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