Interpretation of infrasound generated by erupting volcanoes and seismo-acoustic energy partitioning during strombolian explosions
Johnson, Jeffrey B., 1972-
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Infrasonic signals provide a valuable tool for the study of volcanic eruptions because volcanoes generate the majority of their acoustic energy in the infrasonic bandwidth and infrasound is only slightly affected by propagation filters, transmission losses, dispersion, and instrument site responses. Though changing atmospheric properties can influence infrasonic amplitudes and arrival times, they do not significantly distort the original waveform. Because of the simplicity of acoustic propagation filters (compared to seismic propagation filters), recorded infrasonic pressure waveforms can reveal the overpressure time history at the vent which may be integrated to estimate explosive gas mass flux. Digitized video records are able to substantiate the relationship between infrasound intensity and the rate change of gas mass flux released during an explosion.This dissertation analyzes and interprets the radiated infrasound and seismicity produced by five different active volcanoes. The case studies encompass low-viscosity Strombolian activity (Erebus, Antarctica), medium viscosity Strombolian activity (Karymsky, Russia and Sangay, Ecuador), a more vigorous Vulcanian eruption (Tungurahua, Ecuador), and degassing explosions from an active dacitic dome (Pichincha, Ecuador). The complexity of both the infrasonic and seismic waveforms at these five volcanoes appears related to the viscosity and volatile content of the different magmas. Erebus explosion signals are uniform, short-duration bursts because gas is able to easily escape the low-viscosity magma. Conversely, extended-duration degassing signals at the other volcanoes can be attributed to higher magma viscosity.At both Erebus and Karymsky, arrays of low-frequency microphones and seismometers were deployed within several kilometers of the degassing source to quantify the elastic energy that propagates into the ground and into the atmosphere. Acoustic efficiency (relative to the radiated seismic energy) is attributed to shallow explosion sources with associated impulsive gas outflux. Strombolian explosions at Erebus appear acoustically efficient compared to Karymsky because gas release occurs at the surface of the lava lake. Karymsky explosion sources emanate from shallow depths within the conduit diminishing the impulsivity of gas release from the vent of the volcano. Scatter in the seismic-acoustic energy radiation at Karymsky reveals that conditions in the conduit change during the course of an explosion.
- Geophysics