Blast waves from violent explosive activity at Yasur Volcano, Vanuatu

Abstract

[1] Infrasonic and seismic waveforms were collected during violent strombolian activity at Yasur Volcano (Vanuatu). Averaging~3000 seismic events showed stable waveforms, evidencing a low-frequency (0.1-0.3 Hz) signal preceding 5-6 s the explosion. Infrasonic waveforms were mostly asymmetric with a sharp compressive (5-106 Pa) onset, followed by a small long-lasting rarefaction phase. Regardless of the pressure amplitude, the ratio between the positive and negative phases was constant. These waveform characteristics closely resembled blast waves. Infrared imagery showed an apparent cold spherical front~20 m thick, which moved between 342 and 405 m/s before the explosive hot gas/fragments cloud. We interpret this cold front as that produced by the vapor condensation induced by the passage of the shock front. We suggest that violent strombolian activity at Yasur was driven by supersonic dynamics with gas expanding at 1.1 Mach number inside the conduit.

Acoustic Characterization of Explosion Complexity at Sakurajima, Karymsky, and Tungurahua Volcanoes

“…Before beginning our data analysis, we had originally hypothesized that explosions with larger amplitude would have shorter durations than those with smaller amplitude, that is, peak pressure would vary inversely with event duration. This is because we were expecting the simplest explosion or blast waveforms (Marchetti et al, 2013) would have high amplitudes and very short durations, whereas smaller events would tend to be more sustained degassing events. These figures show that, on the contrary, even largeamplitude explosion events tend to have long-duration codas, consistent with visual observations of sustained gas and/or ash venting following the initial sharp compressional onset at all three volcanoes.…”

“…Note that the main infrasound explosive compression occurs prior to the first snapshot time shown in all cases, as there is a delay between explosion initiation and the visible appearance of tephra. The waveform plot origin times (t 0 s on the x axis) in UTC are as follows: (Marchetti et al, 2013), except (a), which is filtered from 0.05 to 15 Hz to reduce wind and cultural noise. Note the double-peaked blast waveform in (c), corresponding to two explosive emissions.…”

“…Previous conceptual understanding of acoustic signals from explosive eruptions has tended to focus on two endmember signal durations: (1) discrete explosion (blast) waves with relatively simple waveforms lasting from several to tens of seconds (e.g., Firstov and Kravchenko, 1996;Ripepe and Marchetti, 2002;Johnson, 2003;Marchetti et al, 2009Marchetti et al, , 2013 and (2) sustained, broadband, infrasonic tremor signals lasting from minutes to hours (e.g., Vergniolle and Caplan-Auerbach, 2006;Caplan-Auerbach et al, 2010). Commonly, the discrete explosion signals are simply called "explosions" in the volcano acoustics literature and they have been modeled, using linear equivalent source theory, as acoustic monopoles, that is, sources represented as time-varying mass fluxes (e.g., Woulff and McGetchin, 1976;Firstov and Kravchenko, 1996;Johnson, 2003;Delle Donne and Ripepe, 2012;Johnson and Miller, 2014).…”

“…Commonly, the discrete explosion signals are simply called "explosions" in the volcano acoustics literature and they have been modeled, using linear equivalent source theory, as acoustic monopoles, that is, sources represented as time-varying mass fluxes (e.g., Woulff and McGetchin, 1976;Firstov and Kravchenko, 1996;Johnson, 2003;Delle Donne and Ripepe, 2012;Johnson and Miller, 2014). However, it is recognized that in many cases these "explosion" waves have nonlinear characteristics (Yokoo and Ishihara, 2007;Garces et al, 2013;Marchetti et al, 2013), and nonlinear effects will increase with increasing overpressure release rate and supersonic ejection velocities (Needham, 2010). Sustained, broadband infrasonic tremor signals from more sustained eruptions (often described as Vulcanian, sub-Plinian, and Plinian) resemble an infrasonic form of the jet noise produced by smaller scale man-made jets .…”

Using infrasound to constrain ash plume rise

“…A non-linear formulation should be considered instead ). For instance, employing the RankineHugoniot equations for 1D shock wave theory (e.g., Marchetti et al 2013) and acoustic pressures between 50-200 Pa (i.e. within a range compatible with values measured at Mt.…”