Layered Seismogenic Source Model and Probabilistic Seismic-Hazard Analyses in Central Italy

We defined a seismogenic model for central Italy based on three layers of sources and computed the relative seismic-hazard maps. One layer is constituted by individual structures liable to generate major earthquakes (M 5.5). We defined them as seismogenic boxes by using geological information in terms of plan projec- tion of active faults; the seismicity rates associated with an individual source are based on the geometry and kinematics of the fault; the recurrence model is controlled by the earthquake-source association, and, when possible, we defined the occurrence time of the last major event, using it in a time-dependent approach. Another layer is given by the instrumental seismicity analysis of the past two decades, which allows us to evaluate the background seismicity (M �� 5.5); using a sliding-window selec- tion of events, we defined a model of regular adjacent cells of variable a and b values of the Gutenberg-Richter relation. The last layer utilizes all the instrumental earth- quakes and the historical events not correlated to known structures (4.5 � M �� 6), by separating them into seismotectonic provinces shaped on a geological-structural basis. The seismic-hazard computations first use this layered model in a traditional probabilistic scheme. The results indicate a narrow belt of peak ground acceleration (PGA) higher than 0.30g (with standard deviation in attenuation functions) in the axial part of the Apennine chain, with a maximum spot of PGA � 0.40g southeast of the area damaged by the 1997-1998 Umbria-Marche sequence (PGA expected not to be exceeded in 50 years at 90% probability level). The background seismicity gives a nonnegligible contribution to the hazard, at least for first damage levels. Then, a simplified time-dependent hypothesis has been introduced for the individual sources alone, computing the conditional probability of occurrence of characteristic earth- quakes for each source by Brownian passage time distributions. Adopting equivalent fictitious seismicity rates, we obtained maps referring to the next 50 years by using traditional codes. These results show that the contribution of the recently active sources vanishes, and the most hazardous sites are now located south of L'Aquila and in the Sulmona area. We consider that the methodology and results obtained are useful for seismic risk reduction strategies.