# Evidence of megathrust earthquakes and seismic supercycles in subtropical Japan from millennia-old coral microatolls

## TL;DR

Fossil corals in Japan reveal repeated megathrust earthquakes over 5,000 years, indicating long-term seismic supercycles and future hazards.

## Contribution

The study identifies megathrust earthquake supercycles in the Ryukyu subduction zone using coral microatolls.

## Key findings

- Coral records show multiple relative emergence events between 5-4 and 3-2 thousand years ago.
- Elastic modeling links these events to coseismic uplift from megathrust earthquakes.
- The data suggest seismic supercycles with recurrence intervals exceeding 2,000 years.

## Abstract

Megathrust earthquakes in subduction zones often go unreported because they are rare and the historical record is short. On the Ryukyu subduction zone of southwestern Japan, unlike neighboring Nankai Trough, the history and future potential of great interplate earthquakes are not well known. While the geodetic measurements on the islands suggest that the plate coupling is very weak, recent observations of slow seismic events as well as offshore geodetic measurements imply the presence of coupled patches along the megathrust. Furthermore, the historical and geological studies indicate evidence of great tsunamis. Here, we use fossil microatolls in Ishigaki island to reconstruct the relative sea level in the Holocene. The coral record reveals several relative emergence episodes clustering between 5-4 and 3-2 thousand years ago (ka). Elastic modeling shows that the observed motions can correspond to coseismic uplift associated with megathrust earthquakes. The clusters of megathrust events suggest possible supercycles of earthquakes with a recurrence interval of more than 2 ka. Such results imply a strong seismic hazard for the upcoming centuries. The devastating 1771 Meiwa earthquake and associated tsunami may mark the onset of the most recent seismic supercycle.

By studying fossil corals on Ishigaki Island in Japan, Debaecker et al. find evidence of repeating megathrust earthquakes over the last 5,000 years, revealing long-term earthquake supercycles and highlighting significant future earthquake and tsunami hazards.