Approximate decoupling of topographic, stratigraphic and valley effects on the peak seismic acceleration

The seismic amplification of any given area is a combined result of topographic, stratigraphic and valley effects. Nevertheless, quantifying the contribution of each of these three effects in the overall amplification is not straightforward. To this end, this paper proposes a procedure for the approximate decoupling of these effects on the amplification of the peak seismic acceleration at the ground surface. This is performed by setting the pertinent site amplification factor equal to the product of three spatially-variable amplification factors, one for each of these effects. The procedure employs two-dimensional (2D) linear visco-elastic numerical simulations and simplified analytical formulations, the latter aiming at the quantification of the one-dimensional (1D) amplification of soil-bedrock and (solely) bedrock columns. As an example, the decoupling procedure is applied to a slope with irregular topography, characterised by a homogeneous soil layer overlying bedrock. The ground response analyses employ Ricker wavelet excitations, characterised by different predominant periods covering the range of periods related to typical earthquakes, as well as actual seismic recordings. The results depict that the three amplification factors depend on the seismic excitation and vary widely along the study area (taking values that range from 0.6 up to 1.7). The proposed procedure is an approximate, but reliable tool for decoupling the spatially-variable contributions of the aforementioned effects, at least when the visco-elastic conditions are realistic. As such, it may be used as an expeditive tool for identifying regions mostly affected by each of these effects, thus guiding the need for additional site investigation or in-depth analysis.