Basin structure and ground vulnerability derived from HVSR analysis in Pohang, South Korea

The city of Pohang in South Korea is a leading industrial center that has played a significant role in the development of the Korean economy since the 1970s. On November 15, 2017, the city experienced a 5.4 earthquake, which was the second largest, but most damaging, earthquake to occur in Korea since 1904. Ground shaking during the earthquake was strong enough to cause severe damage to infrastructure in the area, and more than 100 people were injured. The earthquake resulted in an estimated USD$52 million in direct and indirect losses. The Pohang area also experienced soil liquefaction following the Pohang earthquake. We collected the ambient noise data (three-component seismic data) from 446 temporary stations. Horizontal-to-vertical spectral ratio (HVSR) method was applied to image the bottom geometry of the Pohang Basin. The resonance frequencies (> 0.33 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 west, and thickens to the southeastern and central parts of the basin. We also estimated the vulnerability of the ground to seismic shaking, Kg. We then compared their spatial distribution with damage and liquefaction, collected and documented by the Korea Meteorological Administration and the Korea Institute of Geoscience and Mineral Resources, respectively, following the earthquake. We confirm that the spatial distribution exhibits a robust correlation with Kg. We also demonstrate that Kg is an efficient proxy for identifying relatively fragile zones during ground shaking and soil liquefaction. The resultant three-dimensional shape of the sedimentary basin will provide crucial information for the micro zonation for seismic risk mitigation and future city planning.