Constraints on localized core-mantle boundary structure from multichannel, broadbandSKScoda analysis

摘要: [1] In recent years, a wide range of geophysical results have offered evidence that Earth's lowermost mantle is characterized by strong lateral variations in material properties. Among the structures particular interest are intermittent ultralow-velocity zones (ULVZs), located directly above core-mantle boundary (CMB), which were originally inferred from distortion teleseismic SPdKS phases. ULVZs been modeled as layers with sharp boundaries and seismic velocity reductions ≥10% interpreted regions partial melt. In this study, we further constrain local ULVZ structure beneath North America signal processing waveform modeling SKS coda recorded at broadband arrays. Secondary phases effectively isolated eigenimage processing. Residual (i.e., SKS-less) data sections various western Pacific events display clear arrivals, followed secondary phase whose timing slowness consistent CMB origins. One-dimensional these reflectivity generalized ray synthetics favors an asymmetric model, present only one intercepts. The preferred P S velocities 18% 50%, respectively, diffuse upper boundary. These characteristics production gravitational sinking dense (e.g., iron rich) melt CMB. We postulate gradational may mark transition domain between upwelling, where more uniform exist, downwelling, either nonexistent or imperceptibly thin.