Using a model of elastic-perfectly plastic plate, I demonstrate that plate unbending combined with dehydration embrittlement can account for the morphology and stresses of double or triple seismic zones (DSZ's or TSZ's) observed in many subducting slabs. Densifying metamorphic reactions may induce tensile stresses as well as high pore fluid pressures in the slab crust and thus generate the upper seismicity band of the TSZ. Limited dehydration may moderately elevate the fluid pressure in the slab mantle, but unbending usually dominates mantle stresses and results in another two seismicity bands. DSZ's are underdeveloped TSZ's.