Seismologists working with Fiberoptics experts at Google have developed a method to use existing underwater telecommunication cables to detect earthquakes. The technique could lead to the improved 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 Earth’s oceans. In the 1980s, telecommunication 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 sought a way to use those submerged cables to monitor seismicity. After all, more than 70 percent of the globe is covered by water, and it is extremely difficult and expensive to install, monitor, and run underwater seismometers to keep track of the earth’s movements beneath the seas. What would be ideal, researchers say, is to monitor seismicity by making use of 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,” fiber optic cables that are not actively being used.
Scientists have come up with a way to analyze the light traveling through “lit” fibers—in other words, existing and functioning submarine cables—to detect earthquakes and ocean waves without the need for any additional equipment.
This new technique can really convert the majority of submarine cables into geophysical sensors that are thousands of kilometers long to detect earthquakes and possibly tsunamis in the future. We believe this is the first solution for monitoring seismicity on the ocean floor that could feasibly be implemented around the world. It could complement the existing network of ground-based seismometers and tsunami-monitoring buoys to make the detection of submarine earthquakes and tsunamis much faster in many cases.
The cable networks work through the use of lasers that send pulses of information through glass fibers bundled within the cables to deliver data at rates faster than 200,000 kilometers per second to receivers at the other end. To make optimal use of the cables—that is, to transfer as much information as possible across them—one of the things operators monitor is the polarization of the light that travels within the fibers. Like other light that passes through a polarizing filter, laser light is polarized—meaning, its electric field oscillates in just one direction rather than any which way. Controlling the direction of the electric field can allow multiple signals to travel through the same fiber simultaneously. At the receiving end, devices check the state of polarization of each signal to see how it has changed along the path of the cable to make sure that the signals are not getting mixed.
In their work, the researchers focused on the Curie Cable, a submarine 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 changes in temperature and even lightning strikes, can change the polarization of light traveling through fiber optic cables. Because the temperature in the deep ocean remains nearly constant and because there are so few disturbances there, the change in polarization from one end of the Curie Cable to the other remains quite stable over time. However, during earthquakes and when storms produce large ocean waves, the polarization changes suddenly and dramatically, allowing the researchers to easily identify such events in the data.
Currently, when earthquakes occur miles offshore, it can take minutes for the seismic waves to reach land-based seismometers and even longer for any tsunami waves to be verified. Using the new technique, the entire length of a submarine cable acts as a single sensor in a hard-to-monitor location. Polarization can be measured as often as 20 times per second. That means that if an earthquake strikes close to a particular area, a warning could be delivered to the potentially affected areas within a matter of seconds.