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Abstract: The Philippine Sea plate is subducting under the Eurasian plate beneath the Chugoku-Shikoku region, southwestern Japan. We have constructed depth contours for the continental and oceanic Mohos derived from the velocity structure based on receiver function inversion. Receiver functions were calculated using teleseismic waveforms recorded by the high-density seismograph network in southwestern Japan. In order to determine crustal velocity structure, we first improved the linearized time-domain receiver function inversion method. The continental Moho is relatively shallow (∼30km) at the coastline of the Sea of Japan and at the Seto Inland Sea, and becomes deeper–greater than 40km–around 35°N and 133.8°E. Near the Seto Inland Sea, a low-velocity layer of thickness 10km lies under the continental Moho. This low-velocity layer corresponds to the subducting oceanic crust of the Philippine Sea plate. The oceanic Moho continues to descend from south to northwest and exhibits complicated ridge and valley features. The oceanic Moho runs around 25km beneath the Pacific coast and 45km beneath the Seto Inland Sea, and it extends to at least to 34.5°N. The depth variation of the Moho discontinuities is in good qualitative agreement with the concept of isostasy. From the configurations of both the continental and oceanic Mohos, we demonstrate that the continental lower crust and the subducting oceanic crust overlap beneath the southern and central part of Shikoku and that a mantle wedge may exist beneath the western and eastern part of Shikoku. The southern edge of the overlapping region coincides with the downdip limit of the slip area of a megathrust earthquake. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]