Using fiber optic cables on the high seas to detect earthquakes

Caltech seismologists working with optics experts at Google have developed a method for using existing submarine telecommunications cables to detect earthquakes. This technique could lead to improved earthquake and tsunami warning systems around the world.

A vast network of more than a million kilometers of fiber optic cables lies at the bottom of the Earth’s oceans. In the 1980s, telecommunications companies and governments began to lay these cables, each of which could span thousands of miles. Today, the global network is considered the backbone of international telecommunications.

Scientists have long searched for a way to use these submerged cables to monitor seismicity. After all, more than 70% of the globe is covered with water, and it is extremely difficult and expensive to install, monitor and operate underwater seismometers to track the movements of the Earth under the seas. The ideal, according to the researchers, would be to monitor seismicity using the infrastructure already in place along the ocean floor.

Previous efforts to use optical fibers to study seismicity have relied on the addition of sophisticated scientific instruments and / or the use of so-called “dark fibers”, optical fiber cables that are not actively used.

Now Zhongwen Zhan (Ph.D. ’13), assistant professor of geophysics at Caltech, and his colleagues have devised a way to analyze light traveling through “lit” fibers – in other words, cables. existing and functional submarines – to detect earthquakes and ocean waves without the need for additional equipment. They describe the new method in the February 26 issue of the journal Science.

“This new technique can really convert the majority of submarine cables into geophysical sensors several thousand kilometers long to detect earthquakes and possibly tsunamis in the future,” Zhan explains. “We believe this is the first ocean floor seismicity monitoring solution that could be implemented globally.” It could complement the existing network of ground seismometers and tsunami monitoring buoys to facilitate the detection of underwater earthquakes and tsunamis more quickly in many cases. “

Wired networks operate through the use of lasers that send pulses of information through glass fibers bundled in cables to deliver data at rates faster than 200,000 kilometers per second to receivers on the other end. . In order to make the best use of cables, that is, to transfer as much information as possible between them, one of the things that operators monitor is the polarization of the light that travels through the fibers. Like any other light that passes through a polarizing filter, laser light is polarized, that is, its electric field oscillates in one direction rather than in any direction. Controlling the direction of the electric field can allow multiple signals to pass through the same fiber simultaneously. Upon reception, the devices check the polarization state of each signal to see how it has changed along the cable path to make sure the signals are not mixed.

In their work, the researchers focused on the Curie Cable, an undersea fiber-optic cable that stretches more than 10,000 kilometers along the eastern edge of the Pacific Ocean from Los Angeles to Valparaiso, in Chile. (Although Zhan says the technique could be used on several of the hundreds of submarine cables that crisscross the globe.)

On earth, all kinds of disturbances, such as temperature changes and even lightning strikes, can alter the polarization of light passing through fiber optic cables. Because the temperature in the deep ocean remains almost constant and there is so little disturbance there, the change in polarization from one end of the Curie cable to the other remains fairly stable over time, Zhan and his colleagues found.

However, during earthquakes and when storms produce large ocean waves, the polarization changes suddenly and drastically, allowing researchers to easily identify such events in the data.

Currently, when earthquakes occur miles offshore, seismic waves can take a few minutes to reach land-based seismometers and even longer for tsunami waves to be checked. Using the new technique, the entire length of a submarine cable acts as a single sensor in a location that is difficult to monitor. Polarization can be measured up to 20 times per second. This means that if an earthquake occurs near a particular area, a warning could be issued to potentially affected areas within seconds.

During the nine months of testing reported in the new study (between December 2019 and September 2020), researchers detected around 20 moderate to large earthquakes along the Curie Cable, including the magnitude 7.7 earthquake. which took place off Jamaica in January. 28, 2020.

Although no tsunamis were detected during the study, the researchers were able to detect polarization changes produced by ocean swells originating in the Southern Ocean. They believe that the polarization changes observed during these events were caused by pressure changes along the seabed when powerful waves passed through the cable. “This means that we can detect ocean waves, so it is plausible that one day we will be able to detect tsunami waves,” Zhan says.

Zhan and his colleagues at Caltech are currently developing a machine learning algorithm that would be able to determine whether the detected polarization changes are produced by earthquakes or ocean waves rather than some other system change, such as a ship or a crab moving the cable. They expect that the entire detection and reporting process can be automated to provide critical information in addition to the data already collected by the global network of land-based seismometers and the Deep-Ocean Assessment and System (DART) buoys. Reporting of Tsunamis), operated by the National Data Buoy Center of the National Oceanic and Atmospheric Administration.

New Science the article is entitled “Detection of seismic and water waves by optical polarization on transoceanic cables”.