Synthetic Seismic Waveform Analysis for Constraining Anisotropy in the D'' Layer Using the PcSdiff Phase

The D'' layer, at the base of Earth's mantle above the core-mantle boundary (CMB), is a region of profound interest due to its complex seismic characteristics and structural heterogeneity. Varying from 100 to 400 km in thickness, it features anomalous seismic velocity gradients, large-scale lateral variations, and ultra-low velocity zones, hinting at dynamic interactions with the core and potential origins for mantle plumes and subducting slabs. The PcSdiff waves, extending beyond the core-reflected PcS wave's ray theoretical limit (~63°), feature a P-wave diffracting along the CMB before being converted into an S wave. The observation of PcSdiff waves offers a novel method for examining the lowermost mantle, particularly the D'' layer, as they enable enhanced resolution in this complex region not readily available from nominal diffracted waves such as P/S diffracted waves. Their unique path can be highly sensitive to the anisotropic properties of the lowermost mantle, allowing for a detailed analysis of both isotropic and anisotropic structures, which help elucidate the dynamics and composition of this enigmatic region. This study focuses on an in-depth exploration of the D'' layer's anisotropic properties, and performs seismic forward modeling against the novel PcSdiff phase. By simulating seismic wave propagation through this region, we aim to provide a novel methodological approach to decode the intricate seismic interactions within the D'' layer. In the first step, we use an extensive synthetic waveform dataset to understand the responses of the PcSdiff phase to the geometry of the D'' layer and its anisotropic features. This process is pivotal in isolating and understanding the PcSdiff phase's nuanced responses to radial and lateral velocity variations due to anisotropy within the D'' layer. We aim to dissect these responses in systematic detail, looking for subtle signatures and patterns that might be indicative of specific anisotropic features. Then, our approach integrates seismic data from the USArray with synthetics produced for 3D models of the internal structure. In pursuing this detailed characterization, our objective is to provide a more nuanced and comprehensive understanding of how the novel PcSdiff wave can be used to probe the anisotropic properties of the D'' layer.