Contour mapping of Italian seismicity

The scope of the present work is to identify the Italian seismic regions, toward a better definition of the active tectonic structures. Spatial filtering is used, together with the recently released PFG-ENEL seismic catalog which covers the period 1000-1980. Any such approach based on catalog data has to acknowledge that catalog reliability and resolving power are functions of time and position. These problems, which can potentially lead to an incorrect regionalization, can be attributed to catalog incompleteness. The effects of the latter are estimated and correlated through a new approach, which determines the completeness interval from the plot of the cumulative number of events versus time, and measures the degree of incompleteness by assuming it to be a function with a continuous first derivative. In this way, all the information present in the catalog can be used in contour mapping by an appropriate weighting of incomplete parts in spatial filtering. On the other hand, the choice of the weight factors involves some arbitrary judgement and therefore introduces a bias. In order to reach an objective result, different weighting schemes covering all feasible choices are tried, and the final regionalization is concluded to be valid only if it is stable. A fair stability is obtained and it is henceforth possible to conclude that the seismic regions identified represent an unbiased picture of the real situation. Interestingly, different areas exhibit a different seismic character, some being capable of frequent activity but never exceeding moderate magnitude values, and others being characterized by a scarcity of moderate events combined with the capability of occasional large earthquakes. © 1987.