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

For urban areas where numerical modelling has been successfully compared with strong motion records, the theoretical computations of synthetic seismograms permit a detailed microzoning based on a set of possible earthquake scenarios. For areas where very limited recordings are available, the synthetic time series can be used to estimate the expected ground motion, thus leading to a pre-disaster microzonation without the need to wait for an earthquake 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 input. In the framework of UNESCO-IUGS-IGCP project 414, `Realistic Modelling of Seismic Input for Megacities and Large Urban Areas', two deterministic approaches have been applied to several urban areas for the purpose of seismic microzoning. The two approaches, valid for anelastic media, are the hybrid technique, based on the modal-summation technique and ®nite differences, and the mode-coupling analytical technique. We show three examples of the application of the deterministic techniques for seismic hazard assessment at urban scale, discussing the results for the cities of Rome, Catania and 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 the prevention aspects of Civil Defence.