Seismic supercycles in the Ryukyus subduction zone: Insights from marine terraces and coral microatolls

Those past decades, the occurrence of major megathrust earthquakes highlighted the need to improve our knowledge on the behavior and seismogenic potential of subduction zones. Located between Japan main island and Taiwan, the Ryukyus island arc follows the Ryukyus trench, where the Philippines Sea plate subducts below the Eurasian plate. Seismic coupling has been questioned here, since the subduction zone present similarities with subduction zones where large Mw9-class megathrust earthquakes occurred, and since recent GPS and seismic data rather correspond to that of an aseismic convergence. In the Ryukyus, we aim to improve our knowledge of subduction zone behavior using surface deformations. The archipelago allows for an unprecented multi-time scale study of the vertical deformation of the overriding plate since they present marine terraces and coral microatolls. Using digital surface model computed from very high-resolution Pleiades satellite images over 13 islands, we investigate the distribution of Pleistocene marine terraces along the arc and the vertical long-term deformation they record. Spatial distribution of those uplifts along the arc revealed the along-arc segmentation, that we related to curvature of the arc, as well as subduction of bathymetric reliefs. Over several decades to several millenia, we documented vertical deformation of the arc using coral microatolls. We sampled and analyzed up to 12 microatolls in the southern Ryukyus, including 4 fossil ones. Combining the fossil study sites and slices permitted to evidence up to 19 past sudden vertical motions. Correlating those movements with coseismic slips on the interplate thanks to elastic modelling led us to suggest seismic supercycles along the Ryukyus islands over the last 5ka. Those supercycles would last several centuries and would be defined by recurrence time for subduction earthquakes of several decades to few centuries. Those past two centuries, modern microatolls recorded uplift motions of up to 3 mm/yr. Variability in the uplift motions highlighted differences between islands, linked with differences in structure and coupling of the plate interface. Those motions have also been observed at shorter time scale through GPS data. Processes promoting such variations are discussed together with features of the subduction zone, such as the seismogenic segments size in the megathrust. Our multi time-scale study of the Ryukyus subduction zone helped improve the seismic hazard previously estimated with GPS data over very short-term, since it documents the relationship between long-term structures of the plate interface, its tectonic behavior at the mid-term time scale, and its seismic potential at short-term.