# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 4 Results
### HALB Site  
The HALB site, situated on the western part of Jeju Island, exhibited characteristics similar to those of the GOS site (Figure 1). Figure 6 presents the observed and synthetic HVSR curves for the optimal Vs structure model at the HALB site. The observed HVSR curve demonstrated a significant peak at a frequency of 0.37 Hz, with a corresponding peak amplitude of 3.70, as noted in Table 4.  
For the inversion process using the MC simulations and variance bounds ranging from 0.25 to 2.5, the misfit sizes were observed to vary from 156.1 to 0.13, with the smallest misfit occurring at a variance of 2.5. Similarly, the SA algorithm yielded misfit sizes for the best-fit inversion model ranging from 120.4 to 0.6 over the same variance bounds. Despite \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Layer**} & \multirow{2}{*}{**Lower Limit Depth of**} & \multirow{2}{*}{**Shear Velocity (m/s)**} & \multicolumn{4}{c}{**RC**} \\ \cline{3-6}  & & & **B (1–2)** & **B (2–3)** & **B (3–4)** & **B (4–5)** & **B (5–6)** \\ \hline
1 & 70–90 & 500–700 & & & & \\
2 & 230–250 & 1300–1500 & & & & \\
**3** & **390–410** & **700–900** & & & & \\
4 & 590–610 & 900–1100 & & & & \\
5 & 690–710 & 1400–1600 & & & & 0.27 \\
6 & basement & 2500–2700 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Depth, shear velocity (Vs), and RC of GOS site. Figure 6: Observed and synthetic HVSRs of HALB site; the red line indicates the observed curve, and the black dotted line indicates the synthetic curve. the use of different inversion algorithms and the wide range of permissible variances, the observed and synthetic HVSR curves displayed a suitable alignment, especially around the peak part of the HVSR curves at frequencies of approximately 0.3 to 0.4 Hz (Table 4). Figure 7 illustrates the Vs profiles at the HALB site, displaying all inverted and final optimal Vs models. In a manner similar to the previous site, the optimal model was determined by focusing on the more frequently mapped darker areas, or the modal model. The Vs models at the HALB site indicate an abrupt reduction in velocity from 1100 to 1300 m/s to 700-900 m/s at depths of 80-100 m. This shift is interpretable as a transition from the surface rock mass to the LVL. Applying the site classification method based solely on the outcrop shear-wave velocity, as indicated in Table 5, the outcrops at the HALB sites (1100-1300 m/s) were categorized as \(\mathrm{S_{B}}\). The LVL remained nearly consistent at depths of 350-370 m. As the depth increased to 590-610 m, a velocity increment was observed, followed by an encounter with the basement at depths of approximately 690-710 m [45]. The Vs profile in Figure 7 illustrates varying degrees of uncertainty across different depth ranges, with notably greater uncertainty observed in velocities below 690-710 m compared to other depth ranges. The discrepancies in the inverse algorithms did not yield significant differences in the basement velocity. Conversely, larger variances in the observed HVSR curve included reduced uncertainties in the inversion, especially in determining velocities below depths of 690-700 m. Consequently, to diminish the uncertainty of the Vs structure, future studies should consider various methodologies, such as augmenting the variance of the observed HVSR curve to a greater extent. Table 8 includes the average RC values derived from all the inversion processes, providing additional insights into the Vs profiles. These average RC values revealed one large negative peak (\(-0.20\)) and three large positive peaks (0.20, 0.14, and 0.22). These peaks corresponded to significant contrasts in the Vs profile, including both decreases and increases in velocity.