Development and verification of site classification system for design ground motions at regions of shallow bedrock

Abstract

In Korea which belongs to the region of low or moderate seismicity, the Korean seismic codes use the site classification system and the corresponding site coefficients that are similar to the UBC 97 and 1997 NEHRP provision which were revised based on the earthquake records in the western United States. However, the use of site coefficients derived from earthquake records which occurred in definitely different site conditions as well as seismicity conditions can result misleading in the motions of ground and structures during earthquakes. In many countries, the soil parameter $V_{s,30}$, which indicates a mean shear wave velocity on the top 30m deep soil, has been widely adopted as a parameter for the site classification. This site classification system is acceptable for the regions with relatively deep bedrock and the soil sites where the transition from soil to hard rock is gradual, which is common geologic condition in the western US. However, it is not acceptable for the regions where the transition from soil to stiff rock is abrupt and the bedrock is located at a depth less than 30m from the ground level, which is common in Korea. Therefore, the $V_{s,30}$ is calculated fallaciously by extrapolating together the undesirable $V_s$ of bedrock to 30m and thus, a higher $V_{s,30}$ is mistakenly assigned to the soil site with a shallow bedrock. For this reason, a suitable site classification and the corresponding site coefficients which can address the local site conditions of shallow bedrock, are required for the reliable estimation of earthquake ground motions in this regions. Site response analyses were performed by using equivalent linear analysis technique for the local geologic and dynamic site characteristics of 125 sites collected in Korean Peninsula. Based on the results of site response analyses, two-parameters site classification system considering separately the depth to bedrock ($\It{H}$) and mean shear wave velocity of soil above bedrock $(V_{...