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Graduate Defense: Bryan Rosenblatt

October 27 @ 3:00 pm - 5:00 pm MDT

Thesis Information

Title: Controls on the Frequency Content of Near-Source Infrasound at an Open-vent Volcano (Villarrica, Chile)

Program: Master of Science in Geophysics

Advisor: Dr. Jeffrey Johnson, Geosciences

Committee Members: Dr. Brian Jackson, Physics, and Dr. Jacob Anderson, Geosciences

Abstract

Assessing the background state of Villarrica and other open-vent volcano’s acoustic signature has important implications for monitoring activity in the likely event of future unrest. Villarrica volcano’s active lava lake produces continuous infrasound with peaked tremor near 1 Hz and excursions of +/- ~0.2 Hz. Past work has shown that the infrasound’s frequency content reveals key volcanic properties such as eruption style and crater shape, and infrasound spectral changes coincided with and indicated a rise in the lava lake level before Villarrica’s most recent paroxysm. As such, quantifying and understanding the regular fluctuation in recorded infrasonic frequencies, from Villarrica and other open-vent volcanoes, is imperative. Based on a week-long period of crater rim infrasound observations associated with stable open-vent activity, we identify two independent source processes: spatter bursting events and lava lake-induced tremor oscillations (broadband discrete signals and ~1 Hz tremor respectively). Using a 3-D finite difference time domain infrasound propagation model (InfraFDTD), we show that sound speed and source spectrum variability can both influence Villarrica’s spectrum, however sound speed variations in the crater–whether diurnal or volcanic in origin–cannot explain the full extent of the observed frequency excursions. Instead, source spectrum variability is primarily responsible for the frequency excursions and these do not obviously correlate with diurnal cycles. This work highlights data collected from a dense distribution of twenty infrasound sensors operating at the summit and is used to validate the effectiveness of the InfraFDTD modeling approach.