Many kilometres off the western coast of the Americas, an undersea cable connects Los Angeles, California to Valparaiso, Chile. Stretched end-to-end, it’s equal to four-fifths of the Earth’s diameter. The cable is fibre optic; it’s a lifeline for data transmitted between the two continents. But according to new research, the cable could easily serve a dual function: mitigating the disastrous impacts of earthquakes and tsunamis.
The results come from an interdisciplinary collaboration between geophysicists and network engineers who looked at disturbances in the polarisation of light being transmitted through the cables. A patent has been filed in connection with the team’s paper on the subject, published on Wednesday in the journal Science.
“There are scientific and societal implications here,” said Zhongwen Zhan, the lead author of the new paper and a geophysicist at the California Institute of Technology, in a video call. “Most of our geophysical sensors for detecting earthquakes and studying what the interior of the Earth looks like are on land, but a lot of the most important geological processes are happening in the ocean. We’re leveraging pre-existing cables in the ocean for a relatively scalable way of detecting earthquakes. We think in the future we can use these for earthquake and tsunami early warnings.”
In the relentlessly online world in which we live, where movies filmed a century ago can be streamed at the touch of a button and you can speak face to face with someone on the other side of the planet, fibre optic cables carry a brunt of that informational load. Such undersea cables as Google’s “Curie” cable are constantly transmitting huge amounts of data at breakneck speeds to keep the world connected.
Inevitable imperfections in the cables means that the light’s polarisation varies as data travels through them in either direction. Other disturbances, like temperature fluctuation and human activity can further mess with the polarisation of the cables. But in the deep sea, temperatures are relatively constant, and there are rarely humans. That means that when a seismic wave undulates through the environment or a large ocean upswell passes through, it’s noticeably detectable in how it warps the undersea cable.
Since seismological research at the bottom of the sea is time-consuming and expensive, reading fluctuations in the polarisation of such deep-sea cables is an inexpensive, expedient alternative, the study authors argue. There are plenty of submarine cables to read such data from. While the Curie cable measures about four-fifths of the Earth’s diameter, the total submarine cable network could circle the planet 20 times. Among a half century of other geophysical events the team recorded, the Curie cable detected the 7.1 magnitude earthquake that struck Oaxaca, Mexico, last June.
When the team first recognised a perturbation in the cable signal and were able to line it up with an earthquake, “it was not expected at all,” Zhan said. “No one had ever detected an earthquake by looking at a telecommunications signal itself.”
During the team’s observations, they were able to recognise 20 earthquakes and 30 ocean swells. Importantly, the team is not yet able to detect the epicentre of any seismic events — the cables merely pick up the disturbance — but Zhan said that down the road, it could be possible to triangulate earthquake epicenters by looking at disturbed polarisations across different cables.
“I think this is going to transform the way we observe the oceans as seismologists,” William Wilcock, a seismologist at the University of Washington who is unaffiliated with the new paper, said in a phone call. Wilcock recently authored a Perspectives article in Science on the work by Zhan’s team. “In my area, there’s a big concern about the Cascadia subduction zone offshore, and there’s been a lot of thought about how to develop infrastructure offshore to improve our monitoring of that. To do that with dedicated systems is hundreds of millions of dollars. But to be able to potentially use commercial cables to do at least some of that is an enormous boon to actually moving forward.”
Whether the method of listening to the Earth is adopted by the telecommunications industry by and large remains to be seen. What’s certain is that this team has shown we can listen to light, using the byproduct of your playing Call of Duty or sending family photos to spy on the planet’s seismic activity, perhaps better preparing us for whatever small one or very, very big one will come next.