SOIL NONLINEARITY AND BEDROCK STRONG MOTIONS ESTIMATED FROM DOWNHOLE ARRAY RECORDS AT KASHIWAZAKI-KARIWA NUCLEAR POWER PLANT DURING THE 2007 NIIGATA-KEN CHUETSU-OKI

Nonlinear soil properties as well as bedrock outcrop strong motions are back-calculated using 4-depth downhole strong motions recorded at the Kashiwazaki-Kariwa nuclear power plant during the 2007 Niigata-ken Chuetsu-oki earthquake. Adopted in the inverse analysis are genetic algorithms (GA) combined with a 1-D equivalent-linear response analysis in which strain-dependent damping ratios are assumed in the frequency domain. The inversion results show that the surface layer down to a depth of 70 m exhibited strong nonlinear behavior with a shear modulus ratio down to about 0.01 and a damping ratio up to about 35%.