Two-thirds of the earth’s surface is covered by water and thus largely inaccessible to modern networks of seismological instruments. A novel use of fiber optic cables could help improve hazard assessment and increase early warning capability. Laser reflectometry using BOTDR (Brillouin Optical Time Domain Reflectometry), 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). This technique 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 Sicily where the recently mapped North Alfeo Fault crosses the Catania EMSO seafloor observatory, 28 km long fiber optic cable. Two other EMSO test sites with fiber optic cables, the 100 km long Capo Passero (SE Sicily) and the 2 km long cable off Molene Island (W France) will also be studied. Initial reflectometry tests were performed on these three cables using a Febus BOTDR interrogator in June and July 2017. Unexpectedly high dynamic noise levels (corresponding to strains of 200 - 500 mm/m) were observed on the Molene cable, likely due to the high-energy, shallow water, open ocean environment. The tests on the EMSO infrastructure in Sicily indicated low experimental noise levels (20 - 30 mm/m) out to a distance of 15 km.
BOTDR observations will have to be calibrated by other independent measurements. Therefore, targeted marine geophysical surveys of the seafloor along the trace of the cable and faults are planned, with the use of seafloor geodetic instruments to quantify fault displacement. Once the BOTDR fault-monitoring technique has been tested, demonstrated and calibrated offshore Eastern 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 (decommissioned) telecommunication cables or development of dual-use cables (two of the anticipated outcomes of the FOCUS project). This represents a potentially tremendous breakthrough in seismology, tectonics and natural hazard early warning capability.
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