Seismicity Preceding Volcanic Explosions During the 2006 Eruption of Augustine Volcano
University of Alaska, Fairbanks
vii, 79 p.
I examine seismicity associated with volcanic explosions using data collected from the 2006 eruption of Augustine Volcano. I present two quantitative tools which, in retrospective analysis, are excellent indicators of imminent eruption. The first tool, referred to as the Frequency Index (FI), quantifies seismicity based on a ratio of high and low frequency energy in an earthquake seismogram. I show that the lowest values of FI, those falling below -1.8, occur exclusively within hours of explosive eruptions. The second tool uses waveform cross correlation to identify repeating earthquakes. While the majority of earthquakes during the eruption have unique waveforms, I find subsets of highly similar earthquakes which are closely associated with volcanic explosions. I investigate waveform evolution which occurs in a cluster of highly similar earthquakes during the early stages of the eruptive sequence. The earthquake cluster occurred prior to magma reaching the surface, and was very well constrained in terms of timing and location. As such it provided an ideal dataset with which to relate waveform evolution to volcanic processes. I show that the waveform evolution was caused by a change in hypocenter location, and propose that it resulted from the gas-driven incremental opening of a crack within the volcanic edifice
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