Inferring Seismogenic Sources by Multi-method Approach: Case-Studies from Nothern-Central Apennines and Adriatic Region

Italy is the country with the highest seismic risk in Europe, with the largest number of earthquakes in the last 500 years (CPTI15, Rovida, A. et al., 2022), and with one of the greatest number of victims since 1900, equivalent to about 160.000 fatalities (Italian Civil Protection Department, 2018). The long-lasting seismic sequence that started on 24 August 2016 (Amatrice-Visso-Norcia) and struck a large area of central Apennine, partially overlapping the areas affected by the 1997 Umbria–Marche (Mw 5.8 mainshock) to the north and 2009 L’Aquila seismic sequence to the south (Mw 6.1 mainshock) (Fig. 1.1) is just one of the very last seismic sequences that affected the central Apennines with catastrophic scenarios. Nine major seismic events with Mw grater then 5 (Mw>5) occurred in few months (ISIDe Working Group, 2007) with epicenters spread over c. 50 km following an NNW–SSE strike along the central Apennines. The two strongest earthquakes Mw 6.0 (24 August 2016) and Mw 6.5 (30 October 2016), with the latter representing the largest earthquake in Italy since the Mw 6.9 1980 Irpinia event (Azzarro et al., 2016), caused 298 casualties, hundreds of injuries and almost 30.000 homeless, with several villages totally destructed (Pucci et al., 2017). Furthermore, the earthquakes severely damaged some strategic infrastructures such as roads, viaducts, bridges, strategic facilities worsening the early emergency phase. In fact, during this sequence, macroseismic (Mercalli–Cancani–Sieberg – MCS) intensities up to X or XI (ruinous or catastrophic) were observed (Galli et al., 2016), and IMCS ≥ VII (very strong) interested an area of about 2.100 Km2 (Galli et al., 2016, available online https://emidius.mi.ingv.it/). The coseismic effects of the Amatrice-Visso-Norcia seismic sequence have been largely measured and studied with more than 4.000 direct and secondary effects associated with the earthquakes identified by the EMERGEO Working Group (Civico et al., 2018). Direct coseismic effects are the result of surface fracturing and faulting processes (Emergeo Working Group, 2018), mainly linked to the reactivation of pre-existing faults (Boncio et al., 2004; Pizzi & Galadini, 2009; Bonini et al., 2016). In turn, gravitational slope deformations such as activation/re-activation of superficial landslides and rockfalls represent secondary coseismic effects, responsible of the interruption of the main traffic circulation across the central Apennines (e.g. the Sasso Pizzuto landslide into the deeply incised Nera River valley; Romeo et al., 2017). The above listed examples show how coseismic effects associated with large earthquakes may affect the communities and their socio-economic systems, and how a strong post-disaster management and resilience is essential to mitigate the seismic risks. In fact, the mitigation of seismic risk of an area requires a deep knowledge of the territory in terms of the environmental and architectural characteristics, fundamental to produce a reliable seismic hazard assessment. For this reason, in a seismic hazard evaluation for a given site, it is necessary identifying the seismic sources, which can generate strong earthquakes, estimating the magnitudes and frequency of the earthquake’s occurrence. Since the pioneering work of Cornell (Cornell, 1968), seismic hazard assessment depends on several factors, among which one of the most important is represented by the delineation of the seismic sources, often poorly understood because extensively covered by Quaternary deposits or located in offshore sites. In fact, their identification depends on the accuracy of the available geological, geophysical, and seismological data combined with tectonic knowledge, near-fault geomorphology, subsurface geophysical techniques, and remote sensing analysis. In the last years, these methodologies have been largely implemented within the active tectonic studies, including a better understanding of the Pleistocene regional geomorphology through the improvement of analytical modeling of large-scale landscape features. In Italy, the active faults identification process, and the direct estimation of their seismogenic potential, is still characterized by uncertainties and debated interpretations (Bosi, 1975; Scandone et al., 1992; Michetti et al., 2000). However, during the last decades new models and methods of investigation have been developed to identify and depict the seismogenic sources associated to large historical earthquakes. Only in recent time, a homogeneous and updatable database of possible seismogenic sources for medium to large size seismic events has been compiled by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). It is a georeferenced repository of tectonic, faults and paleoseismological information for the Italian territory and surrounding area (Database of Individual Seismogenic Sources - DISS Working Group, 2021, online available at https://diss.ingv.it/). Despite the latest important update of the published DISS database, several historical earthquakes still do not have an associated seismogenic source with surface evidence, and at the same time, several faults interpreted as active, do not have historical earthquakes that could prove this assumption. These examples well represent the situation of the central Apennine, where several structures are debated. The absence or the ambiguity of this information make any seismic risk mitigation planning difficult.