SITE EFFECTS OF SHALLOW AND WIDE BASIN

The site effects relating to the amplification of ground motion under earthquake loading are strongly influenced by both the subsurface soil condition and geologic structure. In this study, the site effects at the Gyeongju area in Korea were examined by site investigation including borehole drilling and in-situ seismic tests. Geologic information of ground surface obtained by site visit and collected pre-existing site investigation data were also accumulated into geo-knowledge database. Subsurface of Gyeongju area with abundant historical earthquake events is composed of alluvial soil deposit of a maximum of 40 m thickness overlying weathered residual soils and rock bed. A Geotechnical Information System (GTIS) based on GIS framework were implemented to effectively find out spatial geologic structure of study area and it indicated Gyeongju basin has a shallow and wide shape. 2-dimensional finite element (FE) analyses for two typical cross sections of the study area were performed and 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 due to the surface waves generated by interactions of shear waves with basin geometry. Furthermore, 1-dimensional FE seismic response analyses for representative soil sites in the basins were additionally conducted, and it gives identical results with the 2-dimensional seismic responses at most locations in the basins with the exception of the locations near the basin edges, because the basins in this study are very shallow and wide