Single-path ground motion amplifications during the 2020 Haenam, South Korea, swarm

Two moderate earthquakes (the 2016 Gyeongju and 2017 Pohang earthquakes with local magnitudes of 5.8 and 5.4, respectively) in Korea caused severe damage to buildings and grounds, raising attention to local seismic hazard assessments. However, ground motion amplifications owing to site effects have not been studied yet for Korea. In 2020, the Haenam swarm produced 74 earthquakes with local magnitudes ranging from 0.87 to 3.08, providing great opportunities to investigate single-path ground motion amplifications. We develop a local ground motion model for predicting within-rock motions using the records from nine borehole stations, and evaluate empirical amplification ratios at four surface stations based on a non-reference approach. We check that the peak periods from the horizontal-to-vertical spectral ratios agree well with those from the empirical amplification ratios. Based on the shear wave velocity profiles obtained from downhole tests and multichannel analyses of surface waves, we perform one-dimensional site response analyses for the four surface stations. The amplification ratios estimated from the site response analyses are comparable with those from the empirical approach for the periods shorter than 0.07 s, but not for the periods longer than 0.07 s, raising the necessity of detailed studies on dynamic properties of soils and rocks in Korea.