Three-Dimensional Tomographic Imaging of the Crust and Upper Mantle Beneath the Marmara Region

In this study, three-dimensional P- and S-wave velocity structures and P- and S-wave velocity ratio variations in the crust and upper mantle beneath the Marmara Region and the Sea of Marmara were modeled using the Poisson tomography method in the field of Seismology. Within this scope, P- and S-wave arrival times from a total of 23,672 earthquakes that occurred in the region between 2011 and 2023 were evaluated, and an inversion procedure based on body-wave arrival times was applied. The obtained results indicate that the northern branch of the North Anatolian Fault Zone is the most active segment among its three main branches. In addition, a low-velocity zone characterized by seismic gap features extending from the southern parts of Marmara toward Istanbul was identified. Within these seismic gap zones, P- and S-wave velocities decrease sharply across regions exhibiting strong velocity gradients. It was determined that this low-velocity structure cuts across the North Anatolian Fault, which branches into three segments within the Sea of Marmara, and continues at depths of approximately 15–25 km. Earthquakes were observed to concentrate particularly in areas with high-velocity ratio variations and within transition zones from low to high values. Due to the complex tectonic and stratigraphic structure of the Marmara Region, these low-velocity seismic gap zones within the crust may be associated with segments capable of generating large earthquakes in the future. Therefore, a detailed investigation of the crustal structure of the region provides important insights for understanding regional earthquake hazards.