Realistic modelling of waveforms in laterally heterogeneous anelastic media by modal summation

For urban areas where numerical modelling has been successfully compared wi th strong motion records, the theoretical computations of synthetic seismog rams permit a detailed microzoning based on a set of possible earthquake sc enarios. For areas where very limited recordings are available, the synthet ic time series can be used to estimate the expected ground motion, thus lea ding to a pre-disaster microzonation without the need to wait for an earthq uake to occur. In both cases the use of synthetic computations is necessary to overcome the fact that the so-called local site effects can be strongly dependent on the properties of the seismic source generating the seismic i nput. In the framework of UNESCO-IUGS-IGCP project 414, 'Realistic Modellin g of Seismic Input for Megacities and Large Urban Areas', two deterministic approaches have been applied to several urban areas for the purpose of sei smic microzoning. The two approaches, valid for anelastic media, are the hy brid technique, based on the modal-summation technique and finite differenc es, and the mode-coupling analytical technique. We show three examples of t he application of the deterministic techniques for seismic hazard assessmen t at urban scale, discussing the results for the cities of Rome, Catania an d Augusta (Italy). The data set of synthetic seismograms can be fruitfully used and analysed by civil engineers for design and reinforcement actions, and therefore supplies a particularly powerful, and economical, tool for th e prevention aspects of Civil Defence.