Two case histories are presented which refer to the historical towns of Orvieto and Bisaccia, both located on top of rock buttes overlying a more deformable clayey substratum. The comparison of the two case histories indicates that apparently similar geological conditions do not lead to the same type of seismic response. In fact, the specific physical and dynamic properties of the substratum and the overlying slab can determine different seismic behaviour at the hill top and at the rock-clay interface. In particular, at Bisaccia a deamplification of the seismic motion at the hill top was predicted, as well as the development of excess pore pressures in the clay deposit underneath the conglomerate. On the other hand, at Orvieto significant amplification of surface motion is expected, due to the impedance contrast between the different pyroclastic materials of the rock slab and to the topography effect at the slab edge as well
