Date of Final Oral Examination (Defense)
Type of Culminating Activity
Doctor of Philosophy in Geophysics
Jeffrey B. Johnson, Ph.D.
Brittany D. Brand, Ph.D.
Hans-Peter Marshall, Ph.D.
Paul Michaels, Ph.D.
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This work is licensed under a Creative Commons Attribution 4.0 License.
Real-time study of erupting vents is important for both monitoring and scientific purposes; because direct in-situ study of erupting vents is impractical, our best tools for studying eruptions in real time involve monitoring eruptive products and waves that travel far from the volcano. The atmosphere is a particularly advantageous medium for studying propagation and transport of volcanic waves and products: acoustic waves pass through it with minimal scattering, particles follow predictable trajectories, and the atmospheric structure that affects both is well-monitored. Analyses of acoustic waves and tephra deposits can provide important information on eruptions including total explosive energy, volume, and fragmentation processes. Additionally, the hazards associated with these processes justify the need to understand and be able to model them.
Despite the apparent simplicity of volcanic-atmospheric phenomena, many open questions and difficulties remain. This dissertation aims to address some of the challenges facing us and help develop a better understanding of volcanic-atmospheric phenomena. In this work, I discuss and demonstrate tools to improve our understanding of such phenomena. A general introduction to atmospheric physics and eruptive processes is provided in chapter 1.
A particularly severe problem addressed by this dissertation is analysis of pressure waves from powerful volcanic explosions. Due to theoretical and numerical difficulties associated with shock wave physics and the hazardous environment around exploding vents, existing theory, models, and observations are all insufficient to account for nonlinear shock wave propagation near the vent. This problem adds considerable uncertainty to potentially valuable acoustic inferences of eruptive activity. I address this problem in three ways. In chapter 2, I use numerical models of volcanic explosions to demonstrate a new framework for analyzing nonlinear pressure waves from powerful explosions, showing that tools developed for studying chemical and nuclear explosions can be adapted to study explosive volcanic eruptions. In chapter 3, I use existing acoustic theory and models to investigate an unusually powerful and well-instrumented vulcanian eruption at Volcan Tungurahua (Ecuador), calculating the volume of erupted gas and tephra (~0.5 km3), classifying subsequent tremor into distinct mechanisms by its infrasound, and showing the relationship of volcanic lightning to vent activity. In chapter 4, I describe the development and use of a novel infrasound instrument (the Gem infrasound logger) intended to address limitations of existing instrumentation that particularly affect our ability to record shock waves. As the lowest-cost, lightest, and most flexible infrasound instrument currently available, the Gem is an ideal tool for recording shock waves in remote or hazardous settings where the risk of instrument loss must be tolerated and installation by drones with limited payload capacity may be necessary.
Finally, in chapter 5, I explore numerical modeling tephra transport from severe eruptions, focusing on two case studies in this dissertation. The first eruption, a 2015 lava fountain at Volcan Villarrica (Chile), produced a plume 6-8 km above the vent and deposited tephra in a narrow band extending tens of kilometers downwind. A custom Lagrangian model of tephra transport considering actual wind conditions at the time of the eruption shows good agreement with a map of the deposit obtained from field mapping and satellite imagery, including the finding of tephra grading perpendicular to the wind direction. In the second eruption, the 2013 vulcanian eruption at Tungurahua, I use the same Lagrangian model to calculate ballistic trajectories and times of impact with the ground, and show that coincident infrasound that cannot be explained by other sources probably originates in ballistic impacts. Infrasound due to ballistic impacts (which has previously not been documented) could be used to improve monitoring by enabling estimates of explosive properties to be made within tens of seconds of the eruption onset.
Anderson, Jacob Fortner, "Pressure Waves and Tephra Dispersal from Volcanic Explosions: Models, Observations, and Instrumentation" (2018). Boise State University Theses and Dissertations. 1468.
Available for download on Saturday, December 19, 2020