Résumé
During the MD214 EAGER cruise (Marion Dufresne, June 2018, doi.10.17600/18000520), three long sediment cores (between 12 and 23 m long) and three box-cores were collected in the Hateruma basin. In this small confined basin located in the forearc of the Ryukyus subduction (Japan), we aimed to access to sedimentary record of past extreme events, and specifically earthquakes and tsunamis over the last tens of thousand years. In this area, the Meiwa tsunami in 1771 was the most destructive event ever known in the region. Most islands of the Yaeyama archipelago in the southernmost Ryukyus were washed by that huge tsunami leaving nearly 12,000 victims. A study of available bathymetric and very high resolution seismic profile (CHIRP) data was carried out to determine the geometry of sedimentary bodies in the upper tens of meters of this basin and to identify the source areas of submarine gravity events (canyons, gullies, slump scars...). The analysis of the cores consisted in the sedimentological description (sedimentary facies, types of deposits), the interpretation of chemical and physical data (density, P-wave velocity, magnetic susceptibility and XRF core logs; laser granulometry; X-Ray imaging...). First stratigraphic constraints were obtained by (super 210) Pb analysis on the uppermost centimeters of sediment near the seafloor and by radiocarbon dating of planktonic foraminifera in pelagic layers. Five types of submarine gravity deposits were identified in cores including debrites, fine-grained turbidites, sandy turbidites, ash layers and mega-deposits (homogenites). These various sedimentary facies indicate a large variability of gravity processes in this basin with potentially different sediment sources and triggering mechanisms of gravity instabilities. The first radiocarbon dates allow preliminary age models for the 3 cores and show strong contrast of sedimentation rates between the three sites. Eleven sedimentary beds have been correlated in 3 cores and identified as potential records of large earthquakes during the last 30 kyrs with a return period of 3 +/- 2 kyrs.