HR: 16:45h
AN: S42H-04    [PDF]
TI: Site Amplification Characteristics and Their Relationship with Averaged S-wave Velocities Derived From K-NET and KiK-Net Strong Motion Records in Japan
AU: * Kawase, H
EM: kawase@arch.kyushu-u.ac.jp
AF: Kyushu Univeristy, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Matsuo, H
EM: matsuo@seis.arch.kyushu-u.ac.jp
AF: Hitachi Systems and Services, 3-2-16 Ohmori-kita, Ohta-ku, Tokyo, 143-8545 Japan
AB: To predict strong ground motions for future scenario earthquake in a broad-band frequency, we need to characterize both source spectra and site amplification. For high frequency we can use statistical method based on the observed records and the summation technique. Thanks to the advent of the K-Net and KiK-net (by NIED) in Japan, we have now plenty of weak motion data, enough to construct statistical Green_fs functions. In order to represent long- and intermediate period nature of the strong ground motion in the near field region, we want to use theoretical technique such as three-dimensional finite difference method. Any theoretical methods need S-wave velocity structures that should reproduce the observed site amplification quantitatively. In this study we perform first the analysis to separate the so-called source spectra, attenuation coefficient, and the site amplification factors from about 15,000 K-NET, KiK-Net, and JMA records observed at 1,700 stations in Japan. The separation method is the well-established one of Andrews (1980) and the resultant source spectra are modeled as omega-square spectra. As a reference site we use one rock station of KiK-Net in Yamaguchi Prefecture, from which we remove amplification of shallow surface deposits. Once we obtain site amplification factors, we try to reproduce them by using one-dimensional S-wave velocity structures below each site of K-NET and KiK-Net (in total, 1,300). We use Genetic Algorithm to invert the S-wave structures with fixed S-wave velocities in the shallow part. We succeed to reproduce site amplification factors at about one thirds of the sites very well. The majority of not-so-good sites need modification of S-wave velocities in the shallow part based on logging data. We also perform the same kind of separation analysis for PGA, PGV, and JMA Seismic Intensity. We found that PGA site factors correlate best with 5.0 Hz spectral amplification and PGV 2.5 Hz spectral amplification. We also found that the best correlation of site factors can be obtained with the average S wave velocity only for top 5 m for PGA and 10 m to 20 m for PGV. Since we have a space for better representation of velocity structures, the results reported here are not conclusive. Once we get better S-wave velocity structures at as many sites as possible, we expect to extrapolate them and construct an initial model of the three-dimensional basin structure of the whole Japanese islands.
DE: 7203 Body wave propagation
DE: 7212 Earthquake ground motions and engineering
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting