Seismic Anisotropy and Intraslab Hydrated Faults Beneath the NE Japan Forearc

The water cycle in subduction zones is closely related to the generation of large earthquakes and arc magmas. Outer-rise hydrated faults subducting with the oceanic lithosphere are suspected to be important for the water cycle. However, geophysical evidence for the intraslab hydrated faults beneath forearc is scarce. Here we present high-resolution P-wave anisotropic tomography of the Tohoku forearc derived from arrival-time data recorded at both onshore and offshore seismic stations. Trench-parallel intraslab fast velocity planes of anisotropy are revealed that intersect the slab upper surface at high angles (∼45°–90°), reflecting aligned hydrated faults in the slab. Ruptures of the hydrated faults may cause large intraslab earthquakes. The hydrated-fault associated water entering a large near-trench asperity in the megathrust zone could have triggered the great 2011 Tohoku-oki earthquake (Mw 9.0).