A team of researchers from the UK, Italy and Malta has found a way to use fibre-optic cables already on
In their paper published in the journal ‘Science’ on June 14, 2018, the team describes how they made their discovery and how it would work.
Submarine optical fibre cables are the backbone of international and intercontinental telecommunication. Since the first installations in the 1990s, the number of links has increased exponentially due to growth in the Internet and mobile services.
The current total length of submarine fibre cables is over 1 million km. In 2016 alone approximately 100,000 km of cable was added to the existing network and another 200,000 km is planned by mid-2018.
Optical fibres can detect seismic events over km-scale links using distributed acoustic sensing (DAS) techniques.
An underwater fibre-optic cable stretching from Malta to Sicily sensed a 3.4 magnitude quake in the Mediterranean Sea on September 2, 2017.
Researchers confirmed this detection with two nearby seismometers. One seismometer near the Malta end of the cable, closer to the earthquake’s epicentre, detected the quake shortly before the cable, and a seismometer near the Sicily end identified it shortly after.
The researchers report that one of their members, Giuseppe Marra of Britain’s National Physical Laboratory (NPL) proposes to shine a little light into the oceans by co-opting infrastructure built for an entirely different purpose.
Dr Marra and his colleagues hope to use the planet’s 1m-kilometre network of undersea fibre-optic cables, which carry the internet from continent to continent, as a giant submarine sensor.
Their approach - which would make it possible to sense temblors without installing new seafloor equipment - could permit detection of earthquakes in regions where seismic monitoring has otherwise been difficult, including in subduction zones or in remote ocean regions lacking seismometers.
Dr Marra proposes to use noise to detect earthquakes. The idea is to shine a high-quality laser beam through one of the optical fibres in the cable. At the other end that fibre is connected to another in the same cable for the return journey, forming a loop. The seismic waves from a nearby earthquake will deform the cable minutely, leaving the returning light slightly out of phase with the light emitted by the laser. The discrepancies involved are tiny: on the order of millionths of a metre for a cable several thousand kilometres long. Measuring them requires equipment capable of discriminating between femtoseconds. A femtosecond is a millionth of a billionth of a second, which is roughly to a second what ten cents is to the GDP of the entire planet.
However, the researchers haven’t yet studied how the laser method works on the long fibre-optic cables that run between continents.
But eventually, it could help bolster tsunami detection, monitor earthquakes in remote areas like the Arctic, and more, said the researchers.
Even though 70% of the Earth’s surface is covered with water, almost all seismic stations are on land. As a result, underwater earthquakes remain largely undetected, limiting scientists’ ability to identify the source mechanisms of underwater seismic events.
The existing optical fire cable networks could help expand quake detection capabilities if the fibres therein were used as the sensing element, according to the research paper.