New insights into seismic behavior of building and surrounding soil at Hamaoka nuclear power station during Suruga Bay earthquake in 2009

The Suruga Bay earthquake in 2009 attacked the Hamaoka nuclear power station and stopped the operation. The maximum acceleration 4.38 m/s2 at the foundation of the reactor building of No. 5 unit was four times larger than that of No. 1 unit. It was found that the vibration amplitude at 2.5 Hz is mainly related to that maximum acceleration. The records in the underground support the fact that the vibration amplification was caused in the surface soil from 25 to 100 m beneath the reactor building. The non-stationary Fourier spectra clarified that the frequency of the dominant component shifted from 3.0 Hz to 2.5 Hz in the short transient time. The dual-peak shape in the displacement profile was assumed to consist of the fundamental mode and the local vibration mode and this was identified by the dual Ricker wavelets. This identification indicates that the vibration amplification was caused by the deformation with the amplitude of 20 mm in the underground. The average strain of soil from SRϕ-22 (22 m underground) to SRϕ-100 (100 m underground) reached 0.031% which reduced 30% of shear stiffness. The rocking mode of the foundation was further observed from the vertical deformation of the foundation. By investigating the phase of the displacement profile, it was found that the natural period at the north position was longer than that at the south position, which is related to the nonlinearity of the supporting soil.