Volcano infrasound: A review

Journal of Volcanology and Geothermal Research 206 (2011) 61–69

Contents lists available at ScienceDirect

Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

Review

Volcano infrasound: A review Jeffrey Bruce Johnson a,⁎, Maurizio Ripepe b a b

Dept. of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, United States Dipartimento di Scienze della Terra, Firenze, Italy

a r t i c l e

i n f o

Article history: Received 5 January 2011 Accepted 23 June 2011 Available online 2 July 2011 Keywords: Volcano infrasound Explosive eruptions Infrasound sensors Sensor arrays and networks

a b s t r a c t Exploding volcanoes, which produce intense infrasound, are reminiscent of the veritable explosion of volcano infrasound papers published during the last decade. Volcano infrasound is effective for tracking and quantifying eruptive phenomena because it corresponds to activity occurring near and around the volcanic vent, as opposed to seismic signals, which are generated by both surface and internal volcanic processes. As with seismology, infrasound can be recorded remotely, during inclement weather, or in the dark to provide a continuous record of a volcano's unrest. Moreover, it can also be exploited at regional or global distances, where seismic monitoring has limited efficacy. This paper provides a literature overview of the current state of the field and summarizes applications of infrasound as a tool for better understanding volcanic activity. Many infrasound studies have focused on integration with other geophysical data, including seismic, thermal, electromagnetic radiation, and gas spectroscopy and they have generally improved our understanding of eruption dynamics. Other work has incorporated infrasound into volcano surveillance to enhance capabilities for monitoring hazardous volcanoes and reducing risk. This paper aims to provide an overview of volcano airwave studies (from analog microbarometer to modern pressure transducer) and summarizes how infrasound is currently used to infer eruption dynamics. It also outlines the relative merits of local and regional infrasound surveillance, highlights differences between array and network sensor topologies, and concludes with mention of sensor technologies appropriate for volcano infrasound study. © 2011 Elsevier B.V. All rights reserved.

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Focus of current volcano infrasound studies . . . . . . . . . . . . . 3. Quantification of volcano infrasound . . . . . . . . . . . . . . . . . 4. Eruption dynamics inferred from volcano infrasound observations . . . 5. Multi-disciplinary geophysical studies involving infrasound . . . . . . 6. Local versus regional monitoring and array versus network deployments 7. Volcano infrasound instrumentation . . . . . . . . . . . . . . . . . 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Early volcano airwave observations were recorded with analog microbarometers capable of measuring frequencies lower than about 1 Hz and sampled with limited time resolution (seconds to tens of ⁎ Corresponding author. E-mail addresses: [email protected] (J.B. Johnson), maurizio.ripepe@unifi.it (M. Ripepe). 0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2011.06.006

. . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

61 62 62 64 66 66 67 67 67 67

seconds). Several large eruptions, like the 17 km3 Krakatau eruption of 1883 and the 1 km3 eruptions of Mount St. Helens in 1980 and Agung in 1963 (Pekeris, 1939; Ritsema, 1980; Goerke et al., 1965), produced low frequency (a few mHz) acoustic-gravity waves transmitted through the atmosphere by combined gravitational and elastic forces (Harkrider, 1964). These large events induced atmospheric oscillations that were recorded globally and have been attributed to massive ejections of mass or thermal energy (Kanamori et al., 1994). The 1883 Krakatau eruption, for instance, induced low frequency pressure oscillations that circled the

62

J.B. Johnson, M. Ripepe / Journal of Volcanology and Geothermal Research 206 (2011) 61–69

Earth seven times while also producing intense acoustic waves, including audible sounds of ‘heavy gunfire’, reported at distances as great as 5000 km (Simkin and Fiske, 1983). The Mount St. Helens eruption of May 18, 1980 also produced acoustic gravity waves recorded worldwide (e.g., Bath, 1982; Bolt and Tanimoto, 1981; Donn and Balachandran, 1981), associated ~400 Pa infrasound (of unresolved period due to low resolution analog recordings) that were recorded 54 km from the vent (Banister, 1984; Reed, 1987), as well as audible sounds heard as far as 700 km from the vent (Fairfield, 1980). A network of more than 30 short period seismometers throughout Washington State also responded to the airwaves, which refracted in both the stratosphere and thermosphere before returning to Earth as ground-coupled airwaves (Johnson and Malone, 2007). Other examples of early volcano infrasound studies were made with both microbarometers and infrasound arrays. At regional distances, defined here as tens to several thousand kilometers of an eruption site, Gorshkov (1959) used pressure records to assess explosive yield of Bezymianny in 1959 from data recorded 45 to 760 km from the source. Mauk (1983) recorded signal from El Chichon 1980 in Southern Mexico on microbarometers in the United States and calculated eruption energetics. Wilson et al. (1966) used regional infrasound recorded at several tens of second periods to identify multiple acoustic channels in Alaska from the eruption of Redoubt. This array site was also used to identify and locate activity from eruptions of Redoubt and Trident (Wilson and Forbes, 1969). In the digital age very few eruptions have occurred that can be classified as ‘very large’, greater than or equal to a volcano explosivity index (VEI) of 5 (Newhall and Self, 1982). The VEI 5 Pinatubo eruption of 1991, which erupted 5 km 3 of magma, was recorded with broadband microphones as gravity waves and infrasonic waves at 2770 km in Japan (Tahira et al., 1996), but there were no near-source records of the sound spectrum. Only recently, broadband microphones and microbarographs deployed within 6 km from the Soufriere Hill volcano at Montserrat have detected clear gravity waves, as well as acoustic waves, associated with moderate Vulcanian explosions (Ripepe et al., 2010a). Researchers began to install ‘higher’ frequency sensors capable of recording near infrasound (1–20 Hz) in the late 1980s. Early reports by Dibble et al. (1984), Firstov and Storcheus (1987), and Okada et al. (1990), amongst others, demonstrated the benefit of complementing a local seismic network with infrasound sensors on the volcano itself. Braun and Ripepe (1993) and Vergniolle and Brandeis (1994) used data recorded with infrasound-sensitive microphones to model bubble oscillations and explosive activity at Stromboli. Other early work was pioneered in the 1980s at Japanese volcanoes, including Tokachi, Sakurajima, and Suwanosejima, using infrasound sensors that were sensitive in the band between 0.1 and 1000 Hz (Okada et al., 1990; Iguchi and Ishihara, 1990; Kamo et al., 1994). Although previous work by Richards (1963) and Woulff and McGetchin (1976) had focused on audible-band recordings of volcano sounds, the studies in the 1990s noted that the most intense sounds produced by frequently erupting “laboratory volcanoes”, i.e., VEI b2, lie in the near-infrasound band extending, in some cases, down into the microbarom band (centered at 0.2 Hz (Bowman et al., 2005)). Because frequently exploding volcanoes appear to radiate efficiently in this band it has been a primary focus for volcano infrasound investigation, and most prevalent in the literature of the last decade.

