The marine part of AlpArray Seismic Network is composed by 30 broadband Ocean Bottom Seiemometers, fundamental to extend the ray coverage and the earthquake monitoring to the Ligurian sea. The resulting station distribution makes the AASN the ideal tool for 3-dimensional mapping of structures and physical properties of rock volumes in the crust, the upper mantle and the mantle transition zone. Following varying plans and station availabilities, the final configuration covers most of the Ligurian Sea, only leaving empty areas with less than 1000 m water depth, which cannot be instrumented as they are prone to bottom trawling.

The deployment started in June 2017 a French-German research team from ISTerre, IPGP, Geomar Kiel -- composed by Coralie Aubert (ISTerre), Wayne Crawford (IPGP, head of the mission), Romuald Daniel (IPGP), Anne Paul (ISTerre, scientific responsable), Catherine Péquegnat (ISTerre), Florian Petersen (Geomar), Patrick Schröder (Geomar) et Martina Ulvrova (Univ. Lyon) -- deployed 22 seismometers (60s) from the Alfred Wegener Institute pool and 7 seismometer (240s) from the NSU-IPGP pool.

Each OBS consist of a seismometer; and a pressure meter or a “hydrophone”. They are assembled to an anchor and a floating or “buoyancy” devices, connected to each other via a “releaser”. When a signal is sent to the releaser, it releases the anchor. With the buoy still attached to it, the OBS can float back to the surface of the water.

After almost 8 months of operation the 30 OBSs has been dismateled. In February-March 2018, the same team from France and Germany on the the research vessel Maria S. Merian is collecting back the instrumentations, ready for a new experiment.

Blog and reports from the cruises

Check here the weekly blog written during the OBS collection cruise on board of the Maria S. Merian vessel, as well as the final reports (thanks to Heidrun Kopp, February-March 2018):

- Blog news 2                          - Report from the first week                           
- Blog news 3                          - Report from the third week
- Blog news 4                          - Report from the fourth week
- Blog news 5                          - Final cruise report
- Blog news 6                          - Final data report

Furthermore, the OBS component of AlpArray in the Ligurian Sea has trigged further three marine or amphibious experiments, planned to be finalized in 2018:

• LOBSTER (Ligurian Ocean Bottom Seismology and Tectonics Research), an onshore-offshore refraction experiment with 3 land stations and 35 short-period OBSs, to be performed in February 2018 along two profiles. The additional benefit is that AlpArray OBSs will also record these active sources which will allow to retrieve proper station orientation information;
• OBS-Ligure, a local densification of the AlpArray Seismic Network with 7 short-period OBSs at the Northern Ligurian margin, in an effort to better record the seismicity in a region where faults are known to be capable of generating rare earthquakes in a range of Mw 6-7 (e.g., Larroque et al. 2011);
• SEFASILS, featuring 50-60 OBSs and a dozen onshore sites for refraction and deep reflection studies between Corsica and the toe of the continental slope between Nice and the Gulf of Genoa.