Detecting Small Seafloor Subsidence in the Nankai Trough
- Toward Real-Time Monitoring of Seafloor Crustal Deformation –

1. Key Points

• In the Nankai Trough seismogenic zone, accurate monitoring of seafloor subsidence caused by plate subduction is essential for understanding processes of occurence of large interplate earthquake. In this study, we quantitatively observed, for the first time, the small-scale long-term seafloor subsidence associated with plate convergence int the Nankai trough.
• Seafloor pressure gauges, originally installed in the Nankai Trough for early tsunami detection, are also expected to monitor seafloor crustal deformation caused by plate subduction. However, sensor drift has been a major challenge, making it difficult to quantitatively measure long-term vertical seafloor displacements.
• In this study, we developed an in-situ calibration technique for seafloor pressure gauges, overcoming the long-standing challenge of sensor drift and successfully detecting small vertical seafloor displacements of 1.5–2.5 cm per year. These observations provide important evidence for understanding the state of plate coupling and the progression of earthquakes in the Nankai Trough seismogenic zone, and are expected to contribute to improved disaster prevention measures.

Figure 1. Photos of the pressure standard and the in-situ pressure calibration using the mobile pressure calibrator to the DONET pressure gauge (Left). In-situ pressure calibration is performed a few meters away from the DONET pressure gauge with the mobile pressure calibrator attitude maintained horizontal. (Right) Schematic figure of the in-situ pressure calibration chain.

2. Overview

A research team of Dr. Yuya Machida at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has successfully estimated sensor drift of DONET seafloor pressure gauges, which had long been a major challenge until now. By removing this sensor drift from long-term pressure records, the team revealed that the seafloor in the Nankai Trough seismogenic zone is steadily subsiding at a rate of about 1.5 to 2.5 centimeters per year.

This study aims to elucidate the phenomenon of subsidence of the oceanic plate over time in regions where large interplate earthquakes occur. Steady subsidence of the seafloor contributes to understanding the strain accumulated between plates and its regional characteristics, leading to a better understanding of the mechanisms underlying large Nankai Trough earthquakes and slow slip events.

Until now, detecting such long-term and small-scale subsidence was extremely difficult because the natural rate of subsidence is about the same as the sensor drift in pressure gauges. To solve this problem, the team developed a new calibration method using a mobile pressure calibrator that can bring reference standards directly to the seafloor. Between 2018 and 2024, they carried out repeated calibrations at two DONET stations located in the focal regions of the expected Nankai and Tonankai earthquakes.

As a result, the team succeeded in detecting reliable subsidence rates of about 2.5 cm per year in the Nankai region and 1.5 cm per year in the Tonankai region. This result marks the first time that extremely small, long-term seafloor subsidence has been captured with high precision in a megathrust earthquake zone.

This study shows that even very small, long-term seafloor subsiding can be detected with high accuracy using seafloor pressure gauges. The team plans to expand the number of observatories where in-situ calibration is performed, which will help clarify long-term subsidence patterns across multiple sites. This will make it possible to better assess regional differences in plate coupling and stress buildup. By combining these observations with numerical models that use land-based GNSS and offshore GNSS-A data, researchers hope to gain deeper insights into how stress accumulates in the Nankai Trough.

The new calibration method developed in this study greatly enhances the potential of ocean-floor observation networks such as DONET. In addition to tsunami monitoring, it could make it possible to track long-term crustal movements and subtle phenomena like slow slip in near real time. Ultimately, this may lead to the development of more precise real-time maps of crustal deformation along plate boundaries—knowledge that is directly linked to understanding earthquake cycles and assessing tsunami hazards.

The above results were published in Geophysical Research Letters on September 19, 2025 (JST).