Undersea communications cables reveal ocean earthquakes

The researchers used intermittent data from telecom companies to convert submarine communications cables into deep-sea seismic sensors.

Written by Lauren Melideo, PhD, science writer, (lwritesscience)

Citation: Melideo, El, 2021, Subsea Communications Cables Detect Ocean Earthquakes, Temblor, http://doi.org/10.32858/temblor.167

Earthquakes on Earth often create a wide range of seismic sensors, giving researchers a lot of data to analyze. Using this data, seismologists gain a better understanding of how earthquakes occur and where they are most likely to strike in the future. Unfortunately, most of the planet is covered in water and very few seismic sensors monitor the vast oceans. Now, a new study is exploring whether underwater telecommunications cables can act as stand-alone sensors and monitor large areas of the Earth’s surface.

More than 70% of the Earth’s surface is covered by water, so seismic monitoring equipment is scarce. Credit: Jack Atkinson, Unsplash

A new approach

The idea of ​​using fiber-optic communication cables to sense earthquakes is not new. Researchers have previously used so-called “dark fibers” – which are not actively used inside the cable – on the ground to detect seismic waves on a limited basis underwater. But so far, research with submarine cables has only been done on short sections of cable because dark fibers are so rare underwater, says Zhongwen Zhan, a seismologist at CalTech’s seismic laboratory. These cables are too expensive to have as many dark fibers for other uses. This limits the space over which cables can be used for earthquake detection.

Zan and a team of researchers sought a different way of using fiber-optic cables to seismically monitor the oceans. Zan notes that optical waves used to transmit data over long distances in cables travel in two orthogonal planes, which allows telecom companies to transmit a large amount of information simultaneously. The angle between waves can change if the light signal is disturbed along its journey from one end of the cable to the other. Telecom companies monitor the information at the receiving end of the cables, Zhan says, to ensure that the received signal matches the transmitted signal and that the two orthogonal signals do not overlap with each other along the way. Telecommunications companies have no further use of this information after this confirmation. Zan says the team realized this unused data could have other applications.

The Google submarine cable detects earthquakes

The team switched to the 6,525-mile (10,500 km) Curie Cable, which stretches between Los Angeles, California and Valparaiso, Chile. This cable crosses underwater faults and a seismically active area in the Pacific Ocean. The stable conditions on the sea floor – with some temperature fluctuations or other vibrations common on land – should cause little to no disturbance to the signal that travels through submarine cables. However, the disturbances were still evident in the upcoming signals, Zan notes. The team found that some of these disturbances occurred around the same time that the earthquakes were detected by conventional seismic methods. One such accident was a 7.4 magnitude earthquake near the city of Oaxaca, Mexico, on June 23, 2020.

Korean Cable Publishing. Credit: Google Cloud

“The wonderful part about this research is that they don’t rely on installing additional equipment on cables that are not in use,” says Celine Hadziwano, professor of geophysics at the University of Hamburg and a seismologist.

Since the recorded signal disturbance is integrated along the entire length of the cable, it is not currently possible to know exactly where the earthquake occurred along the cable with this type of sensor, Hadziwano says. Researchers describe the potential use of signal disturbance information from multiple simultaneous cables to locate an earthquake. “This approach is still very promising and could be very robust for future applications of early detection of a distant earthquake,” Hadziwano says. She says that such information is useful. If a major earthquake is detected along an undersea cable, existing earthquake early warning systems can be triggered before seismic waves approach the ground.

“Many big earthquakes are happening outside,” says Zan. “If you only have stations on Earth, then you only look at them from one side and you really get a very limited understanding of those earthquakes.” He says that with the enormous distance between the origins of these oceanic earthquakes and the ground-based sensors, rapid warnings of these earthquakes are not possible, as if a sensor were located near earthquakes.

Another potential application of this method is tsunami warnings, but this is not yet confirmed. Zan says researchers discovered ocean waves during their study. “(A) A tsunami is one type of ocean wave, so we hope that maybe one day it will succeed in detecting tsunamis,” he notes. No major tsunamis occurred during the nine months of observation, so the team does not yet know if submarine cables can detect tsunamis.

Observe the vast ocean

The research holds the promise of expanding the ability of seismologists to observe and identify these earthquakes that are occurring thus far from conventional seismic sensor networks. “This [submarine] The network is already in, ”Zhan says – for a total of more than 1.2 million kilometers, according to CNN. Zan notes that using even a small proportion of these cables for geophysical research would greatly expand the seismic sensing coverage of the Earth’s surface.

References

Zhan, Z., M. Cantono, V. Kamalov, A. Mecozzi, R. Muller, S. Yin & JC Castellanos (2021). Seismic sensing based on optical polarization and water waves on ocean-going cables. Science. https://doi.org/10.1126/science.abe6648

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