Targeted 3-D prestack depth imaging at Legs 190-196 ODP drill sites (Nankai Trough, Japan)

Target-oriented 3-D prestack depth migration (PSDM), tied to known depths of key reflections at three ODP drill sites in the Nankai Trough, provides an improved representation of the subsurface depths and P-wave velocities around the drill holes. The resulting velocity volume is representative of the "in situ" velocities and allows us to extend the 1-D logging and core velocity data from the drill sites to the entire seismic transect. The PSDM velocity gradients closely match those at the drill sites, although the absolute values of the velocity curves are somewhat different. The average PSDM vertical velocity gradient above the décollement decreases from 0.99 s^-1 seaward of the trench axis to 0.87 s^-1 landward in the proto-thrust and frontal thrust zone, with a velocity inversion of about 50-150 m/s at the décollement. Below the décollement, the vertical gradient in the underthrusting section decreases from 0.97 s^-1 in the basin to 0.72 s^-1 beneath the frontal thrust. The lateral velocity gradient in the underthrusting section between the basin and the deformation front is 0.02 s^-1, corresponding to ∼10% thinning in the direction of subduction.