An undersea invention could be critical for early earthquake warnings in California, a study has found.
In California, there are miles and miles of underwater fiber optic cables crossing the state, providing people with the internet. Scientists have found a method that transforms these cables into powerful seismometers, able to predict the location and time of earthquakes as strong as magnitude 6, according to the study published in the scientific journal Nature.
California is one of the most earthquake-prone states in the U.S. The state lies on the San Andreas Fault—the border section between two massive tectonic plates under the surface of the Earth. It stretches for more than 800 miles through California, past San Francisco and nearly as far south as San Diego.
This fault line has been causing concern among experts in recent years as multiple segments appear to be significantly stressed. For this reason, they fear that the "Big One"—a gigantic earthquake—could be imminent, meaning earthquake predation technology is rapidly expanding.
"This technique cannot predict 'big ones.' But the fiber earthquake sensing can play a critical role in earthquake early warning. We could leverage the extensive pre-existing telecommunication fiber network to build a better early warning system," professor of Geophysics at California Institute of Technology and study author Zhongwen Zhan told Newsweek.
Zhan and colleagues tested their method using a 62-mile section of a cable that picked up data from the magnitude 6 earthquake that struck the Antelope Valley in 2021. This small section of cable provided data equivalent to 10,000 seismometers.
The section discovered that this earthquake was sequenced from four smaller ruptures—data that could not be detected by normal seismometers.
"In this study, we convert a [62-mile] already-laid telecommunication fiber cable into an ultra-dense seismic array. This fiber seismic array can achieve a similar resolution in regional rupture imaging to that of conventional dense local seismic arrays, but at a much lower cost," Zhan said.
By attaching laser emitters at either end of the cables, scientists found the cables can measure these earthquakes by pinpointing the area of the fault that ruptured. The laser emitters shoot beams of light through the glass that makes up the cable's core.
The light provides a way for scientists to track imperfections in the glass. This way, scientists can record when seismic waves move through the ground.
"We use a technique called back-projection imaging to image the high-frequency sources during the earthquake rupture process. Conventional back-projection studies use teleseismic P waves to image the ruptures of large earthquakes," Zhan said.
"Teleseismic S waves are not used due to their strong attenuation at high frequencies. Also, teleseismic P-wave back-projection imaging is performed on a 2D horizontal plane and has no depth resolution, since the ray paths of teleseismic P waves are almost vertical in the source region.
"Our study uses both P and S waves to triangulate the source because both P and S waves retain clear signals at high frequencies over a regional distance. We use waveforms from nearby aftershocks to correct for waveform complexities. We also use both P and S phases to obtain a 3D back-projection image that has depth resolution."
Scientists already knew that underwater cables could be utilized to sense earthquakes, but now, utilizing these cables could lead to an increase in early warning systems, notifying officials when a quake is set to occur.
"This is not the first study of using fiber cables to 'detect' earthquakes. But this is the first time using fiber cables to identify the detailed rupture process of a moderate-sized earthquake (magnitude 6 in our study)," Zhan said.
Earthquake technology is being improved all the time, however, it is still not possible to predict the exact date and time for when one will occur. It is also not possible to predict how large it will be.
If research into this area continues, and scientists managed to get broader coverage to measure seismic activity, the way we measure earthquakes could be revolutionized, Zhan said in a statement.
California already has approximately 500 seismometers set up across the state, but each one costs around $50,000, the study reports. By utilizing these existing cables, they could take the place of millions of seismometers. And, be more powerful.
"Though we cannot predict earthquakes, distributed acoustic sensing will lead to a better understanding of the details underlying how the earth ruptures," Zhan said.
Scientists who have warned of a "Big One" hitting the state of California say planning is key.
California as a state is already strict on earthquake measures and readiness. As technologies and scientific investigations into earthquakes continue, it is possible that early warning systems could be in place when the fault does inevitably cause a large earthquake.
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