Global Presence and Absence of Ultra‐Low Velocity Zones as Seen by Sdiff Postcursors

Abstract The core‐mantle boundary (CMB) is a region of significant lateral heterogeneity. Two antipodal large low‐velocity provinces (LLVPs) dominate the lower mantle, while smaller but more extreme ultralow velocity zones (ULVZs) are scattered atop the CMB 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 S diff , an S‐wave that diffracts along the CMB. We hand‐pick over 2 million traces and detect nearly 100,000 postcursors to S diff and over 900,000 robust null observations, achieving 99.8% coverage of the CMB. Nearly all regions of S diff postcursor presence can be explained by six “S diff ‐ULVZs”, which are found to preferentially lie both underneath hotspots and near LLVP edges. There could exist multiple types of ULVZs, and those large‐scale ULVZs that S diff are sensitive to are linked to some hotspots and the edges of LLVPs, while others may not be. Notably, we find that the large majority of the CMB lacks evidence for the presence of large‐scale ULVZs.