Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption

VOLGEO-04787; No of Pages 15 Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

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Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption David Fee a,⁎, Stephen R. McNutt b, Taryn M. Lopez b, Kenneth M. Arnoult a, Curt A.L. Szuberla a, John V. Olson a a b

Wilson Infrasound Observatories, Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775-7320, United States Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775-7320, United States

a r t i c l e

i n f o

Article history: Received 19 May 2011 Accepted 14 September 2011 Available online xxxx

a b s t r a c t The explosive phase of the 2009 Redoubt Volcano eruption produced predominantly short duration, highamplitude infrasound signals recorded up to 4500 km away. All 19 numbered explosive events were recorded at a local microphone (DFR, 12 km), as well as at an infrasound array in Fairbanks, Alaska (I53US, 547 km), most with high signal to noise ratios. The local microphone provides an estimate of the source parameters, and comparison between the two datasets allows the unique opportunity to evaluate acoustic source term estimation at a remote array. High waveform similarity between DFR and I53US occurs during much of the explosive phase due to strong stratospheric ducting, permitting accurate source constraints inferred from I53US data. Cross-correlation analysis after applying a Hilbert transform to the I53US data shows how the acoustic energy has passed through a single caustic, as predicted by ray theory. Similar to previous studies, significant low-frequency infrasound from Redoubt recorded at I53US is coincident with high-altitude ash emissions. The largest events also produced considerable energy at greater than 50 s periods, likely related to the initial oscillations of the volcanic plume or jet. Many of the explosive events have emergent onsets, somewhat unusual for explosive, short-duration eruptions. Comparison of the satellite-derived SO2 emissions with the relative amount of acoustic energy at I53US shows a very high, statistically significant correlation. This study reiterates the utility of using remote infrasound arrays for detection of hazardous emissions and characterization of large volcanic eruptions, and demonstrates how, under typical meteorological conditions, remote infrasound arrays can provide an accurate representation of the acoustic source. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Low latency detection and characterization of hazardous volcanic eruptions remains a challenging task in remote areas, particularly when seismic data coverage is sparse or weather clouds and/or sampling frequency diminish the effectiveness of remote sensing. The Comprehensive Nuclear-Test-Ban Treaty Organization has developed a global network of infrasound arrays as part of the International Monitoring System (IMS) with the goal of detecting clandestine nuclear tests. Recent work has shown that this network can also be utilized to detect and constrain large volcanic eruption parameter estimates. Eruption characterization using the global infrasound network is particularly promising for remote volcanic eruptions, such as the recent Kasatochi and Sarychev Peak eruptions, as the existing monitoring networks in those regions were sparse and cloud cover (Prata, 2009) and icing of ash particles (Rose et al., 1995) often hampers remote sensing's ability to detect volcanic ash and gas. Fee et al. (2010b) combined IMS infrasound data with remote sensing analysis ⁎ Corresponding author. Tel.: + 1 907 474 7564. E-mail address: [email protected] (D. Fee).

to determine eruption source parameters of the 2008 plinian eruptions of Kasatochi and Okmok volcanoes. They observed at least three main eruption pulses and related their remote infrasound data (N2000 km) to eruption source processes at Kasatochi. Arnoult et al. (2010) also analyzed IMS data for these eruptions and identified at least seven arrays that detected the Kasatochi eruption. Matoza et al. (2011a) provided the most detailed eruption chronology available for the 2009 eruption of Sarychev Peak, Kurile Islands using IMS data, as this remote volcano had no seismometers nearby and the satellite sampling frequency was 15 min. Estimates of the source location were also made for the Sarychev Peak and Kasatochi eruptions (Arnoult et al., 2010; Matoza et al., 2011a). The recent eruption of Eyjafjallajökull, Iceland was detected at 14 infrasound arrays and demonstrated that even moderate sized eruptions can produce significant infrasound at remote distances (Matoza et al., 2011b). These eruptions all presented a significant hazard to aviation and caused extensive flight delays and loss of airline revenue. These studies clearly show the IMS infrasound network can assist with the detection and understanding of large eruptions and help mitigate volcanic hazards. The goal of remote acoustic monitoring of volcanic eruptions is to provide constraints on various eruption parameters, such as location,

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Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

The most recent eruptive activity at Redoubt occurred in 2008– 2009 and is split into three phases: precursory, explosive, and effusive (Bull et al., this issue-b). The precursory phase began with increases in gas emissions noted in July 2008, soon followed by elevated thermal activity at the summit. Seismicity and gas emissions increased substantially in January 2009, followed by the first explosive eruption of ash and steam on 15 March 2009 that signaled the onset of the explosive phase. Nineteen magmatic explosive eruptions were detected and classified by the Alaska Volcano Observatory (AVO) (Table 1) between 23 March and 4 April, with many of these events producing hazardous ash plumes to stratospheric altitudes that disrupted local air traffic and deposited ash on local communities. Melting of the Drift River Glacier during the eruptions produced voluminous lahars that inundated the Drift River Valley (Bull et al., this issue-b; Schaefer et al., 2011). Petrological studies indicate crystal-rich andesites were erupted with a range of compositions between 57.5 and 62.5 wt.% SiO2 (Coombs et al., this issue). Seismicity prior to the explosive phase consisted of swarms of repeating events and each of the explosive events coincided with strong co-eruptive tremor (Buurman et al., this issue). Significant SO2 emissions were also detected during the explosive phase (Lopez et al., this issue). Four periods of lava dome growth were detected during the eruption, with three being destroyed during the explosive phase while the final dome grew after Event 19 and persisted during the effusive phase (Bull et al., this issue-a). Additional “reanalysis” events were determined by AVO after the eruption had ceased (McNutt et al., this issue). For this study we focus on the 19 primary events as the reanalysis events were smaller and not recorded as well at the remote infrasound arrays.

timing, duration, and changes in intensity. These constraints can be used to detect and notify civil and aviation authorities of an ongoing eruption (Fee et al., 2010a), as well as provide input for volcanic ash transport and dispersion models (e.g. Mastin et al., 2009). The 2009 eruption of Redoubt Volcano, Alaska provides a unique opportunity to further assess the utility of using infrasound to characterize volcanic eruptions. This eruption had numerous explosive events that produced prodigious infrasound detected by both a local pressure sensor (DFR, 12 km) and remote IMS arrays 547–4600 km away. Unlike previous remote volcano infrasound studies, the availability of local infrasound data provides a representation of the acoustic source waveform. In this manuscript we use the IMS array I53US, located in Fairbanks, Alaska (547 km), to derive source constraints on the explosive events themselves, and compare the local and remote acoustic data to determine the accuracy of these constraints. A thorough understanding of acoustic propagation from source to receiver is necessary for remote volcano infrasound studies, as the atmospheric structure will affect how the sound propagates to the array, thus influencing the acoustic travel times, propagation path, transmission loss, etc. For example, Fee et al. (2010b) showed that uncertainties in the propagation path from Kasatochi volcano to the I53US infrasound array could lead to source timing discrepancies of up to 15 min. For this study we perform basic propagation modeling and analysis of the atmospheric models to help understand the remote data and comparisons to DFR. Further, we compare the satellite-derived SO2 emissions with the relative acoustic energy at I53US and discuss the implications for using infrasound to detect hazardous volcanic emissions. This paper will serve as a companion paper to McNutt et al. (this issue), which discusses in greater detail the local infrasound data and its relation to other datasets.

3. Data and methods 2. Eruption overview

3.1. Infrasound data

Redoubt Volcano (60.4857° N, 152.742 W) is a 3.1 km high active stratovolcano located in the upper Cook Inlet of southern Alaska (Fig. 1). The volcano's relatively high level of activity and proximity to Anchorage, AK (170 km), the Drift River Oil Terminal, and numerous North Pacific air routes make it a hazard to both air and ground civil activity.

