The average shear-wave velocity within the top 30 m from the surface, V_S30, 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_S30 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_S30 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_S30 model for South Korea is determined using the calibrated SPT N models at the nationwide boreholes. The V_S30 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_S30 structure presents high correlation with geological and topographic features. The V_S30 values are low in coastal (low topographic) areas, and high in mountain (high topographic) areas. Apparent linear relationship is observed between V_S30 and topography. The western and southeastern coastal regions may be vulnerable to strong seismic shaking.

This data set is concerned with the paper "A national V_S30 model for South Korea to combine nationwide dense borehole measurements with ambient seismic noise analysis" submitted to Earth and Space Science by Byeongwoo Kim and Tae-Kyung Hong in 2021. We provide data files to produce seven figures in the paper: Figures 3, 8, 9, 11, 14, 17, and 21. All data files are given in the ASCII format.