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

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.