This study examines the dynamic seismic response of the Afyonkarahisar city center basin, situated within the seismically active Afyon–Akşehir Graben in western Turkey. The basin, approximately 250 m deep and 10 km wide, is composed of low-velocity Quaternary alluvial deposits overlying high-velocity Paleozoic bedrock, creating a pronounced velocity contrast that affects seismic wave propagation. Geological and geotechnical properties were characterized through borehole logs, geophysical surveys, and laboratory tests. A two-dimensional nonlinear seismic response model was developed along an east–west cross-section derived from topographic and geological data, incorporating slope variations and bedrock depth changes. An adaptive finite element mesh was applied near steep slopes to capture complex phenomena such as diffraction and focusing, while free-surface and absorbing boundary conditions minimized artificial reflections. Two real earthquake records were used: the 2002 Sultandağı (Mw 6.5) and the 1995 Dinar (Mw 6.1) events, applied at the bedrock base to simulate low-amplitude (near-linear) and high-amplitude (nonlinear) site responses. Acceleration time histories were extracted from multiple basin locations, and peak ground acceleration (PGA) and spectral acceleration (SA) values were computed. Results reveal significant spatial variability in amplification, controlled by basin geometry, alluvial thickness, and input motion characteristics. For smaller-magnitude motions, amplification was greater at the basin center, whereas for larger magnitudes, edge amplification dominated due to wave trapping, diffraction, and lateral interference. These findings highlight the need to jointly consider earthquake magnitude, site geology, and basin-edge effects in seismic hazard assessments, with special attention to edge zones in engineering design for similar sediment-filled basins.