Results of seismic structure modeling along profiles NS1 and NS2. (a, b) Starting models for first arrival tomography. The contour interval is 0.5 km/s, and the inverted triangles represent OBSs. The velocity node spacings for the inversion were 1 km (vertical) and 5 km (horizontal). (c, d) Final P-wave velocity structure models. (e, f) Results of the checkerboard resolution tests. The area with no ray coverage is shaded gray. (g, h) Reflector distributions imaged by the traveltime mapping method.