Inversion of Strain-Dependent Nonlinear Characteristics of Soils Using Weak and Strong Motions Observed by Borehole Sites in Japan

Abstract

During the 1995 Hyogo-ken Nanbu earthquake, strong motions with horizontal peak ground accelerations of 507 cm/sec 2 or more, 187 cm/sec 2 , and 126 cm/sec 2 were observed at surface stations of borehole sites at Amagasaki, Takasago, and Nanko in Japan, respectively. We showed qualitative evidence of nonlinear soil response during strong motions at Amgasaki and Takasago, but not at Nanko, using cross correlation and spectral ratio analyses of weak and strong motions. We evaluated soil nonlinearity by inversion for S -wave velocities and frequency-dependent damping factors. The strain-dependence of shear modulus ratios inverted for gravel is larger than that for clay, which is consistent with laboratory test results. In the shear-strain range from about 10 –4 to 10 –3 , the strain dependence of shear modulus ratios inverted for clay and gravel at Amagasaki and Takasago is consistent with empirical relations based on laboratory test results for clay and gravel, respectively. These results support using laboratory estimates of shear modulus ratios to simulate strong motions at clay and gravel sites. The damping factors for gravel inverted from the mainshock at Amagasaki are the largest among the others, but the strain dependence of damping factors are not as significant as that of shear modulus ratios. The reason why the strain dependence of damping factors is not so significant may be because scattering attenuation is relatively dominated in the low-strain range.