# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 4 Results
### CJD Station  
Figure 2 presents the observed and inverted HVSR curves for the CJD site, situated in one of the seismotectonic provinces of the main Korean Peninsula and located on a small island near the southwestern part of the peninsula (refer to Figure 1). The observed curve is depicted by the red line, and the black dotted line represents the synthetic curve for the optimal velocity model derived from the inversion results. The alignment between the observed and inverted results was notably close. Although there was a minor discrepancy in the low-frequency band, this difference was minimal, particularly when \begin{table}
\begin{tabular}{c c c} \hline \hline
**Site Name** & **Resonance Frequency (Hz)** &
\begin{tabular}{c} **Maximum Amplitude at** \\ **Resonance Frequency** \\ \end{tabular} \\ \hline CID & N/A & N/A \\ GOS & 0.36 and 1.46 & 4.33 and 3.39 \\ HALB & 0.37 & 3.70 \\ JJB & 0.35 & 4.34 \\ JIU & 0.42 & 3.88 \\ MRD & 0.43 & 3.36 \\ SGP & 0.42 & 3.88 \\ SSP & 0.43 & 4.99 \\ UDO & 0.33 & 3.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Resonance frequency and maximum resonance amplitudes at the 9 sites. considering the expanded y-scale and compared to the other sites examined in this study, as discussed subsequently. Both the MC and SA algorithms were utilized to invert the HVSR curves with a wide range of variance bounds (0.25-2.5) against the observed HVSR curves. Utilizing an initial seven-layer model, the MC algorithm revealed a misfit ranging from 3.84 to 22.79 across the entire frequency spectrum (0.1 Hz to 15 Hz), with the misfit value increasing drastically as the variance bound diminished. Similarly, the SA algorithm exhibited misfit values that escalated from 3.49 to 71.88 as the variance bound was reduced from 2.5 to 0.25, with the minimal misfit occurring at a variance bound of 2.5. Despite differences in the inversion algorithms and the extensive range of variances, the inversion results were similar in both the shape and amplitude of the HVSR curves, affirming the stability and reliability of the derived global optimal parameters. In certain instances, HVSR curves may exhibit no pronounced peaks or may display amplitudes smaller than those observed at other sites across the entire frequency range, as exemplified by the CJD site. Owing to the inherent complexities of HVSR characteristics, which complicate the accurate estimation of Vs profiles due to uncertainties in the inversion process, sites with such profiles are often excluded from the inversion analysis. Despite its tectonic and geographic distinction from the Jeju region (as illustrated in Figure 1), the CJD site was incorporated into this study to facilitate a comparative analysis of the Vs structural characteristics between two tectonically divergent regions, i.e., Jeju Island and the inland areas. As depicted in Figure 3, when applying two site classification codes (the national building code for site classification), the outcrops at the CJD sites, with velocities ranging from 1900 to 2000 m/s, were categorized as \(\mathrm{S_{A}}\) (the highest-quality outcrops) (refer to Table 5). This classification signifies that the near-surface sedimentary layer is not well-developed at this site [44]. In Figure 3, the layer profiles do not exhibit a starkly distinct velocity gradient; instead, there is a gradual increase in Vs with a subtle gradient. This pattern suggests a predominantly intact basement stratum structure extending to a depth of approximately 580-610 m. For site CJD, this weak velocity gradient might have contributed to the reduced amplitude of the HVSR across the entire frequency spectrum. Figure 2: Observed and synthetic HVSRs of CJD site; the red line indicates the observed curve, and the black dotted line indicates the synthetic curve. Furthermore, the average RC of the Vs profile (as detailed in Table 6) demonstrated ranges between 0.002 and 0.05 across the seven strata, significantly lower than one-tenth of the RCs of the sites discussed later.