Seismologists use dark fiber to detect undersea earthquakes

subsea fiber optic cable dark fiber earthquake
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ITEM: Researchers say they’ve worked out a potential new use case for the thousands of kilometers of dark fiber crisscrossing the ocean floors – detecting undersea earthquakes and undiscovered fault lines.

According to a new research paper published in Science, a research team at the University of California at Berkeley successfully turned a 20-km section of an existing subsea cable system off the California coast into a row of seismometers.

They accomplished this with a technique called “distributed acoustic sensing”, in which laser pulses are beamed through the fiber and analyzed to detect physical movement of the actual cable.

Result: the team not only recorded a 3.5-magnitude quake, but also discovered a previously unknown fault system.

The idea of using existing fiber to detect quakes has been around for awhile. Last year, Nature Communications published a study from a research team from GFZ German Research Centre for Geosciences that used a similar technique to turn fiber-optic cables into seismic sensors, and with similar results. Another study earlier this year from UC Berkeley and Lawrence Berkeley National Lab also reported success with the distributed acoustic sensing technique.

However, those tests used terrestrial cables. According to Gizmodo, this matters because the distributed acoustic sensing technique requires placing actual sensors (dubbed “interrogators”) along the cable to measure the laser pulses.

Needless to say, that’s a challenge for subsea cables even in relatively shallow waters, let alone cables a couple of kilometres below the surface. The latest UC Berkley test used part of the Monterey Accelerated Research System (MARS) cable in Monterey Bay, where the water was no deeper than 100 meters. And obviously, 20 km is a relatively tiny distance on trans-Pacific cable systems spanning thousands of kilometers.

There are other limitations too, Jonathan Ajo-Frankin from Lawrence Berkeley National Lab told Gizmodo in February:

A single sensing location along the line is less sensitive than a single seismometer, and can only measure strain on the fiber in one direction, though he pointed out that sensing locations would be more closely spaced and far more numerous than present-day seismometers. It also produces a ton of data – 128 terabytes in just three months – which can be difficult to manage, store, and sift through.

Still, the results from UC Berkeley’s subsea test were very encouraging – especially for seismologists who are keen to figure out additional ways to detect and map undersea fault lines.

Put simply, you can’t realistically put that many seismological sensors on the ocean floor. According to a report from AAAS, the National Science Foundation currently monitors a total of 252 ocean-bottom seismometers – which isn’t nearly enough coverage. Moreover, those seismometers have limited battery life and storage space. There are other solutions to fill those gaps, but they’re not cheap or easy to deploy.

That’s why researchers say turning existing infrastructure into vast sensor networks would be a game-changer for seismology in terms of both coverage and cost.

While distance remains an issue – both in terms of how far distributed acoustic sensing can reach and the cost of leasing dark fiber from the US west coast to Asia – that could be offset by initially focusing monitoring efforts along the coastlines, which in itself would be invaluable. The AAAS report notes that the epicentre of the 2011 Tohoku earthquake in March that created in the tsunami that devastated northeastern Japan was around 70 km offshore – and seismologists had no idea the fault in question was capable of producing a 9.1 quake.

In any case, the next phase of the research will involve testing the technology at deeper levels and further distances in different marine environments.

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