Imaging composite dike propagation (Etna, 2002 case)

[ 1] Late on the night of 26 October 2002, a dike intrusion started suddenly at Mount Etna, producing intense explosive activity and lava effusion on the southern flank. Five to six hours afterward, a long field of eruptive fractures propagated radially along the northeastern flank of the volcano, producing marked variations at the permanent tilt network. The dike propagation velocity was inferred by the associated seismicity. We modeled the temporal evolution of the continuously recorded tilt data, both during the vertical dike propagation on the high south flank on 26 October and during the radial propagation along the northeast flank, between 27 and 28 October. The reproduction of the recorded tilt signal allowed us to describe the geometry and characteristics of the two dikes in greater detail than the previous static inversion. We deduced that the eruption was characterized by an unusual composite mechanism, clearly showing a transition from a nearly pure opening mode displacement to a mechanism characterized by an equally strong normal dip-slip component and a smaller left lateral strike-slip component. In this study we demonstrate the interaction between the final segment of the dike and a preexisting structure that was reactivated in response to the intrusion. We show that tilt and its modeling represent a powerful tool to verify and constrain dike intrusions in detail.