Fault segmentation and seismotectonics of active thrust systems: the Northern Apennines and Southern Alps laboratories for new Seismic Hazard Assessments in northern Italy (NASA4SHA)

The PRIN2020 Project aimed at investigating the fault complexities in active thrust systems like those of the Northern Apennines and Southern Alps (NASA; Figure 1), and at quantifying their consequences on standard and pioneer practices of seismic hazard assessments. We established two distinct, but concatenated, major goals. Firstly, we aimed at better defining the 3D geometry and the kinematics of the thrust systems. This goal can be reached by improving our knowledge on the internal fault hierarchy and earthquake occurrence at specific fault segments, based on a multidisciplinary approach, in which geological, geophysical, remote sensing, seismological, palaeoseismological, geodetic, and morphotectonic datasets are integrated and critically confronted among researchers with different background and expertise. Complementary techniques allow investigating the fault characteristics at different scales and with different resolutions, from the entire seismogenic level of the Earth crust, of as many as possible active segments, for better quantifying the seismotectonic parameters like, maximum credible rupture area, slip-rates and aseismic components, on-/off-fault magnitudefrequency distribution. These results have been obtained by a joint effort of some research units (RUs) and represent one major outcome of the project. Secondly, another major goal of the project was the release of a prototypal fault displacement hazard assessment (FDHA) in selected locations of Northern Italy. These brand-new analyses are now feasible, thanks to some prototypal tools developed within the Fault segmentation and seismotectonics of active thrust systems: the Northern Apennines and Southern Alps laboratories for new Seismic Hazard Assessments in northern Italy (NASA4SHA) Project. They represent an important contribution to seismic risk reduction strategies and for planning prioritization of interventions that should be adopted in such a densely populated and economically important region of Italy. All project activities can also represent a contribution to the Italian Interuniversity Center for 3D Seismotectonics with territorial applications (CRUST), to which many of the researchers that worked in the NASA4SHA Project belong to. Indeed, the Center promotes research and teaching in Seismotectonics within Italian universities, involving the analysis of long-term and current tectonic activity operating across various temporal and spatial scales to understand the seismic processes controlling seismic hazards. The collection of short papers included in the present volume is the tip of the iceberg of research performed from May 2022 to May 2025 by ca. 50 research fellows, organized in 5 RUs belonging to the academia, and research institutions. The list of the papers already published as an outcome of the NASA4SHA Project is reported in Appendix at the end of the volume. A short summary of the contents of this volume is given, and a final section summarizes the lessons learned and potential perspectives. The background for this research is represented by seismological observations and global tectonics that have driven to basic identification of earthquake-prone areas, however, just in the last decades and for few regions worldwide the location and seismogenic capacity of faults have been fully included in seismic hazard assessment analyses. Earthquake geology with a variety of disciplines contributes to the quantification of earthquake occurrence for known/ suspected seismic sources. Accurate 3D geometries of active faults, updated segmentation criteria based on observations of slip partitioning and other fault properties, and state-ofthe-art physical models of earthquake behaviour have been proposed in the literature for a more realistic estimate of earthquake recurrences. This kind of multidisciplinary approach is particularly needed in slowly deforming areas, especially in those densely populated and highly industrialized. This is the case of northern Italy where two opposite verging imbricate thrust systems affect the marginal sectors of the Po and Veneto-Friuli plains, which represent our target areas.