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In the present study, a set of three strong motions accompanied by 21 before and after mainshock motions from liquefied soil at the Kushiro Port vertical array site have been obtained. S-wave velocity variation is estimated using a new proposed nonlinear GA inversion technique. This inversion technique is reproductive in assessment for S-wave velocities due to its direct link with the simulation FEM program and matching technique between the observed and simulated waveforms. Layers (6 ∼ 23 m depth) have been found to be responsible for liquefaction at the Kushiro Port site. Stiffness degradation due to liquefaction could be predicted by applying the inversion technique on the horizontal components of the mainshocks at different time-windows and then multiplying the weighting functions by the nonlinear simulated ground motions. These weighting functions were applied to delete the misfit time window of the nonlinear simulated ground motions when these were compared to the observed records. Finally, stacking was applied for the weighted nonlinear simulated ground motions. A strong degree of liquefaction is concluded during the Kushiro-Oki (1993) and the Hokkaido Toho-Oki (1994) earthquakes, whereas a low degree of liquefaction was detected to have occurred during the Tokachi-Oki (2003) earthquake due to high increase of rigidity following a trend that resembles a consolidation curve.

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