In this study, it was investigated how strong ground motions are affected by the multiple reflections, transformation, superposition, and interference with surface waves and trapped seismic waves in the near surface soft soil layer due to the bedrock inclination in narrow alluvial basins. One and two-dimensional (1D and 2D) dynamic analyses were performed to investigate the complex site effects in narrow alluvial basins that have soft soil conditions considering soil nonlinearity and changing geometrical features by numeric models simultaneously. The site classes were defined as ZD and ZE that mostly needed site-specific analysis by classification of Turkish Building Earthquake Code 2018 provisions. The response spectrums and amplifications were calculated under different levels of earthquake excitations. As a result, Spectral Intensity Factors (AGF2D/1D), which are defined for each point depending on the period (T) and the dimensionless distance from the basin edge (x/L), were calculated with the ratio of absolute acceleration spectra of 1D and 2D analyses. Thus, it is considered that the results will guidance to prevent unexpected damages and losses by calculating reliably the spectra that determine the earthquake loads on the structures, contribute to the development of earthquake maps, and ensure safe urbanization and construction.