New seismic evidence for the origin of arc and back-arc magmas

We used high-resolution seismic tomography to probe into the arc and back-arc magmatism and volcanism of the Japan subduction zone. Prominent zones with low-velocity, high Poisson's ratio, high-attenuation and strong seismic anisotropy are revealed in the crust and uppermost mantle beneath the active volcanoes, and they exist in the central portion of the mantle wedge under the volcanic front and back-arc region, roughly parallel with the subducting oceanic plate. The anomalous zones in the mantle wedge are connected with the subducting slab at a depth of 90-150 km, while they also exhibit along-arc variations. These seismological results indicate that the arc and back-arc magmas are caused by a combination of corner flow (convection) in the mantle wedge and fluids resulting from the dehydration process of the subducting oceanic plate.