堆積層上における中・強震動の波群解析の試み

Abstract

In order to make clear the effects of sedimentary layer on strong ground motion due to earthquakes, a tripartite array with strong-motion accelerographs of three orthogonal components has been deployed since 1982 in the campus of Tokyo Gakugei University, Koganei, Tokyo. The array is located at the western suburbs of the Tokyo Metropolis on Pleistocene sedimentary layer which reaches ca. 2000m depth overlying Pre-Tertiary basement. Digital accelerographs with sampling rate of 60Hz and with 12bits/word are installed on the array of 300-400m sides.Several events among all the data acquired by the present time are available for analyses; those are in the magnitude range of 5.7-7.3, in the depth range of 20-400km and in the range of epicentral distance of 47-340km. Although the maximum height and the phase of high frequency acceleration are strongly affected by variety of surficial portions (thickness_??_10m) underlying the foundations of seismographs, ground velocity and ground displacement of each station of the array show coherent signals with same amplitude but poorly discriminated phase lag each other up to the coda of the raw wave trains. However, filtrated waves of the raw data through some narrow pass bands exhibit clearly discriminated phase lags among the three stations throughout the wave trains right after the S-wave onset.Method of sums and the Fourier's analysis are adopted to derive apparent velocities of the later phases which are of considerable amplitude in ground displacement and ground velocity; the frequency range of the phases is 0.2-1.0Hz within which the velocity spectrum is in fairly high level. The results can be interpreted as surface waves excited in the thick sedimentary layer which covers the Tokyo area. Though the frequency dependence of phase velocity is clearly seen, it could not be confidently interpreted as a branch of unified single mode of surface wave. It is more likely to be interpreted as some branches of higher modes.A moving-window analysis with multiple filtering technique has given time-varying frequency spectrum of the strong-ground motion wave trains. It also shows a sequence of average group velocities between seismic foci and the array station. The results in conjunction with those of the phase velocity suggest laterally heterogeneous surface layer in and around the Tokyo area.