The submerged North Anatolian Fault in the Sea of Marmara is lacking any major seismic activity since its last rupture in 1766. The purpose of this acoustic ranging experiment was to determine whether this fault is continuously and aseismically creeping (i.e. slipping) or whether it is locked and thus accumulating stress that could cause a large magnitude earthquake, few tens of kilometers away from Istanbul.In October 2014, a ranging network of 10 acoustic transponders was installed across an active segment of the North-Anatolian Fault in the Marmara Sea at a depth of about 800 m (Figures 1 and 2, see further details).Figure 1: Map showing the location of the geodetic network in the Sea of Marmara (yellow box). Local ocean bottom seismometer (OBS) stations (29/10/2014-25/04/2015 and 26/04/2015-13/04/2016) are indicated with black triangles. Bathymetry in the central Sea of Marmara from Le Pichon et al. (2001) (https://doi.org/10.1016/S0012-821X(01)00449-6) and topography from Ryan et al. (2009) (https://doi.org/10.1029/2008GC002332).Figure 2: Map showing the location of the geodetic stations (red points indicate geodetic transponders from GEOMAR labelled with transponder IDs and green points indicate the French transponders). Bathymetry from AUV mapping (Grall et al. 2018, https://doi.org/10.17882/55744). Baselines of the intercommunicating transponders of Geomar shown with lines. The black baseline (G2302-G2307) has no line of sight. The long baselines with lengths of larger than 1000 m (shown in orange) are configured to work only unidirectional. White lines show bidirectional baselines. Figure taken from cruise report of POS484 (https://doi.org/10.3289/CR_POS_484/1).The acoustic network comprised 4 transponders from the University of Brest, France, and 6 transponders from GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany.The data available through this web site correspond to the data collected by the 6 German stations of GEOMAR's GeoSEA Array only.Data and details of the 4 French stations are accessible here: https://www.seanoe.org/data/00486/59750/Data from the GeoSEA Array available here include sets of acoustic ranging between pairs of stations (i.e. two-way-travel times), in addition to sound-speed, temperature and pressure measurements at each station. Sample rate was 120 minutes during the 2.5 years of data availability. These are the raw, unprocessed data. For instance, distances must be inferred from two-way-travel times and sound-speeds.Deployment and data upload cruises:- Stations were installed during the MARSITE cruise, leg1 (from 28/10 to 01/11 2014) with RV Pourquoi Pas? (https://doi.org/10.17600/14000500).- First data download and visual inspection of the transponders with ROV were made during the MARSITE cruise, leg3 (13 to 16 November 2014).- In April 2015, the data were acoustically downloaded from the seafloor transponders using a modem from the sea surface during RV Poseidon cruise POS484 (https://doi.org/10.3289/CR_POS_484/1).- In April 2016, the data were again downloaded during RV Poseidon cruise POS497 (https://doi.org/10.3289/CR_POS_497).- In May 2017, a third set of acoustic ranging data was downloaded with RV Yunus (cruise YUNUS17).- In January 2018, a download was attempted with RV Yunus (cruise YUNUS18), but the German transponders did not respond, most likely due to empty batteries of the transponders on the seafloor. Three of the French instruments responded which had larger batteries, and two were retrieved from the seafloor because their batteries were almost exhausted.Details on the Sonardyne transponders and user manuals of the 6TD autonomous monitoring system can be found here:https://www.sonardyne.com/product/autonomous-monitoring-system/

AcknowledgementsThe acoustic seafloor monitoring in the Sea of Marmara is a joint project of the Istanbul Technical University (Turkey) together with the University of Brest (France) and GEOMAR Helmholtz Centre for Ocean Research Kiel (Germany). The GeoSEA Array is funded through grant 03F0658I of the BMBF (German Federal Ministry of Education and Research).We want to thank the Captains and crews of RV Pourquoi Pas?, RV Poseidon, and RV Yunus for the successful deployment of the instruments (MARSITE Cruise in 2014) and recovery of the data (cruises POS484 in 2015, POS497 in 2016, YUNUS17 in 2017 and YUNUS18 in 2018), respectively.We thank C. Hammersley and T. Bennetts from Sonardyne for their technical support with the geodetic instruments over the years.Support was provided by the bilateral French-Turkish ANR/TÜBITAK collaborative research project MAREGAMI (ANR-16-CE03-0010-02 and Tübitak Project 116Y371). Support for the cruises for our French colleagues was provided by CNRS-INSU through the European Monitoring Seafloor Observatory programme (EMSO).This work is dedicated to the memory of our French colleague Dr. Anne Deschamps, who passed away shortly after the deployment of the acoustic transponders.DisclaimerThe data was prepared with very great attention on precision and accuracy. However, we will not take any warranty or liability for usage of the data in any applications such as navigation or others.

Supplement to: Lange, Dietrich; Kopp, Heidrun; Royer, Jean-Yves; Henry, Pierre; Çakir, Ziyadin; Petersen, Florian; Sakic, Pierre; Ballu, Valérie; Bialas, Jörg; Özeren, M Sinan; Ergintav, S; Geli, Louis (2019): Interseismic strain build-up on the submarine North Anatolian Fault offshore Istanbul. Nature Communications, 10(1)