Infrasound explosion and coda signal investigated with joint analysis of video at Mount Erebus, Antarctica (original) (raw)
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Bulletin of the …, 1988
Sixty eruptions were recorded from a TV camera on the crater rim, and a 9 station seismic net and 2 infrasonic microphones on the mountain, to test a previous result that eruptions were being triggered by separate earthquakes of depth up to 4 km. The recording period was 16 December 1986 to 7 January 1987. The seismic waveforms of similar large explosions were closely identical, and after stacking to improve the signal to noise ratio, plots of seismic arrival time versus distance from the eruption site showed that the seismic intercept time was 1. 43 ±0. 06 s later than the TV explosion time, and the apparent velocity was 4060 ±92 m/s. This velocity was much higher than that used for focal determinations (2. 1 km/s), and it appears that the errors in reading emergent onsets, plus an erroneously low velocity, were responsible for the previously published pipe-like distribution of explosion earthquake hypocenters extending to 4 km depth. If so, the visible explosions were the source of the seismic waves. Explosions were occurring from areas of 2 to 10 m across in the incandescent or convecting part of the lava lake, and were preceded by updoming for about 1 s. All eruptions ejecting bombs caused earthquakes, but ash eruptions from vents outside the lava lake were almost aseismic. Ejection velocities of bombs calculated from flight times ranged from 10 to 76 m/s. The fastest bombs followed an incandescent ash front expanding at up to 160 m/s. The highest velocity of bombs ejected without ash was 35 m/s. All bombs thrown out of the crater were highly vesicular. Relevelling after explosions took 3-8 s the few times it was seen. More frequently, there was an upwelling at the site 8. 8 ±1. 6 s later. This indicates a viscosity of ca. 104 Pas. High enough for the lava foam itself to explode.
Interpretation and utility of infrasonic records from erupting volcanoes
Journal of Volcanology and Geothermal Research, 2003
In the most basic seismo^acoustic studies at volcanoes, infrasound monitoring enables differentiation between sub-surface seismicity and the seismicity associated with gas release. Under optimal conditions, complicated degassing signals can be understood, relative explosion size can be assessed, and variable seismo^acoustic energy partitioning can be interpreted. The extent to which these points may be investigated depends upon the quality of the infrasonic records (a function of background wind noise, microphone sensitivity, and microphone array geometry) and the type of activity generated by the volcano (frequency of explosions, bandwidth of the signals, and coupling efficiency of the explosion to elastic energy). To illustrate the features, benefits, and limitations of infrasonic recordings at volcanoes, we showcase acoustic and seismic records from five volcanoes characterized by explosive degassing. These five volcanoes (Erebus in Antarctica, Karymsky in Russia, and Sangay, Tungurahua, and Pichincha in Ecuador) were the focus of seismo^acoustic experiments between 1997 and 2000. Each case study provides background information about the volcanic activity, an overview of visual observations during the period of monitoring, and examples of seismo^acoustic data. We discuss the benefits and utility of the infrasound study at each respective volcano. Finally, we compare the infrasound records and eruptive activity from these volcanoes with other volcanoes that have been the focus of previous seismo^acoustic experiments.
1] We describe a multiparameter experiment at Erebus volcano, Antarctica, employing Doppler radar, video, acoustic, and seismic observations to estimate the detailed energy budget of large (up to 40 m-diameter) bubble bursts from a persistent phonolite lava lake. These explosions are readily studied from the crater rim at ranges of less than 500 m and present an ideal opportunity to constrain the dynamics and mechanism of magmatic bubble bursts that can drive Strombolian and Hawaiian eruptions. We estimate the energy budget of the first second of a typical Erebus explosion as a function of time and energy type. We constrain gas pressures and forces using an analytic model for the expansion of a gas bubble above a conduit that incorporates conduit geometry and magma and gas parameters. The model, consistent with video and radar observations, invokes a spherical bulging surface with a base diameter equal to that of the lava lake. The model has no ad hoc free parameters, and geometrical calculations predict zenith height, velocity, and acceleration during shell expansion. During explosions, the energy contained in hot overpressured gas bubbles is freed and partitioned into other energy types, where by far the greatest nonthermal energy component is the kinetic and gravitational potential energy of the accelerated magma shell (> 10 9 J). Seismic source energy created by explosions is estimated from radar measurements and is consistent with source energy determined from seismic observations. For the generation of the infrasonic signal, a dual mechanism incorporating a terminally disrupted slug is proposed, which clarifies previous models and provides good fits to observed infrasonic pressures. A new and straightforward method is presented for determining gas volumes from slug explosions at volcanoes from remote infrasound recordings.
Bulletin of Volcanology
Infrasound signals are used to investigate and monitor active volcanoes during eruptive and degassing activity. Infrasound amplitude information has been used to estimate eruptive parameters such as plume height, magma discharge rate, and lava fountain height. Active volcanoes are characterized by pronounced topography and, during eruptive activity, the topography can change rapidly, affecting the observed infrasound amplitudes. While the interaction of infrasonic signals with topography has been widely investigated over the past decade, there has been limited work on the impact of changing topography on the infrasonic amplitudes. In this work, the infrasonic signals accompanying 57 lava fountain paroxysms at Mt. Etna (Italy) during 2021 were analyzed. In particular, the temporal and spatial variations of the infrasound amplitudes were investigated. During 2021, significant changes in the topography around the most active crater (the South East Crater) took place and were reconstruc...
