Estimation of Spectral Decay Parameters in the Southern Korean Peninsula

Spectral decay parameters, 𝜅 and 𝜅₀ which related to inelastic characteristics or geological conditions of a site, estimated from the slope in semi-log acceleration spectrum are important parameters in the stochastic ground-motion simulation. It is necessary to estimate accurate spectral decay parameters of each site in order to reflect the site condition in ground-motion simulation. Acceleration records of M_L >2.7 earthquakes from January 2003 to February 2019, as listed in the catalog of Korea Meteorological Administration, were used to calculate 𝜅 values of each station. Two horizontal components of acceleration records were rotated to the radial and transverse component to capture characteristics of SH waves. Mean, trend, and instrumental response of the transverse component data were removed. Time windows for extracting the S and Lg waves were applied to the data and then the windowed spectrums were transformed to the semi-log spectrum. In the semi-log spectrum, the frequency range to estimate linear slope of diminution were manually selected. 𝜅 values of each event were computed by dividing the slope with -𝜋. 𝜅₀ values of each station were estimated by linear regression of 𝜅 values on the hypocentral distances. In consequence, 𝜅₀ values for 110 stations were estimated. 𝜅₀ values of surface stations are higher than those of borehole stations in average. It is reasonable result considering that 𝜅₀ value is correlated with the sediment thickness of site. We could more accurately predict the ground-motions of 912 Gyeongju Earthquake sequence (M_W 5.1, 5.5, and 4.4) using the estimated kappa values.