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Alessandro Maria Michetti, Università dell?Insubria (Italy)
Andrea Berlusconi, Università dell?Insubria (Italy)
Franz Livio, Università dell?Insubria (Italy)
Giancanio Sileo, Università dell?Insubria (Italy)
Andrea Zerboni, Università Statale di Milano (Italy)
Mauro Cremaschi, Università Statale di Milano (Italy)
Luca Trombino, Università Statale di Milano (Italy)
Karl Mueller, University of Colorado (United States)
Eutizio Vittori, APAT, Agency for the Environmental Protection and Technical Surveys (Italy)
Cipriano Carcano, ENI (Italy)
Sergio Rogledi, ENI (Italy)
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The discovery of coseismic paleoliquefaction and recurrent surface faulting events at Monte Netto, near Brescia, within the epicentral area of the Christmas 1222 (epicentral intensity IX in the MCS scale, that in the seismotectonic setting of the Lombardia Southern Alps can be related with a Mw of 6 to 6.5) earthquake, is a remarkable example of the value of the paleoseismological evidence for assessing seismic hazards. Moreover, the Monte Netto case history holds significant implications for the identification of capable faults (sensu IAEA, 2002; Michetti et al., 2005) in active compressional tectonic environments, in particular as related to the siting of critical facilities such as nuclear power plants.
Based on the review of literature data, reinterpretation of extensive subsurface information in the Po Plain made available courtesy of ENI E&P, regional geomorphic analyses, and detailed field mapping at selected sites, we were able to definitely confirm that 3 isolated hills (Castenedolo, Ciliverghe and Monte Netto), located few km SW of Lake Garda, are the geomorphic expression of late Quaternary growing anticlines. We show that these structures are controlled by an east-west trending, out-of-sequence backthrust, that has been characterized for the first time during this project in terms of geometry and rates of recent deformation. A quarry excavation at the Capriano del Colle hill allowed to conduct preliminary paleoseismological analyses, supported by new trenching and pedostratigraphic data, on ca. 150 m long, 7 m high, north-south and east -west trending trench walls. Two secondary decametric anticlines (ca. 4 to 9 m of exposed amplitude) have been recognized. The core of the anticlines is composed of a sequence of Middle Pleistocene fluvio-glacial deposits and paleosols. These sediments are overlaid by a Mid to Late Pleistocene loess-paleosols sequence, laterally thickening up to 6 m.
The culmination of the northern anticline is characterized by the presence of a graben-like structure composed by bending-moment faults, which offset also the uppermost Holocene soil. Stratigraphic relations clearly show that the graben structure formed during at least two discrete surface faulting events. Near the central graben the excavation exposed also a liquefaction feature consisting of a sand and gravel dike cutting through ca. 60 cm of the Mid Pleistocene deposits, whose presence has to be considered unequivocal evidence of the coseismic genesis of the described deformations. Seismic reflection profiles across Monte Netto clearly shows in fact that surface folding and faulting are geometrically linked to a secondary flexural slip fault at a depth of ca. 2 km, controlled in turn by the slip of the main Capriano del Colle backthrust at a depth of ca. 5 km beneath the excavated site. This give proofs for the first time of paleoseismic surface faulting phenomena clearly connected to a typical compressional structure in the Po Plain Foredeep.
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