Fig. 1

Schematic view of the boundary region around the trench. The region extends from r₀ to r₁(r-direction), from θ₀ to θ₁ (θ-direction), and from φ₀ to φ₁ (φ-direction). The region is divided into two subregions (oceanic subregion and continental subregion) and a different velocity is imposed on each subregion ( for oceanic subregion and for continental subregion). A finite volume whose size is Δr (r -direction) × Δθ (θ-direction) × Δφ (φ-direction) and whose center is on the boundary (r_PB, θ_PB, φ_PB) is also shown.