Transforming submarine cables into a global monitoring system for seismic and environmental risks

European Union researchers are exploring how undersea communications cables could double as environmental and seismic sensors – a potential game-changer for early warning systems.

Beneath the world’s oceans, a silent revolution is underway. More than 1.48 million kilometers of underwater fiber optic cable carries almost all of the world’s Internet and telephone traffic. Researchers are now showing that these cables can do more than just transmit data – they can listen to the planet.

By picking up small changes in how light travels through them, these cables can detect shifts in motion, vibration, and temperature of the seafloor and water.

A European Union-funded research initiative in the emerging field of fiber-optic seafloor sensing is working on the technology to turn the ocean floor into a vast, real-time observatory. The discoveries made should allow scientists to better monitor climate change, track tectonic activity and improve warnings of tsunamis and earthquakes.

Underwater ground sensor network

About 70% of the Earth’s surface is covered by water, but most of it is inaccessible to conventional seismic instruments.

“We have excellent satellite coverage of the sea surface,” said Marc-André Goucher, a marine geologist at the Geo-Ocean Research Center in Brest, France. “But at depth, where most earthquakes and tsunamis originate, we have very few direct observations.”

This is starting to change thanks to research into how submarine cables can be repurposed as a global sensor network.

Two complementary technologies dominate this field: distributed acoustic sensing (DAS) and Brillouin optical time domain reflectometry (BOTDR).

Goucher led a seven-year, EU-funded research initiative called FOCUS that ended in September 2025.

She explored how these two techniques can detect small deformations – just one or two centimeters – along active deep-sea fault lines.

To test the concept, the team installed a 6-kilometre-long prototype cable across the seafloor along the North Alfio Fault off Catania, Sicily. The area is vulnerable to seismic activity because it is located near Mount Etna, Europe’s largest and most active volcano.

Listen to the seabed

In 1908, a 7.1-magnitude earthquake struck the Strait of Messina between Sicily and mainland Italy, triggering a devastating tsunami that killed more than 80,000 people in one of Europe’s deadliest natural disasters. The researchers’ goal is to better assess seafloor fault movements and help prepare coastal communities before similar events occur again in the future.

Goucher’s team worked with the Italian National Institute for Nuclear Physics (INFN), which agreed to connect the FOCUS cable prototype to the existing submarine cable run from the Seafloor Observatory off the coast of Catania, Sicily.

The cable, which was designed in collaboration with IDIL, a French company specializing in fiber-optic systems, resembles regular communications cables, but includes special sensor fibers that are more sensitive to mechanical disturbances on the seabed.

Measuring just 9mm thick, it combines two types of optical fibres: loose fibers, similar to communications cables, and tightly packed fibres, which are more sensitive to stress (mechanical deformation). The researchers used BOTDR to measure subtle changes in fiber length corresponding to stress in the Earth’s crust.

“Our main goal was to find out what happens before an earthquake occurs, to detect early deformation before sudden rupture,” said Dr. Giovanni Barrica from the University of Catania.

At the moment, no significant movement has been observed, but this is also useful. “This means that the fault is currently closed and may be leading to a buildup of tectonic stress,” Goucher said. “When this tension is relieved, we will be watching.”

The Sicilian cable has already proven its value. In late 2020, it detected a massive ocean current, possibly caused by an underwater landslide, a type of “marine avalanche” that can travel hundreds of kilometers and sometimes trigger tsunamis.

Such events are rarely observed, but fiber-optic data captured their signature in detail. This opens opportunities to monitor and detect secondary hazards that could threaten coastal communities and critical seafloor infrastructure.

From Sicily to the Caribbean

Meanwhile, the FOCUS team also explored the potential of an underwater communications cable network to improve environmental monitoring.

Researchers used local underwater cable networks off the island of Guadeloupe in the Caribbean Sea to monitor changes in water temperature on the sea floor.

Initially, the team had to manually collect data every few months from ground-based fiber optic relay cabinets. Now, thanks to the always-on setup, they can monitor the cables remotely every three hours.

Their measurements record how light is scattered inside the cables. When the cable is disturbed, small defects in the fiber shift slightly, changing the pattern of light. Scientists track these changes to understand what’s happening at the seafloor.

“If there’s anything disturbing the cable — if it’s being pulled, moved, heated or cooled — we can measure that,” Goucher explained.

While analyzing how the optical signal changes with temperature, they discovered an increase of about 1.5 degrees Celsius in shallow water over two years, which is consistent with sea surface temperature measurements made by satellites. At the same time, there was a massive coral bleaching event, with coral losses reaching around 30%.

In deeper waters off Guadeloupe, from 300 to 700 metres, the cables show smaller temperature increases of 0.2 to 1 degree Celsius.

These results have just been accepted for publication (in Geophysical Research Letters) means that thousands of kilometers of communications cables can be used to monitor changes in deep ocean temperatures, adding a new dimension to weather measurements.

“While our initial focus is tectonics, these measurements show how the same cables can track climate-related changes,” Goucher said. “The potential for integrated monitoring of the environment and risks is enormous.”

This technique can also be extended to other earthquake-prone areas such as Japan, Cascadia (along America’s Pacific coast) and other places in the Mediterranean.

DAS systems can detect the initial seismic waves of an earthquake within seconds, while BOTDR systems can track long-term stress that builds up over time. DAS provides immediate earthquake and tsunami warning capability, while BOTDR can provide long-term monitoring of fault deformation, with potential application to earthquake prediction.

“The new secondary use of fiber optic cables could represent a huge advance in seismology and hazard warning,” Goucher said. “We are effectively turning the world’s digital nervous system into an ecosystem.”

With more cooperation and investment, the ocean floor – once almost invisible – could become one of the most powerful scientific tools to protect lives and understand our changing planet.