STUDY ON HIGH-CUT FREQUENCY CHARACTERISTICS OF GROUND MOTIONS FOR INLAND CRUSTAL EARTHQUAKES IN JAPAN

摘要

High-cut frequency characteristics of ground motions for inland crustal earthquakes occurring in Japan are examined. It is very important to make clear spectral decay characteristics in high frequency range for strong ground motion prediction. The target earthquakes are the 2003 Miyagi-ken Hokubu earthquake (Mw:6.1) and the 2005 Fukuoka-ken Seiho-oki earthquake (Mw:6.6). The Butterworth type high-cut filter with cut-off frequency, fmax and its power coefficient of high-frequency decay is assumed to express the high-cut frequency characteristics of ground motions in this study. The four parameters such as seismic moment, corner frequency, cut-off frequency, and its power coefficient are estimated by comparing observed spectra at hard rock sites with theoretical spectra. The theoretical spectra are calculated, based on the omega squared source characteristics convolved with propagation-path effects and fmax filter shapes. In result, the fmax of the mainshock of the 2003 Miyagi-ken Hokubu earthquake is estimated as 8.3Hz and that of the 2005 Fukuoka-ken Seiho-oki earthquake is 6.5Hz, respectively.The power coefficients of high-frequency decay, s, are estimated about 1.0. From these results, the high-cut filter with about 7.0Hz as fmax and 1.0 as the power coefficient of high-frequency decay is earthquakes are estimated in the range about 9Hz and 20Hz. The seismic moment dependency of fmax