Influence of the Kilauea 2018 Fissure 8 Geometry and Eruption Dynamics on Acoustic Jet Noise

Abstract:

The eruption of Kilauea in 2018 produced persistent explosive and effusive activity of fissure 8, located in the lower East Rift Zone. The lava fountain at the fissure reached heights up to 80 m and was active for over two months. During this time an infrasound array of 4 sensors was deployed less than 500 m from the fountain. This array recorded continuous low frequency and audible sound from the fissure that has previously been linked to degassing and effusion (Patrick et al., 2019, Science). Local observers reported that the sound from the fissure also closely resembled a jet engine. Such close and continuous recordings of sustained eruptions, particularly at fissures, are rare and give us the unique opportunity to investigate the acoustic source of a particular type of high-velocity multiphase volcanic jet flow. We compare the acoustic spectrum of this volcanic jet (lava fountain) to the empirical spectrum of turbulent man-made jets. Turbulent mixing noise from a jet can be divided into large scale turbulence (LST) generated by coherent instability waves and fine scale turbulence (FST) attributed to fine-scale eddies. We introduce a new tool to automatically detect and quantify volcanic jet noise by non-linear fitting of the empirical jet-noise spectra, LST and FST, to the eruption data. The volcanic jet noise produced by fissure 8 is relatively well-matched by the empirical spectra, which suggests that turbulence is a dominant source of noise. The spectrum is also remarkably stable during the eruption. However, on June 16 the spectrum changes with a higher proportion of lower frequencies recorded. This coincides with a maximum in lava effusion rate and sound pressure level. After June 16, the effusion rate and sound pressure level decrease while the spectrum still shows the elevated low-frequency amplitudes. We compare the changes in the spectrum to the change in the morphology of the cone that surrounds the fountain and the dynamics and properties of the fountain itself. This work may allow infrasound data to be used to help determine eruption properties such as effusion rate and fissure geometry.