# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 6 Conclusions  
1. An LVL was conclusively identified at all the eight sites surveyed across the eastern, western, southern, and northern regions of the Jeju area. However, the origin of the LVL at the UDO site appears to differ from the other sites. The velocity of the LVL across these sites spans from 700 to 1100 m/s, with an average thickness of approximately 160-300 m. This finding highlights the widespread presence of the LVL throughout the Jeju region, albeit with variations in its geological origin, particularly at the UDO site. 2. As discussed in this study, the unique stratigraphy of the UDO site contrasts with prior assumptions regarding the Seoguipo sedimentary layer. This contrast suggests the existence of a low-velocity layer potentially formed by recent local magmatic intrusion events rather than the anticipated marine-origin U-formation. This hypothesis represents a significant departure from earlier understandings and warrants further exploration in the discussion section. The implication that the LVL at the UDO site might be a result of geologically recent processes, rather than sedimentary deposits, opens up new avenues for understanding the geophysical characteristics of the region. 3. The basement depth in the Jeju region varies from 590 m to 810 m below the surface, with a broad range of values observed. Excluding the UDO site, the basement depths of the other seven sites fall within a more confined range of 590-710 m, demonstrating a relatively narrow range of basement depths. The CJD site, characterized by a geologically distinct tectonic structure, was included primarily for comparison. The Vs profile at the CJD site distinctly differed from the eight Jeju sites in terms of the shear velocity and the absence of an LVL structure, thereby enhancing the reliability of this research. 4. The resolution of the highly ill-posed inversion process was successfully achieved through the implementation of four key strategies. These included the application of two different inversion algorithms, the allowance of broad variance ranges in the observed HVSR curves, the determination of optimal initial parameter value ranges based on prior studies, and the execution of a preliminary random search. Furthermore, this study employed a weighting scheme for more frequently mapped darker profiles and utilized the RC index for interpreting various Vs models. This approach enabled us to achieve an optimal Vs model characterized by a high consistency. 5. This research has yielded valuable geological insights, extending beyond the shallow layers to the basement at depths greater than 700 m. Such comprehensive data significantly contribute to microzonation analysis in the Jeju region, including site amplification and site attenuation (kappa value), thereby aiding in the development of seismic design codes or standards. The findings from this study are instrumental in enhancing our understanding of the overall seismic hazards in the Jeju region. 6. In this study, the number of layers at the nine sites was interpreted to range between four and six layers, thus validating the initial assumption of seven layers as a reasonable starting point. However, the geological aspect of stratigraphic diversity should be considered, and the number of parameters or layers should be minimized for the efficiency of the inversion process. Therefore, determining the appropriate maximum number of layers before inversion through numerous initial trial-and-error inversion processes is of significant importance. Conceptualization, J.K. and G.N. ; methodology, J.K.; software, J.K.; validation, G.N. and H.J. ; formal analysis, J.K.; investigation, G.N. ; resources, H.J. ; data curation, D.P. ; writing--original draft preparation, J.K.; writing--review and editing, D.P. and H.J. ; visualization, D.P. ; supervision, G.N. ; project administration, H.J. ; funding acquisition, D.P. All authors have read and agreed to the published version of the manuscript.