Development of new site classification system for the regions of shallow bedrock in Korea

Abstract

In the current Korean code, the site classification and the corresponding site coefficients were determined similar to the UBC 1997, which were revised based on the results from the investigation of the 1989 Loma Prieta earthquakes in the western United States. Since the site effects are directly dependent on the local site conditions, the use of site coefficients derived from earthquake records which occurred in different site conditions may mislead the earthquake ground motion. Therefore, it is important to compare the local site conditions in Korea to those in the western United States and assess their effects on site coefficients, for the reliable estimation of earthquake ground motions in Korea. In this paper, site response analyses were performed based on equivalent linear technique using the shear wave velocity profiles of 162 sites collected around the Korean peninsula. The site characteristics, particularly the shear wave velocities and the depth to the bedrock, are compared to those in western United States. The results showed that the site-response coefficients based on the mean shear velocity of the top 30 m (V-S30) suggested in the current code underestimates the motion in short-period ranges and overestimates the motion in mid-period ranges. Also the current Korean code based on UBC is required to be modified considering site characteristics in Korea for the reliable estimation of site amplification. From the results of numerical estimations, new regression curves were derived between site coefficients (F and F) and the fundamental site periods, and site coefficients were grouped based on site period. The standard deviations of the proposed method were reasonable compared to site classification based on V-S30. Finally, new site classification system was tentatively recommended based on site periods for regions of shallow bedrock depth in Korea.