[S-CG50] Earthquakes, Tsunamis, Seismotectonics, and Hazard Potential of the Ryukyu Trench and Okinawa Trough

[SCG50-11] Long-term vertical deformations in the Ryukyu subduction zone, Japan

*Sophie Debaecker^1,2,3, Nathalie Feuillet¹, Kenji Satake² (1.Institut de Physique du Globe de Paris, Université de Paris, 2.Earthquake Research Institute, University of Tokyo, 3.Littoral Environnement et Societes, CNRS/Universite de la Rochelle)

Keywords:Subduction zone, Marine terraces, Ryukyus

In subduction zones, the seismic cycle may induce plastic deformations that can accumulate through time leading to the formation of permanent reliefs. Studying long-term deformations can lead to a better understanding of the megathrust seismic cycle, its interface and its areas of past and potential future seismic ruptures. In the Ryukyu subduction zone, the tectonic behavior at the plate interface is not well understood. Its seismic coupling is debated since the recent discovery of both aseismic events and past earthquakes. We propose to investigate long-term behavior of the overriding Eurasian plate to better understand the geometry and behavior of the megathrust, and better assess its seismic hazard. We studied marine terraces, that are exceptionnaly distributed along the archipelago.

Marine terraces are formed by a combination of eustatic variations and tectonic vertical motions. Using very high resolution satellite imagery to compute Digital Surface Models, we obtained unique and new insights on the morphology of 12 islands between Kikai and Ishigaki islands. We identified and mapped a total of 13 Pleistocene terraces. We measured their elevation and correlated them with glacio-eustatic curves, as well as previous absolute datation from literature in order to establish their chronology. An average uplift has been estimated for each studied island, and constrained using reef growth modelling.

Our study hilights the main role of the megathrust in shapping the long-term topography on the overriding plate. Variations in the uplift rates reveal the spatial and vertical segmentation of the megathrust. Finally, we explored the link between long-term uplift and short to very short-term vertical deformations of the Ryukyu subduction zone to discuss its behavior over multiple time scales.