Three‐dimensional inverse‐ray imaging from MVA and triangulation of oblique profiles

Imaging of sub-surfaces based on 3-D inverse rays shooting from three non-collinear sources/receivers is presented. Two take-off angles of a 3-D inverse ray, one along the seismic profile and another orthogonal to the profile, are determined when the inverse rays from three non-collinear sources/receivers possess the common direction of shooting rays. In this paper, migration velocity analysis (MVA) of 2D MCS data and triangulation of picked reflection times along oblique seismic profiles are conducted. Application of 3-D inverse rays to a forearc basin in the southernmost Ryukyu subduction zone show that the thickness of sediment near the western end of the Ryukyu forearc region varies strongly with a maximum thickness greater than 6 km. Furthermore, a major fault with a subsidence of about 1 km is found at the SE portion of the basin. The slab buckling and the basement rise due to the oblique subduction of the Philippine Sea Plate may have generated this fault and induce strong, shallow earthquakes.