Infrasonic jet noise from volcanic eruptions

Abstract

The lowermost section of a Vulcanian or Plinian volcanic eruption column may be thought of as a momentum‐driven, turbulent, free‐shear jet flow. We propose that large‐amplitude and long‐duration infrasonic (<20 Hz) signals recorded at ranges of tens of kilometers during powerful eruptions at Mount St. Helens, USA, and Tungurahua, Ecuador, represent a low frequency form of jet noise. A preliminary test of this hypothesis is made by comparing the observed infrasonic spectra to the empirically‐derived similarity spectra for pure‐air jets. Although the spectral shapes are in approximate agreement, the observed volcanic signals have additional complexities not present in the pure‐air laboratory data. These features may result from multiphase flow containing solid particles and liquid droplets, very high temperatures, and perhaps complex crater morphology. However, the overall similarity between the volcanic signals and jet noise indicates that broadband infrasound measurements at volcanoes may provide a quantitative link to eruption jet dynamics, and would aid substantially in the remote assessment of volcanic hazard.

Introduction to an Open Community Infrasound Dataset from the Actively Erupting Sakurajima Volcano, Japan

“…Infrasonic tremor at Sakurajima is usually coincident with sustained low-level ash emissions typically rising < 1 km above the vent. The tremor may also reflect jetting process and resemble a low frequency form of jet noise (e.g., Matoza, Fee, et al, 2009 waveforms, propagation effects cause station-dependent variability that is regularly observable in tremor waveforms.…”

Infrasonic observations of the June 2009 Sarychev Peak eruption, Kuril Islands: Implications for infrasonic monitoring of remote explosive volcanism

“…Considering that SP explosions produced plumes reaching to altitudes typically of 5--10 km or higher (Table 2), it is possible that the infrasonic source could be extended in altitude. Matoza et al, (2009) proposed that broadband infrasonic tremor recorded during sustained vulcanian and plinian eruptions, such as that observed here at SP (Figure 3), may be generated by similar physical noise--generation mechanisms as those operating in man--made jet flows. It is thought that one dominant noise source in man--made jets is the growth and decay of stochastic instability waves propagating downstream along the edge of the jet flow in the shear layer between the jet flow and ambient atmosphere (Tam and Burton, 1984).…”

“…Here we report atmospheric infrasound observations of the June 2009 SP eruption. Energetic vulcanian and plinian explosions can radiate large--amplitude infrasound directly into the atmosphere (e.g., Garces et al, 2008;Matoza et al, 2009;Fee et al, 2010aFee et al, , 2010b. In contrast, seismicity (eruption tremor) recorded on dedicated volcano--seismic networks during vulcanian and plinian explosions may result from subsurface processes and/or limited air--ground acoustic--seismic coupling (Matoza, 2009).…”

Application of the Monopole Source to Quantify Explosive Flux during Vulcanian Explosions at Sakurajima Volcano (Japan)

“…Although the monopole source is postulated as a suitable approximation for purely explosive transients, higher order dipole and quadrupole acoustic sources (Woulff and McGetchin, 1976;Johnson et al, 2008;Kim et al, 2012) and jet noise (Matoza et al, 2009;Taddeucci et al, 2014) have been proposed for a range of volcanic eruptive activities. Both jet noise and dipole type sources radiate sound less efficiently than a monopole source (for given eruption energetics) and are thought to radiate sound with an axisymmetric symmetry (Dowling, 1998).…”