German researchers have also discovered that fibreoptic cables can also pick up seismic signals from hammer shots, passing cars or wave movements in the ocean. Philippe Jousset and Thomas Reinsch, from GFZ German Research Centre for Geosciences, carried out the investigation together with colleagues from Island, UK, Berlin, Germany, and Potsdam, Germany.
The scientists sent pulses of laser light through an optical fibre, which was part of a 15 km long cable deployed in 1994, within the telecommunication network on Reykjanes peninsula, SW Island, crossing a well-known geological fault zone in the rift between Eurasian and American tectonic plates.
The light signal was analysed and compared to data sets from a dense network of seismographs. "Our measurements revealed structural features in the underground with unprecedented resolution and yielded signals equalling data points every four meters", says Jousset, adding: "This is denser than any seismological network worldwide."
After presenting preliminary ideas at several conferences since 2016, Philippe was told that the new method was a 'game changer for seismology'. Although the method is not new in other applications, having been used for in boreholes for reservoir monitoring for years, the team is the first to have carried out such measurements along the surface of the ground for seismological objectives, and with such a long cable.
Their current study not only shows well-known faults and volcanic dykes. The scientists also found a previously unknown fault below the ground surface. Furthermore, the team measured subsurface deformation taking place over a period of several minutes. Small local earthquakes, waves originating from large distant quakes, and microseism of the ocean floor were also recorded via fibreoptic cable. "We only need one strand of a modern fibreoptic line", says Charlotte Krawczyk, Director of GFZ's Geophysics Department.
Fibreoptic cables can be used to detect earthquakes and other ground movements. The data cables can also pick up seismic signals from hammer shots, passing cars or wave movements in the ocean (Image: Philippe Jousset for GFZ German Research Centre for Geosciences)
The advantages of the new method are enormous as there are countless fibreoptic cables spanning the globe in the dense telecommunication network, especially beneath megacities with high seismic hazards, such as San Francisco, Mexico City, Tokyo, or Istanbul. Such cables could provide a cost efficient and widely spread addition to existing seismological measuring devices.
Future studies are planned to investigate whether deep sea cables can also be used for seismic measurements, and the scientists are optimistic. They think that the cables on the sea floor will detect submarine earthquakes, tectonic plates’ ground motions, and water pressure variations, helping seismologists, as well as oceanographers.
This study was published in the journal Nature Communications on July 3, 2018.