Deep-sea internet cables could help sense distant earthquake rumbles

Ocean covers more than two-thirds of Earth’s surface. For seismologists, oceanographers, and others who want to continually monitor our planet’s motions, this fact poses a problem. The seas can be dim and murky places where important data—on things like earthquakes and seismic hazards—are hard to come by. 

But just because the oceans are mysterious doesn’t mean it lacks infrastructure: for one, the over 750,000 miles of telecommunications cables that let the internet cross continents. Scientists know this too. They’ve begun to play with that infrastructure for detecting earthquakes. 

Their latest step in doing so: using a trans-Atlantic cable to find earthquakes, as they did in a paper published in Science on May 20. The researchers, led by Giuseppe Marra at the UK’s National Physical Laboratory, detected two earthquakes, one of which had originated half the world away.

“We have very limited sensing offshore. Very limited. It’s ridiculous, what we have,” says Zack Spica, a seismologist at the University of Michigan, who was not one of this paper’s authors. “But, now, we are realizing that we have, actually, thousands of possible sensors out there, so we could possibly start digging into it and start watching what’s going on.”

Today, telecommunications companies have woven optical fibers into an intricate web cast across the globe. These cables are hidden yet crucial components that make the internet tick. Not only do they bridge hemispheres, they bring critical connectivity to more isolated parts of the world. 

(Just ask Tonga, whose cable link was torn by a volcanic eruption earlier this year. People and relief efforts in the islands often had to rely on snail-like 2G satellite internet until the cable was repaired.)

Using cables for underwater sensing isn’t a new idea. At first, the idea relied on bespoke, specialized cables. The US Navy toyed with them in the early Cold War as a way of detecting Soviet submarines. Scientists in both California and Japan began testing cables for earthquake detection from the 1960s. 

But installing specific equipment is expensive, and in the 21st century—helped by the telecoms industry’s increased reception to the idea—scientists have begun to take advantage of what is already there.

[Related: Earthquake models get a big shakeup with clues buried in the San Andreas fault]

Perhaps the most established method is a technique known as distributed acoustic sensing (DAS). To do this, scientists shoot short light pulses of light from one end of the cable. If an earthquake, for instance, shakes the cable, the tremors will reflect some of that light back to the sender, who can use it to reconstruct what happened and where.

Many scientists have embraced DAS, but it has a key limitation: distance. As light (or any other signal) travels along a line, it attenuates, or loses strength. So it’s hard to use DAS to sense beyond a few dozen miles. That is no small feat, but what if you wanted to see into, say, the middle of the ocean, thousands of miles from shore?

In 2021, researchers led by Zhongwen Zhan, a seismologist at Caltech, tested another method on Curie, a Google-owned cable running from Los Angeles to Valaparaíso, Chile, parallel to the highly active Pacific coast of the Americas. That team studied the fingerprints of earthquakes on regular signal traffic through the cable. 

But their method had a flaw: They couldn’t tell how far away something had happened, only that it had. “They detected earthquakes, but…they didn’t know where it was coming from,” says Spica.

Of course, if you’re chatting with your friend overseas, your voices can reach each other with no issue at all. That’s because these cables are outfitted with devices called repeaters. Like players in a grand game of telephone (only far, far more reliable), repeaters take an incoming signal and amplify it to send it along to the next one. 

For several years, some scientists have supported a proposal, called SMART, to outfit new repeaters on future cables with inexpensive seismic, pressure, and temperature sensors. Telecoms firms are now paying attention: One SMART project—a cable linking Portugal’s mainland with its Atlantic islands—is slated to enter service in 2025.

But seafloor cables’ submerged repeaters already have a second function: To help cable operators locate potential issues, the repeaters can send some of their signal back.

Marra and his colleagues harnessed that existing failsafe. They sent an infrared laser through the cable and examined the signals that returned from each repeater. In doing so, they could break an ocean-crossing cable into bite-sized chunks a few dozen miles long. 

“I know others have been thinking about how to do this,” says Bruce Howe, an oceanographer at the University of Hawai’i who also wasn’t involved in this paper, “but they did it.”

Marra’s group tested their technique on a trans-Atlantic cable running between Southport in North West England and Halifax in Atlantic Canada. They were able to detect not just earthquakes—one originating from northern Peru and another originating from all the way in Indonesia—but also the noise from water moving in the ocean.

There are a few catches. For one, says Howe, this sort of detection is different from what seismologists are accustomed to. Marra and colleagues weren’t yet able to measure the magnitude of an earthquake. And discerning an earthquake from, say, ocean temperature shifts may prove difficult. This is where multiple methods—for instance, this latest technique  plus SMART—could work in tandem.

Many scientists are excited about cables’ potential. “I really feel that the greatest breakthroughs [in seismology] are going to be done offshore, because there is so much to explore,” Spica says. They could vastly improve our tsunami warning systems. They might help geologists peer into poorly understood places where tectonic plates are coming together or pulling apart, such as mid-ocean ridges. And they might be able to help oceanographers monitor what’s happening in warming oceans.

“Money is, as always, the main obstacle,” Howe says, “but recent progress indicates we can overcome this.”

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