Crustal attenuation characteristics in western Turkey

We analysed 1764 records produced by 322 micro- and moderate-size local earthquakes in western Turkey to estimate crustal attenuation characteristics in the frequency range of 1.0 < f < 10 Hz. In the first step, we obtained non-parametric attenuation functions and they show that seismic recordings of transverse and radial S waves exhibit different characteristics at short and long hypocentral distances. Applying a two-step inversion, we parametrized Q( f ) and geometrical spreading exponent b( f ) for the entire distance range between 10 and 200 km and then we estimated separately Q and b values for short (10-70 km) and large (120-200 km) distance ranges. We could not observe significant frequency dependencies of b for short distance range, whereas the significant frequency dependence of b was observed for large distances. Low Q(0) values (similar to 60) with strong frequency dependence of Q (similar to 1.4) for short distances suggest that scattering might be an important factor contributing to the attenuation of body waves in the region, which could be associated to a high degree of fracturing, fluid filled cracks, young volcanism and geothermal activity in the crust. Weak Q frequency dependence and higher Q(0) values for large distances manifest more homogenous medium because of increasing pressure and enhanced healing of cracks with increasing temperature and depth. Q anisotropy was also observed for large hypocentral distance ranges