GNGTS 2014 - Atti del 33° Convegno Nazionale

historical earthquakes and tsunamis (CPTI Working Group, 2004; Tinti et al. , 2004; Tinti, 2007), knowledge of source parameters is relatively poor, particularly for seismogenic structures extending offshore. The multiscale analysis of geophysical data integrated with analysis of onland geology and paleoseismology provided new insights to compile an inventory of first-order active faults that may have produced past seismic events (Polonia et al. , 2011, 2012, 2013). The proposed seismogenic faults include: 1) the NNW-SSE trending transtensive Alfeo-Etna fault system; 2) the Ionian Fault system, crossing the Messina Straits region close to Capo d’Alessio- Ali’ Terme. This fault accommodates margin segmentation, the outward movement of Calabria relative to Sicily and divergence in the Messina Straits area. It is a lithospheric structure representing an incipient plate boundary in the central Mediterranean Sea. These faults which possibly connect onland with the shear zone between Taormina and Tindari are likely to be source regions for future large magnitude events as they are long, deep and bound sectors of the margin characterized by different deformation and coupling rates on the plate interface. The length of the proposed seismogenic faults (Polonia et al. , 2012) may be used to predict the magnitude of the expected earthquake (Wells and Coppersmith, 1994), although the uncertainties are very high. Assuming that: (a) active faults are not segmented along their length, at least at depth, and (b) single fault ruptures affect the entire fault length, the maximum expected moment magnitude (M) for earthquakes occurring on the proposed seismogenic fault systems spans from about 7.2 to 7.6 (fault length 80-150 km). This magnitudes are associated with a maximum displacement of about 1-2 m, under the assumption of on-fault slip uniformity. Considering that the convergence rate reported by GPS is between 2 and 4 mm.a -1 (D’Agostino et al. , 2008; Devoti et al. , 2008) the first approximation of recurrence time of such earthquakes is about 250-1000 years (Polonia et al. , 2012). The repeat time may decrease if slip rates on single fault strands are higher than expected by regional models. This calls for a better understanding of seismic behavior of the active faults in the Messina Straits region. Monitoring and analysis of geodetic, seismological, tectonic and geological parameters may provide fundamental information on fault length, fault segmentation, slip rates on single fault strands and overall convergence rates for a better assessment of seismic hazard in this densely populated and industrialized area of the central Mediterranean Sea. Seismic shaking, tsunami generation and slope instability. The multidisciplinary investigation of the effects of historical earthquakes on marine sedimentation through the analysis of the turbidite record (Polonia et al. , 2013a, 2013b, 2014b) suggests that major historical earthquakes recorded in the area (i.e. the 1908, 1693 and 1169 events) triggered mass flows and turbidite emplacement. The source for all the historical turbidites discovered in the deep basin, as inferred from their mineralogy, is the metamorphic basement outcropping in the Messina Straits region. These findings suggest that seismically/tsunami triggered turbidites represent more than 90% of sedimentation in the deep basin and this is related to the occurrence of frequent, medium scale, earthquakes in the working area. Marine sediments may thus be considered as seabed archives of paleo-earthquakes capable of reconstructing seismicity back in time, during several earthquake cycles (10,000-30,000 years). The peculiar pre-collisional tectonic setting and landlocked configuration of the Ionian Sea make this region prone to slumps and submarine landslides because of uplifting coastal mountain belts behind the basin (Southern Apennines) and steep continental margins discharging into the basins large quantities of sediments that increase the potential for mass failures (Polonia et al. , 2013). This makes the northern margins of the Ionian Sea more prone to tsunami hazard, because of the high susceptibility to mass failures and the frequent occurrence of medium to large earthquakes that could generate landslide-induced tsunamis. GNGTS 2014 S essione 1.2 167