Earthquake probabilities and probabilistic shaking in Italy in 50 years since 2003: trials and ideas for the 3rd generation of Italian seismic hazard maps

In the frame of the activities promoted by the National Group for the Defence against Earthquakes (GNDT), a four-year national project has been funded on the topic of determining priorities in seismic risk mitigation at the national scale. This paper describes the efforts to introduce individual faults and time-dependent issues in the seismic hazard assessment developed within this project. The elaborations refer to the national scale and use some original data produced and released for the project’s purposes, namely, the Database of Italy’s Seismogenic Sources and a compilation of the Italian instrumental earthquakes from 1981 until 2002. An integrated seismic hazard model that combines the individual earthquake sources recognised by geological and seismological studies with the information obtained by the national seismographic network is proposed and applied to the seismic hazard assessment. Individual sources are assumed to follow the characteristic earthquake model and their rates of occurrence derived from geometric and kinematic considerations. The minor seismic activity defined in terms of background sources is represented by Gutenberg-Richter relationships calibrated on the instrumental dataset. Then, the conditional probability of occurrence of characteristic earthquakes for each individual source is modelled by the Brownian Passage Time distribution. The simple time-dependent hypotheses introduced are used to derive equivalent fictitious seismicity rates: they can be entered into traditional seismic hazard codes for having maps that are referred to the time when the analysis has been performed. The results are heavily controlled by some arbitrary choices like the regional distribution of slip rate (applied to all individual sources lacking detailed information), or the uncertainties a priori attributed to the mean recurrence time. Nevertheless, the maps of conditional probability of earthquake occurrence and the seismic hazard maps, under Poisson and time-dependent hypotheses, enhance the role of moderate earthquakes in driving the seismic hazard. The databases have been updated during the life of the project and the new versions became public after the project ended. The elaborations presented here refer to the data available during the project and were not updated to be consistent with the final products released by the project. The results obtained must therefore, be considered mainly for their methodological approach to the problem, so their application to seismic protection strategies has to be done with great care.