Fitting Jet Noise Similarity Spectra to Volcano Infrasound Data. Issue 11 (10th November 2021)

• Record Type: Journal Article
• Authors: Gestrich, J. E.; Fee, D.; Matoza, R. S.; Lyons, J. J.; Ruiz, M. C.
• Is Part Of: Earth and space science. Volume 8:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-10
• Subjects: jet noise -- infrasound -- volcano -- turbulence -- acoustic -- monitoring; Space sciences -- Periodicals; Geophysics -- Periodicals; 500.5
• DOI: 10.1029/2021EA001894
• Languages: English
• ISSNs: 2333-5084

Abstract

Abstract: Infrasound (low‐frequency acoustic waves) has proven useful to detect and characterize subaerial volcanic activity, but understanding the infrasonic source during sustained eruptions is still an area of active research. Preliminary comparison between acoustic eruption spectra and the jet noise similarity spectra suggests that volcanoes can produce an infrasonic form of jet noise from turbulence. The jet noise similarity spectra, empirically derived from audible laboratory jets, consist of two noise sources: large‐scale turbulence (LST) and fine‐scale turbulence (FST). We fit the similarity spectra quantitatively to eruptions of Mount St. Helens in 2005, Tungurahua in 2006, and Kīlauea in 2018 using nonlinear least squares fitting. By fitting over a wide infrasonic frequency band (0.05–10 Hz) and restricting the peak frequency above 0.15 Hz, we observe a better fit during times of eruption versus non‐eruptive background noise. Fitting smaller overlapping frequency bands highlights changes in the fit of LST and FST spectra, which aligns with observed changes in eruption dynamics. Our results indicate that future quantitative spectral fitting of eruption data will help identify changes in eruption source parameters such as velocity, jet diameter, and ash content which are critical for effective hazard monitoring and response. Plain Language Summary: Detecting and quantifying the amount of ash and other material that erupted is important for effective hazard mitigation. One method to observe eruptions is to record low‐frequency sound that is produced during the eruption, called volcano infrasound. However, the source of volcano infrasound produced by energetic eruptions is complex. Tam et al. (1996, https://doi.org/10.2514/6.1996-1716 ) showed that small jets in the laboratory generate sounds with two different but characteristic spectral shapes that are produced by two kinds of turbulence: fine‐scale turbulence and large‐scale turbulence. Volcanoes produce large jets with complex eruption flows, and here we examine the similarity of their spectral shape compared to laboratory jets using a nonlinear spectral fitting technique. We find that volcanic jets generally produce noise similar to laboratory jets, while times of background noise have a different spectral shape. We also develop a method that shows which frequency bands have the most similar shape to laboratory jets, and find that the jet noise frequencies change as the eruption dynamics evolve. Our results may allow scientists to determine when an eruption is occurring using the infrasound spectral shape, as well as infer volcanic eruption parameters, such as jet velocity and ash content. Key Points: We introduce a technique to quantitatively fit empirical jet noise spectra to volcano infrasound data from different volcanoes A decrease in misfit during eruptions suggests that spectral fitting can distinguish between eruption and noise Spectral fitting shows jet noise frequencies change as the eruption dynamics evolve