A 10,000-kilometre-long fibre-optic cable owned by Google that is at the bottom of the Pacific Ocean can be used to detect deep-sea seismic activity and ocean waves.

Zhongwen Zhan at the California Institute of Technology in Pasadena, California, and his colleagues, including researchers at Google, used traffic data from one of the tech giant’s optical fibres to measure changes in pressure and strain in the cable. Using this data, they could detect earthquakes and ocean waves called swells generated by storms.

Over a nine-month period, the team recorded around 30 ocean storm swell events and around 20 earthquakes over magnitude 5 – strong enough to damage buildings – including the magnitude 7.4 earthquake event near Oaxaca, Mexico, in June 2020. The team had wanted to measure a tsunami, but none occurred during the monitoring.

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Deploying and maintaining geophysical instruments on the seafloor is difficult and expensive, so underwater seismic stations are relatively rare. Anthony Sladen at the University Côte d’Azur in France says that the study is “a major step in exploiting the benefits of existing cables”.

There have been previous efforts to use fibre-optic cables as seismic sensors, but these required specialised laser-detection equipment at both ends of the cable or the use of dedicated fibres within the cables. Such fibres are in short supply on deep-sea cables, so dedicating one to measuring earthquakes would be difficult.

Zhan says his team’s approach of using an existing traffic fibre is more flexible and scalable as it doesn’t need new infrastructure. “This is exciting as if only a fraction of the million kilometres of submarine fibre-optic networks could be used as sensors, there would be vast improvements in the amount and coverage of seismic data.”

Journal reference: Science, DOI: 10.1126/science.abe6648