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

在本项目为期一年的延长期内，我们打算分析马尔马拉海东部周围宽带站记录的地震环境噪声场，以研究地震各向异性。我们打算利用传统的面波分析技术，首次研究该地区各向异性的方位角变化。GONAF提供的新数据现在才允许进行如此详细的分析。我们将超越之前的工作（最初的两年阶段），不仅通过分析各向异性，而且通过使用包括垂直阵列在内的更密集的数据来改进现有的工作。以前所未有的细节研究各向异性也将有助于探测NAFZ马尔马拉段的应力积累。在本工程的初始阶段，已经得到了该区域的各向同性频散曲线和平均横波速度结构。在这里，我们计划使用所有宽带环境噪声记录重新计算色散曲线，并将其反演为各向异性参数。这种计算将导致两个主要测量：快波的方向和振幅，以及面波速度的方位依赖性。此外，由于我们使用的是表面波，我们将能够确定地壳和上地幔各向异性的深度，这在使用剪切波分裂的局部各向异性研究中是无法确定的。这些信息还将使我们能够详细说明研究区域各向异性的起源，即应力或结构诱导。此外，考虑到各向异性结果，该地区的P波和s波速度模型将得到改进，从而进一步提高伊斯坦布尔近海王子群岛段当地地震活动性的震源精度。

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.