Controls on the Frequency Content of Near-Source Infrasound at an Open-Vent Volcano (Villarrica, Chile)

The acoustic signals from open-vent volcanoes can contain specific information related to that volcano’s eruption dynamics and future activity. Thus, studying a specific volcano’s acoustics may provide critical warning mechanisms, signaling impending eruptions. Villarrica volcano, located in southern Chile, has an active lava lake that produces continuous infrasound with spectral peaks near 1 Hz and excursions of +/- ~0.2 Hz. The infrasound’s frequency content reveals key volcanic properties such as eruption style and crater shape. Leading up to Villarrica’s most recent paroxysm in 2015, infrasound spectral changes coincided with and indicated a rise in Villarrica’s lava lake level. As such, quantifying and understanding the regular fluctuation in recorded infrasonic frequencies, from Villarrica and other open-vent volcanoes, is imperative. A week-long period of crater rim infrasound observations associated with stable, open-vent activity, revealed two independent source processes: spatter bursting events and lava lake-induced tremor oscillations (broadband discrete signals and ~1 Hz tremor respectively). Comparison of these acoustic signals with results from a 3-D finite-difference time-domain wave propagation model (InfraFDTD) shows that sound speed and source spectrum variability can both influence Villarrica’s spectrum. Yet, sound speed variations in the crater--whether diurnal or volcanic in origin--cannot explain the full extent of the observed frequency excursions. Instead, source spectrum variability is primarily responsible for the frequency excursions. This work highlights the utility of data collected from a dense distribution of twenty infrasound sensors operating at the summit and validates the effectiveness of the InfraFDTD modeling approach