The geotechnical data base in Tsukuba area was prepared and microzoning of ground amplification was made. Geological and geotechnical surveys had been carried out at boreholes of about 2600 sites. For more than 1500 boreholes, the following values were digitized; (1) depth of geological discontinuty, (2)
N-value, (3) depth where
N-value was measured, (4) class of soil, (5) age of soil, (6) coordinate of site and (7) level of site. In order to calculate the response spectrum of ground, the underground structure of S wave velocity (
Ns) and density (ρ) are necessary for each site. It is possible, in general, to introduce empirical formulas proper to the area concerned, which give S wase velocity and density of the ground from geological data, through correlativity between results of seismic exploration and geological data. Unfortunately, there were not so many data of seismic exploration as to be able to create such formulas in Tsukuba area. In the present report, therefore, the formulas of Iida et al. were used. Response spectrum for each site was calculated by Haskell's method, when soil was treated as visco-elastic media of Voigt type. Analyses were made for almost all of segments in the area of about 21km in the latitude (35°58′45″ N-36° 10′00″N) and about 15km in the longitude (140°1′45″E-140°11′45″E), when the size of one segment was 380m in longitude and 460m in latitude. Regional distributions of three predominant periods in the period range from 0.1sec to 1.0sec and the corresponding peak amplitudes were obtained. The underground structure of P wave velocity in the depth of less than 400m was determined by seismic exploration which was carried out by the Geographical Survey Institute to investigate the ground condition in and around the borehole for earthquake observation. On the basis of S wave velocity, which was estimated from the above P wave data, response spectrum for the deep underground structure was obtained and predominant periods of 1.3sec and 2.6sec were recognized.
View full abstract