Variations of shear-wave splitting parameters in the source region of the 2023 Türkiye doublet earthquakes

In this study, the shear-wave splitting parameters of local seismic events from the source regions of the 2023 Türkiye M_W7.7 and M_W7.6 doublet earthquakes (event 1 and event 2, respectively) were measured from June 1, 2022, to April 25, 2023, and their spatiotemporal characteristics were analyzed. The results revealed clear spatial and temporal differences. Spatially, the dominant fast-wave polarization direction at each station shows a strong correlation with the direction of the maximum horizontal principal compressive stress, as characterized by focal mechanism solutions of seismic events (M_W≥3.5) near the station. The dominant fast-wave polarization direction and the regional stress field also showed a strong correlation with the intermovement of the Arabian Plate, African Plate, and Anatolian Block. Along the Nurdagi-Pazarcik fault zone, the seismic fault of event 1, stations closer to the middle of the fault where the mainshock occurred exhibited notably greater delay times than stations located towards the ends of the fault and far from the mainshock. In addition, the stations located to the east of the Nurdagi-Pazarcik fault and to the north of the Sürgü fault also exhibited large delay times. The spatial distribution of shear-wave splitting parameters obtained from each station indicates that the upper-crust anisotropy in the source area is mainly controlled by the regional stress field, which is closely related to the state of the block motion. During the seismogenic process of the M_W7.7 earthquake, more stress accumulated in the middle of the Nurdagi-Pazarcik fault than at either end of the fault. Under the influence of the M_W7.7 and M_W7.6 events, the stress that accumulated during the seismogenic process of the earthquake doublet may have migrated towards some areas outside the aftershock intensive area after the earthquakes, and the crustal stress and its adjustment range near the outer stations increased significantly. With the exception of two stations with few effective events, all stations showed a consistent change in shear-wave splitting parameters over time. In particular, each station showed a decreasing trend in delay times after the doublet earthquakes, reflecting the obvious intensification of crustal stress adjustment in the seismogenic zone after the doublet earthquakes. With the occurrence of the earthquake doublet and a large number of aftershocks, the stress accumulated during the seismogenic process of the doublet earthquakes is gradually released, and then the adjustment range of crustal stress is also gradually reduced.