Tsunamis are one of the most devastating natural disasters that can strike coastal areas, causing massive damage, displacement, and death. In 2004, the Indian Ocean tsunami killed nearly 228,000 people in 14 countries. In 2011, the Tohoku earthquake and tsunami triggered the Fukushima nuclear disaster and claimed over 15,000 lives in Japan.
To prevent such tragedies, early warning systems are essential to alert people and authorities of the impending danger and evacuate them to safety. However, the current systems rely on expensive and sparse buoys that monitor the ocean for signs of tsunamis, such as changes in water pressure or surface height. These buoys are costly to install and maintain, and they may not cover all the possible sources and directions of tsunamis.
But what if there was a cheaper, more comprehensive, and more reliable way to detect tsunamis using existing infrastructure? That is the idea behind a new technique developed by researchers from the University of Michigan and the California Institute of Technology, who have shown that fiber optic cables that line the ocean floors can be used as an effective tsunami warning system.
Fiber optic cables are thin strands of glass or plastic that transmit data using pulses of light. They are widely used for telecommunication, internet, and cable TV services, and they span over 1 million miles across the ocean floors, connecting continents and islands. These cables are sensitive to sound waves, temperature changes, and vibrations, which can affect the properties of the light signals traveling through them.
The researchers used a technique called distributed acoustic sensing, or DAS, to tap into these signals and extract information about the surrounding environment. DAS involves sending a laser beam through a fiber optic cable and measuring the backscattered light that reflects from tiny imperfections along the cable. By analyzing the frequency, phase, and amplitude of the backscattered light, the researchers can infer the strain and displacement of the cable caused by external factors, such as seismic waves, ocean currents, or tsunamis.
Using DAS, the researchers were able to detect two tsunamis that occurred in 2019 and 2020, one originating from the South Sandwich Islands and the other from the Kuril Islands. The researchers used a DAS interrogator unit installed in Florence, Oregon, that tapped into a 20-kilometer-long segment of a fiber optic cable that runs along the continental shelf of the Pacific Ocean. The DAS unit recorded the changes in the cable’s strain and displacement caused by the passing tsunami waves, which were also detected by the conventional buoys and tide gauges.
The researchers compared the performance of the DAS system with the existing tsunami detection systems and found that the DAS system had several advantages. First, the DAS system provided a much higher spatial resolution, with one sensor every 10 meters along the cable, compared to one buoy every several hundred kilometers. This means that the DAS system can capture more details and variations of the tsunami waves and their propagation. Second, the DAS system had a lower latency, with data being transmitted in real time, compared to the buoys that have a delay of several minutes. This means that the DAS system can provide faster and more accurate alerts. Third, the DAS system had a lower cost, since it used existing fiber optic cables that are already deployed and maintained by telecommunication companies, compared to the buoys that require dedicated installation and upkeep.
The researchers believe that the DAS technique can be applied to any fiber optic cable that runs along the ocean floor, creating a global network of sensors that can monitor and warn of tsunamis. They also suggest that the DAS technique can be used for other purposes, such as studying ocean dynamics, marine life, and climate change.
