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The shear wave velocity, Vs, profile of the subsoil is one of the most important controlling parameters of the seismic site response (Rathje et al., 2010), i.e. the motion modification during the wave propagation through the stratigraphic series, from the bedrock upward to the ground surface. Despite the high heterogeneities and complexities of the geological conditions, generally the shear wave velocity increases with depth due to the effects of geologic age, cementation and overburden stress. This is the condition indicated by the Italian Building Code (ItBC2018) for the application of the simplified approach, based on subsoil categories, to define the seismic action at the ground surface. They exist, however, particular geological settings for which this condition does not occur and specific seismic site response analyses are reqired by the ItBC2018. This is the case, for instance, of the shear wave velocity inversion, when the top layer is stiffer (i.e. exhibiting higher Vs) than the one below (i.e. exhibiting lower Vs) overlaying the bedrock. The present work identifies the most common Italian geological settings where Vs profile inversions are possible, associating typical mechanical and geotechnical properties to each layer. Based on this preliminary subsoil models, 1D site response analyses have been then performed to evaluate the influence of the stiffness inversion on ground shaking in terms of shear strain profiles and acceleration response spectra at the ground surface. In particular, stochastic 1D analyses were carried out via Monte Carlo method assuming assigned statistical distribution for Vs profiles and nonlinear G/G0-? and D-? curves.

The identification of areas susceptible to different co-seismic instabilities is an important issue of the seismic zonation at urban scale finalized to the territory planning and its protection. Among the co-seismic permanent deformations caused by seismic shaking, the fractures, the landslides, the settlements due to liquefaction or compression/densification can be recognized. The seismic compression or densification is a phenomenon producing permanent ground settlements in dry cohesionless soils (clean sands and sands with fine content) inducing damages to structures, infrastructures and lifelines, accordingly with well documented post-earthquake damages of past events. The susceptibility to this co-seismic instability in presence of dry clean sand, silty sand and sandy silty has been evaluated in the present work through the evaluation of the expected permanent ground settlements by means of non-simplified uncoupled methods computing volumetric strains from cyclic shear strains evaluated by means of site response analyses. This procedure was integrated into a parametric study of 1D seismic site response analyses varying relative density (or shear wave velocity) and thickness of compressible layers, intensity of input ground motion, depth of the seismic bedrock. The results have been then processed to define simplified charts differentiated for three different levels of input peak ground acceleration values and for the three considered lithologies (clean sands, silty sands and sandy silts). These latter are mainly finalized to be used at urban scale, in the perspective of Seismic Microzonation (SM) studies requiring input-data commonly available in level 2 and 3 studies that have a strategic application in land use planning in the perspective of the territory protection. A specific methodology was proposed by means of guideline based on a procedure with increasing complexity: 1) preliminary screening; 2) level 1 analyses; 3) level 3 analyses. The areas potentially susceptible to seismic compression identified in this preliminary phase are to be studied in the level 1 of SM, that identifies attention zones by checking the presence of predisposing conditions to the phenomenon. In the level 3 of SM, the susceptible zones and respect zones are identified through the estimation of the settlements by means of the charts proposed in the present work and the seismic site response analysis, respectively.

One of the most important controlling parameters of the seismic site response in terms of earthquake ground motions modification is the shear wave velocity (Vs) profile of the subsoil, that generally increases with depth because of geological age, cementation and overburden stress. There exist, however, geological settings where the velocity profile is characterized by inversions, when a stiffer layer (exhibiting higher Vs) overlies a softer one (with a lower Vs). When referring to the Italian territory, these conditions are widespread and require specific seismic site response analyses, both for structures and infrastructures design and for land planning. The aim of the present work is to study the effect of Vs inversion on seismic site response in recurrent Italian geological domains for application in seismic microzonation studies, i.e., at the miunicipality scale. Firstly, the most common Italian geological domains characterized by inversion of Vs profiles are identified. Afterwards, one-dimensional subsoil columns representative of each identified geological domain are defined, based on Italian database of the seismic microzonation. In the end, based on the subsoil models defined at previous steps, numerical simulations of 1D site response are performed, in order to evaluate the influence of Vs inversion on ground shaking in terms of shear strain profiles and acceleration response spectra at the ground surface. To take into account the variability in the input parameters, stochastic 1D analyses are carried out via Monte Carlo method assuming assigned statistical distribution for Vs profiles and nonlinear variation curves of the normalized shear modulus and the damping ratio with the shear strain.

Evaluation of bedrock stiffness and thickness effect on ground motion modification represent a key-issue in the perspective of seismic risk reduction policy, since the maximum depth of site prospection generally does not extend down to the stiff bedrock location. Site conditions were identified from the Italian database of seismic microzonation studies. Simplified and realistic numerical models were adopted, i.e. extended down to the soft and stiff bedrock, respectively, in order to perform seismic site response analyses. Results indicate that simplified numerical model provides a quantification of the surface ground motion modification with an error generally equal to ±10% with respect to the realistic model. Hence, the evaluation of seismic performance of sites based on simplified numerical models represents a support tool for seismic risk management at large area.

An integrated subsoil model for seismic microzonation in the Central Archaeological Area of Rome is presented in this study. This model was created in the framework of a research project aimed at evaluating the geohazard level affecting Palatine hill, Roman Forum and Coliseum. A multidisciplinary survey was carried out mainly in 2010 including continuous-coring boreholes, in situ and laboratory geotechnical tests, MASW, Cross and Down-Hole tests, ambient noise measurements, Electrical Resistivity Tomographies, Ground Penetrating Radar surveys. The surveying methodologies adopted in the surveying campaign are also presented here. A three step procedure adopted for creating the integrated subsoil model of Palatine hill and surrounding areas is described in detail. This procedure first involves the construction of a geological model starting mainly from boreholes' data interpretation and correlation. Geological formations are here interpreted in terms of lithofacies following their sedimentological features and then characteri-zed by means of their main physical properties (i.e. grain size distribution, void ratio, unit weight, plasticity index) to define a scheme of lithotypes. The lithotypes represent the "framework" to be characteri-zed in terms of dynamical properties to produce the final inputs for 1D and 2D numerical modeling of ground motion amplification effects aimed at the construction of seismic microzonation maps.