Fault-induced seismic anisotropy by hydration in subducting oceanic plates

In subduction zones, the anisotropic fast shear-wave component is generally observed to be orientated parallel to the strike of the trench. The interpretation of this shear-wave splitting above subduction zones has, however, been controversial and none of the inferred models appears to be explain the whole range of anisotropic patterns observed worldwide. Manuele Faccenda and colleagues now show that the amount and geometry of seismic anisotropy measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals formed along steeply dipping faults and from the larger scale vertical layering consisting of dry and hydrated crust-mantle sections whose spacing is several times smaller than teleseismic wavelengths. Faults orientations and estimated delay times are consistent with the observed shear-wave splitting patterns in most subduction zones.