Correction for Site Effects of Earthquake Ground Motions Using Horizontal-to-vertical Spectral Ratio of Ambient Noise

Horizontal-to-vertical ratio (HVSR) method is employed to remove site effects on ground motions recorded on the surface stations. HVSR calculated at each station was used to correct the site effect as a site response function. Ambient noise data on the surface broadband stations administered by Korea Meteorological Administration and Korea Institute of Geoscience and Mineral Resources were used to calculate HVSR. A moving window, with a duration of 60 seconds and a time shift of five seconds, was applied to the continuous waveforms of two hours with no significant signal. A ratio of spectral amplitudes between two horizontal components and vertical component was computed and mean of calculated results for each windowed data was determined as the representative of HVSR. The representative HVSR of each surface station was used to correct the site effects from ground-motion records of the 2016 Gyeongju earthquake sequence (Mw 5.1, 5.5, and 4.4). The discrepancies in the amplitude of ground-motion between surface stations and borehole stations located on similar distance were reduced after the site correction using HVSR result. It could imply that the observed ground-motions were successfully corrected to the bedrock level, assuming that the influences of the site effects at the borehole station is insignificant.