GNGTS 2013 - Atti del 32° Convegno Nazionale

coseismic fracturing associated with the hidden faults of Mt. Etna is, thus, a crucial topic in the seismic microzonation analyses to validate the location of seismogenic sources and assess their potential. In this paper we discuss the results of the geological andmorphostructural investigation that has been performed across the Fiandaca, Santa Tecla and Santa Venerina hidden faults, in order to define their role in the active kinematic picture of the eastern flank of Mt.Etna. Seismotettonic setting. The eastern flank of Mt. Etna is dissected by two distinct sets of active structures. The prominent NNW-oriented fault belt, representing the northern termina- tion of the Ionian-Etnean Branch of the Siculo-Calabrian Rift Zone (inset in Fig. 1; Catalano et al. , 2008), consists of a half graben that includes the Acireale-S. Alfio master fault, the syn- thetic San Leonardello Fault and the antithetic Trepunti Fault. The epicenters of several main earthquakes (A.D. 1881; 1911; 1971) have been located at hangingwall of the rift zone (Mona- co et al. , 1997; Azzaro, 1999; Azzaro et al. , 2009). On the footwall of this rift zone, discrete NW-oriented fracture systems, represented by the Fiandaca, Santa Tecla and Santa Venerina faults (ff, STF and SVF in Fig. 1; Barreca et al. , 2013), have been described as alignments that are usually affected by both diffuse aseismic creep (Azzaro, 1999; Monaco et al. , 2012) and coseismic remobilisation. The updated models on the active tectonics and the seismicity of Mt. Etna (Azzaro, 1999; Azzaro et al. , 2012; Barreca et al. , 2013) refer to the Fiandaca (A.D. 1894, 1984), S. Tecla (A.D. 1865, 1914) and SantaVenerina (A.D. 1879, 2002) faults some relevant seismic events, which have been otherwise attributed to the NNW oriented faults of the rift zone (Monaco et al. , 1997). The most recent episode of remobilization of a “hidden” fault has been referred to the 29 th of October 2002 earthquake (Mm=4.1; Azzaro et al. , 2006), which was associated to the reactivation of an historical ground fracture zone (A.D. 1879), crossing through the village of Santa Venerina (SVF in Fig. 1; Azzaro et al. , 2006; Barreca et al. , 2013). Stratigraphy of the eastern flank of Mt. Etna. Along the eastern flank of Mt. Etna, the volcanostratigraphic succession includes the products of all the stages of the etnean volcanism (Gillot et al. , 1994). The volcanic cover rests, as a whole, on marine deposits, consisting of a monotonous Early-Middle Pleistocene marly-clay succession (Di Stefano and Branca, 2002) that bears submarine tholeiitic lava horizons, ranging in age from 0.58 to 0.46 Ma (Gillot et al. , 1994). The marine succession is capped by subaerial transitional products from tholeiitic to alkaline, ranging in age from about 225 ka B.P. to 168 ka B.P. (Gillot et al. , 1994; Corsaro et al. , 2002). In a large area, going from Acireale to Giarre (inset in Fig. 1), a buried clastic wedge, which was exclusively detected by geophysical data (Cassinis et al. , 1970; La Delfa et al. , 2007), is interposed between the top of the marly clays and the outcropping etnean volcanic cover. The clastic deposits, here designed as Basal Clastic Wedge, fill a prominent triangular shaped tectonic depression, the Giarre Basin, which is bordered by two culminations of the Middle Pleistocene marly-clay substratum: the Acitrezza Ridge and the Nunziata High (inset in Fig. 1). The sequence of the alkaline products can be divided in four distinct units, which are separated by epiclastic deposits marking first order erosion surfaces. The oldest alkaline products, showing radiometric age from 140 ka to 132 ka (Gillot et al. , 1994; Branca et al. , 2007) have been grouped in the pre-Tyrrhenian alkaline lavas. These are unconformably covered by huge volumes of epiclastic deposits, here named Acireale Lahars that concealed the pre-existing volcanic topography, reaching a maximum thickness of about 150 m, in the area of Acireale. The Acireale Lahars shows distinctive reddish horizons that have been drilled in different sites of the southeastern flank of Mt. Etna, indicating a wide distribution beneath the recent volcanic covers, from Acireale to Santa Venerina. On top of the Timpa of Acireale, the Acireale Lahars are capped by coastal marine deposits that form the wide terrace of the 5.5 OIS (125 ka), extending towards the Acitrezza Ridge. The marine terrace is covered by alkaline lavas, ranging in age from 120 ka to 60 ka. These have been referred to the Tyrrhenian alkaline lavas, grouping the products of several spread edifices (Branca et al. , 2007). A major epiclastic horizon, here designed as Milo Formation, conceals the deeply entrenched erosion surface that models the Tyrrhenian alkaline lavas, in the whole northwestern sector of the Giarre Basin. The backbone 41 GNGTS 2013 S essione 1.1