Abstract
A study has been made on amplification characteristics of earthquake ground motions in the period range 1 to 10 sec in whole parts of Japan, as one of the basic investigations for dynamic earthquake-resistance of very high-rise and large-scale structures. The data used in this analysis were obtained by strong motion displacement seismographs with natural period of 6 sec, operating at about 100 local observatories of Japan Meteorological Agency. All the shallow and moderate-to-large earthquakes occured in and around Japan in recent 15 years were selected for analysis. Total number of earthquakes was 138. Plots of maximum amplitude as function of epicentral distance show large scatters, which are attributed to the site conditions. An attenuation curve was, using a least squares method, approximated to the relation between maximum amplitude and epicentral distance for each earthquake. At each site the deviation from this curve was regarded as an index to express ground characteristics. Deviations for all earthquakes were assessed and the histogram was made. The mean value of histogram was defined as the most probable amplification of seismic ground motions at each station. The obtained mean values were plotted on the map of Japan, which facilitates a general view on ground characteristics in Japan. Attention must be paid on the fact that markedly high values are seen at large cities in Japan such as Tokyo, Osaka, and Niigata. A detailed survey on the ground characteristics was carried out in Kanto plain. The results revieled that the amplification increases with increasing depth to the basement at the site, and that the depth relating to ground motions in the period of 1 to 10sec are from 100m to several 1000m.
