Melting and seismic structure beneath the northeast Japan arc

Transportation of H2O associated with subduction of the Pacific plate benea th the northeast Japan are is modeled to predict distribution of aqueous so lution and melt, and the consequent P-wave velocity structure. The observed velocity structure coincides well with the model for equilibrium transport of H2O: most of H2O subducted is brought down to 150-200 km depth along th e slab, then the aqueous fluid generated ascends to initiate melting, which explains the low velocity regions (similar to 6% reduction) observed benea th the backarc, rather than beneath the volcanic front. If equilibrium tran sport of H2O occurs, initiation of melting beneath the backarc with deep su bduction of H2O is likely to be the case also for other subduction zones wi th slabs older than several tens of m.y., cold enough to stabilize serpenti ne to a great depth.