In Korea, several problems have been identified in the design standards for earthquake resistance, within a high-frequency range, due to the inappropriate characterisation of local site amplification characteristics. Among the methods that have been suggested for analysis of site amplification characteristics is one that uses the H/V spectral ratio of ground motions, which was used in this study. The method can be applied to S-waves, coda waves, and background noise. Based on a series of 15 aftershocks of the Fukuoka earthquake of 20 March 2005, analyses were conducted for a total of 267 sets of ground motion data observed at seismic stations in Korea. Most of the seismic stations suggested consistent site amplification characteristics among the three types of seismic energies despite a logarithmic difference of three or larger in the Fourier transform value of the observed waves, and also despite long epicentre distances, even greater than 600 km. The stations KHD, GSU, HSB and TJN showed no dominant resonance frequencies because they are located on solid bedrock or have borehole seismic sensors. It is necessary to use care when considering stations GKP1, KRA, and SNU, which have distinct resonance frequencies. The results of this study are essential for the effective design of earthquakeresistant structures and also for reliable seismic source and crustal attenuation studies.
The problem has been pointed out that the domestic design response spectrum does not reflect site amplification, particularly in the high frequency bands, including the fact that site specific response spectrum from the observed ground motions appears relatively higher than design response spectrum. Among various methods, this study applied H/V spectral ratio of ground motion for estimating site amplification. This method, originated from S waves and Rayleigh waves, recently has been extended to Coda waves and background noise for estimating site amplification. For limited time of periods, 4 electric substation sites had operated seismic stations at two separate locations (bedrock and borehole) within each substation site. H/V spectral ratio of S wave, Coda wave, and background noise, was applied to 36 accelerations of 3 macro earthquakes (Odaesan, Jeju and Gongju earthquakes), larger than magnitude 3.4. observed simultaneously at each bedrock location within 4 electric substation sites. Site amplifications at the bedrock location of 4 sites were compared among S wave, Coda wave energy, and background noise, and then compared to the previous results from the borehole location data. The site classification was also tried using resonancy frequency information at each site and location. The results suggested that all the electric substation sites showed similar site amplification patterns among S wave, Coda wave, and background noise. Each station showed its own characteristics of site amplification property in low, high and specific resonance frequency ranges. Comparison of this study to other results using different method can give us much more information about dynamic amplification of domestic sites characteristics and site classification.
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