This study utilizes infrasound array data from the IMS network and a single local microphone deployed by AVO (station DFR). Fig. 1a shows the locations of the arrays relative to Redoubt, while Table 2 lists the stations and arrays within 5000 km that detected at least one eruptive event, as well as their azimuth and range from the volcano. The DFR infrasound station consists of a single Chaparral

a)

b) 60.7

90° N

I18DK 60.6

DFR

60.5

Redoubt

75° N

I44RU

60° N

I53US

I10CA

Redoubt I56US

45° N

I57US 120° W

° 180

0

10

20 km

60.4

150° W 15° N I59US

60.3 -153.1

-152.9

-152.7

-152.5

-152.3

Fig. 1. Map of study area. a) IMS stations (black circles) within 4600 km of Redoubt Volcano, Alaska (black triangle). The closest array is I53US at 547 km NE of Redoubt. b) Redoubt Volcano and local infrasound station DFR, 12.2 km NE of the summit.

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

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Table 1 List of AVO defined events and detection summary at I53US. Onset indicates either emergent (E) or impulsive (I). Event times and DFR parameters are used from McNutt et al. (this issue), while plume heights are from Schneider and Hoblitt (this issue). Event

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

2009 date

Time

23 Mar 23 Mar 23 Mar 23 Mar 23 Mar 24 Mar 26 Mar 26 Mar 27 Mar 27 Mar 27 Mar 28 Mar 28 Mar 28 Mar 28 Mar 28 Mar 28 Mar 29 Mar 4 Apr

6:35:16 7:01:52 8:14:05 9:38:52 12:30:21 3:40:35 16:34:29 17:24:14 7:47:11 8:28:22 16:38:58 1:34:43 3:24:18 7:19:39 9:19:24 21:40:00 23:29:25 3:23:31 13:57:05

DFR pressure

DFR duration

(Pa)

(min)

25 151 38 70 173 76 7 100 31 54 83 146 138 78 59 28 67 49 88

26 3 13 8 16 12 1 7 15 4 4 2 3 2 2 2 3 83 31

I53US Onset

I53US pressure

I53US energy

I53US duration

(Pa)

(Pa2∙ s)

(min)

7:09:10 7:30:10 8:44:23 10:08:46 12:57:51 4:13:41 17:10:23 17:53:20 8:14:41 8:54:16 17:07:40 2:01:25 3:52:36 7:47:57 9:48:06 22:09:54 23:57:19 3:52:37 14:29:47

0.99 2.80 1.29 3.70 4.57 3.41 0.28 3.07 2.62 4.86 4.25 6.76 6.66 6.22 3.54 0.83 2.13 1.43 2.50

1.37 8.70 23.98 191.02 470.06 186.72 0.18 69.58 29.66 83.51 53.67 109.94 273.02 127.04 59.36 0.83 10.28 5.52 75.6

16.8 14 19.6 27.2 21.6 10.4 2.8 12.8 49.2 22 7.2 8.4 8 18.8 10.8 3.2 16.8 5.2 31.2

Physics Model 25 microphone connected to multiple porous hoses for wind noise reduction, deployed 12.2 km NE of Redoubt's summit (Fig. 1b). The Model 25 has a flat frequency response between 0.1 and 50 Hz. I53US is the nearest array to the volcano (547 km) and is composed of 8 infrasound elements deployed as a pentagon enclosing a small triangle. Each element consists of a Chaparral Physics Model 5 microphone (flat frequency response between 0.02 and 50 Hz) connected to a rosette pipe array to reduce wind noise. Seven other infrasound arrays detected parts of the eruption as well: I56US (Newport, Washington), I44RU (Petropavlovsk-Kamchatsky, Russia), I18DK (Qaanaaq, Greenland), NVIAR (Mina, Nevada), I10CA (Lac Du Bonet, Canada), I57US (Pinon Flat, California), and I59US (Kona, Hawaii). For this study, we focus on data from DFR and I53US, as they are the two stations closest to the volcano and have the highest signalto-noise ratio (S/N) and lowest atmospheric perturbation effects. All of the stations are sampled at 20 Hz, except NVIAR (40 Hz) and DFR (100 Hz). All times listed are in UTC.

Correlation coefficient NaN 0.47 0.37 0.48 0.51 0.66 0.31 0.54 0.66 0.74 0.62 0.75 0.89 0.78 0.80 0.30 0.42 0.43 0.19

Plume height

Onset

ULP energy

E I E E I E N/A E E I I I I I I I I E E

N Y? ? ? Y ? N Y N Y Y Y Y Y Y N Y Y Y

(km) 5.5 13.4 13.1 13.1 14.9 18.3 6.7 18.9 11 14.9 15.5 11.9 15.2 11.9 13.1 5.2 12.2 12.5 15.2

of the signal velocity in the plane of the array) and azimuth (geographic bearing opposite the propagation direction across the array) are determined for each time window using a least-squares solution for plane waves traversing the array (Szuberla and Olson, 2004). The Fisher statistic (F-stat) (Melton and Bailey, 1957), a common signal processing detector for infrasound array data (Olson and Szuberla, 2008), is used as the signal detector. F-stat processing performs a comparison of the variances of both signal and uncorrelated noise, and effectively estimates the signal-to-noise ratio for spatiotemporally correlated signals. First the array data are time shifted for an incoming acoustic wave, which corresponds to the dominant azimuth and trace velocity found using the aforementioned least squares approach (Szuberla and Olson, 2004). Then the F-stat is found by computing the variances within each sensor record and between the array sensors using the following relationship: F¼

Vb =ðM−1Þ Vw =ðMðn−1ÞÞ

ð1Þ

3.2. Signal processing and detection methods Array processing is performed on all array data to detect signals from Redoubt. Data are divided into 150 s windows with 80% overlap, then band-pass filtered between 0.075 and 2 Hz. This frequency band is where the majority of acoustic signals are concentrated for the explosive eruptions. Lower frequency energy is apparent for some explosions, but signal contamination from wind is more common at these low frequencies. After filtering, the trace velocity (component Table 2 Table of infrasound arrays that recorded the Redoubt eruption. Redoubt is located at 60.4857° N, 152.742 W. Azimuth is the bearing from the station to the volcano, in degrees from north. Station

DFR I53US I56US I44RU I18DK NVIAR I10CA I57US I59US

Lat.

60.592 64.875 48.264 53.106 77.476 38.430 50.201 33.606 19.592

Lon.

− 152.686 − 147.861 − 117.126 157.714 − 69.288 − 118.304 − 96.027 − 116.453 − 155.894

Azimuth

Range

(°N)

(km)

195 209 314 55 285 327 310 329 2

12.2 547 2625 3041 3385 3416 3628 3960 4546

where Vb is the variation between the sensor recordings, Vw is the variation within a single sensor recording, M is the number of samples, and n is the number of sensors (Melton and Bailey, 1957; Olson and Szuberla, 2008). Melton and Bailey (1957) show that the S/N (PS/N) can then be derived from the F-stat and number of sensors: PS=N ¼

F−1 : n

ð2Þ

Data segments with an F-stat above a threshold are then counted as detections (Olson and Szuberla, 2008). Here we choose a PS/N of 1 as the detection threshold, corresponding to F = 9 for an 8-element array. We follow the method of Blandford (1974) and apply the Fstat method to overlapping data segments. Redoubt detections are further restricted to segments originating from ±15° of the theoretical azimuth to the volcano and must have an acoustic trace velocity (0.25–0.45 km/s). Fig. 2 shows an example of signal detection processing for one of the Redoubt's explosive eruptions (Event 13). The relative acoustic energies of the eruptions are also estimated. For a spherical source in free space the acoustic energy (Ea) can be calculated by: Ea ¼

4πr 2 T 2 ∫ Δp ðtÞdt ρc 0

ð3Þ

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

Event 13, 28 March 2009 Pressure (Pa)

a)

2 0 −2 800

F−stat

b)

600 400 200

0.4 0.35 0.3 0.25

Azimuth (°)

d)

Vel (km/s)

0

c)

