Depth-dependent anisotropy along northwest segment of the North Anatolian fault zone : evidence for paleo-tectonic structures contributing to overall complexity

Abstract

The North Anatolian Fault Zone (NAFZ) is a prominent tectonic structure with a significant impact on the observed active deformation in Türkiye. Detailed knowledge of the seismic anisotropy in the crust and mantle along this nascent shear deformation zone provides insights into the kinematics associated with past and present tectonic events. We employed teleseismic earthquakes observed by the Dense Array North Anatolia seismic network to map 3- D variations in crustal and mantle anisotropy in/around the NW segment of the NAFZ. To achieve this, we first performed a harmonic decomposition analysis of P-receiver functions. The results were then used as a priori information to conduct an anisotropic receiver function inversion with the Neighborhood Algorithm that enabled imaging of the actual orientation and geometry of anisotropic structures. SKS splitting measurements are further used to make a comparison between the anisotropic behavior of crustal and mantle structures. Crustal anisotropy parameters estimated in our analyses/models well identify the signature of deformation caused by accumulated strain in the earthquake cycle through the strike of shallow cutting faults in the brittle crust beneath the NAFZ. Diffuse intense anisotropic energy at lower crustal depths was attributed to lattice preferred orientation of crystals or partially molten lenses elongated along the shear direction. Strong harmonic energy variations beneath the northern part of the Istanbul Zone likely reflect imprints of LPO-originated frozen fabric at shallow depths (0–20 km) associated with the palaeotectonic Odessa Shelf, Intra-Pontide Suture Zones or remnants of the Tethys Ocean.