Numerical modeling of seismic site effects in a shallow alluvial basin of the Northern Apennines (Italy)

The main purpose of this work is to investigate the basin-edge effects on a cross section of an alluvial basin in the northern Apennines (Italy) and to compare the results with those obtained by applying existing approximate criteria to account for valley effects in cases of simple geometrical and stratigraphic conditions. The combined influence of geometry and layering of the basin was evaluated by performing 1D and 2D numerical analyses. The examined site is located in one of the highest seismicity zones of Tuscany. Geological, geophysical, and geotechnical information is obtained from extensive surveys carried out in the area as part of a pioneering Italian project (1998–2004) promoted and supported by the Regional Government of Tuscany to address microzoning studies on an urban scale. Five actual accelerometric signals were adopted in the analyses, consistent with a site reference spectrum defined according to Italian building code. To estimate the additional valley effects on the ground surface of the basin, numerical results from a 2D model were compared with the corresponding 1D numerical results obtained for an isolated soil column at the same location. A large number of vertical soil profiles were analyzed along the cross section, and different types of amplification factors were computed with reference to both peak ground accelerations and elastic response spectra. The results obtained were compared with those of existing approximate approaches for estimating basin effects on basic geometric and geological conditions (sine- or trapezoidal-shaped valley, homogeneous soil deposits), fitting the actual cross section by a simplified symmetrical trapezoidal scheme.