What geophysical signatures indicate hydrous mantle regions

Hydrous and fluid-rich mantle wedges commonly exhibit

(1) reduced seismic velocities (Vp and Vs) and elevated seismic attenuation (Q⁻¹), reflecting partial melt and/or fluids;

(2) high electrical conductivity in magnetotelluric (MT) surveys due to interconnected aqueous fluids, saline components, or small melt fractions;

and (3) spatial correlation of these anomalies with the fore-arc to arc region above the slab’s dehydration front.

Joint interpretation of seismic and MT data improves discrimination of fluids versus temperature effects, because low velocities alone can be non-unique. Double seismic zones within the slab and ETS depths often track major dehydration reactions, further supporting fluid presence.

Together, low Vp/Vs and high conductivity volumes in the mantle wedge are strong indicators of hydrous conditions and fluid pathways in subduction systems.