Regional scale geophysical parametrization for the development of simplified method to assess the 1D seismic amplification: the case of Piedmont Region (Northern Italy)

In the framework of Italian Seismic Microzonation studies, regional authorities have equipped themselves with simplified procedures, named as “seismic abacuses”, aimed at estimating the amplification of seismic motion. In particular, the purpose of these abacuses is to perform the quantitative characterization of the amplification phenomena expected in lithostratigraphic situations characterized by flat and horizontal layers. These are essentially tables in which a set of values ​​of geophysical parameters considered diagnostic are uniquely associated with expected values ​​of the ground motion amplification in terms of "Amplification Factor" (AF) with respect to a reference motion. The abacuses were defined on the basis of 1D seismic response analyses of real cases considered characteristic and significant of the local lithological, geotechnical and geophysical context of each region. The AF values ​​reported in the regional abacuses correspond to different percentiles of the total number of cases for the period interval considered.The development of the seismic abacuses generally includes 4 steps: geological/geotechnical characterization of the regional territory, parameterization, numerical simulations, statistical analysis and construction of representative abacuses. This work is devoted to describe and summarize the various phases of the procedure done to develop the seismic abacuses of the Piedmont region (Northern Italy).In the first phase, geological, geotechnical and geophysical information collected from regional authority’s repositories were used to define the Geological Domains (GDs), i.e., the areas characterized by similar tectonic and/or depositional history. In the second phase, the seismic, geometric and geotechnical parametrization of the cover terrains for each GD is performed: in particular, the shear-wave velocity (Vs) profiles collected were used to define several models describing the trend of these values as a function of the depth following a power law. As concerns the geotechnical aspects, since the lack of laboratory data, shear modulus reduction and damping ratio curves as a function of shear strain provided for the cover terrains in the Italian territory were considered. The numerical simulations, which represent the third step, were carried out using the NC92Soil software considering an equivalent-linear approach and following the Inverse Random Vibration Theory procedure. In this step, the power-law models previously defined were used to generate via randomization procedure a set of 5000 seismo-stratigraphical profiles for each GD. The last step was devoted to the drawing-up of the abacuses. In this phase, AF values were computed and their population obtained for three period intervals, for each of the GDs and for each of the hazard levels was statistically analyzed and each AF value was classified according to the respective values ​​of geophysical proxy parameters chosen. At the end of the whole procedure, an ensemble of 39 tables was provided to the regional authority.