Use deep-sea fiber optic cables to detect earthquakes

Use submarine cables to detect earthquakes

Credit: California Institute of Technology

Caltech seismologists working with optical experts at Google have developed a method to use existing underwater telecommunication cables to detect earthquakes. The technique could lead to improved earthquake and tsunami warning systems around the world.

A large network of more than a million kilometers of fiber optic cable lies at the bottom of the Earth’s oceans. In the 1980s, telecommunications companies and governments began laying these cables, each of which could stretch thousands of miles. Today, the global network is considered the backbone of international telecommunications.

Scientists have long been looking for a way to use the submerged cables to monitor seismicity. After all, more than 70 percent of the earth is covered by water, and it is extremely difficult and expensive to install, monitor and manage seismometers underwater to monitor the movements of the earth under the sea. According to researchers, the ideal is to monitor seismicity by using the infrastructure that already exists along the seabed.

Previous attempts to use optical fibers to study seismicity have depended on the addition of sophisticated scientific instruments and / or the use of so-called “dark fibers”, optical fibers that are not actively used.

Now Zhongwen Zhan (Ph.D. ’13), assistant professor of geophysics at Caltech, and his colleagues have devised a way to analyze the light moving through ‘lit’ fibers – in other words, existing and functioning submarine cables – to detect earthquakes and tidal waves without the need for additional equipment. They describe the new method in the February 26 magazine Science.

“This new technique can really transform the majority of submarine cables into geophysical sensors that are thousands of miles long to detect earthquakes and possible tsunamis in the future,” says Zhan. “We believe this is the first solution to seabed seismicity monitoring that could potentially be implemented around the world. It could complement the existing network of ground seismometers and tsunami monitoring buoys to detect submarine earthquakes and tsunamis. to make it faster in many cases. ‘

The cable networks work by using lasers that send pulses of information through glass fibers bundled in the cables, to deliver data at speeds of more than 200,000 kilometers per second to receivers on the other end. Making optimal use of the cables – that is, transmitting as much information as possible about them – is one of the things that drivers monitor, the polarization of the light moving inside the fibers. Like other light passing through a polarizing filter, laser light is polarized – this means that its electric field moves in only one direction, rather than in any way. By controlling the direction of the electric field, multiple signals can move through the same fiber simultaneously. On the receiving side, devices check the polarization state of each signal to see how it has changed along the cable path to make sure that the signals do not get confused.

In their work, the researchers focused on the Curie cable, a fiber optic cable for submarines that stretches more than 10,000 kilometers along the eastern edge of the Pacific Ocean from Los Angeles to Valparaiso, Chile. (Although Zhan says that the technique can be used on many of the hundreds of submarines that cross the world.)

On land, all sorts of disturbances, such as changes in temperature and even lightning, can alter the polarization of light moving through fiber optic cables. Because the temperature in the deep ocean remains almost constant and because there are so few disturbances, the change in polarization from one side of the Curie cable to the other remains fairly stable over time, Zhan and his colleagues found.

However, during earthquakes and when storms produce large ocean waves, the polarization changes suddenly and dramatically, allowing researchers to easily identify such events in the data.

Currently, when earthquakes occur miles off the coast, it can take time before the seismic waves reach seismometers on land, and even longer before any tsunami waves are verified. Using the new technique, the entire length of a submarine cable serves as a single sensor in a location that is difficult to monitor. Polarization can be measured up to 20 times per second. This means that if an earthquake strikes near a specific area, a warning can reach the potential areas within a few seconds.

During the nine months of testing reported in the new study (between December 2019 and September 2020), the researchers detected about 20 moderate to large earthquakes along the Curie cable, including the earthquake with a magnitude of 7.7 which took place outside Jamaica in January. 28, 2020.

Although no tsunamis were detected during the study, the researchers were able to detect changes in the polarization caused by the swell of the ocean that formed in the Southern Ocean. They believe that the changes in polarization observed during the events were caused by pressure changes along the seabed as powerful waves moved past the cable. “It means we can detect ocean waves, so it’s plausible that one day we’ll be able to detect tsunami waves,” Zhan said.

Zhan and his colleagues at Caltech are now developing a machine learning algorithm that could determine whether detected changes in polarization are caused by earthquakes or ocean waves, rather than by another change to the system, such as a ship or crab which moves the cable. They expect the entire detection and notification process to be automated to provide critical information, in addition to the data already collected by the worldwide network of land seismic meters and buoys in the Deep-Ocean Assessment and Reporting of Tsunamis (DART) system. by the National Data Buoy Center of the National Ocean and Atmospheric Administration.

The new Science paper is entitled “Optical polarization-based seismic and water wave detection on transoceanic cables.”


Use submarine cables to detect earthquakes


More information:
Zhongwen Zhan et al. Optical polarization-based seismic and water wave detection on transoceanic cables, Science (2021). DOI: 10.1126 / science.abe6648

Provided by California Institute of Technology

Quotation: The use of deep-sea fiber optic cables to detect earthquakes (2021, 26 February) detected on 27 February 2021 from https://phys.org/news/2021-02-submarine-cables-earthquakes.html

This document is subject to copyright. Except for any fair trade for the purpose of private study or research, no portion may be reproduced without the written permission. The content is provided for informational purposes only.

Source