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Large earthquakes occur rather orderly in space and time; hence they can be somehow anticipated, and their effects can be projected into the future. The modern practice of seismic hazard assessment rests on these principles and may rely on them, but also requires a detailed knowledge of the location and characteristics of individual earthquake sources. We discuss how this knowledge base can be constructed, with an eye on the geological history , which provides a record of the faults capable of generating large earthquakes, and one on the human history , which supplies evidence for whether, where and how these faults have actually caused damaging earthquakes in the past. How do these two records interact with each other? It is now accepted that identifying active and potentially seismogenic faults in Italy is especially hard. The geological record may be clear and honest when dealing with processes at the scale of several million years, but can be very difficult to decipher if we are concerned with contemporary geological processes, such as earthquakes. Shortening the tim E-W indow of observation of earthquake activity is why Historical Seismology is so crucial for constructing a seismogenic source model. To this end we exploited a number of key Italian destructive earthquakes, each of which illuminates a recent geological process that may not offer a discernible surface signature. Our findings led us to reconsider the tectonic style of large areas, changed our perception of their earthquake potential, hinted at the existence of unknown seismogenic zones, and even led to downsizing the magnitude of the largest Italian historical earthquakes. We maintain that the complexity of the geological setting may be counterbalanced by the richness of the historical earthquake record. We also believe that our experience in the combined investigation of Italy's historical earthquakes and seismogenic sources may be replicated in all earthquake-prone countries that exhibit a history of human settlement throughout the millennia. [ABSTRACT FROM AUTHOR]

Abstract: We designed a new seismic source model for Italy to be used as an input for country-wide probabilistic seismic hazard assessment (PSHA) in the frame of the compilation of a new national reference map. We started off by reviewing existing models available for Italy and for other European countries, then discussed the main open issues in the current practice of seismogenic zoning. The new model, termed ZS9, is largely based on data collected in the past 10 years, including historical earthquakes and instrumental seismicity, active faults and their seismogenic potential, and seismotectonic evidence from recent earthquakes. This information allowed us to propose new interpretations for poorly understood areas where the new data are in conflict with assumptions made in designing the previous and widely used model ZS4. ZS9 is made out of 36 zones where earthquakes with M w >=5 are expected. It also assumes that earthquakes with M w up to 5 may occur anywhere outside the seismogenic zones, although the associated probability is rather low. Special care was taken to ensure that each zone sampled a large enough number of earthquakes so that we could compute reliable earthquake production rates. Although it was drawn following criteria that are standard practice in PSHA, ZS9 is also innovative in that every zone is characterised also by its mean seismogenic depth (the depth of the crustal volume that will presumably release future earthquakes) and predominant focal mechanism (their most likely rupture mechanism). These properties were determined using instrumental data, and only in a limited number of cases we resorted to geologic constraints and expert judgment to cope with lack of data or conflicting indications. These attributes allow ZS9 to be used with more accurate regionalized depth-dependent attenuation relations, and are ultimately expected to increase significantly the reliability of seismic hazard estimates. [Copyright &y& Elsevier]