Decrease in volcano jet noise peak frequency as crater expands

Volcanic jet noise is the sound, often <20 Hz and termed infrasound, generated by momentum-driven fluid flow through a volcanic vent. Assuming the self-similarity of jet flows and audible jet noise extends to infrasonic volcanic jet noise, the Strouhal number, St, connects frequency changes to those in jet length (expanded jet diameter) and/or velocity scale (jet velocity). We examine the infrasound signals from the June 2019 eruptions of Raikoke, Kuril Islands and Ulawun, Papua New Guinea volcanoes with changes in crater geometry. We use data from the International Monitoring System infrasound network and pre- and post-eruption satellite data (PlanetScope imaging). We observe a decrease in infrasound peak frequency during the transition to a more intense phase, which remains through the end of both eruptions. With the PlanetScope data, we measured a crater area increase of ∼50 000 m2 at Raikoke and ∼31 000 m2 at Ulawun. We use crater diameter as a proxy for expanded jet diameter. Our analysis suggests that the increases in crater diameter alone cannot account for the decreases in peak frequency for a constant St. This suggests the jet velocity also increased, which fits satellite data, and or the fluid properties (e.g., particle loading, nozzle characteristics, etc.) changed. This is reasonable as the eruptions intensified, which likely involved an increase in jet velocity and erosion of the crater walls. This is the first study to corroborate the decrease in infrasound peak frequency with documented increase in crater area.