by Morgan Sherburne, University of Michigan Locations of the subsea DAS (Distributed Acoustic Sensing) system, DART (Deep-ocean Assessment and Reporting of Tsunamis) observation systems, BPRs (bottom pressure recorders), tide gauge, and two tsunami events. Panel (b) indicated by the blue box in panel (a), shows the precise locations of the DART and tide gauge. Panel (c) zooms in on the red box in panel (b), revealing the water depth and position of the DAS system.

Fiber optic cables that line ocean floors could provide a less expensive, more comprehensive alternative to the current buoys that act as early warning systems for tsunamis, says a University of Michigan researcher.

A system called DART, or Deep-ocean Assessment and Reporting of Tsunamis, is composed of specialized buoys that monitor for tsunamis. Overseen by the National Oceanic and Atmospheric Administration, the buoys cost about $500,000 to install, with another $300,000 annually for upkeep. Thirty-two detection buoys dot the perimeter of the Pacific Ocean, resulting in millions of dollars per year in upkeep—costly, but vital upkeep.

Now, U-M seismologist Zack Spica and colleagues at California Institute of Technology have used a technique called distributed acoustic sensing, or DAS, to tap into a cheaper, more ubiquitous way to keep tabs on the natural disasters: the roughly 1 million miles of fiber optic cables that crisscross ocean floors.

“Telecommunication companies have been laying down these fiber optic cables for the last 30 years, and have spent hundreds of billions of dollars to do that,” said Spica, U-M assistant professor of Earth and environmental sciences. “Now, thanks to advanced photonics and great computing power, we can turn fiber optic cables into super dense, high fidelity arrays of sensors.”

Tsunamis are a series of massive waves triggered by sudden displacement of ocean water, most typically caused by the sudden ground motion of the sea floor. Tsunamis can be minor, or they can be devastating, such as 2004’s Indian Ocean tsunami, which killed nearly 228,000 people.

In a study published in Geophysical Research Letters, Spica and colleagues show that fiber optic cables can be used as an early tsunami warning system.

“Unlike earthquakes that happen suddenly and are hardly avoidable, even though some early warning systems exist, tsunamis generally take more time to build up and reach the coast,” Spica said. “This means that early warning systems are more efficient for tsunamis. Yet, what is hard is to assess the magnitude of a tsunami before it reaches the coast. Therefore, offshore instrumentation is needed, which is costly and hard to maintain.”

Over the previous five years, Spica and his fellow researchers installed DAS interrogator units in fiber optic telecommunication companies in Alaska, Japan, Spain and Lake Ontario that tap into underwater fiber optic cables. Using one of the devices placed in Florence, Oregon, the team was able to detect a tsunami that originated in an island chain nearly 1,300 miles east of the tip of South America.

“This was a major earthquake in the Sandwich Islands that generated a large tsunami. It wasn’t even in the same ocean as the cable and device on which we detected it,” Spica said. “By the time the tsunami arrived in Oregon and Alaska, it had a run-off of only a few centimeters, which didn’t produce any damage.”