Double Wadati-Benioff zones are a global feature of subduction zones in the 150-200 km depth range, consisting in two parallel planes of seismicity separated by 15-30 km. The upper plane corresponds to seismicity in the dehydrating oceanic crust. The lower plane seismicity has been attributed to dehydration embrittlement associated with the destabilisation of the hydrous mineral serpentine or to periodic shear instabilities in ductile mantle shear zones. Dehydration embrittlement implies hydration of the subducted slab while periodic instabilities can occur in an anhydrous mantle. Low seismic velocities are associated with the lower seismicity plane. I will argue that they can be explained by seismic properties of anhydrous rocks, suggesting a periodic shear-instability mechanism may be active in an essentially anhydrous mantle. A dry lithospheric mantle accounts for the high velocities of the mantle between oceanic crust and lower seismicity plane. Implications for the water budget of subduction zones wil be discussed.