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.