Thoughts of the February 20, 2023 Defne aftershock

On February 6 2023, two large earthquakes with magnitudes of Mw 7.8 (Pazarcık) and Mw 7.6 (Elbistan) occurred consecutively along the East Anatolian Fault Zone in eastern Turkey, causing enormous casualties and heavy damage. This devastating sequence of earthquakes was followed by the Defne aftershock on February 20 near Antakya province, which increased the damage and loss of life. In this study, the teleseismic broadband P velocity waveforms have been inverted in order to obtain the coseismic finite-fault slip distribution of the February 20, 2023 Defne aftershock. It was found that the rupture was controlled by the failure of a single asperity with the largest displacement of approximately 0.75 m, which occurred between 6 and 20 km depth. The source mechanism indicated a dominant left-lateral faulting with a significant normal component and released a total seismic moment of 5.85x1018 Nt.m. Coseismic Coulomb stress changes modelling showed that the Defne aftershock rupture was triggered by the earthquake sequence and that the February 6 Pazarcık earthquake had a dominant effect. In the stress modelling carried out on the Dead Sea Fault, the northern segment of the fault remained in the region of significant positive stress loading. Considering the positive stress load over 1 bar created by the earthquake sequence and the Defne aftershock ruptures, as well as the fact that no major earthquake has occurred for more than 600 years, it is clear that the probability of rupture in the northern part has increased significantly and the seismic hazard is high.