Ocean bottom pressure-gauge (OBP) records play an important role in seafloor geodesy, but oceanographic fluctuations in OBP data are a major source of noise in seafloor transient crustal deformation observations, including slow slip events (SSEs), so it is important to evaluate them properly. To extract the significant characteristics of the oceanographic fluctuations, we applied principal component analysis (PCA) to the 3-year Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) OBP time series for 40 stations during 2016–2019. PCA can separate several oceanographic signals based on the characteristics of their spatial distributions, although transient tectonic signals could not be clearly confirmed from the observed pressure records. The higher-order modes of the principal component reflected the oceanographic variation along the sea depth, and we interpreted that they were caused by the strength or weakness and meandering of ocean geostrophic currents, based on a comparison to the global ocean model ECCO2 by “Estimating the Circulation and Climate of the Ocean” (ECCO) consortium. In addition, to evaluate the ability of PCA to separate transient crustal deformation from oceanographic fluctuations, we conducted a synthetic test assuming an SSE by rectangular faults. The assumed synthetic tectonic signal can be separated from the oceanographic signals and included in the principal component independently depending on its amplitude. We proposed a transient event-detection method based on the spatial distribution variation of a specific principal component with or without a tectonic signal. This method can detect transient tectonic signals larger than moment-magnitude scale MW 5.9 from OBP records.  

The spatial variation of azimuthal S-wave phase velocity anisotropies caused by differential horizontal stress along the subducting plate at the Nankai Trough was analyzed to understand the stress state of the overhung block of the forearc region, off Kii Peninsula, Japan. We conducted controlled-source seismic surveys along the circumference of a 3 km diameter circle centered at each seismometer of a cabled earthquake observatory installed on the seafloor above the Kumano basin of the Nankai Trough subduction zone. We applied an anisotropy semblance method to estimate the orientation of fast and slow S-wave velocities of both shallow sediments and deep accretionary prism using the multi-azimuth seismic dataset acquired at each seismometer location. The estimated orientations of fast S-wave velocity are parallel to the convergent direction of the subducting place beneath the Kumano basin in the deeper accretionary prism while perpendicular to the convergent direction in the shallow sediments inside the Kumano basin. The orientations of these fast S-wave polarization show good agreement with those of horizontal maximum stress orientations estimated in situ borehole measurements in the observation area Then differential horizontal stress field in the Nankai Trough region was estimated from obtained S-wave anisotropy using a simple crack model. The azimuths of fast S-wave polarization and the derived differential stresses could be explained well by the tectonics of the Nankai Trough subduction zone. These results strongly suggested that the S-wave azimuthal anisotropy measurements could be used to monitor the subsurface stress field as a function of time.