Repurposed fibre optic cables show promise as seismometers

This is the claim of scientists at the California Institute of Technology (CALTECH), who have measured a magnitude 6 (M6) earthquake, determining the time and location of four individual asperities that led to the rupture. Their work is detailed in Nature.

California is crisscrossed by thousands of miles of fibre optic cables that deliver digital services to its residents. Professor of Geophysics Zhongwen Zhan and his team used a 100km section of cable to precisely understand the complex mechanics behind a particular 2021 earthquake, suggesting that access to more cables would enable improved understanding of earthquake physics and better earthquake early-warning systems.

"If we can get broader coverage to measure seismic activity, we can revolutionise how we study earthquakes and provide more advance warning," Zhan said in a statement. "Though we cannot predict earthquakes, distributed acoustic sensing will lead to a better understanding of the details underlying how the earth ruptures."

There are about 500 seismometers throughout the roughly 56,500 square miles in Southern California, and each one costs up to $50,000. Utilising fibre optic cables throughout the state could be equivalent to blanketing it with millions of seismometers, the team believes.

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To use a fibre optic cable as a seismometer, laser emitters are placed at one end of the cable to direct light through the glass strands of the cable's core.

According to CALTECH, the glass contains imperfections that reflect a minuscule portion of the light to the source, where it is recorded. This way, each imperfection acts as a trackable waypoint along the fibre optic cable, which is typically buried just below ground level. Seismic waves moving through the ground cause the cable to move slightly, which changes the travel time of light to and from the waypoints. In this way, the imperfections act like thousands of individual seismometers that allow seismologists to observe the motion of seismic waves.

In this new study, the team examined the light signatures travelling through a stretch of fibre optic cable located in the Eastern Sierra Nevada during the 2021 Antelope Valley M6 earthquake. The section of cable was equivalent to 10,000 seismometers and was able to discover that the M6 was made up of a sequence of four smaller ruptures. These so-called sub-events could not be detected by a conventional seismic network.

In collaboration with the laboratory of Nadia Lapusta, the Lawrence A. Hanson, Jr., Professor of Mechanical Engineering and Geophysics, the team was able to create an accurate model of the M6 earthquake based on the measured seismic activity. The model showed the timing of the four sub-events and pinpointed their exact locations on the fault region.

"Using fibre optic cable as a series of seismometers reveals aspects of earthquake physics that have long been hypothesised but difficult to image," said Zhan.