Seafloor topography of the forearc off northeastern Japan. (A) The red circled area represents the mid-slope terrace. (B) Cross section of the forearc wedge in the Japan Trench (modified from Kimura et al. ). The slope surface is separated into four slopes by the tilt angle: the deep-sea terrace, the upper slope, the middle slope, and the lower slope. The area surrounded by the blue dotted line is the frontal wedge. (C) The profile of MY102 drawn by seismic refraction data. Within the outer wedge beneath the middle slope, a clear reflector exists on the seismic profile that is due to the velocity contrast between the trenchward side and landward side of the wedge. This reflector is named the back-stop interface (Tsuru et al. ). (D) Enlarged figure of the frontal wedge whose geometry is analyzed in this research.