Site characterization combining in-situ borehole blow counts and seismic spectral ratios: application to the Korean Peninsula

Abstract:

The average shear wave velocity for the upper 30 m depth, V_S30, is an essential parameter for building code and seismic design. However, the V_S30 structure in the Korean Peninsula has been poorly resolved. We develop a V_S30 model for the Korean Peninsula. We calculate the horizontal-to-vertical spectral ratio (H/V spectral ratio) of ambient seismic noises. To determine an the shear wave velocity structure beneath seismic stations, we compare the H/V spectral ratios with the theoretical site amplification based on one-dimensional equivalent linear method. We evaluate the initial structure for the theoretical amplification using the standard penetration test N values (SPT N) in adjacent boreholes. We determine the optimal shear-wave velocity structure that has minimum difference between theoretical site amplifications and H/V spectral ratios. The correlation between shear wave velocity and SPT N values are revised. We apply the process to all broadband seismic stations available in the Korean Peninsula. We collect ambient seismic noises from 20 seismic stations, and SPT N values at 175,619 boreholes. The V_S30 model is calculated using the shear wave velocity structures that converted from boreholes data. The V_S30 is relatively low in coastal regions, while relative high in inland regions. The model shows correlation with a USGS model based on topographic slope analysis. The V_S30 model is expected to be useful for systematic seismic design and soil classification.