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

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