eruption physics, locate various types of volcano infrasound sources, infer vent and conduit geometries, and/or quantify the outflux of volcanic materials. These goals also have important implications for volcano monitoring. 3. Quantification of volcano infrasound Local infrasound observations enable quantitative intercomparison of volcano eruptive behavior. Although the atmosphere is nonhomogeneous and dynamic it is relatively uncomplicated compared to the solid medium through which seismic waves propagate. In general, the volcanic edifice is highly scattering and attenuating to seismic waves and comprised of large impedance contrasts. For short propagation distances, the atmosphere, is relatively homogeneous and isotropic such that infrasonic pressure records are relatively representative of source processes occurring at the volcano. At local recording distances acoustic amplitude and power, coda duration, signal envelope, and frequency spectra are easily quantified. This enables a comparison of infrasound for suites of eruptions either at a single volcano or for a range of volcanoes. Signals are commonly displayed as time series waveforms (analogous to seismograms) or as spectrograms, which indicate evolving frequency content in a signal (e.g., see Fig. 1). Qualitative insight into infrasound signals may also be realized by speeding up infrasound in to the audio band. Although the human ear does not have a flat frequency response it is sensitive to subtle variations in tone. Peak pressure amplitudes for infrasound data are generally provided in pascals along with the specified distance from the volcano vent where they were recorded. For sensors deployed local to a volcano, i.e., on the slopes and within about 100 m to 10 km from the infrasound radiator, recorded pressures are often assumed to be linear acoustic waves. Amplitude fall-off for near-infrasound is primarily due to geometric spreading because intrinsic attenuation at infrasonic wavelengths is minimal. For local propagation distances, and under assumption of isotropic radiation from a monopole point source, excess pressure amplitude diminishes as the inverse of propagation distance. To quantitatively compare pressure amplitudes from different volcanoes and different eruptions it is thus useful to scale from the excess pressure recorded at distance r to a reduced pressure that would have been recorded at a common distance rred(Kinsler et al., 2000; Johnson, 2003): p red = p ×

ð1Þ

The reduced pressure pred is analogous to a reduced displacement of body waves (Aki and Koyanagi, 1981) that is commonly used in volcano seismology for intercomparison of different episodes of volcanic tremor. In the case of reduced pressure, the reduction distance rred is arbitrary and has been specified in the literature at 1 m (e.g. Ripepe et al., 2007) and 1 km (e.g., Johnson et al., 2004) for local deployments. For locally recorded sound at volcanoes it is also useful to quantify the total acoustic power associated with an eruption signal for a specified passband. The time-varying power P(t) near to the source is proportional to the sound intensity integrated across a surface Ω normal to the propagation vector. The sound intensity for volumetric sources, e.g. an idealized volcanic explosion at the free surface, is proportional to the mean squared excess pressure p2 divided by the impedance of the atmosphere ρc(e.g., Dowling, 1998), such that

2. Focus of current volcano infrasound studies Intense infrasound is produced by erupting volcanoes and has been used for three primary purposes including: 1) studying eruption dynamics, 2) monitoring of restless volcanoes in order to assess and mitigate hazards, and 3) probing of the atmosphere. By far the most common theme in the literature is the use of infrasound to understand

r rred

P ðt Þ = Ω

p2 ðt + r =cÞ ρc

ð2Þ

In the case of isotropic radiation into a half space the surface normal to radiation at radius r is Ω = 2πr 2, but for sound extending into the atmosphere above a volcano with steep slope, the solid angle

J.B. Johnson, M. Ripepe / Journal of Volcanology and Geothermal Research 206 (2011) 61–69

63

Fig. 1. 30-minute infrasound waveform examples and cumulative energy, spectrograms, and power spectra from 6 select volcanoes for data filtered from 0.5 to 50 Hz. Waveforms (top panels) are reduced to 1 km (Eq. (1)) and amplitudes are indicated by scale bars. Cumulative energy (time integration of Eq. (2)) is shown as a red curve (top panel) and indicated in red text. Spectrograms are calculated with 10 s moving windows and 90% overlap and show logarithmic power density (dB/Hz) relative to the most intense pixel in each colormap. Acoustic power spectra are calculated as the mean spectrogram spectra and are shown with linear amplitude scale. Detail of these data is shown in Fig. 2. Power and energy statistics are provided in Table 1.

may be larger. For instance, for isotropic radiation from the summit of a stratovolcano with slope 30° the solid angle would be Ω = 3πr 2. It is important to note that Eq. (2) is not applicable to directional sources, such as acoustic dipoles or quadrupoles, which preferentially radiate

along the axis of a dipole (Dowling, 1998). Nor is Eq. (2) appropriate for large-scale jet noise, which is thought to produce shock radiation along the jet flow (Matoza et al., 2009a; Fee et al., 2010b). Dipole and quadrupole radiations, associated with jetting, have been proposed as

64

J.B. Johnson, M. Ripepe / Journal of Volcanology and Geothermal Research 206 (2011) 61–69

a component of the radiated acoustic energy in many eruptions (Woulff and McGetchin, 1976; Vergniolle and Caplan-Auerbach, 2004). Time-integrated acoustic power, or acoustic energy, is another useful metric that can be used to compare and contrast various eruptions. Table 1 provides a power comparison of local volcano infrasound (filtered between 0.5 and 50 Hz) made in recent years during the VEI 0 and 1 eruptions shown in Fig. 1. These examples reflect the diverse character of the respective volcanic activities ranging from pyroclasticladen explosions from a dome at Santiaguito, vigorous buoyant plume generation at Reventador, degassing from an open magma column at Halemàumàu (Kilauea), degassing from a roiling lava lake at Villarrica, Strombolian eruptions at Fuego, and Vulcanian ash-rich eruptions from Tungurahua. The dramatic range in radiated acoustic power for these different eruptions (summarized in Table 1) is not easily attributed to the intensity of eruption. The diverse quality of infrasound instead appears controlled by diverse atmospheric ensonification mechanisms for different styles of eruption (Fig. 2). 4. Eruption dynamics inferred from volcano infrasound observations Compact volumetric sources are the most efficient sources of volcano infrasound. Under this condition the dimension of the volcanic vent, or acoustic radiator, is small compared to the wavelength of radiated sound. A simple source, or monopole source approximation, may then be made and the resultant sound field will be proportional to the source strength, or change in rate of mass injection (Lighthill, 1978): pM ðr; t Þ =

r Q ðt−r = cÞ Ω

ð3Þ

Here Q is the mass acceleration at the source, which is the effective volumetric acceleration of the atmosphere at the source. Under the assumption of a monopole source mechanism the mass flux history and cumulative explosive flux may be recovered from single and double time integration of Q. Several studies have made use of variants of Eq. (3) to invert for infrasound wave generation in the atmosphere due to accelerations of solid rock, fluid magma, or expanding gas at the atmosphere free surface. Studies that are attributed to monopole sources include Strombolian explosions at Erebus and Karymsky (Johnson et al., 2004; Johnson, 2007) as well as oscillations of bubbles at the surface of fluid lava surfaces at Shishaldin, (Vergniolle et al., 2004), Stromboli (Vergniolle and Brandeis, 1994), and Erta Ale (Bouche et al., 2010). At more silicic volcanic systems volumetric sources have been attributed to gas release from fractures on a dome at Unzen (Yamasato, 1998; Oshima and Maekawa, 2001), swelling of a lava plug followed by explosive emission of gas at Sakurajima (Yokoo et al., 2009), inflation of the vent region prior to eruption at Suwanosejima (Yokoo and Iguchi, 2010), and rapid dome uplift at Santiaguito (Johnson and Lees,

