Ground motion prediction for huge subduction earthquakes in Osaka basin, Japan, using an advanced stochastic Green's function

The hybrid technique combined theoretical calculation with stochastic one is powerful in predicting broad-band strong ground motions from future large earthquakes. Kamae et al. (1998) has demonstrated its powerfulness through some simulations of near-source strong ground motions in the 1995 Kobe earthquake, Japan. Then, in the stochastic calculation for high-frequency ground motions, the stochastic simulation method by Boore (1983) has been applied taking into consideration the amplitude characteristics on corresponding sedimentary site. However, in application of the stochastic calculation to prediction of ground motions inside the basin, we need to consider not only amplitude characteristics but also phase ones at target sites. We found that the characteristics of the frequency dependent envelope as well as the Fourier spectra at sedimentary sites inside the basin change depending on the source direction, source distance, source depth and local geological conditions. In general, seismic waves in the basin structures elongate the duration time of ground motion by the effect of the seismic waves secondarily induced at the basin edges and so on. In this study, after characterizing empirically phase and spectral amplitude, we examine the possibility for stochastic simulation of strong ground motions at sedimentary sites using empirical frequency dependent envelope functions and amplitude spectra at each site. Then, frequency dependent envelope functions in Osaka basin are specified by the source direction, source distance, source depth and local geological conditions. This method is verified by reproduction of recordings with long duration in Osaka basin due to some earthquakes. Finally, Strong ground motions from future huge subduction earthquakes (e.g. Nankai earthquake, Tonankai earthquake) are predicted by the hybrid technique using this advanced stochastic simulation technique for high-frequency motion.