Upper-Crustal Seismic Anisotropy and Its Geological Controls in Southeastern Korea from Local Earthquake Data

We investigated upper-crustal seismic anisotropy in the southeastern Korean Peninsula using local-earthquake shear-wave splitting (SWS) recorded by the Gyeongju Hi-density Broadband Seismic Network. A total of 4611 reliable SWS measurements were analyzed to characterize fast-polarization directions (ϕ) and delay times (δt) in a tectonically complex intraplate setting. Spatial averaging reveals a dominant northeast–southwest (NE–SW) fast orientation across much of the study area, with local rotations to north–south to north-northeast–south-southwest (NNE–SSW) around the Yangsan fault system and to east–west (E–W) near the Ulsan fault; east of the Ulsan fault, NE–SW and E–W orientations coexist. Delay-time tomography shows elevated δt values near the 2016 Gyeongju earthquake sequence and east of the Ulsan fault, where the high-anisotropy zone terminates along the Yeonil tectonic line. These patterns indicate that upper-crustal anisotropy is controlled by multiple sources. Localized high anisotropy near the Naenam fault suggests fault-related deformation associated with the Gyeongju sequence. East of the Ulsan fault, anisotropy likely reflects a combination of inherited Miocene basin-related structures and the present regional stress field. The widespread NE–SW component, therefore, is not attributed to a single origin but is likely to record domain-dependent contributions from inherited structures and, locally, shallow stress conditions distinct from the deeper east-northeast–west-southwest regional compression. These results provide a new baseline for understanding upper-crustal anisotropy in a diffuse intraplate setting.