220 210 200 03:51

03:53

03:55

03:57

03:59

04:01

04:03

UTC Time (HH:MM)

Fig. 2. Infrasound detections at I53US for Event 13. a) The array data is beamformed and filtered, then divided into 150 s windows for array processing and detection which produces the b) F-stat, c) trace velocity, and d) azimuth. Data windows exceeding the F-stat threshold (F = 9), having acoustic trace velocities, and within ± 15° of the theoretical azimuth to the volcano (dashed red line) are considered a detection and are denoted in red. Event 13 begins with an impulsive onset and has two primary energy pulses with high Fstat. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

where r is the source–receiver distance, ρ the air density, c the sound speed, Δp pressure perturbation, and T the source duration (Pierce, 1981). Acoustic energy calculations have proven effective in detecting changes in eruptive vigor and ash cloud height (Fee et al., 2010a; Steffke et al., 2010), discriminating between types of volcanic activity (Woulff and McGetchin, 1976; Marchetti et al., 2009), and comparing the relative seismic and acoustic energy release from explosions (Johnson and Aster, 2005). However, Eq. (3) assumes purely spherical spreading of the waveforms in free space, which is incorrect at remote distances. Further it does not account for absorption (Sutherland and Bass, 2004) or energy loss through ground reflection (Attenborough et al., 2006), and is thus not valid at remote distances. Here we use a modified form of Eq. (3) to calculate the relative acoustic energy (Ear) from Redoubt by disregarding the propagation (4πr 2) and acoustic impedance (ρc) terms, and simply integrating the squared pressure over the time interval: T

2

Ear ¼ ∫0 Δp ðtÞdt:

ð4Þ

The relative acoustic energies presented here are thus useful in comparing the eruptive vigor between explosions at Redoubt but not to actual acoustic energies for other volcanoes. The units of Ear are Pa 2∙s, and it is calculated only for time segments that meet the detection thresholds previously detailed. Cross-correlation is performed between DFR and I53US waveforms. DFR waveform segments are selected beginning one minute prior to the acoustic onset and ending five minutes after the acoustic signal has ceased. Onsets and durations from McNutt et al. (this issue) are used. The longer event duration is used to fully capture the I53US signals. DFR data are also resampled to 20 Hz to match the I53US sample rate. The corresponding I53US waveform for crosscorrelation is selected by looking 1823 s after the event onset, which corresponds to a typical stratospheric celerity of 0.30 km/s.

Celerity is an often used term that describes the great-circle distance between the source–receiver divided by the travel time. A lag of 100 s in the I53US data is allowed to align the waveforms. All signals were filtered using a 4-pole, acausal Butterworth filter. Power spectral density (PSD) estimates are made using Welch's modified periodogram method. Delay and sum beamforming was performed for all waveforms (where indicated) to increase the S/N (Johnson and Dudgeon, 1992). For the signals with periods between 50 and 200 s, the instrument response at I53US has been removed. This band is referred to as the Ultra Long Period (ULP) band (Chouet, 1996), and corresponds to long-duration atmospheric oscillations. ULP signals at I53US were distinguished from noise by comparing waveforms between all eight array elements. Acoustic signal durations are determined using the aforementioned detection thresholds. The acoustic onsets were derived by McNutt et al. (this issue) to the nearest second using data from the local station DFR. 3.3. Propagation modeling Infrasound may propagate long distances due to the relatively low amount of attenuation at infrasonic frequencies and multiple atmospheric waveguides (ducts). The propagation of acoustic energy in the atmosphere is predominantly governed by vertical wind and temperature gradients. The effective sound speed in the atmosphere can be estimated by: ceff ¼

pffiffiffiffiffiffiffiffiffi → → γRT þ v • n

ð5Þ

where γ is the specific heat ratio, R the universal gas constant, T the temperature, v the horizontal wind vector, and n the ray normal (Whitaker and Norris, 2008). The latter term is necessary due to the translational effects of winds in the atmosphere (advection of sound). The sound speed for a typical windless atmosphere of 20 °C at sea level is 343 m/s. Following Snell's Law and Eq. (5), sound

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

waves propagating from a source near the earth's surface will generally be refracted upward as the temperature (and thus sound speed) often decreases with height (Fig. 3b). However, strong winds and/or temperature gradients may cause the sound speed at altitude to exceed that at the source, causing the sound to refract downward, creating a waveguide. In this study we compare the maximum ceff in the stratosphere (40–70 km) to that at the source (3 km heightRedoubt's summit) to obtain an estimate of the strength of the stratospheric duct (ceff ratio). A ceff ratio N1 indicates that a stratospheric duct should be present, causing rays to refract back to the ground. The two most common long-range atmospheric ducts occur in the stratosphere (~ 50 km) and thermosphere (~100 km) (Fig. 3b). Thus, the two most common long-range atmospheric returns are stratospheric and thermospheric, termed Is and It, respectively. Stratospheric arrivals usually have higher amplitudes due to the shorter propagation paths and the higher attenuation levels above ~60 km. Tropospheric (~b10 km height) arrivals are unlikely at distances N250 km. Both Is and It arrivals will likely have different characteristics at the array. Thermospheric returns arrive with higher incidence angles due to refraction at higher altitudes. Trace velocity (apparent horizontal phase velocity) at the array, v, is related to the incidence angle, θ, through v = c/cos(θ), where c is the sound speed. It arrivals should therefore have higher trace velocities. Azimuthal deviations will also vary between Is and It arrivals, as the two propagation paths will experience different horizontal wind translations. Typical celerities for Is phases range between ~0.28 and 0.31 km/s, while It phases have lower celerities between ~ 0.22 and 0.26 km/s due to the longer propagation path through the thermosphere. We perform basic propagation modeling to interpret the remote infrasound recordings. Travel times and propagation paths are estimated using ray theory, a type of geometric model which relies on a high frequency approximation to represent propagation paths as rays. This method is useful for estimating travel times and identifying propagation paths. However, it does not account for diffraction and scattering and thus often incorrectly predicts “shadow zones” where no sound should propagate. We utilize the InfraMAP software program developed and maintained by BBN Technologies (Gibson and Norris, 2002) to run the 3-D Hamiltonian Ray Tracing Program for Acoustic Waves in the Atmosphere (HARPA), modified from Jones et al. (1986). HARPA accounts for horizontal and vertical translation of the acoustic wave by winds. For this study, theoretical rays are launched from a 3 km source height (Redoubt summit) between 10° and 60° at 2° intervals. Ray tracing modeling used here is range-

b)

a) 120

Altitude (km)

100

5

dependent, meaning updated atmospheric variables are used along the propagation path, compared to range-independent modeling where only the source profile is considered. Atmospheric specifications are provided by the Naval Research Laboratory (NRL) ground to space (G2S) models (Drob et al., 2003). The G2S models provide temperature, wind, and atmospheric composition estimates from the earth's surface to 140 km every 6 h at 1° × 1° intervals, and include a recent revision to the upper atmosphere (Drob et al., 2008). 4. Results 4.1. Infrasound array observations and eruption constraints We detected all 19 numbered Redoubt explosive events at I53US and most have a high S/N. Table 1 lists the explosive events and the detection characteristics for DFR and I53US, as well as the plume heights determined from radar measurements (Schneider and Hoblitt, this issue). Fig. 4 shows the beamed, 0.02 to 2 Hz waveforms for the events at I53US, time aligned according to the onset at the array. Coherent time periods are outlined in black, while uncorrelated noise is shown in gray. At I53US, most events have relatively short durations (b15 min) and high amplitudes (N2 Pa peak pressure) considering the distance (547 km). Durations range from 2.8 to 31 min, although the durations (as determined here) are likely overestimates of the source duration due to multi-pathing (multiple ray paths arriving at different times). Peak amplitudes vary from 0.28 to 6.76 Pa and relative acoustic energies range between 0.18 and 470 Pa 2∙ s. Fig. 5 is similar to Fig. 4, except that I53US data are band-pass filtered between 0.005 and 0.02 Hz. The 2009 Redoubt eruption explosive phase consists of four main groups of explosive events, classified solely on the similarity of acoustic characteristics (Table 1, Figs. 4 and 5): (1) Events 1 and 9; (2) Events 2–6, and 8; (3) Events 10–18; and (4) Event 19. Group 1 consists of two events (1 and 9) with relatively long durations (N16 min) and multiple pulses/explosions. Both of these events have low relative acoustic energies (1.37 and 29.7 Pa 2∙s), low ash clouds (b11 km) and no ULP energy. Group 2 comprises Events 2–6 and 8. Events 2–6 occur over a span of 21 h on 23–24 March and have similar acoustic signals. Each event has a relatively long duration (N10 min) and sustained infrasound where the amplitude does not vary significantly during the event. All Group 2 events produced high acoustic energies and ash clouds (N13 km). ULP energy is also clear for Events 2 and 5, and possibly for Events 3, 4, and 6. Event 5

c) c ceff

Zonal Meri GCP

80 60 40 20 0 -100 -50

0

50 100

Velocity (m/s)

