The response spectra of multidimensional analyses are compared with the one-dimensional (1D) local models to couple the irregular soil stratification effect in a site. In recent studies, the surface motion spectra ratios of 2D/1D or 3D/1D are defined as spectral aggravation factors for each region in a site. Particularly in alluvial basins, where the soil media is typically formed by fault ruptures or topographic depressions filled with sediments, the inclination of the rock outcrop in the edge of the basin has a considerable effect on the site response, and such effect has not yet been taken into consideration of recent seismic building codes and general engineering applications. In this study, the natural alluvial basin near the North Anatolian Fault in Gemlik, Maramara Region, Turkey, was investigated by 40 seismic site tests and 4 validation borings. The 2D and 1D nonlinear response history analyses in north-south and east-west directions of the Gemlik basin were performed by numerical model on finite difference scheme considering nonlinear elasto-plastic material behaviors and geometric discontinuities. 22 strong ground motions recorded on rock site are excited vertically as SH waves. The numerical results exhibited the narrow basin effects are derived not only by reflection, refraction, and shifting behavior but also by focusing and superposition of the seismic waves propagating from both opposite basin edges. As a result, the site-specific spectral aggravation factors, SAF2D/1D defined by the ratio between the 2D and 1D acceleration response spectra for each period and any location on the site, were proposed for the Gemlik basin. The variations of the aggravation factors were observed as increasing values to 1.2–2.2 on the near edge and basin center.

Several studies have revealed that the inclined bedrock at the sides of the basins bring about the concentration of damages, which well known as basin edge effect. As a seismically active area much of the Turkey lies on the Anatolian Plate. This small plate bounded by two major strike-slip fault zones, the North Anatolian Fault and East Anatolian Fault. During the history, Turkey has been the site of devastating earthquakes. Severe structural damage at basin sides during recent major earthquakes around the world strongly pointed out the importance of basin edge effect. Some of its notable examples can be tracked in Turkey. With respect to the fact that Turkey is among the top 20 percent of all countries exposed to earthquake hazard with regard to mortality and economic losses, the necessity of the investigation and consideration of the basin edge effect in earthquake design codes is revealed. In this study, the dynamic behavior at the edge of two real basins is investigated. In this study, at the first, the basin edge model of the basins are extracted from the data obtained from extensive microtremor surveys and geotechnical investigations as well as insitu studies including SPT, CPT, PS logging tests. Then, the idealized geometry of the Dinar and Duzce basins edge models are proposed. Afterwards, the basins are subjected to the collection of strong ground motions using a fully nonlinear method which works based on explicit finite difference scheme FLAC3D code. The 5566 COMPDYN 2019 7 th ECCOMAS Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineering M. Papadrakakis, M. Fragiadakis (eds.