Fibre optic cables that can detect earthquakes

4th August 2023
Paige West

In a recent research endeavour conducted at Caltech, researchers have detailed their utilisation of a segment of fibre optic cable to precisely gauge the complex particulars of a magnitude 6 earthquake.

This approach allowed them to accurately determine both the timing and the precise locations of four distinct asperities, which are essentially the immobilised segments of the fault responsible for causing the rupture.

Over the course of multiple years, Professor of Geophysics Zhongwen Zhan (PhD '14) and his research team have been dedicated to demonstrating that the reutilisation of fibre optic cables presents a straightforward approach to significantly enhance our capacity for measuring seismic events. This is achieved by creating an extensive array of improvised seismometers through a technique known as distributed acoustic sensing.

Published in the journal Nature, the recent research harnessed a mere 100km segment of cable to gain a meticulous comprehension of the intricate mechanisms underlying a specific earthquake that occurred in 2021. This underscores the potential for enhanced insights into earthquake physics and, consequently, the advancement of more effective earthquake early-warning systems through the utilisation of additional fibre optic cables.

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

Across the expanse of approximately 56,500 square miles in Southern California, approximately 500 seismometers are distributed. The procurement cost for each of these seismometers can reach up to $50,000. In contrast, the utilisation of fibre optic cables across the entire state could offer an equivalent effect to saturating it with a multitude of millions of seismometers.

To repurpose a fibre optic cable as a seismometer, the process involves placing laser emitters at one end of the cable. These emitters project beams of light through the cable's elongated and slender glass strands that compose its core. Within the glass, there exist minute imperfections that reflect a tiny fraction of the light back to its source, where it is then captured and recorded. Consequently, each of these imperfections serves as a traceable point of reference along the fibre optic cable, which is commonly situated just below the surface of the ground.

The propagation of seismic waves through the ground induces subtle movements in the cable, resulting in slight wiggles. These movements subsequently alter the travel time of light as it journeys to and from these reference points. This phenomenon causes the imperfections distributed along the cable's length to function akin to numerous discrete seismometers. This arrangement empowers seismologists to meticulously monitor the trajectory of seismic waves.

In this fresh investigation, the research team scrutinised the patterns of light traversing a segment of fibre optic cable positioned within the Eastern Sierra Nevada region at the time of the 2021 Antelope Valley earthquake, which registered a magnitude of 6. This particular cable section effectively emulated the functionality of approximately 10,000 individual seismometers. The study unveiled that the magnitude 6 earthquake was composed of a series of four smaller ruptures, referred to as ‘sub-events.’ These sub-events, akin to miniature earthquakes, were imperceptible through a traditional seismic network.

Through a collaborative effort with the research group led by Nadia Lapusta, the Lawrence A. Hanson, Jr., Professor of Mechanical Engineering and Geophysics, the team successfully constructed a precise model of the magnitude 6 earthquake (M6). This model was crafted by leveraging the data obtained from the observed seismic phenomena. The model not only elucidated the temporal sequence of the four sub-events but also precisely identified the geographical coordinates of their occurrences within 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," says Zhan. "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."

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