Seismogenic sources in Central Italy: from causes to effects

In the last few years, Italy has been facing the problem of a new generation of hazard maps, with the transition from standard probabilistic seismic hazard estimates to time-dependent seismic hazard assessments. These innovative methodologies require the detailed knowledge of the parameters of individual sources (seismogenic structures), so that some constraints on the seismicity characterisation, based on independent geological evidence, can be introduced. This work aims at analysing the methodological assumptions that lead to assigning seismicity rates to a new zonation of the Umbria-Marche region (Central Italy), and to exploring the consequences in terms of seismic hazard assessment. We have used «seismogenic boxes», that are the surface projection of active normal master faults identified with structural geologic criteria as the sources, entered directly into the hazard computation. The parameters associated to the boxes (maximum magnitude, recurrence times, seismicity model) have been evaluated in three ways: the first is a quite traditional rating based on the earthquake catalogue; the other two derive from the geometrical characteristics of the sources, using the empirical relationships that correlate the energetic characteristics to the shape and extension of the seismogenic boxes in different ways. The seismicity model of the sources is usually of a Gutenberg- Richter type, with the exception of two cases, for which we have defined the seismicity rate considering the characteristic earthquake model. The results are given in PGA hazard maps, following the standard stationary approach, and refer to the 90% probability level of non-exceedence in 50 years. The results presented in this paper represent a sensitivity analysis to appreciate how much all the geological markers may aid in reducing the intrinsic uncertainties of estimates like the maximum magnitude and recurrence time, first-order ingredients in the seismic hazard assessment.