Journal of Volcanology and Geothermal Research 259 (2013) 31–44

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Journal of Volcanology and Geothermal Research

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Seismic observations of Redoubt Volcano, Alaska — 1989–2010 and a conceptualmodel of the Redoubt magmatic system

J.A. Power a,⁎, S.D. Stihler b, B.A. Chouet c, M.M. Haney a, D.M. Ketner a

a Alaska Volcano Observatory, U.S. Geological Survey, Volcano Science Center, 4210 University Drive, Anchorage, AK 99508, USAb Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775, USAc U.S. Geological Survey, Volcano Science Center, 345 Middlefield Rd., Menlo Park, CA 94025, USA

⁎ Corresponding author.E-mail address: jpower@usgs.gov (J.A. Power).

0377-0273/$ – see front matter. Published by Elsevier Bhttp://dx.doi.org/10.1016/j.jvolgeores.2012.09.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 June 2011Accepted 22 September 2012Available online 28 September 2012

Keywords:Redoubt VolcanoDeep Long-Period EventMagmatic System ModelEruption Forecasting

Seismic activity at Redoubt Volcano, Alaska, has been closely monitored since 1989 by a network of five to tenseismometers within 22 km of the volcano's summit. Major eruptions occurred in 1989–1990 and 2009 andwere characterized by large volcanic explosions, episodes of lava dome growth and failure, pyroclastic flows,and lahars.Seismic features of the 1989–1990 eruption were 1) weak precursory tremor and a short, 23-hour-long, in-tense swarm of repetitive shallow long-period (LP) events centered 1.4 km below the crater floor, 2) shallowvolcano-tectonic (VT) and hybrid earthquakes that separated early episodes of dome growth, 3) 13 additionalswarms of LP events at shallow depths precursory to many of the 25 explosions that occurred over the morethan 128 day duration of eruptive activity, and 4) a persistent cluster of VT earthquakes at 6 to 9 km depth.In contrast the 2009 eruption was preceded by a pronounced increase in deep-LP (DLP) events at lower crust-al depths (25 to 38 km) that began in mid-December 2008, two months of discontinuous shallow volcanictremor that started on January 23, 2009, a strong phreatic explosion on March 15, and a 58-hour-longswarm of repetitive shallow LP events. The 2009 eruption consisted of at least 23 major explosions betweenMarch 23 and April 5, again accompanied by shallow VT earthquakes, several episodes of shallow repetitiveLP events and dome growth continuing until mid July. Increased VT earthquakes at 4 to 9 km depth beganslowly in early April, possibly defining a mid-crustal magma source zone.Magmatic processes associated with the 2009 eruption seismically activated the same portions of theRedoubt magmatic system as the 1989–1990 eruption, although the time scales and intensity vary consider-ably among the two eruptions. The occurrence of precursory DLP events suggests that the 2009 eruption mayhave involved the rise of magma from lower crustal depths. Based on the evolution of seismicity during the1989–1990 and 2009 eruptions the Redoubt magmatic system is envisioned to consist of a shallow system ofcracks extending 1 to 2 km below the crater floor, a magma storage or source region at roughly 3 to 9 kmdepth, and a diffuse magma source region at 25 to 38 km depth. Close tracking of seismic activity allowedthe Alaska Volcano Observatory to successfully issue warnings prior to many of the hazardous explosiveevents that occurred in 2009.

Published by Elsevier B.V.

1. Introduction

During the spring of 2009 Redoubt Volcano erupted for the secondtime since close seismic monitoring of the volcano began in 1989. The2009 eruption was preceded by almost 10 months of increasingunrest that included ground deformation, increased gas and heatflow, volcanic tremor, and swarms of shallow seismic events. Themagmatic phase of the eruption began on March 22, 2009 with theextrusion of a short-lived lava dome and was characterized by theextrusion of lava and large magmatic explosions that produced ashplumes that reached altitudes of 14 km and spread ash over much

.V.

of south central Alaska (Wallace et al., 2013). The final explosiveevent occurred on April 5, 2009 and the lava dome continued togrow until July 2009 (Bull and Buurman, 2013).

Rudimentary seismic monitoring of Redoubt Volcano began in1971 and since 1989 the Alaska Volcano Observatory (AVO) hasoperated a network of 5 to 10 permanent seismic sensors surround-ing the volcano. In 2009 the permanent network was augmentedwith four broadband seismometers that recorded on site. Eruptive se-quences are characterized seismically by repetitive swarms of shal-low Long-Period (LP) events, hybrid events, Volcano-Tectonic (VT)earthquakes, explosion signals and volcanic tremor (Chouet et al.,1994; Lahr et al., 1994; Power et al., 1994; Stephens et al., 1994).The inter-eruptive period is characterized by persistent VT earth-quake activity at 3 to 8 km depth and high levels of shallow seismic

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activity, much of which is likely associated with ice movement on theheavily glaciated Redoubt cone.

In this paper we develop a general description of the seismicityobserved at Redoubt Volcano between October 1989 and December2010 that is based principally on earthquake hypocenters and seismicwaveform character. First we review seismic instrumentation, dataacquisition, and standard data analysis used by AVO during this peri-od. We then develop a more detailed seismic chronology for the peri-od between July 2008 and January 2010 that spans the 2009 eruption.In this chronology we include information on the forecasts and publicwarnings issued by AVO through the aviation color code (Gardnerand Guffanti, 2006). This chronology is based on earthquake hypo-centers, helicorder records, Real-Time Seismic Amplitude Measure-ments (RSAM) (Endo and Murray, 1991), the durations of explosiveevents, and spectral measurements. Many of these techniques areidentical to those used to describe the 1989–1990 eruption and aredeveloped here to provide a direct comparison of the seismicityassociated with the two eruptive sequences. We then offer a volcano-logical interpretation of the long-term patterns and temporal devel-opment of the volcanically-induced seismic activity and concludewith a review of the role that these basic seismic parameters playedin efforts to forecast the 2009 eruption. Additional analyses of theseismicity associated with the 2009 eruption sequence are presentedby Buurman et al. (2013), Haney et al. (2013), Ketner and Power(2013) and Hotovec et al. (2013).

