Balance Between Deformation and Seismic Energy Release: The Dec 2018 ‘Double-Dike’ Intrusion at Mt. Etna

Dikes are the primary mechanisms to transport magma and feed eruptions. Investigations
of the surface deformation and seismicity caused by a dike can potentially provide useful
information to forecast the expected propagation and associated hazard. On December
24, 2018, a dike intrusion reached the summit of Mt. Etna, feeding an effusive fissure. The
intrusion was accompanied by a seismic swarm, with hypocenters beneath the summit
craters and eruptive fissure, and by ground deformation. The seismicity continued the
following day, with the hypocenters deepening to 3 kmb.s.l. due to the propagation of a
deeper and thicker dike. This situation generated the fear of feeding a more dangerous
eruption in the medium-low flank. Recently it was found an equation that relates the
average thickness and dimension of the dike with the expected released mechanical
energy and, therefore, to the seismic moment. By using this updated application, it is
shown that the observed seismicity could not be accounted for by the first dike. Instead,
the cumulative effect of both dikes indicates a total moment from available energy
expected that balances the moment recorded by the seismicity. The proposed
approach proved very useful in the specific case of Etna volcano eruptions, resulting
an effective tool to monitor the state of the intrusion of the magma and, therefore, to predict
if a dike has enough energy to continue propagating or to stop.