Caltech research reveals underground fiber optic cables as effective earthquake detectors

(UI) — In California, an extensive network of underground fiber optic cables spans thousands of miles, serving as a vital conduit for internet connectivity. However, these underground cables possess an intriguing additional capability: they can function as earthquake sensors, detecting and quantifying seismic activity, according to a study conducted by California Institute of Technology (Caltech).

Researchers at Caltech have recently conducted an investigation that utilized a segment of fiber optic cable to intricately analyze the characteristics of a magnitude 6 earthquake. The study successfully identified the precise timing and location of four distinct asperities along the fault line, which are regions where movement is hindered and eventually leads to the rupture of the fault.

Zhongwen Zhan, a geophysics professor at Caltech, and his committed team have spent years demonstrating how the use of repurposed fiber optic cables may dramatically improve our ability to track seismic occurrences. Their strategy makes use of distributed acoustic sensing, a method that turns a network of makeshift seismometers out of fiber optic cables, allowing for the thorough monitoring of seismic activity.

The study was limited to a 62-mile (100 km) stretch of fiber optic cable. The team was able to learn comprehensive details on the complex dynamics underpinning a particular earthquake that took place in 2021 thanks to this piece. The results highlighted the possibility that easier access to more fiber optic connections would open the door to a better understanding of earthquake physics, advancing seismic early-warning systems.

In Southern California's about 56,500 square miles, there are around 500 seismometers, each of which may cost up to $50,000. However, installing fiber optic lines all across the state may be like covering it with millions of seismometers, the study concluded.

Laser emitters placed at one end of a fiber optic cable send light beams through the long, thin glass strands that make up the cable's core in order to use the cable as a seismometer. Tiny flaws in the glass cause a very small amount of light to be reflected back to the source, where it is captured.

Each flaw serves as a trackable waypoint along the fiber optic cable in this way. The cable is normally buried close below ground. The cable wiggles somewhat as a result of seismic waves traveling through the earth, altering the distance traveled by light to reach these waypoints. Seismologists may examine the motion of seismic waves because the flaws throughout the length of the cable function as thousands of tiny seismometers.

During the magnitude 6 Antelope Valley earthquake of 2021, the team of researchers looked at the light signatures passing through a section of fiber optic cable in the Eastern Sierra Nevada. The cable's equal to 10,000 seismometers allowed researchers to determine that the M6 earthquake was really composed of a series of four minor ruptures. A typical seismic network was unable to identify these so-called "sub-events," which were similar to tiny earthquakes.

The team was able to produce an accurate model of the M6 earthquake based on the recorded seismic activity thanks to collaboration with Nadia Lapusta's group, led by Lawrence A. Hanson, Jr., Professor of Mechanical Engineering and Geophysics. The time and precise positions on the fault zone of the four sub-events were displayed by the model.

"Using fiber optic cable as a series of seismometers reveals aspects of earthquake physics that have long been hypothesized but difficult to image," Zhan stated in the Caltech study. "As an analogy, imagine your everyday backyard telescope. You can see Jupiter, but you probably can't see its moons or any details. With a really powerful telescope, you can see the fine details of the planet and moon surfaces. Our technology is like a powerful telescope for earthquakes."

This story was originally published by California Institute of Technology (Caltech).

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