Joshua P. Richardson

Varying seismic-acoustic properties of the fluctuating lava lake at Villarrica volcano, Chile

Villarrica volcano outgasses through an open lava lake, with bubbles ranging in size from submillimeter to several meters, the largest of which produce strombolian bursting events that are visible from the crater rim. Thousands of shallow strombolian events identified through seismic waveform cross correlation were found to produce discrete and repetitive long-period seismic and infrasonic signals. We identified variations of up to 0.7 s in seismic-acoustic arrival delay times between April and July 2010 at a station ~750 m from the vent, which we interpret as due to fluctuations in the level of lava lake. During time periods interpreted as having high lava lake levels, based on reduced time delays, interevent times were also reduced, and average seismic amplitude measurements, seismic and acoustic event energies, and volcano acoustic-seismic ratios were all high as compared to times when the lava lake was lower. The crater is also a source of nearly continuous, monotonic infrasonic tremor. We found that the peak frequency of this infrasonic tremor, typically around 0.5–1.0 Hz, was inversely correlated with seismic-acoustic delay times and therefore an indicator of lava lake level. We use this correlation to propose a new model for infrasonic tremor generation, namely, using crater geometry to approximate a Bessel horn. We interpret the two clearest cycles of elevated seismicity and lava lake level as due to an increase in exsolved gas, resulting from an injection of volatile-rich magma or an overturn in a deeper magma reservoir.