2. Instrumentation and data analysis

Following the establishment of AVO in 1988 stations DFR, NCT,and RDN were added to stations RDT and RED which had beeninstalled in 1971 and 1974, respectively, to form the Redoubt seismicnetwork (Fig. 1). Station DRE was installed on February 7, 1990 totrack lahars in the Drift River Valley and it operated until September7, 1990. Stations REF, RWS, and RSO were added to the network inMarch of 1990 to improve the network's capability to track shallowseismicity associated with the 1989–1990 eruption. In September1990 station RDW replaced station RWS. In 1995 station RDW waspartially abandoned, however the antenna mast and electronicsenclosure were left on site so that the station could be reoccupiedrapidly should Redoubt reawaken.

The Redoubt network operated in this configuration until the fallof 2008. Unfortunately the network experienced numerous stationoutages between roughly 1995 and 2008 that were principally causedby difficulties in receiving FM radio signals on the Kenai Peninsula.The time periods that individual stations were used to determine

Fig. 1. Locations of seismic stations that operated surrounding Redoubt Volcano between1989 and 2010. Triangles correspond to short-period vertical instrumentswhile hexagonsreflect 3-component broadband instruments. Contours reflect 3000, 6000, and 9000 footelevations.

P-arrival times are summarized in the annual AVO earthquake cata-logs and provide some indication of when individual stations wereoperational. The most recent AVO earthquake catalog was publishedby Dixon et al. (2011).

In response to increasing volcanic unrest, digital three-componentbroadband seismometers were installed at RDJH and RDWB on Febru-ary 2 and February 4, 2009, respectively. These stations used digitalspread spectrum radios to transmit signals to AVO offices in Anchor-age. On March 21, 2009, three-component broadband sensors weredeployed at stations RD01 (collocated with REF), RD02, RD03 andRDW, and operated until June 9, June 10, June 10, and July 1, respec-tively. These stations used Guralp 6-TD seismometers (0.02–30 s)that recorded data on site at 50 samples per second. To monitor laharsin the Drift River a short-period seismometer was operated at RDEfrom February 4 until July 1, 2009 when it was replaced by stationRDDR (Fig. 1). Locations and periods of operation of the various seis-mic instruments operated at Redoubt during the 2009 eruption aresummarized by Dixon et al. (2010) and Ketner and Power (2013).

2.1. Data acquisition and processing

FromOctober 12, 1989 to March 1, 2002, a personal computer (PC/AT based) system was used to digitize and detect seismic events. Thissystem used the programMDETECT from October 12, 1989 to January1, 1994 when the acquisition program was switched to XDETECT (Leeet al., 1988). XDECTECT remained in operation until it was replacedby an Earthworm system (Johnson et al., 1995) on March 1, 2002.The Earthworm system remained in operation throughout the re-mainder of the study period. The Earthworm system was determinedto be more effective at detecting events in a volcanic environmentthan the earlier programs and more events were located followingits adoption than in earlier years (Dixon et al., 2004).

Phase arrivals andmaximumwaveform amplitudes and periods weredetermined using the programXPICK (Robinson, 1990). Hypocenters andlocal magnitudes were determined using the program HYPOELLIPSE(Lahr, 1999) with a flat layered velocity model and station correctionsto account for local variations in seismic wave velocities. The velocitymodel and station corrections are listed in Tables 1 and 2 respectively.The model was configured in a similar manner to that described by Lahret al. (1994), andwas developed to locate earthquakes at Redoubt follow-ing the 1989–1990 eruption. In this paper we have allowed hypocentersto range as high as 3 km above sea level, which roughly correspondsto the elevation of the Redoubt summit. In this system, hypocentraldepth is referenced to sea level and negative depth refers to elevationabove sea level. Earthquake locations and other seismic events reportedin this paper are referenced to Universal Time (UT). To convert toor Alaska Daylight Time (AKDT) and Alaska Standard Time (AKST) sub-tract 8 or 9 hours, respectively. Improved earthquake hypocenters usingthree-dimensional velocity models and improved phase arrivals for vari-ous subsets of located events between 1989 and 2010 are presented byBenz et al. (1996) and Deshon et al. (2007).

In addition to event detected data we also recorded data from theRedoubt network in a variety of continuous formats. A separate RSAMacquisition system was operated from October 1989 through 1998(March and Power, 1990). With the implementation of the Earthworm

Table 1One-dimensional velocity model used to locate earthquakes at Redoubt Volcano (Lahret al., 1994).

Layer number Vp(km/s)

Top of layer Vp/Vs

1 2.90 −3.00 1.812 5.10 −1.70 1.803 6.40 1.5 1.724 7.00 17.0 1.78

Table 2Station corrections used to locate earthquakes at Redoubt Volcano in this study.

Station P-delay(s)

S Delay(s)

ILI 0.44 0.78ILM 0.44 0.78ILN 0.44 0.78RDT 0.00 −0.02RDW 0.00 −0.02RED −0.02 −0.05

Fig. 2. Map (A) and east–west cross section (B) showing earthquake hypocenterscalculated at Redoubt Volcano between 1989 and 2010. All calculated hypocentersare shown regardless of quality. Hypocenters classified as deep long-period (DLP)events are shown as shaded diamonds.

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system, continuous data for AVO stations were archived on DVDsbetween September 10, 2002 and December 31, 2010. Starting onApril 25, 2005 AVO seismic data were stored on a Winston waveserver(Cervelli et al., 2004) operating in Anchorage. Continuous data forRedoubt stations were also archived at the Incorporated Research Insti-tutions for Seismology (IRIS) beginning on December 13, 2008, and alsoon an Antelope data base at the Alaska Earthquake Information Centerin Fairbanks beginning in 1995. An automated alarm system was alsoused to track seismic events during the 2009 eruption (Thompson andWest, 2009).

