To get vibration characteristics over 23 wards in Tokyo, measurements of “S” and “P” wave velosities were taken at 8 places. From these measurements, more considerable scattering in “S” wave velosities are observed in sandy layers, and gravel and sand layers comparing with clay Formation. And also, as for the “S” wave velosities of upper Yurakucho Layers, we found that N-Value is separated into 2 groups by the values of “N=O” and “N≥1”. The density of subsoil measured in the field using scattering γ-ray is coincided with the density obtained in our laboratory test. It is possible that the “Kazusa-Group” is one of the strongest seismic baserocks in Tokyo. It was found that the “Kazusa-Group” is thick, for instance, in the Edogawa area, it is over 2, 000m and in the Tama area, over 500m. It means that the layer is physically fixed by referring the resistivity test by electrical logging and the densities of subsoil measured with γ-ray. “S” wave velosities in the Kazusa-Group measured in Denenchofu area, one of the 8 places, are 580m/s in the field test, and by ultrasonic 650m/s.
This Areai, as shown in Fig. 3, has a basin structure, surrounded by faults or Precambrian basement rocks. The basin is fflled by alluvium deposits chiefly composed of sands, gravels, clays and calcareous deposits, with some amounts of lavas and pyroclastic rocks originated from Kilimanjaro Volcan. These deposits which are roughly estimated at more than 130m in thickness in the south and seem to become thicker toward north are major aquifers in this area. As shown in Fig. 9 the total quantity of water recharged in the Kahe-Miwaleni groundwater basin is estimated to be 300, 000, 000m3/year and the quantity of water consumed by surface runoff (mainly from the Miwaleni Spring) and pumping works is estimated 110, 000, 000m3/year. About half the quantity of recharge may be regarded as the object of the future groundwater exploitation. However, it is still dangerous to think that all of 190, 000, 000m3/year can be expoited in future. It is technically impossible to pump out all the potential quantity due to hydrogeological restrictions and power limit of the pumping facilities. The quantity of water that can be practically drawn up should be estimated at most 50, 000, 000m3/year, less than half the potential quantity.