2010). In the last study, because the source region was observed to be large relative to the wavelength of the primary radiated infrasound, a finite element distribution of monopole sources was used to model infrasound synthetics. The monopole approximation has also been used by Moran et al. (2008a) to infer air mass displaced by a large rock fall event associated with volcanic activity. Infrasound from many volcanic eruptions begins with an abrupt compressional phase followed by a similar amplitude rarefaction (e.g., Morrissey and Chouet, 1997). Often these bi-polar pulses are shaped like N-waves, which are similar in appearance to a chemical explosion shock wave after it has decayed to an acoustic wave. Such N-shaped explosion waveforms are a common feature of the onset of many explosive eruptions, including those at Fuego (Lyons et al., 2010), Erebus (Rowe et al., 2000), Sakurajima (Morrissey et al., 2008), Shishaldin (Caplan-Auerbach and McNutt, 2003; Petersen and McNutt, 2007), Augustine (Petersen et al., 2006), Tungurahua (Ruiz et al., 2006), Karymsky (Johnson et al., 1998), Arenal (Hagerty et al., 2000), Stromboli (McGreger and Lees, 2004), Klyuchevskoi (Firstov and Kravchenko, 1996), Etna (Ripepe et al., 2001b), and many other volcanoes. In many cases a substantial infrasonic coda follows the N-wave pulse and this coda may be manifested as broadband or harmonic tremor (e.g., Johnson and Lees, 2000). A type of harmonic tremor referred to as ‘chugging’, when accompanied by audible pulses, is common at diverse volcanoes and is often characterized by frequency spectra with a fundamental peak at around 1 Hz and well-defined integer overtones (e.g., Fig. 1B). This harmonic tremor has also been seen in the seismic wavefield at Karymsky, Arenal, Sangay, Reventador, and elsewhere (Benoit and McNutt, 1997; Lees et al., 2008; Ruiz et al., 2006; Johnson and Lees, 2000). Such harmonic tremor has been qualitatively explained as a succession of pressure pulses at the surface (Lees and Ruiz, 2008), and has also been proposed as resonance modes of fluid-filled conduits (Buckingham and Garces, 1996; Garces, 2000). Time-varying changes in the fundamental frequency, known as ‘gliding’, have been attributed to both changing geometry and acoustic velocities within the volcano conduit as bubbles nucleate or grow (Garces and McNutt, 1997; Hagerty et al., 2000). Other types of sustained infrasound signals are not necessarily presaged by an explosive onset. Such tremors are often evident at openvent volcanic systems and reflect degassing styles (e.g., Fig. 1C–D). At Stromboli, for instance, infrasonic records reveal persistent degassing pulses, occurring every ~1–2 s while the magmatic column is in a nonequilibrium condition (Ripepe et al., 1996). Gas appears unable to compensate the melt pressure and the magma is degassed by small gas bursting events referred to as “puffing”, which accounts for nearly 50% of the total gas budget (Ripepe et al., 2007; Harris and Ripepe, 2007). Intense, long-lived infrasonic tremor with a consistent peaked frequency, but no clear overtones, is also observed at other volcanoes, typically those that are open-vent systems with convecting lava lakes. Observations at Kilauea and Villarrica show a long-lived resonant monotonic tremor source that originates from skylights above lava tubes (Garces et al., 2003; Matoza et al., 2010a) and central vents (Ripepe et al., 2010b;

Table 1 Statistics for filtered (0.5 to 50 Hz) infrasound amplitude, power, and energy from select volcanoes (Figs. 1 and 2). Maximum power is calculated as peak values for 1 s, 10 s, and 60 s running power averages calculated according to Eq. (2).

a) Santiaguito b) Reventador c) Kilauea d) Villarrica e) Fuego f) Tungurahua

Trace reference start time (yyyy:ddd:hh)

Station distance (km)

Max pressure (Pa)

Reduced pressure at 1 km (Pa)

Max power (MW) averaged over 1 s

Max power (MW) averaged over 10 s

Max power (MW) averaged over 1 min

30-minute total energy (MJ) from Fig. 1

3-minute total energy (MJ) from Fig. 2

2009:001:11 2005:236:16 2008:190:09 2010:22:12 2007:117:13 2009:165:15

0.4 1.7 2.4 0.1 7.0 5.5

4.4 0.9 1.3 58 5.9 27

1.8 1.5 3.2 2.9 41 150

0.012 0.016 0.076 0.059 1.9 160

0.007 0.011 0.021 0.021 0.29 18

0.003 0.008 0.004 0.01 0.07 3.1

0.18 6.8 0.59 12 4.1 240

0.16 0.69 0.28 1.1 4.0 190

J.B. Johnson, M. Ripepe / Journal of Volcanology and Geothermal Research 206 (2011) 61–69

65

Fig. 2. 3-minute infrasound waveform examples and cumulative energy, spectrograms, and power spectra from 6 select volcanoes. This figure shows detail of data presented in Fig. 1. Caption is the same as for Fig. 1.

Fee et al., 2010a; Goto and Johnson, 2011). The sources of resonance have been attributed to vortices in the jet shear layer akin to a whistling tea kettle (Matoza et al., 2010a), oscillations in the gas flux (Ripepe et al., 2010b), and/or to Helmhotz resonance (Fee et al., 2010a; Goto and Johnson, 2011). The Helmholtz mechanism has also been invoked to explain tremor at Shishaldin (Vergniolle and Caplan-Auerbach, 2004) and Etna (Montalto et al., 2010).

Broadband tremor extending throughout the near-infrasound band is also routinely observed and often occurs during an eruption event. The spectral envelope of the broadband tremor has similarities to jet noise (Matoza et al., 2009a), and studies by Caplan-Auerbach et al. (2010), Vergniolle and Caplan-Auerbach (2006), and Woulff and McGetchin (1976) have proposed that power from a presumed dipole source can be used to infer eruption velocities at Shishaldin and

66

J.B. Johnson, M. Ripepe / Journal of Volcanology and Geothermal Research 206 (2011) 61–69

Acatenango fumaroles. Johnson et al. (2008) used a local network of infrasonic microphones to infer that the impulsive short-duration eruptions of Erebus had a component of both dipole and monopole radiation. Contrasting with the simple monopole source, dipole acoustic radiation has an associated directionality and an axisymmetric radiation pattern. Inverting for the volcano acoustic physical source requires sensors located local to a volcano to reduce influence from time-varying propagation effects as the winds and temperatures in the atmosphere change. Even at distances of about 12 km, diurnal variations in the wind have been shown to strongly affect the infrasound recorded from Kilauea (Fee and Garces, 2007). More work is required to understand the distance and frequency band where volcanic signal is lost and, further, to understand the potential effects of source and site response for local infrasound recordings. 5. Multi-disciplinary geophysical studies involving infrasound Interpretation of volcano infrasound signals is well complemented by other coincident geophysical measurements and observations, including seismic, thermal cameras, Doppler radar, geodetic, and visual-band observations. In particular, conjoint seismo-acoustic observations afford improved understanding of diverse types of infrasound sources (Arrowsmith et al., 2010), including volcanic eruptions. Joint volcano seismo-acoustic analysis has facilitated the interpretation of long period earthquakes at Stromboli (Braun and Ripepe, 1993; Ripepe et al., 2001a) and Santiaguito (Johnson et al., 2009), ‘drumbeat’ long-period earthquakes at Mount St. Helens (Matoza et al., 2009b), tremor associated with dome collapse at Montserrat (Green and Neuberg, 2005), explosive activity at Stromboli (Ripepe et al., 1996) and Etna (Ripepe et al., 2001b), and seismic precursors to a large eruption at Miyake-jima (Kobayashi et al., 2005). For eruptive events the ratio of radiated acoustic to seismic energy is hugely variable both within a suite of events and in the intercomparison of energy partitioning at different volcanoes (Johnson and Aster, 2005). This volcano acoustic seismic ratio may provide insight into changing magma impedance Hagerty et al., 2000; Rowe et al., 2000; Garces et al., 1998, and/or amount of overlying material muffling the acoustic source. Energy ratios of acoustic signals and thermal transients recorded with ground-based infrared sensors have also been used to discriminate between eruptive behaviors at Santiaguito, Stromboli, Villarrica, and Fuego (Johnson et al., 2004; Marchetti et al., 2009a). The relative timing of these phases provides information about depth of magma fragmentation assuming that infrasound is radiated coincidentally with the release and upward flow of a hot gas or ash plume (Ripepe et al., 2001a). Such thermal acoustic studies have been carried out at Santiaguito (Sahetapy-Engel et al., 2008), Etna (Gresta et al., 2004), and Stromboli (Ripepe et al., 2002) and generally found magma fragmentation sources to occur at depths of tens to a few hundred meters. Understanding of volcano infrasound has been further complemented in recent years by careful visual and video observations (e.g., Johnson et al., 2009; Yokoo and Iguchi, 2010), geodetic measurements (Ripepe and Harris, 2008; Di Grazia et al., 2009), and Doppler radar (Scharff et al., 2008; Gerst et al., 2008). 6. Local versus regional monitoring and array versus network deployments Infrasound monitoring of volcanoes can be accomplished with local or regional sensors and/or with sensors deployed in array or network configurations. The primary advantage of local monitoring is that weather-dependent propagation effects are minimized and sound radiation may be assumed to be largely radial such that shadow zones and multi-pathing effects can be discounted. Local deployments also generally benefit from increased signal amplitude close to the source,

