# Geo-Proxy-Based Site Classification for Regional Zonation of Seismic Site Effects in South Korea
## 3 Development of Site Classification with Geo-Proxies Using Geotechnical Database in Seoul  
### Study Areas and Geotechnical Database  
To establish an empirical relationship for presenting seismic response based on site-specific geotechnical characteristics (normalized as spatial grid information), reliable quantitative calculation of site effect parameters in the target region must first be performed. This study selected the western metropolitan areas (the Seoul and Incheon regions) in South Korea, where reliable geotechnical datasets (borehole) and land coverage information (satellite images and digital elevation models) were well distributed spatially and constructed as a geo-spatial DB (Figure 2). The Seoul and Incheon regions are the largest and second-largest cities in South Korea, respectively. First, the borehole datasets and DEM in the Seoul area were applied as backbone datasets to develop the seismic zonation with seismic response parameters based on the validated classification system and to normalize the correlations between seismic response parameters and DEM for developing geo-proxy criteria. In the Seoul area, estimates of the spatial geotechnical layers across the extended area were collected from a total of approximately 22,300 existing borehole datasets and approximately 1700 surface geo-knowledge datasets. To estimate soil layers spatially, the optimized site-specific interpolation method [18] was applied to the Seoul area (53.0 km East-West, 39.0 km North-South). The Seoul area was separated into 100 m mesh areas, yielding 225,835 spatial grid squares (interpolation grid squares) including the extended area of the Seoul administrative area. Component mesh-unit data was created for each spatial grid square [25]. The subsurface soil layers identified from borehole data were classified into five categories: fill, alluvial soil, weathered soil, weathered rock and bedrock. In the river basin of the Seoul area, the alluvial soil is thicker (up to 70 m) and the depth to bedrock is deeper (up to approximately 85 m) than in the surrounding mountain areas (Table 2). Soil development in the river basin is mainly a result of fluvial landform processes. Such zones of thick soil and deep depth to bedrock are susceptible to ground motion amplification due to site effects during earthquakes. For validation of the classification system based on the site response parameters and proposed geo-proxies, the Incheon area near Seoul was selected for cross-validation. The existing borehole datasets included in the geotechnical DB in the Incheon area were obtained from approximately 7500 boreholes and the additional geo-knowledge-based geotechnical layer data [26] were acquired from 340 boreholes. The extended area in Incheon is 41 km in the east-west direction and 34 km in the north-south direction. Similar to Seoul, in order to estimate soil layers spatially, the Incheon area was separated into 100 m mesh areas, yielding 189,320 spatial grid squares (interpolation grid squares) including the extended area. As shown in Table 2, plains with coasts and rivers as well as thick alluvial soil in some hills are developed in the study area and the maximum thickness is approximately 24 m. In addition, the weathered layer, which is developed by long-term weathering, is also found in the target area and has a thickness of up to 32 m. These map data can provide intuitive information for solving geotechnical engineering problems and making decisions. Thirty-seven rock types are found in the Seoul area and their geological ages can be classified into Precambrian, unknown, Triassic, Jurassic, Cretaceous and Quaternary. Among them, distributive ratios decrease in the order of Jurassic Daebo granites, Precambrian banded gneiss from the Gyeonggi gneiss complex and Quaternary alluvium, which in total account for about 83.7% of the rock types in the area.