Integrating faults and past earthquakes into a probabilistic seismic hazard model for peninsular Italy

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 seis- micity is modelled using a number of seismotectonic zones and the occurrence of earthquakes is assumed uniform. How- ever, in the past decade, efforts have increasingly been di- rected 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 us- ing 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 seis- mic 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 max- imum magnitude and a truncated Gutenberg–Richter model. The second earthquake source category involves grid-point seismicity, with a fixed-radius smoothed approach and a his- torical catalogue were used to evaluate seismic activity. Un- der 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 de- creases by a fault-size-driven factor. Additionally, we com- puted 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 acceler- ations that are almost uniformly distributed across large re- gions. In addition, we conducted sensitivity tests to deter- mine 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 method- ology used in the field of fault-based regional seismic hazard modelling in Italy.