On the combined effect of topographic irregularities and wave passage on the spatial variation of seismic ground motion

The spatial variation of ground motion (SVGM) can significantly affect the response of distributed infrastructure systems including bridges, dams, and pipelines. SVGM-related phenomena are the result of the combination of three effects: (1) wave passage, (2) geometric incoherence of the input, and (3) ground- and site-effects due to soil layering, the presence of topographic irregularities, and/or alluvial basins. Existing models are able to capture wave passage and geometric incoherence effects, while ground- or site-effects are either not modeled, or treated in a simplified manner. We perform a numerical experiment by means of the finite element method, exploring the combined effect of wave passage and the presence of 2D topographic features. Such combined effects were observed following recent earthquakes, but were never analyzed using a comprehensive set of numerical analyses. We explore two regularly shaped canyons (semi-circular and V-shaped) and a real case study for a canyon crossed by a multi-span bridge in Italy: the Viadotto Italia. We show that the combined effect of wave passage and topographic features can modify the amplitude and shape of the input motion, altering soil-structure interaction processes involving bridge piers. We also show that path effects, related to the direction of the input motion, have an influence on how input ground motions are modified. For all canyon shapes explored, there is a significant amplification of the vertical component of the motion that sometimes becomes comparable to the horizontal components. We anticipate that outcomes from this research would improve future engineering models and/or site-specific studies.