Undersea internet cables can detect earthquakes

The ocean covers more than two-thirds of the Earth’s surface. For seismologists, oceanographers, and others who wish to constantly monitor the motions of our planet, this fact poses a problem. Seas can be bleak, dark places where important data — about things like earthquakes and seismic hazards — are hard to come by.

But just because the oceans are murky doesn’t mean they lack infrastructure: for example, the more than 750,000 miles of communication cable that allow the internet to cross continents. Scientists know this, too. They are starting to play with this earthquake detection infrastructure.

Their latest step in doing so: using a transatlantic cable to find earthquakes, as they did in a research paper published in Science on May 20. The researchers, led by Giuseppe Mara at the National Physical Laboratory in the United Kingdom, discovered two earthquakes, one of which originated in half the world.

“We have very limited sensing outside. Very limited. It’s absurd, what we have,” says Zach Spica, a seismologist at the University of Michigan who was not one of the authors of this paper. “But, now, we realize that we have, in fact, thousands of the possible sensors out there, so we can start digging into them and start observing what’s going on.”

Today, telecommunications companies have woven optical fibers into an intricate network molded around the world. These cables are the hidden but crucial components that keep the Internet moving. Not only do they connect the two hemispheres, but they provide a critical connection to more isolated parts of the world.

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

Using cables for underwater sensing is not a new idea. Initially, the idea relied on specialized, bespoke cables. They were manipulated by the US Navy in the early Cold War as a way to spot Soviet submarines. Scientists in both California and Japan have been testing cables for earthquake detection since the 1960s.

But installing certain equipment is expensive, and in the 21st century – aided by the telecoms industry’s growing receptiveness to the idea – scientists are beginning to take advantage of what already exists.

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

Perhaps the most well-established method is a technology known as Distributed Acoustic Sensing (DAS). To do this, the scientists fire short pulses of light from one end of the cable. If an earthquake, for example, shakes the cable, the tremors will reflect some of that light back to the transmitter, who can use it to reconstruct what happened and where.

DAS has been adopted by many scientists, but it has a major limitation: distance. When light (or any other signal) travels along the line, it attenuates or loses strength. So it is difficult to use DAS to sense after a few dozen miles. This is not an easy matter, but what if you wanted to see, for example, the middle of the ocean, thousands of miles from the shore?

In 2021, researchers led by Zhongwen Zhan, a seismologist at Caltech, tested another method on Curie, a Google-owned cable that runs from Los Angeles to Valaparaíso, Chile, parallel to the hyperactive Pacific coast of the Americas. This team studied the fingerprints of earthquakes on the movement of regular signals through the cable.

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

Of course, if you’re talking with your friend outside, your voices can reach each other without any problem at all. That’s because these cables are equipped with devices called repeaters. Like the players in the great game of telephone (just far, far more reliable), repeaters take an incoming signal and amplify it to send it to the next signal.

For several years, some scientists have supported a proposal called SMART to equip new repeaters in future cables with inexpensive seismic, pressure and temperature sensors. Now telecoms companies care: Project SMART – a cable connecting mainland Portugal to its Atlantic islands – is due to enter service in 2025.

But repeaters submerged in seafloor cables actually have a second function: To help cable operators identify potential problems, repeaters can transmit some of their signals again.

Mara and his colleagues took advantage of this existing safety. They sent an infrared laser through the cable and checked the signals coming back from each repeater. In doing so, they can break an ocean-crossing cable into tiny bits a few tens of miles long.

“I know others have been thinking about how to do it,” says Bruce Howe, an oceanographer at the University of Hawaii who was also not involved in this research paper, “but they did.”

The Mara group tested their method on a transatlantic cable running between Southport in northwest England and Halifax in Atlantic Canada. They were able to detect not only earthquakes – one originating from northern Peru and one originating all the way in Indonesia – but also noises from the movement of water in the ocean.

There are a few catches. This type of finding is different from what seismologists are used to, Howe says. Mara and her colleagues have not yet been able to measure the magnitude of the earthquake. Distinguishing an earthquake from ocean temperature fluctuations, for example, can be difficult. This is where multiple methods – for example, this state-of-the-art technology plus SMART – can work in tandem.

Many scientists are excited about the capabilities of cables. “I really feel like the greatest breakthrough [in seismology] It will be done outside, because there is so much to explore,” says Spica. They can greatly improve tsunami warning systems. They may help geologists look at poorly understood places where tectonic plates meet or break apart, like mid-ocean ridges. And they may be able to help oceanographers monitor what is happening in the warming oceans.

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

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