GNGTS 2015 - Atti del 34° Convegno Nazionale

1D modelling of site effects. The local seismic response is a very important task as the ground shaking caused by earthquakes can be significantly amplified by the soil geotechnical characteristics, causing localized heavy damage to buildings and population. 1D and 2D modelling techniques are applied to quantify the soil amplification with respect to the expected ground motion at bedrock. From the geotechnical model (Rivera et al ., 2013), we have obtained 225 stratigraphic models for the whole study area (15 km × 15 km). We have considered a grid of points every 250 m, associating each of them to a stratigraphic model. The uniform hazard response spectra (UHRSs) at bedrock calculated by Alvarez et al . (2015) have been taken as ground shaking, considering a partition of the study region into 6 sub areas. Two techniques for soil modelling have been applied: a 1D modelling of seismic wave propagation in a stratified halfspace, and a 2D modelling using the boundary element technique in a lineal analysis. The 1D modelling [software PSHAKE by Sanò and Pugliese (1991)] was applied in sectors where the geotechnical model shows a homogeneous soil behaviour in depth (almost flat parallel layers). The 2D modelling [software BESOIL by Sanò (1996)] was used in areas of complex stratification with strong lateral variations; this code applies a fast numerical calculation of the seismic wave propagation in space. Both modelling techniques request the geometry of the different soil layers and their geotechnical properties (V S , density, Poisson coefficient, damping). The modelling of the wave motion has been calculated from the bedrock depth to the free surface. The AF for each model has been estimated as the ratio of the output over the input spectrum in the range of periods from 0.1 to 0.5 s. Fig. 3 shows the values of the AFs obtained for Santiago de Cuba. No amplification is expected along the coast of the Caribbean Sea with the exception of a small portion to the SW, where Quaternary sediments appear. A similar behavior can be seen to the north and west of the bay, where volcanogenic sedimentary rocks outcrop. A small sector located to the east of the bay shows deamplification: it coincides with more than 100-m thick sandstones. The largest amplifications refer to low compact soils (for example clay, sand or gravel). Comparison between local AFs and those of building codes. AFs based on the NERHP (BSSC, 2001) soil classification have been defined in addition to the local AFs calculated by modelling. The AFs computed by soil modelling (Fig. 3) differ from those obtained from the soil classification (Fig. 2) by the NEHRP provisions (BSSC, 2001) because both 1D and 2D modelling techniques take into account various soil parameters than shallow geological characteristics only, they are: the complete stratigraphic section from the free surface to the bedrock, the thicknesses of the various layers with the respective values of density and velocity, the degradation curves of each material and the specific UHRS for each site. Fig. 2 – AFs for the Santiago de Cuba broader region according to the NERHP (BSSC, 2001) provisions on the basis of soil types derived from surficial geology. 174 GNGTS 2015 S essione 2.2