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Overview
A researcher at the University of Michigan says seabed fiber-optic cables could provide a cheaper, more comprehensive alternative to the buoy-based tsunami early-warning systems currently in use.
Current Buoy System and Costs
The DART (Deep-ocean Assessment and Reporting of Tsunamis) system consists of dedicated tsunami-monitoring buoys. Under the supervision of the U.S. National Oceanic and Atmospheric Administration (NOAA), deployment of each buoy costs on the order of $500,000, with about $300,000 in annual maintenance. Dozens of detection buoys deployed around the Pacific incur multi-million-dollar annual maintenance expenses.
Using DAS and Submarine Fiber
Seismologist Zach Spica and colleagues at the University of Michigan have applied distributed acoustic sensing, or DAS, to leverage the roughly one million miles of submarine fiber-optic cables as an alternative sensing network for natural hazards.
Tsunami Characteristics
Tsunamis are waves produced by sudden displacement of large volumes of seawater, commonly triggered by abrupt seafloor motion. Tsunami impacts can range from minor to catastrophic; for example, the 2004 Indian Ocean tsunami caused about 228,000 deaths.
Research Findings
In a study published in Geophysical Research Letters, Spica and colleagues demonstrated that fiber-optic cables can function as part of an early-warning system for tsunamis.
Unlike sudden earthquakes, which can produce short-duration high-frequency signals, tsunamis typically require more time to develop and to reach the coast, making early detection via different sensor modalities potentially valuable.
Field Deployments
Over the past five years the team installed DAS interrogators on submarine telecom fibers in locations including Alaska, Japan, Spain, and Lake Ontario. Using an instrument deployed near Florence, Oregon, the researchers were able to detect a tsunami triggered by a large earthquake in the South Sandwich Islands, nearly 1,300 miles east of the southern tip of South America.
According to the team, the detected event originated in a different ocean basin than the cable and equipment used for detection.
How DAS Works
DAS monitors light pulses sent through a fiber-optic cable and measures backscattered photons. When light propagates as a wave through the fiber, a portion is scattered back toward the source. The intensity of the returned light over time is proportional to strain along the cable, allowing the system to infer dynamic deformations along the cable length.
Detection Challenges
Initial DAS applications focused on detecting earthquakes, which release large amounts of energy in short time intervals. The key question for tsunami detection is whether DAS can sense the much lower-frequency, longer-duration motions associated with tsunami waves, where wave crests can be separated by tens of minutes and span many miles.
Earthquakes typically produce higher-energy, faster vibrations, whereas tsunami waves are much broader and lower frequency.
Implications and Next Steps
DAS could offer telecom operators an alternative use for submarine cables as network architectures evolve. The research team notes that while telecom companies are aware of sensing applications for fiber, widespread operational adoption remains premature.
The study builds on prior work investigating whether fiber-optic networks can detect ground motion during earthquakes. The next step identified by the researchers is development of software capable of recording and processing tsunami signals from submarine cables in real time.
