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The city of Pohang in South Korea experienced substantial damage following the ML 5.4 earthquake on 15 November 2017. Damage surveys immediately after the earthquake revealed significant spatial variations in damage intensity across the epicentral area. We collected ambient noise data (three‐component seismic data) from 124 locations in Pohang, covering a total area of 17 km × 20 km, and applied the horizontal‐to‐vertical spectral ratio technique to estimate the resonance frequency, amplification factor and vulnerability of the ground to seismic shaking. Spatial variations exhibited by these three parameters are strongly correlated and consistent with variations in local geology. By comparing the distribution of the ground vulnerability index (Kg) with the damage distribution (or intensity) documented by the Korea Meteorological Administration following the earthquake, we confirm that the site effect is the prevailing effect that influences damage intensity in this region. We also demonstrate that Kg is an efficient proxy for identifying relatively fragile zones during ground shaking: in areas with a reported damage intensity of VIII, the proportion of the area with Kg > 20 is as high as 78.2%. Given that the study area has experienced a recent moderate earthquake, with significant economic consequences, our new results and analytical techniques will provide valuable information for future urbanization projects. [ABSTRACT FROM AUTHOR]

The town of Haenam in southwest Korea lies partially on reclaimed coastal land and experienced an unprecedented earthquake swarm during April and May 2020. Construction of a new town in the area means that there is demand to evaluate the seismic hazard caused by site-specific amplification of seismic ground motions by artificial unconsolidated cover. We used a microtremor horizontal-to-vertical spectral ratio (HVSR) method to identify resonance frequencies, image depths to bedrock, and assess seismic ground vulnerability across the epicentral area of the recent earthquake swarm. Microtremor measurements were taken at 144 sites across an 18.5 km × 12.5 km study area. The HVSR analysis shows resonance frequencies as low as 1.1 Hz. Using mean shear-wave velocities obtained by Multi-Channel Analyses of Surface Waves (MASW), we find that resonance frequencies correspond to depths to bedrock of between 3 m and 56 m. There are clear relationships between the distribution of resonance frequencies, depths to bedrock, surface geology, and areas of development in recent land reclamation projects. Higher resonance frequencies characterize areas of thin sedimentary cover or hard rock sites, while relatively low resonance frequencies characterize reclaimed land or greater depths to bedrock. We quantify the seismic hazard potential across the study area using the seismic vulnerability of ground index (Kg), and observe a clear association between zones of high Kg and areas of reclaimed land. This study demonstrates the value of the HVSR technique in characterizing the subsurface and assessing seismic vulnerability in areas of reclaimed land, where it can be used as a viable and low-cost alternative to large-scale active seismic refraction or MASW measurements. The results can also be used to inform and guide urban planning projects. [ABSTRACT FROM AUTHOR]