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.