## 5 Results and Discussion
### Comparison of the Results with the Reg. Guide 1.60  
As shown in Figure 11, we compared the mean and mean plus 1\(\sigma\) response spectra for the horizontal and vertical components of the eight seismic stations based on the ground motions observed in the Jeju region to that of the Reg. Guide 1.60, which mandates separate standards for vertical and horizontal ground motions and is used as the seismic design standard for nuclear power plants and related facilities in Korea, the US, and other nations. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Standards**} & \multicolumn{2}{c}{**V/H Ratio**} \\ \cline{2-3}  & **High Frequency** & **Low Frequency** \\  & **(Short Period)** & **(Long Period)** \\ \hline Eurocode 8 (Type 1) & 0.9 & 0.9 \\ Eurocode 8 (Type 2) & 0.45 & 0.45 \\ USNRC & 1 & 2/3 \\ ASCE 4–16 & 2/3 & 2/3 \\ NEHRP & 0.7 & 1/2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The vertical-to-horizontal (V/H) ratios of seismic standards. Figure 9: Mean and individual V/H ratios of the 8 seismic stations. The \(x\)-axis represents frequency and the \(y\)-axis the V/H ratios. According to the internal regulations, the vertical spectra of Reg. Guide 1.60 are scaled to two-thirds that of the horizontal spectra. While we analyzed the response spectra of up to 50 Hz, Reg. Guide 1.60 only covered bands up to 33 Hz. To compare the observed response spectra to the design standards of Reg. Guide 1.60, both the horizontal and vertical spectra of Reg. Guide 1.60 were scaled down to 0.3 g, corresponding to the level of the design earthquake (the Safe Shutdown Earthquake [SSE]) of all Korean nuclear facilities. The response spectra for both components were significantly lower than those of the design acceptance guidelines in the low-frequency band (0.6 Hz or less), indicating that the design standards are adequately conservative. However, for the frequency bands ranging from 0.6 Hz or higher, the response spectra for both components exceeded those of  
Figure 11: Comparison of normalised response spectra at 8 stations with the horizontal and vertical response spectra of Reg. 1.60 scaled to 0.3 g. The \(x\)-axis represents frequency and the \(y\)-axis is the amplification factor, equivalent to the response spectrum acceleration. Figure 10: Mean V/H ratios of the 8 seismic stations for three groups of earthquake magnitude: M\({}_{\rm L}\) 6.0–7.3 (black solid), 5.3–5.8 (red solid), and 4.6–5.0 (blue solid), respectively. The \(x\)-axis represents frequency and the \(y\)-axis the V/H ratios. Reg. Guide 1.60 scaled to 0.3g, indicating that the current seismic design standards for the specified frequency bands are not adequately conservative, particularly in the Jeju area. In contrast, [PERSON] and [PERSON] (2016) from Fukuoka events series, [PERSON] et al. (2018) from Pohang event series (ML5.4 or less), and [PERSON] et al., (2019) from Gyoungju event series (ML 5.8 or less) demonstrate that the horizontal response spectra exceeded the seismic design standards with the same scale of 0.3 g and in frequency bands greater than \(\sim\)7 Hz ([PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016). As suggested in the previous section, the exceeded response spectra in the frequency bands from 0.6 Hz or higher could be attributed to the seismo-tectonics and shallow crustal characteristics of the Jeju area. The higher responses than standard for broad ranges of frequencies are presumed to be caused by the large elastic impedance contrast between the fairly thick low-velocity marine sediment layer interposed in the shallow crust ([PERSON] and [PERSON], 2012) and the basaltic/granitic bedrock ([PERSON], et al., 2007) in the Jeju island, however, more studies are needed ([PERSON] et al., 2016; [PERSON] et al., 2016). Furthermore, compared to the design acceptance guidelines, the response spectra from all eight stations showed relatively stronger amplitudes in frequency bands greater than 25 Hz, which mainly affected small devices attached to walls having short resonance periods.