Abstract

We analyze the ambient seismic‐noise field in order to investigate the crustal structure at the North Anatolian fault zone (NAFZ) in northwest Turkey. We focus on the eastern Sea of Marmara section, where the NAFZ is in the final phase of the seismic cycle prior to an expected major (M>7) earthquake. We apply cross‐correlation analysis of the seismic ambient noise to determine the spectral dependence of the seismic velocity in order to image the crustal structure at seismogenic depth. Time‐domain cross correlations are calculated for available station pairs in the target area. Interstation distances span 0.3–90 km. Here, the vertical component is analyzed in order to recover fundamental‐mode Rayleigh waves in the 0.05–1.1 Hz frequency range. Group velocity dispersion curves are obtained for selected correlation paths in particular to address the azimuthal dependence of the velocity field. In the frequency band of interest, average group velocities range between ∼1.8 and 3.5  km/s. Dispersion curves corresponding to the north–south‐trending paths crossing the main NAFZ fault branch below the eastern Sea of Marmara show low group velocities between ∼1.5 and 1.8  km/s, which is well explained by the 3–4 km‐deep Çınarcık basin, located between the two major fault branches, the Princes Islands and Armutlu fault segments. In contrast, ray paths restricted to within the mainland of Istanbul and the Armutlu peninsulas (primarily trending east–west) show higher group velocities up to 3.2  km/s. By averaging the dispersion curves, we determine an optimized 1D S‐wave velocity model for the eastern Sea of Marmara region, allowing for a significant improvement in hypocenter determination for local seismicity.

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