3. Redoubt seismicity

In describing the seismicity at Redoubt we use the terminologyadvanced by Lahr et al. (1994) and refer to individual events asVolcano-Tectonic (VT) earthquakes and Long-Period (LP) events. Thisclassification scheme is intended to categorize individual shocks basedon the source process involved rather than waveform appearance. Inthis study, we describe events based simply on the appearance of thewaveform in the time domain. We recognize that numerous factors af-fect the appearance of seismograms in volcanic environments and thatclassification in this manner requires several assumptions. Additionalstudies of seismicity at Redoubt in 2009 that make use of more rigorousclassification techniques are presented by Buurman et al. (2013) andKetner and Power (2013).

Between October 12, 1989 and December 31, 2010, AVO calculatedhypocenters for more than 10,400 earthquakes within roughly 12.5 kmof the summit of Redoubt Volcano. Most of these hypocenters concen-trate beneath the volcano's summit and range in depth from −3.0 to10 km (Figs. 2–4) and heightened rates of earthquake activity occurredin this depth range during and following both the 1989–1990 and2009 eruptions. Located seismic events in the area between sea leveland −3.0 km depth are generally confined to eruptive periods (Fig. 3)and characterized by repetitive swarms of LP and hybrid events and oc-casional VT earthquakes (Lahr et al., 1994; Power et al., 1994; Stephensand Chouet, 2001). Most located earthquakes range in magnitude from0.5 to 1.5. The largest earthquake observed at Redoubt Volcano is a mag-nitude 3.5 that occurred on April 9, 2009 at a depth of 4.5 km roughly5 km west of Redoubt's summit (Fig. 4). LP and VT earthquakes arealso observed at depths of 25–35 km. Hypocenters in this depth rangescatter broadly beneath the Redoubt edifice and increased significantlyin association with the 2009 eruption (Figs. 2 and 3).

3.1. 1989–1990 Seismic chronology

The 1989–1990 eruption of Redoubt Volcano was characterized bylarge explosions, episodes of lava dome growth, periods of repetitiousdome failure associated with high altitude ash plumes, and lahars in theDrift River Valley. The eruption can be described as consisting of fivephases, 1, precursory phase (October 1989–December 14, 1989), 2, ventclearing phase (December 14–December 19, 1989), 3, first dome buildingphase (December 19, 1989–January 2, 1990), 4, second dome buildingphase (January 2–February 15, 1990) and 5, the repetitious dome build-ing/failure phase (February 15–June 15, 1990) (Power et al., 1994).

The seismicity observed prior to the 1989–1990 eruption was rel-atively subtle and short in duration, although stations RDN, DFR, andNCT in the Redoubt network were not available for analysis prior toOctober 12, 1989, as telemetry to the recording site had not beencompleted. Precursory seismicity consisted of two short episodes oftremor on November 14 and 20 that lasted just 45 and 6 minutes,respectively, on station RED. A slow steady increase in small shallowevents was observed in helicorder records from station RDN betweenNovember 10 and December 13, 1989 (Power et al., 1994). This in-crease in seismicity was accompanied by increased fumarolic activityand snow melt in the Redoubt summit crater (Gardner et al., 1994).

The first clear seismic precursor of the 1989–1990 eruption was aroughly 23-hour-long swarm of repetitive LP events that began lateon December 13, 1989. This swarm immediately preceded the firstexplosive events on December 14. The characters of these eventsare analyzed in detail by Chouet et al. (1994), Stephens et al.(1994), Lahr et al. (1994) and Stephens and Chouet (2001). Therelationship between LP seismicity and magma degassing was inves-tigated by Morrissey (1997) and a model of the source process of the

Fig. 3. Focal depth of earthquake hypocenters versus time beneath Redoubt Volcano between 1989 and 2010. Only hypocenters that occur within roughly 2.5 km of the volcano'ssummit and have standard horizontal and vertical errors of less than 5 km are shown. Events classified as Deep Long-Period (DLP) events are shown as shaded diamonds and alllocated events are shown. Events with hypocenters shallower than 10 km are plotted without distinction of source origin. Shaded time periods in 1989–1990 and 2009 correspondto periods when the volcano was in eruption.

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initial LP swarm was elaborated by Morrissey and Chouet (1997). Be-tween December 14, 1989 and February 15, 1990 Redoubt Volcanoexperienced nine major explosive events that were large enough tobe recorded on seismic station SPU located 80 km north of RedoubtVolcano. Many of these explosions produced large tephra plumesand lahars in the Drift River Valley. An actively growing lava domewas observed in the summit crater between many of the explosionsfrom December 19, 1989 to April 21, 1990 (Gardner et al., 1994;Miller, 1994). This period of eruptive activity is accompanied by acomplex mix of VT, hybrid, and LP events that took place at shallowdepth (Lahr et al., 1994; Power et al., 1994). The explosions onDecember 15, 1990 initiated a strong sequence of volcano-tectonicearthquakes between 4 and 9 km depth beneath the volcano. Theonset of these earthquakes was attributed to a stress response fromthe removal of magma from this depth range (Power et al., 1994).

Fig. 4. Histogram (A) of located earthquakes per day and magnitudes (B) between October 1map area shown in Fig. 2A are included in this plot and shocks with magnitude below 0.0

Earthquake activity in this depth range intensified during the 1989–1990 eruption and increased activity continued through at least1996 (Fig. 3).

Following the February 15 explosion Redoubt entered a period ofrepetitious dome growth and failure that included 11 separate explo-sive events, the last of which occurred on April 21, 1990. Many of theexplosions were preceded by shallow swarms of LP events that began34 to 3 hours prior to the explosion (Stephens et al., 1994). Most ofthese swarms of LP events exhibited emergent arrivals and wererecorded on too few stations to reliably locate. The time periodbetween successive dome failures was remarkably regular first aver-aging 4.5 days between February 15 and March 14, and then7.6 days until the final failure on April 21. Page et al. (1994) suggestthese repetitive dome failures resulted from changes in the shape ofthe crater floor following the large explosion on February 15, 1990,

2, 1989 and December 31, 2010. Only earthquakes with hypocenters that lie within theare not shown. Total number of earthquakes located on March 22, 2009 is 1125.

