GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 GNGTS 2023 Furthermore, 3D numerical simulation allows us to evaluate two additional ground motion parameters of engineering interest that cannot be captured in 2D analysis: the polarization and rotational motion. Figs. 3a and 3b show the peak values of the two horizontal components of the time-domain response function corresponding to a soliciting impulse of the same direction. It can be seen that the N-S component has the largest peak values along the hilltop. This behavior could be related to the vibration of the ridge in the component perpendicular to its extension. On the other hand, we can observe that the peak values of the E-W component have local maxima on the secondary N-S oriented ridges, which slope down from the main ridge in agreement with the outcrop of arenaceous lithofacies (Fig. 1b). Moreover, the Borgo valley does not seem to be affected by polarization effects. Following the calculation method devised by Stupazzini et al. (2009), the possible occurrence of torsional motion is verified. Fig. 3c shows the scaling factor between the site-induced angular velocity about the horizontal axis (i.e., torsional motion) and the translational acceleration of the vertically incident plane wave. The figure shows that both steep relief and basin edges are susceptible to torsional motion. As with the amplitude of seismic motion, the presence of deposits and fractured layers increases the magnitude of torsional motion caused by seismic excitation. Fig. 3. (a) The distribution of the peak values of the time-domain response function horizontal E-W components (a) and horizontal N-S components; (b) The distribution of the peak values of the time-domain response function horizontal components (a) and horizontal E-W components; (c) Scaling factor A between the site-induced torsional motion and the translational acceleration of the vertically incident plane wave, averaged in the frequency band 4–8 Hz.