Characteristics and Distribution of Infrasound Signals at Tungurahua Volcano, Ecuador

Four different types of infrasound signals have been observed at Tungurahua volcano during nine years of non-steady eruptive activity. These signal types are: a) Explosions: the most frequent type is a short-lived impulsive pressure event related to explosive eruptions and usually followed by ash columns and ballistic fragments. Pressure histories of these events show a sharp impulse followed by a slow rarefaction. Duration of compression-rarefaction sequence is about 4-5 s. Explosion swarms with thousands of discrete events preceded large eruptive episodes. b) Infrasound tremors: major Tungurahua eruptions (July 14-15, 2006; August 16-17, 2006; February 6-8, 2008) have infrasonic and seismic tremor with irregular waveforms lasting several hours. Spectral content of infrasonic signals display peaks in the range of 0.5 to 2 Hz. These signals were accompanied by sustained degassing and occasionally by pyroclastic flows. Moderate episodes of sustained degassing also exhibit infrasound tremors with the same spectral composition although without confirmation of pyroclastic flows. c) Roars: spindle-shaped pressure signals associated with roaring sounds have been related to non-impulsive degassing. These signals are characterized by a high-frequency content, small pressures, and short-durations. d) Clear episodes of chugging events have been recorded in Tungurahua. These events followed large explosions in the aftermath of the July 14-15, 2006 eruption and after moderate infrasound tremors in July 2008. Up to six overtones and gliding frequencies have been observed. Chugging records at different sites show significant signal degradation with distance.