# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 4 Results
### JJB Site  
The JJB site is situated in the northern region of Jeju Island, as illustrated in Figure 1. Figure 8 displays the observed and synthetic HVSR curves for the optimal Vs model at this site. The observed HVSR curve demonstrated a notable peak at a frequency of 0.35 Hz, with a corresponding peak amplitude of 4.34, as detailed in Table 4. For the variance bounds ranging from 0.25 to 2.5, the misfit sizes from the MC simulations varied from 106.22 to 2.09. Similarly, the SA algorithm produced misfit sizes for the best-fit inversion model ranging from 168.60 to 1.45 over these variance bounds. Consistent with the observations at previous sites, the misfit size decreased with increasing the variance bounds for both algorithms. Despite the use of different inversion algorithms and the extensive range of permissible variances, the synthetic HVSR curve for the optimal Vs model closely aligned with the observed curve, particularly around the peak frequencies of approximately 0.3 to 0.4 Hz. Figure 9 presents the Vs profiles at the JJB site, showcasing all inverted and optimal Vs models. In determining the optimal shear velocity profile, greater emphasis was placed on  
Figure 8: Observed and synthetic HVSRs of JJB site; the red line indicates the observed curve, and the black dotted line indicates the synthetic curve. \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**} & \multicolumn{4}{c}{**RC**} \\ \cline{3-6}  & & & **(m/s)** & **B (1–2)** & **B (2–3)** & **B (3–4)** & **B (4–5)** \\ \hline
1 & 80–100 & 1100–1300 & \multirow{3}{*}{**(–) 0.20**} & \multirow{3}{*}{**(–) 0.20**} & \multirow{3}{*}{**(–) 0.14**} & \multirow{3}{*}{**(–) 0.22**} \\
2 & **350–370** & **700–900** & & & \\
3 & 590–610 & 1100–1300 & & & \\
4 & 690–710 & 1500–1700 & & & \\
5 & Basement & 2400–2600 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Depth, shear velocity (Vs), and RC of HALB site. the darker segments, which represent the more frequently mapped areas. Based on all the inversion results, excluding several geologically implausible values, the Vs profile up to a depth of 70-90 m showed the first layer with a velocity of 900-1100 m/s. Employing the site classification method based on surface shear-wave velocity, as indicated in Table 5, the outcrops at the JJB sites (600-700 m/s) were categorized as \(S_{\text{B}}\). The second layer extended to depths of 240-260 m with a velocity of 1200-1400 m/s. The third layer, exhibiting a significant velocity reduction of approximately 600 m/s to 600-800 m/s, was identified as the LVL, extending from 240-260 m to 540-560 m in depth. With increasing depth, the fourth layer extended to 640-660 m, where the deep basement commences [45]. The average RC values at the JJB site, as listed in Table 9, exhibited three large positive peaks (0.13, 0.22, and 0.37) and one large negative peak (\(-\)0.30). The first significant positive RC value of 0.13, occurring at a depth of 70-90 m, corresponded to the transition from the sedimentary outcrop to the surface basement. The second largest RC, \(-\)0.30, at a depth of 240-260 m, indicates a sudden decrease in velocity, marking the encounter with the LVL. With increasing depth, the average RC was 0.22 at 540-560 m and ultimately 0.37 at 640-660 m. Consequently, the final velocity model at the JJB site was interpreted as comprising five layers, including the basement, as illustrated in Figure 5. This interpretation was derived by considering both the frequently mapped models, indicated by their relatively darker color, and the average RC values. The depth, sign, and magnitude of the RC aligned well with the depth of transition, velocity jump, and LVL at the JJB site. Although the depth range and thickness of the low-velocity layer varied among sites, the presence of the LVL was similarly identified at the JJB site.