4aPA – Using Sound Waves to Quantify Erupted Volumes and Directionality of Volcanic Explosions
Alexandra Iezzi – amiezzi@alaska.edu
Geophysical Institute, Alaska Volcano Observatory
University of Alaska Fairbanks
2156 Koyukuk Drive
Fairbanks, AK 99775
David Fee – dfee1@alaska.edu
Geophysical Institute, Alaska Volcano Observatory
University of Alaska Fairbanks
2156 Koyukuk Drive
Fairbanks, AK 99775
Popular version of paper 4aPA
Presented Thursday morning, May 16, 2019
177th ASA Meeting, Louisville, KY
Volcanic eruptions can produce serious hazards, including ash plumes, lava flows, pyroclastic flows, and lahars. Volcanic phenomena, especially explosions, produce a substantial amount of sound, particularly in the infrasound band (<20 Hz, below human hearing) that can be detected at both local and global distances using dedicated infrasound sensors. Recent research has focused on inverting infrasound data collected within a few kilometers of an explosion, which can provide robust estimates of the mass and volume of erupted material in near real time. While the backbone of local geophysical monitoring of volcanoes typically relies on seismometers, it can sometimes be difficult to determine whether a signal originates from the subsurface only or has become subaerial (i.e. erupting). Volcano infrasound recordings can be combined with seismic monitoring to help illuminate whether or not material is actually coming out of the volcano, therefore posing a potential threat to society.
This presentation aims to summarize results from many recent studies on acoustic source inversions for volcanoes, including a recent study by Iezzi et al. (in review) at Yasur volcano, Vanuatu. Yasur is easily accessible and has explosions every 1 to 4 minutes making it a great place to study volcanic explosion mechanisms (Video 1).
Video 1 – Video of a typical explosion at Yasur volcano, Vanuatu.
Most volcano infrasound inversion studies assume that sound radiates equally in all directions. However, the potential for acoustic directionality from the volcano infrasound source mechanism is not well understood due to infrasound sensors usually being deployed only on Earth’s surface. In our study, we placed an infrasound sensor on a tethered balloon that was walked around the volcano to measure the acoustic wavefield above Earth’s surface and investigate possible acoustic directionality (Figure 1).
Figure 1 [file missing] – Image showing the aerostat on the ground prior to launch (left) and when tethered near the crater rim of Yasur (right).
Volcanos typically have high topographic relief that can significantly distort the waveform we record, even at distances of only a few kilometers. We can account for this effect by modeling the acoustic propagation over the topography (Video 2).
Video 2 – Video showing the pressure field that results from inputting a simple compressional source at the volcanic vent and propagating the wavefield over a model of topography. The red denotes positive pressure (compression) and blue denotes negative pressure (rarefaction). We note that all complexity past the first red band is due to topography.
Once the effects of topography are constrained, we can assume that when we are very close to the source, all other complexity in the infrasound data is due to the acoustic source. This allows us to solve for the volume flow rate (potentially in real time). In addition, we can examine directionality for all explosions, which may lead to volcanic ejecta being launched more often and farther in one direction than in others. This poses a great hazard to tourists and locals near the volcano and may be mitigated by studying the acoustic source from a safe distance using infrasound.
Figure 1. a) A satellite image of Sakurajima Volcano, adapted from Kim and Lees (2014). Five stand-alone infrasound sensors were deployed around Showa Crater in July 2013, indicated by inverted triangles. b) An image of a typical explosive eruption observed during the field campaign.
Figure 2. a) Sound pressure level in dB relative to the peak pressure at the source position. b) Variation of infrasound waveforms across the network caused by propagation path effects.
Figure 3. Volume flux history inferred from infrasound data. In Case 1, the Green’s function is computed by 3-D numerical modeling considering volcanic topography. In Case 2, the analytic solution of the wave equation in a half-space is used, neglecting the topography.