Relationship between infrasound-derived and buoyancy-derived eruption plume volume estimates

Abstract

Volume estimations for discrete plumes accompanying short-lived eruptions are potentially possible using infrasound signals. Plumes emitted during short-lived (discrete) eruptions have been modeled as discrete thermals or rooted thermals. Based on the one-dimensional model for the rise of a discrete thermal, the initial buoyancy, F0, of a spherical thermal can theoretically be estimated from the maximum plume height and a vertical profile of ambient air density. We here examine the relationship between infrasound-derived volume, Vinf, and the buoyancy-derived volume, Vb, as derived from F0, which depends on gravity acceleration, and the difference in density between the thermal and surrounding air, to understand how Vinf relates to the dynamics of the eruption plume. We analyze infrasound data accompanying Vulcanian and phreatic eruptions at Aso, Shinmoedake, and Lokon-Empung volcanoes to estimate Vinf, and consider Vinf from other volcanoes. We obtain a representative ratio of Vb/Vinf of 16 by examining 53 events. Because the analyzed infrasound signals share a prominent pressure pulse at the onset, we regard Vinf as the volume of the plume at initiation, i.e., as a jet. Instead, we consider Vb as the thermal volume when it has entrained a sufficient amount of the surrounding air and obtains F0. Comparison of the bulk density of the jet and the discrete thermal yields a rate of volume change between both regimes by a factor of 1.8–32, which is consistent with the ratio of Vb/Vinf. Our result provides an effective index to constrain erupted plume volume using infrasound data with real-time monitoring systems.