Nihal Akyol

Site amplification of S-waves in Bursa City and its vicinity, Northwestern Turkey: comparison of different approaches

Microearthquake recordings of local events have been used to quantify the site effects in the vicinity of Bursa City, northwest Turkey. Since the city is located near the southwest branch of the western extension of the North Anatolian Fault (NAF) zone, the importance of the seismic hazard in the region becomes progressively more important. The accelerograms of 69 earthquakes that were recorded on different geologic units, massive limestone, slope deposit and Quaternary sediment were analyzed to estimate the response of the recording sites. Site amplification functions were obtained by using three different approaches (standard spectral ratio, SSR; horizontal to vertical, H/V ratio and generalized inversion method, GIM) and the differences between the methods were also evaluated. We found large discrepancies between the SSR and H/V ratio methods, specifically; the former yields almost three times higher amplitudes than those obtained in the latter approach. Station located within the Bursa Quaternary basin (SCKR) is characterized by the largest estimates of the amplification amplitudes (8.0, 4.5 and 4.0 for SSR, H/V ratio and GIM, respectively) in all the three methods. On the other hand, stations located on deep limestone geological unit (SIGD and SKAY) show the least amplification level, ranging between 1.0 and 1.6. Three methods are able to identify resonant frequencies of the sites, although the absolute amplitudes of the amplification function are obtained different from each method.

Separation of source, site and near-surface attenuation effects in western Turkey

We analyzed 1764 records from 322 micro- and moderate-size local earthquakes in the central west Turkey to separate source, site and near-surface attenuation effects by utilizing a generalized inversion technique (GIT) to the spectra. GIT site transfer functions were compared with horizontal-to-vertical spectral ratio (HVSR) estimates by Akyol et al. (Pure Appl Geophys 170(12):2107–2125. doi: 10.1007/s00024-013-0661-2, 2013). Large amplitude values of vertical component GIT site transfer functions were obtained at different frequency bands for some of the sites. These results imply contaminations of HVSR estimates due to basin geometry induced waves caused by deep or shallow lateral heterogeneities. GIT source function estimates were interpreted as Brune source model. Weak epicentral dependence of near-surface attenuation parameter κ with large scattering could be attributed to the source and propagation path complexities along different paths arriving to the stations. Large fe values with large κ0 values emphasize near-surface weathered zones attenuation effect at high frequencies.

Site term from single-station sigma analysis of S-waves in western Turkey

The main aim of this study is to obtain site terms from single-station sigma analysis and to compare them with the site functions resulting from different techniques. The dataset consists of 1764 records from 322 micro- and moderate-size local earthquakes recorded by 29 stations in western Turkey. Median models were derived from S-wave Fourier amplitude spectra for selected 22 frequencies, by utilizing the MLR procedure which performs the maximum likelihood (ML) estimation of mixed models where the fixed effects are treated as random (R) effects with infinite variance. At this stage, b (geometrical spreading coefficient) and Q (quality factor) values were decomposed, simultaneously. The residuals of the median models were examined by utilizing the single-station sigma analysis to obtain the site terms of 29 stations. Sigma for the median models is about 0.422 log10 units and decreases to about 0.308, when the site terms from the single-station sigma analysis were considered (27% reduction). The event-corrected within-event standard deviations for each frequency are rather stable, in the range 0.19–0.23 log10 units with an average value of 0.20 (± 0.01). The site terms from single-station sigma analysis were compared with the site function estimates from the horizontal-to-vertical-spectral-ratio (HVSR) and generalized inversion (INV) techniques by Akyol et al. (2013) and Kurtulmuş and Akyol (2015), respectively. Consistency was observed between the single-station sigma site terms and the INV site transfer functions. The results imply that the single-station sigma analysis could separate the site terms with respect to the median models.