however disadvantages to local deployment may include higher ambient noise levels (in windy mountainous environments), and the risk that sensors might be destroyed by vigorous eruptive activity (Moran et al., 2008a; Marchetti et al., 2009b). The latter disadvantage can be circumvented by redundant deployment of infrasound stations at various azimuths and distances. As with local seismic studies at volcanoes, local infrasound sensor distribution may be arranged in array or network topology. Array deployments enable signal beam forming and discrimination of activity at various volcanic vents from uncorrelated noise or infrasound sources that are external to the volcano. Network deployments allow precise localization of an infrasonic transient or tremor. At Stromboli and Etna infrasound arrays within a few km of the vent are used to discriminate which craters and which vents are active as a function of time (Marchetti et al., 2009b; Ripepe et al., 2007; Ripepe and Marchetti, 2002). A local array at Stromboli, for instance, was effective at documenting events leading up to the eruption that occurred in 2007 (Ripepe et al., 2009), as were local network stations at Sakurajima, which recorded activity leading up to a Vulcanian eruption (Garces et al., 1999). At Stromboli, Etna, and also at Erebus, local network deployments of sensors within a few kilometers of the vent enable localization of vent activity (Johnson, 2005; Cannata et al., 2009a; Cannata et al., 2009b; Jones et al., 2008; Montalto et al., 2010). Local networks and arrays have also proven beneficial for tracking moving sources such as pyroclastic flows at Unzen (Yamasato, 1997), Montserrat (Ripepe et al., 2010a), and large rock falls such as those occurring at Mount St. Helens (Moran et al., 2008b). Many rock fall and pyroclastic flow events tend to have relatively low amplitude infrasound signal and require identification through semblance signal stacking across the array. Johnson et al. (2010) used network and array semblance to identify explosions as well as very low amplitude volcanic rockfalls and degassing events at Santiaguito. Also at Santiaguito, Jones and Johnson (2010) used a network of infrasound arrays to track dynamic source locations across the extensive (~200 m diameter) dome region. Regional and global monitoring of volcanoes in the infrasound band has proven to be effective for identifying periods of eruptive activity at numerous volcanoes. A dedicated volcano infrasound array in Ecuador, for instance, was capable of distinguishing between activity at three volcanoes, Galeras, Sangay, and Tungurahua (Garces et al., 2007). The array, located 37 km from Tungurahua, proved effective for continuously tracking activity, identifying more than 20,000 explosions between 2006 and 2008, and characterizing trends in activity ranging from Strombolian to a Plinian paroxysm (Fee et al., 2010c). Arrays in Washington State at 13 and 250 km from Mount St. Helens also provided comprehensive coverage of infrasound radiation during the 2004–2008 eruption (Matoza et al., 2007; Moran et al., 2008a). International Monitoring System (IMS) stations (Campus, 2006; Le Pichon et al., 2009), as well as other infrasound research arrays, also routinely pick up signal from erupting volcanoes, including Sarychev at 640–6400 km (Matoza et al., 2010b), Kasatochi and Okmok Volcanoes at 2000–5000 km (Fee et al., 2010b), Yasur at 400 km (Antier et al., 2007), Ambrym at 670 km (Le Pichon et al., 2005), Augustine at 675 km (Wilson et al., 2006), Etna at 1774 km (Evers and Haak, 2005), Erebus at 25 km (Wilson et al., 2003; Dibble, 1989, Hekla Liszka and Garces, 2002) and Manam at 9358 km (Wilson and Olson, 2005). Volcano infrasound recorded at regional distances has great potential as a remote sensing tool for probing atmospheric conditions and understanding how low frequency sound refracts in the stratosphere and thermosphere (e.g., Wilson et al., 2006). Discrepancies between infrasound wavefront backazimuth and true volcano backazimuth can be used to understand regional wind effects (e.g., Evers and Haak, 2005; Matoza et al., 2010b). High intensity and/or continuous volcano infrasound activity has been used to observe variations in seasonal winds in the upper atmosphere (Le Pichon et al., 2005; Antier et al., 2007), whereas local studies, using a network of infrasound sensors distributed

J.B. Johnson, M. Ripepe / Journal of Volcanology and Geothermal Research 206 (2011) 61–69

about a volcano, can provide information on low altitude atmospheric conditions (Marcillo and Johnson, 2010). Integrated local and regional monitoring of continuously active volcanoes has potential for remotely measuring changing atmospheric conditions (Antier et al., 2007; Le Pichon et al., 2010).

7. Volcano infrasound instrumentation Precise calibration of infrasound recording devices is an important component of volcano infrasound studies and is complicated by a proliferation of sensor types. Calibration is of greatest importance for the researcher focused on waveform modeling or precise energy calculations, however uncalibrated sensors can be effectively used for many monitoring applications including event counting and assessment of relative event magnitudes. Investigators have used a wide array of sensors, including high fidelity IMS-style sensors, such as the MB2000 microbarometer and very low noise floor Chapparal Physics sensors. These instruments provide excellent signal-to-noise, which is desirable for regional or global low wind recording environments, but may not always be necessary for local volcano seismo-acoustic deployment sites, where ambient noise is often unavoidable and signal levels are typically high. In the interest of economy, some independent researchers and volcano observatories have developed and calibrated their own lowcost sensors using both electret condenser elements and microelectromechanical systems (MEMS) silicon chips. In general electret condenser microphone elements have a response that is flat only in the audible band down to the upper near-infrasound, often falling off at 1–10 Hz. Accurate pressure waveforms can be recovered by careful calibration of microphone elements in the lab and through application of a digital instrument response correction (Ripepe et al., 2004; Johnson et al., 2003). Nonetheless, electrets have the disadvantage in that linearity at high pressures (tens of Pa) is suspect and that they respond to the change in pressure rather than its absolute value (e.g. Ripepe et al., 2004). MEMS technology has resulted in low-cost transducers with flat responses down to DC frequencies, however with a higher level of electronic noise than electret condenser microphones. In many volcano deployments, however, the higher instrument noise floor of MEMS transducers is still below the ambient noise level. These transducers can make for ideal local volcano infrasound sensors because MEMS allows for high amplitude pressure transients from volcanoes (hundreds of Pa) to be recorded on scale. AllSensors™ produces a MEMS pressure transducer that has 16 bits of dynamic range from about +/−2 mPa (RMS noise floor at 0.5 to 2 Hz) to +/−125 Pa. A capillary bleed valve is typically attached to allow passage of sound pressure waves while filtering out ultra long period pressure changes (e.g., barometric changes). Additional new solid state pressure sensing technologies, such as the quartz crystal oscillators manufactured by Paroscientific, also show promise. Sensors intended for other applications have also proven utility for detecting volcano infrasound. It has long been recognized that seismometers can act as uncalibrated infrasound sensors since high intensity airwaves, such as those at Fuego (Yuan et al., 1984), Pavlof (McNutt, 1986; Garces et al., 2000), Langila (Mori et al., 1989), Stromboli (Braun and Ripepe, 1993) and Mount St. Helens (e.g. Johnson and Malone, 2007), are capable of shaking the ground through their impact at the free surface. Even GPS technology has served witness to the propagation of volcano infrasound waves such as during a 2003 Montserrat eruption, which perturbed the ionosphere sufficiently to generate timing anomalies that progressed away from the volcano at acoustic velocities (Dautermann et al., 2009). In extreme cases structural damage, such as broken windows, serves as a poignant testament to the intense power of acoustic waves radiated by volcanic explosions.

