Water flow to the mantle transition zone inferred from a receiver function image of the Pacific slab

Abstract

Variations in seismic velocity near the subducting slab provide constraints on the thermal structure and the distribution of hydrous minerals and water in a subduction zone. We investigated the seismological structure in the upper mantle beneath Japan using both radial and transverse receiver functions (RFs). Radial RF can image horizontal layers of velocity contrast while transverse RF can image dipping layers. The combination allows us to simultaneously image both horizontal and dipping velocity discontinuities. We investigated the records of 45 teleseismic events observed with tiltmeters at 678 Hi-net stations. The frequency band we used is from 0.02 to 0.16 Hz. The resultant RF image shows an elevated 410 km discontinuity and, more importantly, the top surface of the Pacific slab down to below the 410 km discontinuity. With forward modeling, we determined that the mantle wedge is about 8% slower in shear-wave speed than the subducting slab at depths deeper than 200 km. The seismic velocity contrast is presumably caused by a sequence of hydrous minerals at the base of the mantle wedge which receives water released by dehydration reactions in the oceanic crust. We employed numerical simulations to determine the distribution of water in and around the subducting slab. The result suggests that hydrous minerals are continuously stable above the subducting slab in relatively cool conditions, and carry water to the mantle transition zone.

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