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such that it could no longer support a large dome as it had earlier inthe eruption. The Redoubt lava dome continued to slowly growuntil approximately June 15, 1990 (Miller, 1994).

3.2. Inter-eruptive Seismicity 1991–2008

Between 1991 and 2008 most well-located earthquakes, those withstandard vertical and horizontal errors less than 5.0 km, occurred be-tween 3 and 9 km depth beneath the summit of the volcano (Fig. 3).Most of these shocks exhibit well defined P- and S-phases typical of asource associatedwith brittle failure of rock and are commonly referredto as VT earthquakes in volcanic environments. VT earthquakes werefirst observed in this area when the Redoubt seismic network becameoperational in October 1989. Increases in rate of earthquake activity inthis depth range were observed in association with both the 1989–1990 (Power et al., 1994) and 2009 eruptions (Fig. 3). We suggest thelong-term earthquake activity in this depth range results from volcanicprocesses associated with magma accumulation thought to occur with-in this depth range (Power et al., 1994). The changes in earthquake ratebetween 2000 and 2008 (Fig. 3) are more a reflection of seismic net-work health rather than variation in magmatic processes.

During inter-eruptive periods stations RSO and REF typically recordhigh rates of asNCT, DFR and RED (Fig. 1). These high rates ofmicroseis-mic activity are not unexpected in light of the locations of RSO and REF(Fig. 1) high on the heavily glaciated Redoubt cone and given the longestablished association between glaciers and microseismic activity(Weaver and Malone, 1979; Roux et al., 2008).

3.3. 2009 Precursory seismicity

The first observed sign of unrest prior to the 2009 eruption was astrong sulfur smell reported downwind of the volcano in mid-July andagain in mid-September of 2008 (Schaefer, 2012). Anomalous snowmelt was observed in the Redoubt Crater in mid-September and in-creased fumarolic activity was observed through late 2008 (Bull andBuurman, 2013). Retrospectively, Grapenthin et al. (this volume) iden-tified anomalous ground deformation starting in April orMay of 2008 atthe continuous GPS station AC17 operated by EarthScope roughly

Fig. 5. Representative waveforms for a deep long-period (DLP) earthquake that occurred benand the magnitude is 1.6.

26 km northwest of the summit of Redoubt Volcano. Nounusual earthquake activity was observed in association with theseearly signs of unrest on seismic stations close to the volcano(Ketner and Power, 2013) or in the earthquake catalog producedby AVO, although several stations in the Redoubt seismic networkwere not operational during this period (Dixon et al., 2010). Asmall number of LP events were identified in October and Novemberof 2008 with hypocenters between −3 and 5 km depth (Fig. 3).Based on the continued observations of gas flux and snow melt inthe summit crater AVO raised the aviation color code to yellow onNovember 5, 2008.

On December 12, 2008 we began to locate deep long-period (DLP)events and VT earthquakes at 28 to 35 km depth beneath the volcano(Figs. 2 and 3). DLP events are characterized by emergent P and S phases,an extended coda, and a velocity spectrum that is peaked between 1 and3 Hz (Fig. 5). Calculated hypocenters for Redoubt DLP events range fromroughly 28 to 35 km in depth and magnitudes from 0.3 to 1.5. BetweenDecember 12, 2008 and December 31, 2010, 30 DLP events were locatedbeneath Redoubt (Fig. 2). The average standard horizontal and verti-cal errors calculated by HYPOELLIPSE are 1.92±0.99 km and 0.79±0.41 km respectively. Uncertainties in these error estimates represent 1standard deviation from themean. Similar error estimates were obtainedbyNichols et al. (2011) for locations of DLP events at volcanoes in Oregonand Washington using a variety of network configurations. Results fromseismic tomography (Ebberhardt-Phillips et al., 2006) suggest DLP eventsbeneath Redoubt are occurring within the lower portions of the crust oruppermost portions of the mantle.

The shallow magmatic system at Redoubt became seismically ac-tive on January 24, 2009 when two notable episodes of volcanic trem-or occurred at 4:11 and 21:27. The durations of these two bursts were48 and 41 minutes, respectively, as measured from the onset of thesignal to the approximate time the signals amplitude returned totwice the normal background level at station RSO (Fig. 6). Strongtremor resumed on January 25 at 02:14 and marked the beginningof a more continuous seismic activation of the shallow portions ofthe Redoubt magmatic/hydrothermal system that would wax andwane, and occasionally quiet completely, until the onset of magmaticexplosions on March 23, 2009. Helicorder style plots of station RSO

eath Redoubt Volcano on January 23, 2009. The calculated hypocentral depth is 32 km

Fig. 6. Velocity seismic records from station RSO from (A) January 20 to February 20, and (B) February 21 to January 23, 2009, illustrating the 60-day period of precursory volcanictremor prior to the 2009 eruption of Redoubt Volcano. (C) RSAM 10-minute average amplitudes showing the relative amplitude of tremor during this period. Station RSO was dis-abled on March 23 as a result of explosive activity.

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illustrating the intensity of these episodes of tremor are shown inFig. 6. In response to this tremor AVO raised the aviation color codeto orange early on January 25, 2009.

Notable periods of increased volcanic tremor occurred betweenJanuary 25 and 27, January 30 and 31, and February 5 and 26. AnRSAM plot from station RSO from January 19 through March 23shows the relative intensity of the various episodes of tremor duringthe precursory period (Fig. 6). To characterize the frequency of theprecursory tremor we have calculated a spectrogram covering theperiod from January 23 to March 23, 2009 (Fig. 7). This spectro-gram was assembled from day-long spectrograms calculated using25-min-long windows with 50 percent of overlap between windows.A vector of zeros (zero padding) was appended to each window and aHanning taper was applied prior to performing the Fourier Transform.The result is a day-long spectrum with a high degree of resolutionalong the frequency axis. To reduce the frequency resolution to a rea-sonable level for plotting over a two-month time period, we apply amoving average smoother over a frequency interval of 0.066 Hz(200 frequency samples) along each column of the spectrogram(constant time slice), and decimate the frequency axis by samplingthe frequencies at intervals of 0.0066 Hz instead of the original sam-pling interval of 0.00066 Hz. To obtain a spectrogram that is normal-ized at each time, we divide each column of the spectrogram by itsmaximum value. Finally, we plot the normalized spectrogram usinga logarithmic plot in which amplitude is given in decibels accordingto the relation 20log10 (A). This technique provides a method tocapture the spectral properties of the 58-day period of tremor.

