A National VS30 Model for South Korea to Combine Nationwide Dense Borehole Measurements With Ambient Seismic Noise Analysis

The average shear‐wave velocity within the top 30 m from the surface, V S 30 , represents site characteristics including the soil classification and site amplification that are essential information for building codes and seismic design. A novel method to determine a V S 30 model based on a composite analysis of borehole standard penetration test numbers (SPT N) and horizontal‐to‐vertical (H/V) spectral ambient noise ratios is introduced. A national V S 30 model for South Korea is determined using the method. The shear‐wave velocity structures beneath 20 nationwide broadband seismic stations are determined using the H/V analysis. The SPT N data are collected from 175,619 nationwide densely‐distributed boreholes. The shear‐wave velocity models from SPT N values are calibrated for the local reference velocity models from H/V analysis. A representative relationship between the SPT N values and shear‐wave velocities is introduced. A national V S 30 model for South Korea is determined using the calibrated SPT N models at the nationwide boreholes. The V S 30 model is verified by comparisons with local field measurements. The proposed model is consistent with the USGS model based on a surface slope analysis. The V S 30 structure presents high correlation with geological and topographic features. The V S 30 values are low in coastal (low topographic) areas, and high in mountain (high topographic) areas. Apparent linear relationship is observed between V S 30 and topography. The western and southeastern coastal regions may be vulnerable to strong seismic shaking.