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

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Abstract

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.

Introduction

In early 2006 two infrasound arrays were deployed in Ecuador as part of the proof-of-concept Acoustic Surveillance for Hazardous Eruptions (ASHE) project (Garces et al., 2008) to monitor and mitigate the significant volcanic ash hazard to aviation in this region. The initial goal of the ASHE project was to determine the feasibility of acoustically detecting significant atmospheric ash emissions and rapidly notifying civil defense authorities (ideally, within 5 min). The feasibility study has been successfully completed (Garces et al., 2008), and this paper provides details on the methods, salient scientific results, capabilities, and vulnerabilities of this remote sensing technology.

The Washington, DC Volcanic Ash Advisory Center (VAAC) is responsible for ash monitoring for aviation in this region. Existing seismic (Kumagai et al., 2007), gas (Arellano et al., 2008), satellite (Carn et al., 2008), and other technologies currently monitor Ecuador, but the persistently poor visibility, elevated eruptive activity, and remoteness of the region make the task of detecting ash emissions and notifying the necessary authorities challenging. Low-frequency (< 20 Hz) sound waves (infrasound) propagate long distances with little attenuation and are not affected by the dense cloud cover often present in the region. Further, infrasound is a direct measurement of pressure release into the atmosphere, in this case the eruption of pressurized gas, ash, and lava, and is thus a good indicator of explosive volcanic activity. Accurately differentiating and identifying the character of eruptive pressure release at the volcano is the most difficult and crucial aspect.

Between March 2006 and February 2008 near constant and diverse infrasound from Tungurahua Volcano was recorded by the two ASHE arrays. This paper provides a detailed chronology and characterization of the eruptive activity using infrasound and satellite imagery. We expand on the ash monitoring results presented in Garces et al. (2008) and present a companion paper to the satellite-based ash plume observations of Steffke et al. (in review), from which the ash clouds heights and dimensions listed here are derived. We focus on five time periods of volcano-acoustic activity. These periods are representative of a common eruption style and/or significant eruption at Tungurahua thus far observed during the experiment: Strombolian (January 2008), Vulcanian (May 2006, February 2008), Sub-Plinian (July 2006), and Plinian (August 2006). These recordings are noteworthy in that they are some of the highest quality and diverse infrasound measurements of explosive volcanism.

Beyond the monitoring aspect, this manuscript seeks to demonstrate the capability of correlating acoustic records with satellite-derived observations to constrain source mechanisms. For Tungurahua Volcano, we focus on three main aspects: (1) timing: onset, duration, and end of activity; (2) evolution: changes in intensity and character of the signal, in both the time and frequency domain; and (3) source: the physical generation of acoustic energy and how this relates to ash, gas, and pyroclastic density current (PDC) production. The relationship between PDCs and their associated acoustic signals is not currently understood, and this paper provides a unique opportunity to study the infrasound produced from sustained vs. collapsing columns. Further, high quality infrasonic records from energetic silicic eruptions are rare, and this project provides the first detailed, continuous, high fidelity acoustic recordings of all stages of a Plinian eruption.

Section snippets

Tungurahua Volcano

Tungurahua is one of the most active stratovolcanoes in the Ecuadorian Andes (Fig. 1). At 5023 m, the steep sloped volcano has experienced infrequent but dangerous sector collapses and immense debris flows in its history. Recent activity at the andesitic–dacitic volcano has been characterized by near continuous activity from the central crater in the form of PDCs, lava flows, and ash emissions. Although not located near a major city, over 25,000 residents live in close proximity and within the

Equipment

Two four element infrasound arrays were deployed with the aim of detecting and differentiating between multiple volcanoes and other infrasonic sources at regional distances. The RIOE array (Fig. 1a) is located 36.75 km southwest of Tungurahua Volcano, 43 km from Sangay Volcano, 214 km from Reventador Volcano, and ∼ 170 km south of Quito, Ecuador. This is the primary array used for this study, as it is the closest to Tungurahua, which is by far the most acoustically active volcano during the study

Experiment results

Between March 2006 and February 2008, 19,865 explosions were detected at RIOE, with over 3500 of those detected at LITE as well. The peak pressures of these explosions at RIOE range from 0.03 to 24.43 Pa (Fig. 3a). The largest explosion saturated the sensors (> 25 Pa) on July 17, 2006, and thus is even more energetic. Assuming spherical spreading, this correlates to a peak pressure of over 900 kPa at the vent and is comparable to some of the largest explosions recorded to date by infrasound

Constraining silicic eruptions using infrasound

Increases in acoustic power (a possible proxy for jetting intensity) during the major, sustained eruptions at Tungurahua between 2006 and 2008 are broadly consistent with increases in ash cloud height. Two exceptions are August 17, 2006 0300–0415 and July 15, 2006 0130–0300. Possible changes in the vent diameter, atmosphere, or multiphase eruptive mix may be responsible for these inconsistencies. Increases in acoustic power also correlate well with total ash cloud extent, which may not be as

Conclusions

Two infrasound arrays deployed in Ecuador provide a continuous record of the activity at Tungurahua Volcano between 2006 and 2008. A system was set up to automatically detect significant volcanic activity and notify the VAAC of a possible aviation hazard. After two large eruptions in 2006 were used to refine the automated ASHE algorithms, the onset of the Subplinian February 6, 2008 eruption was detected and a notification was sent ∼ 5.8 min after the acoustic onset. Acoustic energy from

Acknowledgements

The authors are grateful to the entire ASHE team for making this work possible, with special thanks to the Geological Survey of Canada for their technical and logistical support and to the dedicated staff of the Instituto Geofisico for their persistent and invaluable monitoring of Tungurahua. Patricio Ramon at the IG was exceptionally helpful in the preparation of this manuscript. Rene Servranckx at the Montreal VAAC and the helpful staff of the Washington DC VAAC provided invaluable feedback

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