Site characterization on geologic and soil conditions was performed for evaluating the site effects relating to the site-specific seismic response characteristics at a small urbanized area, Hongseong, in Korea, where structural damages were recorded by an earthquake of magnitude 5.0 on October 7, 1978. In the field, various geotechnical site investigations composed of borehole drillings and seismic tests for determining shear wave velocity (Vs) profile were carried out at 16 sites. Based on the geotechnical data from site investigation and additional collection in and near Hongseong, an expert information system on geotechnical information was implemented with the spatial framework of GIS for regional geotechnical characterization across the entire study area. For practical application of the GIS-based geotechnical information system to estimate the site effects causing seismic hazards for the Hongseong area, spatial seismic zoning maps on geotechnical parameters, such as the bedrock depth and the site period, were created at the area of interest. Furthermore, seismic zonation of site classification according to the mean Vs to a depth of 30 m was also performed for seismic design at any location in the area of interest. From the spatial geotechnical information and seismic zonations, the capability of seismic amplification was examined at plain and hill locations in Hongseong.