We apply the single-station microtremor horizontal-to-vertical spectral ratio (HVSR) method to image the geometry of Pohang Basin, South Korea, which experienced the greatest earthquake damage in Korea during a series of anthropogenic earthquakes between November 2017 and February 2018. We collected and analysed the ambient seismic noise at 124 temporary stations. The resonance frequencies, which vary significantly across the area (0.35–19.86 Hz), were inverted to constrain the depth of the major impedance contrast, which is interpreted to be the sediment–bedrock interface beneath each station. The sedimentary layer thickness is generally thin in the north and thickens to the southern and central parts of the basin, where the depth to bedrock is up to 340 m. We compare the HVSR results with six borehole observations in the area, whose depth to bedrock ranges from 189 to 359 m. The sediment thicknesses obtained via the direct borehole measurements and HVSR method are comparable with each other. The resultant three-dimensional shape of the sedimentary basin provides crucial information for the microzonation of the Pohang area for seismic risk mitigation. It also provides a realistic initial velocity model for three-dimensional tomographic inversions to elucidate the detailed subsurface structure of the region.

中文翻译：

---

韩国浦项市非均质浅沉积结构的微震HVSR分析

我们采用单站微震水平与垂直光谱之比（HVSR）方法对韩国浦项盆地的几何图像进行成像，该浦项盆地在2017年11月至2018年2月的一系列人为地震中遭受了韩国最大的地震破坏。我们收集并分析了124个临时站点的环境地震噪声。共振频率在整个区域（0.35–19.86 Hz）之间发生显着变化，因此被反转以限制主要阻抗对比的深度，这被解释为每个台站下方的沉积物-基岩界面。北部的沉积层厚度通常较薄，而盆地的南部和中部则较厚，那里到基岩的深度最大为340 m。我们将HVSR结果与该地区的六个钻孔观测结果进行了比较，到基岩的深度范围为189至359 m。通过直接钻孔测量和HVSR方法获得的沉积物厚度彼此可比。沉积盆地的三维形状为浦项地区的微区化提供了重要信息，以减轻地震风险。它还为三维断层扫描反演提供了一个现实的初始速度模型，以阐明该区域的详细地下结构。