We perform a shear‐wave splitting analysis using seismograms of aftershocks following the 1999 Mw 7.4 Izmit earthquake to delineate between stress‐ and structure‐controlled anisotropic mechanisms in the upper crust beneath the northwest North Anatolian fault zone (NAFZ). The splitting analysis is performed on direct shear‐wave arrivals using a cross‐correlation technique to calculate the fast shear‐wave polarization and the delay time between fast and slow shear‐wave arrivals. Five stations located both within (<1  km) and near (>1  km) the NAFZ yield a combined 333 quality shear‐wave splitting measurements. The shear‐wave splitting measurements indicate that it is occasionally possible to delineate between the two anisotropic mechanisms. Stations greater than 1 km from the NAFZ tend to have dominant fast shear‐wave polarizations parallel or subparallel to the regional strike of the maximum horizontal compressive stress, which suggests a stress‐controlled anisotropic mechanism. The single station within 1 km of the NAFZ yields a bimodal dominant fast shear‐wave polarization that is both parallel and perpendicular to the strike of the nearby Mudurnu fault. We interpret the fault‐parallel mode as representing a structure‐controlled anisotropic mechanism from the Mudurnu fault. The average delay time for all earthquakes is 0.038±0.031 s, and anisotropic intervals do not appear to extend below 8 km. The fast shear‐wave polarizations and delay times suggest a heterogeneous anisotropic distribution within the recording array and that the physical properties of the upper crust vary between the NAFZ and surrounding area.

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