Long-Term Evolution of the Seismic Activity Preceding the 2015 Seismic Crisis at Deception Island Volcano, Antarctica (2008-2015)

Deception Island is an active volcano located in the South Shetland Islands, Antarctica. Although the last eruptions occurred in 1967-1970, the volcano has undergone periods of seismic unrest in 1992, 1999, and 2015. In this work, we analyze continuous seismic data obtained by a permanent station for the period 2008-2015 preceding the 2015 unrest. We identify different types of seismic signals including tectonic and volcano-tectonic (VT) earthquakes and long-period (LP) seismicity, using a combination of visual and automated techniques. The temporal evolution of the seismicity displays three differentiated stages. In Phase 1 (2008-2010) the volcano was in a dormant state characterized by a moderate level of seismicity dominated by low-energy LP seismicity, and very few VT earthquakes. In Phase 2 (2011-2014), there was a gradual increase in the level of LP events and tremor, and an acceleration of the number and energy of VT earthquakes. In Phase 3 (2014-2015) the seismicity reached a climax, with the occurrence of seismic swarms comprising thousands of earthquakes, displaying maximum activity in September-October 2014 and February 2015. We propose that the change from Phase 1 to Phase 2 was due to the recharge of magma into the plumbing system at Moho depths. The emplacement of fresh magma increased the amount of gas (and heat) permeating the volcano edifice, triggering VT earthquakes in brittle regions and favoring the occurrence of LP seismicity in the shallow hydrothermal system. During Phase 2 magma ascended through the plumbing system, as evidenced by the acceleration of the seismicity rates, the detection of thermal anomalies, and a change in the deformation pattern that for 2013-2015 corresponded to an inflation process. Finally, in Phase 3 the magma intrusion reached its shallowest point before stalling at a depth of 6-10 km. The stress perturbations propagated through crustal fluids and produced a distal VT swarm SE of Livingston, as well as proximal VT swarms and very intense LP seismicity at Deception Island.