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 aqueous 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. ¿ 2000 American Geophysical Union