Résumé
Taiwan undergoes extremely high seismic and typhoon hazards because of its specific geodynamical and climatic context. The island is located at the junction of two subduction zones resulting in high seismicity, but also along the "typhoon alley" with the occurrence of 4 typhoons per year on average. Exceptional events (M>8 earthquake, mega-typhoon, tsunami) are rare at historical timescale, and it is essential to establish time series of several thousands years to better understand the factors controlling them. The marine sediment provides a continuous and datable record of these events by submarine gravity deposits (turbidites, debrites) contrasting with slow pelagic sedimentation. During the MD214 EAGER oceanographic cruise (Marion Dufresne, 06/2018; doi.10.17600/18000520), 32 sediment cores were collected in the deep-sea domain around Taiwan. The objective was to access sedimentary record of extreme events (e.g., earthquakes, typhoons, and volcanic eruptions) during the last tens of thousand years, to constrain their cyclicity and variability, and better assess natural hazards. We present preliminary results of cores collected offshore eastern Taiwan, in the Ryukyu Forearc basins, accretionary prism, and trench. Taiwan is perpendicular to the Ryukyu subduction zone, and therefore laterally supplies the forearc basins and the trench with sediment via numerous submarine canyons and channels directly connected to their supply rivers. Sediment contains hemipelagites interbedded between numerous turbidites (<1m thick). Radiocarbon dating in one core of the Nanao basin reveals a mean recurrence of the turbidites of 30yr. Two types of turbidites support large typhoons/floods with a mean recurrence of approximately 40yr, and approximately 120yr for larger deposits, compatible with nearby paleoseismic record. Eastwards, offshore Hateruma, turbiditic sequences show different gravity deposits, including Megaturbidites/Homogenites classically interpreted as earthquake induced, and debrites with reworked sediment from coral reefs. Those deposits may represent the first marine record of tsunamis in this area. Finally, deposits in piggy-back basin show large intervals of hemipelagic deposits suitable for palaeo-oceanographic investigation, interrupted by scarce MTDs probably related to earthquakes, and ash layers.