Strong variations in degassing rates at Villarrica Volcano (Southern Chile): Consequences for deep-seated processes

Villarrica is one of the most active volcanoes in Chile, has a persistent lava lake within its crater, and is presently characterized by continuous degassing and high-level seismicity. We use a multiparameter approach based on high time-resolution gas flux measurements and seismic data to characterize magmatic and tectonic processes controlling the volcanic activity. The instrumentation includes 3 ground-based NOVAC-type scanning Mini-DOAS spectrometers for the quantification of SO2 fluxes, installed at the volcano in March 2009, which are complemented with seismic data from the catalogue of the Volcanic Observatory of the Southern Andes (OVDAS in Spanish) and from the SFB 574 temporary volcanic network. During the last three years, we have detected activity variations at Villarrica occurring on several time scales, which can be related to distinct processes at depth. The monthly averages in degassing rates increased from 400 tons/d SO2 before December 2009, to about 1200 tons/d SO2 in April 2010. Superimposed on this pattern are strong pulses of seismic LP events occurring every 3 to 4 months, accompanied by peaks in degassing rates, interpreted as periodic influx of fresh magma batches to the deeper levels of the plumbing system. The degassing rates further show cyclic variations with a periodicity of 5 to 7 days. We interpret the latter variations to represent the turnover time for a conduit convection cycle of magma from a mid-crustal reservoir, where the degassing peaks correspond to the arrival of comparatively undegassed magma at shallow levels. In addition, irregular regional earthquake events are typically followed by increased degassing activity at Villarrica 2-4 days later, interpreted as due to increased bubble nucleation in the magmatic system at depth. The period of strongly increasing degassing rates between December 2009 and April 2010 encompasses the M 8.8 Maule earthquake, which occurred on February 27, 2010. However, the degassing variations show a complex pattern, including a steady increase already 3 months before the Maule earthquake. Degassing rates dropped off sharply 7 days before the earthquake, and was followed by several weeks of comparatively low degassing during the aftershock phase. At the end of March, degassing rates picked up to a factor of 3 above the pre-earthquake level culminating with a peak of 2800 tons/d SO2 at the beginning of April. The persistence of high degassing rates even two years later indicates a permanent change in the magma plumbing system. Further, the observation that the Maule earthquake rather interrupted than enhanced the trend of increased degassing requires a fundamental increase in the magma supply to Villarrica commencing months before the rupture event. We tentatively assign the increased availability of magma to pre-rupture creep in the lower crust or at mantle depths, enabling a pressure gradient for influx of magma. We suggest that such a change in the volume of incoming magma would probably have led to an eruptive cycle at a closed vent system like e.g. the neighboring Llaima volcano.