In Situ Soil-Specific Nonlinear Properties Back-Calculated from Vertical Array Records during 1995 Kobe Earthquake

Abstract

In situ nonlinear soil properties are investigated based on vertical array records obtained during the 1995 Hyogoken Nambu earthquake (Kobe earthquake) at soft soil sites near the earthquake fault zone. Spectrum ratios between ground surface and deeper levels are calculated for the main shock and associated small shocks and S-wave velocities and damping ratios in surface soil layers to best reproduce them are back-calculated by means of an inversion analysis (extended Bayesian method) assuming one-dimensional vertical horizontal shear (SH)-wave propagation. Obvious differences in S-wave velocities and damping ratios are found between the main shock and the small shocks. Clear strain-dependent modulus degradations which can be differentiated for different soil types are recognized. The degradations are essentially consistent with some of laboratory test results to date for each soil type at least for $G∕G_0\approx 0.5$ or larger, though for gravelly soils back-calculated values tend to show milder degradations than laboratory test results presumably due to large inclusion of fines in actual ground. Back-calculated damping ratios show essentially the same trend as in laboratory tests, although the absolute values are in most case a few percent higher in the strain range smaller than ${10}^{-4}$ .