Deconvolution of site effects in ground-motion using site response function derived from HVSR method

Abstract

Site effects caused by unconsolidated sediments laying on the bedrock amplify or attenuate ground motions propagated to the surface. Site effect is unique site-by-site, and thus, makes analyzing actual attenuation characteristics of ground motions difficult. In the southern Korean Peninsula, 17 seismic stations administered by KMA and KIGAM were equipped with a pair of accelerometers; one is installed at the surface, and the other at the borehole (namely SB station). We estimate the site response functions of the stations using ambient noise data. First, the horizontal-to-vertical spectral ratios (HVSR) of the stations were calculated. Then, calibration ratios to adjust the amplitude of HVSR to that of surface-to-borehole spectral ratio (SBSR) were estimated and applied to the amplitude of HVSR. These amplitude-corrected HVSRs are used as the site response function to correct linear site effects in ground motions. To deconvolve the site effect of ground motions, we designed linear zero-phase FIR filters based on the site response functions. Then we divided the spectral amplitudes of the ground motions by the frequency response of the FIR filter. For the SB stations, site response functions of ten stations were obtained, and ground-motion data of 39 events with ML > 2 were corrected using these site response functions. In the result, the peak ground motion (PGA) of corrected ground motions at the surface was reduced by 20-76% on average compared to raw ground motions. Comparing ground motions of the borehole and surface sensors, the corrected PGA of the surface was 1.8-4.9 times bigger than the raw PGA of the borehole. For the whole surface stations of 176, the site-response functions of 75 stations were estimated, and ground-motion data of 210 events with ML > 3 were corrected by their site-response functions. We found that surface ground motions are deamplified to the level of borehole ground motions through the site effect correction.