Shear wave splitting along a nascent plate boundary: the North Anatolian Fault Zone

The North Anatolian Fault Zone (NAFZ) is a transform structure that constitutes the boundary between the Anatolian Plate to the south and the Eurasia Plate to the north. We analysed the properties of the upper-mantle strain field and mantle anisotropy in the vicinity of NAFZ via splitting of SKS and SKKS phases. We used data from the North Anatolian Fault (NAF) passive seismic experiment. This is the first study that analyses the upper-mantle anisotropy in this region and our results indicate that the observed upper-mantle strain field is uniform underneath the array with consistent NE–SW polarization directions for fast split waves. The measured lag times between the arrivals of the fast and slow split waves varies from 0.5 to 1.6 s for the study area. Smaller lag times are obtained consistently in the eastern part of the array. However, we do not observe any significant variation in either the polarization directions or the delay times across the plate boundary (NAFZ). The uniformity of the fast polarization directions throughout the study area and the strength of anisotropy favour an asthenospheric source for the anisotropy. The regional tectonic framework favours a SW direction of asthenospheric flow due to the forces acting on the upper-mantle exerted by the slab-roll-back taking place along the Aegean and the Cyprean Subduction Zones.