Understanding Outgassing Activity with Measurements of SO2 Fluxes and Seismicity at Villarrica Volcano, Chile

Villarrica is an open-vent stratovolcano characteristically basaltic to basaltic-andesite in composition. Since at least 1984 it has shown persistent gas plume emission and bubble-burst activity. This activity takes place at the surface of a lava pond located a few tens of meters below the crater rim. In order to study the relation between gas fluxes and volcanic activity, we have combined SO2 flux measurements, visual observations of the lava pond, and seismic data collected at a short-period seismic station 3.7 km NW of the volcano. The gas data was obtained with Flyspec UV spectometers by doing traverses around the volcano and measuring the gas plume directly from the crater rim. We applied new field methodologies to get high resolution time series of the outgassing at the vent. During periods with higher levels of activity, a good correlation is found between higher SO2 fluxes and RSAM values. Four different styles of bubble bursting have been identified: seething magma, strombolian explosions, gas jetting, and small lava fountains. The first is continuous in time, whereas the last three are discrete events that correlate with higher amplitude tremor bursts and SO2 concentration time series. This integrated analysis shows that, at different time scales, changes in the amount and style of outgassing correlate with tremor amplitude as well as visually observations of the activity. This is interpreted as a strong link between tremor and the outgassing activity taking place in the shallowest part of the open plumbing system, which is a result of the melt-gas dynamics within the conduit.