Stress Field Estimation From S‐Wave Anisotropy Observed in Multi‐Azimuth Seismic Survey With Cabled Seafloor Seismometers Above the Nankai Trough Megathrust Zone, Japan.

The spatial variation of azimuthal S‐wave phase velocity anisotropies caused by differential horizontal stress along 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 radius circle centered at each seismometer of a cabled seafloor observatory installed in the Nankai subduction zone. We applied an anisotropy semblance method to estimate the orientation of fast and slow S‐wave velocities of 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. These fast S‐wave polarization orientations in shallow sediments show good agreement with horizontal maximum stress orientations estimated in situ borehole measurements in the observation area. We estimated the differential horizontal stress field in the Nankai Trough region from obtained S‐wave anisotropy using a simple crack model. Using this method, we first suggest the convergent‐parallel horizontal maximum compression in the deeper part of the accretionary prism above the locked portion. Assuming obtained S‐wave anisotropy is attributed to the tectonic loading by the subducting plate, these results suggest that the S‐wave azimuthal anisotropy measurements can be used to monitor the subsurface stress field as a function of time. Plain Language Summary: In the Nankai Trough, Japan, giant megathrust earthquakes repeatedly occurred. Spatio‐temporal distribution of stress state in the subduction zone is a key parameter to understand the earthquake preparation and generation cycle. In the present study, we estimate the stress field along the Nankai Trough subduction zone from the S‐wave anisotropy analysis using active seismic data acquired by cabled seismic observatories deployed in the Nankai Trough. S‐wave anisotropy can be a proxy of the stress state if stress‐aligned microcracks are vertically distributed in the anisotropic medium. We estimate differential horizontal stress along the subduction zone using a new robust S‐wave analysis method and a simple stress‐induced microcrack model. The obtained orientations and amplitudes of the differential horizontal stress have good agreement with the existing results obtained by in‐situ borehole observations and other seismic surveys in the shallow sediment layer. In the deeper part, the accretionary prims layer above the plate interface, our results in the outer arc of the Kumano forearc basin indicate the plate convergence direction as orientation of maximum horizontal stress. The results are new insights, and can reflect the present stress state accumulating interseismic strain along the seismogenic fault. Key Points: S‐wave anisotropy is obtained from multi‐azimuth seismic dataset observed by cabled seismometers deployed in the Nankai Trough, JapanDifferential horizontal stress field in the Nankai Trough region is estimated from obtained S‐wave anisotropy using a simple crack modelThe estimated horizontal stress field can reflect the present stress state accumulating interseismic strain along the seismogenic fault [ABSTRACT FROM AUTHOR]