# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 4 Results
### Sgp  
Figure 14 presents both the observed and synthetic HVSR curves for the SGP site, located in the southern Jeju region, similar to the MRD site (Figure 1). The observed HVSR curve featured a distinct peak at a frequency of 0.42 Hz, with corresponding amplitudes of 3.88, as indicated in Table 4. To assess the misfit sizes of the inversion using the MC simulations, variance bounds ranging from 0.25 to 2.5 were applied, yielding values from 10.67 to 0.36. The SA algorithm also produced misfit sizes for the best-fit inversion model ranging from 2.36 to 0.21 within the same variance bounds. In line with findings from previous sites, an increase in the variance bounds correlated with a decrease in the misfit size for both algorithms. Despite the utilization of different inversion algorithms and the broad range of variances, both the observed and synthetic HVSR curves exhibited a strong correlation, particularly at higher frequencies, with minor deviations around 10 Hz and at resonance frequencies. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Layer**} & \multicolumn{2}{c}{**Lower Limit Depth of**} & \multirow{2}{*}{**Shear Velocity (m/s)**} & \multicolumn{4}{c}{**RC**} \\ \cline{3-4}  & & & **B (1-2)** & **B (2-3)** & **B (3–4)** & **B (4–5)** & **B (5–6)** \\ \hline
1 & 60–80 & 900–1100 & \multirow{2}{*}{**0.13**} & \multirow{2}{*}{**0.22**} & \multirow{2}{*}{**0.21**} \\
2 & 130–150 & 1200–1400 & & & & \\
3 & **390–410** & **600–800** & & & \\
4 & 490–510 & 1000–1200 & & & & \\
5 & 690–710 & 1600–1800 & & & & \\
6 & basement & 2200–2400 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Depth, shear velocity (Vs), and RC of MRD site. Figure 13: Estimated shear-wave velocity (Vs) profile from the inversion of MRD site; the thin lines indicate all the inversion results, and thick gray line indicates the optimal Vs profile. The Vs profile illustrated in Figure 15 presents the observed and optimal Vs models at the SGP site, with darker sections being given higher weights and indicating higher RC values. The subsurface analysis identified distinct stratigraphic layers. The first layer, extending from the surface to depths of 260-280 m, was characterized by velocities of 1000-1200 m/s. Utilizing the site classification method based on the outcrop shear-wave velocity, as indicated in Table 5, the outcrops at the SGP sites (1000-1200 m/s) were classified as S\({}_{\text{B}}\). The second layer, which extended to depths of 480-500 m, featured a low-velocity layer (LVL) with velocities of 900-1100 m/s, slightly reduced compared to the first layer. The third layer, extending to depths of 680-700 m, marks the onset of the deep basement layer, with velocities ranging from 2100-2300 m/s [45]. This study emphasized the examination of RC values and their correlation with other layer boundaries. The average RC values at the SGP site, as shown in Table 12, included two sizeable positive values (0.20 and 0.19) and a relatively small negative value (\(-\)0.05). Notably, the negative RC value of \(-\)0.05 at the SGP site was cons  
Figure 14: Observed and synthetic HVSRs of SGP site; the red line indicates the observed curve, and the black dotted line indicates the synthetic curve. Figure 15: Estimated shear-wave velocity (Vs) profile from the inversion of SGP site; the thin lines indicate all the inversion results, and thick gray line indicates the optimal Vs profile. to those at the other sites. Furthermore, the transition in velocity from the first layer to the LVL (second layer) at the SGP site was minimal, as indicated by the small RC value. The final velocity model for the SGP site was interpreted as comprising four distinct layers, including the deep basement layer. This interpretation was formulated by thoroughly analyzing both the frequently mapped modal models and the average RC values. Comparative analyses with other sites, such as the MRD in the southern region, provided valuable insights into regional variations in both the basement depth and LVL characteristics.