Seismological structure of subduction zones and its implications for arc magmatism and dynamics

In this paper I review recent seismological findings on the structure, magmatism, and dynamics of subduction zones. High-resolution seismic tomography has revealed prominent low-velocity (low-V) and high-attenuation (low-Q) zones that exist in the crust and uppermost mantle just beneath active arc volcanoes and extend to 400 km depth in the mantle wedge. The low-V/low-Q zones are located in the central portion of the mantle wedge and lie 30-50 km above the subducted oceanic slab. The mantle wedge low-V/low-Q zones also exhibit strong seismic anisotropy. These results suggest that arc magmatic systems are not limited to the near-surface areas, but are related to the deep processes, such as the convective circulation in the mantle wedge and dehydration reactions in the subducted slab. The low-V/low-Q zones beneath the arc and back-arc are separated at shallow levels but merge at depth > 100 km, indicating that the slab components of the arc and back-arc magmas occur through mixing at these depths. These low-V/low-Q bodies form the deep roots and sources of the arc magmatism and volcanism. Large crustal earthquakes in Japan are found to occur around low-V zones that may represent weak sections of the seismogenic crust. The crustal weakening is thought to be closely related to the subduction of the Pacific and Philippine Sea plates in this region. Along the volcanic front and in back-arc areas, the crustal weakening may be caused by the active volcanism and the presence of magma chambers. In the forearc areas, fluids were detected in the earthquake source areas, which may have contributed to the rupture nucleation and may be related to the dehydration of the subducted slab. These results suggest that large crustal earthquakes may not strike anywhere, but only anomalous areas which may be detected with geophysical methods. Suggestions are also provided for future directions of seismological research of subduction zones.