Global presence and absence of ultra-low velocity zones as seen by Sdiff postcursors

James Robin Atkins, Carl Martin, Sanne Cottaar 

The core-mantle boundary is a region of significant lateral heterogeneity. Two antipodal large low-velocity provinces (LLVPs) dominate the lower mantle, while smaller but more extreme ultra-low velocity zones (ULVZs) pepper the lowermost mantle in a variety of morphologies. These ULVZs have been linked to both the edges of LLVPs and to plume-related hotspots, the latter of which may plausibly be fed or anchored by ULVZs.

In this study, we produce a global map of ULVZs using the phase Sdiff, picking over 2 million traces and detecting nearly 100,000 postcursors to Sdiff and over 900,000 null observations. Nearly all regions of ULVZ presence can be explained by six ‘Sdiff-ULVZs’, which are found to preferentially lie both underneath hotspots and near LLVP edges. It is possible that there exist multiple types of ULVZs, and those large-scale ULVZs that Sdiff are sensitive to appear to be linked to some hotspots and the edges of LLVPs, while others may not be. Notably, we find that the large majority of the core-mantle boundary lacks evidence for the presence of large-scale ULVZs