Geoscientists Discover New Use for Global Fiber Optic Cable Network: Seismic Monitoring
This network is sensitive enough to pick up even the slightest seismic activity, including our footsteps
Hiya!
Most of modern life today — the internet, phones, television, and other high-speed communications — relies on a world-spanning network of fiber-optic cables. These cables connect our utilities, neighborhoods, mass transit, traffic cameras, and more. However, scientists found a creative new use for this vast network.
Rather than transmitting information, geoscientists are using the cables to gain a better understanding of seismic activity and even peer inside the Earth's interior. These cables are all around better than typical seismometers, including being sensitive enough to track our footsteps.
Fiber-Optic Cables
Beneath our feet lies a worldwide network of optical fibers, which are composed of strands of glass or plastic thinner than a human hair, bundled into groups of two to several hundred to form a cable.
In a way, fiber-optic cables have made our species more connected than ever before by removing the obstacle of distance. Physical distance used to be a considerable limitation, even after postal services and the telephone emerged. But now, thanks largely to fiber-optic cables, we can connect with anyone, anywhere, at any time. Easy peasy.
Some estimates suggest over 95 percent of all the data that moves around the planet travels through these cables. They span oceans and snake beneath and between cities and continents.
In February 2025, Meta (formerly Facebook) announced their “most ambitious subsea cable endeavor yet,” which they named Project Waterworth. They explain in their statement:
“Once complete, the project will reach five major continents and span over 50,000 km (longer than the Earth’s circumference), making it the world’s longest subsea cable project using the highest-capacity technology available.”
If successful, Meta will utilize an undersea high-capacity, fiber-optic cable that is longer than the planet is round. The Earth’s circumference is 24,901 miles (40,07 km), but Meta’s cable will stretch for 31,000 miles (50,000 km).
For most of us, these cables are out of sight, out of mind as long as we can stream our shows, scroll social media, and receive calls without interruption. According to John Ballato, a materials scientist at Clemson University who specializes in glass fibers,
“Nearly 500 million kilometers [over 3 million miles] of optical fibers are made per year, and hardly anyone realizes that today’s modern conveniences wouldn’t exist without them.”
However, scientists are continually finding new ways to utilize this ever-growing, Earth-spanning network of cables, like the geoscientists who have begun using them as seismic sensors to gain a deeper understanding of the planet's interior.
How the Cables Work
Now that we’ve discussed the broad purpose of fiber-optic cables, let’s unravel the particulars about how they work before I tell you about the geoscientist’s ingenuity. Because if you’re like me, you may be a bit confused about how hair-thin fibers, which are designed for telecommunication, could serve as seismic sensors. It seems a bit contradictory, or at least unrelated.
Thankfully, it’s relatively simple to grasp.
For standard telecommunications, data is encoded into light pulses that are sent through the fibers, which are bundled into cables, and then decoded at the receiving end. Therefore, the fibers serve as the conduit that delivers information from one point to another.
But, sometimes, minor, randomly located defects or warping within the fibers act like mirrors, scattering the light and causing distortions in the information. These defects cause subtle delays in the light reflecting back toward the beginning of the cable. This is known as the backscatter, or Rayleigh scattering.
For instance, if I were chatting with you on the phone and an earthquake occurred in the area where the fiber-optic cable connecting us was located, the vibrations could alter the frequency of my voice, making it sound distorted.
Experts can use a device called an interrogator to interpret shifts in backscatter to determine precisely where the vibrations deflected the fiber and by how much.
Through the use of interrogators, it was discovered that the backscatter from fiber-optic cables could be applied to detect sounds.
Distributed Acoustic Sensing
In the 1980s, the Navy pulled fiber optic cables behind ships to detect the sounds of nearby enemy submarines. This technique, referred to as distributed acoustic sensing (DAS), involves measuring acoustic waves at many points along a cable’s length.
By the early 2000s, the oil and gas industry had begun experimenting with DAS by snaking fiber-optic cables down wells to monitor tremors and seismic activity as they drilled, and using backscatter lasers to locate sharp temperature changes, indicating a ruptured pipe or well.
Since then, experts have discovered many other applications for this approach, ranging from measuring the speed of waves moving through the ground to monitor changes in soil moisture to tracking the movement of animals, including humans, by their footsteps (which I’ll discuss in more detail soon).
As Andreas Fichtner, a researcher and Professor for Seismology and Wave Physics at ETH Zurich in Switzerland, told Paul Voosen of Science in 2021,
“You have a new hammer, you have to go for every nail you can possibly find.”
One limitation of DAS, however, is that “lit cables,” which are actively carrying information, can’t typically be used to take reliable measurements because the two signals interfere with each other. But then scientists discovered another option.