Fiber

Fiber-optic cables make up the vast underground nervous system that meets our growing demand for high-speed Internet and communication services. However, signals in the cables can occasionally suffer vibrations from cars driving overhead, nearby construction, or even earthquakes. Researchers have previously proposed harnessing those perturbations to convert the thousands of kilometers of underground cable into sensitive seismic arrays.

In a new study, researchers from the California Institute of Technology show that fiber-optic cables not only detect quakes—they can measure nuances and complexities of the seismic events as well. In one instance, using a 100-kilometer stretch of cable, the researchers were able to pinpoint the time and location of four smaller miniquakes that made up a magnitude-6 earthquake.

“This work is not just detection, it’s beyond detection,” says Jiaxuan Li, a postdoctoral researcher in geophysics at Caltech and coauthor of the paper published 2 August in Nature. “We are imaging the details of the rupture process of an earthquake.”

“We could transform those very dense [fiber] networks into seismic arrays that we can use for early warning.” —Jiaxuan Li, Caltech

By harnessing more cables and getting even more data, seismologists could gain a better understanding of earthquakes. And while an array of fiber-optic cables is unlikely to predict earthquakes before they happen, researchers could use the technique to help develop better early warning systems that save lives, Li says.

California typically gets two or three earthquakes each year that aresevere enough—meaning magnitude 5.5. or higher—to cause moderate damage to structures. There are over 700 seismometers throughout the state. Each one costs up to US $50,000, and the network of detectors is expensive to maintain.

Having a sensor as close as possible to an earthquake source is important for early detection. This is not possible with expensive seismometers, Li says. Optical-fiber cables, on the other hand, are already laid in the ground, crisscrossing everywhere, providing a dense, low-cost seismic sensor network. “We have a very extensive fiber-optic network in cities and between cities,” he says. “We could transform those very dense networks into seismic arrays that we can use for early warning.”

Li and colleagues used a technique called distributed acoustic sensing (DAS), which, while new to the world of seismology, is already used to monitor pipelines and power cables for defects. The method involves sending laser light pulses over optical fibers and measuring the intensity of the signals reflected back from imperfections in the fiber. Slight stretching or contracting of the fiber (say, from an earthquake) can change the reflected signals.

Based on the pulse’s time of return, you can pinpoint when and where along the cable the disturbance occurred. Because light gets reflected from thousands of imperfection points along fibers, a kilometers-long stretch of cable can act as thousands of seismometers. This means significantly more seismic data, leading to higher resolution, which allows pinpointing the location of smaller seismic activity.

The Caltech researchers have converted preexisting optical cables into a DAS array. Telecom companies usually lay down more fiber than they need, and the research team taps into some of this “dark” unused fiber. With permission from the California Broadband Cooperative, the team set up a DAS transceiver at one end of a length of fiber-optic cable along the border between California and Nevada.

In their study, the researchers analyzed light signals from two 50-kilometer sections of optical-fiber cable that recorded the 2021 Antelope Valley magnitude 6 earthquake. The sections of fiber cable were located to the north and south of the town of Old Mammoth. Overall, the 100 km of cable provided data equivalent to that from 10,000 seismometers.

Using the high-resolution data, the researchers found that the quake was made up of a sequence of four smaller ruptures, called “subevents,” which could not be detected by a conventional seismic network. By creating a computer model of the earthquake based on the data, the researchers were able to detail the precise time and location of these subevents.

The fiber used in the study was 100 km away from the earthquake center, Li says. “The resolution of our image can be increased a lot by having fibers in other directions, on the east and west side of the earthquake as well.”

Processing and storing the large amounts of data produced by a dense fiber-optic seismic array would be a tough technical challenge, he says. But the “very first and hardest” challenge would be getting access to fibers to create an expansive DAS array, for which they would have to negotiate with telecom companies. “It’s not an easy process, but I hope that based on this work telecom companies can see the advantage of collaboration.”