The Cephalonia Island (Greece) area is located in the north-western end of the Aegean subduction frontal thrust that is linked to the dextral Cephalonia Transform Fault (west of Cephalonia). As the mean slip rate and the length of the Cephalonia Transform Fault are large, the seismic hazard is high in terms of earthquake frequency and magnitude. The Plio-Quaternary Koutavos-Argostoli basin site was selected within the French Research Agency PIA SINAPS@ project (www.institut-seism.fr/projets/sinaps/) to host a vertical accelerometer array. The long-term goal is to validate three-dimensional nonlinear numerical simulation codes to assess the site¬specific amplification and nonlinearity in the framework of seismic hazard assessment. Geological and geophysical surveys were carried out from 2011 to 2017. The aim of this work is to present i) the geological and structural context, ii) the complementary geological, geotechnical, and iii) the geophysical investigations that led to the identification of the main geotechnical bodies and their structures. The characterization of the three-dimensional structure of the stratigraphic units was achieved by coupling geological cross-sections (i.e., depth geometry) and HVSR/AVA geophysical surveys that were designed to identify the sedimentary body thickness. The data gathered allowed to build a three-dimensional geological model of the Argostoli basin. Starting from this point, a 3D numerical model on the same area was constructed using spectral element code known as SEM3D. Several simulations of seismic scenarios were performed, for the purpose of highlighting the importance of the use a representative model of the complexity of the wave path on the hazard assessment.