# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 4 Results
### JJU Site  
The JJU site, also located in the northern region and similar to the JJB site, is depicted in Figure 1. Figure 10 presents the observed and synthetic HVSR curves for the optimal Vs model. The observed HVSR curve demonstrated a prominent peak at a frequency of 0.42 Hz, with a corresponding peak amplitude of 3.88, as noted in Table 4. For the variance bounds ranging from 0.25 to 2.5, the misfit sizes from the MC simulations varied from 342.74 to 3.33. The SA algorithm similarly produced misfit sizes for the best-fit inversion model ranging from 316.85 to 3.15 over the same variance bounds. As observed at the previous sites, the misfit size decreased with increasing variance bounds for both algorithms. Despite the differences in the inversion algorithms and the extensive range of permissible variances, the observed and synthetic HVSR curves showed a strong correlation, with the exception of certain frequency ranges (3-9 Hz). Figure 11 displays the Vs profiles at the JJU site, showcasing both the inverted and optimal Vs models. In determining the optimal shear velocity profile, greater emphasis was placed on the darker segments, which represent the more frequently mapped areas. The first layer formed a velocity range of 700-900 m/s up to a depth of 160-180 m. Utilizing the site classification method based on the surface shear-wave velocity, as indicated in Table 5, the outcrops at the JJU sites (700-900 m/s) were classified as S\({}_{\text{B}}\). The second layer, with velocities of 1300-1500 m/s, extended from 160-180 m to depths of 290-310 m. The third layer began with a slightly lower velocity of 900-1100 m/s, averaging a reduction of 400 m/s, and extended to depths of 490-510 m. This layer, exhibiting lower velocities, was consistent with the typical LVL identified across the Jeju region compared to the upper and lower strata. Finally, the velocity significantly increased to 2300-2500 m/s at depths of 690-710 m, indicative of the deep basement [45]. Figure 10: Observed and synthetic HVSRs of JJU site; the red line indicates the observed curve, and the black dotted line indicates the synthetic curve. The average RC values at JJU, as outlined in Table 10, consisted of three large positive values (0.30, 0.26, and 0.17) and one large negative RC (\(-\)0.17). The initial large positive RC of 0.30 correlated with the transition from the surface outcrop to the rock mass or surface bedrock. At depths of 290-310 m, a substantial negative RC of \(-\)0.17 suggests the presence of the LVL. This was followed by another large positive RC (0.26) at depths of 490-510 m. Ultimately, as the depth increased, there was a transition to the basement at approximately 690-710 m, indicated by an RC of 0.17. The final velocity model at JJU was interpreted as comprising five layers, inclusive of the deep basement, based on an analysis that encompassed both the models frequently represented in darker colors and the average RC values, as depicted in Figure 6. Similar to the other three sites previously discussed, a low-velocity layer was identified at the JJU site. When comparing the JJB site with the JJU site, both situated in the northern region, notable differences in the LVL were observed. At the JJB site, the LVL begins at depths of 540-550 m, with a thickness of approximately 100 m and velocities ranging from 600 to 800 m/s. Conversely, at the JJU site, the LVL starts at depths of 490-510 m, spans around 200 m in thickness, and exhibits velocities of 900-1100 m/s. Although the starting depths of the LVL at both sites are relatively similar, there are discernible differences in both thickness and velocity. Figure 11: Estimated shear-wave velocity (Vs) profile from the inversion of JJU site; the thin lines indicate all the inversion results, and thick gray line indicates the optimal Vs profile.