Nonlinear Soil Behavior Observed at Vertical Array in the Kashiwazaki-Kariwa Nuclear Power Plant during the 2007 Niigata-ken Chuetsu-oki Earthquake

The Kashiwazaki-Kariwa nuclear power plant suffered extreme shaking during the 2007 Niigata-ken Chuetsu-oki earthquake. Accelerograms observed at the dense seismometer array in the plant are now open to the public and will provide valuable knowledge. The temporal changes of S-wave velocity were examined by using normalized input-output minimization (NIOM) method based on the vertical array records observed during the mainshock and the events before and after it. It was found that the S-wave velocity in the layers (surface-50 m depth and 50-100 m depth) decreased significantly during the principal motion of the mainshock (indicating nonlinear behavior), whereas nearly linear behavior was observed in the bedrock layer (below 100 m depth). It was also found that the S-wave velocity increased in the layers above 100 m depth soon after this motion, indicating no major liquefaction in these layers. Finally, the relationship between the shear modulus and shear strain was examined based on the obtained S-wave velocities. The normalized shear moduli (G/G(0)) in the surface and intermediate layers decreased to about 0.2 at a strain level of 1 x 10(-3) to 1 x 10(-2) and about 0.6 at a strain level of 1 x 10(-3) to 2 x 10(-3).