HR: 0800h
AN: S31B-0444    [Abstracts]
TI: Long-Period Ground Motion Characteristics in Osaka Basin, Japan - Examination of 3D Basin Structure Models
AU: * Iwaki, A
EM: iwaki@egmdpri01.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan
AU: Iwata, T
EM: iwata@egmdpri01.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan
AB: Osaka, the most populated area in western Japan, is located inside a sedimentary basin whose size is approximately 60km x 40km and the maximum bedrock is 3km. As there are many long-period structures such as skyscrapers and oil tanks, it is important to understand long-period ground motion characteristics in this area in order for the reliable prediction of long-period ground motions during future mega-thrust earthquakes in the subduction zone of Philippine Sea plate. Iwaki and Iwata (2007) evaluated the applicability of the two existent 3D basin structure models (Kagawa et al., 2004 and Horikawa et al., 2003) by conducting a long-period ground motion simulation of an event that occurred near the hypothetical subduction zone events. Both models reproduced the response spectra of the observed ground motions well, within factor of 2, at most stations. However, at some stations, the predominant periods of the observed ground motions were not well reproduced, which suggests the need for the models to be improved. In this study, we show observed long-period ground motion characteristics in Osaka basin from a number of events, and estimate the effects of 3D basin structure on the characteristics by model simulations. We first examined site-specific characteristics of long-period ground motions using the strong motion records of seven major inland earthquakes (M>6) and four deep earthquakes (M>7). We took the coda part (60s after the S-wave arrival) and calculated horizontal-to-vertical Fourier amplitude spectral ratio (H/V). The H/Vs of 11 events were stable at each station, which suggests they represent the long-period ground motion characteristics controlled by the underground structure rather than events. Then ground motion simulations were carried out by 3D finite-difference method (Pitarka, 1999). The target events were: 1) the largest aftershock of the 2004 Off Kii peninsula earthquake (M_JMA6.5) and 2) the 2000 Tottori-ken Seibu earthquake (M_JMA7.3), both 2-300km away from Osaka basin but of different backazimuths. Coda H/Vs calculated from the synthetic waveforms of two events are similar to each other, which could be corresponding to the observed characteristics. We compared the observed H/V, 3D simulated H/V, and the theoretical Rayleigh wave H/V obtained from 1D velocity structure beneath each station. We found that 3D simulated H/V explained the observed one better than the 1D H/V at most stations. This reconfirms the importance of the 3D underground structure on long-period ground motion characteristics inside the basin. Time- dependent characteristics of H/V would be discussed in order to improve the basin structure models. We used strong motion data provided by COERKA, NIED, JMA, Denkyo-net, and Osaka City Waterworks Bureau.
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: 2007 Fall Meeting