A semiquantitative petrogenetic grid in the system MgO-SiO2-H2O up to 30 GPa and 1600¿C was constructed using Schreinemakers analysis on previous experimental data. The grid includes stability relations of hydrous wadsleyite, hydrous ringwoodite, and dense hydrous magnesium silicates (DHMSs): phase A, phase D, phase E, and superhydrous phase B. A sequence of chemical reactions among these hydrous phases was clarified. In the mantle transition zone (410--660-km depth), hydrous wadsleyite and hydrous ringwoodite are stable even at a standard mantle temperature of 1600¿C, whereas in the other depths, nominal hydrous phases including DHMSs are stable below 1400¿C. Newly found phase transitions in hydrous wadsleyite and hydrous ringwoodite are multiphase and multireaction because their compositions are on neither MgO-SiO2 nor Mg2SiO4-H2O tie lines, different from single transition in the dry Mg2SiO4 system. The grid indicates that water in the subducting slab peridotite would be transported to the bottom of the upper mantle by several DHMSs and hydrous polymorphs of olivine, via solid-solid reactions after antigorite decomposition. Finally, DHMSs would dehydrate to liberate free water at the upper mantle-lower mantle boundary layer or deeper level. We also examined a relation between the predicted depth distributions of dehydration reactions in slab peridotite along several possible pressure-temperature paths and the mode of seismic frequency in each subduction zone. The result suggests a possible origin of intermediate to deep seismicity by the dehydration of hydrous phases in the subducting slab.