Seafloor Communication Cables, a New Breed of Earthquake-Detectors

One of the most devastating aspects of the tsunami that lashed the coasts of South Asia in 2004 was the lack of an early warning system. The waves unleashed by the earthquake travelled at 700 kilometers per hour, but they didn’t arrive to the coast of India and Sri Lanka until four hours later. The tragedy was unavoidable, but a timely evacuation would have no doubt saved many lives. The main paradox is that, despite two thirds of our planet being covered by water, most seismographers are found on land. The reason for this is that installing such systems on the seafloor, miles below the surface, can be a costly and complex affair. However, we do have the required infrastructure already waiting to be used: the thousands of submarine communication cables which allow you to read this article anywhere in the planet. And, as with the legendary Newton’s apple, the discovery that they could detect earthquakes was born out of coincidence.

Giuseppe Marra works as a metrologist at the National Physical Laboratory in Teddington (U.K). There he analyses the fiber optics connections between the network of European atomic clocks. These subterranean cables usually carry noise from their surroundings, such as vibrations from traffic aboveground. However, while examining the data from October 2016, Marra noticed a peculiar reading. When he studied the date of the event he realized that it could be attributed to a 5.9-magnitude earthquake that had shaken the center of Italy during that month. This would prove to be his eureka moment: fiber optics cables could operate as seismographers thanks to the subtle changes of phase in laser beams when they are affected by external vibrations. However, if this system for the detection of earthquakes was to work, they would have to test it in areas safe from human-made disturbances. That’s when he thought of the submarine cables. Marra and other scientists put their idea to test with the submarine cable joining Malta and Sicily and were able to discern a 3.4-magnitude quake. They also found that while one cable could detect the phenomenon, at least three of them were needed to pinpoint the exact location of the event.

Currently, there are more than a million kilometers of submarine cables covering the main seas and oceans of the planet. The best part is that putting that infrastructure to work for detect earthquakes would require a comparatively modest investment: just 40.000 € to retrofit each end of the cables with a small cabinet of lasers and optical equipment. But there is more to the story.

A new seafloor mapping system

There are many unknowns regarding the ocean floor in most of the world. It is an unchartered continent that can reach depths of eleven kilometers in some places like the Mariana Trench. Nevertheless, by using the new technology developed by Marra and his team, the mantle and behavior of tectonic plates could be mapped and provide valuable information, such as how seismic waves vary depending on the density of rock they pass through. The resulting 3D image would resemble a computer tomography (CT) scan. For the time being, though, the system has yet to be fine-tuned before it can go beyond the detection of earthquakes.

Source: Science Mag