Anisotropy of the upper mantle beneath the southern Korean Peninsula from splitting of core refracted shear waves

Abstract:

Dynamics in the upper mantle can be inferred by measuring shear wave splitting, or a fast axis of olivine crystal and delay time between polarized waves. In this work, various core refracted shear wave phases (e.g., SKS, SKiKS, SKKS, and PKS) recorded in all available broadband stations in the southern Korean Peninsula (SKP) are used. We retrieve teleseismic data with the distance range of 80º-145º and magnitude greater than 5.5 from 1999 to 2018. Each phase is visually identified and splitting is measured using cross correlation, transverse component minimization, and eigenvalue method for cross validation. Results from these methods are consistent and show similar complex local heterogeneities. In particular, an interesting back-azimuthal pattern of the fast axis is observed in data from stations located on the western part of the SKP that the fast direction varies significantly for rays from the south (~150º) while the consistent north-south polarization at other incoming directions. This pattern is generally not observed in stations on the eastern part of SKP with north-south polarization. In all the measurements, delay times are generally small (~ 1.0 s). It could indicate current flow directions at the upper mantle affected by the preexisting thick lithosphere at the southwestern part of SKP with possible fossil anisotropy in the thick lithosphere.