Abstract
The deep underground seismic characteristics which will play an important role in assessing the earthquake resistance of high-level waste geological disposal facilities were examined based on the results of seismic observations conducted at the Shiroyama Underground Hydroelectric Power Station and Hosokawa Mine, and the following results were obtained.
(1) Little amplification is recognized in the acceleration of seismic motion from the deep underground part to the neighborhood of 100m below the surface, but amplification in the seismic motion increases in the surface layer. That is, the maximum acceleration in the underground part deeper than 100m below the surface decreases to about 1/2 of that on the surface part.
(2) The frequency spectra of seismic motion in the underground part have a simplified shape as compared with those in the surface part. Furthermore, the values of the response spectra in the underground part decrease more than those on the ground surface as the epicentral distance becomes shorter.
(3) The vertical angle of incidence of seismic waves in the deep underground part is about 35°max. against the vertical direction. On a seismic design input to geological disposal facilities, therefore, it will be necessary to examine the incidence directin of seismic waves as a parameter.
(4) In the caverns whose size is similar to that of tunnels in the geological disposal facilities, it can be judged that there is little amplification in the seismic motion due to the effect of the caverns.
