High-Resolution Crustal Velocity Images along the North Anatolian Fault Zone in NW Turkey Obtained from Ambient Seismic Noise Tomography

Having knowledge of 3-D high-resolution physical properties for the crustal structure along the North Anatolian Fault Zone (NAFZ) in NW Turkey will greatly contribute to future seismic hazard scenarios, which will be developed for a region including the Sea of Marmara. In this study, we provide the first detailed shear wave velocity structure of NW Turkey, including the NAFZ estimated from ambient seismic noise tomography. For the first time in this region, a Transdimensional Bayesian tomography method has been applied to obtain a high-resolution image of the crustal structure. Over 20,000 Green's functions are retrieved from the seismic ambient noise cross-correlations. For the forward estimation of travel times, we employed the Fast Marching Method. We also performed a simple and computationally fast 2-D nonlinear tomography inversion on the same data-set to gain a rough idea of group velocity variation in the regions. Besides the manual extraction of Green's functions, we have also tested an auto-picking approach introduced by Hawkins&Sambridge (2019) that ultimately enables estimates of interstation group velocities.

Lateral variation of group velocities inferred from Rayleigh waves between periods 1 and 10 s reveals the basin structures such as the Çınarcık, Tekirdağ, and Adapazarı Basins that are characterized by typically low group velocities. The low-velocity regions appear to delineate the weak crustal zones along the NAFZ and the complex structure due to the active tectonics of Anatolia. Our final velocity maps highlight the contrast between the Istanbul-Zonguldak Zone and the Armutlu-Almacık Block, as they are separated with a sharp high-to-low velocity change in the N-S direction.