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 (SPTN) 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. Plain Language Summary Seismic ground motions are an important factor to control the seismic damages. The seismic amplification is highly dependent on the shear-wave velocities at shallow depths <= 30 m, V-S30. We introduce a novel method to determine a V-S30 model based on the standard penetration test numbers (SPT N) from nationwide 175,619 boreholes and horizontal-to-vertical spectral ambient noise ratios (H/V ratios) from 20 broadband seismic stations. We determine a V-S30 model for South Korea. The shear-wave velocity structures beneath broadband seismic stations are used for local reference velocity models. The shear-wave velocity models from SPT N values are calibrated for the local reference velocity models from the seismic stations. We determine a representative relationship between the SPT N values and shear-wave velocities. We determine a high-resolution V(S30 )model using the calibrated SPT N models at boreholes. The proposed model is verified by comparisons with other results. The V-S30 structure presents high correlation with geological and topographic characteristics. The V-S30 values are low in coastal areas, and high in mountain areas. The western and southeastern coastal regions may be vulnerable to strong ground motions during earthquakes.
