# 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
### Seismic Zonation Based on Site Response Parameters in Seoul  
When only the borehole data within the administrative boundaries of the Seoul region are displayed on the terrain surface information using the geotechnical DB, the geo-spatial grid information, such as the geotechnical layer information, was constructed with the optimized geostatistical interpolation method (Figure 2). Seoul has a relatively large amount of data compared to other regions of interest, as well as a relatively even data distribution level. In this study, the spatial prediction of site response parameters and the spatial reliability assessment for various regions of interest in South Korea, other than Seoul, were performed based on the constructed geotechnical DB. In addition, various site response parameters were derived and a correlation analysis was performed for these parameters. It was found that Seoul generally exhibited higher correlation between parameters. Accordingly, Seoul was selected as the base region for deriving an effective evaluation technique for the site-specific site seismic response based on its data and spatial computation results and the geo-spatial grid information was secured and analyzed.  
For Seoul, site response characteristics were evaluated and presented as spatial information based on the geotechnical DB [23]. Figure 3 shows bedrock depth (\(H\)), spatial distribution of site period (\(T_{G}\)) and site classification information according to the site period as major site response parameters presented as fundamental information to be used for seismic strategies of Seoul. Bedrock depth was generally shallow in mountains and hills and site period was therefore evaluated to be low. As a result, the site classification B, in which ground motion amplification does not occur, was dominant. On the other hand, plain land adjacent to the Han River and its tributaries was found to have site classifications C and D with large ground motion amplification. In particular, some water-joining regions and large-scale landfill regions of the western part of Seoul, which corresponds to the lower part of the Han River, were identified as site classification E with the largest site amplification. Such spatial information was predicted and constructed at a grid square interval of 100 m \(\times\) 100 m in the Seoul administrative area [18]. This study aimed to derive an efficient region-specific site classification technique by calculating the site response parameters for the geo-spatial grid in Seoul and identifying the correlations between these parameters.  
Figure 3: _Cont._The interdependent tendencies and correlations between site response parameters related to site effects were compared and analyzed. Among such correlations, the relationship between \(V_{S30}\) and \(T_{G}\) or fundamental frequency (\(f_{0}\)), which corresponded to unified reference parameters of the site classification system, was quantitatively analyzed and preliminary analysis for developing a site-specific empirical site classification technique was conducted. Figure 4 is a comparative correlation to determine the relationship between \(T_{G}\) and \(V_{S30}\). In addition, Figure 5 shows the relationship between \(f_{0}\), which has a reciprocal relationship with the site period and \(V_{S30}\). A relationship between the two parameters was confirmed, as could be derived from the relationship between \(T_{G}\) and the site period. Therefore, an empirical method for an efficient site classification system based on the site period (or fundamental frequency), which can reasonably represent the site-specific geotechnical characteristics of South Korea, was derived.  
Figure 3: Site response spatial information for Seoul: (**a**) bedrock depth, (**b**) site period, (**c**) site classification (modified after [PERSON] et al. (2018)).