GNGTS 2013 - Atti del 32° Convegno Nazionale

subsurface investigation have evidenced that the ground fracture zone is aligned with the axis of a buried paleovalley, whose flanks represent sharp lateral discontinuities between an alternation of lavas and clastic deposits, infilling the valley, and the more homogeneous epiclastic sequences of the substratum. In this context, the coseismic fractures originated at almost right-angle to the valley orientation, being centred on its axis, and extend for a length which is consistent with the reconstructed width of the valley at depth. Finally, the fracture alignment abruptly interrupts at the mouth of the valley, which is controlled by a NE-SW oriented buried fault zone. The direct connection between their geometry and that of the buried valley strongly suggests interpreting the coseismic fracture zones of Santa Venerina as a site effect of the ground motion, determined by major lateral mechanical discontinuities of the substratum that likely produced a differential ground motion between the valley infilling and the surrounding terrains. As well, the active coseismic ground deformation along the Fiandaca and Santa Tecla faults, where cumulative features are absent, could be explained in terms of differential site response, due to the sharp lateral discontinuities of the rock mechanics on the opposite sides of the two major structures. The two faults, located at the southwestern border of the prominent Giarre Basin, can represent mechanical barrier along which the ground motion generated by seismic events located along the main seismogenic faults of the SCRZ, cutting through the Giarre Basin, concentrate their effects. 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