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

Abstract

Qualitative and quantitative analysis of seismic waveforms sensitive to the core–mantle boundary (CMB) region reveal the presence of ultralow-velocity zones (ULVZs) that have a strong decrease in compressional (P) and shear (S) wave velocity, and an increase in density within thin structures. However, understanding their physical origin and relation to the other large-scale structures in the lowermost mantle are limited due to an incomplete mapping of ULVZs at the CMB. The SKS and SPdKS seismic waveforms is routinely used to infer ULVZ presence, but has thus far only been used in a limited epicentral distance range. As the SKS/SPdKS wavefield interacts with a ULVZ it generates additional seismic arrivals, thus increasing the complexity of the recorded wavefield. Here, we explore utilization of the multi-scale sample entropy method to search for ULVZ structures. We investigate the feasibility of this approach through analysis of synthetic seismograms computed for PREM, 1-, 2.5-, and 3-D ULVZs as well as heterogeneous structures with a strong increase in velocity in the lowermost mantle in 1- and 2.5-D. We find that the sample entropy technique may be useful across a wide range of epicentral distances from 100° to 130°. Such an analysis, when applied to real waveforms, could provide coverage of roughly 85% by surface area of the CMB.

Improved Characterization of Ultralow‐Velocity Zones Through Advances in Bayesian Inversion of ScP Waveforms

“…This is similar to the ULVZ strength parameter defined in Pachhai, Thorne, et al. (2022) and Rondenay et al. (2010) and provides another way of assessing whether or not a ULVZ is required to fit these data.…”

“…We generated random noise by convolving a Gaussian auto‐correlation function (ACF) with a series of random numbers similar to that in Pachhai, Thorne, et al. (2022). Here we define the Gaussian ACF as: $$$\mathrm{A}\mathrm{C}\mathrm{F}(t)=\sigma {e}^{-{t}^{2}/{T}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{n}\mathrm{e}\mathrm{r}}^{2}},$$$ where T corner is the corner period, and σ is the root‐mean‐square (RMS) of the noise.…”

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

“…It has also been suggested that ultra‐high velocity zones (UHVZs) may exist along the CMB as well (e.g., Yu et al., 2018; Garnero et al., 2020; Pachhai et al., 2022b). The PdP reflection from this type of structure could potentially look similar to the ULVZ DAS results since the difference in polarity is removed when the PcP waveforms are enveloped (Section 2.5).…”

“…Unlike ULVZs, the seismic characteristics of UHVZs have not been extensively investigated. Studies that have examined UHVZs have used S‐wave phases (e.g., Yu et al., 2018; Garnero et al., 2020; Pachhai et al., 2022b) and have estimated that the corresponding V S is ∼10% higher than PREM. That said, these investigations provide no constraints on the potential V P increase within UHVZs.…”

“…That said, these investigations provide no constraints on the potential V P increase within UHVZs. Given this, both δV S and δV P were varied from +5% to +10% in +5% increments in our UHVZ models, but similar to Pachhai et al (2022b), we did not let δV P exceed δV S . UHVZ density variations and thicknesses are largely unconstrained; therefore, we set δρ = 0%, and we used thicknesses of 5, 10, and 20 km.…”