Undersea telecom cables detect ocean earthquakes

Researchers used throwaway data from telecom companies to turn submarine telecommunications cables into deep-sea earthquake sensors.
 

By Lauren Milideo, Ph.D., science writer, (@lwritesscience)
 

Citation: Milideo, L., 2021, Undersea telecom cables detect ocean earthquakes, Temblor, http://doi.org/10.32858/temblor.167
 

Earthquakes on land often rattle a wide array of seismic sensors, giving researchers plenty of data to analyze. Using these data, seismologists gain a better understanding of how earthquakes occur and where they are more likely to strike in the future. Unfortunately, most of the planet is covered by water and very few seismic sensors monitor the vast oceans. Now, a new study explores whether underwater telecommunications cables can serve as stand-in sensors and monitor immense spans of Earth’s surface.
 

 

A fresh approach

The idea of using fiber-optic telecommunications cables to sense earthquakes is not new. Researchers have previously used so-called “dark fibers” — fibers not being actively used within a cable — on land to detect seismic waves and on a limited basis underwater. But until now, research utilizing submarine cables has taken place only on short bits of cable because dark fibers are so rare underwater, says Zhongwen Zhan, a seismologist at CalTech’s Seismological Laboratory. These cables are too expensive to have a large number of dark fibers available for other use. This limits the area over which cables can be used to detect earthquakes.

Zhan and a team of researchers sought a different way to use optical-fiber cables to seismically monitor the oceans. Light waves used to transmit data long distances in cables travel in two perpendicular planes, allowing telecom companies to send a large amount of information at once, notes Zhan. The angle between the waves can change if the light signal is perturbed along its journey from one end of the cable to the other. Telecom companies monitor the information at the receiving end of the cables, says Zhan, to ensure that the signal received matches the signal sent and the two perpendicular signals have not interfered with each other along the way. Telecom companies have no other use for this information after this confirmation. The team realized this unused data may have other applications, Zhan says.
 

Google’s submarine cable detects quakes

The team turned to Google’s 6,525-mile-long (10,500-kilometer) Curie cable, which runs between Los Angeles, California and Valparaiso, Chile. This cable traverses underwater faults and a seismically active zone in the Pacific Ocean. The steady conditions on the seafloor — with few temperature fluctuations or other vibrations common on land — should cause little disruption in the signal traveling through submarine cables. Yet perturbations were still evident in the arriving signals, notes Zhan. The team found that some of these perturbations occurred at the same time as earthquakes detected by more traditional seismological means. The magnitude-7.4 quake that occurred near Oaxaca, Mexico, on June 23, 2020, was one such occurrence.
 

 

“The cool part about this research is that they don’t rely on installing extra instrumentation on cables that are not being used,” says University of Hamburg Institute of Geophysics professor and seismologist Céline Hadziioannou.

Because the recorded signal perturbation is integrated along the entire cable length, it is not currently possible to know exactly where along the cable a quake occurred using this type of sensing, says Hadziioannou. The researchers describe the potential use of signal perturbation information from several cables at once to determine a quake’s location. Hadziioannou says that “the approach is still very promising and could be quite powerful for future applications of early detection of the fact that there has been a remote earthquake.” Such information is useful, she says. If a large quake is detected along an undersea cable, existing earthquake early warning systems could be triggered before the seismic waves approach land.

“Many of the bigger earthquakes are happening offshore,” Zhan says. “If you only have stations on land, then you are only looking at them from one side and you are really getting very limited understanding of those earthquakes.” He says that with the tremendous distance between these ocean quakes’ origins and land-based sensors, quick warnings of these quakes are not possible, as they would be if a sensor were located closer to the earthquakes.

Another potential application of this method is tsunami warnings, but this is not yet certain. The researchers did detect ocean waves during their study period, Zhan says. “(A) tsunami is one kind of ocean wave, so we are hopeful that maybe one day it will work for detecting tsunamis,” he notes. No major tsunamis occurred during their nine months of observation, so the team does not yet know if submarine cables can detect tsunamis.
 

Monitoring the vast oceans

The research holds promise in expanding how seismologists are able to view and learn about these quakes happening so far from traditional seismic sensing networks. “This [submarine] network is already there,” says Zhan – a total of over 1.2 million kilometers, according to CNN. Using even a small percentage of these cables for geophysical research would greatly expand the seismic sensing coverage of Earth’s surface, Zhan notes.
 

References

Zhan, Z., M. Cantono, V. Kamalov, A. Mecozzi, R. Muller, S. Yin & J.C. Castellanos (2021). Optical Polarization-Based Seismic and Water Wave Sensing on Transoceanic Cables. Science. https://doi.org/10.1126/science.abe6648