Separation of Source, Propagation and Site Effects from Observed S-wave of Bursa City and Its Vicinity in the Northwestern Anatolian Fault Zone, Turkey

Abstract

Micro and moderate-sized earthquake recordings (ML=1.8-4.1) of local events have been used to quantify attenuation and site effects in the vicinity of the Bursa City of Marmara region, Turkey. We used digital records from 69 local events recorded at short hypocentral distance (r < 60 km) in the region. The digital seismograms analyzed were recorded by temporary network of six seismic stations each of them deployed in a different geological site. Non-parametric inversion method was applied to acceleration records from tectonically active Bursa region and results obtained 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 Qs = 38.26f 0.73. The attenuation functions including anelasticity of heterogeneous medium and geometrical spreading effect were used to correct the S-wave spectral records. At the second step, the corrected amplitudes were inverted to separate source and site response for 21 different frequencies selected between 0.5 and ~25 Hz. The near surface attenuation was also estimated and found that the spectral decay parameter  is smaller for stations located on rock, Igdır, (SIGD, ~0.006) compared to stations located on soft sediment, Hamitler, (SHMT, ~0.02). Çukurca, (SCKR) is the most important site having about 3.5 amplification value at 1.8 Hz. Demirtaş, (SDEM) amplifying the spectral amplitudes about 2.8 times at 2.5 Hz, SHMK about 3 times at between 2.5 and 3.5 Hz and SHMT about up to 3 times at between 1.5 and 4 Hz. However, stations located on the Uludağ Mountain Masif (SKAY and SIGD), which correspond to a deep Limestone geological unit, have the smallest amplification, that values between 0.6 and 1.4.