, Bercovici said. Another limitation is that the model only fully applies to harmonic tremors while volcanoes are erupting, said Dr. Steve McNutt, a research professor of volcano seismology at the Alaska Volcano Observatory.

The new model’s large applicability would be an improvement over other models, which are often tailored to a certain volcano’s structure, McNutt added.

“They took a novel approach,” McNutt said of the model’s creators. “Most people consider the molten rock itself, but they considered the magma.”

The model introduces the “magma wagging” theory of eruptions in violent volcanoes, in which the frequency of tremors is due to a column of magma plugging the volcano’s closed central pipe, Bercovici and Currier said. The column of magma then “wags” back and forth against gaseous material lining the volcano’s sides, Currier said. The wagging could be due to minor intermittent explosions beneath the column, Bercovici said.

Despite the progress the new model has made, experts stressed that the model is only a preliminary step in understanding what happens in harmonic tremor. But Suckale said that the model looks promising, and that it simply needs to be tested more.

“Many people think of one volcano like any other volcano, while other evidence suggests that some volcanoes are very, very different. The interesting thing will be to see whether this applies for all. And that’s difficult.”

Marsh agreed that the model can be developed.

“I think it’s a nice piece of work,” Marsh said. “It’s not just pie in the sky.”

When nearing eruption, the harmonic tremor of most explosive volcanoes averages about one Hz, Bercovici said.

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