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    Broadband ground-motion simulation of the 24 May 2014 Gokceada (North Aegean Sea) earthquake (Mw 6.9) in NW Turkey considering local soil effects
    (Springer, 2018) Karagoz, Ozlem; Chimoto, Kosuke; Yamanaka, Hiroaki; Ozel, Oguz; Citak, Seckin
    On 24 May 2014, a Mw 6.9 earthquake occurred in the west of Gokceada Island, northern Aegean Sea. The earthquake was close to Canakkale, Enez, Tekirdag cities, and damaged 300 buildings in the Marmara Region, NW Turkey. We simulated its broadband (0.1-10 Hz) ground motions including 1D deep and shallow structures soil amplification effects at the 12 strong ground motion stations in the western Marmara Region. The 1D deep velocity structures from the focal layer to the engineering bedrock with an S-wave velocity of 0.78 km/s in different azimuthal directions were tuned by comparing the observed group-velocity dispersion curves of Rayleigh and Love waves from the mainshock with theoretical ones. We also added the shallow parts from previous surveys into the 1D models. Synthetic seismograms on the engineering bedrock were generated using the discrete wave number method with a source model and the 1D deep velocity structures. Then the surface motion was generated considering shallow soil amplification. The synthetic seismograms are generally in good agreement with the observed low and high-frequency parts at most of the stations indicating an appropriateness of the source model and the 1D structural model.
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    Empirical Attenuation Relationships for Western Anatolia, Turkey
    (Tubitak Scientific & Technological Research Council Turkey, 2009) Akyol, Nihal; Karagoz, Ozlem
    Seismic hazard studies have become progressively more important for earthquake engineering applications in western Anatolia, which contains one of the world's best examples of a rapidly extending intra-continental tectonic regime. A two-stage regression analysis was applied to peak ground acceleration and 5%-damped spectral acceleration values of 168 recordings from 49 earthquakes in order to develop empirical attenuation relationships which can be used to predict ground motion for western Anatolia. Moment magnitudes for earthquakes range between 4.0 and 6.4 while the hypocentral distances range between 15 and 200 km in our dataset. Site classifications, as one of the predictor variables for the regression analysis, were obtained using horizontal to vertical spectral ratio estimates. These estimates, together with empirical attenuation relationship predictions, have shown that soil amplification is significant in western Anatolia. Attenuation relationship models that are obtained explicitly account for nonlinear sediment effects for deep soil sites in the region. Nonlinear effects of deep soil sites at lower periods are significant at the higher levels of shaking and manifest over-prediction for acceleration values, while they manifest lower prediction values at lower levels of shaking. Both results from the horizontal to vertical ratio method and the regression analysis show that the number of strong motion stations located on the rock sites in the region should be increased and the present site classification of strong motion stations in Turkey should be re-evaluated in detail. When obtained attenuation relation models were compared with the attenuation relationships based on data from tectonically similar regions, the attenuation relations modelled for a specific region could not, in general, be used in engineering applications realized for another region. Our results also indicate that the spectral acceleration model defined in the Turkish Building Code cannot adequately explain magnitude and distance dependencies in western Anatolia.
