Pattern of Crustal Anisotropy Along the East Anatolian Fault, the Sürgü-Çardak Fault and Surroundings Associated with the 2023 Kahramanmaraş Earthquakes, SE Türkiye

The devastating earthquakes of Mw 7.8 and Mw 7.7 with left-lateral strike-slip mechanisms occurred on the 6^th February 2023 along the East Anatolian Fault (EAF) and the Sürgü-Çardak Fault (SÇF) in southeastern Türkiye. Observed intense aftershock activity triggered by the 2023 doublet provides a valuable opportunity to study the upper crustal anisotropy along the EAF, the SÇF, and surrounding rock volumes. In this study, we measured the shear-wave splitting parameters of several local earthquakes that occurred between 1^st July 2022 and 31^st August 2023 - approximately seven months before and after the 2023 mainshocks. To improve the accuracy of the event locations, we initially relocated 10.823 earthquakes (M > 2) using the HypoDD code, building up a catalogue of high-precision earthquake locations. Subsequently, the splitting parameters, including the fast polarization direction (FPD) and the delay time (DT), were estimated for ~1.615 events recorded at 34 broadband seismic stations operated by AFAD (Turkish National Seismic Network). Only event-station pairs with an incidence angle of less than 45° and an event-station distance of smaller than 1° (~111 km) were considered to be suitable for detailed analysis. The spatial variations in both FPD and DT imply a complex anisotropic structure beneath the study region, likely caused by structure-induced mechanisms. At each station, the fast polarization directions are closely aligned with geometry of mapped faults and active faulting mechanisms which vary along the structurally intricate deforming zones in SE Türkiye. The overall observation suggests that the crustal anisotropy is predominantly controlled by the fault-related structures within the region of study. Besides, the delay times (~0.2s) are significantly larger at stations in close proximity to the active fault-lines.