Source Mechanisms of Infrasonic Airwaves Generated by Strombolian Activity
Abstract
Scaled laboratory experiments on the ascent of individual gas slugs in a stationary liquid produce atmospheric pressure traces that display considerable similarity with published infrasonic waveforms from Stromboli (Figure 1). By scaling for gas expansion, the fluid dynamics behind the rise, expansion and bursting of gas slugs in the confines of a conduit can be characterised into different regimes relevant to the generation of infrasonic signals from strombolian activity. Quiescent expansion occurs for small gas masses, where negligible dynamic gas over-pressure develops during slug ascent and, prior to burst, the role of the meniscus is important. With increasing gas mass, a transition regime emerges where dynamic gas over-pressure is significant. For larger gas masses, this regime transforms to fully explosive behaviour, where gas over-pressure dominates and slug bursting is not a critical factor in the generation of infrasonic signals. Whereas previous work has related the magnitude of infrasonic pulses from very large strombolian events to the square root of the gas mass involved, we find that, for gas masses more typical of activity at Stromboli volcano, and over the regimes described here, the magnitude of infrasonic pulses is linearly related to gas mass. The experimental analyses allow volcanic slug dimensions, gas mass and over-pressure to be quantified from impulsive infrasonic signals generated by strombolian events. Figure 1. Experimental pressure (P) behaviour is used as a proxy for the mass flux (q) of gas through the one-dimensional tube during the experiment (Lighthill, 1978). On emerging into the atmosphere this mass flux drives acoustic signals whose P can be represented by dq/dt (Lighthill 1978, Johnson 2003). Experimental dP/dt is shown (black line) for experiments (0.1 s scale bar), giving an indication of the infrasound waveform shape that may result from the fluid dynamic source mechanism of slug rise, expansion and burst. Reprinted from McGreger and Lees (2004) (with permission from Elsevier) and Ripepe and Marchetti (2002) (with permission from AGU) are infrasonic signals measured at Stromboli volcano (grey lines) for comparison. Fitting of the strombolian infrasonic waveforms to those of the synthetic experimental waveforms results in collapse of the infrasonic time base to a common value (0.8 s scale bar).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V11B2758L
- Keywords:
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- 8409 VOLCANOLOGY / Atmospheric effects;
- 8419 VOLCANOLOGY / Volcano monitoring;
- 8445 VOLCANOLOGY / Experimental volcanism