The ocean covers more than two-thirds of the earth’s surface. For seismologists, oceanographers and others who want to constantly monitor the movements of our planet, this is a problem. The seas can be dark and muddy places where important data is difficult to obtain – about things like earthquakes and seismic hazards.
But the fact that the oceans are mysterious does not mean that they lack the infrastructure: first, more than 750,000 miles of telecommunications cables that allow the Internet to traverse the continents. Scientists know that too. They started playing with that earthquake detection infrastructure.
Their last step: using a transatlantic cable to find earthquakes, as they did in an article published in Science May 20. Scientists led by Giuseppe Marry of the British National Physics Laboratory recorded two earthquakes, one of which originated in the middle of the world.
“We have very limited sensing at sea. Very limited. It’s ridiculous what we have,” says Zack Spica, a seismologist at the University of Michigan who wasn’t one of the authors of this article. so we can start digging into it and start watching what’s going on. “
Today, telecommunications companies have woven fiber optics into complex networks around the world. These cables are hidden but important components that make the Internet work. Not only do they bridge the hemispheres, but they bring critical connectivity to more isolated parts of the world.
(Just ask Tonga, whose cable connection was broken by a volcanic eruption earlier this year. People and humanitarian efforts in the islands often had to rely on the snail’s 2G satellite internet until the cable was repaired.)
Using cables for underwater sensing is not a new idea. At first, the idea relied on custom, specialized cables. The US Navy played with them at the beginning of the Cold War as a way of exposing Soviet submarines. Researchers in California and Japan have been testing earthquake detection cables since the 1960s.
However, the installation of a specific device is expensive, and in the 21st century – with the increased acceptance of this idea in the telecommunications industry – scientists have begun 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 established method is the technique known as Distributed Acoustic Sensing (DAS). To do this, the scientists shoot short pulses of light from one end of the cable. For example, if an earthquake shakes a cable, the shakes bounce some of that light back to the sender, who can use it to reconstruct what happened and where.
Many scientists have adopted the DAS, but it has major limitations: distance. As light (or any other signal) moves along the line, it weakens or loses strength. So it’s hard to use DAS to shoot more than a few tens of miles. That’s not a small thing, but what if you wanted to look, say, in 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 leading from Los Angeles to Valaparaíso, Chile, in parallel with the highly active Pacific coast of America. This team studied earthquake fingerprints on normal cable signal traffic.
But their method was wrong: They couldn’t tell how far something had happened, just that it had happened. “They found an earthquake, but … they didn’t know where it was coming from,” Spica says.
Of course, if you chat with your friend overseas, your voices can connect with each other without any problems. This is because these cables are equipped with devices called repeaters. Like players in a big phone game (only much, much more reliable), repeaters receive an incoming signal and amplify it to send it to the next.
For several years, some scientists have supported a proposal called SMART to equip new repeaters on future cables with inexpensive seismic, pressure and temperature sensors. Telecommunications companies are now paying attention: One SMART project – a cable connecting the Portuguese mainland with the Atlantic islands – is due to become operational in 2025.
But submerged cable repeaters on the seabed have a second function: To help cable operators locate potential problems, repeaters can send back part of their signal.
Marra and his colleagues took advantage of this existing security. They sent an infrared laser over the cable and examined the signals returning from each repeater. They could break a cable across the ocean into tens of miles.
“I know others have been thinking about how to do it,” said Bruce Howe, an oceanographer at the University of Hawai’i who also did not participate in the article, “but they did.”
Marr’s group tested their technology on a transatlantic cable between Southport in northwest England and Halifax in Atlantic Canada. They were able to detect not only earthquakes – one from northern Peru and the other from all over Indonesia – but also noise from the water moving in the ocean.
There are a few catches. First, says Howe, this type of detection is different from what seismologists are used to. Marra and colleagues have not yet been able to measure the magnitude of the earthquake. And recognizing earthquakes from, say, ocean temperature shifts can be difficult. Here, several methods – such as this latest technique plus SMART – can work in tandem.
Many scientists are excited about the potential of cables. “I really feel like the biggest breakthroughs.” [in seismology] they will be done at sea because there is so much to explore, ”says Spica. They could significantly improve our tsunami warning systems. They can help geologists look at misunderstood places where tectonic plates join or separate, such as mid-ocean ridges. And maybe they could help oceanographers keep track of what’s happening in the warming oceans.
“Money is, as always, a major hurdle,” says Howe, “but recent progress suggests we can overcome it.”