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Abstract: We examine the upper mantle and transition zone beneath the western Pacific using multiple ScS reverberations. A low-velocity zone (LVZ) is found above the 410-km discontinuity oceanward of the subducting Honshu slab at an average depth of 356km, with a thickness that ranges from 50 to 75km assuming the LVZ continues to the 410-km discontinuity, which is locally elevated. The low-velocity region is evident in previous tomographic studies, and our results suggest that the anomaly is best explained by a layer of partial melt. The layer may be entrained from above by subduction or produced in situ by the combined effects of water and temperature. A self-consistent model that explains local P-wave velocities (Obayashi, M., Sugioka, H., Yoshimitsu, J., Fukao, Y., 2006. High temperature anomalies oceanward of subducting slabs at the 410-km discontinuity. Earth and Planetary Science Letters 243, 149–158) and our observations calls for a maximum temperature anomaly of ∼150°C and a resulting maximum olivine water content after melting of 0.100wt. %. [Copyright &y& Elsevier]

Abstract: Multiple ScS reverberations are used to search for mantle reflectors beneath the Tasman and Coral Seas with a hierarchical waveform-inversion/migration method. In addition to the major transition zone discontinuities, a low-velocity layer above the 410-km discontinuity is detected. The top of the low-velocity layer lies at an average depth of 352 km, indicating that the layer could be more than 70-km thick if it persists to the 410-km discontinuity, which occurs at an average depth of 420 km along paths containing the low-velocity layer. We attribute the low velocities to partial melt resulting from volatile-induced melting. The considerable thickness of the partial melt layer may require thin films of a hydrous melt with a zero-degree dihedral angle surrounding grains or the combined effect on melting of the addition of both water and carbon to the deep upper mantle via subduction. Although the depths of the transition zone discontinuities do not indicate that the transition zone itself is rich in water, the impedance contrasts do contain a subtle signature that could be related to transition zone water, namely a decrease in the impedance contrast across the 410-km discontinuity and a relatively strong 520-km discontinuity. [Copyright &y& Elsevier]