Characterization of the seismic velocity field and crustal anisotropy in the Marmara region, NW Turkey, using ambient noise

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Within the here proposed one-year extension of the project we intend to analyze the seismic ambient noise field recorded at the broadband stations surrounding the eastern Sea of Marmara in order to study seismic anisotropy. We intend to apply traditional surface-wave analysis techniques in order to study the azimuthal variation of anisotropy in the region for the first time. The new data provided by GONAF only now allow to conduct such a detailed analysis. We will go beyond previous work (initial two-year phase) not only by analyzing anisotropy but also by refining existing work with denser data including vertical arrays. Studying anisotropy with unprecedented details will also contribute to detect stress accumulations along the Marmara segment of the NAFZ.The isotropic dispersion curves and an average S-wave velocity structure for the region were already derived during the initial phase of this project. Here, we plan to recalculate dispersion curves using all broad-band ambient noise recordings and invert these for anisotropic parameters. This calculation will lead to two principal measurements: the direction of the fast wave and its amplitude as well as the azimuthal dependency of surface-wave velocity. In addition, since we are using surface waves, we will be able to identify the depth of crustal and uppermost mantle anisotropy, which could not be determined in a local anisotropy study using shear-wave splitting. This information will also allow us to elaborate on the origin of the anisotropy in the study region, i.e. stress- or structure-induced. Moreover, P- as well as S-wave velocity models for the region will be refined taking into consideration the anisotropy results allowing for further improved hypocentral precision of local seismicity along the Princes Islands segment offshore Istanbul.