The response spectra of multidimensional analyses are compared with one-dimensional (1D) local models to couple the irregular soil stratification effect at a site. In recent studies, the surface motion 2D/1D or 3D/1D spectra ratios are defined as the spectral aggravation factors for each region of 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 at the edge of the basin has a considerable effect on the site response, and such an effect has not yet been taken into consideration in recent seismic building codes and general engineering applications. In this study, the natural alluvial basin near the North Anatolian Fault in Gemlik, Marmara region, Turkey, was investigated by 40 seismic site tests and 4 validation borings. The 2D and 1D nonlinear response history analyses in the north-south and east-west directions in the Gemlik basin were performed by numerical models on a finite difference scheme considering nonlinear elastoplastic material behaviors and geometric discontinuities. Twenty-two strong ground motions recorded at the rock site were excited vertically as SH waves. The numerical results showed that narrow basin effects were derived not only by reflection, refraction, and shifting behavior but also by the focusing and superposition of the seismic waves propagating from opposite basin edges. As a result, the site-specific spectral aggravation factors SAF(2D/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 in the aggravation factors were observed as values increasing to 1.2-2.2 near the edge and at the basin center.