Rendiconti online Soc. Geol. It., Vol. n (2012), n-n
1 RIASSUNTO
Fratture cosismiche durante i terremoti di Ferrara del 20 e 29 maggio 2012: controllo morfo-litologico, origine tettonica o entrambi?
L’analisi del trend delle fratture cosismiche sviluppatesi a seguito dei terremoti di Ferrara del 20 e 29 maggio 2012 (Mw 6.1 e 6.2), confrontato con le direzioni assiale del doming co-sismico rilevato dai dati interferometrici, suggerisce un possibile controllo tettonico sull’origine delle fratture stesse. In particolare, viene ipotizzata una riorganizzazione degli assi principali dello sforzo che determina lo sviluppo di un sistema di cross-jointing, le cui manifestazioni superficiali sono amplificate dalle condizioni geomorfologiche e litostratigrafiche locali.
KEY WORDS: Coseismic ground rupture, Ferrara Earthquakes, tectonic.
On 20 and 29 May 2012, a large area located some kilometers WNW of the city of Ferrara was struck by two relevant E-W compressional events, with Mw 6.1 and Mw 6.0, respectively. These events were followed by five events with Mw > 5.0 (two on 20 May and three on 29 May) and by hundreds of events of lower magnitude distributed along a WNW-ESE-elongated area of ~500 km2 (ISIDe database at
http://iside.rm.ingv.it/ iside/standard/index.jsp.).Soon after the 20 and 29 May earthquakes, we carried out two geological surveys within a large area approximately coinciding with the boundary of the VI MCS mesoseismal area (GALLI et alii, 2012), in order to verify the occurrence of coseismic surface ruptures (LAVECCHIA et alii, 2012). A large number of ground fractures was mapped mainly occurring within loose or poorly dense Quaternary alluvial sandy deposits, prevailing along the suspended paleo-levees of abandoned river channels. The most intense liquefaction processes were associated to the 20 May event and observed between the localities of S. Agostino and Mirabello, along the paleo-channel of the Reno River.
The liquefaction processes associated to the 29 May event were less important, mainly concentrated close to the new epicenter, with some reactivation in the area between the 20 and 29 May epicenters (e.g., Finale E., San Felice S.P.).
The observed coseismic ground deformation comprised: 1- linear or en echelon opened fractures, e.g. diaclasis, with opening up to 80 cm and with or without a vertical offset (maximum observed offset of 60 cm) and often, but not systematically, accompanied by liquefaction (alignment of sand blows, sand volcanoes, ground settlements). 2-linear ground cracks, e.g. fissures with a very small or unappreciable opening and, generally, not accompanied by liquefaction; 3 - scattered liquefaction phenomena not directly related to fractures (isolated or aligned sand volcanoes, well overflows). The strike distribution of the compiled fracture data set varies over a wide range of directions, from N15° to N135°, with a dominant set oriented NNE-SSW/NE-SW. The fracture direction well correlates with the prevailing orientation of the present and paleo-river channels.
The Ferrara 2012 coseismic fractures certainly do not represent surface expression of the seismogenic faults, being rather nearly perpendicular to them. Nevertheless, evidence suggests that the fractures may be of tectonic origin. In fact, the N20-40° trend of the major fracture zones (e.g S. Agostino and Mirabello, ~6 km long and Cavezzo ~2 km long) is nearly perpendicular to the hinge of two coseismic hanging wall anticlines nucleated above the aftershock zones of the 20 and 29 May main shocks (InSAR data from COSMO SkyMed, SALVI et
alii, 2012). During periclinal folding, we can reasonably expect
flexure-induced, near-surface tensional stresses oriented both orthogonal and parallel to the fold hinge. The tensional stress orthogonal to the hinge is also parallel to the nearly N-S regional maximum compression (σ1) driving the geodynamics of the area (MONTONE et alii, 2012). Therefore, the component of tension is likely largely overcome by the regional compression and is not sufficient to originate hinge-parallel tensional fractures, as observed. Conversely, the tensional stress parallel to the hinge is nearly parallel to the regional σ2 and may be sufficient to reduce the σ2 to σ3, producing hinge-orthogonal tension fractures. In
Coseismic ground rupture during the 20 and 29 May 2012 Ferrara
earthquakes: morpho-lithologic control, tectonic origin or both?
GIUSY LAVECCHIA (*), DANIELE CIRILLO (*), PAOLO BONCIO (*), RITA DE NARDIS (*-**) & FRANCESCO BROZZETTI (*)_________________________
(*) Laboratorio di Geodinamica e Sismogenesi, DiSPUTer, Università «G. d’Annunzio», Campus Universitario, 66013 Chieti Scalo, Italy (**) Dipartimento della Protezione Civile, Roma, Italy
Rend. online Soc. Geol. It., Vol. 22 (2012), pp. 118-119, 0 figg. © Società Geologica Italiana, Roma 2012
2 other words, the observed fractures may be interpreted as the cross jointing of the coseismic folds (LAVECCHIA et alii, 2012 and references therein). The non-perfect orthogonality between fractures and hinges (80°) and the fact that the fractures did not occur systematically close to the highest domal uplift, where tensional stresses should be maxima, suggest that tectonic stresses may explain the first order features and that local conditions (river morphology, fluvial facies distribution) played an important role in producing and propagating the ruptures.
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