Réponses sur la sismicité régionale
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The GROSMARIN experiment aims at investigating the deep structure of the North-Ligurian margin. This rifted margin is singular in that it lies immediately next to the Alpine orogenic arc. It is furthermore seismically active and can experience destructive earthquakes such as that of Imperia in 1887, which resulted in more than 600 casualties, at a time when the coastal area was much less densely populated than nowadays (2.5 Millions of inhabitants). In order to quantify seismic hazard along the French Riviera and the Ligurian region, the identification of active structures – an important part of which being located at sea – is required.
Plain lines are shots lines.
The objective of the GROSMARIN cruise is dual:
• first to characterize the North-Ligurian margin from a structural standpoint – mode and degree of the crustal stretching that preceded oceanic accretion, segmentation along strike, subsequent evolution in an orogenic context –;
• second, to identify zones of active crustal deformation at sea that are likely to generate earthquakes.
In order to reach these goals, we deployed 21 ocean-bottom seismometers (OBS) on a grid spanning 50 km along strike and 25 km across, located between Nice, France, and Imperia, Italy, and ranging from mid-slope to the deep basin.
This network was extended on land by 13 portable stations that were deployed in direct continuity during the whole experiment. The permanent stations of the French and Italian networks have complemented the array. Station interval was thus comparable offshore and on land. All these instruments recorded the shots of a seismic source so as to identify the main structures of the margin through a crustal 3D tomography and to fulfil the first aforementioned objective.
The acquisition of 10 shot profiles corresponding to the axes of the receiver array was achieved. Fifteen of the instruments subsequently remained in operation for the following 6 months in order to record the seismicity of active structures from close range, hence allowing a precise relocation of these events and a mapping of active faults – this addresses the second objective.
Some of the sea and land instruments were fitted with broadband sensors to allow teleseismic imaging of deep lithospheric discontinuities.
The cruise is now over and 16 out of 21 marine stations have been recovered. The OBS deployed are part of IRD pool developed and operated at Géoazur. The array was complemented by 5 stations rented to the Alfred Wegener Institute, through a collaboration with the IFM-GEOMAR.
This cruise is realised in the larger frame of the BALIZ project that was initiated in the Geosciences-Azur laboratory. The purposes of this programme come to a better understanding of the deformation mechanisms across the margin, at depth and in shallow areas. This is a prerequisite to quantify the hazard linked to these deformations, and eventually, to provide some founding to a policy of risk mitigation, in France and Italy. Some Italian laboratories are implicated on the different aspects of this programme: DipTeRis (Università di Genova) and DiSGAM (Università di Trieste). The GROSMARIN cruise is the second that takes place in that frame. It is focused on the deep structure. The shallow structure was addressed by the MALISAR cruises in 2006-2007 : MALISAR1 and MALISAR2.
Person in charge (sea) : Jean-Xavier Dessa (Géoazur) and Nicole Béthoux (Géoazur)
Team on board : Marie-Odile Beslier (Géoazur), Frauke Kligelhoefer (IFREMER-Brest), Olivier Bellier (CEREGE), Mauro Pavan (DIPTERIS, Univ. de Gênes), Davide Scafidi (DIPTERIS, Univ. de Gênes), Kevin Manchuel (Géoazur), Elodie Verdier (Géoazur), Yann Hello (Géoazur), Olivier Desprez de Gésincourt (Géoazur), Mario Ruiz Fernandez (Géoazur), Marten Lefeldt (IFM-GEOMAR).
Person in charge (ground) : Anne Deschamps (Géoazur) and Nicole Béthoux (Géoazur)
Team on ground : Didier Brunel (Géoazur), Françoise Courboulex (Géoazur), Laetitia Honoré (Géoazur), Julien Balestra (Géoazur), Alexandre Dano (Géoazur), Yvonne Font (Géoazur), Guust Nolet (Géoazur), Robert Pillet (Géoazur), and Géoazur maintenance team on ReNaSS and RAP stations .
Collaboration and maintenance of italian stations : Stéfano Solarino, Elena Eva, Claudio Eva, Daniele Spallarossa, Gabriele Ferretti, Daniele Cavanna and the maintenance team on RSNI (Regional seismic Network of Northwestern Italy).