Depth variations in seismic velocity in the subducting crust: Evidence for fluid-related embrittlement for intermediate-depth earthquakes

We investigated seismic wave velocity in the subducting crust of the Pacific slab beneath eastern Hokkaido, northern Japan. To detect depth-dependent properties of the seismic velocities in the crust, we analyzed guided waves that propagate in the crust and estimated P wave velocity (V_p) of 6.5–7.5 km/s and S wave velocity (V_s) of 3.6–4.2 km/s at depths of 50–100 km. The results show that the obtained V_p and V_s are 10–15% lower than those expected for the fully hydrated mid-ocean ridge basalt, suggesting the existence of aqueous fluids by ~1 vol % in the crust at this depth range. Our observations suggest that overpressurized fluids channeled in the subducting crust plays as a dominant factor for facilitating the genesis of crustal earthquakes at intermediate depths.