Articles

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.

Séminaire de Pierre Bonnand (Laboratoire Magmas et Volcans / Université Clermont-Auvergne)

Une histoire de Redox avec les isotopes radiogéniques et stables du Cérium

Séminaire GM/LGO de Philippe JOUSSET (GFZ Potsdam)

Fibre optic Distributed Acoustic Sensing for seismology, volcanology and submarine research

 

Volcanic and seismic activities produce a variety of phenomena that put population at risk and disrupt our societies. Distributed Acoustic Sensing (DAS) technology has the ability to revolutionize seismic and volcanic monitoring and our understanding of the underlying processes. After a short review of the principle benefits and issues of the new DAS technology and its applications, I will focus on Mount Etna, Italy, a complex volcanic structure, where volcano-tectonic processes interact.

We applied DAS to monitor and explore faults at several locations around Mount Etna volcano in 2018 and 2019, in collaboration with the Instituto Nazionale di Geofisica e Volcanologia (INGV Catania). Distributed Acoustic Sensing (DAS) technology has been tested for the first time in 2018 (and also in 2019), as a new tool for monitoring the complex tectonic and volcanic interactions of Etna volcano from the summit to the sea floor. We connected up to 3 iDAS interrogators, sometimes simultaneously, to optical cables close to the summit, in urban areas and offshore. Each iDAS measured the dynamic strain rate along the whole length of the optical fibre, from the interferometric analysis of the back-scattered light.

At the summit area, we connect an iDAS interrogator inside the Volcanological Observatory of Pizzi Deneri (2800 m elevation close to Etna summit) to record dynamic strain signals along a 1.5 km-long fibre optic cable that we deployed in the scoria of Piano delle Concazze. We recorded signals associated with various volcanic events, local and distant earthquakes, thunderstorm, as well as many other anthropogenic signals (e.g., tourists). To validate the DAS signal, we collocated along the fibre cable multi-parametric arrays (comprising geophones, broadband seismometers, infrasonic arrays). During the survey periods, Etna activity was mainly characterized by moderate but frequent explosive and/or effusive activity from summit craters, as monitored by INGV. Our observations suggest that DAS technology can record volcano-related signals (in the order of tens nanostrain) with unprecedented spatial and temporal resolutions, opening new opportunities for the understanding of volcano processes. I show several recent results.

In urban environments, we took advantage of the existence of fibre optic telecommunication infrastructures, we connected iDAS interrogator to fibre optic cables, known to cross active faults linked to the volcano eastern flank dynamics. We recorded dynamic strain rate along several telecom lines in villages on the slopes of Etna, where faults cross villages (e.g., 4 km cable for about 20 days in Zafferana village; 12 km-long cable running from Linera to Fleri; 40 km-long fibre optic telecommunication cable on the western side of the volcano, at the border between the sedimentary layer and the volcano edifice).

On the sea floor, we connected an iDAS interrogator to a 30-km long fibre within a cable transmitting data from sub-marine instrumentation to INFN-LNS facility at the Catania harbor in collaboration with the University of Brest, the CNRS and IFREMER (FOCUS project). We record dynamic strain signals from local and regional earthquakes and look for faults offsetting the sea floor below the eastern flank of the volcano.

Our preliminary results demonstrate that DAS technology is able to contribute significantly to the monitoring system of earthquake and volcanic phenomena at Etna volcano from the summit to submarine environment, and thereby improves assessment of volcanic and seismic hazard at volcanoes.

Summit of Etna volcano August 2018.

Submarine DAS strain rate record of an earthquake M5.6 in Albania, 21.09.2019

Les webinaires GM (Ifremer)

Pendant la période de confinement où il n’était pas possible d’organiser des séminaires en présentiel, nos voisins et collègues du département Géosciences Marines (IFREMER) ont organisé des webinaires (ou séminaires en distanciel).

Nous étions conviés à participer à ces rendez-vous hebdomadaires riches d’échanges scientifiques.

28 mai 2020, Stephan JORRY (IFREMER)

Origin of modern atolls: Challenging the deeply engrained Darwin’s Theory

4 juin 2020, Stéphanie DUPRE (IFREMER)

The Aquitaine Shelf edge (Bay of Biscay): a primary outlet for microbial methane release

11 juin 2020, Morelia URLAUB (GEOMAR)

Characterising deformation of submerged volcanoes

18 juin 2020, Christophe BASILE (ISTerre – Université de Grenoble Alpes)

Séminaire GM/LGO d’Océane FOIX (GM, Ifremer, Brest)

The 3 ‐ D Velocity Models and Seismicity Highlight Forearc Deformation

Due to Subjecting Features (Central Vanuatu)

 

The central Vanuatu forearc is characterized by a reduced convergence rate at the trench, signifcant uplif of the overriding plate, and the presence of large forearc islands. Volcanic actvity and intermediate ‐ depth seismicity behind the forearc are among the highest on Earth. These features are presumed to be associated with the subducton of a large seamount chain and an immersed ridge. We used a catalog of P and S arrivals from a local seismological network to construct the frst 3 ‐ D velocity model of the region and to relocate earthquakes beneath the forearc. The 3 ‐ D model reveals a highly heterogeneous velocity distributon in the frst 40 km beneath the surface. Trench ‐ parallel low P and S velocity zones in the upper tens of kilometers beneath the western edges of the two largest forearc islands correlate to the major features entering into subducton and suggest highly fractured and probably water ‐ infltrated features. Trench ‐ parallel high ‐ velocity zones at 5–15 ‐ km depth, further to the east, may be part of a contnuous consolidated rock structure that acts as a backstop. Thick overriding plate crust (29 ± 3 km) in the forearc is consistent with the presence of contnental remnants. The earthquake distributon is generally heterogeneous, suggestng a complex fault structure and variable stress. Earthquakes are, however, well aligned at the plate interface in between the subductng features, where they constrain the angle of subducton to be 15° on average, down to 10–15 ‐ km depth.

6 mars 2020