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Abstract

RV SONNE cruise SO244-2 sailed offshore northern Chile from Nov. 27 to Dec. 13, 2015 to install the seafloor geodetic network GeoSEA (Geodetic Earthquake Observatory on the SEAfloor) on the marine forearc and outer rise of the South American subduction system around 21°S. This segment of the Nazca-South American plate boundary has last ruptured in an earthquake in 1877 and was identified as a seismic gap prior to the 2014 Iquique/Pisagua earthquake (Mw=8.1). The southern portion of the segment remains unbroken by a recent earthquake and is currently in the latest stage of the interseismic phase of the seismic cycle. Seafloor geodetic measurements provide a way to monitor crustal deformation at high resolution comparable to the satellite-based GPS technique upon which terrestrial geodesy is largely based. The GeoSEA Network consists of autonomous seafloor transponders installed on 4 m high tripods, which were lowered to the seabed on the deep-sea cable of RV SONNE. The transponders within an array intercommunicate via acoustic signals for a period of up to 3.5 years. An additional component of the network is GeoSURF, a self-steering autonomous surface vehicle (Wave Glider), which monitors system health and is capable to upload the seafloor data to the sea surface and to transfer it via satellite. We have chosen three areas on the middle and lower slope and the outer rise for the set-up of three sub-arrays. The array in Area 1 on the middle continental slope consists of 8 transponders located in pairs on four topographic ridges, which are surface expressions of faults at depth. Area 2 is located on the outer rise seaward of the trench where 5 stations monitor extension across plate-bending related normal faults. The third area is located at water depth >5000 m on the lower continental slope where an array of 10 stations measures diffuse strain build-up. Data from all networks and all stations were successfully uploaded to GeoSURF and/or a HPT modem lowered into the water from RV SONNE. The seabed installation of a total of 23 transponders was completed by December 07, when we proceeded to deploy a total of 14 ocean bottom seismometers (OBS) on the forearc between 19.2°-21.6°S. These instruments will be recovered by RV LANGSETH in Spring/Summer 2016.