Assessment of site effects of a shallow and wide basin using geotechnical information-based spatial characterization

The site effects relating to the amplification of ground motion under earthquake loading are strongly influenced by both the subsurface soil condition and the geologic structure. For site characterization at the Gyeongju area in Korea, in this study, site investigations were carried out, including borehole drilling and in-situ seismic tests. In addition, geologic information of ground surface obtained by site visits and pre-existing site investigation data were combined into a geo-knowledge database. The subsurface of this area, which has a history of abundant historical earthquakes, is composed of an alluvial soil deposited up to 40 m thick, overlying weathered residual soils, which rest on bedrock. A geotechnical information system (GTIS), based on a geographic information system (GIS) framework, was implemented to effectively determine the spatial geologic structure of the study area. The results indicated that the Gyeongju basin has a shallow and wide shape. Based on this GTIS, two-dimensional (2D) finite-element (FE) analyses were performed to evaluate the site effects related mainly to the subsurface geologic structure of two typical basin sections in the study area. The resulting seismic responses show that the earthquake ground motions were amplified during the propagation of shear waves through the soil layer overlying the bedrock, and the duration of shaking near the basin edges was prolonged owing to the surface waves generated by interactions of shear waves with basin geometry. Furthermore, one-dimensional (1D) FE seismic response analyses for representative soil sites in the basin were additionally conducted. The results of the 2D seismic responses at most sites in the basin are similar, except for the sites near the basin edges, because the basin in this study is very shallow and wide.