# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 5 Discussion  
This study aimed to confirm the presence of an LVL across various sites in the Jeju area. The inversion analysis results substantiated the existence of an LVL at all eight sites examined, aligning with findings from previous research, with the exception of the CJD  
Figure 19: Estimated shear-wave velocity (Vs) profile from the inversion of UDO site; the thin lines indicate all the inversion results, and thick gray line indicates the optimal Vs profile. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Layer**} & \multirow{2}{*}{\begin{tabular}{c} **Lower Limit Depth of** \\ **Each Stratum (m)** \\ \end{tabular} } & \multirow{2}{*}{**Shear Velocity (m/s)**} & \multirow{2}{*}{**\begin{tabular}{c} **RC** \\ **B (1–2)** \\ \end{tabular} } & \multirow{2}{*}{**\
\begin{tabular}{c} **RC** \\ **B (2–3)** \\ \end{tabular} } & \multirow{2}{*}{**
\begin{tabular}{c} **B (3–4)** \\ \end{tabular} } & \multirow{2}{*}{**B (4–5)**} & \multirow{2}{*}{**B (5–6)**} \\ \hline
1 & 140–160 & 400–600 & & & & \\
2 & 240–260 & 900–1100 & & & & \\
3 & 390–410 & 1200–1400 & & & & \\
4 & 590–610 & 1500–1700 & & & & (\(\sim\) 0.23 \\
5 & **790–810** & **900–1100** & & & & 0.38 \\
6 & Bedrock & 2100–2300 & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 14: Depth, shear velocity (Vs), and RC of UDO sites. site. The analysis of the multilayer structure revealed that the velocity of the LVL varied substantially, ranging from 600 to 1100 m/s, and was distributed across depths of from 320 to 810 m from the surface, as detailed in Tables 7-14. The identification of the LVL at the sites in the eastern, western, southern, and northern regions of Jeju confirms its widespread presence throughout the area. Note that of the eight sites considered in this research, two are islands, and six are located closer to the coast than those examined in previous studies. Regarding the SSP site, most prior studies have suggested that both the U-formation, originating from marine deposits, and the Seoguipo sedimentary layer exhibit low velocity, gravity, and resistivity. These characteristics are likely related to the LVL due to their shared sedimentary origin [28; 30; 32; 46]. However, some researchers have posited that the area east of the Bukcheon-Pyoseon line lacks a Seoguipo sedimentary layer [31; 47]. As indicated in Table 4 and Figure 17, the SSP site exhibited the highest resonance frequency (0.43 Hz) and largest resonance amplitude (4.99) among all the eight sites in the Jeju region, including the CJD site. Additionally, the LVL at the SSP site was located at a relatively shallow depth (320-340 m below the surface) and had the lowest Vs values (600-800 m/s). These characteristics, especially the proximity of the LVL to the surface and its notably large resonance amplitude, are likely directly related to the decision by the KMA to close this site to minimize waveform distortion in seismic monitoring. The UDO site, situated east of the Bukcheon-Pyoseon line like the SSP site, falls within a region where previous studies did not anticipate the presence of a Seoguipo sedimentary layer. Consequently, the low-velocity layer (LVL) identified at the UDO site is likely a formation of marine origin, akin to the U-formation. However, when compared to other sites, the UDO site exhibits unique stratigraphic features, as detailed below (refer to Table 14 and Figure 19):  
1. The UDO site has the deepest basement among the eight sites examined, with the lowest basement velocity, ranging from 2100 to 2300 m/s. 2. The LVL at the UDO site shows a significant velocity reduction of approximately 600 m/s compared to the layer above it and possesses a relatively high thickness of around 200 m.
3. Notably, the LVL is situated directly above the basement, whereas the layer immediately above the basement across the Jeju region is typically a tuff layer that is present throughout the Jeju region.