May 28 , 2022
Caltech seismologists collaborated with Google optics experts to create a system for detecting earthquakes using existing undersea telecommunication cables. The method could lead to more effective earthquake and tsunami warning systems all around the world.
At the bottom of the world's seas is a massive network of fiber optic cable stretching over a million kilometers. These cables, which can stretch thousands of kilometers, were first laid in the 1980s by communications firms and governments. The worldwide network is now regarded as the telecommunications industry's backbone.
Scientists have been looking for a way to use the buried cables to monitor seismic activity for a long time. After all, water covers more than 70% of the globe, and installing, monitoring, and operating underwater seismometers to measure the earth's movements beneath the seas is highly complex and expensive. Researchers believe that using the equipment already in place throughout the ocean floor to monitor seismicity would be optimal.
Previous attempts to research seismicity with optical fibers relied on the insertion of sophisticated scientific gear and/or the use of "dark fibers," or fiber optic connections that aren't in use.
Now, Zhongwen Zhan (PhD '13), an assistant professor of geophysics at Caltech, and his colleagues have devised a method for analyzing light traveling through "lit" fibers—that is, existing and functioning submarine cables—in order to detect earthquakes and ocean waves without the use of additional equipment. The novel approach is described in the journal Science on February 26.
“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.”
When earthquakes strike miles away, it can take minutes for seismic waves to reach land-based seismometers, and even longer for tsunami waves to be confirmed. In a difficult-to-monitor area, the full length of a submarine cable acts as a single sensor using the new technology. The rate at which polarization can be measured is up to 20 times per second. That means that if an earthquake occurs at a specific location, a warning might be sent to those who may be affected in a couple of seconds.
The researchers were able to detect changes in polarization caused by ocean swells that originated in the Southern Ocean, despite the fact that no tsunamis were reported throughout the investigation. Pressure fluctuations along the seafloor as big waves flowed past the cable, they believe, caused the variations in polarization recorded during those episodes. "Because we can identify ocean waves, it's possible that we'll be able to detect tsunami waves one day," Zhan says. “This means we can detect ocean waves, so it is plausible that one day we will be able to detect tsunami waves,” says Zhan.