GNGTS 2019 - Atti del 38° Convegno Nazionale

384 GNGTS 2019 S essione 2.2 EFFECT OF SHEAR WAVE VELOCITY INVERSION ON 1D SEISMIC SITE RESPONSE S. Fabozzi 1 , G. Falcone 1 , M. Moscatelli 1 , A. Pagliaroli 2 , E. Peronace 1 , A. Porchia 1 , G. Romagnoli 1 1 Institute of Environmental Geology and Geoengineering IGAG - National Research Council of Italy CNR, Italy 2 Department of Engineering and Geology – University of Chieti Pescara G. D’Annunzio, Italy Introduction. 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 definte 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 identifys 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/G 0 -g and D-g curves. Geological settings with Vs profile inversion. Three main geological-depositional settings in Italian territory, generally exhibiting Vs inversion profiles, have been identified in this work as it follows: 1) FLUVIALAND MARINE TERRACES It is the case, for example, of Floridia village area (Panzera et al. , 2019). - STIFF LAYER → FLUVIALTERRACES: terraced alluvial deposits mainly consisting of sandy-silty gravels, a mixture of gravels, sands and silts. MARINE TERRACES: terraced marine deposits consisting of breccias, conglomerates and slightly cemented calcarenites. The finer fluvial and marine terraced deposits (sands and silts) are excluded since they are characterized by lower Vs and can hardly give rise to phenomena of Vs reversal profile. - SOFT LAYER → Plio-Pleistocene marly-clays and clays deposits, covered by fluvial and marine terraced deposits, generally distributed near the coast lines. 2) CLIFFS AND LAVA ROCK LAYER It is the case, for example, of Orvieto, Gerace and Bisaccia cliffs (Lanzo et al. , 2004) and Catania (Catalano et al. , 2017). - STIFF LAYER: tuffs and lava rocks; - SOFT LAYER: Plio-Pleistocene marly-clays and clays deposits. 3) TRAVERTINE PLATEAU ON ALLUVIAL PLAINS It is the case, for example, of Rieti hill where, in the east side of the intermontane plain, a large plateau of travertine, is located above a silty-sandy alluvial and lacustrine deposits. - STIFF LAYER: travertine or similar lithic layer; - SOFT LAYER: sand and silt with clayey intercalations and peaty levels. As it can be deduced from the above description, the identified geological domains could exhibit different geometrical and geotechnical properties. A detailed analysis of numerous data in such domains, extrapolated from the data-base of the seimic microzonation studies ( DB-MS )