# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 1 Introduction  
The thickness of the sedimentary cover overlaying the earth's surface and the local shear-wave velocity structure, extending to depths of approximately 2-3 km, are critical parameters in both geotechnical engineering and geosciences. These parameters are influential in controlling site amplification and site attenuation (kappa) of ground motion during seismic events. The horizontal-to-vertical spectral ratios (HVSRs) of environmental noise have demonstrated effectiveness in investigating shear velocity profile layers across diverse geological contexts [1]. In this investigation, the local shear-wave velocity structures of the Jeju region were examined using HVSRs of environmental noise, aiming to identify the presence of a significant low-velocity layer (LVL) or a consortium of LVLs. Historically, conventional approaches for describing the mechanical properties of the ground for seismic microzonation have relied upon borehole drilling and active seismic prospecting. Over the past two decades, substantial advancements have been achieved in mapping the mechanical properties of both shallow subsoil and bedrock for local seismic microzonation, utilizing environmental noise recordings. This methodology involves employing a single station to estimate the HVSR curves. Numerous studies employing HVSR curves for site amplification characterization have utilized data sourced from background noise [2; 3; 4; 5], coda wave seismic energy [6; 7; 8; 9; 10], and S-wave energy [11; 12], thus allowing for amplification and attenuation in seismic microzonation in the volcanic region [13]. Specifically, the long-period HVSR of environmental noise has been effectively applied to ascertain the shear-wave velocity (Vs) of deep sedimentary layers in various global locations, including the Kanto Plain in Japan [14], the Santa Clara Valley in California [15], and the Rhine Embayment in Germany [16]. Additionally, array configurations have been employed in certain studies [17] to estimate surface wave dispersion curves. Until recently, the HVSR curves of environmental noise were interpreted semi-empirically, viewed as either representing Rayleigh wave ellipticity or the amplitude ratio of the sum of Rayleigh and Love waves [18; 19]. The advancements in diffuse-field theory [20; 21] have led to the proposition that a theoretical formulation, instead of an empirical one, should be applied to multilayer stratum models with depth-dependent shear velocity. Research applying diffuse-field theory in this domain is being actively conducted by [PERSON] et al. [22], [PERSON] et al. [23], [PERSON] et al. [24], and others, focusing on horizontally homogeneous multilayer models. Passive seismic techniques based on the measurements of environmental noise have proven effective in scenarios where alternative methods are impractical owing to either elevated environmental seismic noise or constrained budgets. These techniques offer substantial lateral coverage at manageable costs, rendering them particularly advantageous for seismic microzonation in regions of low seismicity and in developing countries. The present study aimed to analyze the shear velocity profile of a multilayer structure by applying diffuse-field theory. Jeju Island, situated on the continental shelf south of the Korean Peninsula, measures approximately 70 km (EW) by 30 km (NS). As a volcanic island, it exhibits the geochemical characteristics of oceanic island basalt, originating from Pleistocene eruptions. Geologically, the Jeju region is markedly distinct from the inland areas of the Korean Peninsula, comprising the Halla volcanic edifice in its central region, a lava plateau in the east, and numerous cinder cones dispersed across the island [25]. Previous geophysical studies have established the existence of an LVL beneath the extrusive volcanic outcrops of Jeju Island. This LVL, varying in thickness from approximately 100 to 300 m and occurring at different depths, is believed to comprise quaternary marine sediments.