Structure Heterogeneity Around the Western End of the 1999 Izmit Earthquake Rupture Revealed by Distributed Acoustic Sensing

We use distributed acoustic sensing along a submarine fiber‐optic cable to provide the first high‐resolution two‐dimensional shear‐wave velocity (Vs) model of the North Anatolian Fault (NAF) in the Marmara Sea. Using 7 days of ambient noise recordings, we extract Scholte wave dispersion curves via slant‐stacking and invert for the Vs structure, identifying a pronounced low‐velocity layer (∼0.6 km/s), indicative of unconsolidated sediments. Autocorrelation and natural migration profiles reveal a ∼3‐km‐wide fracture zone (WFZ), which we interpret as a legacy of long‐term slip on the NAF. The low Vs and structural complexity suggest the WFZ functions as a soft barrier. Hence, it could have contributed to arrest rupture propagation during the 1999 M7.6 Izmit earthquake, as well as partitioning stress between fault segments. We also resolve a flower‐like structure of faults. These results may demonstrate how fault zone heterogeneity controls rupture dynamics and stress accumulation, offering mechanistic insights into seismic segmentation.