Caltech's seismologists, in collaboration with Google's optics experts, have developed a method for using existing underwater telecommunication cables to detect earthquakes.
A vast network of over one million kilometers of fiber optic cables is found at the bottom of Earth's oceans. In the 80s, telecommunications companies and governments began laying these cables, each of which can stretch thousands of kilometers. Today, the global network is considered the backbone of international telecommunications.
Scientists have long been looking for a way to use those submerged cables to monitor the seismicity. After all, more than 70% of the globe is covered by water, and it is extremely difficult and expensive to install, monitor and operate underwater seismometers to track earthquakes under the sea.
To detect earthquakes and tsunamis we already have a network to exploit
The ideal, researchers say, is to monitor seismicity using existing infrastructure along the ocean floor.
now Zhongwen Zhan, PhD, assistant professor of geophysics, has devised with his colleagues at Caltech a way to analyze light traveling through “lit” fibers (existing, working undersea cables) to detect earthquakes and ocean waves without needing additional equipment.
The new method is described in the journal Science.
“This new technique can actually convert most underwater 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 earthquake monitoring on the ocean floor that could feasibly be implemented worldwide. It could complement the existing network of ground-based seismometers and tsunami monitoring buoys to make detection of underwater earthquakes and tsunamis much faster.
Zhongwen Zhan, Caltech
How the method to detect tsunamis and earthquakes works with underwater telecommunication cables
Cable networks work through the use of lasers that send pulses of information. They do this through glass fibers that deliver data at speeds exceeding 200.000 kilometers per second to receivers at the other end.
In their work, the researchers focused on 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, Chile.
On land, all sorts of disturbances, (such as temperature changes and even lightning) can change the polarization of light traveling through fiber optic cables. Since the temperature in the deep ocean remains nearly constant and there are few disturbances there, the change in polarization from one end of the Curie cable to the other remains quite stable over time.
Except during earthquakes and when storms produce large ocean waves. In that case, the polarization changes suddenly and drastically, allowing researchers to easily identify such events in the data.
How this method can change tsunami and earthquake detection
Today it takes minutes for seismic waves from earthquakes that occur kilometers away from the coast to reach Earth's seismometers. And tsunamis take even longer. Using this new technique, the entire length of a submarine cable acts as a single sensor, measuring the situation up to 20 times per second. This means that if an earthquake strikes near a particular area, a warning could be delivered to potentially affected areas within seconds.
During the nine months of testing reported in the new study, the researchers detected about 20 moderate to large earthquakes along the Curie cable. Among them, the 7,7 magnitude earthquake that took place off the coast of Jamaica on January 28, 2020.
Zhan and his colleagues at Caltech are now developing a machine learning algorithm. It would be able to determine whether the detected changes in polarization are produced by earthquakes or tsunamis, or just by ships or a crab moving the cable.
