Title: Using seismo-acoustic observations to characterize volcanic processes
Abstract: Volcanic eruptions are violent phenomena that can wreak havoc on local populations and infrastructure and threaten global air traffic and economies. Volcanologists and observers use geophysical signals (seismic, infrasound, lightning, and remote sensing data) to probe the subsurface and eruptions that are otherwise inaccessible or too hazardous to be near. Magma and gas movement in the subsurface generates seismic waves. When an explosive eruption occurs, it generates infrasonic waves (sound below human hearing), an eruption plume frequently observable with satellites, and often lightning. I use these geophysical signals to understand the volcanic processes that generate them.
Sustained eruptions generate complex, directional sound known as volcanic jet noise. Jet noise is the sound produced by momentum-driven flow through a nozzle or vent, and jet noise from a volcano has similar time and frequency characteristics to jet noise from rockets, although at lower frequencies. Volcanic jet noise is a new area of research and is notably more complex than discrete explosion infrasound. An important area of research is connecting infrasound frequency to flow velocity and in turn mass eruption rate (MER; the mass and speed of ejected material) as it is a key control on ash plume dynamics and hazards. The manufactured jet noise community established that jet noise varies as a function of the angle to the jet axis, which is parallel to jet flow, and derived empirical equations through extensive laboratory testing relating jet noise and jet parameters including velocity, diameter, and temperature. However, these equations may not wholly extend to volcanoes, as volcanic jets are multiphase (gas, ash, and tephra) and time-variable. I examine the infrasound signal characteristics from two large eruptions in June 2019 (Raikoke, Kuril Islands and Ulawun, Papua New Guinea volcanoes) with changes in crater geometry from PlanetScope satellite imagery. During both eruptions, I observe a decrease in infrasound peak frequency during an increase in eruptive intensity, which remains through the end of the eruptions. The satellite data show an increase in the crater area at both Raikoke and Ulawun. I investigate the relationship between the frequency and crater area changes. This is the first study to corroborate the decrease in infrasound peak frequency with documented increase in crater area. The fortuitous satellite overpass timing, clear skies, and high spatial resolution enables this quantitative examination of jet noise frequency with jet flow parameters.
Very long period seismic signals (VLPs) are commonly used for interpreting volcanic plumbing systems and eruption dynamics. The standing conceptual model for VLPs links them to the driving mechanism for strombolian explosions, involving a gas slug rising in the conduit, generating a VLP, and bursting at a free surface. The model for strombolian eruptions is based on observations of bubble bursting at an observable lava lake at Heimaey Volcano, Iceland, in the 1970s, and has formed the basis for decades of detailed analysis and interpretation of VLPs at volcanoes worldwide, including Kilauea (Hawai’i), Erebus (Antarctica), Yasur (Vanuatu), Fuego (Guatemala), and Stromboli (Italy) volcanoes. In my study, I carefully document a paucity
of coincident explosions with repeating VLPs at Stromboli Volcano, Italy, through contemporaneous analysis of seismic (VLP) and acoustic (explosion) data. I demonstrate that the prevailing model linking VLPs to strombolian explosions does not hold during a period of intense study at Stromboli in 2018, and further argue that the simple ‘slug’ model for VLPs is not applicable to volcanoes where strong crystallization occurs in the upper conduit. My data and observations are wholly consistent with a recently proposed conceptual model for Stromboli’s shallow conduit and eruption dynamics, in which a semipermeable plug of partially-crystallized magma acts as a control on gas escape. This result has critical implications for the interpretation of seismicity as a precursor to major explosions at Stromboli and similar systems and provides insight into the mechanics of shallow degassing.