Kopp, Heidrun, Lange, Dietrich, Hannemann, Katrin, Krabbenhoeft, Anne, Petersen, Florian and Timmermann, Anina, eds. and Scientific Crew SO244-II (2016) RV SONNE Fahrtbericht / Cruise Report SO244/2, GeoSEA: Geodetic Earthquake Observatory on the Seafloor, Antofagasta (Chile) – Antofagasta (Chile), 27.11.-13.12.2015. . GEOMAR Report, N. Ser. 034 . GEOMAR Helmholtz Centre for Ocean Research, Kiel, 86 pp. DOI 10.3289/GEOMAR_REP_NS_34_2016.

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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.

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