Fiber Optic Cable Use for Seafloor studies of earthquake hazard and deformation

Informations projet

FOCUS

N° de convention de subvention: 786304

DOI

10.3030/786304

Projet clôturé

Date de signature de la CE 28 Mai 2018

Date de début 1 Octobre 2018

Date de fin 30 Septembre 2025

Financé au titre de

• EXCELLENT SCIENCE - European Research Council (ERC)

Coût total

€ 3 487 910,63

Contribution de l’UE

€ 3 487 910,50

Coordonné par

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 5 - FOCUS (Fiber Optic Cable Use for Seafloor studies of earthquake hazard and deformation)

Période du rapport: 2024-10-01 au 2025-09-30

Résumé du contexte et des objectifs généraux du projet

Two-thirds of the Earth’s surface is covered by water and thus largely inaccessible to modern networks of seismological instruments. The FOCUS project aims to improve seismic monitoring of the seafloor through a novel use of fiber optic cables to improve hazard assessment and increase early warning capability. Laser reflectometry using BOTDR, commonly used for structural health monitoring of large-scale engineering structures (e.g. - bridges, dams, pipelines, etc.), can measure very small strains (< 1 mm) at very large distances (10 - 200 km). It has never been used to monitor deformation caused by active faults on the seafloor. The objective of the FOCUS project is to demonstrate that this technique can measure small (1 - 2 cm) displacements on a primary test site offshore Catania, Sicily, where a 29 km long fiber optic cable crosses the recently mapped North Alfeo Fault. BOTDR observations must be calibrated by other independent measurements. Therefore, targeted marine geophysical surveys of the seafloor along the trace of the cable and faults were planned, with micro-bathymetry, high-resolution seismics, seafloor seismic stations and use of seafloor geodetic instruments to quantify fault displacement. Once the BOTDR fault-monitoring technique has been tested and calibrated offshore Sicily, the goal is to expand it to other fiber optic cable networks, either existing research networks in earthquake hazard zones (Japan, Cascadia) or to the Mediterranean region through access to retired telecommunication cables, or through the development of dual-use cables with industry partners, (two of the anticipated outcomes of the FOCUS project). The novel secondary use of fiber optic cables as described by FOCUS represents a potentially tremendous breakthrough in seismology, tectonics and natural hazard early warning capability, one that could turn Earth’s future undersea communication infrastructure into a seismological monitoring network of unprecedented scale.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

Our primary study area was offshore Catania, Sicily (near Mount Etna), an urban region of about 1 million, and struck by strong historical earthquakes (like in 1693 with 60,000 victims). 25 km offshore, a newly mapped, linear fault (like the San Andreas or North Anatolian Faults) called the North Alfeo Fault, was the target of our investigation. Here, using a Remotely Operated Vehicle (ROV) and a deep-sea plow, we deployed a specially designed and constructed, 6-km-long fiber optic cable, and crossed this submarine fault in 4 places. This large-scale experimental set-up allowed us to perform measurements from land, from the port laboratory of INFN-LNS in Catania. We detected a disturbance of the cable in Nov. 2020 (about 1 month after cable deployment). There was elongation at the first fault crossing (and a bit at the third fault crossing) of about 1.5 cm and 0.5 cm, respectively, and at first it looked like we might have detected movement (slip) along the fault. But the acoustic baselines (calculated distances) from a set of 8 acoustic beacons (mounted on 2.5-m-high metal tripods) and deployed on the seafloor, 4 on either side of the fault, indicated the fault had not moved. These results were reported in a 2023 paper published in Earth and Planetary Science Letters (Gutscher et al., 2023). A second signal we detected with our optical measurements on the strain cable was a manmade signal. In Sept. 2021, using an industry ROV, ze tried to improve the coupling between the cable and the seafloor by putting down almost 100 weight bags (17 bags containing steel pellets, and 79 bags containing sand) each weighing 25 kg on the portions of the cable that were poorly buried and/or spanning. These bag deployments (typically spaced 5 m apart, over 120 m long segments) pushed the cable down into the soft mud and caused a clear elongation signal in these places. (The bag drop signals / results were also reported in the 2023 EPSL article.)

Our second study area is a commercial submarine telecommunications network that connects the islands of the Guadeloupe archipelago. The Conseil Regional of Guadeloupe (the regional governing body) and Orange Caraïbe (the internet and telecom cable operator) agreed to grant access to the cable network so the members of the FOCUS project could perform field measurements using the BOTDR technique. The first measurements were performed in June 2022, which allowed us to establish our initial baseline. We then performed measurements every 6 months for the next 2 years, typically in late May / June and in late Nov. / Dec. We observed systematic annual changes in the Brillouin frequency, that appeared to be constant over large portions (20 km long segments) of the cable located on the shallow-water coral reef / platform. Our preliminary interpretation was that these fluctuations were temperature changes and when we compared these predicted temperature changes to the sea-surface temperature (as measured by satellites) there was an excellent correlation (to within 0.1°C). And for the third year (from June 2024 -June 2025) we also added an extra campaign in Sept. 2024 and in March 2025 (to catch the maximum and minimum annual water temperature). We also measured a strong (1.5°C) temperature increase between 2022 and 2024, which correlates with a period of severe coral bleaching (mortalities of about 30%) in the French Antilles (Martinique and Guadeloupe). These results were recently published in Geophysical Research Letters in Oct. 2025 (Gutscher et al., 2025).

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

As described above, the FOCUS project provided two main results regarding the use of BOTDR (Brillouin Optical Time Domain Reflectometry) on submarine cables: 1) for the strain cable aspect - we demonstrated we can measure strain (out to distances of 40 - 50 km) caused by natural and man-made disturbances. 2) for monitoring temperature at the seafloor (and with an accuracy of 0.1°C). As explained in the major achievments section while both these results were theoretically expected, they represent the first time such studies were performed in a marine environment, over such large distances (50 - 60 km) and over such long periods (3 - 5 years).
There were other very interesting results, some of them less relevant to the fiber optic measurements, I will mention them briefly here: 1) the type of sensor fiber we use (whether loosely buffered in a gel, or tightly buffered surrounded by a hard resin) has a strong impact on the performance for strain sensing (this was one of the main conclusions of Giuseppe Cappelli’s PhD thesis which he just completed in Dec. 2025); 2) the fine-scale structure (morphology from near seafloor bathymetric mapping performed with submarine drones/robots AUV and ROV) and sub-surface imaging and sediment cores of the upper 20 m below the seafloor, all conclusively show that the fault we were studying offshore Catania is fresh and moves by several meters per ten thousand years, and is thus capable of generating magnitude 6 earthquakes (this is the topic of a manuscript currently under review in the journal Tectonics) 3) we recorded small to medium size earthquakes using our network of OBS on the seafloor (from magnitude 2 to magnitude 4.3) in and around the fault we were studying. While no earthquakes occurred along the fault in the direct vicinity of the FOCUS cable (within 10 to 20 km), there is a band of seismicity along the fault trace, further to the SE (this is unpublished research in progress).

seafloor geodetic station (Canopus acoustic beacon on tripod)

microbathymetric map with North Alfeo fault and submarine landslide

deep-sea plow with 6km of cable and the cable-end-module (yellow)

connection of the FOCUS cable to the Y-junction box, using the ROV Victor6000