Ultra-Low Velocity Zones Beneath the Southern Hemisphere Imaged With Double-Array Stacking of PcP Waveforms

Ultra-low velocity zones (ULVZs) are anomalous structures, generally associated with decreased seismic velocity and sometimes an increase in density, that have been detected in some locations atop the Earth's core-mantle boundary (CMB). A wide range of ULVZ characteristics have been reported by previous studies, leading to many questions regarding their origins. The lowermost mantle beneath Antarctica and surrounding areas is not located near currently active regions of mantle upwelling or downwelling, making it a unique environment in which to study the sources of ULVZs; however, seismic sampling of this portion of the CMB has been sparse. Here, we examine core-reflected PcP waveforms recorded by seismic stations across Antarctica using a double-array stacking technique to further elucidate ULVZ structure beneath the southern hemisphere. Our results show widespread, variable ULVZs, some of which can be robustly modeled with 1-D synthetics; however, others are more complex, which may reflect 2-D or 3-D ULVZ structure and/or ULVZs with internal velocity variability. Our findings are consistent with the concept that ULVZs can be largely explained by variable accumulations of subducted oceanic crust along the CMB. Partial melting of subducted crust and other, hydrous subducted materials may also contribute to ULVZ variability. Earth's core-mantle boundary (CMB), the interface between the solid silicate mantle and the molten iron-rich outer core, is associated with a range of anomalous structures, including ultra-low velocity zones (ULVZs). While generally associated with reduced seismic wave velocities and sometimes increased density, prior studies have reported a wide range of ULVZ characteristics, leading to many questions regarding their origins. The lowermost mantle beneath the southern hemisphere provides a unique environment to study ULVZs because it is located away from regions of large-scale mantle upwelling and downwelling. Our study uses core reflected P-waves (PcP) recorded by seismic stations in Antarctica to investigate this portion of the CMB for ULVZ presence. We find widespread evidence for variable ULVZ structure. Some of the imaged ULVZs can be modeled with a single layer, but others are more complex. We suggest that the ULVZs beneath the southern hemisphere are predominantly associated with subducted oceanic crust that has variable accumulations along the CMB. In some regions, hydrated subducted materials may also experience partial melting, which may contribute to the complicated ULVZ structures imaged in some locations. Core-reflected P-waves are used to investigate ultra-low velocity zones in the lowermost mantle beneath the southern hemisphere Results show widespread evidence for variable ultra-low velocity zones, some of which may indicate layered or gradational structure Variable accumulations of subducted oceanic crust with localized partial melting can explain these anomalous structures