## 1 Introduction  
Relatively near-surface ground conditions can have a profound effect on earthquake ground motions at a particular site. Depending on such conditions, traveling earthquake ground motions may either be amplified, such as when the near-surface conditions are considered softer than deeper layers, or may be de-amplified or attenuated, such as when near-surface conditions are considered stiffer than deeper layers. Therefore, understanding near-surface ground conditions is important for understanding the seismological, geological, and geotechnical conditions for a site or region. For seismically stable South Korea, this importance was magnified when two consecutive earthquakes, the 2016 \(\mathrm{M_{W}}\) 5.5 Gyeongju and 2017 \(\mathrm{M_{W}}\) 5.5 Pohang earthquakes [1, 2], struck a region that housed several nuclear power plant complexes. Media showed videos of the effects of strong-motion ground shaking and renewed the community's curiosity in how prepared the region was against earthquakes. Ground motion amplification for vertically propagating seismic waves is typically accounted for with site amplification factors [3, 4, 5, 6] or with the site term of modern ground motion prediction equations [7, 8, 9, 10, 11]. Virtually all of these parameters are dependent on the time-averaged shear wave velocity in the top 30 m of surficial ground material, \(V_{S30}\), with a few others integrating varying depth parameters [7, 8, 9, 10]. In general, a site characterized with a lower \(V_{S30}\) would be subjected to greater ground motion amplification relative to a site with a higher \(V_{S30}\). However, obtaining \(V_{S30}\) for a site is no easy task, and there are several methods of doing so. These methods are typically categorized as active or passive and invasive or noninvasive. Active methods use a wave source that is controlled, while passive methods do not. Invasive methods permanently disturb the ground media, while noninvasive methods do not. Relevant to vertical amplification, the preferred method is an active invasive one known as downhole PS-suspension logging. This is the preferred method as it directly records shear wave travel times within the travel medium [12; 13]. The main drawbacks to the in situ method are time and cost. Other noninvasive methods are generally less costly but perform indirect measurements. One set of noninvasive methods includes surface wave processing, such as spectral analysis of surface waves and multichannel analysis of surface waves [14; 15; 16]. Another set includes processing ground motion data from microtremors [17; 18; 19; 20; 21]. However, the aforementioned methods have shown varying levels of uncertainty as well as limitations with respect to their use at different site conditions [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. A relatively more popular technique in estimating \(V_{\text{S30}}\) at a site is the horizontal-to-vertical spectral ratio, HVSR, method [21; 34; 35; 36; 37; 38; 39; 40; 41]. The method is based on the assumption that the amplification in the horizontal component will be significantly higher than the amplification in the vertical component during seismic wave propagation in a half-space. Any form of amplification in the vertical components, including both P and SV waves, is counterbalanced by the effects of refracting ray paths towards the vertical [42]. Therefore, the peaks of HVSR are associated with the fundamental frequencies of a site and thus an estimate for \(V_{\text{S30}}\). Studies that have correlated \(V_{\text{S30}}\) data with HVSR information generally derive their HVSR from either acceleration Fourier amplitude spectra or acceleration response spectra. Some of these response spectra studies utilized acceleration waveforms from international compilations [42], central and eastern North America [43], California, United States [44], and Iran [45]. However, applying HVSR-\(V_{\text{S30}}\) techniques is challenging as the user would need to know both horizontal and vertical ground motions at a site.