Frequency Fluctuations of Volcanic Infrasound: Why Does Volcán Villarrica's Peak Frequency Waver?

Abstract:

Infrasound from Villarrica was recorded on January 12-19, 2020 with a large-N deployment of 21 stations located at the summit in and around the crater. This study focuses on the frequency content of these infrasonic waves, which can be used to constrain processes and properties, including eruption dynamics, crater geometry, and volumetric gas flux. Villarrica is an open-vent volcano in Chile with an active lava lake that produces continuous monotonic infrasound centered at about 1 Hz, but with excursions of +/- ~0.2 Hz. Past studies have correlated a rise in Villarrica’s infrasonic frequency to a rise in its lava lake level prior to an eruption, so accounting for regular fluctuation in recorded frequency is imperative. During our week-long experiment, for which volcanic activity was stable, we argue that the short-term fluctuations in frequency content could be due to atmospheric conditions and not source mechanics. The main atmospheric condition likely to cause this recorded frequency ambiguity is the changing temperature gradient of the atmosphere both inside and outside of the crater. To test the temperature controls, we ran simulations over Villarrica’s topography using a 3-d finite difference time domain infrasound propagation model (InfraFDTD). Multiple sound speeds were calculated as a function of the possible crater temperatures and implemented into the model to develop synthetic signals. The synthetic signal arrival times and frequency outputs were then compared to signals we observed, showing how temperature within the crater is likely to be the most important effect on the recorded signal frequency content. We use the large-N array to demonstrate that frequency variations are consistent across the crater region.