University of Michigan (U-M) researchers have demonstrated the potential for using existing networks of buried optical fibers as an inexpensive observatory for monitoring and studying earthquakes. The study provides new evidence that the same optical fibers that deliver high-speed internet and HD video to homes could one day double as seismic sensors. “Fiber optic cables are the backbone of modern telecommunications, and we have demonstrated that we can turn existing networks into extensive seismic arrays to assess ground motions during earthquakes," said U-M seismologist Zack Spica, first author of a paper published in the journal JGR Solid Earth. The study was conducted using a prototype array at Stanford University, where Spica was a postdoctoral fellow before recently joining the U-M faculty as an assistant professor in the Department of Earth and Environmental Sciences. “This is the first time that fiber optic seismology has been used to derive a standard measure of subsurface properties that is used by earthquake engineers to anticipate the severity of shaking,” said geophysicist Greg Beroza, a co-author of the paper. To transform a fiber optic cable into a seismic sensor, the researchers connected a laser interrogator to one end of the cable and shot pulses of laser light down the fiber. The light bounces back when it encounters impurities along the fiber, creating a backscatter signal analyzed by a interferometer device. Changes in the backscatter signal can reveal how the fiber stretches or compresses in response to passing disturbances, including seismic waves from earthquakes. According to Beroza, the U-M team’s research extends previous work with the 3-mile Stanford test loop by producing high-resolution maps of the shallow subsurface, which scientists can use to see which areas will undergo the strongest shaking in future earthquakes. The study also demonstrates that optical fibers can be used to sense seismic waves and obtain velocity models and resonance frequencies of the ground. Spica and his colleagues said their results concur with an independent survey that used traditional techniques. “What's great about using fiber for this is that cities already have it as part of their infrastructure, so all we have to do is tap into it,” Beroza said. Many urban centers are built atop soft sediments that amplify and extend earthquake shaking. The near-surface geology can vary considerably from neighborhood to neighborhood, highlighting the need for detailed, site-specific information. However, deploying thousands of seismometer arrays across these urban landscapes is a challenge. “In urban areas, it is very difficult to find a place to install seismic stations because asphalt is everywhere,” Spica said. “In addition, many of these lands are private and not accessible, and you cannot always leave a seismic station standing alone because of the risk of theft.” Spica said that fiber optics might someday mark the end of such large-scale and expensive experiments because the cables are buried under the asphalt and crisscross the entire city, avoiding the typical disadvantages of surface seismic stations. The next phase of the project involves a much larger test array. A 27-mile loop was formed recently by linking optical fibers on Stanford’s historic campus with fibers at several other nearby locations. The 3-mile Stanford fiber optic array and data acquisition were made possible through a collective effort by Stanford IT services, Stanford Geophysics, and OptaSense Ltd. Financial support was provided by the Stanford Exploration Project, the U.S. Department of Energy and the Schlumberger Fellowship.