Vertical array records obtained at Kobe Port Island during the Hyogo-ken nanbu earthquake were analyzed. Acceleration records at four different depths were obtained. The peak horizontal acceleration was minimum at the ground surface and increased with depth, whereas, the peak vertical acceleration, the peak horizontal velocity, and displacement all decrease with depth.
The de-amplification of the horizontal acceleration was due to non-linear effect of soils.The amplification of the vertical acceleration was caused by small P-wave velocity in the unsaturated fill layer which resulted in a large velocity contrast at the boundary between the unsaturated and saturated layer.
Using ground response analysis, the equivalent linear soil parameters were obtained for 50% of the rigidities of the shear wave velocity with PS-logging.
A method of synthesis of strong ground motion by summation's aftershock records was applied to simulate the main shock at the ground surface and at a depth at GL-83 m. Synthesized strong motion at GL-83 m compared relatively well to the recorded one. Strong ground motion synthesized at the ground surface indicated twice larger peak acceleration than the observed. The smaller amplitude observed at the ground surface was due to the non-linear effect of soils.