According to The Japan Times on September 28, a Japanese research team reported continuous annual sinking of the seabed along the Nankai Trough off the Pacific coast.
The discovery was made using data from water pressure gauges in the “Donet” underwater earthquake and tsunami observation network, installed off the Kii Peninsula in central Japan.
Researchers said long-term observations at multiple points will help better assess the risk of major earthquakes and tsunamis, as the Nankai Trough is where the oceanic plate subducts beneath the continental plate. This gradual sinking could lead to sudden slips along the plate boundary, triggering powerful quakes and tsunamis.
Previously, detecting small-scale subsidence—only a few centimeters per year—was difficult due to potential instrument errors. However, the team led by Yuya Machida from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) developed a highly precise water pressure calibration device. It was installed near existing sensors off the Kii Peninsula and checked every 6 to 12 months using laser measurements.
Results showed that the seabed southeast of the peninsula is sinking by an average of 1.5 cm per year, and the southern area by about 2.5 cm per year.
The plate boundary in this region is not firmly locked and often experiences small earthquakes or slow slips without noticeable shaking.
Machida said, “In the future, we hope to increase the number of calibration sites for underwater pressure gauges to better understand long-term subsidence patterns.”
