Il terremoto del 1905 in Calabria: revisione della distribuzione degli effetti e delle ipotesi sismogenetiche

Il terremoto dell’8 settembre del 1905 è sempre stato un evento un po’ negletto dalla storiografia sismologica italiana, oscurato dagli effetti del catastrofico evento del 1908; eppure è il terremoto per il quale è stato calcolato il più alto valore strumentale della magnitudo in Italia (tra Ml=7.9 e Ms=7.47). Per questo lavoro sono state recuperate tutte le fonti già citate nei precedenti studi, oltre a molte altre sconosciute, tra cui una relazione ministeriale molto voluminosa, che nel complesso hanno permesso di ricostruire la distribuzione degli effetti per 895 località dell’Italia meridionale in termini di scala MCS. Il quadro che ne deriva è percettivamente diverso da quelli precedentemente delineati e permette di avanzare ipotesi in parte inedite sugli scenari sismogenetici della Calabria. In particolare, 1) la grande estensione dell’area di danneggiamento, 2) l’apertura verso il Tirreno del piano quotato, 3) l’insorgenza di un debole maremoto e la sua natura bipolare, 4) l’assenza del purché minimo indizio di fagliazione di superficie a terra, 5) e il confronto con la distribuzione degli effetti di altri terremoti profondi del basso Tirreno ci conducono ad ipotizzare la presenza di una sorgente subcrostale nell’offshore del Monte Poro.

In the first hours of the night of September 8th, 1905, the villages of Mt. Poro (southern Calabria) and many others along the Tyrrhenian edge of Calabria, were struck by the most powerful earthquake even occurred in Italy (at least in terms of recorded magnitude: Ml=7.9, DUNBAR et al., 1992; Ms=7.47; MARGOTTINI et al., 1993; GRUPPO DI LAVORO CPTI, 1999), that caused around 600 casualties. In this paper, thanks to hundreds original sources collected in archives and library all over Italy, but particularly to a huge, unpublished contemporary government report, we reevaluated the macroseismic intensity distribution (MCS scale) for 895 localities in southern Italy (Fig. 7; see details in GALLI AND MOLIN, 2007). The resulting framework of effects differs perceptively from those presented by previous Authors, allowing to hypothesize an unedited hypothesis as far as the Calabrian seismogenetic scenarios are concerned. Firstly, the earthquake had a very large damage area, typical of deep event, the 7th MCS degree extending well beside 170 km in a NESW direction (Fig. 8), that is more than the neighbour epochal earthquake of February 5, 1783. The maximum intensity reached 10-11 MCS in the Mt. Poro villages (Fig. 1), but differently from the others strong Apenninic earthquakes, in the mesoseismic area the highest intensities (10-11 MCS) alternated to very low effects (6-7 MCS). The macroseismic framework is also complicated by the presence of environmental effects (landslides and liquefaction, surficial breaks; Figg. 10-12) affecting 25% of the damaged municipalities (85% of villages with Is?10 MCS). On the other hand, microtremors analyses performed in the most damaged localities showed a scarce relationship between disruption level and frequency peaks falling in the range of engineering interest (i.e., suggesting low site amplification effects; Fig. 13). From the seismogenetic point of view, no surficial trace of coseismic faulting have been observed in the mesoseismic area, notwithstanding the magnitude value would imply a source dimension (~50 km length) and a coseismic slip (> 1m) well detectable for a inland crustal structure (e.g. along the faults quoted in previous papers; see TORTORICI et al., 1995; GALLI AND BOSI, 2002; CUCCI AND TERTULLIANI, 2006). Moreover, the intensity distribution is clearly opened toward the Tyrrhenian Sea, where along the coasts a weak tsunamis was also observed after the earthquake (Figg. 8-9). Therefore, 1) the very large damage extension, 2) the opening of isoseismal lines toward the sea, 3) the occurrence of a bipolar tsunamis, 4) the lack of inland surface faulting and 5) the comparable shape and effects distribution of others studied deep-Tyrrhenian earthquakes (Fig. 4) allow us to hypothesize the presence of a sub-crustal source in the Mt. Poro offshore, responsible for the 1905 event. In facts, it is well known that the Calabrian Arc is subducted by an active, Ionian lithospheric slab that, in the offshore of the Tyrrhenian coast dips ~70° toward NW. Most of the slab seismicity (Figg. 5-6) is due to down-dip compression mechanisms (events M>7 at depth > 150 km), but it is possible that also the hinge of the slab, due to flexural-extension processes, might release high values of magnitude, such the one recorded for the 1905 earthquake (Fig. 14).