Analisi di fenomeni deformativi di pendii e rilievi in condizioni sismiche: il caso di Gerace

The case history presented in this study is referred to the historical centre of Gerace, located on a soft rock cliff in the most hazardous seismic zone in Calabria (Southern Italy). The study describes the hazard analysis, the subsoil model and the numerical simulations of the seismic sequence occurred in the area during 1783, when five strong earthquakes in less than two months induced large deformations and slope failure along the hill. The reference seismic scenario was simulated through a detailed analysis of hystorical and seismogenic data; a multiple-approach estimate of synthetic ground motion parameters allowed to choose a set of acceleration time histories to simulate the seismic sequence. The subsoil model for the dynamic seismic response analyses was set up by combining data collected from previous investigations and experimental results gathered by the authors through dynamic in situ (Down-Hole, MASW and SASW) and laboratory (RC, CTS, CSS) tests. 1D and 2D seismic response analyses were carried out by EERA and FLAC 5.0 on two sections developing along the longitudinal and transversal axes of the town. For each section, the ground motion was analysed in terms of horizontal and vertical profiles of peak accelerations, and response spectra; the deformation pattern was interpreted by means of peak and residual shear strain distributions, and contours of displacements. The 2D analyses evidenced the stratigraphic amplification mechanisms affecting the subsoil response: the ground motion resulted even attenuated in the thick varicolour clay formation beneath the soft rock slab, but amplified in the overlying marl and clay layers. Furthermore, the analyses showed that the distribution of peak ground acceleration was characterised by fluctuations and discontinuities, depending on the outcropping of different materials along the topographic surface. The comparison between the results of both 1D and 2D analyses, expressed in terms of response spectra, permitted to assess the influence of topography on the ground motion in the town centre. Finally, the pattern of calculated permanent strains and displacements permitted to justify the occurrence of large deformations along the slopes, reported by the chronicles.