Volcanic eruptions observed with infrasound
Geophysical research letters, 2004
1] Infrasonic airwaves produced by active volcanoes provide valuable insight into the eruption dynamics. Because the infrasonic pressure field may be directly associated with the flux rate of gas released at a volcanic vent, infrasound also enhances the efficacy of volcanic hazard monitoring and continuous studies of conduit processes. Here we present new results from Erebus, Fuego, and Villarrica volcanoes highlighting uses of infrasound for constraining quantitative eruption parameters, such as eruption duration, source mechanism, and explosive gas flux.
Volcano infrasound: progress and future directions
Bulletin of Volcanology
Over the past two decades (2000–2020), volcano infrasound (acoustic waves with frequencies less than 20 Hz propagating in the atmosphere) has evolved from an area of academic research to a useful monitoring tool. As a result, infrasound is routinely used by volcano observatories around the world to detect, locate, and characterize volcanic activity. It is particularly useful in confirming subaerial activity and monitoring remote eruptions, and it has shown promise in forecasting paroxysmal activity at open-vent systems. Fundamental research on volcano infrasound is providing substantial new insights on eruption dynamics and volcanic processes and will continue to do so over the next decade. The increased availability of infrasound sensors will expand observations of varied eruption styles, and the associated increase in data volume will make machine learning workflows more feasible. More sophisticated modeling will be applied to examine infrasound source and propagation effects from...
The source of infrasound associated with long-period events at Mount St. Helens
J. geophys. Res, 2009
1] During the early stages of the 2004-2008 Mount St. Helens eruption, the source process that produced a sustained sequence of repetitive long-period (LP) seismic events also produced impulsive broadband infrasonic signals in the atmosphere. To assess whether the signals could be generated simply by seismic-acoustic coupling from the shallow LP events, we perform finite difference simulation of the seismo-acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere. The effects of topography, velocity structure, wind, and source configuration are considered. The simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of P/SV and Rayleigh wave energy converted locally along the propagation path, and acoustic energy originating from the source epicenter. Although the horizontal acoustic velocity of the latter is consistent with our data, the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations, and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source. The observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic LP events. The observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated, highly permeable material. Heating by magmatic activity causes pressure to rise, periodically reaching the pressure threshold for rupture of the ''valve'' sealing the crack. Sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack, initiating resonance of the remaining fluid. Subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13.4 km to the NW of the volcano are attributed primarily to atmospheric boundary layer propagation effects, superimposed upon amplitude changes at the source.
Journal of Volcanology and Geothermal Research, 2011
On 13 May 2008 an eruptive fissure opened on Mount Etna's eastern flank feeding both explosive activity and lava effusion from multiple vents for about 14 months. During the investigated May-September 2008 eruptive period, infrasound recordings from a 4 station-sparse network allowed tracking of the explosive activity in terms of location and dynamics. In order to focus on activity from the eruptive fissure, the infrasonic events generated by the summit craters were selected by using both spectral features and time delays between pairs of stations and excluded from our analysis. Then, to accurately locate events from the fissure, we used a composite method, based on the semblance and brightness functions. This enabled the study of the co-existence of more than one infrasound source and/or its migration along the eruptive fissure. Hence, results permitted us to discriminate the number of active vents and their location along the fissure even when, due to poor weather conditions, it was not possible to access the vents or carry out direct observations. The eruptive activity was characterised by variations in the number of active vents according to the overall intensity of the eruptive event. Variability of the infrasound waveforms highlighted either that distinct vents produced signals with different waveforms, or that single vents generated different events during distinct periods of time, or finally both the previous phenomena. We applied the strombolian bubble vibration model to model waveform differences and attributed the signal variations to bubble radius changes.► Infrasound investigation provides insights into eruptive fissure activity. ► Joint semblance-brightness technique allows very precise locations of eruptive vents. ► Infrasound waveform variability shows different active vents or their time changes.
Tracking eruptive phenomena by infrasound: May 13, 2008 eruption at Mt. Etna
Geophysical Research Letters, 2009
Active volcanoes produce inaudible infrasound due to the coupling between surface magmatic processes and the atmosphere. Monitoring techniques based on infrasound measurements have been proved capable of producing information during volcanic crises. We report observations collected from an infrasound network on Mt. Etna which enabled us to detect and locate a new summit eruption on May 13, 2008 when poor weather inhibited direct observations. Three families of signals were identified that allowed the evolution of the eruption to be accurately tracked in real-time. Each family is representative of a different active vent, producing different waveforms due to their varying geometry. Several competitive models have been developed to explain the source mechanisms of the infrasonic events, but according to our studies we demonstrate that two source models coexist at Mt. Etna during the investigated period. Such a monitoring system represents a breakthrough in the ability to monitor and understand volcanic phenomena.