In-situ monitoring of strain on the seafloor: an overview and results of the GeoSEA projects

The ocean floor is the outer solid shell for over 70% of our planet, which is continuously moved and deformed in the course of global plate tectonics. These processes lead to tectonic stresses building up in the seafloor, which over long periods of time become so large that they suddenly and usually (still) unexpectedly discharge in an earthquake. In the marine environment, the seafloor cannot be studied with the established tools of tectonic geodesy, as water is not a suitable medium for geodetic systems that depend on the relatively unperturbed transmission of electromagnetic waves. During the past three decades, advances made by using space geodetic systems, such as GPS and InSAR, have revolutionized our ability to precisely track actively deforming areas onshore in high spatial and temporal resolution. Offshore, seafloor geodesy aims at precise underwater measurements of interstation distances, absolute positions, water depth, and tilt. Seafloor displacement occurs in the horizontal (x,y) and vertical direction (z) as a function of time (t). The vertical displacement is measured by monitoring pressure variations at the seafloor. Horizontal seafloor displacement can be measured either using an acoustic/GPS combination to provide absolute positioning or by long-term acoustic telemetry between different beacons fixed on the seafloor to determine relative distances by using the travel time observations to each other, which is the technique used in the framework of the GeoSEA project (Geodetic Earthquake Observatory on the SEAfloor) with the aim to record deformation directly on the seafloor. Acoustic direct path measurements by the GeoSEA Array were conducted across the North Anatolian Fault in the Sea of Marmara, on the flank of Mt Etna in the Ionian Sea, and on the North-Chilean subduction zone in the eastern Pacific. The goal of these observations is to be able to directly measure the stress buildup and use it to refine estimates of the hazard situation. Since the expected deformation rates are low (a few cm/year at most), the stations have to remain on the seafloor for several years, where they measure autonomously in water depths up to 5,500 m with a precision of 5 mm, allowing for precise measurements of strain build-up in the seafloor. The results from the different campaigns reveal the range and degree of coupling as well as the distribution of deformation, ranging from fully locked to slow-slip movement.