Spectral probabilistic seismic hazard assessment for Italy

During the last decade the Italian ″Gruppo Nazionale per la Difesa dai Terremoti″ has undertaken a project for assessing seismic hazard on probabilistic bases in the national territory , to be used as scientific background for the revision of the current seismic zonation; the seismic hazard was expressed there in terms of peak ground acceleration and macroseismic intensity. A more detailed analysis is now performed. Probabilistic spectral seismic hazard maps at different frequencies of engineering interest demonstrate that the soil condition is a first order factor influencing the level of the expected shaking, with an average increase of about 0.2 9 for the prediction related to a 475-year return period, when passing from rock to soft soil. The attenuation model similarly conditions the results, but in a more magnitude-dependent fashion. Here, the two relationships selected account for the effects of strong rare seismicity and moderate frequent earthquakes differently: differences going from 20% to 80% of the predicted spectral accelerations are, therefore, model-dependent. The uniform hazard response spectra for some major Italian towns are then compared to the elastic response spectra of the present Italian seismic zonation, indicating that two important towns such as Rome and Naples are not adequately represented by the present code. The effective peak acceleration is considered, finally, a good synthetic parameter for representing seismic hazard in most engineering applications.