Fig. 8

a Comparison of peak frequencies between surface-to-borehole spectral ratios for horizontal components (H-to-H) (x axis) and vertical components (V-to-V) (y axis) during weak-motions. b Comparison of peak frequencies between surface-to-borehole spectral ratios for vertical components (V-to-V) during weak-motions (x axis) and strong-motions (y axis). The data points are color-coded by DNL values computed based on vertical component spectral ratios (V-to-V). Some data points that deviate largely from the diagonal line in b are provided with their site codes close to the data points