Research on the attenuation characteristics of 1 second or shorter period earthquake ground motion in the Kanto-Tokai districts in Japan

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Since 1979, the Research Committee of Strong-Motion Earthquake Instrument Arrays on Rock Sites has continued the seismological array observation in and around the Tokyo metropolitan area of Japan. The four main sites, that is, Higasi-matsuyama in Saitama Pref., Choshi and Tateyama in Chiba Pref. and SHuzenji Site in Izu Peninsula were set in 1978, and the three supplementary sites were added in 1983. The four main sites have the local array network respectively, so the present total number of observation points is 21 points. Since the beginning of the observation, a lot of seismograms have been recorded. Out of the total data set, only the accelerograms with comparatively large amplitude were used for the data set in this study, that is, 4966 accelerograms recorded in 305 earthquake events were used for analyses.In the first place, the empirical formula of ground motion attenuation was proposed in frequency domain. Ground motion attenuation may be represented by the following formula.log_<10>Sv(T)=A(T)-M+[B(T)-X+log_<10>X]+C(T)+DELTA_<ij>(T)Here, Sv(T), M and X represent velocity response spectrum, earthquake magnitude and hypocentral distance, respectively, and A(T), B(T)and C(T)are the regression coefficients. B(T)may be transformed into Q-values, and after then the Q values were compared to those by the past seismological research. As the result of the regression analyses, the Q values in the Kanto-Tokai districts of the Central Japan were evaluated as the function of frequency of ground motion, and it was shown that the Q values are proportional to frequency with the exponent of 0.73, that is, Q(f)*f^<0.73>.In conclusion, using these empirical formulae as the standard scale for evaluation of ground motion intensity in frequency domain, a simple method to understand the regional characteristics with respect to ground motion attenuation was proposed.