A Matlab code to decipher the lowermost mantle (D″ layer) Anisotropy

Abstract:

The lowermost mantle region ~200-300 km above the Core Mantle Boundary, known as the D″ layer plays a crucial role in understanding the dynamic behaviour of the earth’s interior such as mantle convection, hot plume generation and fate of subducted slabs/graves. Though many global studies indicated the large- and small-scale heterogeneities in the D″ layer, it’s origin is still contentious and this region is one of the poorly understood regions of Earth’s interior. Seismic anisotropy inferred from the reflected shear phases (ScS) has emerged as an effective technique to establish not only the existence of D” layer but also provides additional constraints on the nature of D″ layer. However, there is no well-established/published codes/programmes to decipher the D″ layer anisotropy (г_D″: δt & Φ) using the core reflected phases. Thus, we developed a Matlab based code to explore the D″ layer anisotropy (S-ScS splitting algorithm) using eigenvalue method. In this method, the SKS- and S-phases are used to account for the receiver (г_R) and source (г_S) side upper mantle anisotropy. The Matlab code calculates the residual splitting in ScS-phase related to D” layer by correcting the г_R before the analysis and г_S during the analysis. In principle, the estimated splitting parameters from the source/receiver corrected ScS waveforms truly represent the lowermost mantle anisotropy. Further, we also included the sensitivity of the user provided г_R and г_S measurements on the final splitting parameters (г_D″). A case study indicates that the D’’ layer anisotropy parameters (г_D″) are highly dependent on the correctness of г_R and г_S, which are utilised to account for the receiver and source-side anisotropy and it is observed that a 5% error in г_R may reflect as ~2.5% error/change in г_D″.