One-dimensional seismic response of two-layer soil deposits with shear wave velocity inversion

The paper presents the results of a parametric study with the purpose of investigating the 1D linear and equivalent linear seismic response of a 30 meters two-layer soil deposits characterized by a stiff layer overlying a soft layer. The thickness of the soft layer was assumed equal to 0.25, 0.5 and 0.75 H, being H the total thickness of the deposit. The shear wave velocity of the soft layer was assumed equal to Vs=90 and 180 m/s while for the stiff layer Vs=360, 500 and 700 m/s were considered. Six accelerograms extracted by an Italian database characterized by different predominant periods ranging from 0.1 to 0.7 s were used as input outcropping motion. For the equivalent liner analyses, the accelerograms were scaled at three different values of peak ground acceleration (PGA), namely 0.1, 0.3 and 0.5 g. The numerical results show that the two-layer ground motion is generally deamplified in terms of PGA with respect to the outcrop PGA. This reduction is mainly controlled by the shear wave velocity of the soft layer, being larger for lower Vs values, by the amount of nonlinearity experienced by the soft soil during the seismic shaking and, to a minor extent, by the thickness of the soft soil layer.