The spectral character of the precursory tremor varied consider-ably between January 25 and March 23 as seen at station RSO(Fig. 7). To ensure that the frequencies shown in Fig. 7 were the resultof the source rather than a path effect we also calculated long-termspectrograms for stations REF and RDN and recovered similar tempo-ral patterns for tremor frequency. Between January 25 and February 5tremor had frequencies as low as 1 Hz, while the period from February5 to 26 had a broader spectrum with frequencies between 3.5 and4.5 Hz. Tremor with a lower frequency character returned betweenFebruary 27 and March 23. Determining the cause of the observedshifts in tremor frequency is beyond the intended scope of this study.

Heightened periods of shallow discrete seismic events occurredintermixed with this tremor. Notable periods of discrete events tookplace on January 25, January 30–31, February 2, and February 26–27.Hypocentral depths during this time period ranged from −3 to 5 kmdepth. The most notable of these periods occurred on February 26 and27 and consisted of both LP and VT type events (Ketner and Power,2013). This swarm contained five events that had high amplitudesand had poorly developed phases. These waveform characteristicshave been associated with explosion events at Redoubt in the past(Power et al., 1994), although there was no visual confirmation of ex-plosive events during this period. OnMarch 10, after 10 days of relativequiescence (Fig. 6C) AVO dropped the aviation color code to yellow. Fol-lowing a small phreatic explosion on March 15 that generated a smallplume of steam and ash (Bull and Buurman, 2013), AVO again raisedthe color code to orange, where it remained until March 18, when itwas returned to yellow.

Fig. 7. Spectrograms from station RSO showing the frequency of tremor from (A) January 23 to February 21, and (B) February 22 to March 24, 2009.

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3.4. 2009 Eruption seismicity

This phase of the 2009 eruption is characterized by energetic swarmsof shallow repetitive seismic events, strongexplosion signals, pyroclasticflows and lahars, and periods of lava dome growth. A helicorder stylerecord (Fig. 8) shows the progression of seismicity between March 20and April 5. During this phase of the eruption five prominent swarmsof shallow seismic events were identified by Ketner and Power (2013)and Buurman et al. (2013), although many of these events were toosmall or emergent to be reliably located.

The 2009 eruption of Redoubt Volcano began with a pronouncedswarm of seismic events that preceded the first explosive event at06:38 on March 23 by about 58 hours. On March 20, 2009 smallrepetitive seismic events began to occur at shallow depth. These eventshad fairly broad spectra with energy between 1 and 15 Hz and calculat-ed depths range from−3 to 4 km depth andmagnitude range from 0.0to 1.5. In response to this swarmAVO raised the color code to orange andadvised that an eruption could occur within days to weeks. A lava domewas observed growing in the summit crater at about 17:00 onMarch 22(Bull and Buurman, 2013). This seismic swarm intensified until the firstmagmatic explosion occurred at 06: 38 onMarch 23 (Fig. 8). AVO imme-diately raised the color code to red following this initial explosive event.This first explosion disabled station RSO (Fig. 1), whichwas not repaireduntil April 24, 2009. Analyses by Buurman et al. (2013) and Ketner andPower (2013) indicate this swarm contained a great diversity of seismicevent types and more than 14 identifiable earthquake families.

To characterize the individual explosive events during the 2009eruption we have measured the duration of the signal on stationSPU located 80 km north of Redoubt Volcano near Mount Spurr.This method was chosen to match the measurements of explosion du-ration made during the 1989–1990 eruption (Power et al., 1994). Wehave measured the time interval during which the amplitude of the

seismic signal was at least twice its normal background level onhelicorder type displays. Durations were rounded to the nearest min-ute and are summarized in Table 3. More detailed studies of explosiveevents during the 2009 eruption sequence are presented by Buurmanet al. (2013) and Fee et al. (2013).

The explosive phase of the 2009 eruption occurred between 06:38on March 23 and 18:36 on April 5. In total 23 explosive events wererecorded that met the duration criteria at station SPU (Table 3).Large explosive events principally occurred in two clusters or timeperiods. The first was between 06:38 on March 23 and 03:41 onMarch 24 and the second was between 17:24 on March 26 and17:44 on March 30, 2009 (Fig. 8; Table 3). After the first period ofexplosive activity on March 23–24, AVO lowered the color code to or-ange late on March 25. The return to code red was made early onMarch 26 when explosive activity resumed. Two additional explosiveevents occurred at 14:01 and 18:36 on April 4 and 5. Explosions at06:38 on March 23, 08:29 on March 27, 03:24 on March 29, and14:01 on April 4 were immediately preceded by swarms of shallowrepetitive seismic events (Fig. 9). Smaller swarms of events precededexplosions at 8:14 March 23, 03:41 March 24, 8:29 March 27, 1:35March 28, and 3:24 March 29 (Ketner and Power, 2013). Of particularnote during this phase of the eruption are two swarms on March 24and 29 that did not immediately precede explosive events. The fre-quencies of swarm events gradually shifted from broader spectrum(1 to 15 Hz) to a narrower spectrum with energy concentrated be-tween 0 and 7 Hz through the explosive phase of the eruption.More in depth analysis of the waveform character is presented byBuurman et al. (2013) and Ketner and Power (2013).

