Italian intensity hazard maps: a comparison of results from different methodologies

Most of the seismic hazard studies of the Italian territory developed over the last decade make use of the standard Probabilistic Seismic Hazard Analysis (PSHA), based on the well known methodology proposed by Cornell (standard approach). Recent studies on this methodology check the weight and the importance on the final results of the choices made in the computational method and introduce different procedures for the evaluation of the uncertainties. In this paper, hazard estimates for Italian municipalities, provided by an improved version of the standard methodology, have been compared with those obtained through a new procedure based on a different approach. Basic aspects of this approach (site approach) are the extensive use of intensity data, extracted from documentary sources available at the investigated localities and a more complete and coherent treatment of the different sources of uncertainty involved in seismic hazard evaluations. The site approach maps, which have to be considered as equivalent and alternative to those obtained with standard PSHA, show significant differences from the latter ones. These differences appear in no way systematic and homogeneous as previous studies showed. Hazard estimates obtained from the site approach are generally more spatially heterogeneous than those computed by the standard approach. Furthermore, site estimates are generally higher than standard ones in low seismicity areas and where local information about effects of past earthquakes is relatively poor. Standard estimates, instead, are higher in high seismicity areas and, also, where better information exists about the local seismic history.