Seismic velocity structures have been estimated using ambient noise records observed by distributed acoustic sensing (DAS) technology. Previous studies have obtained S-wave velocity (Vs) structures along submarine cables using surface waves; however P-wave velocity (Vp) structure have seldom been constructed because ambient noise primarily contains surface waves. Here, we show P and Rayleigh wave extractions from ambient seafloor noise observed by DAS, and also estimate the Vp and Vs structures along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. To extract the P waves, we calculated the cross-correlation functions (CCFs) of the ambient noise and applied a frequency-wavenumber filter to the CCFs to remove the effects of slower surface waves. The results indicate that similar features can be obtained in the Vp and Vs profiles, and that the P wave extractions can be performed on days with poor weather conditions.

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.

Seismic wave extractions have been performed using ambient noise records observed by distributed acoustic sensing (DAS) technology. Extractions of microseisms can be investigated at a local scale using such DAS records observed in the ocean. Here, we show P and Scholte wave extractions from ambient seafloor noise observed by DAS along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. The P waves can be observed at a frequency band of 0.1–0.3 Hz and up to a distance of 6–7 km. The distance in which the P waves can be observed is controlled by the P incident angle and the DAS sensitivity to the observable apparent velocity. The effective extractions of P and Scholte waves are performed in correspondence to large significant wave heights of the sea surface, which indicate that these waves are originated from fluid distturbances at the sea surface.