Applications of the modal summation technique to the theoretical site response estimation

A practical definition of site effect, that accounts for both the seismolog ical and engineering point of view, is the one given by Field(7): "the uniq ue behavior of a site, relative to other sites, that persists given all (or most) of the potential sources of earthquake ground motion in the region". This definition implicitly indicates the difficulties connected with a cor rect estimate (or prediction) of the site effect. We estimate theoretical s ite responses adopting several techniques and using different seismic motio n types. Our results confirm that the identification of the site behavior w ith a set of resonance frequencies can be a very difficult task, especially when the amplification levels seem to be "azimuthally" dependent. We show that the adoption of simplified models can lead to misleading conclusions c oncerning the seismic response of sedimentary basins. The results suggest t hat, in order to perform an accurate estimate of the site effects in compli cated geometries, it is necessary to make a parametric study that takes int o account the complex combination of the source and propagation factors.