# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 5 Discussion
Given these distinct stratigraphic features, several geological hypotheses could explain the presence of the LVL at the UDO site, differing from the expected U-formation. The first hypothesis considers the possibility of seawater fluid injection or groundwater storage due to the proximity of the site to the coast, although this is unlikely due to the considerable depth of the LVL. A second possibility involves the development of a local fault or fracture zone, yet this is deemed improbable due to the lack of significant orogen in the area. The third and most plausible hypothesis attributes the LVL to local intrusive magma that is yet to fully consolidate. Considering the frequent occurrence of intrusive magma injections in the Jeju region over extended geological periods, it is highly likely that the LVL at the UDO site was formed by a recent local magmatic intrusion rather than by a U-formation of marine origin. Future research may provide further insights into this topic, potentially yielding a clearer understanding of the unique characteristics of the UDO site. The findings from the eight Jeju sites, particularly the SSP and UDO sites, demonstrate that a substantial thickness of the LVL is a consistent feature across all sites, high-lighting a complex multilayer structure. Consequently, the existence of such layers may present challenges to various geophysical interpretation methods. These include borehole analysis [32], gravity and magnetic surveys [27], gravity studies [28], paleomagnetic research [29], seismic investigations [30], magnetotelluric (MT) assessments [31], and deep resistivity soundings [32]. Specifically, these challenges become evident when these methods are applied with oversimplifications, such as assuming a two-layer structure. The basement depth in the Jeju area varies, ranging from 590 to 810 m below the surface, with basement velocities of 2100-2700 m/s, as detailed in Tables 7-14. This variability is especially pronounced when excluding the UDO site, which has a depth of 790-810 m. Excluding the UDO site, the basement depth of the remaining seven sites lies within a relatively narrow range of 590-710 m. This uniformity in the basement depth was observed across all Jeju sites, with the exception of the UDO site. Notably, only the CJD site, which possesses a geologically distinct tectonic structure, was included for comparative purposes. The Vs profiles indicate significant differences between the eight Jeju sites in terms of the shear velocity and the presence or absence of an LVL structure, thereby enhancing the reliability of this study. The resolution of the highly ill-posed inversion process in this study was achieved through the implementation of four key strategies. These strategies included the application of two distinct inversion algorithms, the allowance of broad variance ranges in the observed HVSR data, the determination of an optimal initial parameter value range based on previous studies, and the inclusion of a preliminary random search phase. These innovative strategies effectively addressed the challenges inherent in the highly ill-posed inversion process, leading to satisfactory results. In addition, this study explored the interpretation of various Vs models by employing two specific approaches. These approaches comprised a weighting scheme for the more frequently mapped darker profiles as well as the use of the RC index. Combining these tactics facilitated the attainment of an optimal Vs model with a high degree of consistency.