Crustal Anisotropy Variations Revealed by Local S-wave Splitting in the Source Region of 2023 Kahramanmaraş Earthquakes along the East Anatolian Fault Zone

The 2023 Kahramanmaraş earthquake doublet (Mw 7.8 and Mw 7.7) produced an extensive rupture along several segments of the East Anatolian Faults (EAF), and triggered intense aftershock activity in southeastern Türkiye. This seismic sequence therefore provides an exceptional dataset to investigate the crustal anisotropy in such a complex tectonic area. In this study, we evaluated the crustal anisotropy in the source region of these catastrophic earthquakes by conducting a detailed local S-wave splitting (SWS) analysis on a relocated earthquake catalogue. We have performed shear-wave splitting measurements over several thousand local-S waveforms recordings of 31 permanent broadband seismic stations operated by AFAD (Turkish Earthquake Data Center) that were extracted using precise relocation of aftershock activity. After a visual quality control procedure for each splitting analysis, a total of 486 high-quality measurements were obtained. The results reveal a highly heterogeneous anisotropic pattern, with fast direction oriented from N80°W to N79°E and mean fast direction for the whole dataset of N16°E, reflecting the lateral variations in the regional stress field along the EAF, the Sürgü-Çardak Fault (SÇF), and the Malatya Fault. A transition between stress-induced and structure-related anisotropy is clearly identified across different segments of the EAF. Stations in close proximity to the EAF exhibit a dominant structure-induced anisotropic signature, characterized by the strict alignment of FPDs parallel to the fault geometry. Overall, the obtained results provide a comprehensive perspective on how the upper crust responds to substantial stress release, thus offering critical insights into the mechanical behaviour of complex fault networks where the regional collisional stress regimes and strike-slip faulting systems converge.