Infrasound from Tungurahua Volcano 2006–2008: Strombolian to Plinian eruptive activity

Strombolian to Plinian activity from Tungurahua Volcano, Ecuador has been recorded by the autonomous infrasound arrays of the Acoustic Surveillance for Hazardous Eruptions (ASHE) project since early 2006. Our studies suggest that acoustic energy release during large eruptions does appear to broadly scale with eruption intensity. This manuscript provides a detailed chronology and characterization of Tungurahua's eruptive activity between 2006 and 2008 and demonstrates the ability to constrain source parameters of significant eruptions, such as onset, duration, and escalation, at regional distances by combining infrasound and remote sensing techniques. The ASHE system in Ecuador automatically detected over 20,000 volcanic explosions at an array 37 km from Tungurahua and was successful at notifying the onset, escalation, and cessation of a hazardous February 2008 eruption with a latency of 5 min. Elevated infrasonic energy from sustained and intense Tungurahua eruptions correlates well with ash column heights and their lateral extent during the study period. The spectra of these sustained explosive eruptions appear to be recurrent, readily recognizable, and indicative of volcanic jetting and significant atmospheric ash injection. The paroxysmal Plinian phase of the August 2006 eruption produced an ash cloud that extended well into the stratosphere (> 24 km), coinciding with a shift of the dominant jetting frequency from 0.25 Hz to below 0.1 Hz, and radiation of over 5 × 107 W of acoustic power. Transient explosions were often marked by minor or no ash release and are presumed to be more gas-rich. A change in the acoustic spectrum of volcanic jetting was also detected in the transition from a sustained to collapsed eruption column at the end of the July 14, 2006 eruption. The jetting spectrum at Tungurahua during a period of sustained pyroclastic density current production changes from a typical double-peaked to a single-peaked spectrum, suggesting remote acoustic monitoring can help ascertain the stability and dynamics of an eruptive column.