Recent Relative Sea‐Level Changes Recorded by Coral Microatolls in Southern Ryukyus Islands, Japan: Implication for the Seismic Cycle of the Megathrust

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 24; no. 6
• Main Authors: Debaecker, S., Feuillet, N., Satake, K., Sowa, K., Yamada, M., Watanabe, A., Saiki, A., Saurel, J.‐M., Nakamura, M., Occhipinti, G., Yu, T.‐L., Shen, C.‐C.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.06.2023; AGU and the Geochemical Society; Wiley
• Subjects: Accumulation; Archipelagoes; Aseismic zones; Corals; Earthquakes; Geological hazards; Islands; Microatolls; Navigation; Navigation satellites; Navigation systems; Navigational satellites; Sciences of the Universe; Seismic activity; Seismic hazard; Slabs; Subduction; Subduction zones; Tropical environments; Japan
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010587

The seismic hazard related to megathrust earthquakes in the Ryukyus (southern Japan) is poorly constrained as no large earthquake has been reported there. The Meiwa tsunami impacted the coasts of the Yaeyama and Miyako islands in 1771 but its origin is still debated. Global Navigation Satellite Systems measurements indicate that strain is accumulating along the plate interface but the observation period is short. It is thus crucial to gain information on the seismic potential of the megathrust. The islands of the Ryukyu archipelago are located in a tropical region and surrounded by reefs where numerous microatolls are growing. They preserved the record of variations of the relative sea‐level in their skeleton. We mapped seven sites over five islands and subsequently selected and sampled eight slabs of modern microatolls. The corals have emerged slowly at a rate of 0.7–2.8 mm/yr due to the long‐term interseismic loading on the megathrust up to 40 km in deep. The coupling rate estimated from elastic back‐slip models ranges between 10% and 100%. We also identified multi‐decadal relative sea‐level changes of a few cm/yr, likely due to very long duration slow‐slip events (SSE) along the shallow or deep parts of the megathrust. Those SSEs occur each 10–40 years and have accommodated 50% of the convergence rate in the last 250 years. Our study provides new constraints on the seismic cycle of the Ryukyu megathrust and on the seismic hazard in this region and suggests that a large megathrust earthquake could occur in the area in the future. Plain Language Summary We aim to better constrain the seismic and tsunamigenic hazard related to large subduction earthquakes in the Ryukyu archipelago (southern Japan). The behavior (seismic vs. aseismic) of this subduction zone is still debated and we do not know if it has the capacity to rupture during a large thrust earthquake in the future. This is mainly because the historical catalog of seismic events is very short (only few centuries) in this area. Our goal was to find geological markers able to preserve, over several decades or centuries, the trace of past vertical surface deformations of the upper plate above the megathrust. Those deformations are controlled by deep processes along the megathrust, either due to interseismic strain accumulation in between earthquakes or release during the earthquake (coseismic slip). To this goal, we sampled massive corals that are able to record centimetric variations of the sea‐level. All corals have emerged at a rate of 2 mm/yr because the plate interface is locked and accumulates strain that may be released in a future large earthquake. We also show that long‐duration transient slow‐slip events likely occur along the shallower portion of the megathrust. Our study provides new constraints on the seismic hazard in this area. Key Points We document relative sea‐level history in the southern Ryukyus from modern coral microatoll records Interseismic uplift rates indicate a coupling of up to 40% of the deep portion of the megathrust up to 40 km We evidenced possible decadal‐and decametric‐scale slow‐slip events below the seismogenic part of the megathrust