# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 4 Results
### GOS Site
The Vs profile revealed a marked increase in velocity at depths of 70-90 m, with the velocities ranging from 500 to 700 m/s. This sharp velocity increase can be interpreted as a transition from surface sedimentary outcrops to a rock mass. Utilizing the site classification from the national building code based on the shear-wave velocity, as delineated in Table 5, the outcrops at the GOS sites with velocities of 500-700 m/s were classified as S\({}_{\text{C}}\). Beyond this, the velocity, ranging from 1300 to 1500 m/s, remained relatively constant up to depths of 230-250 m [44]. The third layer started at depths of 390-410 m, exhibiting significantly lower velocities (700-900 m/s) compared to the upper layers, indicative of the presence of the LVL. The identification of the LVL in the Vs profile represents one of the principal objectives of this study. Numerous previous studies have identified the LVL in Jeju, correlating it with marine-origin sediments of both the U-formation and the overlying Seoguipo sedimentary formation. [PERSON] et al. [26] observed that marine sediments (U and Seoguipo formations) extensively overlay granitic basement rock across Jeju. Magnetotelluric (MT) studies [31], deep-resistivity soundings [32], and earlier MT surveys [27] have typically revealed a three-layered model: a resistive superstrate, a notably conductive substrate (with a thickness of approximately 100 m or more, varying by site), and a high-resistivity layer (basement). These conductive layers were consistently identified with marine-origin sediments, as confirmed by [PERSON] et al. [26]. With increasing depth, there was a rapid doubling of the velocity observed at depths of approximately 590-610 m and 690-710 m. The latter depth range marks the onset of the deep basement. In evaluating the frequency of the mapping models, along with the average RC values derived from the two different algorithms and the broad ranges of variances against the observed HVSR curve, several significant RC values were noted, as outlined in Table 7. These included four large positive peaks (0.43, 0.11, 0.20, and 0.27) and one large negative peak (\(-\)0.30). The first substantial positive RC, 0.40, was associated with the transition from the surface outcrop to the rock mass or surface bedrock, extending to a depth of 230-250 m. Note that larger RC values indicate a greater velocity differential across layer boundaries. Figure 5: Estimated shear-wave velocity (Vs) profile from the inversion of GOS site; the thin lines indicate all the inversion results, and thick gray line indicates the optimal Vs profile. At a depth of 230-250 m, the third layer begins, marked by a substantial negative RC of \(-\)0.30, indicative of the LVL. The Vs profile further revealed two additional velocity increments at depths of 390-410 m (RC = 0.11) and 590-610 m (RC = 0.20). Ultimately, as the depth extends and merges into the deep basement at approximately 690-710 m, an RC of 0.27 was observed. The depth, sign, and magnitude of these RC values aligned well with the transitions, velocity jumps, and the LVL identified in the site profile of the GOS. Therefore, the final velocity model for the GOS site was interpreted as comprising six layers, including the deep basement. This interpretation was based on a comprehensive assessment of not only the modal models that were frequently mapped, as denoted by their relatively darker color, but also the average RC values, as depicted in Figure 5. The Vs profile suggests the existence of several distinct layers, including the surface sedimentary layer, shallow rock masses, LVL, and the deep basement.