The cable itself provides a potential new technology for monitoring seismic activity using lasers

Under the complex seabed of the Earth, not only is it rich in marine biological resources, but it has also quietly built a giant network of cable communication spanning the world. However, earthquakes and tsunamis may cause invisible damage to these important cables. Currently, over 70% of the Earth's surface is water - which undoubtedly poses a challenge for scientists monitoring seismic activity.

However, the cable itself provides a potential new technology for monitoring seismic activity using lasers. Recently, Geo Ocean researchers and their collaborators published a study in the Earth and Planetary Science Letters, using a laser technology called Brillouin Optical Time Domain Reflectometer (BOTDR) that can monitor and diagnose the "health status" of these cable structures in almost real-time.

Typically, this technology is used to monitor large-scale engineering projects, such as dams and bridges. Implementing this technology in the underwater environment means that there is finally a new "diagnostic" solution for cables that are very prone to damage, which can be identified and repaired in a timely manner.

Scientists deployed a 6 kilometer long cable remotely to measure the strain at 2100 meters below sea level. In order to measure the seismic activity of the Catania coast of Sicily, it is connected to a huge 29 km long optical fiber cable. This includes emitting a laser (consisting of hundreds of thousands of pulses within two hours) through one end of the fiber optic cable and detecting the points where light scatters from the defect, highlighting the area where the cable is deformed in some way.

In the 21 months starting from October 2020, they found an average increase of 2.5 centimeters in the length of the cable, which is due to the deformation caused by the tension of the cable passing through the North Alfeo Fault fault on the seabed.

This area is prone to strike slip faults (staggered sliding displacement of two tectonic plates), although the study section has experienced the subduction of a narrow plate under the Ionian Sea in northeastern Sicily, and caused the strain in both directions to reach 500 meters from the point where the fault passes.

In further testing, the aforementioned scientific team placed 79 sandbags every 120 meters along the cable, each weighing 25 kilograms. When the cable sank into the soft sediment below, the sandbags were stretched by 4 centimeters. Importantly, this stretching is not only limited to the area where sandbags are placed, but actually affects twice the spacing length. This is an important discovery because communication cables are buried under heavy objects, such as blocks or sediment landslides that move during fault movement, strong underflow or turbid currents (water sediment mixtures rapidly flow down slopes at a speed of 20 meters per second), which can seriously affect the ability of communication cables to withstand this tension and maintain their integrity.

However, there is also a positive side - because the elastic properties of the cable mean that some strains dissipate over time; Therefore, the cable has a certain potential to 'correct' the damage, so it is unlikely to completely break.

The Italian region under study has experienced devastating earthquakes in the past: for example, the earthquake in 1693 caused 12000 deaths in Catania alone; More recently, the 1908 earthquake (in the Calabrian arc region where Catania is located) killed as many as 82000 people. The new use of laser technology may mean identifying seismic activity before earthquakes occur, allowing time to develop strategies to mitigate damage to life, infrastructure, and communication.

Source: OFweek