An international study led by researchers from the Institute of Earth Physics of Paris reveals that the Ryukyu subduction zone in southwestern Japan is the site of millennial seismic supercycles, including major earthquakes capable of generating devastating tsunamis.
Subduction zones are among the regions most exposed to earthquakes and tsunamis worldwide. Yet, some of them remain poorly characterized in terms of their long-term seismic behavior. This is the case for the Ryukyu subduction zone, which extends between southern Japan and Taiwan. Although modern geodetic measurements suggest weak mechanical coupling, geological and historical records report major tsunamis, notably the Meiwa tsunami of 1771.
Artist's illustration of a fault splitting a city in two.
In a study published in Nature Communications, an international team of researchers, including scientists from the Institute of Earth Physics of Paris (IPGP), reconstructed the seismic history of this zone over an exceptional period (5,000 years) by analyzing fossil coral microatolls located on Ishigaki Island in the Ryukyu archipelago.
Corals as natural archives of large earthquakes
Microatolls are massive corals whose growth is tightly controlled by the relative sea level. Their morphology and internal stratigraphy allow them to record vertical ground motions, particularly those caused by earthquakes, with centimeter-scale precision. By combining field mapping, geomorphological analyses, X-ray imaging, tomography, and uranium-thorium dating, the researchers reconstructed a detailed chronicle of relative sea-level variations on a millennial scale.
The results highlight several episodes of rapid island uplift, notably grouped around 5,000-4,000 years ago and 3,000-2,000 years ago. These uplifts are interpreted as the signature of major subduction earthquakes.
Seismic supercycles spanning several millennia
The data reveal the existence of seismic supercycles: long periods of relative quiescence, followed by concentrated sequences of earthquakes, culminating in major events capable of producing tsunamis. Elastic modeling indicates that some of these earthquakes may have reached magnitudes comparable to the largest known events in other active subduction zones.
Several episodes identified in the corals coincide with tsunamis already documented in the region's geological and historical records, strengthening the hypothesis of a cyclic recurrence on a scale exceeding 2,000 years.
A major challenge for seismic and tsunami hazard assessment
This work challenges the idea that the Ryukyu subduction is primarily aseismic. It shows, on the contrary, that this zone is capable of producing major earthquakes with significant tsunamigenic potential. Understanding these long cycles is essential for improving seismic and tsunami hazard assessment in this densely populated and highly exposed region.
By providing new insight into the long-term dynamics of the Ryukyu subduction, this study illustrates the key role of natural archives in understanding major geophysical risks and underscores the importance of integrating long timescales into prevention strategies.