Strain-Dependent Soil Properties Estimated from Downhole Array Recordings at Kashiwazaki-Kariwa Nuclear Power Plant during the 2007 Niigata-Ken Chuetsu-Oki Earthquake

Strain-dependent shear moduli and damping ratios are estimated for the deposit of the Service Hall at the Kashiwazaki-Kariwa nuclear power plant based on shear stress-strain time histories that can be directly determined from its downhole array recordings during the 2007 Niigata-ken Chuetsu-oki earthquake. The Holocene dune sand with V_s = 310–350 m/s at depths less than about 70 m shows strong nonlinearity with a shear modulus ratio of about 0.3 and a damping ratio of about 30 % at a maximum shear strain of about 3 x 10^–1%. The reduction in shear modulus ratio is consistent with previous laboratory test results for sand consolidated under similar confining pressures but the corresponding damping ratio seems to be slightly higher than that of the previous studies. The mudstone with V_s =500 m/s located at depths greater than about 100 m does not show any strong nonlinearity with a shear modulus ratio of about 0.6 and a damping ratio of about 10 % at a maximum shear strain of about 1.5 x 10^–1%. The strong nonlinearity of the Holocene dune sand during the main shock might have lowered the amplitude ratios between the near-surface layer and the bedrock in the short period range significantly compared with those of other smaller events.