Behavior of semi-underground power station by near field strong earthquakes observation

The earthquake observation has been carried out at Ochiai semi-underground hydroelectric power station installed in a cylindrical pit of 15 m in diameter, 22 m in depth in rock mass in purpose of evaluating the earthquake resistance of semi-underground nuclear power plants. The behavior of the cylindrical pit has been analysed mainly by the observed seismic waves of the near field strong earthquakes, which happend in August, 1983. And some of the obtained results are as follows. (1) Rations of subsurface maximum accelerations to surface maximum accelerations were almost in range from 1/2 to 1 in radial direction and 7/10--1 in vertical direction. (2) The power spectrum in high frequency domain of more than 8 H sub(Z) in radial direction and more than 11 H sub(Z) in vertical direction decrease according to the depth from the ground surface. (author)