Using such submerged cables - which comprise a global network considered the backbone of international telecommunications - to monitor seismicity has long been a goal of scientists, as they present an infrastructure already in place along the ocean floor. However, say the researchers, previous efforts 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.
Now, say the researchers, working with optics experts at Google, they 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," says Zhongwen Zhan (PhD '13), assistant professor of geophysics at Caltech. "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 - i.e., 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 - i.e., 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. While on land there are many disturbances - such as changes in temperature and even lightning strikes - that can change the polarization of light traveling through fiber-optic cables, in the deep ocean the temperature remains nearly constant and there are few disturbances, so the change in polarization from one end of the Curie Cable to the other was found to remain quite stable over time.
However, say the researchers, during earthquakes and when storms produce large ocean waves, the polarization changes suddenly and dramatically, allowing them 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, say the researchers, 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, meaning 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.
During the nine months of testing reported in their study (between December 2019 and September 2020), the researchers say they detected about 20 moderate-to-large earthquakes along the Curie Cable, including the magnitude-7.7 earthquake that took place off of Jamaica on January 28, 2020. Although no tsunamis were detected during the study, the researchers say they were able to detect changes in polarization produced by ocean swells that originated in the Southern Ocean.
They believe the changes in polarization observed during those events were caused by pressure changes along the sea floor as powerful waves traveled past the cable.
"This means we can detect ocean waves," says Zhan, "so it is plausible that one day we will be able to detect tsunami waves."
The researchers say they are now developing a machine learning algorithm that would be able to determine whether detected changes in polarization are produced by earthquakes or ocean waves rather than some other change to the system, such as a ship or crab moving the cable. They expect that the entire detection and notification process could be automated to provide critical information in addition to the data already collected by the global network of land-based seismometers and the buoys in the Deep-ocean Assessment and Reporting of Tsunamis (DART) system, operated by the National Oceanic and Atmospheric Administration's National Data Buoy Center.
For more, see the study: "Optical polarization–based seismic and water wave sensing on transoceanic cables."
'Billion sensor' earthquake observatory uses existing optical fibers
'Living sensors' hold promise for ocean surveillance, says DARPA
Forecasting earthquake aftershock locations with AI
Earth's atmosphere as a global sensor