In addition to providing a vast network of cables that span the globe and serve as a communication aid, the ocean’s depths are home to a complex marine ecosystem. These crucial cables may be damaged by earthquakes and tsunamis. Scientists who are keeping an eye on this seismic activity face a challenge because more than 70% of the Earth’s surface is water.
However, the cables themselves provide a potential new method for using laser light to monitor seismic activity. A new report by specialists at Geo-Sea and their colleagues revealed in Earth and Planetary Science Letters has utilized laser innovation, named Brillouin optical time space reflectometry (BOTDR), to screen the underlying wellbeing of these links in close to continuous This method has typically been used to monitor large engineering projects like bridges and dams. When this technology is used in a submarine environment, damaged cables can be quickly identified and repaired.
To measure strain 2,100 meters below the sea surface, the researchers deployed a 6-kilometer-long cable with a remotely operated vehicle. To measure earthquake activity offshore of Catania, Sicily, it was attached to a massive 29-kilometer fiber optic cable. This required passing a laser through one end of the fiber optic cable for two hours, firing hundreds of thousands of pulses at a time, and locating the points where the light scattered off imperfections, bringing to light areas where the cable was deformed in some way.
They discovered that the deformation caused by tension at the point where the cable crosses the North Alfeo Fault on the seafloor caused the cable to experience an average length increase of 2.5 cm over 21 months beginning in October 2020. Although the study section experienced subduction of a narrow plate beneath the Ionian Sea under northeast Sicily, which resulted in strain reaching 500 m in both directions from the point of crossing the fault, this region is prone to strike-slip faults (where two tectonic plates slide past each other).
Undersea fiber optic cables are getting sandbags. Credit: Gutscher and others, 2023
In additional tests, 79 25-kg sand bags were placed 120 meters apart from the cable. This caused the cable to stretch up to 4 cm as it sank into the soft sediments below. Importantly, this stretching affected twice the length of the intervals, not just the area where the sandbags were dropped. This is an important finding because strong bottom currents or turbidity flows (a water-sediment mixture flowing rapidly down a slope at speeds up to 20 meters per second) could have a significant impact on the ability of communications cables to withstand such tension and maintain their integrity when they are buried beneath a heavy load, such as caused by moving blocks or sediment slumping during fault movement.
Despite this, there is good news because the cable has elastic properties, which means that some of the strain will go away over time. As a result, the cable is less likely to break completely and can “correct” some of the damage.
The study’s region of Italy has experienced devastating earthquakes in the past, including the 1693 event that killed 12,000 people in Catania alone and the 1908 earthquake that killed up to 82,000 people in the Calabrian Arc, which includes Catania. Due to the novel application of laser technology, seismic activity may be detected in advance of an earthquake, allowing time to plan mitigation strategies for life, infrastructure, and communications.
More information: Marc-André Gutscher et al, Detecting strain with a fiber optic cable on the seafloor offshore Mount Etna, Southern Italy, Earth and Planetary Science Letters (2023). DOI: 10.1016/j.epsl.2023.118230
