A Japanese research team recently said that they have detected annual seafloor subsidence ¡ª or gradual caving ¡ª near the Nankai Trough off the country¡¯s Pacific coast for the first time.

The team used water pressure gauges of the Donet underwater earthquake and tsunami observation network to detect the changes in two locations off the Kii Peninsula in central Japan.

Long-term observations at multiple sites could aid in assessing the risk of major earthquakes and tsunamis.

In the Nankai Trough, the sea-side plate sinks under the land-side plate, causing the seafloor to lower. Over time, this process can lead to a sudden slip at the plate boundary and result in a major earthquake and tsunami.

For the issuance of tsunami warnings, the Meteorological Agency uses Donet water pressure gauges that translate increases in water pressure from a tsunami into height measurements.

While subsidence can be detected through water pressure changes, the slight variations equivalent to a few centimeters per year had previously been indistinguishable from mechanical errors.

The team led by Yuya Machida, a researcher at the Japan Agency for Marine-Earth Science and Technology, or JAMSTEC, has developed a mobile high-precision water pressure calibration device.

They installed the device near a water pressure gauge at two locations off the Kii Peninsula, using laser measurements to assess minute height differences and comparing water pressure every six months to a year.

The team found that the seabed at locations off the southeast and south of the Kii Peninsula sank 1.5 centimeters and 2.5 centimeters per year, respectively.

The boundary between the plates is not fixed and small earthquakes can occur intermittently. The plates sometimes slip slowly without triggering seismic activity.

¡°In the future, we hope to increase the number of sites where seabed water pressure gauges will be calibrated and clarify long-term subsidence patterns,¡± Machida said.