In the quiescent period following the 17:44 explosion on March30, AVO again lowered the color code to orange late on April 2. Thiscolor code change was made about the time that small repetitiveevents resumed. This swarm intensified until approximately 22:00

Table 3Explosion onset time and duration measured at station SPU.

Date UTC Time* Duration at SPU(Min)

March 23, 2009 06:38 2.0March 23, 2009 07:02 7.0March 23, 2009 08:14 20.0March 23, 2009 09:39 38.0March 23, 2009 10;52 8.0March 23, 2009 12:30 20.0March 23, 2009 12:58 3.0March 24, 2009 03:41 15.0March 26, 2009 17:24 14.0March 27, 2009 08:29 7.0March 27, 2009 08:42 7.0M arch 27, 2009 16:39 8.0March 28, 2009 01:35 2.0March 28, 2009 03:25 4.0March 28, 2009 07:20 2.0March 28, 2009 09:20 2.0March 28, 2009 10:00 7.0March 28, 2009 21:40 6.0March 28, 2009 23:29 6.0March 29, 2009 03:24 11.0March 30, 2009 17:44 3.0April 4, 2009 14:01 31.0April 5, 2009 18:36 3.0

Fig. 8. Velocity seismic record from station REF from March 19 through April 6, 2009 showing the seismicity recorded during the explosive phase of the 2009 eruption of RedoubtVolcano. Letters A, B, and C correspond to periods of explosive activity between A, 06:38 March 23 and 03:34 March 24, B, 17:24 March 26 and 17:44 March 30, and C, the explosiveevent on April 4 at 14:01. Numbers correspond to shallow earthquake swarms that occurred between 1, March 20 and 23, 2, March 26 and 27, 3, March 29, and 4, April 2 to April 4.

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on April 3, when the swarm began a slow decline that ended with theexplosion at 14:01 on April 4 (Fig. 8). Events in this swarm werelower in amplitude than earlier swarms and had lower frequencycontent between 1 and 5 Hz. The color code was raised to red imme-diately following the 14:01 April 4 explosion and was lowered to or-ange on April 6.

Starting on April 2, VT earthquakes began to occur at depths of 4 to9 km. Like the earthquakes at similar depths in 1989–1990, we attri-bute these shocks to a stress response associated with the removalof magma from this zone. The onset of these events suggests that sub-stantial amounts of magma were removed from the depths of 4 to9 km while a dome was growing in the crater between March 31and April 4, 2009.

An additional swarm of LP events took place between May 5 andMay 8 when the final dome was growing in the summit crater. Thisswarm occurred during a period of accelerated dome growthreported by Diefenbach et al. (2013). A helicorder-style recordshows the progression of this swarm (Fig. 10). The color code wasnot raised during this swarm as it was already at orange. These eventshad emergent waveforms, lacked distinct phases, and recorded wellonly on the stations located on the volcanic edifice. Ketner andPower (2013) report that this swarm was dominantly composed ofa single family of events.

Following the May 5–8 swarm the shallow Redoubt magmaticsystem grew increasingly quiet seismically, while elevated rates of VTearthquakes in the 4 to 9 km depth range continued throughout 2010(Fig. 3). The lava dome continued to grow slowly until mid-July(Diefenbach et al., 2013). The aviation color code was downgraded toyellow on June 30 as dome growth slowed, and to green on September29 as the lava dome showed no signs of instability and allmonitoring in-formation had shown declining activity for several months.

Two final small swarms of shallow repetitive events began onDecember 28, 2009 and April 5, 2010 that were primarily visible on sta-tion RSO (Fig. 1). These swarms were much less energetic and hadlower events rates (maximum of 1 event every 1.5 minutes) thanthose seen in March through May of 2009. These two swarms wereboth relatively short lived, lasting only about 28 and 32 hours,

Fig. 9. Summary of explosive phase of the 2009 Redoubt eruption that shows (A) the time history of explosion event duration at station SPU, (B) RSAM average amplitudes at stationREF, (C) and earthquake hypocenter focal depth at Redoubt Volcano between March 19 and April 5, 2009. Periods when shallow earthquake swarms and periods when a lava domewas known to be present in the Redoubt summit crater are shown as bars at the base of sections A and B, respectively.

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respectively, and no obvious changes in the dome or summit craterwere noticed. In response to these swarms AVO raised the color codeto yellow between December 28, 2009, and January 5, 2010 and April5 and April 12, 2010.

4. Interpretation and discussion

4.1. Conceptual model of the redoubt magmatic system

Based on our long-term (1989–2010) observations of seismicity atRedoubt Volcano we suggest the underlying magmatic system is com-posed of three parts or sections. These sections include a deep mag-matic source zone at 28 to 32 km depth, a mid-crustal magmastorage or source zone at roughly 3 to 9 km depth, and a shallow sys-tem of cracks that extends from near the crater floor downward for 1to 3 km. A schematic diagram illustrating the subsurface componentsof the Redoubt magmatic system is shown in Fig. 11.

The onset of DLP events and VT earthquakes at 28 to 32 km depth(Fig. 3) in December of 2008 suggests the 2009 eruption involvedmagma that originated in the lower crust or upper mantle. The widescatter in hypocentral locations for these events suggests that thesource region for this magma may be broadly distributed beneath theRedoubt edifice (Fig. 2). Similar DLP events have been associated withmagma movement at mid-crustal to upper mantle depths at volcanoessuch as Izu-Ooshima, Japan, (Ukawa and Ohtake, 1987), Mammoth

Mountain, California (Hill et al., 1990; Pitt et al., 2002), Mount Pinatubo(White, 1996), Mount Spurr and Shishaldin volcanoes, Alaska (Poweret al., 2004), Mauna Loa, Hawaii (Okubo and Wolfe, 2008) and else-where. The onset of DLP events in December provides a very clear tem-poral link between this activity at 28 to 35 km depth and the 2009eruption. The identification of these events in December of 2008 andtheir continued occurrence through January–March, 2009, figuredstrongly in the public warnings and eruption forecasts issued by AVO.

