Preliminary investigations into two-dimensional aggravation phenomena of seismic ground response in two-layered shallow sedimentary basins

The influence of local geological and soil conditions on ground motion and earthquake-induced damage is widely recognised, and provisions accounting for local site effects are included in some seismic building codes. However, while most of the standards provide simplified procedures to consider the 1D stratigraphic effects and some also consider 2D effects of surface topography, the effects related to subsurface geometry are essentially neglected. This paper illustrates the main results drawn from an initial investigation into 2D aggravation effects in symmetric basins overlain by two-layered linear visco-elastic deposits. A preliminary investigation is conducted on 2D aggravation factors (AGFs), defined as the ratio between the pseudo-acceleration response spectra obtained from 2D and 1D numerical ground response analyses performed on several simplified schemes. The results include: 1) the qualitative visual representation of the relationships between the AGF and a set of dimensionless parameters descriptive of basin geometry, stratigraphy, soil properties, and input motion; and 2) the outcomes of a non-parametric statistical test aimed at quantifying the statistical dependency between the AGF and the above-mentioned dimensionless parameters. Useful indications are obtained for carrying out subsequent analyses aimed at the development of simplified procedures for the inclusion of basin effects in design codes.