Since 1962, the authors have been carrying out earthquake observations at the ground surface and underground at rocky ground in the mountains of the northern part of the Kanto Region, approximately 150 km north of Tokyo. Several papers have already been published regarding the results of these earthquake observations. There are several thin weathered layers interspersed in the bedrock at the observation site and it was found from observation results that earthquake ground motions were influenced by these layers, and this is discussed in the present paper.
The geology of the observation site consists of green tuff formed from the Tertiary to Quaternary periods which is relatively homogeneous and with core recovery rate of roughly 100% at the bore hole used in observations. The site is located at a slope of approximately 19 deg. The condition of the ground as obtained from the results of boring is given in Table 1 with clay layers of thicknesses of 30 cm, 20 cm, and 10 cm produced by weathering of the green tuff found at depths of approximately 16 m, 36 m and 47 m, respectively, from the surface. Also, a number of cracks is found at depths of about 50 m and 52 m.
There is a vertical shaft for access to an underground powerhouse at a point approximately 14 m distant from the bore hole, About 10 m of the surface layer is comparatively weathered at this point with clay layers at depths of 13.8 m and 14.25 m, while no core was recovered between the depths of 53.4 m and 54.8 m.
Based on the above, it is surmised that ground layers run roughly parallel with the surface of the slope.
As shown in Fig. 1, eight strain gage-type accelerometers have been installed horizontally in the E-W direction down to a depth of 67.2 m and in a manner to sandwich clay layers. The accelerometer frequency characteristics are more or less flat from 0 to 10 Hz, and the mutual differences between characteristics of accelerometers are slight.
Five accelerometers have been installed in the vertical shaft. These are electro-magnetic-type seismometers of natural period of 0.3 sec having integrated frequency characteristics nearly flat in range of 1 to 24 Hz, and are located at intervals of approximately 17 m from the ground surface to a depth of 67.2 m. Measurements are made in the horizontal, E-W direction.
Since these two varieties of seismometers have different frequency characteristics, dissimilarities are produced in waveforms at comparatively high frequency components, while there are also differences in absolute values.
Recording is done by oscillographs activated by starters and paper speed is 10 cm/sec.
Acceleration records obtained at the bore hole are studied in this paper.
Approximately 340 earthquakes have been recorded up till the present, these earthquakes mainly having occurred in the Kanto Region, Tohoku Region and the neighboring sea area.