Subsea cables that connect the world’s internet could be used to track seismic activity following a new discovery.
Geoscientists may have found a way to create vast seismic activity detectors using existing subsea internet cables. In a paper published to Nature Communications, an international team was able to show that fibre optic cables on the bottom of the North Sea detected a magnitude-8.2 earthquake near Fiji in 2018.
While a proof of concept, using subsea cables could provide significant benefits for earthquake protection. Oceans cover two-thirds of the earth’s surface, but placing permanent seismometers under the sea is prohibitively expensive.
“Fibre optic communications cables are growing more and more common on the sea floor. Rather than place a whole new device, we can tap into some of this fibre and start observing seismicity immediately,” said Ethan Williams of Caltech, a lead author of the study.
Key to the breakthrough is a technology called distributed acoustic sensing (DAS). Developed for energy exploration, DAS sensors shoot a beam of light down a fibre optic cable.
‘With the flip of a switch, we have an array of 4,000 sensors that would’ve cost millions to place’
– ETHAN WILLIAMS
Tiny imperfections in the cable reflect back minuscule amounts of the light, allowing the imperfections to act as ‘waypoints’. When seismic waves jostle the cable, these waypoints shift minutely in location, thereby changing the travel time of the reflected light waves and allowing scientists to track the waves’ progression.
As part of the North Sea experiment, the researchers used a 40,000-metre section of subsea cable that connects a windfarm to land. There are millions of tiny imperfections in the cable, so they averaged out the imperfections in each 10-metre segment, creating an array of more than 4,000 virtual sensors.
The North Sea array’s fine degree of sensitivity tracked tiny, non-earthquake-related seismic noises and found evidence that supports a longstanding theory that these events – called microseisms – are the product of ocean waves.
“With the flip of a switch, we have an array of 4,000 sensors that would’ve cost millions to place,” Williams said.