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By Donna Lu

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Fibre-optic cables are good for speedy downloads – and for detecting earthquakes.

Nathaniel Lindsey at the University of California, Berkeley, and his colleagues temporarily turned 20 kilometres of existing underwater fibre-optic cables into a row of seismic sensors along the floor of the Pacific Ocean.

The researchers recorded a 3.5-magnitude earthquake and discovered a new fault system off the coast of California.

Fibre-optic cables transmit information in the form of light. An extensive underwater network of these cables connects all the continents except Antarctica and transmits telecommunications data, including telephone and internet traffic.

The researchers used a technique called distributed acoustic sensing, which works by sending pulses of light through the cable and analysing the light that returns to detect slight movements.

“If you start moving a certain portion of the fibre because there’s some seismic wave propagating, you’ll be able to see that seismic wave strain on the cable,” says Lindsey.

During a four-day experiment, the team happened to measure an earthquake, as well as the scattering of the earthquake’s wave fronts by previously unknown faults – fractures in Earth’s crust – in Monterey Bay.

We still don’t know the locations of all the faults on Earth, particularly in hard-to-measure locations such as under cities or on the sea floor.

Distributed acoustic sensing could be used to turn other existing cables that aren’t in use into offshore seismic sensors to map underwater faults and earthquakes more extensively, says Lindsey.

“The number of sensors we have on the sea floor on the west coast of North America can be counted on one hand,” he says. “There is a lot of cable in the ocean relative to the amount of sensors that we currently have.”

The cables in this study were located in a shallow coastal area, with a maximum depth of around 100 metres. In future, the researchers plan to test the technique in areas where the sea floor is much deeper or more sloped.

Journal reference: Science, 10.1126/science.aay5881