Analysis of explosive activity of Tungurahua volcano using seismic-acoustic data

The character and mechanism of small vulcanian eruptions, often seen in andesitic volcanoes, have not been studied thoroughly. Activity of Tungurahua volcano, Ecuador, is presented here using data from a temporal deployment of broad-band seismic stations and infrasonic microphones. This study focuses on three areas of volcano dynamics: (a) structure of the magmatic plumbing system, (b) the process of magma fragmentation inside the volcano conduit, and (c) propagation of infrasound waves through a real medium. Using shear wave splitting of regional tectonic earthquakes, we found that anisotropic structures (micro-cracks alignments) are approximately perpendicular to expected tectonic orientations. The perpendicular orientation of these structures is likely related to magma intrusions in the upper part of the volcano conduit. Degassing at Tungurahua is characterized by frequent short-duration explosions, typical of vulcanian style eruptions. Seismic and infrasound arrival times constrain the source of Tungurahua fragmentation to the shallowest section (upper 200 m) of the volcano edifice. While infrasound events cluster into families of similar waveforms, we found no temporal correlation nor relationship of event size with cluster association. The variation of amplitudes and arrival time differences between seismic and acoustic records suggests that either the shear fragmentation occurs at different loci inside the conduit, or that the seismic and acoustic signals do not share the same source-time-function. In contrast to previous infrasound studies on volcanoes, we analyze the effects of wind and rough topography on the propagation of infrasound waves. Numerical simulations using Finite-Difference Time-Domain methods confirm that wind has a significant influence on arrival times at stations around the volcano. Acoustic reduction is observed if infrasound is impeded by topographic barriers with dimensions similar to or larger than the infrasound wavelength. This research has a direct application to the important effort of volcano explosion monitoring and hazard reduction.