GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 2.1 GNGTS 2024 The use of historical and geological data in seismic hazard assessment: available data, modeling opportunities, and uncertainties involved Gianluca Valensise INGV, Rome, Italy Italy features what is probably the longest and most accurate earthquake record of the planet, plus one of the few compilations of seismogenic sources available worldwide. Not only these data are available to everyone, as they were collected by government agencies with the financial contribution of Italy’s Department for Civil Protection; they are also stored in effective GIS-based databases, which makes it easy to explore them and extract the information needed by seismic hazard assessment (SHA) practitioners. Nevertheless, they are not error-free, and their uncertainties may reverberate on the quality of SHA at all scales. Italian historical earthquake data are subject to uncertainties that concern the observed intensities and hence the focal parameters derived from them, or the ability to separate individual events within complex earthquake sequences – a common occurrence for over 50% of the country’s earthquakes. Although there exist data documenting earthquakes that occurred in the Middle Ages, the record is well populated – i.e., complete – for only a few centuries back, depending on geographic regions and on their recording history; as the characteristic recurrence interval of most Italian quakes is in the order of a millennium, we are likely to have no record of the activity of many prospective seismogenic sources, which makes it hard to achieve the necessary completeness. In their turn, Italian seismogenic sources are known to be hard to find and investigate; most of them are very deep or blind, some lie offshore. The country’s geology is especially deceitful, as older faults are systematically more evident than their active and seismogenic counterparts, and there are very few cases of documented historical surface faulting. Also in this case, achieving completeness is definitely hard. In addition to the mere identification of a large earthquake of the past, or of a large potential seismogenic fault, the potential inaccuracies of the historical and geological-tectonic records extend to the elaborations derived from them. Examples