Characteristics of ground motions in the Jeju Island during the 2021 ML4.9 Jeju earthquake, South Korea

Abstract

An earthquake of local magnitude (M-L) 4.9 occurred on the southwestern seafloor of Jeju Island on December 14th, 2021. The geology of Jeju Island comprises granitic bedrock beneath a muddy marine sediment on top of which basalt and trachyte layers has formed, being expected to result in unique ground shaking characteristics. Analyzing accelerograms from eight seismic stations for 22 earthquakes, we explored intensity and frequency of ground motion and effects of earthquake source and path on ground motion intensity measures (IMs) across Jeju Island. Significantly higher ground motions were consistently recorded at several stations compared with other stations of similar source-to-site distances, indicating notable site amplification effects. Irrespective of seismometer installation depth, the dependence of IMs on M-L for M-L >= 3 is typically analogous to the magnitude scaling of the existing ground motion models (GMMs). Dependence of IMs on source-to-site distance in Jeju Island indicates a relatively faster decay compared with mainland Korea, implying that the marine sediments between the granitic bedrock and basaltic island may strongly attenuate the surface ground motion amplitude. These findings highlight the necessity for a region-specific GMM and site effect models to effectively account for Jeju Island's distinctive ground motion characteristics in engineering applications.