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    Estimation of 1D Deep Vs Models in canakkale and Surrounding Area (Biga Peninsula, NW Turkey) Verified with Numerical Ground Motion Simulation of Moderate-Sized Earthquakes
    (Springer Basel Ag, 2022) Karagoz, Ozlem
    This study aims to determine the 1D deep S-wave velocity structure for canakkale Province and the surrounding area (Biga Peninsula, NW Turkey) using the moderate (M >= 4.0) earthquakes from the last decade. A total of 540 velocity seismograms with a high S/N ratio are obtained from 218 three-component acceleration records of the 10 earthquakes (4.0 <= M-w <= 5.3) that occurred in the areas of Ayvacik, Saros, and can between 2010 and 2018. A total of 34 strong ground motion stations operated by AFAD are grouped in 27 azimuthal directions, and fundamental mode surface wave group velocity dispersion curves are obtained using the multiple-filter method. First, the observed dispersion curves are utilized for the inversion application to define the 1D deep Vs model. Then they are compared with the theoretical curves of the tuned 1D deep Vs models with the trial-and-error forward method after inversion. The RMS misfits between observed and calculated surface group velocities decrease from 0.6 to 0.2 on average. The dispersion analyses allow for improved seismic velocities and thicknesses of especially the uppermost 4-5 km. The defined 1D deep Vs model of 202 source-station paths are also inferred to obtain an average pseudo-3D deep Vs model. In addition, the velocity models are verified with 1D numerical ground motion simulations for 0.05-1 Hz, including the characterized source models of the earthquakes and 1D shallow soil amplifications. The simulation results are quantitatively evaluated with goodness-of-fit measures considering different frequency bands. Fairly good agreement for waveform first arrival and spectral amplitude (0.05-1 Hz) is achieved. However, the later wave packages at the sites located on thick sediment basins cannot be modeled because of the reverberations in the sediment overlying the engineering bedrock. The test of the pseudo-3D Vs model using broadband (0.05-10 Hz) simulation of the 2017 Lesvos mainshock (M-w 6.3) also indicates that both the phase arrival times (< 1 Hz) and the amplitude spectral decay in the high-frequency range of 1-7 Hz are well modeled.
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    Estimation of shallow S-wave velocity structure and site response characteristics by microtremor array measurements in Tekirdag region, NW Turkey
    (Springer Heidelberg, 2015) Karagoz, Ozlem; Chimoto, Kosuke; Citak, Seckin; Ozel, Oguz; Yamanaka, Hiroaki; Hatayama, Ken
    In this study, we aimed to explore the S-wave velocity structure of shallow soils using microtremors in order to estimate site responses in Tekirdag and surrounding areas (NW Turkey). We collected microtremor array data at 44 sites in Tekirdag, Marmara Ereglisi, Corlu, and Muratli. The phase velocities of Rayleigh waves were estimated from the microtremor data using a Spatial Autocorrelation method. Then, we applied a hybrid genetic simulated annealing algorithm to obtain a 1D S-wave velocity structure at each site. Comparison between the horizontal-to-vertical ratio of microtremors and computed ellipticities of the fundamental mode Rayleigh waves showed good agreement with validation models. The depth of the engineering bedrock changed from 20 to 50 m in the Tekirdag city center and along the coastline with a velocity range of 700-930 m/s, and it ranged between 10 and 65 m in Marmara Ereglisi. The average S-wave velocity of the engineering bedrock was 780 m/s in the region. We obtained average S-wave velocities in the upper 30 m to compare site amplifications. Empirical relationships between the AVs30, the site amplifications, and also average topographic slopes were established for use in future site effects microzonation studies in the region.
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    Probabilistic Seismic Hazard Analysis of the Izmir Bay Area
    (Springer Science and Business Media Deutschland GmbH, 2024) Albayrak, Kubilay; Askan, Aysegul; Karagoz, Ozlem; Tan, Onur
    Probabilistic Seismic Hazard Analysis (PSHA) is an analysis that quantifies the probability of exceedance or the rate of various ground motion levels at a specific site or an area by using all the possible earthquakes. Since earthquakes are very intense in specific regions, it is important to identify the seismicity level of these areas. The Izmir Bay area is believed to be one of the most critical areas in Türkiye in terms of high seismicity. So, PSHA of the Izmir Bay area was constructed by using 34 SPAC locations which are used to obtain the average shear wave velocity of the upper 30 m (VS30). The diameter of the area to identify the seismic sources is selected as 200 km. Since PSHA mainly results in Peak Ground Acceleration (PGA) values as a quantifier of ground motion intensity measurement, PGA values are based on 15 different periods. Moreover, Response Spectrum results are based on 50-, 100-, 475-, 975-, and 2475-year recurrence intervals for mean in terms of 16%, 50%, and 84% quantiles. Finally, the seismicity contour maps based on 475- and 2475-year recurrence intervals are prepared based on PGA values to visualize the resulting seismicity of the specified region. Since the Izmir Bay area is mainly comprised of low VS30 values, the results of this study are believed to be important to take precautions for disaster resilience considerations. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024.