In situ assessment of soil dynamic parameters for characterizing nonlinear seismic site response using KiK-net vertical array data.

In this study, we utilize acceleration time histories data at eight KiK-net vertical arrays in Japan, recorded between January 2009 to June 2014, to analyze nonlinear seismic site response. Ground motion records provided by seismic vertical arrays allow estimation of in situ soil dynamic parameters. First, the shear modulus is computed from the shear wave velocity using seismic interferometry by deconvolution. Considering each record as an in situ cyclic test under seismic loading, the shear strain is computed as the velocity ratio between two sensors and the shear stress is assumed as a spatial function of accelerations. Using classic hyperbolic regression models, we quantitatively assess the relationships between the in situ soil dynamic parameters and the level of ground motion. We found nonlinearity starts at a quite low level of shear strain of 5 × 10−6–5 × 10−5, and becomes much more significant beyond a relatively high level of strain of 1 × 10−4. The threshold strains from ground motion observations are well consistent with those from previous laboratory tests. Finally, a parametric analysis is presented to discuss the influence of plasticity index on nonlinear soil behavior evaluations. It is concluded that the threshold acceleration of soil nonlinear behavior (generally ranging from 20 to 100 cm/s2) is mainly controlled by the stiffness and plasticity index, and the degree of soil nonlinear behavior is mainly controlled by the stiffness, plasticity index and the level of ground motion. [ABSTRACT FROM AUTHOR]