Complex Upper Mantle Dynamics beneath the Southern Korean Peninsula from Azimuthal Seismic Anisotropy Analysis

Seismic anisotropy, which mainly occurs by anisotropic minerals alignment (e.g., Olivine), is an indicator of dynamical processes of the upper mantle. The southern Korean Peninsula (SKP), located at the eastern margin of the Eurasian plate, has undergone complex tectonic processes (e.g., amalgamation of continental blocks, subduction, and back-arc extension), showing lithospheric thickness variations (> 50 km) across different geological units. However, the role of lithospheric structures on local mantle flows, and the interaction between the lithosphere and asthenospheric upper mantle beneath the SKP are not well understood. We constrained upper mantle seismic anisotropy beneath the SKP based on the shear wave splitting analysis (SWS) using shear wave core phases (PKS, SKS, SKKS, and SKiKS) recorded by 53 local broadband seismic stations. The SWS resulted in a seismic anisotropy dataset consisting of a total of 1058 source-receiver pairs. The constrained seismic anisotropy was compared with azimuthal variations of teleseismic P-wave residuals measured by 254 local seismic stations (broadband, short-period, accelerometer sensors), which recorded relative P-wave arrival times of earthquakes. We modeled an anisotropic layer using SWS parameters with a method suggested in Eakin et al. (2019). Based on our results, we separated the SKP into the southwestern and eastern parts in terms of the direction of modeled anisotropic layer. The generally NW-SE directions with some complexities of an anisotropic layer were modeled in the southwestern SKP with a relatively thick lithosphere (< 130 km). In contrast, the relatively simple N-S directions of an anisotropic layer were modeled in the eastern SKP with a relatively thin lithosphere (< 80 km). We performed 3-D mantle flow modeling using a known lithospheric structure with temperature-dependent rheology to interpret this anisotropic pattern regarding the upper mantle dynamics. Based on our results, we suggest that the NW-SE direction of dominant regional mantle flow deflects around the locally thick lithosphere in the southwestern SKP, potentially exhibiting complex 2-D or 3-D flows around this local structure.