Seismic microzonation by means of finite element approaches

When an earthquake occurs, seismic waves radiate away from the source and travel rapidly through the earth's crust. The motion recorded at the ground surface of an area could be really different, in terms of duration and frequency content, from the reference outcrop motion due to the following site conditions: sequence of soil layers, velocity contrast between soil layers, thickness of each layer, dynamic behaviour of the soils, topography and geometry of the sub-interface. Concerning the site-effects, it is possible to discuss about one-dimensional (1D) effects, two-dimensional (2D) and three-dimensional (3D) ones. One-dimensional ef-fects are induced in case of horizontally layered deposits with a horizontal ground surface (vertically heterogeneous media). Two-dimensional site-effects are generated in case of a complex stratigraphic sequence (vertically and laterally heterogeneous media) and/or in case of an uneven ground surface. In presence of stratigraphic and topographical surface varying in any direction (vertically, laterally and transversally heterogeneous media), it is necessary to refer to three-dimensional site-effects. Seismic microzonation (SM) studies are used to assess local geological and ge-otechnical site conditions and to identify earthquake characteristics. A SM study can be undertaken according to three different levels of details, as reported in the Italian guidelines. In particular, numerical analyses are requested for a level III SM to quantify the reference motion modification. This work aims to evaluate complex site-effects for a real case study, i.e. the Bo-vino village, located in South of Italy. This case study has been chosen due to the presence of a soft soil valley surrounded by rock outcrop hills. As a consequence complex site-effects are expected. Essential ingredients for predicting site effects are: topography, stratigraphy, interface between soil layers and dynamic soil behaviour. Moreover it is necessary to select the reference seismic event and to define properly the input motion used in the numerical analyses. The present thesis addresses how to perform a site response analysis using the finite element (FE) method. Two different FE codes, in the time domain, have been used: QUAKE/W, based on the equivalent-linear approach, and PLAXIS 3D, which adopts a non-linear Hardening Soil model with small strain stiffness (HSs). At first, some numerical approaches to simulate 1D site response are defined with reference to ideal case studies. These approaches have been validated by compari-son with results obtained with the code EERA, which is based on an equivalent-linear approach in the frequency domain. 1D schemes have also been used to clarify the definition of reference motion and numerical input motion. The numerical simulations of seismic site response for the Bovino case study are then presented. Before discussing the results of these analyses, local geology and topography are described. The geotechnical model is subsequently defined, based on field investigations and laboratory data. The reference outcrop motion is then selected, according to the Italian probabilistic seismic hazard maps. Seven real accelerograms have been selected as reference outcrop motions. The results of the 2D analyses performed using QUAKE/W, with reference to 22 sections, are presented first. The results of the 2D analyses are compared with those of 1D analyses performed with reference to 42 soil columns, extracted along two sections. Finally, results of 1D, 2D and 3D analysis performed with PLAXIS 3D, assuming two reference motions, are discussed. The analyses allowed to identify the effects of dimensional scheme to seismic site response, the dependency of each amplification pattern to the selected reference motion, the ground motion modification due to dif-ferent topography and soil layers interface, the comparison between different consti-tutive approaches to the same problem (i.e. linear equivalent and non-linear).