The triple junction as the preferred nucleation site on the main East Anatolian Fault for the 2023 M_w 7.8 Kahramanmaraş Earthquake: Insights from 3D dynamic rupture simulations

Understanding rupture transfer across fault junctions is critical for interpreting complex multi-fault earthquakes. For the 2023 M_w 7.8 Kahramanmaraş event, we construct three dynamic rupture models constrained by surface fault traces, relocated aftershocks, kinematic slip inversions, near-field strong-motion records, and GPS data to distinguish among potential nucleation sites along the East Anatolian Fault (EAF). Our simulations reveal that although the final slip distributions are insensitive to the precise nucleation point owing to complex fault geometry, the dynamic conditions required for rupture initiation on the EAF differ significantly. Nucleation at the triple junction is facilitated by dynamic Coulomb stress triggering from the Narlı branch fault. Although the supershear rupture velocity is different along the Narlı branch fault of Model 1 and Model 3, both models support nucleation at the triple junction. In contrast, nucleation at 9 km southwest of the triple junction requires artificial stress concentrations, as shown in Model 2. Model 3 shares the same stress concentration along the EAF; however, its near-P-wave supershear speed along the branch fault narrows the Mach cone and directly impedes nucleation to the southwest. All the models show good correspondence with the near-field seismogram and GPS observations. Additionally, we compare the source time functions of the three models with the U.S. Geological Survey result, which imply that the near-P-wave supershear speed along the branch fault is less likely. Our results suggest the triple junction is the most likely nucleation site, and although the 9 km southwest of the triple junction is still possible, it requires highly localized initial stress concentrations.