The Nankai Trough, just offshore of Japan, is one of the world’s most hazardous earthquake zones. To better understand how stress builds before large earthquakes, it is important to measure slow movements of the seafloor. Instruments in the DONET network were originally installed to detect tsunamis, but they also record tiny pressure changes that can reveal vertical crustal motion. A major difficulty, however, is that these sensors gradually drift, making it hard to detect true seafloor changes.

In this study, we applied a new calibration method using a portable pressure standard placed directly on the seafloor to correct sensor drift. Between 2018 and 2023, we carried out repeated calibrations at two DONET stations. After correcting the data, both sites showed clear long-term subsidence: about 2.5 cm per year at one site and 1.5 cm per year at the other. The difference may reflect how strongly the tectonic plates are locked beneath each site.

This study demonstrates that continuous DONET pressure data can be repurposed for geodetic monitoring. Such capability provides a new way to observe offshore crustal deformation and could contribute to future earthquake forecasting and the detection of slow slip events.