We describe the main structure and outcomes of the new probabilistic seismic hazard model for Italy, MPS19 [Modello di Pericolosità Sismica, 2019]. Besides to outline the probabilistic framework adopted, the multitude of new data that have been made available after the preparation of the previous MPS04, and the set of earthquake rate and ground motion models used, we give particular emphasis to the main novelties of the modeling and the MPS19 outcomes. Specifically, we (i) introduce a novel approach to estimate and to visualize the epistemic uncertainty over the whole country; (ii) assign weights to each model components (earthquake rate and ground motion models) according to a quantitative testing phase and structured experts’ elicitation sessions; (iii) test (retrospectively) the MPS19 outcomes with the horizontal peak ground acceleration observed in the last decades, and the macroseismic intensities of the last centuries; (iv) introduce a pioneering approach to build MPS19_cluster, which accounts for the effect of earthquakes that have been removed by declustering. Finally, to make the interpretation of MPS19 outcomes easier for a wide range of possible stakeholders, we represent the final result also in terms of probability to exceed 0.15 g in 50 years.

Faults have been increasingly integrated into seismic‐hazard assessments. We have developed a package of MATLAB (http://www.mathworks.com/products/matlab, last accessed January 2016) tools (called FiSH), designed to help seismic‐hazard modelers analyze fault data. These tools enable the derivation of expected earthquake rates, given common fault data, and allow researchers to test the consistency between the magnitude–frequency distributions (MFDs) assigned to a fault and some available observations. The basic assumption of FiSH is that the geometric and kinematic features of a fault are the expression of its seismogenic potential. Three tools have been designed to integrate the variable levels of information available: (1) the first tool allows users to convert fault geometry and slip rates into a global budget of the seismic moment released in a given time frame, taking uncertainties into account; (2) the second tool computes the recurrence parameters and associated uncertainties from historical and/or paleoseismological data; and (3) the third tool outputs time‐independent or time‐dependent earthquake rates for different MFD models. We present a test case to illustrate the capabilities of FiSH, on the Paganica normal fault in central Italy that ruptured during the 2009 L’Aquila earthquake sequence (mainshock M_w 6.3). The source codes are open, and we encourage users to handle the scripts, communicate with us regarding bugs, and/or suggest further improvements. Our intent is to distribute these tools in order to help researchers to pinpoint potential inconsistencies and obtain reliable fault‐based seismic‐hazard evaluations.

Italy is one of the most seismically active countries in Europe. Moderate to strong earthquakes, with magnitudes of up to ∼ 7, have been historically recorded for many active faults. Currently, probabilistic seismic hazard assessments in Italy are mainly based on area source models, in which seismicity is modelled using a number of seismotectonic zones and the occurrence of earthquakes is assumed uniform. However, in the past decade, efforts have increasingly been directed towards using fault sources in seismic hazard models to obtain more detailed and potentially more realistic patterns of ground motion. In our model, we used two categories of earthquake sources. The first involves active faults, and using geological slip rates to quantify the seismic activity rate. We produced an inventory of all fault sources with details of their geometric, kinematic, and energetic properties. The associated parameters were used to compute the total seismic moment rate of each fault. We evaluated the magnitude–frequency distribution (MFD) of each fault source using two models: a characteristic Gaussian model centred at the maximum magnitude and a truncated Gutenberg–Richter model. The second earthquake source category involves grid-point seismicity, with a fixed-radius smoothed approach and a historical catalogue were used to evaluate seismic activity. Under the assumption that deformation is concentrated along faults, we combined the MFD derived from the geometry and slip rates of active faults with the MFD from the spatially smoothed earthquake sources and assumed that the smoothed seismic activity in the vicinity of an active fault gradually decreases by a fault-size-driven factor. Additionally, we computed horizontal peak ground acceleration (PGA) maps for return periods of 475 and 2475 years. Although the ranges and gross spatial distributions of the expected accelerations obtained here are comparable to those obtained through methods involving seismic catalogues and classical zonation models, the spatial pattern of the hazard maps obtained with our model is far more detailed. Our model is characterized by areas that are more hazardous and that correspond to mapped active faults, while previous models yield expected accelerations that are almost uniformly distributed across large regions. In addition, we conducted sensitivity tests to determine the impact on the hazard results of the earthquake rates derived from two MFD models for faults and to determine the relative contributions of faults versus distributed seismic activity. We believe that our model represents advancements in terms of the input data (quantity and quality) and methodology used in the field of fault-based regional seismic hazard modelling in Italy.