Four down-hole array records obtained during the 1995 Hyogoken Nambu earthquake are used to investigate the nonlinear characteristics of amplifications of vertical ground motions in various soils. Significant vertical accelerations were observed at some sites in this earthquake where soil deposits strongly behaved nonlinearly. The soil parameters under strong ground motions are identified by combining downhill simplex optimum method with multiple reflection theory. The nonlinear soil properties

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... are evaluated by comparing the identified dynamic soil parameters with those under PS logging. The nonlinear seismic responses of both horizontal and vertical ground motions are calculated based on the soil parameters identified.