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We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip fault system extending ∼1200 km across northern Turkey that poses a high level of seismic hazard, particularly to the city of Istanbul. We obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high-resolution images of S-wave velocity in the upper 10 km of a 70 × 30 km region around Lake Sapanca. We observe low S-wave velocities (<2.5 km s -1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we image higher velocities (>3.2 km s -1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the strike of the fault at periods shorter than 4 s. At longer periods up to 10 s, the fast direction aligns with the direction of maximum extension for the region (∼45 ∘). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results support the conclusion that the development of the NAFZ has exploited this pre-existing contrast in physical properties. [ABSTRACT FROM AUTHOR]

Turkey poses a complex crustal structure as a result of the convergence of the Arabian and African plates. The North Anatolian Fault Zone (NAFZ) represents 1600 km long active right-lateral strike-slip fault system of the country that has caused several destructive earthquakes. Location of historical earthquakes indicates that the activity on the fault tends to migrate westward. In the western part of NAFZ, there is a seismic gap inside the Sea of Marmara. Reliable knowledge of physical properties of the crustal structure of the unruptured segment within the Sea of Marmara is crucial for proper seismic hazard assessments considering that the megacity Istanbul with its millions of habitants, is closely located to this seismic gap. In this study, ambient seismic noise tomography, for the first time, has been applied to obtain a high-resolution image of the crustal structure of northwestern Turkey. For forward estimation of traveltimes, we employed the Fast Marching Method. A Bayesian framework was utilized to invert traveltime residuals extracted from Green's functions for the Rayleigh wave group velocity models at various frequencies. Besides the Transdimensional Bayesian tomography imaging, we also applied a rather simple and computationally fast 2-D nonlinear tomography scheme on the same data-set to gain a rough idea on group velocity variation in the regions. Our high-resolution tomographic images included group velocities of Rayleigh waves resolved between 1 and 20 s. Lateral variation of group velocities successfully revealed the basin structures such as Çınarcık, Tekirdağ, and Adapazarı Basins between periods 1 and 10 s that are characterized by typically low group velocities with low uncertainties. The low-velocity zones appear to mark well weakness zones along the NAFZ in the crust and the complex structure due to the active tectonics of the Anatolia. Finally, different tectonic units, in particular, to the east were identified from each other as our maps highlight Istanbul-Zonguldak Zone from Armutlu-Almacik Block with a sharp high to low-velocity change in N-S direction. [ABSTRACT FROM AUTHOR]