Estimates of Spectral Acceleration Amplification of Observation Stations in the Iwate-Miyagi and Niigata Regions, Japan

Speaker: Rami Ibrahim
We estimated empirical amplification factors for seismic stations in the Iwate-Miyagi and Niigata regions, Japan for period ranges of 0.05–5 s. We adopted the method of Si et al. (2010) in which the empirical amplification factors are estimated by averaging ratios of observed and predicted ground motions using reference ground motion attenuation models (GMPEs). The model of Kanno et al. (2006) was used to estimate the 5%-damped acceleration spectra on the bedrock. The results show large amplification at periods of 0.1 s or shorter at stations located in mountainous area and it is reduced for longer periods. However, at stations located on basins, large amplification is found at long periods of 1–5 s. The tendency of our results agrees well with theoretical transfer functions. The empirical amplification factors of spectral accelerations were used to correct for the observed strong motion data of two crustal earthquakes; the 2008 Iwate-Miyagi Nairiku earthquake (Mw 6.9) and the 2004 Niigata-ken Chuetsu earthquake (Mw 6.6). We also corrected the same database using the site correction relation of Kanno et al. (2006). The corrected data by our empirical amplification factors fit better with GMPEs. To check the validity of our analysis, we carried out error analyses by estimating the residual and the standard deviation before and after corrections. We found that both residual and standard deviation were reduced after correction. The method is efficient to calculate site effect at observation stations where imperfect velocity structure is available.