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Caltech seismologists working with optics experts at Google have developed a method to use existing submarine telecommunications cables to detect earthquakes. The technique could lead to the improvement of earthquake and tsunami warning systems around the world.
A vast network of more than a million kilometers of fiber optic cable lies at the bottom of the Earth’s oceans. In the 1980s, telecommunications companies and governments began laying these cables, each of which can span thousands of kilometers. Today, the global network is considered the backbone of international telecommunications.
Scientists have long searched for a way to use those submerged cables to monitor seismicity. After all, more than 70 percent of the world is covered by water, and it is extremely difficult and expensive to install, monitor, and run underwater seismometers to track the movements of the land under the seas.
The ideal, say the researchers, is to monitor seismicity by making use of the infrastructure that already exists along the ocean floor.
Now Zhongwen Zhan, PhD, assistant professor of geophysics at Caltech, and his colleagues have devised a way to analyze the light traveling through “illuminated” fibers – in other words, existing and working submarine cables – to detect earthquakes and waves. oceanic without the need for any additional equipment. They describe the new method in the February 26 issue of Science magazine.
“This new technique can really turn most submarine cables into geophysical sensors thousands of kilometers long to detect earthquakes and possibly tsunamis in the future,” says Zhan. “We believe this is the first solution for monitoring seismicity at the bottom of the ocean that could be feasibly implemented worldwide. It could complement the existing network of land seismometers and tsunami monitoring buoys to make the detection of submarine earthquakes and tsunamis much faster in many cases. “
Cable networks work by using lasers that send pulses of information through bundled glass fibers within cables to deliver data at speeds in excess of 200,000 kilometers per second to receivers at the other end, where devices check the polarization state of each signal to see how it has changed along the cable run to make sure the signals don’t get mixed up.
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In their work, the researchers focused on the Curie Cable, a fiber optic submarine cable that stretches for more than 10,000 kilometers along the eastern edge of the Pacific Ocean from Los Angeles to Valparaíso, Chile.
On the ground, all kinds of disturbances, such as changes in temperature and even lightning, can change the polarization of the light that travels through fiber optic cables. Because the temperature in the deep ocean remains nearly constant and because there are very few disturbances there, the change in polarization from one end of the Curie cable to the other remains fairly stable over time, Zhan and colleagues found. .
However, during earthquakes and when storms produce large ocean waves, the polarization changes suddenly and dramatically, allowing researchers to easily identify such events in the data.
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Currently, it takes minutes for seismic waves from earthquakes that occur miles offshore to reach ground seismometers, and even longer for any tsunami wave to be verified. With the new technique, the entire length of a submarine cable acts as a single sensor for difficult-to-monitor locations. Polarization can be measured up to 20 times per second. That means if an earthquake occurs near a particular area, a warning could be sent to potentially affected areas in a matter of seconds.
During the nine months of testing reported in the new study, the researchers detected around 20 moderate to large earthquakes along the Curie Cable, including the 7.7 magnitude earthquake that took place off Jamaica on January 28, 2020. .
Although no tsunamis were detected during the study, the researchers were able to detect changes in polarization produced by ocean swells that originated in the Southern Ocean. They believe that the changes in polarization were caused by pressure changes along the seafloor as 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,” says Zhan.
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Zhan and his colleagues at Caltech are now developing a machine learning algorithm that could determine whether the detected changes in polarization are caused by earthquakes or ocean waves rather than some other change in the system, such as a ship or crab moving the cable. They hope that the entire detection and reporting process can be automated, adding to the network of land-based seismometers and buoys in the DART (Deep Ocean Tsunami Assessment and Reporting operated by NOAA) system.
– Fountain: Caltech News by Kimm Fesenmaier
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