We note that the observed onset of DLP events in December of2008 was not the earliest sign of increased unrest. Sulfur odor wasreported downwind of Redoubt in mid-July 2008, increased snowmelt in the crater was reported in mid-September 2008 (Bull andBuurman, 2013), and subtle ground deformation also began atabout this time (Grapenthin et al., this volume). These signs of unrestpreceded the observed onset of DLP events by at least six months.This suggests that at Redoubt the onset of DLP events probably doesnot mark the earliest rise of magma and/or associated volatiles fromlower crustal depths. This delay suggests that conditions needed togenerate DLP activity may only occur once certain threshold condi-tions are achieved and magma or gas has been evacuated from thiszone. Petrologic evidence from 2009 erupted material suggests the2009 eruption involved magma that was stored at mid-crustal depthsand was remobilized by a heating event (Coombs et al., 2013). Wesuspect the heat source may have been hotter more mafic magmathat rose from depths of 28–35 km, triggering the DLP events in the

Fig. 10. Velocity seismic record from station REF for May 4 through May 8, 2009, showing the swarm of shallow seismic events that peaked in rate on May 6 while a lava dome wasactively growing within the summit crater.

40 J.A. Power et al. / Journal of Volcanology and Geothermal Research 259 (2013) 31–44

process. The passage of this magma from the lower crust to depths of3 to 9 km seems to have occurred aseismically.

In hindsight, it seems that a similar increase in DLP events mayhave accompanied the 1989–1990 eruption, however the earthquakedetection algorithm used in 1989–1990 lacked sufficient sensitivity todetect as many lower crustal events as may have occurred. Between1989 and 1995 we located more DLP events at Redoubt than between

Fig. 11. Inferred north–south cross section of the Redoubt Volcano magmatic systembased on seismic and supporting geologic and geophysical information. Principal compo-nents are a shallow system of cracks that may extend several km below the RedoubtCrater floor, a mid-crustal magma storage area between 4 and 9 km depth, and a deepermagma source zone at 28 to 35 km depth.

1995 and 2008 (Fig. 3) even though an improved triggering algorithmcame into use in early 2002 (Dixon et al., 2004).

The mid-crustal magma accumulation at 3 to 9 km depth is thesource of most located earthquakes at Redoubt Volcano between 1989and 2010. Seismic activity in this depth range intensified followingthe onset of magmatic explosions in both 1989 and 2009 suggestingthese earthquakes represent stress adjustments associated with theremoval of magma from this depth range. Tomographic inversionsand high-precision earthquake relocations by Benz et al. (1996) andDeshon et al. (2007) provide supporting evidence for the long-termstorage of magma within this portion of the crust. Benz et al. (1996)identified “a low-velocity near-vertical pipe-like structure” that extend-ed from 1 to 6 km below sea level that aligned with the majority of thelocated earthquakes at 3 to 6 km depth. This study did not identify alarge lowvelocity zone that could be associatedwith amagma reservoirand suggested magma storage most likely occurred in a plexus of dikesand sills within this depth range.Workingwith data from 1989 throughNovember 2005, DeShon et al. (2007) inferred slightly higher compres-sional wave velocities at depth. They suggested these higher velocitiesmight coincide with a mid-crustal magma storage zone and proposedthat VT earthquakes took place in thewall rock surroundingmagma ac-cumulations that likely occurred within dikes and sills. Supplementaryevidence for this mid-crustal magma accumulation zone comes fromgeochemical investigations by Browne and Gardner (2006) whosuggested a storage depth of 3 to 6 km for Redoubt magma ejected inthe early stages of the 1989–1990 eruption. Coombs et al. (2013) sug-gest a storage depth of roughly 4 km for magma erupted in 2009.

We surmise the mid-crustal magma chamber is connected to thesurface by an interconnected system of dikes and sills that likelybroadens within the last 2 or 3 km beneath the crater floor. This sys-tems of cracks is the source region for the repetitive swarms of LPevents, hybrid events, and volcanic tremor observed during the1989–1990 eruptions (Chouet et al., 1994; Lahr et al., 1994; Poweret al., 1994; Stephens et al., 1994), as well as the during the 2009

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eruption (Buurman et al., 2013; Ketner and Power, 2013). This regionis also the source of VLP energy associated with explosions during the2009 eruption (Haney et al., 2013). Comparison between waveformsfrom the 1989–1990 and 2009 eruptions (Ketner and Power, 2013)indicates that events from the two eruptive periods have differentwaveforms and do not belong to a single family. This suggests thatmagma may ascend to the surface along numerous pathways at shal-low depth beneath the crater floor. This zone was also identified ashaving a high VP/VS ratio by Benz et al. (1996). The shallow portionsof the Redoubt magmatic system appear to only activate seismicallyin association with eruptions in 1989–1990 and 2009 (Fig. 3), pre-sumably reflecting the rise and eruption of magma and associatedvolatiles.

4.2. Comparison of seismicity of 1989–1990 and 2009 Eruptions

Magmatic processes associated with the 1989–1990 and 2009 erup-tions of Redoubt Volcano seismically activated many of the same por-tions of the magmatic system although time scales and intensity varyconsiderably between the two eruptions. To provide a graphical com-parison of the progression of the two eruptions we show in Fig. 12plots of the duration of explosion events at station SPU along with thefocal depths of located earthquakes at the same scale. The principal

Fig. 12. Comparative plots showing explosion durations measured at station SPU and earthqizontal lines beneath the duration plots indicate estimated periods of lava extrusion.

similarities are 1, the activation of VT earthquakes at 4 to 9 km depththat defines themid-crustal magma storage zone, 2, swarms of shallowLP, VT and hybrid earthquakes at shallow depth beneath the summitcrater, and 3, shallow swarms of repetitive events preceding many ofthe explosions. Strong differences include 1, the protracted 58-day peri-od of shallow tremor that preceded the 2009 eruption, 2, shallowswarms of events on February 26–27, March 24, March 29, and May 6to 10, 2009 that did not culminate in explosions, 3, a period of explosiveactivity in 2009 thatwasmuch shorter than in 1989–1990, 4, an onset ofVT earthquakes at mid-crustal depths that began more slowly in 2009compared to 1989–1990, and 5, some early explosions that were ofmuch greater duration in 1989–1990 than in 2009.

