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Waveform inversion of shallow repetitive long period events at Villarrica Volcano, Chile

Villarrica volcano is presently characterized by an active lava lake in the summit crater, with mild strombolian style eruptions of basaltic‐andesitic products. The lava lake acts as a permeable barrier where gas‐rich magma releases volatiles and then descends through the process of conduit convection to a deep storage reservoir. We identified thousands of repetitive long‐period (LP) seismic events within a three‐year time period using a matched filter; each was discovered to be associated with an infrasonic emission, and hence outgassing from the lava lake. Using this catalog of events for timing, we stacked signals over the three‐year deployment from 2010‐2012 to produce representative seismic and infrasonic signatures, and performed a full‐waveform seismic inversion using nine stations from the network. The data are well represented by a single horizontal force source‐time function, oriented N75°E and dipping 7°W from horizontal, with a maximum amplitude of 34.4 MN. The source is stable in time, and can be explained by a force couple system produced by a bursting bubble in the lava lake and the responsive drag force of the lava sloshing to fill in the void produced by the gas emission in an asymmetrical crater. Path effects lengthen the coda of the events with increasing distance from the source, resulting in emergent tremor‐like signals at distant seismometers. Since the occurrence of LP events correlates with tremor intensity, we suggest much of the tremor results from the superposition of discrete LPs.