On New Year’s Day, Zhongwen Zhan joined hundreds of thousands of spectators in Pasadena, California, for the Rose Parade. But Zhan, a seismologist at the California Institute of Technology, was there for his work as much as for entertainment. While he held his young daughter and watched marching bands and flower-covered floats go by, Zhan knew two buried fiber-optic strands thinner than a human hair were recording vibrations from the parade.
Sprawling networks of fiber-optic cables, which power today’s phone, TV, and internet services, fan out like roots underneath cities. Since they were first developed in the 1970s to replace copper cables, millions of kilometers of fiber optics have been installed in the United States alone. They relay data by bouncing light within thin strands of glass, but not all strands are in use at any one time. Such “dark fiber” can be repurposed as low-cost seismic sensors.
In November 2019, Pasadena officials granted Zhan and his collaborators access to two strands of dark fiber, each 37 kilometers long. The researchers connected lasers that fired pulses of near-infrared light into one end of each strand. Most of the light barreled down the fibers, but a small amount was reflected by defects in the glass. The researchers measured when that backscattered light arrived at their instruments. “You listen for echoes from those defects,” Zhan says.
By repeating those measurements over time and looking for changes in the timing of the echoes, the researchers could deduce when vibrations had stretched individual meter-long sections of the fibers by as little as a few billionths of a meter. “Suddenly, you have tens of thousands of sensors,” Zhan says. “All you need to do is get permission to connect one instrument.” This technique, called distributed acoustic sensing, was originally developed by the U.S. military in the 1980s for detecting submarines. It’s now being expanded to study just about anything that involves vibrations.
Earthquake monitoring is an obvious application, but distributed acoustic sensing can also be used to study how water moves through glaciers and the formation of sinkholes, for example. “There are a ton of different applications,” says Jonathan Ajo-Franklin, a geophysicist at Rice University who has used distributed acoustic sensing to track permafrost thawing near Fairbanks, Alaska.
Zhan and his colleagues decided to use the Rose Parade to test the Pasadena network. They recorded the seismic signatures of passing marching bands and floats, some of which weighed upward of 15,000 kilograms. The seismically “loudest” marching band—the one that shook the road the most—was Southern University and A&M College from Baton Rouge, Louisiana, the team reported last week in Seismological Research Letters. (You can watch a video of their performance here.)
Zhan and other researchers envision a future in which distributed acoustic sensing is ubiquitous. It could be used not only for monitoring earthquakes, but also for recording traffic patterns, revealing slow leaks in buried pipelines, and pinpointing trespassers. “You can lay it around a fence,” Ajo-Franklin says. “If somebody tries to come underneath the fence, you can detect that vibration.”
That future might not be too far off. Large cities such as Los Angeles already have tens of thousands of kilometers of fiber, Zhan says. “If we can convert just a few percent of that into seismic sensors, that’s a million sensors.”