NONLINEARITY, LIQUEFACTION, AND VELOCITY VARIATION OF SOFT SOIL LAYERS IN PORT-ISLAND, KOBE, DURING THE HYOGO-KEN NANBU EARTHQUAKE

Clear nonlinear behavior is analyzed from the acceleration records of the 1995 Hyogo-ken Nanbu earthquake at Port Island, Kobe, From four tr iaxial instruments placed at four different depths, the surficial effe cts during strong ground motions were compared with those during weak motions before and after the mainshock. We used a spectral ratio techn ique and a nonlinear inversion for velocity structure to analyze the d ata. From the spectral analysis, we observed a large variation of the spectral ratios between the surface and different depths during the st rong ground motions and during the Liquefied state. The spectral ratio s after the mainshock (i.e., after the liquefied state) are different from those before the mainshock. The peak frequencies in the spectral ratios after the mainshock are shifted to lower frequencies with respe ct to those in the spectral ratios before the mainshock. We inverted t he S-wave velocities using a genetic algorithm technique to determine the velocity structure before, during, and after the mainshock. The S- wave velocity structure before and after the mainshock was found to be different. Specifically, the S-wave velocity of the second layer (5 m to 16 m depth) after the mainshock was 20% lower than before. Our ana lysis shows that the liquefied state remains at least 3 hr after the m ainshock but no more than 24 hr. The rigidity of the soil decreased cl ose to zero when liquefaction happened and later increases gradually f ollowing a trend that resembles a consolidation curve. The strong infl uence of nonlinearity during the mainshock yielded a big reduction of the horizontal surface ground motions, so that the observed horizontal peak acceleration was only about 25% of the peak acceleration expecte d from the linear theory. However, the nonlinear effects in the vertic al peak acceleration were not significant.