Separation of source, propagation and site effects from S waves of local earthquakes in Bursa region, northwestern Turkey

We have used micro-earthquake recordings (M=1.8-4.1) of local events in the distance range of 5-60 km in order to quantify the attenuation and site effects in the vicinity of the Bursa city, Marmara region, Turkey. The data set consists of 120 three-component recorded accelograms from 69 earthquakes, recorded at six stations. Each station is deployed on different geologic units, such as massive limestone, slope deposit and Quaternary young sediments, in the framework of the Marmara Poly-Project. In this study a nonparametric inversion method was applied to acceleration records from the Bursa region to estimate source, site and path effects using a two-step inversion. At the first step, we determined attenuation functions by analyzing the distance dependence of the spectral amplitudes and retrieved values of Q(s)(f)=46.59(0.67). At the second step, the corrected S-waves spectral records for the attenuation function, including the geometrical spreading effect, were inverted to separate source and site response for 21 different frequencies selected between 0.5 and similar to25 Hz. The near-surface attenuation, kappa value, was also estimated by using the model proposed by Anderson and Hough (1984) at each site. We observed that kappa(0) is smaller for stations located on rock site (Igdur, SIGD, kappa(0)similar to0.004) compared to the one that is located on Neogene sediment (Cukurca, SCKR, kappa(0)similar to0.018). Site amplifications from inversion showed that the station located within the Bursa basin, Cukurca (SCKR), is the most important site with about 4.0 amplification value at 1.8 Hz. Demirta (SDEM) amplifies the spectral amplitudes about 3.0 times at 2.0 Hz, SHMK about 3.5 times between 2.5 and 3.5 Hz and SHMT nearly reaching 3.5 times between 1.5 and 4.0 Hz. However, stations located on the Uludag Mountain Massif (SKAY and SIGD), which correspond to a deep limestone geological unit, have the smallest amplification, that values between 0.6 and 1.4.