RV MARIA S. MERIAN Fahrtbericht / Cruise Report MSM71 LOBSTER: Ligurian Ocean Bottom Seismology and Tectonics Research, Las Palmas (Spain) – Heraklion (Greece) 07.02.-27.02.2017

RV MARIA S MERIAN cruise MSM 71 sailed in the Ligurian Sea from Feb 07 to Feb 27, 2018 to de-install a network of broadband ocean bottom seismometers (OBS) deployed in June 2017 and to acquire two refraction seismic profiles using short period OBS. The long-term network comprising 30 broadband seismometers forms an integral part of the European AlpArray initiative and the German DFG Priority Programme SPP ‚Mountain Building in 4 Dimensions‘ MB 4D (http://www.spp-mountainbuilding.de/). The network consists of stations provided by the DEPAS-Pool as well as the institutes IPGP, GEOMAR, and GeoAzur. The aim of MSM 71 was to recover 29 of the stations. The instruments are deployed with a mean distance of 25 nm (46km) to the neighboring station (28 nm (51 km) to neighboring three stations) west and north of the island of Corsica. The westernmost instruments are located near the Gulf of Lions and the northernmost instruments were installed off the Ligurian coast. The working area lies in French and Italian territorial waters and covers an area of around 85.000 km2, hence almost equaling the area of the northern German states. The refraction seismic profiles with a length of 270 km (147 nm) and 136 km (73 nm) were shot with a 84 l G-gun array. A multi channel seismic streamer was deployed along both profiles to record the upper sedimentary sequences in the subsurface. These data will be used as a priori information during refraction data analysis. Shot signals were also recorded by the broadband stations in the network in order to facilitate the exact positioning and orientation of the stations on the sea floor during post-cruise processing. Therefore an additional shot profile into the network was acquired. The stations in the array recorded the shot signals over distances of 290 km (172 nm). The main refraction profile consisting of 35 ocean bottom seismometers and ocean bottom hydrophones was extended by land stations on the island of Corsica in a shoreline crossing mode. At the beginning of the cruise it became clear that the flash lights and radio beacons on a number of broadband stations did not work reliably so that these instruments could only be recovered during daylight hours. Due to military operations of the French navy and the concurrent closure of marine areas we were forced to re-schedule our working plan a number of times. Unfortunately, two broadband stations (A406A, A408A) as well as an ocean bottom hydrophone (OBSH134) could not be recovered. The data quality on the broadband stations is superb and the majority of stations along the refraction profiles recorded the oceanic Moho. These data will for the first time allow the determination of the Moho depth and the reconstruction of the oceanic crust in the Ligurian Sea on a regional basis.