GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 1.1 7 SEISMOGENIC FAULTS, LANDSLIDES AND TSUNAMIS IN THE IONIAN SEA: A CRITICAL ASSESSMENT A. Argnani 1 , A. Armigliato 2 , F. Zaniboni 2 , G. Gallotti 2 , C. Angeli 2 , M. Zanetti 2 , S. Tinti 2 1 ISMAR-CNR, Bologna, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italia 3 Department of Physics and Astronomy “A. Righi”, University of Bologna, Italy Rationale. The active tectonics of the Ionian region, and in particular of its western side, close to the coast of Sicily and facing the Mt. Etna volcano, have recently received a great deal of attention. In the last century this region has been characterized by a low-to-moderate seismicity, though in historical times it had been the site of the largest earthquakes witnessed in Italy, often accompanied by devastating tsunamis. Altogether the western Ionian and Calabrian regions can be considered as subject to a great geological hazard. New data acquisition, particularly swath bathymetry and seismic profiles, both reflection and refraction, have contributed to clarify several tectonic aspects of this large and complex region; however, some issues remain elusive and discordant interpretations are often encountered in the abundant literature. Here we aim at addressing the historical record of earthquakes and tsunamis and at attempting to discuss the critical issues related to the potential tsunami sources (faults and landslides). Of faults and landslides generating tsunamis. The seismological record of the great earthquakes that affected eastern Sicily and south Calabria is minimal: the most recent event is the December 1908 Messina earthquake, for which only limited instrumental records are available. Despite the broadly confined epicentral location in the offshore portion of the strait, and notwithstanding some seismological information (e.g., Pino et al., 2009), the seismogenic fault has not yet been unambiguosly identified. Various faults have been proposed, often very different one from the other, and sometimes poorly constrained by data (e.g., Argnani, 2022). Irrespective of the proposed seismogenic fault, there is some convergence among the authors on the fact that coseismic slip alone is unlikely to be responsible for the observed tsunami (Piatanesi et al. , 1999; Tinti e et al. , 2008; Schambach et al. , 2020, 2021; Argnani, 2021). Some additional contribution is required either by landslides or by a secondary fault within a multiple rupture scheme (e.g., Gusman et al. , 2018). The critical analysis of the tsunamigenic sources proposed in the Messina Strait suggests that alternative hypotheses can still be explored. For the older large tsunamigenic earthquakes (namely January 1693, December 1542 and February 1169; see the EMTC 2.0 by Maramai et al. , 2019) only chronicles are available, leaving even larger uncertainties. Partly because of this the 1542 and 1169 events have never received particular attention in the literature, and little is knownon the sources of earthquakes and tsunamis. Instead, the potential sources of the 1693 earthquake and tsunami have been largely addressed. Although the epicentral location of the 1693 earthquake is still debated, with some researchers that would locate it onshore (Sirovich and Pettenati 1999; Lavecchia et al. , 2007; Pirrotta and Barbano, 2020), it seems hard to escape the necessity of a seismogenic fault located offshore. A study conducted on a dense grid of seismic profiles along the Malta Escarpment has revealed the presence of an active extensional fault, about 40 km long, just off Augusta (Argnani and Bonazzi 2005; ME in Fig.1). Modelling indicates that this fault can originate the 1693 tsunami, roughly matching the observed regional distribution of maximum wave elevation (Argnani et al. , 2012). In the same study a large landslide that occurred at the fault scarp has also been modelled; its tsunamigenic contribution, however, remains limited, and cannot alone originate the tsunami waves observed along the coast of eastern Sicily. Therefore, the hypothesis that for the 1693 event a tsunamigenic earthquake originated offshore seems supported by modelling results, as well as by studies of inundation record