Volcano-tectonic activity along structures of the unstable NE flank of Mt. Etna (Italy) and their possible origin

We describe the prehistoric-historic activity of the main and minor volcano-tectonic structures of the northeastern flank of Mt. Etna, giving new data which provide a clearer view of how destabilized volcanoes react to magma pressure and gravity forces. Data are mostly based on new lithostratigraphic surveys and on offset measurements of late Pleistocene-Holocene deposits and historic man-made features. The North-East Rift (NE Rift) is a volcanic rift that has guided mostly explosive, but also secondary effusive, activity since the end of the Pleistocene. Monogenetic cones and explosive craters are strongly elongated and aligned along 66 main surface fractures and several others, hidden, but inferred, from sinkhole alignments. The opening direction is locally oblique, with a left-lateral component in the north-eastern segment of the rift. At the NE tip of the rift, the movements are relieved by the normal left-lateral Pernicana Fault (PF), an 11-km-long structure comprising E-W fault segments connected by right-stepping en echelon faults. Our study demonstrates that the very high slip rates at a centennial scale of these structures are well constrained and similar (NE Rift = 0.6-1.8 cm/a, PF = 0.5-2.2 cm/a) and have been also recognized in the Latest Pleistocene-Holocene (2.7 cm/a). The E-W Fiumefreddo Fault (FF) has normal left-lateral kinematics and a historic slip rate of 0.1 cm/a. Ripa della Naca Faults strike NE and are covered by unfaulted < 15-ka-old lavas. Relative sinking to the SE gives a late Pleistocene slip rate of 0.2 cm./a. Newly recognized NNE- to NE-striking active normal faults interfere at the PF-FF junction zone. The active East-North-East Rift, which is described in detail for the first time, is another volcanic rift affecting the eastern flank of Mt. Etna. This rift shows left-lateral normal motions and has guided the location of several pyroclastic cones since at least the end of the Pleistocene. Stratigraphic and structural data prove the presence of a North-South Rift which has existed across the Mt. Etna summit zone during at least the last 14 ka. Our data coupled with published seismological data indicate that these structures are the surface expressions of the seaward sliding of the eastern flank of Etna along two slip surfaces in a two-component principal stress field, produced by gravity loading and magma intrusion. A shallower slide surface can be activated following dominance of gravity instability triggered by the mass of the volcano flank, or movements can occur along a deeper surface triggered by injection phases, or the two surfaces can move at the same time by a combination of effects. The existence of this double system of deformation can explain several apparently contradictory phenomena, such as discontinuous correlation of deformation, earthquakes and eruptions, and different earthquake foci, focal mechanisms and fault slip rates. (C) 2002 Elsevier Science B.V. All rights reserved.