300 350 400

0

Sound Speed (m/s) Redoubt

100

200

300

Range (km)

400

500

600

IS53

Fig. 3. Environmental profiles and ray tracing from Redoubt to I53US for Event 13. a) Winds above Redoubt for 28 March 06:00. Zonal winds (solid black line) show a maximum in the stratosphere between 40 and 65 km height and are minor throughout the rest of the profile. Meridional winds (gray line) are weak up to 120 km. Great circle path (GCP) winds (dotted black line) between Redoubt and I53US have a peak in the stratosphere (~40–65 km). b) Sound speed (c, solid black line) above Redoubt shows a typical high-latitude shape. The effective sound speed (ceff, dotted black line) exceeds that of the source at ~ 40 km, and has a broad maximum in the stratosphere, primarily due to the zonal winds. c) Ray tracing from Redoubt to I53US, with the majority of the rays being refracted in the stratosphere. The eigenray (dark black line) is refracted around 45 km height and experiences a single ground reflection around 225 km.

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

4 1

23−Mar 06:35

2

23−Mar 07:01

3

23−Mar 08:14

4

23−Mar 09:38

5

23−Mar 12:30

6

24−Mar 03:40

7

26−Mar 16:34

8

26−Mar 17:24

9

27−Mar 07:47

10

27−Mar 08:28

11

27−Mar 16:38

12

28−Mar 01:34

13

28−Mar 03:24

14

28−Mar 07:19

15

28−Mar 09:19

16

28−Mar 21:40

17

28−Mar 23:29

18

29−Mar 03:23

19

04−Apr 13:57

0

Pressure (Pa)

-4

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

Time past event onset (mins) Fig. 4. I53US 0.02–2 Hz beamed waveforms for all 19 explosive events. Waveforms are aligned relative to the detected onset (0 min), with the next 31 min being plotted. Amplitude scale is the same for all events. Event number is listed on the left and event time denoted on the right. Black waveform portions correspond to segments that meet the Redoubt detection criteria. Most events have relatively short durations (b20 min) and relatively high amplitudes considering the distance. The events are divided into four main groups based on similar infrasound characteristics: 1) Events 1 and 9; 2) Events 2–6, and 8; 3) Events 10–18; and 4) Event 19. Event 7 is only weakly detected.

(23 March 12:30) produced the most acoustic energy of any event and highest amplitude at DFR, clipping the microphone at N173 Pa. This corresponds to N2.1 × 10 6 Pa at 1 m from the vent (assuming spherical spreading), or roughly 20 atm overpressure. After a lull in activity for ~ 15 h, Event 6 (24 March 03:40) begins with an emergent onset and moderate acoustic energy. This event produced an ash cloud greater than 18 km altitude. No infrasound is detected from Redoubt over the next two days. Event 7 (26 March 16:34) is only weakly detected at I53US, consistent with the low pressure amplitudes for this event at DFR (11 Pa). Due to the low S/N and weak detection, Event 7 is not assigned to any group. Event 8 (26 March 17:24) signals the return of explosive activity, with a large amplitude, high energy eruption that produces the highest ash cloud of the eruptive period (18.9 km). Fig. 6 shows the detections for Event 8 at I53US where a) is the beamed and band-pass filtered waveform (0.075– 2 Hz), b) F-stat, c) trace velocity, and d) azimuth. Red circles indicate time segments that meet the detection criteria. Event 8 is characterized by high amplitude infrasound for ~ 10 min, with low amplitude infrasound at the beginning and end of the event. The acoustic onset is emergent and has noticeable ULP energy (Fig. 5). Group 3 consists of Events 10–18, which represent a significant shift in acoustic activity at Redoubt from the first nine events. Events 10–18 have relatively short durations, high acoustic energies,

impulsive onsets, and peak frequencies of ~0.1 Hz. Events 10–15 in particular share similar characteristics, and occur over a span of ~25 h. These six events have high amplitudes (N3.5 Pa), relatively short durations (b10 min), significant ULP energy, and relatively high ash clouds (N12 km). Fig. 2 shows the detections for Event 13 at I53US. The event begins with low amplitude signal for ~2 min, followed by energetic infrasound and very high F-stat for the next ~4 min. Lower level infrasound is detected for another 2 min. These characteristics are similar for Events 10–15. Events 16–18 have some similar characteristics to Events 10–15, primarily duration and frequency content, but are of lower amplitude and energy. Event 16 is the only event of Group 3 without noticeable ULP energy. After Event 18 (29 March 03:23), there is a lull in explosive activity for the next 4.5 days and no large explosive events occur. During this period a new lava dome developed and grew extensively (Bull et al., this issue-a). On 4 April 13:57, the final explosive event began (Event 19Group 4), and consists of two pulses of infrasound. The first eruptive pulse has an emergent onset and lasts for ~ 15 min, producing an ash plume to 15.2 km. The second pulse begins ~2 min after the first and has a similar duration, but higher amplitudes and significant ULP energy (Fig. 5). No infrasound from Redoubt is detected by the remote arrays after Event 19, although some minor signals were recorded at DFR during the effusive phase (McNutt et al., this issue).

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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Time past event onset (mins) Fig. 5. I53US 0.005–0.02 Hz (200–50 s) beamed waveforms for all 19 explosive events. The format is similar to Fig. 4. This frequency band is referred to as Ultra Long Period (ULP) and likely corresponds to relatively long duration oscillations of the volcanic jet or plume. All events with noticeable ULP energy (Events 2,5,8,10–15, and 17–19) also have significant ash plumes (N11 km).

The majority of the explosive events are detected at multiple arrays. Fig. 7 shows the waveforms and detections at all IMS arrays within 4000 km of Redoubt for 28 March 00:00–15:00, corresponding to Events 12–15. The dotted lines indicate acoustic propagation for 0.3 km/s celerity, typical of stratospheric ducting. Waveform segments highlighted in red indicate coherent acoustic detections at the array corresponding to Redoubt azimuths. Amplitudes for each array are normalized by the maximum over the time period. I53US (547 km, 209°) records all four explosive events with high S/N. I56US (2625 km, 314°), NVIAR (3416 km, 327°), and I10CA (3628 km, 310°) clearly record the four events as well. I18DK (3385 km, 285°) records Event 12–14 but not 15, likely due to slightly higher noise levels during this period. I44RU (3041 km, 55°) did not record any of the events during this interval. I57US and I59US did not record any 28 March events, but did record at least one event. 4.2. Propagation modeling and atmospheric profiles First-order propagation modeling and analysis of the relevant atmospheric structure is performed to aid in the interpretation of the remote recordings. Fig. 3 shows the wind, sound speed, and ray tracing modeling at Redoubt for 28 March 06:00, coincident with a high level of explosive activity and the closest atmospheric profile for Events 13–14. The winds are defined as: zonal (east–west, positive