67

8. Conclusion Modern volcano geophysical studies integrate remote sensing and ground-based observations of geodetic, gas flux, thermal anomalies, and elastic wave radiation. Traditionally volcano surveillance has been grounded with in-situ seismic networks because they afford a continuous record of the earthquakes and mechanisms that occur internal to and on the surface of a volcano. This paper summarizes why seismic surveillance is well complemented by infrasound, which is specifically attuned to sources occurring at, or near, the surface of a volcano and which can be recorded at local, regional, and global distances. Much more work is necessary within the field of volcano infrasound. In many cases, acoustic studies afford tracking of eruptive activity and localization of eruption or secondary sources, but precise quantification of material fluxes and the relation between eruption style and sound radiation is still unclear. Our understanding of how changing atmospheric conditions affect, disperse, scatter, and attenuate infrasound signals is also still poorly constrained for propagation to various distances. Finally, our volcano infrasound observations are weighted towards regional observations of mid-size eruptions or local study of easily-approached small-scale ‘laboratory’ eruptions. More infrequent Plinian eruptions will occur in the future and when they do they will stretch our understanding of the generation of low frequency sound waves. Acknowledgments This work was made possible in part through support from a grant from the National Science Foundation (EAR #0738802). We thank two anonymous reviewers for careful and insightful comments. References Aki, K., Koyanagi, R., 1981. Deep volcanic tremor and magma ascent mechanism under Kilauea, Hawaii. J. Geophys. Res 86 (B8), 7095–7110. Antier, K., Le Pichon, A., Vergniolle, S., Zielinski, C., Lardy, M., 2007. Multiyear validation of the NRL-G2S wind fields using infrasound from Yasur. Journal of Geophysical Research-Atmospheres 112 (D23). Arrowsmith, S.J., Johnson, J.B., Drob, D.P., Hedlin, M.A.H., 2010. The seismo-acoustic wavefield: a new paradigm in studying geophysical phenomena. Rev. Geophys. 89 (D3), 4895–4904. Banister, J.R., 1984. Pressure wave generated by the Mount St. Helens eruption. J. Geophys. Res. D3, 4895–4904. Bath, M., 1982. Atmospheric waves from Mount St. Helens. EOS Trans. AGU 63, 193. Benoit, J.P., McNutt, S.R., 1997. New constraints on source processes of volcanic tremor at Arenal Volcano, Costa Rica, using broadband seismic data. Geophysical Research Letters 24 (4), 449–452. Bolt, B.A., Tanimoto, T., 1981. Atmospheric oscillations after the May 18, 1980 eruption of Mount St. Helens. EOS Trans. AGU 62, 529–530. Bouche, E., et al., 2010. The role of large bubbles detected from acoustic measurements on the dynamics of Erta 'Ale lava lake (Ethiopia). Earth and Planetary Science Letters 295, 37–48. Bowman, J.R., Baker, G.E., Bahavar, M., 2005. Ambient infrasound noise. Geophysical Research Letters 32 (9). Braun, T., Ripepe, M., 1993. Interaction of seismic and air waves as recorded at Stromboli Volcano. Geophys. Res. Lett. 20, 65–68. Buckingham, M.J., Garces, M.A., 1996. Canonical model of volcano acoustics. Journal of Geophysical Research—Solid Earth 101 (B4), 8129–8151. Campus, P., 2006. Monitoring volcanic eruptions with the IMS infrasound network. Inframatics 15, 6–12. Cannata, A., Montalto, P., Privitera, E., Russo, G., Gresta, S., 2009a. Characterization and location of infrasonic sources in active volcanos: Mt. Etna, September–November 2007. J. Geophys. Res. 114 (B08308), 15. doi:10.1029/2008JB006007. Cannata, A., Montalto, P., Privitera, E., Russo, G., Gresta, S., 2009b. Tracking eruptive phenomena by infrasound: May 13, 2008 eruption at Mt. Etna. Geophys. Res. Lett. 36 (L05304). Caplan-Auerbach, J., McNutt, S.R., 2003. New insights into the 1999 eruption of Shishaldin volcano, Alaska, based on acoustic data. Bull. Volcan. 65 (6), 405–417. Caplan-Auerbach, J., Bellesiles, A., Fernandes, J.K., 2010. Estimates of eruption velocity and plume height from infrasonic recordings of the 2006 eruption of Augustine Volcano, Alaska. Journal of Volcanology and Geothermal Research 189 (1–2), 12–18. Dautermann, T., Calais, E., Mattioli, G.S., 2009. Global Positioning System Detection and Energy Estimation of the Ionospheric Wave Caused by the 13 July 2003 Explosion of

68

J.B. Johnson, M. Ripepe / Journal of Volcanology and Geothermal Research 206 (2011) 61–69

