Migration Velocity Analysis and Inverse-Ray Imaging of 3D Sedimentary Structures

Major horizons from stacked sections and velocity analysis of the time migrated sections along eighteen oblique profiles in the southernmost Ryukyu forearc region are integrated based on the inverse-ray method. Inverse-ray imaging starts from shooting three inverse rays at three non-collinear sources/receivers, propagating through structural layers and arriving at a planar reflector. All possible triangulations of the picked reflection times between neighboring profiles are applied for 3-D inverse-ray imaging of the Ryukyu forearc structures and in a layer-stripping approach. The results show that the top of the oceanic crust subducts deepest near the Nanao forearc basin and the maximum thickness of the accretionary prism south of the Nanao forearc basin is greater than 9 km whereas that south of the East Nanao forearc basin is only greater than 5 km. These may imply the most active subduction in the southernmost Ryukyu forearc region is probably along the Nanao forearc basin which is also supported by the active earthquakes below the Nanao forearc basin.