For urban areas where the numerical modelling has been successfully compared with strong motion records, the theoretical computations of synthetic seismograms permit a detailed microzoning based upon a set of possible scenario earthquakes.
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 having 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 upon the properties of the seismic source generating the seismic input.
The data set of synthetic seismograms can be fruitfully used and analysed by civil engineers for design and reinforcement actions, and therefore supply a particularly powerful, and economical, tool for the prevention aspects of Civil Defence.
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 finite-differences, and the mode-coupling analytical technique.
We show some examples of the application of the deterministic techniques and of empirical methods for seismic hazard assessment at urban scale and for site effects estimation.