A Japanese research group has, for the first time, measured yearly seafloor sinking near the Nankai Trough off the Pacific coast, The Japan Times reports. Using advanced water pressure gauges from the Donet earthquake and tsunami monitoring network, the team identified gradual subsidence at two points off the Kii Peninsula in central Japan.

The Nankai Trough is a tectonic boundary where the oceanic plate descends beneath the continental plate, gradually lowering the seabed. Over long periods, this process can culminate in sudden shifts along the plate boundary, sparking powerful earthquakes and tsunamis. Data from Donet is already crucial for Japan’s Meteorological Agency, which relies on water pressure changes to issue tsunami warnings.

Detecting small-scale movements has been a challenge, as shifts of only a few centimetres each year are easily mistaken for device errors. To overcome this, a team led by Yuya Machida of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) created a portable, highly accurate calibration tool. Installed at two sites, it uses laser-based measurements to verify subtle height differences.

Findings revealed seabed sinking of 1.5 cm and 2.5 cm annually at the southeast and south of the Kii Peninsula. Researchers aim to expand monitoring to better map long-term subsidence patterns. This breakthrough not only sharpens earthquake and tsunami risk assessment but also underscores how precision technology is becoming vital in safeguarding coastal populations.