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The direct-path acoustic ranging method provides relative positioning by using high-precision acoustic transponders [Sonardyne Autonomous Monitoring Transponders (AMT)]. Multiple transponders installed at the seafloor measure the time of flight of acoustic signals between them with a microsecond resolution and water sound speed, temperature, and absolute pressure. Travel time observations were converted into distances with millimetric precision. Pressure measurements provided information on vertical displacement. Dual-axis inclinometers detected changes in instrument tilt. Repeated interrogations over months to years allowed the determination of displacements and, hence, deformation of the seafloor inside the network for extended periods, depending on battery capacity.

Here, we used five transponders from GEOMAR’s GeoSEA array. The transponders communicated with 8-ms phase-codes pulses and an 8-kHz bandwidth with a centered frequency of 18 kHz. The acoustic ranges were calculated by cross-correlation of the interrogation and receiving signals. The AMTs logged pressure, temperature, tilt, and sound speed. The log period for each transponder was set to 90 min. We noted instability in the sound speed measurement and recalculated the sound speed using the high-resolution temperature and pressure measurements at each transponder and assuming a constant salinity of 34 practical salinity units (37). We removed the tide signals from the pressure data using the data provided by the Istituto Superiore per la Protezione e la Ricerca Ambientale tide gauge in the port of Catania (www.mareografico.it). Pressure was converted to depth with the seawater density of 1024 kg/m3. For better comparison to the relative distance measurements obtained by acoustic telemetry, and because we are mostly interested in the relative movement of the unstable sector compared to the stable sector, we only showed relative vertical displacement between transponder pairs. These were obtained by subtracting the time series recorded by one transponder from that of another transponder.

The autonomous monitoring transponders were located at the outcrop of a fault at the seafloor. Locations for individual transponders were chosen on the basis of a closely spaced high-resolution two-dimensional (2D) seismic survey and swath bathymetric data. The network design ensures that at least two AMTs sit at each side of the fault and are in acoustic sight of each other. The AMTs were mounted on anchored buoyancy bodies. The deployed trapeze-shaped setup results in 10 monitored baselines. Besides transponder 1, all baselines were recorded in two directions (forward and backward measurements), resulting in six bidirectional baselines and four unidirectional baselines. Distances for forward (for example, measuring the travel time from AMT 1 to 2 and return) and backward measurements (measuring from AMT 2 to 1 and return) closely agree for all transponder pairs.

We deployed the transponders in April 2016 during RV Poseidon expedition POS496 at meter precision using ultrashort baseline acoustic positioning in water depths of 950 to 1180 m. Data stored in each station were uploaded from the seafloor to the surface with an acoustic modem.

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