Spatial Variation in Stress Orientation around Türkiye: Rupture Propagation across the Stress Regime Transition in the 2023 Mw 7.8 Türkiye Earthquake

Summary On 6 February 2023, an Mw 7.8 left-lateral strike-slip fault earthquake occurred on the East Anatolian Fault Zone (EAFZ) in Türkiye. This study examined the spatial variation of the stress field around Türkiye better to understand the generation process of this event. We first combined focal mechanisms around Türkiye, created a dataset consisting of 2984 focal mechanisms, and conducted stress tensor inversion. The results showed that the maximum compressional axis near the EAFZ was oriented north-south and slightly varied along the strike. Moreover, the relative magnitude of north-south compressional stress gradually increases from south to north, and the stress regime changes from a normal fault stress regime to a strike-slip fault regime. The stress change caused by the Mw 7.8 mainshock does not explain this lateral pattern, implying that this stress regime transition existed before the mainshock. This suggests that shear stress on the EAFZ was low in this southern segment because it was unfavourably oriented to the regional stress field. Previous studies have reported that the Mw 7.8 mainshock rupture started at a splay fault, first propagated through the central and northern segments and then backpropagated with a time delay toward the southern segment, where it caused a significant but relatively small slip. The preexisting along-strike shear stress variation on the fault may have contributed to the smaller and delayed coseismic slip in the southern segment than in the central and northern segments. Moreover, the mainshock rupture possibly caused stress rotation locally near the central segment where the magnitudes of the vertical and north-south compressional stresses were almost equal.