east), meridional (north–south, positive north), and the vector wind component along the great circle path (GCP) between Redoubt and I53US. The GCP winds are added to the sound speed in Eq. (5) to obtain the effective sound speed (ceff). Winds above Redoubt for this time period consist of an easterly zonal wind jet in the stratosphere of ~55 m/s at 60 km. This wind jet is common in the stratosphere, and for northern latitudes, is predominantly positive in the winter (easterly) and negative (westerly) in the summer (Drob et al., 2003). Numerous studies have shown the significant influence of the stratospheric wind jet on global infrasound propagation (e.g. Le Pichon et al., 2009). Meridional winds are mostly minor, with a slight northerly component. The GCP winds from Redoubt to I53US (bearing ~24° N) are mostly positive, with a peak in the stratosphere around 40 km/s at 55 km and thermosphere around 110 km. The sound speed and ceff above Redoubt in Fig. 3b show a typical profile for early spring at high latitudes. Sound speed decreases with height in the troposphere (~0–10 km), slowly increases through the stratosphere (~10–50 km), decreases again in the mesosphere (~50– 90 km), and increases in the thermosphere (N90 km). The ceff profile shows a significant stratospheric duct that begins at ~40 km height, where ceff first exceeds ceff at the source. The ceff ratio for this time and location is ~1.14, indicating a strong stratospheric duct. Of note is the relatively low sound speed at the source (~ 320 km/s), primarily due to the cold temperatures at this latitude and the elevated source

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

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height. Matoza et al. (2011a) discussed how an elevated source at Sarychev Peak (and potentially other volcanoes) decreases the effective sound speed at the source and increases the likelihood of stratospheric ducting. We note that the cold temperatures often present at high latitudes will also decrease the effective sound speed at the source and enhance stratospheric ducting.

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2009/3/28 UTC Hour Fig. 7. Infrasound detections for the Redoubt events on 28 March 2009 00:00–15:00 UTC (Events 12–15). For the array data, red waveform segments indicate coherent energy surpassing the aforementioned detection thresholds. DFR onsets and durations are from McNutt et al. (this issue). Amplitudes are normalized and the dotted lines denote the travel time for an acoustic wave propagating from Redoubt at 0.3 km/s. I53US detects all four events with high S/N. I56US, NVIAR, and I10CA also detect all four events but with lower S/N. I18DK detects Events 12–14 but not 15, likely due to increased noise levels. I44RU does not detect any events due to its location to the west of the volcano. I57US and I59US do not detect any of these events, but detected at least one event on another day. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3c shows numerous rays being turned by the stratospheric duct, with higher launch angle rays either being turned in the thermosphere or not refracting back to the ground. The combination of an easterly zonal wind jet, minor meridional winds, and low sound speed at the surface all contribute to the deep stratospheric duct between Redoubt and I53US for this time period. The ray outlined in Fig. 3c is the ray which best connects the source and receiver within a specified tolerance (5 km), and is thus termed the eigenray. This ray has a turning height of ~45 km, travel time of 1834 s (celerity 0.298 km/s), and a single ground reflection (bounce). The majority of the acoustic energy for Event 13 arrives at I53US at 28 March 03:55:12 (Fig. 2), corresponding to a travel time of 1856 s (calculated celerity 0.295 km/s). The modeled travel of 1834 s (celerity of 0.298 km/s) for the eigenray is consistent with the observed travel time. Faster arrivals for Event 13 are possible from other propagation paths, in particular diffracted arrivals from faster stratospheric returns. The predicted azimuthal deviation at I53US for the eigenray at I53US is − 4.7°, primarily due to the easterly zonal stratospheric winds deflecting the sound. The measured azimuthal deviation for the stratospheric arrivals for Event 13 is − 3° to −5° (Fig. 2d), in good agreement with the model predictions. Atmospheric variables above Redoubt for the explosive phase are shown in Fig. 8, including (a) zonal winds, (b) meridional winds, (c) ceff, and (d) the ceff ratio for propagation to I53US. Explosive events are identified by black arrows above (a). The easterly stratospheric zonal wind jet peaks around 26 March and decreases with time. Meridional winds are mostly minor and slightly positive (northerly) during the study period. A moderate-strong stratospheric duct is present (ceff ratio N1) during the entire explosive period, particularly between 23 March and 3 April. Significant stratospheric ducting of infrasound to I53US and other arrays east of the volcano is therefore predicted for most of the explosive period. Westerly propagation from Redoubt (e.g. I44RU) should be inhibited by the stratospheric jet, with only thermospheric arrivals predicted. Diurnal variations in the thermosphere are due to solar tides (Drob et al., 2003). These results should be fairly typical of wintertime propagation from volcanoes in southwest Alaska and the Aleutian arc. Summertime propagation will be enhanced to the west.

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

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Fig. 8. Atmospheric profiles for the duration of the explosive phase (23 March–6 April) including a) zonal winds, b) meridional winds, c) effective sound speed, and d) ceff ratio for Redoubt to I53US. The ceff ratio is derived by dividing the maximum ceff in the stratosphere (40–70 km) to that at the source to estimate of the strength of the stratospheric duct. Stratospheric zonal winds are moderately strong and blow to the east (positive values) for the entire experiment. Meridional winds are mostly minor with predominantly positive values in the stratosphere (northerly). The effective sound speed and ceff ratio are high for the majority of the explosive phase, and are likely responsible for the strong atmospheric ducting observed at I53US. Black arrows along the top of the figure denote explosive event times.

The detections for 28 March at other IMS arrays (Fig. 7) are also consistent with strong stratospheric ducting to the east. Events 12– 15 are all detected at I53US, I56US, I18DK, NVIAR, and I10CA (highlighted waveform portions in Fig. 7), with the timing consistent with stratospheric arrivals. I44RU is the third closest IMS array (3041 km), but did not detect the 28 March events, likely due to its location to the west of the volcano. These results are consistent for the events during the entire Redoubt explosive phase and illustrate the importance of the stratospheric duct. Trace velocities for numerous events at I53US show a transition from moderate (~ 0.33 km/s) values at the onset to high values (~0.4 km/s) later in the wave train. Event 8 infrasound and detections are shown in Fig. 6. Trace velocities for the onset and majority of the event lie between 0.33 and 0.35 km/s (Fig. 6c). However, trace velocities for the final ~ 2 min of the event transition to 0.4 km/s, likely indicating the presence of thermospheric arrivals. Ray tracing for this event predicts primarily stratospheric arrivals (similar to that for Event 13). However, thermospheric arrivals with longer propagation paths and travel times are also predicted. Thermospheric arrivals should produce lower signal levels due to increased attenuation from longer propagation paths and higher absorption in the thermosphere (Sutherland and Bass, 2004), which is consistent with the observations in Fig. 6a. Further, azimuthal deviations should also differ for thermospheric arrivals due to the thermospheric acoustic energy encountering different winds. Estimated azimuths (and hence azimuthal deviations) for the end of Event 8 detections differ from the earlier, likely stratospheric arrivals (Fig. 6d). The observation of trace velocities transitioning from moderate to high values was also observed at I53US for signals from the 2006 Augustine eruption (Wilson et al., 2006), and was interpreted to represent the transition from stratospheric to thermospheric arrivals. Similar observations were also made for the 2009 Sarychev Peak eruption (Matoza et al., 2011a). Note multiple ducted arrivals will increase the measured duration estimate. 4.3. Local and remote infrasound data comparison Nearly all major features of the DFR waveform (and hence acoustic source) are apparent at I53US, including the impulsive signal onset.

