Monitoring Deformation Offshore Northern Chile Using a Seafloor Geodetic Network (GeoSEA)

The seafloor stores crucial information on sub-seafloor processes, including stress, elastic strain, and earthquake and tsunami generation. This information may be extracted through the emerging scientific field of seafloor geodesy. The target of the GeoSEA array (Geodetic Earthquake Observatory on the SEAfloor) is to measure deformation in mm-scale between instruments located in three target areas on the marine forearc and outer rise of the North Chilean subduction zone 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 this segment is still unbroken. The GeoSEA network consists of autonomous seafloor transponders installed on 4 m high tripods. In December 2015, they were lowered to the seabed on the deep-sea cable of RV SONNE. The transponders measure acoustic distance, tilt, temperature and pressure for a period of 3.5 years. As additional component of the network, GeoSURF, a self-steering autonomous vehicle (wave glider), monitors system health and is able to upload data from the seafloor stations and to transfer it via satellite. We set up 3 subarrays with interstation distances of up to 2000 m. On the middle continental slope in 2300 m water depth, an array consisting of 8 transponders measures across crustal faults seen in AUV mapping. A second array of 5 stations located on the outer rise monitors extension across normal plate-bending faults. The deepest deployment in 5000 m water depth located on the lower continental slope with 10 stations is designed to measure diffuse strain build-up. We expect a precision of ± 4mm for each station pair. The data of 11 months will be downloaded in November 2015 during a cruise of RV Langseth and will reveal the deformation between the transponders located on the seafloor.