Complex Anisotropy Signature Beneath Western Anatolia Inferred from Shear Wave Splitting Analyses

Abstract:

Relative northward converging motion of the Arabian and African plates with respect to Eurasian plate plays a key role in controlling the geodynamic evolution and complex surface deformation along the Anatolian microplate. A better understanding of deformation patterns in the mantle can significantly contribute to clarify its relation with complex structures on the surface and underlying geodynamic processes. We employ shear wave splitting (SWS) analysis to detect possible signature of anisotropy within the upper mantle of western Anatolia and central Greece. We applied both tangential energy minimization and eigenvalue approaches to digital waveforms extracted from 36 broad-band seismic stations in western Anatolia and Aegean. In total, 391 teleseismic earthquakes were analysed during SKS splitting measurements. We observe a notable counter-clockwise rotation on FPDs from the NW Anatolia in the north to the Gulf of Gökova in the south. We attribute this to the effect of existing slab tear on the eastern edge of the Hellenic Trench that may change asthenospheric flow pattern. A comparison on delay times (1.55 sec on average) exhibits a decrease from north (~ 2s) to south (~ 1s) suggesting relatively thick anisotropic layer and/or high degree of mantle anisotropy at the north. Back-azimuthal variations of splitting parameters obtained from long-time operating permanent broadband stations (e.g. particularly located nearby the North Anatolian Fault Zone, NAFZ and North Aegean Trough, NAT) allow us to elucidate complicated anisotropic structure (e.g. two-layer) beneath Western Anatolia. Multi-layer anisotropy was tested via Multisplit approach (e.g. Eken and Tilmann, 2014) that employs a grid search scheme for two-layer anisotropy. Our results show a good accordance with theoretical two-layer curves. To better evaluate variations in two-layer models, we performed a simple jackknife test that runs the grid search 200 times over each sub-dataset established via randomly selected 70 percent of entire dataset. Our two-layer models yield anisotropic orientations aligning with the strike of the North Anatolian Fault (NAF) and North Anatolian Trough (NAT) in the lithosphere. N-S orientations in the asthenosphere can be explained by the flow pattern developed in response to the retreat of the Hellenic trench.