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Webinaire Marc-André Gutscher (LGO)

The FocusX1 expedition:

a first deployment of a fiber optic « strain » cable across an active submarine fault (offshore Catania, Sicily)


he FocusX1 marine expedition in October 2020 onboard the R/V PourquoiPas successfully deployed a 6 km long fiber optic cable and 8 seafloor geodetic stations across the North Alfeo fault, offshore Catania Sicily. This is the first time a dedicated strain cable has been deployed across an active submarine fault. Using the ROV Victor6000 we performed ultra-high resolution micro-bathymetric mapping of a roughly 4km2 region of the seafloor, covering an existing seafloor observatory TSS (Test Site South) and a roughly 2.5 km long segment of the fault and conducted a video-camera survey of the planned fault-crossings. The fault is very sharp, characterized by a roughly 10 – 20 m high escarpment featuring rugged seafloor relief (slopes of 10 – 30° and locally, even near-vertical cliffs). A new seafloor junction box was connected to the TSS and then via a cable-end-module was connected to the new (9 mm diameter) fiber-optic strain cable. Using ROV Victor600 we were able to deploy the full 6080m cable length along the seafloor by means of a dedicated plow system, and cross the strike-slip fault in 4 different locations. Additionally, 8 seafloor geodetic stations (Canopus acoustic beacons manufactured by iXblue) were deployed, 4 on each side of the fault, to measure acoustic baselines within this network and thus provide an independent measure of any relative movements caused by displacement of the North Alfeo Fault. The cable is now providing continuous BOTDR (Brillouin Optical Time Domain Reflectometry) acquisition using two active fibers, along the full distance (28 km from Catania to TSS and then two loops of 12.4 and 18.6 km length, respectively. If fault movement occurs it should be detected at multiple locations along the cable and then later calibrated by the seafloor geodetic network. During the deployment some DAS (Distributed Acoustic Sensing) measurements were also performed by Philippe Jousset (GFZ Potsdam). The next marine expeditions planned are FocusG1 with the Tethys (tentatively scheduled in summer 2021) to download the seafloor geodetic data. Thereafter (probably in early 2022) the FocusX2 expedition plans to deploy a network of 25 ocean-bottom seismometers (which will be complemented by a network of landstations run by INGV). Further objectives include performing sediment coring and light seismics in order to better characterize the current seismic behavior of the North Alfeo fault as well as its long-term seismicity (paleo-seismology) and image the deformation pattern recorded in the shallow seafloor sediments.