the Soufriere Hills Volcano. Montserrat, Journal of Geophysical Research—Solid Earth. 114. Di Grazia, G., et al., 2009. A multiparameter approach to volcano monitoring based on 4D analyses of seismo-volcanic and acoustic signals: the 2008 Mt. Etna eruption. Geophysical Research Letters 36, 6. Dibble, R.R., 1989. Infrasonic recordings of Strombolian eruptions of Erebus, Antarctica, March–December 1984, covering the jump in activity on 13 September 1984. In: Latter, J. (Ed.), Volcanic Hazards, Assessment and Monitoring. Springer-Verlag, Berline, pp. 536–553. Dibble, R.R., Kienle, J., Kyle, P.R., Shibuya, K., 1984. Geophysical studies of Erebus Volcano, Antarctica, from 1974 December to 1982 January. New Zealand Journal of Geology and Geophysics 27 (4), 425–455. Donn, W.L., Balachandran, N.K., 1981. Mount St. Helens eruption of 18 May, 1980: air waves and explosive yield. Science 213, 539–541. Dowling, A.P., 1998. Steady-state radiation from sources. In: Crocker, M. (Ed.), Handbook of Acoustics. John Wiley & Sons, New York, pp. 99–117. Evers, L.G., Haak, H.W., 2005. The detectability of infrasound in the Netherlands from the Italian volcano Mt. Etna. Journal of Atmospheric and Solar-Terrestrial Physics 67 (3), 259–268. Fairfield, C., 1980. OMSI sound project, the acoustic effects of the Mount St. Helens eruption on May 18, 1980. Oregon Geol 42 (12), 200–202. Fee, D., Garces, M., 2007. Infrasonic tremor in the diffraction zone. Geophysical Research Letters 34 (16). Fee, D., et al., 2010a. Infrasonic harmonic tremor and degassing bursts from Halemaumau Crater, Kilauea Volcano, Hawaii. J. Geophys. Res. Fee, D., Garces, M., Steffke, A., 2010b. Infrasound from Tungurahua Volcano 2006–2008: Strombolian to Plinian eruptive activity. J. Geophys. Res. 115 (D00L10), 15. doi:10.1029/ 2009JD013621. Fee, D., Steffke, A., Garces, M., 2010c. Characterization of the 2008 Kasatochi and Okmok eruptions using remote infrasound arrays. J. Geophys. Res. 115 (D00L10). Firstov, P.P., Kravchenko, N.M., 1996. Estimation of the amount of explosive gas released in volcanic eruptions using air waves. Volcanol. Seismol. 17, 547–560. Firstov, P.P., Storcheus, A.V., 1987. Acoustic signals that accompanied the March–June 1983 eruption at Klyuchevskoy Volcano. Volcanol. Seismol. 5, 66–80. Garces, M.A., 2000. Theory of acoustic propagation in a multi-phase stratified liquid flowing within an elastic-walled conduit of varying cross-sectional area. Journal of Volcanology and Geothermal Research 101 (1–2), 1–17. Garces, M.A., McNutt, S.R., 1997. Theory of the airborne sound field generated in a resonant magma conduit. Journal of Volcanology and Geothermal Research 78 (3–4), 155–178. Garces, M.A., Hagerty, M.T., Schwartz, S.Y., 1998. Magma acoustics and time-varying melt properties at Arenal Volcano, Costa Rica. Geophysical Research Letters 25 (13), 2293–2296. Garces, M.A., et al., 1999. Infrasonic precursors to a Vulcanian eruption at Sakurajima Volcano, Japan. Geophys. Res. Lett. 26 (16), 2537–2540. Garces, M.A., McNutt, S.R., Hansen, R.A., Eichelberger, J.C., 2000. Application of wavetheoretical seismoacoustic models to the interpretation of explosion and eruption tremor signals radiated by Pavlov Volcano, Alaska. J. Geophys. Res. 105 (2), 3039–3058. Garces, M., et al., 2003. Infrasonic tremor observed at Kilauea Volcano, Hawai'i. Geophysical Research Letters 30 (20). Garces, M., et al., 2007. Prototype ASHE volcano monitoring system captures the acoustic fingerprint of stratospheric ash injection. Inframatics 17, 1–6. Gerst, A., Hort, M., Kyle, P.R., Voge, M., 2008. 4D velocity of Strombolian eruptions and man-made explosions derived from multiple Doppler radar instruments. Journal of Volcanology and Geothermal Research 177 (3), 648–660. Goerke, V.H., Young, J.M., Cook, R.K., 1965. Infrasonic observations of the May 16, 1963, volcanic explosion on the Island of Bali. J. Geophys. Res. 70 (24), 6017–6022. Gorshkov, G.S., 1959. Gigantic eruption of the Volcano Bezymianny. Bull. Volcan 20, 77–109. Goto, A., Johnson, J.B., 2011. Monotonic infrasound and Helmholtz resonance at Volcan Villarrica (Chile). Geophys. Res. Lett. 38 (L06301). Green, D.N., Neuberg, J., 2005. Seismic and infrasonic signals associated with an unusual collapse event at the Soufriere Hills volcano, Montserrat. Geophysical Research Letters 32 (7). Gresta, S., et al., 2004. Seismoacoustic measurements during the July–August eruption of Mt. Etna volcano, Italy. J. Volc. Geotherm. Res. 137, 219–230. Hagerty, M.T., Schwartz, S.Y., Garces, M.A., Protti, M., 2000. Analysis of seismic and acoustic observations at Arenal Volcano, Costa Rica, 1995–1997. Journal of Volcanology and Geothermal Research 101 (1–2), 27–65. Harkrider, D.G., 1964. Theoretical and observed acoustic-gravity waves from explosive sources in the atmosphere. J. Geophys. Res. 69, 5295–5321. Harris, A.J.L., Ripepe, M., 2007. Temperature and dynamics of degassing at Stromboli. J. Geophys. Res. 112, 8713–8727. Iguchi, M., Ishihara, K., 1990. Comparison of earthquakes and airshocks accompanied with explosive eruptions at Sakurajima and Sawunosejima volcanoes. Ann. Disaster Prevention Research Inst., Kyoto Univ 33 (B-1), 1–12. Johnson, J.B., 2003. Generation and propagation of infrasonic airwaves from volcanic explosions. Journal of Volcanology and Geothermal Research 121 (1–2), 1–14. Johnson, J.B., 2005. Source location variability and volcanic vent mapping with a smallaperture infrasound array at Stromboli Volcano. Italy. Bull. Volcan. 67, 1–14. Johnson, J.B., 2007. On the relation between infrasound, seismicity, and small pyroclastic explosions at Karymsky Volcano. Journal of Geophysical Research— Solid Earth 112 (B8). Johnson, J.B., Aster, R.C., 2005. Relative partitioning of acoustic and seismic energy during Strombolian eruptions. Journal of Volcanology and Geothermal Research 148 (3–4), 334–354.

