S-wave velocities and anisotropy in sediment entering the Nankai subduction zone, offshore Japan

P and mode-converted S waves from airgun shots recorded along a line of ocean bottom seismometers were used to construct a 2-D velocity section across the Nankai Trough and the toe of the accretionary complex, offshore southeast Japan. The site was chosen because of plans to drill into the seismogenic zone of the main subduction thrust, which has a history of M similar to 8 earthquakes. The section is constrained by well-log data from three Ocean Drilling Program boreholes. S-wave velocities of the sediments over the entire section, down to > 1.1 km below seafloor, range from 107 to 915 m s-1. Shear wave splitting indicates anisotropy in all the sediments except possibly the distal trench fill. If interpreted conventionally as being due to vertical stress-aligned cracks, the anisotropy in both the trench and the toe of the accretionary wedge is consistent with crack orientation NW-SE, parallel to the direction of maximum tectonic compression. Crack density is similar to 0.01-0.03, consistent with closeness to fracture criticality.