Seismic structure and subduction dynamics of the western Japan arc

Recent findings on the seismic structure and subduction dynamics of the western Japan arc are reviewed. High-resolution seismic velocity and attenuation tomography has imaged the subducting Pacific and Philippine Sea (PHS) slabs clearly. Low-velocity (low-V) and high-attenuation (low-Q) anomalies are revealed in the crust and mantle wedge beneath the volcanic front and back-arc areas, which reflect arc magmas produced by corner flow in the mantle wedge and slab dehydration. Low-V and low-Q zones in the forearc mantle wedge reflect mantle serpentinization due to abundant fluids from the young and warm PHS slab. Low-V anomalies appear below the PHS slab, which reflect upwelling flow in the big mantle wedge above the Pacific slab and may heat the PHS slab from below, causing aseismic PHS slab at depths >100 km under Shikoku and Chugoku. Results of anisotropic tomography show that the Pacific and PHS slabs exhibit mainly trench-parallel fast-velocity directions (FVDs), which may reflect lattice-preferred orientation of anisotropic minerals as well as shape-preferred orientation of steep normal faults produced at the outer-rise area near the trench axis. Trench-normal FVDs are generally revealed in the back-arc mantle wedge and subslab asthenosphere, reflecting mantle flows entrained by the plate subductions. Large megathrust earthquakes nucleated in or around patches with high-V, high-Q and low Poisson's ratio along the slab interface, which may represent strongly coupled areas in the megathrust zone, suggesting that structural heterogeneities in the megathrust zone, such as the subducting seafloor topography and compositional variations, control the nucleation of megathrust earthquakes. The PHS slab has subducted aseismically down to ~450 km depth. The lithospheric age of the PHS slab ranges from 15 to 43 Myr in western Japan. The Beppu-Shimabara graben was produced by joint effects of northward extension of the opening Okinawa Trough, westward extension of the Median Tectonic Line, and hot and wet upwelling flow in the mantle wedge. The generation of large crustal earthquakes is affected by structural heterogeneities in the crust and upper mantle, in particular, arc magma and fluids. Future advances of seismic imaging mainly depend on the progress in seismic instrumentation.