This is not a common occurrence given the distance between source (DFR) and I53US (e.g. Herrin et al., 2008). Fig. 9 displays the 0.075– 2 Hz filtered waveforms and PSD estimates for DFR (red) and I53US (black) for Event 13 (28 March 03:24:18). The I53US waveforms have been time-aligned and beamed to permit comparison. The Event 13 infrasound signal consists of two pulses of activity, the first lasting ~ 1.5 min and peaking about 1 min after the onset (~138 Pa at DFR, 4.2 Pa at I53US). The second infrasound pulse is of lower energy and lasts ~ 3 min. A PSD comparison (Fig. 9c) shows similar frequency content between both stations, consisting of a single broad peak frequency at ~0.1 Hz with a gradual roll-off at higher frequencies. The spectral shapes differ above ~3 Hz, where higher frequency source energy likely experiences greater attenuation along the propagation path (Sutherland and Bass, 2004). Note the DFR microphone response begins to roll-off below 0.1 Hz, so comparisons below this frequency must be taken with care. The energy for this event is well above the median IMS noise model (Bowman et al., 2009) at I53US above 2 Hz, and well above the high noise model up to 7 Hz, illustrating the high S/N of the event. Cross-correlation between the waveforms is now presented to quantitatively examine the similarity between DFR and I53US. Fig. 10a shows the time-aligned, filtered DFR (red) and I53US (black) waveforms from Event 13. The cross-correlation coefficient for this segment is 0.83, a high value considering the distance between the stations. Note although the cross-correlation is relatively high, the waveforms appear out of phase. Geometric acoustics theory predicts that as closely spaced propagating rays progressively refract, they will eventually focus into regions where they intersect, termed “caustics” (Fig. 11a). Here some of the assumptions of ray theory are no longer valid. The wave passing through the caustic will undergo a 90° phase shift, equivalent to taking the imaginary part of the Hilbert transform of the signal (Fig. 11). Geometric acoustics states that a wave will encounter a caustic after each ground bounce (reflection) and its subsequent turning point (Kinney and Pierce, 1980). Although primarily theoretical, phase shifts due to acoustic waves passing through caustics have also been confirmed by experiment (Talmadge et al., 2008). Propagation from Redoubt to I53US is predicted to encounter a single ground bounce at ~ 225 km (Figs. 3c, 11), corresponding to passage through a single caustic after the

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

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Fig. 9. Event 13 local and remote infrasound comparison. Waveforms for a) DFR (12 km, red) and b) I53US (547 km, black) show very similar features. c) PSD comparison for the event shows a single, broad frequency peak at ~ 0.1 Hz and roll-off at higher frequencies for both stations. The spectral shape is similar between the two stations except for at higher frequencies where absorption is greater for I53US. The dashed spectra in c) represent the high, median, and low noise models of the IMS network (Bowman et al., 2009). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ground reflection (and subsequent 90° phase shift). We Hilbert transform the I53US waveform and repeat cross-correlation with DFR in Fig. 10b. The cross-correlation value increases to 0.89, and visual inspection reveals all principal DFR waveform features (and phases) are now apparent at I53US. Note that high waveform similarity is uncommon in long range infrasound propagation, and likely occurs due to the deep stratospheric duct and only one ground bounce. Increased attenuation is also predicted due to the ground bounce, although this effect is relatively minor at low frequencies (Attenborough et al., 2006). Propagation to the other arrays will encounter numerous ground bounces and increased multi-pathing, leading to much lower waveform correlations at greater distances. Waveform similarity varies considerably from event to event (Table 1). However, even when correlation values are relatively low, principal source features are still apparent at I53US. Fig. 12 shows the aligned DFR (red) and I53US (black) waveforms and PSD for Event 8 (26 March 17:24). Although the cross-correlation value is only 0.54, the main source waveform features are apparent at I53US, particularly the acoustic energy arriving as a function of time. PSD comparisons for the explosive events show similar energy distribution as well. The lower cross-correlation value is likely due to a more complex, extended source waveform. 4.4. Acoustic energy and hazardous emissions In this section we compare the infrasound energy with satellite derived SO2 mass, and use the acoustic energy integral as a proxy for the eruptive vigor. Previous studies have shown a broad correlation between infrasound energy and ash cloud height for continuous emissions (Fee et al., 2010a; Steffke et al., 2010); however, the shorter duration explosive events here do not permit a similar detailed correlation, as the satellite sampling interval is often greater than the event duration. We do note that all events with significant low frequency infrasound (b0.5 Hz) produced hazardous ash clouds, consistent with previous infrasound studies of large eruptions (Fee et al., 2010a; Fee et al., 2010b; Steffke et al., 2010). McNutt et al. (this issue) provide a detailed look at the relationship between infrasound parameters at DFR versus plume heights and seismic data, and similar correlations using the remote infrasound data should yield comparable results due to the high waveform similarity for most of the events.

Daily SO2 mass estimates presented here are made using data from the Ozone Monitoring Instrument (OMI). Due to OMI's temporal resolution (~1–3 passes per 24 h at high latitudes) it is difficult to differentiate the SO2 release between the relatively short duration explosive events, thus the SO2 mass produced by Redoubt is estimated on a daily basis. All SO2 estimates used here are from Lopez et al. (this issue), and the reader is referred to that manuscript for a more detailed explanation of the SO2 estimation methods and interpretations. Relative acoustic energies at I53US are calculated on an hourly basis following the methods outlined in Section 3.2. There is a high correlation between the cumulative amounts of infrasound energy and SO2 produced during the 2009 Redoubt eruption explosive phase between 22 March and 6 April. Fig. 13 shows the cumulative daily SO2 mass estimates (red) and cumulative relative infrasound energy (Ear—black) for the explosive phase. Black arrows at the top of the figure indicate numbered explosive events. The relative amounts of daily SO2 mass detected agree well with the Ear recorded at I53US. Events 2–5 produced extensive infrasound on 23 March (total Ear = 1357 Pa 2∙s), compared to the ~ 54 kilotonnes (kt) of SO2. A similar amount of 60 kt SO2 is detected the next day, presumably erupted mostly during Event 6 (23 March 03:40, Ear = 186.7 Pa 2∙s). Note the amount of measured SO2 for this day is greater than for the previous day. Rather than being a more SO2 rich eruption, it is likely that some of the measured SO2 from 24 March is residual SO2 from the previous day, and essentially “double counted”, as the shape of the SO2 plume on 24 March is similar to 23 March and the bulk of the plume is far from Redoubt (Lopez et al., this issue). After a lull in activity over the next day and a half in which no infrasound energy or SO2 emissions were detected, 12 explosive events occurred between 26 March 16:34 and ~29 March 04:00. Event 7 is quite small (Ear = 0.18 Pa 2∙ s, plume height 6.7 km) and likely did not produce significant emissions. Events 8–15 have relatively high acoustic energies, with events 10–15 having similar characteristics (Section 4.1). Overall relatively similar amounts of cumulative SO2 (~ 72 kt) and Ear (~806 Pa 2∙s) were generated by Events 8–15. Similarly, Events 16– 18 (28 March 21:40–29 March 03:23) produced low (and comparable) amounts of both acoustic energy (~17 Pa 2∙s) and SO2 (b200 t on 29 March). Minor but significant amounts of SO2 (~1.5–5 kt) were detected between 30 March and 4 April, coincident with low Ear and lava dome growth (Bull et al., this issue-a). The final explosive event

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

D. Fee et al. / Journal of Volcanology and Geothermal Research xxx (2011) xxx–xxx

a) Event 13, 28−Mar−2009 03:24:26, Corr= 0.83 1 Local I53

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on 4 April (Event 19) produced Ear = 179.5 Pa 2∙s and ~24 kt of SO2, relatively similar amounts. Variations in the amount of acoustic energy ducted in the stratosphere between events may contribute to some of the disparity between the two datasets. To quantitatively assess the correlation between the cumulative infrasound energy and SO2 mass, we apply the Spearman Rank Correlation Test (SRCT) (Freund and Williams, 1966). The SRCT provides a non-parametric, statistical means of assessing the relationship between two quantities. It produces a correlation coefficient, r, that varies between ±1, with these maxima corresponding to either a perfect negative or positive correlation, respectively, while r = 0 indicates no correlation. The SRCT is a similar measure to the well-known Pearson correlation, but is applicable to quantities where the fit may be nonlinear or non-algebraic (Freund and Williams, 1966). Because the two datasets here are unevenly sampled (daily SO2 mass vs. hourly Ear), we resample the cumulative infrasound energy by selecting the first value after the SO2 estimate (black circles in Fig. 13). The time

