La sismicità del settore atesino delle Alpi centrali (Italia settentrionale): alcuni problemi aperti, limiti ed implicazioni dell'approccio multidisciplinare (geologico, storico-sismologico, archeosismologico e archeologico-architettonico)

Abstract

The seismicity of the central Alps in the Adige river sector (northern Italy): some open problems, limits and implications of the multi-disciplinary approach (geological, historical-seismological, archaeoseismological and archaeological-architectural). Earthquakes with M>5 affect the southern sector of the Alpine chain in most of the Italian central-eastern Alps. In contrast, in the easternmost Alpine domain (Friuli region, Tagliamento river area) destructive earthquakes also occur in the inner zones of the Alpine edifice. This raises the question if the occurrence of destructive earthquakes in the inner Alpine sectors can be completely excluded for the rest of the Italian Alps. The July 17, 2001 earthquake (ML 5.2), which struck the Meran area, demonstrated that earthquakes above the damage threshold may occur within the inner Alpine domain of the Adige river region. Further information on this point can be derived from archaeoseismological investigations made in the second half of the 90s. During this period, archaeological excavations at Egna-Neumarkt in the Bolzano province uncovered the remains of a mansio showing traces of destruction and displacement of the walls. This event has been related to an earthquake of the half of the 3rd century AD. The walls were shifted horizontally (about 30 cm) and vertically (about 60 cm). Evidence for a "crisis" during the 3rd century (and particularly at the half of this century) is widespread and proved by the numerous cases archaeologically documented of reconstruction, rebuilding, restoration or abandonment. However, this period of modification has been traditionally attributed to the Aleman invasions. The superposition of the effects of political instability on the possibly strong coseismic damage makes the deciphering of the earthquake characteristics difficult. Archaeological excavations in the cloister of the Capuchin Monastery in Bolzano uncovered remains of a tower which was part of the Wendelstein Castle. Evidence of earthquake damage was represented by the expulsion of a corner and the launch of stones at several metres from the structure. The tower was probably built in the 13th century and, based on the sparse archaeo-chronological information, it may have been destroyed by the 1348 earthquake (which strongly damaged the Friuli region, east of Bolzano) or by another unknown event. Attributing the damage to the 1348 event implies the necessity to better understand the reasons of the strong damage in Bolzano, considering that this town is far from the epicentral area of the earthquake. In the meantime, investigations (e.g. archaeology of the architecture) should be performed in order to understand if the damage was caused by an unknown event of local origin. Recent archaeological data (both from excavations and from archaeology of the architecture) suggest that the 1117 earthquake is actually a complex seismic sequence which affected most of northern Italy. The evidence of strong damage in Trento, derived from the excavation of the San Lorenzo church, suggests the possibility that one of the shocks of the sequence originated north of the southern Alpine border, due to the activation of a seismogenic source still uncovered. Though the available information suggest that destructive earthquakes may also occur in the inner sectors of the Alpine chain east of the Friuli region, the identification of the seismogenic sources potentially responsible for these events remains problematic. Geomorphological factors related to the recent geological history of the Alps hinder the investigation on the seismogenic behaviour of the region. Indeed, the rate of the exogenic processes related to the Last Glacial Maximum and (particularly) the river dynamics following the LGM is evidently higher than the rate of tectonic modelling of the landscape. This means that the identification of the seismogenic sources can only be obtained through a better knowledge of the Alpine seismicity. Considering the exiguousness of the historical information for the earthquakes of the Antiquity and of the Middle Ages, and the mentioned problems of the geological investigation, it is evident that the future archaeoseismological research will play a key-role in better defining the seismogenic behaviour of the Alpine region.