New undersea cable tech is like 'a powerful telescope for earthquakes'

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In a new study at Caltech, the unassuming underground fiber optic cables that crisscross California's landscape have revealed a special bonus: sensing and measuring earthquakes.

While these cables provide internet connectivity, a section of fiber optic cable was repurposed to delve into the intricate mechanics of a magnitude 6 earthquake, shedding light on the details of its rupture process.

The findings, published in the journal Nature on August 2, illuminate the potential of fiber optic cables to enhance earthquake understanding and early-warning systems, provided broader cable coverage is achieved.

Just to clarify, earthquake sensing by underwater cables is not a new concept. In fact, Interesting Engineering (IE) previously reported how at least 400 of them can be leveraged globally, as revealed in a Q&A session with an expert.

Now, in a recent press release, a team led by Professor of Geophysics Zhongwen Zhan has introduced an innovative method called distributed acoustic sensing.

By reimagining fiber optic cables as makeshift seismometers, they harnessed a mere 62-mile (100-kilometer) cable section to unlock insights into the complex dynamics of a specific 2021 earthquake.

"If we can get broader coverage to measure seismic activity, we can revolutionize how we study earthquakes and provide more advance warning," explained Zhan in a press release.

"Though we cannot predict earthquakes, distributed acoustic sensing will lead to a better understanding of the details underlying how the Earth ruptures."

In Southern California, around 500 seismometers span the vast expanse of approximately 56,500 square miles, each with a price tag of up to 50,000 dollars. In contrast, leveraging fiber optic cables could virtually blanket the region with millions of cost-effective seismometers.

The ingenious technique involves placing laser emitters at one end of the cable, sending light beams through the glass strands comprising the cable's core.

Imperfections in the glass reflect a fraction of the light back to the source for recording. This transforms the cable into a network of trackable waypoints, responding to ground movements by altering light travel times between these points.

Courtesy of Z. Zhan

Essentially, the cable's imperfections act as thousands of individual seismometers, enabling seismologists to capture seismic wave motion.

In the study, the team focused on light signatures within a segment of fiber optic cable located in the Eastern Sierra Nevada during the 2021 Antelope Valley magnitude 6 earthquake.

This 10,000-seismometer equivalent section unraveled the earthquake's complexity, revealing a sequence of four smaller ruptures, or "subevents." These elusive sub-events, akin to mini earthquakes, eluded conventional seismic networks.

Collaborating with Nadia Lapusta's laboratory, the team constructed an accurate model of the M6 earthquake using the measured seismic data. The model precisely delineated the four subevents' timing and fault region locations.

"As an analogy, imagine your everyday backyard telescope. You can see Jupiter, but you probably can't see its moons or any details," said Zhan.

"With a really powerful telescope, you can see the fine details of the planet and the moon's surfaces. Our technology is like a powerful telescope for earthquakes."

The potential of repurposing fiber optic cables as seismic instruments holds immense promise, offering a cost-effective means to expand our understanding of earthquakes — and ideally mitigate their impacts.

As science continues to bridge technological innovation with Earth's natural processes, the future of earthquake research grows brighter with every pulse of light coursing through buried cables.

The full study was published in Nature on August 2.