Typical infrasound signals from vulcanian eruptions are characterized by high-amplitude, impulsive onsets, and durations on the order of seconds to minutes (e.g. Marchetti et al., 2009; Yokoo et al., 2009; Fee et al., 2010a). Vulcanian eruptions are often assumed to be a result of the explosive failure of a “capped” conduit (lava plug) and/or the interaction of magma and external water (Self et al., 1979; Morrissey and Mastin, 2000). Numerous Redoubt events could be classified as vulcanian according to their infrasound signatures, particularly Group 2 (Events 10–18). However, many of the other events have emergent onsets, with the peak pressures occurring tens of seconds after the onset (Fig. 4, Table 1). Petersen et al. (2006) and McNutt et al. (2010) noted similar emergent “vulcanian” events at Augustine Volcano in 2006. They attributed the emergent onset as being produced by one of two scenarios: 1) the relatively rapid rise of gas-rich magma with a uniform distribution, creating a steady gas release throughout the eruption; or 2) slow ascent and release of gas through a “leaky” system, most likely to occur after a few days of inactivity (and dome growth). This second scenario might release gas too slowly to produce the substantial infrasound present for the majority of the events. A third scenario involving the coalescence of a large amount of gas under a sealed cap was presented as causing the short duration, impulsive signals more characteristic of vulcanian eruptions, and appears plausible for the more impulsive events here as well. As outlined in Section 4.1, the 19 explosive events at Redoubt can be divided into four main groups based on infrasound characteristics (Table 1, Figs. 4–5): 1) Events 1 and 9; 2) Events 2–6, and 8; 3) Events 10–18; and 4) Event 19. Group 1 has similar acoustic characteristics to many strombolian eruptions (e.g. Marchetti et al., 2009), with multiple pulses of short-duration, impulsive infrasound. The higher acoustic energy and duration for Event 9 make it more similar to some vulcanian eruptions. Subplinian eruptions typically have highenergy, broadband, sustained infrasound signals (e.g. Caplan-Auerbach and McNutt, 2003; Fee et al., 2010a). Due to the extended duration and lack of impulsive onset for many of the Group 2 events, we classify them somewhere between vulcanian and subplinian. Events 10–18 (Group 3) all produced similar infrasound signals, with Events 16–18 having much less energy. These events have relatively short durations (b10 min), high acoustic energies, peak frequencies of ~0.1 Hz, relatively impulsive onsets, and ULP energy for the larger events, and are thus most similar to vulcanian eruptions. These events also show similar, distinguishable characteristics in other datasets. Wallace et al. (this issue) find that the tephra from Events 9–18 are distinct from other events as they are all very finegrained, and Coombs et al.(this issue) also find a change in lithology starting with Event 9. Hotovec et al. (this issue) show how these events have characteristic gliding tremor preceding the explosive events not found during the rest of the eruption. A change to a more viscous magma after Events 8–9 may be responsible for the shift in character, as this could have created a more defined magma plug that would inhibit degassing and allow higher pressure to build within the shallow conduit. After a critical pressure threshold

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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Fig. 11. Ray tracing and passage through a single caustic. a) Five closely spaced rays (0.05° spacing) launched from Redoubt for 28 March 06:00. The rays with higher launch angles refract at slightly higher elevations and thus reach the ground at longer distances. After the first ground reflection, the rays intersect and pass through a caustic around 350 km distance, highlighted in b). Dotted box in a) highlights the geographic grid in b).

jet noise (Tam et al., 2008; Matoza et al., 2009). Interactions of the jet with the volcanic crater may also be important in controlling the infrasound produced (Fee et al., 2010a). Note that volcanic jetting does not require an impulsive source. The impulsive onset for some of the events may be due to the explosive outburst of highly overpressurized material at the vent more typical of vulcanian explosions. Due to the amount and duration of acoustic energy produced by the majority of the explosive events, and the acoustic source likely being a high-velocity jet, we interpret Events 2, 8 and 19 to have destroyed the existing lava dome. After lava dome growth for the preceding 10 h (Bull et al., this issue-a), Event 1 occurs with several relatively low amplitude pulses of infrasound (b25 Pa at DFR, b0.2 Pa at I53US) (Fig. 4) and relatively little ash and lightning (Behnke et al., this issue; McNutt et al., this issue). Event 2 begins just minutes later with a strong, impulsive onset (151 Pa at DFR, 1.2 Pa at I53US) followed by N3 min of jetting (Fig. 4). Extensive lightning and ash

is exceeded, the vulcanian explosion would occur producing the impulsive acoustic onset, short duration, and fine-grained deposits. Event 19 is notably different from the other events, as it has a long duration and two main pulses of activity with emergent onsets, and can best be classified as subplinian. We interpret the majority of the high-amplitude infrasound from Redoubt above 0.02 Hz as the product of volcanic jetting in the gasthrust region of the plume. Recent work has proposed that the high-amplitude, continuous infrasound from sustained explosive eruptions results from the turbulence structures associated with high-velocity volcanic jets, primarily based on the acoustic frequency spectra (e.g. Matoza et al., 2009; Fee et al., 2010a). The majority of the Redoubt explosive events have high-amplitude signals with durations greater than a few minutes, therefore requiring a sustained source. Further, the spectra consist of a single broad spectral peak rolling off at higher frequencies (Figs. 9c, 12c), similar to that predicted for

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Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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UTC Date Fig. 13. Cumulative relative acoustic energy (black) and SO2 mass (red) comparison for the Redoubt explosive phase. A high correlation exists between the relative amounts of acoustic energy detected at I53US and SO2 measured by OMI. SO2 is calculated on a daily basis, while the relative acoustic energy is calculated hourly, with the black circles indicating the hourly acoustic energy value nearest in time to the SO2 estimate. Black arrows at the top of the figure denote the explosive event times. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

were also detected (McNutt et al., this issue). It is unlikely a lava dome could have survived this type of explosive event. The impulsive acoustic onset likely represents the destruction of the dome, followed by sustained jetting. The second dome grew between Events 6–7 (Bull et al., this issue-a), and may have survived the relatively minor Event 7. Event 8 produced the highest ash plume (18.9 km) and extensive infrasound (Figs. 6 and 12) and thus likely destroyed the dome. Event 19 occurs after ~ 6 days of lava dome growth (Bull et al., this issue-a). The event begins with an emergent, relatively high amplitude pulse of infrasound for ~ 15 min, followed by a more energetic pulse of a similar duration but greater ULP energy (Fig. 5). The emergent onset in the infrasound and seismic data (Buurman et al., this issue) suggest that the dome may have failed progressively (e.g. Carn et al., 2004), rather than being destroyed by an impulsive, conduit-related explosion. 5.2. ULP infrasound Numerous Redoubt explosive events produced significant ULP infrasound with dominant periods of ~ 120 s (Fig. 5). All ULP infrasound signals from Redoubt are coincident with significant ash plumes (Table 1—Events 2, 5, 8, 10–15, 17–19), indicating their importance for eruption monitoring and hazard mitigation. Ripepe et al. (2010) observed ULP infrasound from the 28 July and 3 December 2008 eruptions of Soufrière Hills Volcano, Montserrat, and attributed the signals to the initial oscillations of the rising ash plume after a lava dome collapse. The ULP oscillations detected here occur primarily within the first 5 min of the eruption and coincide with extensive ash plumes (Fig. 5), thus they may similarly be produced by the initial rise and oscillation of the plume with periods of ~ 120 s. Longduration pulsations of the volcanic jet may also produce the ULP signal. Most volcanic jetting signals have been associated with higher frequency infrasound (N0.1 Hz) (Matoza et al., 2009; Fee et al., 2010a; Fee et al., 2010b), but pulsations of the jet (not the turbulent structures within them) have not been considered. High-resolution imaging or sampling of the volcanic plume and jet is necessary to confirm either theory, but is unavailable for Redoubt. Longer period pressure oscillations from volcanoes (e.g. N200 s, termed acoustic-gravity waves) may be produced by excitation of the atmosphere by the large amount of thermal energy generated by the eruption (Kanamori et al., 1994; De Angelis et al., 2001). These acoustic-gravity waves propagate near the surface at the local

