Seismic and tsunami hazard assessment of a nearshore active normal fault compared with the historical Calabria 1905 earthquake (SE Tyrrhenian Sea).

Calabria is one of the Italian regions with the highest tectonic activity and the seismic moment release, as suggested by the abundant and highly destructive historical earthquakes that struck the region. The region is geodynamically determined by the NW subduction of the Ionian oceanic crust that since the Tortonian controls the southeastward opening ot the back-arc Tyrrhenian basin and the migration of the Appennies chain. Since the Middle Pleistocene, the Calabrian Arc experienced a rapid uplift of up to ca. 1 mm/yr in part accommodated by major NE-trending normal faults (Monaco & Tortorici, 2000). Based on recorded seismicity and historical seismicity, the current deformation affecting the Calabria region is mainly controlled by an extensional tectonic regime. Subsidence of the western Calabria is also confirmed by recent GPS vertical-component measurements (Serpelloni et al., 2013). Over the last decades the scientific community invested great energy and resources for seismic and tsunami hazard assessment, that can actually improve with knowledge of those active structures capable of generating destructive earthquakes and, if located offshore, also tsunamis. A more reliable hazard assessment of the exposed regions can be done if forecasting models can be constrained with instrumental or historical data. This is the case of the Calabria 1905 earthquake, which caused 557 deaths, injured more than 2000 people and was followed by a tsunami that, even if feeble, was felt in the whole Southern Italy including Sicily (Tinti et al., 2004). Considering the several uncertanities, concerning the seismogenetic source and the epicentral location of this event (Loreto et al., 2013 and references therein), during the summer of 2010 a geophysical survey within the Sant'Eufemia Gulf was carried out. New acquired data allowed us to identify an active structure, named Sant'Eufemia Fault (Loreto et al., 2013), located within the gulf facing the western Calabria (SE Tyrrhenian Sea). Numerous observations and models support the hypothesis that this structure could be the seismogenic source of the 1905 earthquake and, being active, capable to trigger a new event. Analyzing all available geophysical (multichannel seismic profiles, sub-bottom chirp profiles, high resolution morpho-bathymetry) and geological (fieldwork survey) data, we defined the most credible geometry of the Sant'Eufemia fault: a N31°-oriented, 38°-dipping and ca. 40 km-long normal fault plane. Using the Wells & Coppersmith (1994) empirical relationship, we assigned to this fault a width equal to 17 km and the potential for an Mw 7 earthquake. Based on these parameters we performed the parametric KF scenario (Sirovich & Pettenati, 2009). We produced an Intensity map that can be directly compared with intensities distribution of the Calabria 1905 earthquake. This fault is also used to perform tsunami modelling and to compare the numerical results with the historical data available on run-up and inundation. Main results obstained are: the Sant'Eufemia fault is reasonably compatible with the historical information on the 1905 event; and an eventual tsunami wave impact on the western Calabrian shoreline is better constrained. We consider the thorough assessment of the different types of hazards associated with the Sant'Eufemia fault as a very important result for the Tyrrhenian coasts of Calabria, which have not received so far the attention reserved to other Italian coastal areas despite the growing infrastructure facilities and tourism activities.