Johnson, J.B., Lees, J.M., 2000. Plugs and chugs — seismic and acoustic observations of degassing explosions at Karymsky, Russia and Sangay, Ecuador. J. Volc. Geotherm. Res. 101 (1–2), 67–82. Johnson, J.B., Lees, J.M., 2010. Long Period Infrasound at Santiaguito Produced by Rapid Dome Inflation. Geophys. Res, Lett. Johnson, J.B., Malone, S.D., 2007. Ground-coupled acoustic airwaves from Mount St. Helens provide constraints on the May 18, 1980 eruption. Earth Planet. Sci. Lett. 258 (1–2), 16–31. Johnson, J.B., Lees, J.M., Gordeev, E.I., 1998. Degassing explosions at Karymsky Volcano, Kamchatka. Geophys. Res. Lett. 25 (21), 3999–4042. Johnson, J.B., et al., 2003. Interpretation and utility of infrasonic records from erupting volcanoes. Journal of Volcanology and Geothermal Research 121 (1–2), 15–63. Johnson, J.B., Aster, R.C., Kyle, P.R., 2004. Volcanic eruptions observed with infrasound. Geophysical Research Letters 31 (14). Johnson, J.B., Aster, R., Jones, K., Kyle, P., McIntosh, W., 2008. Acoustic source characterization of impulsive Strombolian eruptions from the Mount Erebus lava lake. J. Volc. Geotherm. Res. 177, 673–686. Johnson, J.B., et al., 2009. Dissection of a composite volcanic earthquake at Santiaguito, Guatemala. Geophys. Res. Lett. 36 (L16308). Johnson, J.B., Lees, J.M., Varley, N., 2010. Characterizing complex eruptive activity at Santiaguito Volcano, Guatemala using infrasound semblance in networked arrays. J. Volc. Geotherm. Res. 199, 1–14. Jones, K.R., Johnson, J.B., 2010. Mapping complex eruptive activity at Santiaguito, Guatemala using network infrasound semblance. J. Volc. Geotherm. Res. 199, 15–24. Jones, K., Johnson, J.B., Aster, R., Kyle, P., McIntosh, W., 2008. Infrasonic tracking of large bubble bursts and ash venting at Erebus volcano, Antarctica. J. Volc. Geotherm. Res. 177, 661–672. Kamo, K., Ishihara, K., Tahira, M., 1994. Infrasonic and seismic detection of explosive eruptions at Sakurajima Volcano, Japan, and the PEGASAS-VE early-warning system, First International Symposium on Volcanic Ash and Aviation Safety. U.S. Geological Survey Bulletin, pp. 357–365. Kanamori, H., Mori, J., Harkrider, D.G., 1994. Excitation of atmospheric oscillations by volcanic eruptions. Journal of Geophysical Research—Solid Earth 99 (B11), 21947–21961. Kinsler, L.E., Frey, A.R., Coppens, A.B., Sanders, J.V., 2000. Fundamentals of Acoustics. John Wiley and Sons, Fourth Edition. Kobayashi, T., Ida, Y., Ohminato, T., 2005. Small inflation sources producing seismic and infrasonic pulses during the 2000 eruptions of Miyake-jima, Japan. Earth and Planetary Science Letters 240 (2), 291–301. Le Pichon, A., et al., 2005. Infrasound monitoring of volcanoes to probe high-altitude winds. Journal of Geophysical Research-Atmospheres 110 (D13). Le Pichon, A., et al., 2009. Assessing the performance of the International Monitoring System's infrasound network: geographical coverage and temporal variabilities. J. Geophys. Res. Atmos. 114, D08112. Le Pichon, A., Blanc, E., Hauchecorne, A., 2010. Infrasound Monitoring for Atmospheric Studies. Springer, Netherlands. Lees, J.M., Ruiz, M., 2008. Non-linear explosion tremor at Sangay, Volcano, Ecuador. Journal of Volcanology and Geothermal Research 176 (1), 170–178. Lees, J.M., Johnson, J.B., Ruiz, M., Troncoso, L., Welsh, M., 2008. Reventador Volcano 2005: eruptive activity inferred from seismo-acoustic observation. Journal of Volcanology and Geothermal Research 176 (1), 179–190. Lighthill, M.J., 1978. Waves in Fluids. Cambridge University Press, New York. 504 pp. Liszka, L., Garces, M.A., 2002. Infrasonic observations of the Hekla eruption of February 26, 2000. J. Low Frequency Noise Vibration and Active Control 21 (1), 1–8. Lyons, J.J., Waite, G.P., Rose, W.I., Chigna, G., 2010. Patterns in open vent, Strombolian behavior at Fuego volcano, Guatemala, 2005–2007. Bulletin of Volcanology 72 (1), 1–15. Marchetti, E., Ripepe, M., Harris, A.J.L., Delle Donne, D., 2009a. Tracing the differences between Vulcanian and Strombolian explosions using infrasonic and thermal radiation energy. Earth and Planetary Science Letters 279 (3–4), 273–281. Marchetti, E., Ripepe, M., Ulivieri, G., Caffo, S., Privitera, E., 2009b. Infrasonic evidences for branched conduit dynamics at Mt. Etna volcano, Italy. Geophysical Research Letters 36, 5. Marcillo, O., Johnson, J.B., 2010. Tracking near-surface atmospheric conditions using an infrasound network. J. Acoust. Soc. Am. Lett. 128 (1), EL14–EL19. Matoza, R.S., Hedlin, M.A.H., Garces, M.A., 2007. An infrasound array study of Mount St. Helens. Journal of Volcanology and Geothermal Research 160 (3–4), 249–262. Matoza, R., et al., 2009a. Infrasonic jet noise from volcanic eruptions. Geophys. Res. Lett 36 (L08303). Matoza, R.S., et al., 2009b. The source of infrasound associated with long-period events at Mount St. Helens. J. Geophys. Res 114 (B04305). Matoza, R., et al., 2010a. Infrasonic observations of the June 2009 Sarychev Peak eruption, Kuril Islands: implications for infrasonic monitoring of explosive volcanism. J. Volc. Geotherm. Res. 200, 35–48. Matoza, R.S., Fee, D., Garces, M.A., 2010b. Infrasonic tremor wavefield of the Pu'u 'O'o crater complex and lava tube system, Hawaii in April 2007. J. Geophys. Res. Mauk, F.J., 1983. Utilization of seismically recorded infrasonic-acoustic signals to monitor volcanic explosions — the El Chichon sequence 1982 — a case study. J. Geophys. Res. 88 (B12), 10,385–10,401. McGreger, A.D., Lees, J.M., 2004. Vent discrimination at Stromboli Volcano, Italy. Journal of Volcanology and Geothermal Research 137 (1–3), 169–185. McNutt, S.R., 1986. Observations and analysis of B-type earthquakes, explosions and volcanic tremor at Pavlof volcano, Alaska. Bull. Seismol. Soc. Am. 76, 153–175. Montalto, P., Cannata, A., Privitera, E., Gresta, S., Nunnari, G., 2010. Towards an automatic monitoring system of infrasonic events at Mt. Etna: strategies for source location and modeling. Pure Appl. Geophys. 167, 1215–1231.