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sound speed. We do not detect any pressure perturbations N200 s associated with Redoubt, but are somewhat limited by the sensor response. Future studies of large volcanic eruptions should employ microphones with flat responses down to N200 s to record ULP and acoustic-gravity waves. 5.3. Infrasound and hazardous emissions Low-latency, high-resolution detection and quantification of volcanic emissions is difficult due to a number of factors, but is primarily limited by the relatively low temporal and spatial resolutions of satellite imagery (e.g. Prata, 2009). Detection of volcanic ash is primarily based on the thermal contrast between the volcanic cloud and the earth's surface. This method is largely hindered by three factors: 1) opaque ash clouds, 2) the ash particles becoming coated with ice (icing) (e.g. Rose et al., 1995), and 3) not enough thermal contrast between the ash cloud and the background (Carn et al., 2009). Ash cloud heights are often difficult to determine as well (Prata, 2009). Because of these difficulties, SO2, which poses its own risk to aviation, can be used to track hazardous plumes that may contain volcanic ash. Although SO2 is often considered easier to detect and track than ash, satellite detection of SO2 is currently limited by the instrument sensitivity to SO2 as well as the spatial and temporal resolution of the satellite sensor employed (Carn et al., 2009). In this paper we show that significant low-frequency acoustic energy is indicative of hazardous emissions. Section 4.1 demonstrates how all significant Redoubt emissions coincided with high amplitude infrasound below 0.5 Hz, including considerable ULP energy for the largest eruptions. Additionally, the strong correlation between infrasound and detected SO2 at Redoubt (Section 4.4) reiterates the utility of remote infrasound arrays in constraining volcanic eruptions, supplementing existing eruption monitoring and detection systems, and providing input for volcanic ash transport and dispersion models. Infrasound is able to detect eruptions in real-time (plus propagation and minor processing time), and is not hampered by weather clouds or darkness. However, as shown in this study, a thorough understanding of atmospheric structure and infrasound propagation is necessary before remote infrasound can be fully utilized. Propagation time from the volcano to remote arrays may also be significant and hamper low-latency detection and characterization, as an acoustic wave will take ~ 25–30 min to travel 500 km. Local noise levels are also important to consider (Matoza et al., 2011b). The actual relationship between SO2 production and infrasound energy is currently not well understood. Quantification of degassing rates and infrasound has been examined for low-level Strombolian activity at Pacaya Volcano (Dalton et al., 2010). They found degassing estimates from a UV camera agreed to an order of magnitude with the infrasound-derived SO2 estimates, but longer term degassing rates did not agree well. 5.4. Remote infrasound source term estimation This study, in conjunction with other recent work, has shown how infrasound arrays at distances b700 km can provide high-quality source estimates and event chronologies. Matoza et al. (2011a) were able to derive the most detailed eruption chronology for the remote 2009 Sarychev Peak eruption from high S/N infrasound signals at 644 km, and this study demonstrates how the I53US data at 547 km provides an accurate representation of the acoustic source. However, detailed comparison of I53US and DFR infrasound data demonstrate how phase identification and inferences on source processes from long range infrasound data should be taken with care, as even relatively simple propagation paths perturb the signal. For example, passage through caustics will affect the phase of the signal. However, knowledge of the propagation path between the source and receiver (strong stratospheric ducting with a single ground

Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012

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reflection for Redoubt and I53US) allows straightforward determination of the likely source waveform by application of a single Hilbert transform. The principal propagation path (e.g. stratospheric with 1–2 ground reflections) can be estimated with reasonable confidence for source–receiver distances ~b700 km, but becomes increasingly convoluted and uncertain at greater distances. Longer and more complex propagation paths will complicate remote phase identification. Source duration estimates from remote data will also likely be overestimated by multi-pathing. One potential solution is to utilize relative changes in trace velocity (Section 4.2) to differentiate between stratospheric and thermospheric arrivals and reduce duration overestimation by disregarding the thermospheric portion. It is also worth noting that remote infrasound arrays often have lower noise levels than local microphones due to more favorable site location within dense tree cover and lower wind noise, which may permit more accurate source estimation. Detailed propagation modeling and signal identification may also assist with source characterization. 6. Conclusions The explosive phase of the 2009 Redoubt eruption produced significant infrasound recorded at a local microphone and numerous global infrasound arrays. Most explosive events have relatively short durations (b20 min) and high ash plumes (N11 km). We divide the events into four groups based on similar acoustic characteristics. Two main groups of events exist, with the first consisting of N10 min duration, high-energy eruptions of sustained infrasound and the second being shorter duration, impulsive events more typical of vulcanian eruptions. The change in acoustic signature occurs after Event 9. Comparison of the local and remote data shows that the remote infrasound data at I53US strongly resembles the local data and provides a very good measure of the acoustic source. The high waveform integrity at I53US is due to the strong stratospheric duct present during the study period. This duct was caused by moderately strong easterly zonal winds in the stratosphere, an elevated acoustic source (the volcanic vent at N3 km), and cold temperatures at the surface due to the high latitude. Cross-correlation analysis reveals that the sound wave between Redoubt and I53US likely passed through a single caustic, as evidenced by the highest cross-correlation values and best phase alignment of the data observed following application of a Hilbert transform. This result is consistent with basic ray tracing predicting a single ground bounce between Redoubt and I53US. Similar to previous studies, all significant Redoubt explosive events produced extensive low frequency infrasound (b0.5 Hz). ULP infrasound is also apparent for all events with major ash plumes, possibly related to oscillations of the volcanic jet or plume during the first few minutes of the event. Comparison between the cumulative acoustic energy and satellite-derived SO2 mass from Redoubt show a strong, statistically significant correlation. The presence of significant SO2 in the atmosphere is commonly used to infer that a magmatic eruption and hazardous emissions have occurred. Here we show that significant low frequency acoustic energy is also indicative of hazardous emissions and SO2, further demonstrating the potential to use infrasound as a proxy for gas and ash emissions hazardous to aviation. Future monitoring and detection of volcanic eruptions using infrasound may be improved by incorporating well-known climatological and forecast models of atmospheric structure to predict propagation to remote infrasound arrays. Near real-time, highresolution G2S models could also be used to estimate the amount of ducting and predicted waveform integrity. Acknowledgments This manuscript was greatly improved through conversations with Kate Bull, as well as numerous other members of AVO including

Michael West and Helena Buurman. Roger Waxler graciously helped with some of the propagation interpretations. Robin Matoza provided a helpful early review. Doug Drob provided the invaluable G2S models and Paul Golden of Southern Methodist University provided the NVIAR data. Reviews by Stephanie Prejean and an anonymous reviewer greatly improved the manuscript. Funding was provided by NSF grant EAR-1113294 and the Geophysical Institute. References Arnoult, K.M., Olson, J.V., Szuberla, C.A.L., McNutt, S.R., Garces, M.A., Fee, D., Hedlin, M.A.H., 2010. Infrasound observations of the 2008 explosive eruptions of Okmok and Kasatochi volcanoes, Alaska. Journal of Geophysical Research-Atmospheres 115, D00L15. doi:10.1029/2010jd013987. Attenborough, K., Li, K.M., Horoshenkov, K., 2006. Predicting Outdoor Sound. Spon, London. Behnke, S.A., Thomas, R.J., McNutt, S.R., Krehbiel, P.R., Rison, W. and Edens, H.E., this issue. 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Please cite this article as: Fee, D., et al., Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption, J. Volcanol. Geotherm. Res. (2011), doi:10.1016/j.jvolgeores.2011.09.012