Thousands of miles of fiber-optic cables, already in place across California, can shake up the study of earthquakes.
New research out of Caltech used a section of telecommunication fiber to sense and measure a magnitude 6 earthquake, repurposing the assemblies to extract sharper details about the temblor.
Zhongwen Zhan, professor of physics at Caltech, said the same fiber optic network used for internet and television can serve as a dense network of makeshift seismometers in a method called DAS, distributed acoustic sensing.
“If we can get broader coverage to measure seismic activity, we can revolutionize how we study earthquakes and provide more advance warning,” Zhan said. “Though we cannot predict earthquakes, distributed acoustic sensing will lead to a better understanding of the details underlying how the earth ruptures.”
The new study, published Tuesday in “Nature,” examined the light signatures traveling through a stretch of fiber optic cable in the Eastern Sierra Nevada during the magnitude-6 Antelope Valley quake in 2021.
Researchers said that section of cable acted like 10,000 seismometers and was able to provide detailed characteristics of the tremor. For example, it showed the 2021 earthquake was made up of a sequence of four smaller sub-events or ruptures, mini-quakes that could not be detected by a conventional seismic network.
“Using fiber optic cable as a series of seismometers reveals aspects of earthquake physics that have long been hypothesized but difficult to image,” Zhan said.
Imagine using an everyday backyard telescope and training it on Jupiter, he added. You can see the planet, but not 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,” Zhan said.
There’s an economic advantage, too. Southern California has about 500 seismometers scattered across 56,500 square miles. Each one costs up to $50,000. Using fiber optic cables throughout the state would be like blanketing California with millions of ready-made seismometers. Access to more cables can lead to improved understanding of earthquake physics and better earthquake early-warning systems.
Zhan’s team got access from the city of Pasadena to use its fiber optic network for the study. They used laser emitters on one end of the cable to shoot beams of light through the long, thin glass strands that make up the cable’s core. Tiny imperfections in the glass reflect the light back to the source, where it is recorded. Each imperfection then produces a trackable waypoint or stopping place along the cable.
During an earthquake, seismic waves traveling through the ground causes the cable to wiggle, changing the travel time of light to and from the waypoints. The scientists used the information they gleaned from these imperfections to observe the motion of seismic waves.
Aside from Zhan, the Caltech researchers include Jiaxuan Li, first author; Nadia Lapusta, professor of mechanical engineering and geophysics at Caltech; graduate student Teaho Kim and scientist Ettore Biondi. The National Science Foundation paid for the study.