A Compositional Component to the Samoa Ultralow‐Velocity Zone Revealed Through 2‐ and 3‐D Waveform Modeling of SKS and SKKS Differential Travel‐Times and Amplitudes

Abstract

Thin regions sitting on top of the core-mantle boundary (CMB) with P-and S-wave velocity reductions as large as 25%-45% respectively, known as ultralow-velocity zones (ULVZs) have been investigated using seismic techniques for nearly three decades. Evidence continues to accumulate that some of the ULVZs discovered may be linked to deep seated roots of whole mantle plumes that give rise to hot spot volcanism. Seismic waveform modeling has inferred large-scale ULVZs beneath hot spots such as Hawaii (Cottaar &

Quantification of Small-Scale Heterogeneity at the Core–Mantle Boundary Using Sample Entropy of SKS and SPdKS Synthetic Waveforms

“…However, the MSE technique may also be able to pick up on the more subtle SE changes across all distances. Using the data set collected in [26,58], as much as 85% of the CMB by surface area could potentially be scrutinized, whereas 57% of the CMB was covered in Thorne et al [59] using the more limited epicentral distance range. Thus, this method could be utilized to explore a much larger range of the CMB than has been previously explored, one that is nearing global coverage.…”

“…In the previous section we showed how measurements of sample entropy varies for 1-D ULVZ and UHVZ models. However, ULVZs are not observed globally [44,89,90] but have finite dimensions [26,87,91]. In order to assess the effect of finite ULVZ/UHVZ size on SE measurements, we computed synthetic seismograms using the 2.5-D axi-symmetric finite difference (PSVaxi) code [51,92,93].…”

“…We evaluate SE in a large suite of 2.5-D ULVZ box-car shaped models that have been computed in part for a series of previous studies [26,51,91] but significantly expanded upon here. The model space is computed for ULVZs with the following characteristics: (1) We allowed δVS decreases of 10%, 20%, 30%, and 40%.…”

“…Although an exact definition of what constitutes a ULVZ is currently lacking, these features are commonly described as being thin (typically less than 40 km) with strong decreases in S-(up to 50%) and P-(up to 25%) wave velocities, with an associated increase in density of up to 30% [21]. Size constraints on ULVZs are typically lacking, but they may span a wide variety of length scales from a few 10s of km [22][23][24] up to several hundred km [25][26][27]. Although ULVZ structures are small, the study of their physical origin is likely essential for understanding the chemical and dynamical processes in the lowermost mantle, and their link to large-scale mantle structures [28,29].…”

“…Our synthetic experiments suggest that the SE increases with the strength and size of ULVZ structures in a systematic manner that could be used to detect ULVZs across a much wider range of epicentral distance than have been used in previous studies. In a follow-up paper, we then apply this approach to a global data-set of SKS/SPdKS waveforms [26,58,59].…”