Melting and seismic structure beneath the northeast Japan arc

Transportation of H₂O associated with subduction of the Pacific plate beneath the northeast Japan arc is modeled to predict distribution of aqueous solution 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 the slab, then the acqueous fluid generated ascends to initiate melting, which explains the low velocity regions (˜6% reduction) observed beneath the backarc, rather than beneath the volcanic front. If equilibrium transport of H₂O occurs, initiation of melting beneath the backarc with deep subduction of H₂O is likely to be the case also for other subduction zones with slabs older than several tens of m.y., cold enough to stabilize serpentine to a great depth.