Systematic analysis of crustal anisotropy along the Karadere–Düzce branch of the North Anatolian fault

S U M M A R Y We perform a systematic analysis of crustal anisotropy along and around the Karadere–Düzce branch of the North Anatolian fault (NAF), which ruptured during the 1999 Mw 7.4 İzmit and Mw 7.1 Düzce earthquakes. A method consisting of an iterative grid search for the best shear wave splitting parameters in sliding time windows is applied to ∼22 000 measurements recorded in the 6-month period after the İzmit main shock. Based on objective criteria, ∼6600 measurements are assigned high quality and used for further detailed analysis. Most stations near the rupture zone have fast polarization directions that are parallel to, and change with, the nearby fault strike. The average delay times for ray paths that propagate along the rupture zone are larger than for the other paths. These results suggest the existence of an approximately 1-km broad zone around the Karadere–Düzce branch with fault-parallel cracks or shear fabric. However, some fault zone (FZ) stations record bimodal or scattered polarization directions, while stations near large structural complexities (e.g. branching and offsets) show average fast polarization directions that are almost perpendicular to the local fault strike. The average fast polarization directions from ray paths that propagate inside the Almacik block, south of the Karadere–Düzce branch, are neither parallel to the local fault strike nor to the expected regional maximum compressive stress direction. The large overall spatial variations of the results imply that multiple structures and mechanisms contribute to the observed crustal anisotropy in our study area. Most stations do not exhibit a clear dependency of shear wave splitting delay time with increasing depth and hypocentral distance, indicating that the anisotropy is confined primarily to the top 3–4 km of the crust. Using the observed average delay time at FZ stations and assumed propagation distance of 3.5 km, we estimate the apparent crack density in the damaged shallow FZ rock to be approximately 7 per cent.