The 2004-05 Etna eruption: implications for flank deformation and structural behaviour of the volcano

The development of the 2004-05 eruption at Etna (Italy) is investigated by means of field surveys, to define the current structural state of the volcano. In 2004-05, a fracture swarm, associated with 3 effusive vents, propagated downslope from the SE summit crater towards SE. Such a scenario is commonly observed at Etna, as a pressure increase within the central conduits induces the lateral propagation of most of the dikes downslope. Nevertheless, some unusual features of this eruption (slower propagation of fractures, lack of explosive activity and seismicity, oblique shear along the fractures) suggest a more complex triggering mechanism. A detailed review of the recent activity at Etna enables us to better define this possible mechanism. In fact the NW-SE trending fractures formed in 2004-05 constitute the south-eastern continuation of a N-S trending fracture system which started to develop in early 1998, to the east of the summit craters. The overall 1998-2005 deformation pattern forms therefore an arcuate feature, whose geometry and kinematics are consistent with the head of a shallow flank deformation on the E summit of Etna. Similar deformation patterns have also been observed in analogue models of deforming volcanic cones. In this frame, the 2004-05 eruption was possibly induced by a dike resulting from the intersection of this incipient fracture system with the SE Crater. A significant acceleration of this flank deformation may be induced by any magmatic involvement. The central conduit of the volcano is presently open, constantly buffering any increase in magmatic pressure and any hazardous consequence can be expected to be limited. A more hazardous scenario may be considered with a partial or total closing of the central conduit. In this case, magmatic overpressure within the central conduit may enhance the collapse of the upper eastern flank, triggering an explosive eruption associated with a landslide reaching the eastern lower slope of the volcano