J.B. Johnson, M. Ripepe / Journal of Volcanology and Geothermal Research 206 (2011) 61–69 Moran, S.C., et al., 2008a. Seismic and acoustic recordings of an unusually large rockfall at Mount St. Helens, Washington. Geophys. Res. Lett. 35 (L19302). Moran, S.C., McChesney, P.J., Lockhart, A.B., 2008b. Seismicity and infrasound associated with explosions at Mount St. Helens, 2004–2006. In: Sherrod, D.R., Scott, W.E., Stauffer, P.H. (Eds.), A Volcano Rekindled: The Renewed Eruption of Mount St. Helens, 2004–2006. USGS Professional Paper, pp. 111–128. Mori, J., et al., 1989. Seismicity associated with eruptive activity at Langila Volcano Papua New Guinea. J. Volc. Geotherm. Res. 38 (3–4), 243–255. Morrissey, M.M., Chouet, B.A., 1997. Burst conditions of explosive volcanic eruptions recorded on microbaragraphs. Science 275 (5304), 1290–1293. Morrissey, M., Garces, M., Ishihara, K., Iguchi, M., 2008. Analysis of infrasonic and seismic events related to the 1998 Vulcanian eruption at Sakurajima. Journal of Volcanology and Geothermal Research 175 (3), 315–324. Newhall, C.G., Self, S., 1982. The volcanic explosivity index (VEI): an estimate of explosive magnitude for historical volcanism. J. Geophys. Res. 87 (C2), 1231–1238. Okada, H., Nishimura, Y., Miyamachi, H., Mori, H., Ishihara, K., 1990. Geophysical significance of the 1988–1989 explosive eruptions of Mt. Tokachi, Hokkaido, Japan. Bull. Volcanol. Soc. Japan, Ser. 2 35 (2), 175–203. Oshima, H., Maekawa, T., 2001. Excitation process of infrasonic waves associated with Merapi-type pyroclastic flow as revealed by a new recording system. Geophysical Research Letters 28 (6), 1099–1102. Pekeris, C.L., 1939. The propagation of a pulse in the atmosphere. Proc. R. Soc. Lond. 171, 434–449. Petersen, T., McNutt, S.R., 2007. Seismo-acoustic signals associated with degassing explosions recorded at Shishaldin Volcano, Alaska, 2003–2004. Bulletin of Volcanology 69 (5), 527–536. Petersen, T., De Angelis, S., Tytgat, G., McNutt, S.R., 2006. Local infrasound observations of large ash explosions at Augustine Volcano, Alaska, during January 11–28, 2006. Geophysical Research Letters 33 (12). Reed, J.W., 1987. Air pressure waves from Mount St. Helens eruptions. J. Geophys. Res. 92 (D10), 11979–11982. Richards, A.F., 1963. Volcanic sounds: investigation and analysis. J. Geophys. Res. 68 (3), 919–928. Ripepe, M., Harris, A.J.L., 2008. Dynamics of the 5 April 2003 explosive paroxysm observed at Stromboli by a near-vent thermal, seismic and infrasonic array. Geophysical Research Letters 35 (7). Ripepe, M., Marchetti, E., 2002. Array tracking of infrasonic sources at Stromboli Volcano. Geophysical Research Letters 29 (22). Ripepe, M., Poggi, P., Braun, T., Gordeev, E.I., 1996. Infrasonic waves and volcanic tremor at Stromboli. Geophys. Res. Lett. 23 (2), 181–184. Ripepe, M., Ciliberto, S., Schiava, M.D., 2001a. Time constraints for modeling source dynamics of volcanic explosions at Stromboli. J. Geophys. Res. 106 (B5), 8713–8727. Ripepe, M., et al., 2001b. Seismic and infrasonic evidences for an impulsive source of the shallow volcanic tremor at Mt. Etna, Italy. Geophysical Research Letters 28 (6), 1071–1074. Ripepe, M., Harris, A.J.L., Carniel, R., 2002. Thermal, seismic and infrasonic evidences of variable degassing rates at Stromboli Volcano. Journal of Volcanology and Geothermal Research 118 (3–4), 285–297. Ripepe, M., Poggi, P., Marchetti, E., 2004. Small aperture infrasonic array monitors activity of Stromboli Volcano. Inframatics 7, 1–14. Ripepe, M., Marchetti, E., Ulivieri, G., 2007. Infrasonic monitoring at Stromboli Volcano during the 2003 effusive eruption: insights on the explosive and degassing process of an open conduit system. Journal of Geophysical Research-Solid Earth 112. Ripepe, M., Delle Donne, D., Lacanna, G., Marchetti, E., Ulivieri, G., 2009. The onset of the 2007 Stromboli effusive eruption recorded by an integrated geophysical network. Journal of Volcanology and Geothermal Research 182 (3–4), 131–136. Ripepe, M., De Angelis, S., Lacanna, G., Voight, B., 2010a. Observation of infrasonic and gravity waves at Soufriere Hills Volcano, Montserrat. Geophys. Res. Lett. 37 (L00E14). Ripepe, M., et al., 2010b. Monochromatic infrasonic tremor driven by persistent degassing and convection at Villarrica Volcano, Chile. Geophys. Res. Lett. 37 (L15303).

69

Ritsema, A.R., 1980. Observations of Mount St. Helens eruption. EOS Trans. AGU 61, 1201–1202. Rowe, C.A., Aster, R.C., Kyle, P.R., Dibble, R.R., Schlue, J.W., 2000. Seismic and acoustic observations at Mount Erebus Volcano, Ross Island, Antarctica, 1994–1998. Journal of Volcanology and Geothermal Research 101 (1–2), 105–128. Ruiz, M., Lees, J.M., Johnson, J.B., 2006. Source constraints of Tungurahua volcano explosion events. Bulletin of Volcanology 68 (5), 480–490. Sahetapy-Engel, S.T., Harris, A.J.L., Marchetti, E., 2008. Thermal, seismic and infrasound observations of persistent explosive activity and conduit dynamics at Santiaguito lava dome, Guatemala. Journal of Volcanology and Geothermal Research 173 (1–2), 1–14. Scharff, L., et al., 2008. Eruption dynamics of the SW crater of Stromboli Volcano, Italy — an interdisciplinary approach. Journal of Volcanology and Geothermal Research 176 (4), 565–570. Simkin, T., Fiske, R.S., 1983. Krakatau 1883— The Volcanic Eruption and Its Effects. Smithsonian Institute Press, Washington D.C.. 464 pp. Tahira, M., Nomura, M., Sawada, Y., Kamo, K., 1996. Infrasonic and acoustic-gravity waves generated by the Mount Pinatubo eruption of June 15, 1991. In: Newhall, C.G., PUNONGBAYAN, R.S. (Eds.), Fire and Mud. University of Washington Press, Seattle, pp. 601–614. Vergniolle, S., Brandeis, G., 1994. Strombolian explosions. 1. A large bubble breaking at the surface of a lava column as a source of sound. Journal of Geophysical Research— Solid Earth 101 (B9), 20433–20447. Vergniolle, S., Caplan-Auerbach, J., 2004. Acoustic measurements of the 1999 basaltic eruption of Shishaldin volcano, Alaska—2. precursor to the Subplinian phase. Journal of Volcanology and Geothermal Research 137 (1–3), 135–151. Vergniolle, S., Caplan-Auerbach, J., 2006. Basaltic thermals and Subplinian plumes: constraints from acoustic measurements at Shishaldin volcano, Alaska. Bulletin of Volcanology 68 (7–8), 611–630. Vergniolle, S., Boichu, M., Caplan-Auerbach, J., 2004. Acoustic measurements of the 1999 basaltic eruption of Shishaldin volcano, Alaska: 1. origin of Strombolian activity. J. Volc. Geotherm. Res. 137, 135–151. Wilson, C.R., Forbes, R.B., 1969. Infrasonic waves from Alaskan volcanic eruptions. J. Geophys. Res. 74 (18), 4511–4522. Wilson, C.R., Olson, J.V., 2005. I53US and I55US signals from Manam Volcano. Inframatics 9, 31–35. Wilson, C.R., Nichpare, S., Forbes, R.B., 1966. Evidence of 2 sound channels in polar atmosphere from infrasonic observations of eruption of an Alaskan volcano. Nature 211 (5045), 163–165. Wilson, C.R., Olson, J.V., Oborne, D.L., Le Pichon, A., 2003. Infrasound from Erebus Volcano at I55US in Antarctica. Inframatics 4, 1–8. Wilson, C.R., et al., 2006. Infrasonic array observations at I53US of the 2006 Augustine Volcano eruptions. Inframatics 13, 11–25. Woulff, G., McGetchin, T.R., 1976. Acoustic noise from volcanos — theory and experiment. Geophysical Journal of the Royal Astronomical Society 45 (3), 601–616. Yamasato, H., 1997. Quantitative analysis of pyroclastic flows using infrasonic and seismic data at Unzen volcano, Japan. Journal of Physics of the Earth 45 (6), 397–416. Yamasato, H., 1998. Nature of infrasonic pulse accompanying low frequency earthquake at Unzen Volcano, Japan. Bull. Volcanol. Soc. Japan 43, 1–13. Yokoo, A., Iguchi, M., 2010. Using infrasound waves from eruption video to explain ground deformation preceding the eruption of Suwanosejima volcano, Japan. J. Volc. Geotherm. Res. 196 (3–4), 287–294. Yokoo, A., Tameguri, T., Iguchi, M., 2009. Swelling of a lava plug associated with a Vulcanian eruption at Sakurajima Volcano, Japan, as revealed by infrasound record: case study of the eruption on January 2, 2007. Bulletin of Volcanology 71 (6), 619–630. Yuan, A.T.E., McNutt, S.R., Harlow, D.H., 1984. Seismicity and eruptive activity at Fuego volcano, Guatemala: February 1975–January 1977. J. Volc. Geotherm. Res. 21, 277–296.