Influence of geometrical and geophysical parameters on the seismic site amplification factor

An evaluation of the intrinsic of 1D modelling for estimation of the site seismic response (DAF, Dynamic Amplification Factor) in the case a non-horizontal basement is presented and discussed. 1D models are still commonly used in engineering practice. Therefore it is important to understand the limits of the 1D modelling in order to simplify the problem without dangerous errors when evaluating the seismic response. Up-to-date and robust visco-elastic non-linear 1D and 2D algorithms were used. The reference accelerogram is relative to an Umbria earthquake (Colfiorito - Italy, 26/09/97, M=5.6), recorded at the Department of Geophysics and Volcanology, University of Naples Federico II. The results indicate that in the case of a dipping bedrock, if the angle is less than 10°and the S-wave velocity contrast within the overburden is included between 1 - 0.5, the evaluation of the site response given by both the algorithms is similar. For dip values exceeding 10°, the DAF spectra start to diverge, both in terms of amplitude and peak periods. If the S-wave velocity contrast is less than 0.5 the 1D and 2D DAF spectra are substantially different even if the bedrock is horizontal and this is due to the different method of DAF calculation used by the two algorithms. The results obtained with theoretical models were confirmed by comparing the 1D and the 2D response in two test sites characterised by a dipping volcanic tuff basement, in the city of Naples. The response of the 1D models in such geological conditions is inaccurate. Therefore the use of 1D algorithms for DAF evaluation during seismic site planning should be restricted to those sites having geophysical and geometrical properties compatible with those illustrated in the results.