The December 2015 Mount Etna eruption: An analysis of inflation/deflation phases and faulting processes

Abstract During the first days of December 2015, there were four paroxysmal events at the “Voragine” crater on Mount Etna, which were among the most violent observed during the last two decades. A few days after the “Voragine” paroxysms, the Pernicana – Provenzana fault system, located near the crater area, underwent an intense seismic swarm with a maximum “local” magnitude M L of 3.6. This paper investigates the relationship between the eruptive phenomenon and the faulting process in terms of Coulomb stress changes. The recorded seismicity is compatible with a multicausal stress redistribution inside the volcano edifice, occurring after the four paroxysmal episodes that interrupted the usual trend of inflation observed at Mt. Etna. The recorded seismicity falls within the framework of a complex chain of various and intercorrelated processes that started with the inflation preparing the “Voragine” magmatic activity. This was followed with the rapid deflation of the volcano edifice during the paroxysmal episodes. We determined that the recorded deflation was not the direct cause of the seismic swarm. In fact, the associated Coulomb stress change, in the area of seismic swarm, was of about −1 [bar]. Instead, the fast deflation caused the rarely observed inversion of dislocation in the eastern flank at the same time as intense hydrothermal activity that, consequently, underwent an alteration. This process probably reduced the friction along the fault system. Then, the new phase of inflation, observed at the end of the magmatic activity, triggered the faulting processes.