A Japanese study found that the “closed zone” of the Nankai Trough earthquake changes over time

This image shows the projected source area of ​​the fearsome Nankai Trough earthquake extending off the coast of Shizuoka Prefecture, captured in June 2025 from a Mainichi Shimbun helicopter. (Mainichi)

TOKYO – A Japanese research team has discovered that the “closed zone” in the projected source area of ​​the terrifying Nankai Trough earthquake has changed over time. The region, where tectonic plates stick together tightly and pressure builds up until they are released by earthquakes, is key to understanding the potential for a huge earthquake in the future.

The research team, which includes members from the University of Tokyo’s Institute of Industrial Science and the Japanese Coast Guard, announced the findings in a study conducted on the Nankai Basin, where major earthquakes have occurred frequently over periods of 100 to 200 years. The team said data on closed areas forms the basis for earthquake and tsunami forecasting, and the results could lead to a re-evaluation of large earthquakes expected in the future that better reflects actual conditions.

In the Nankai Basin, the Philippine Sea plate is sinking beneath the Eurasian plate, on which the Japanese archipelago lies, at a rate of 4 to 6 centimeters per year. The team conducted a detailed analysis of crustal deformation data from the seafloor and land from 2013 to 2023. They calculated how slowly the plate was moving compared to its normal rate of sliding and used the result to estimate the degree of locking of the plate.

The results showed that some areas remained closed continuously, while other areas changed over time. The closed area extended continuously from an area about 50 kilometers from the Bongo Channel to a location about 50 kilometers from the Tokai area, with a particularly strong closure off the city of Tosa in Kochi Prefecture, and off the Kii Channel.

At the same time, areas that changed over time were distributed across a wide area 50–100 km offshore on the trench side in continuously closed areas.

The study suggests that rocks along plate boundaries in the Nankai Basin may be more homogeneous than in other areas, with a structure in which locking occurs at a certain depth and becomes less likely at shallow depths. The areas that changed over time were on the shallow side, where temperatures and pressures were lower, and water may have been involved. The accumulation of data over more than 10 years made it possible for the first time to analyze changes as temporal.

Previous giant earthquakes are said to have been caused by slow earthquakes, where plates gradually slide. “For the Nankai Basin, it was generally assumed that rupture of locked areas would start from a slow earthquake zone,” said Yusuke Yokota, associate professor of seafloor geodesy at the Institute of Industrial Science at the University of Tokyo. “However, the areas where locking changed over time did not necessarily overlap with areas where slow earthquakes occur. In other words, this suggests that release of plate locking can occur anywhere.” He added that considering new scenarios for the Nankai Basin is necessary.

The research results were published in the international academic journal Earth, Planets and Space in the field of Earth and Planetary Sciences (https://doi.org/10.1186/s40623-026-02472-1).

(Japanese original written by Yurika Tarumi, Lifestyle, Science and Environment News Section)

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