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I terremoti del gennaio 1117. Ipotesi di un epicentro nel cremonese

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Collocazione:
Il Quaternario Italian Journal of Quaternary Sciences, 18(2), 2005, 87-100
Autore/i:
Paolo GALLI
Abstract:

Il terremoto del 1117 può essere considerato l’evento distruttivo di riferimento per la storia sismica di una vasta parte della Pianura Padana. Tuttavia, a causa della lontananza temporale delle fonti e della oggettiva complessità dell’evento (terremoto multiplo), la sua attuale ubicazione epicentrale (Verona) non appare rappresentativa della distribuzione degli effetti. Sulla base del riesame completo delle fonti primarie conosciute e di quelle poche non note nei precedenti studi, nonché da evidenze di carattere archeosismologico, viene proposto un quadro indiziario degli effetti che, confrontato con le possibili strutture sismogenetiche sepolte della Pianura Padana, permette di ipotizzare l’esistenza di un secondo epicentro in area basso-padana, verosimilmente ubicato nel Cremonese.

The 1117 earthquake is probably the strongest event which occurred in northern Italy during the historical period. Although its eco, and perhaps its effects were felt in a vast area of central and southern Europe (i.e., from Reims to Montecassino) its epicenter is still uncertain and strongly debated, and it is presently placed near Verona by the Italian seismic catalogue (GRUPPO DI LAVORO CPTI, 1999).
Actually, we know that it was a long seismic sequence, characterized by more than one mainshock, with damage areas in the Adige valley (south of Trento), in the Verona territory, and in the area NW and SE of Cremona (ENEL, 1986; MAGRI E MOLIN, 1986; BOSCHI ET AL., 1995; GALADINI ET AL., 2001b). Damage were also reported in a village near August (Germany) and Pisa, being related to earthquakes which occurred by chance in the same days, but without any possible seismotectonic significance with respect to the earthquake cited universally by the historical source as happened in “Longobardia” (the land of northern Italy ruled by Longobards during Middle ages).
In northern Italy, the damaged localities explicitly quoted by the local synchronous sources (excluding those mentioned repetitively in transalpine chronicles: Verona …. similiter Parma et in Venetia) are only Verona, Ronco all’Adige (a small village SE to Verona) and Cremona, while three memorial stones witness effects in Padua, Costozza and Nonantola (villages near Padua and Modena, respectively).
A strong effect is also recorded in Milan and its province.
From the seismotectonic point of view, GALADINI ET AL. (2002) placed the source of the 1117 earthquake on the so-called Thiene- Bassano thrust, which is located between 110-130 km away from Cremona, along the southalpine front.
As for the effect in Cremona, new archaeoseismic data deduced from archaeological excavation carried out in the past years in the cathedral confirm and enrich the frame depicted by the synchronous sources (collapse of the new (?) church under construction during the 1117 earthquake, burying of the remains of the patron Saint Himerius under the ruins, and their recovery only twelve years later).
Moreover, this data seems to demonstrate that the current cathedral was entirely rebuilt following the earthquake, none of the wall or structures in fieri during the 1117 having been saved after the collapse. The original design of the cathedral started in 1107 was changed, and only few sculptures and architectural elements in situ before the earthquake were re-employed afterwards.
Based on the above, it is very difficult that damage and collapses in places located up to 100 km away from each other may be accounted for only one seismic source. Considering the existence of more than one mainshock between 2-3-4 January (as reported by the historical sources), the event which hit southern Lombardy and neighboring places before 5 p.m. (local time) of January 3 could be related to a local seismogenic structure, and not to the fault which caused damage in the same hours/days in the Verona-Padua area (i.e., the mentioned Thiene-Bassano thrust). This structure could be identified with one of the thrusts existing along the buried front of the Apennine chain which show traces of recent tectonic activity, as the one affecting the buried Piadena anticline, just under the province of Cremona.

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