# Geo-Proxy-Based Site Classification for Regional Zonation of Seismic Site Effects in South Korea
## 2 Site Classification System with Seismic Response Parameters  
### Seismic Response Parameters  
A comprehensive geotechnical DB has been constructed and expanded by collecting existing borehole datasets for major metropolitan cities and areas of interest in South Korea. This DB was designed to be utilized by geotechnical earthquake engineering and to secure additional geotechnical  
Figure 1: Dual framework for site classification system with site response parameters and geo-proxies.  
layer data from geotechnical expert-based site surveys for regions lacking in borehole datasets. In addition, it was applied in combination with geotechnical dynamic properties, such as the \(V_{S}\) of the target area, to derive and suggest site-specific seismic response information essential for establishing local strategies for a seismic mitigation plan. Figure 1 shows a schematic framework for constructing a geotechnical DB to derive site response parameter information of target regions, which have the potential for serious earthquake disasters. The site period (\(T_{G}\)) was representatively considered as a parameter and spatial zoning information on site classification was presented.  
This study first examined and established various geotechnical engineering parameters that can quantify the amplification characteristics of ground motion, which is the site-specific geotechnical response. The average \(V_{S}\) up to 30 m (\(V_{S30}\)) can be defined as a parameter that distinguishes the amplification degree of ground motion. Unlike \(V_{S30}\) that considers the quantitative geotechnical stiffness distribution of the target location, the depth to bedrock (\(H\)), which reflects geometric conditions based on geotechnical layer conditions and correlated stiffness, can be considered as a parameter. In addition, the site period (\(T_{G}\)), which can be determined using the bedrock depth as well as the geotechnical stiffness variation up to the depth among the geometric conditions of the target site, is also defined as a site response parameter. The average shear wave velocity of the bedrock topsoil layers (\(V_{S,soil}\)), which has recently emerged as a combined parameter according to the relative location of the bedrock depth, can be considered as a parameter of the site effects. Furthermore, considering that \(V_{S30}\) is an indicator that takes 30 m as the empirical depth, the average shear wave velocities with less depth (\(V_{S10}\), \(V_{S15}\) and \(V_{S20}\) or \(V_{S}Ds\)) can also be considered as site response parameters [4]. The examined site response parameters are the same variables but they can also be represented in other expressions or terms. In particular, \(T_{G}\) can be expressed as the fundamental frequency that corresponds to the resonance frequency (\(f_{0}\)) by using its inverse. The calculation equations for the site effect parameters can be summarized as follows. First, \(V_{S30}\), which is mainly applied in the current site classification system, can be calculated as shown in Equation (1). \(d_{i}\) and \(V_{Si}\) represent the thickness and average \(V_{S}\) of the \(i\)-th layer up to 30 m underground, respectively. In this case, the sum of \(d_{i}\) is 30 m.  
\[V_{S30}=30/\sum_{i=1}^{n}\frac{d_{i}}{V_{Si}} \tag{1}\]  
In addition to the existing \(V_{S30}\), various site response parameters are examined and identified, including \(T_{G}\), \(V_{S}Ds\) (\(D_{S}\) < 30 m) and \(V_{S,soil}\)[4; 6; 18]. \(T_{G}\) and \(V_{S,soil}\) can be calculated by Equations (2) and (3), respectively. \(D_{i}\) and \(V_{Si}\) represent the thickness and shear wave velocity of the \(i\)-th layer distributed over the bedrock, respectively. In addition, the site period rises as the bedrock depth (\(H\) = \(\sum D_{i}\)) increases.  
\[T_{G}=4\sum_{i=1}^{n}\frac{D_{i}}{V_{Si}} \tag{2}\]  
\[V_{S,soil}=\left(\sum_{i=1}^{n}D_{i}\right)/\left(\sum_{i=1}^{n}\frac{D_{i}} {V_{Si}}\right) \tag{3}\]