An investigation of seismic anisotropy in the crust beneath the western part of the North Anatolian Fault Zone

Abstract

The North Anatolian Fault (NAF) is one of the most well-known continental strike slip faults, however, the details related to the geodynamic processes and the extent of deformation in the crust remain poorly understood. Within the context of the Dense Array for North Anatolia (DANA) project, a comprehensive data set was gathered from a dense temporary seismic network consisting of 70 stations that were deployed in early May 2012 and operated for 18 months in the Sakarya region and the surroundings. With the aim of further exploring the crustal deformation near the fault mainly caused by the strike slip motion also resulting in the alignment of minerals, the crustal seismic anisotropy beneath the western segment of NAF was investigated by local shear wave splitting analysis. Out of 1371 events, 90 well located earthquakes were extracted with magnitudes >1.4 corresponding to a total of 645 splitting measurements. This method makes use of earthquakes nearby or directly below each station benefiting from the fast polarization direction and the relevant delay time parameters as the main output. Despite the scattered patterns, the fast orientations are dominantly E-W parallel to the fault strike. Delay times between the fast and slow components of the shear wave vary between 0.01 and 0.3 s clearly revealing the existence of crustal anisotropy. In particular, measurements at each station exhibits spatial variations across the fault where it splays into two main branches beneath the study area separating different geologic units. A strong correlation could not be established between small scale faults and azimuthal anisotropy. The measurements presented here constrain crustal anisotropy above the deepest earthquakes at approximately 15 km depth.