Just outside the bustling city of Naples, Italy is a ticking time bomb—a restless volcano that, were it to erupt, could claim the lives of hundreds of thousands of people.
But there’s no need to fret: This broad, ocean-flanked caldera, named Campi Flegrei, is one of the most heavily monitored volcanic systems on Earth, and there’s no evidence that it’s going to blow its top any time soon. Still, Campi Flegrei is very much active, and volcanologists are constantly on the lookout for new ways to forecast dangerous activity.
One technique is in development through the work of Luca de Siena, a geophysicist at Johannes Gutenberg University of Mainz. By tuning into how sound waves from the nearby sea reverberate through the ground beneath Campi Flegrei, de Siena has found a way to map where magma and hot fluids appear to be accumulating—and if they may soon be headed for the surface.
Campi Flegrei is perhaps less well-known than Mount Vesuvius, its cousin to the east. But despite its relative anonymity, this volcano and its stewing cache of magma are a massive hazard: Half a million people have made their home in its vicinity—and all signs point to the fact that someday (likely in the distant future), it will erupt again.
Keeping tabs on Campi Flegrei is no easy task, though. Volcanologists depend heavily on seismic data, but the region’s earthquakes have been mild and infrequent in the past several decades. And generating sound waves artificially by, say, igniting dynamite or other explosives, is somewhat frowned upon in such thickly settled slices of the world.
As an alternative, de Siena pinpointed a source of sound waves that the Earth had already installed billions of years ago: the ocean. As seawater ricochets off of Naples shoreline, sound waves course through the landscape, where they can be picked up by seismic instruments already rooted into Campi Flegrei’s flanks.
“This is all about getting as much as we can from existing instruments,” says Alison Graettinger, a volcanologist and geoscientist at the University of Missouri, Kansas City who was not involved in the research.
Sound waves move at different speeds through different materials: Though they zip through solid rock, they tend to slow to a relative crawl upon meeting pockets of fluid or semi-fluid material. Monitoring how quickly batches of sound waves reach seismic instruments, therefore, can yield crucial intel on a volcano’s subterranean shape.
By reading a series of measurements taken over the course of three years, de Siena and his team constructed an image of Campi Flegrei’s hidden physique, and homed in on regions where sound waves appeared to lag. These “slow zones” might indicate where the Earth’s crust has begun to stockpile magma or hot fluids, though it’s still difficult to distinguish between the two.
“This is a great study… because this data hasn’t really been looked at before,” says Janine Krippner, a volcanologist at Concord University who was not involved in the research. “It’s really neat that people are now thinking outside the box, especially in systems that can impact so many people.”
And if this data continues to be collected over long periods of time, it may begin to paint a picture of Campi Flegrei’s regular rumblings—and what constitutes a concerning deviation from the norm.
“To monitor volcanoes, we’re interested in change, and timescales,” Graettinger says. “A difference from normal is important, but first you need enough normal to know what a difference is.”
For instance, now that de Siena has a decent idea of where magma may amass, the next step is to see if those pockets begin to move or expand. “If we see that, that could be scary,” de Siena says. “Because it means there might be a fluid reservoir that is filling and become bigger.”
That’s not the case right now. Even if it were, a change in a single indicator isn’t enough to raise alarm bells. “Figuring out what’s causing these subsurface systems to change is still difficult,” Graettinger says. “Just because you see something change in a volcanic system, it doesn’t mean it’s going to erupt—it’s not clear cut.”
The key, she adds, is taking into account other types of data—especially considering how much volcanoes naturally evolve over time. Researchers must deal in all types of currency when it comes to monitoring their fickle subjects of study, collecting cues from chemistry, GPS, and other aspects of geophysics. And it never hurts to add another technique to the toolkit.
“We’re lucky, in that volcanoes usually give us signals when something is happening,” Krippner says. “But if we’re not listening, we’re not going to hear it.”
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