Stress- and Structure-Induced Shear-Wave Anisotropy along

Abstract We perform a shear-wave splitting analysis using seismograms of after-shocks 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 sta-tions 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 mechan-ism. 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 heteroge-neous anisotropic distribution within the recording array and that the physical proper-ties of the upper crust vary between the NAFZ and surrounding area