The similar seismic characteristics shared by the 1989–1990 and2009 eruptions are not surprising. The whole rock magma compositionwas found to be similar for both eruptions (Coombs et al., 2013) andpresumablymagma followed a similar path to the surface in both erup-tions. Perhaps more of a surprise is that the two eruptions differ asmuch as they do. Conceptually many of the observed differences inseismicity between the 1989–1990 and 2009 eruptions could resultfrom the smaller erupted volume in 2009 (81 to 120×106 m3, Bullet al., 2013) as compared to the 1989–1990 volume (150 to250×106 m3, Gardner et al., 1994). Additional petrologic evidencealso suggests the triggering mechanism for the two eruptions may

uake focal depth versus time for (A) the 1989–1990, and (B) the 2009 eruptions. Hor-

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have been different. The 1989–1990 eruption is thought to have beentriggered by a magma mixing event (Nye et al., 1994; Swanson et al.,1994;Wolf and Eichelberger, 1997),while Coombs et al. (2013) suggestthe 2009 eruption could have been triggered by a heating event thatremobilized magma. This heating event was accompanied by a largeCO2 flux (Werner et al., 2013) in January–March thatmay have been re-sponsible for the strong periods of observed tremor at this time (Fig. 6).The other key difference is the shallow swarms of events on February26–27, March 24, March 29, and May 6–10 that did not immediatelyprecede explosions. These behaviors should be considered when for-mulating warnings and eruption forecasts during future periods ofunrest.

Further differences between the two eruptions were the observedincrease in DLP events and lower-crustal VTs in 2009, which were notobserved in 1989–1990, and the repetitive period of lava domegrowth and failure in the later stages of 1990, which was absent in2009. We suggest the most likely explanation for the lack of lowercrustal seismicity in 1989–1990 was that the event detectionalgorithm used at that time was not tuned properly to capture eventsin this depth range (Dixon et al., 2004). The lack of repetitive periodsof dome growth and failure in 2009 may be a result of changes in thegeometry of the crater floor as suggested by Page et al. (1994), ratherthan changes in an underlying magmatic process. Alternatively, thisdifference could result from differences in magma composition orviscosity.

4.3. 2009 Eruption forecasting

In this sectionwe review the role that seismic observations played informulating eruption forecasts and public warnings, and we providerecommendations for evaluating future episodes of unrest at RedoubtVolcano. The forecasting strategy used byAVOduring the 2009 eruptionrelied on the synthesis of data from several monitoring techniques thatincluded seismic observations (hypocenters, seismicity rate, RSAM,continuous spectral measurements, and waveform characteristics),visual observations (from both overflights and webcams), satelliteobservations, gas measurements, thermal infrared measurements, andgeodetic data. This information was supplemented with extensiveknowledge from the 1989–1990 eruption sequence that included chro-nologic information on seismicity (Lahr et al., 1994; Power et al., 1994),as well as detailed supporting seismic investigations (Chouet et al.,1994; Benz et al., 1996; Morrissey and Chouet, 1997; Stephens andChouet, 2001; Deshon et al., 2007), and eruptive products (Gardneret al., 1994; Miller, 1994; Nye et al., 1994; Swanson et al., 1994; Wolfand Eichelberger, 1997; Browne and Gardner, 2006). Regardless, exten-sive challenges remained in forecasting volcanic activity and associatedhazards during the 2009 eruptive sequence.

Forecasts and public warnings in 2009 were principally communi-cated using the volcano alert notification system. This two part sys-tem uses an aviation color code to communicate hazards to aircraftand a volcano alert level to communicate hazards to individuals andcommunities on the ground surrounding volcanoes (Gardner andGuffanti, 2006). The specific methods used by AVO for disseminatinghazards information are reviewed by Neal et al. (2010). The volcanoalert notification system is an expanded and standardized systemfor distributing volcano hazard related information and represents arefinement of the Level-of-Concern Color Code system that wasdeveloped during the 1989–1990 eruption of Redoubt Volcano(Brantley, 1990). In this discussion we focus only on the aviationcolor code as the aviation color code and alert level were alwayschanged together during the 2009 Redoubt eruptive sequence.

Our forecasting efforts in 2009 were largely successful as the colorcode was raised in advance of or shortly following major explosionsthat produced hazardous ash clouds or lahars. A long-term warningwas issued when the color code was changed from green to yellowon November 5, 2008, 136 days prior to the first magmatic explosion.

The color code was also successfully raised to orange roughly 1 dayprior to the first explosion on March 23 and was raised to red withinminutes of the occurrence of large explosive events on March 22,March 26, and April 4. The color code remained at red through thetwo major explosive period on March 23–24 and March 26–30 andwas also red for the small final explosive event on April 5 (Table 1),and additional warnings were issued following individual explosiveevents during these periods in the manner described by Neal et al.(2010).

While the forecasting effort was successful in the overall sense it isimportant to note the instances when the color code was raised andhazardous volcanic activity did not occur as expected. The most nota-ble of these was on January 25, 2009 when the color code was raisedto orange at the onset of the period of precursory tremor (Fig. 6)when hazardous ash clouds and lahars were still 56 days away.Much of the reason for this change was the very short 23-hour longswarm prior to the onset of explosive activity in 1989–1990 and, con-sequently, Redoubt has always been used as an example of a volcanothat could reawaken quickly. The color code was also raised to yellowon December 28, 2010 and April 5, 2010 when no further surficialactivity occurred. Because our forecasting strategy relies in part onan incomplete physical understanding of the seismic expression ofmagmatic processes that lead to the rise and eruption of magma, aswell as pattern recognition, such false alarms are likely unavoidable.A period of repetitious dome failures did not occur in 2009 andconsequently we did not employ a statistical forecasting approachas Page et al. (1994) developed for the 1989–1990 eruption.

In evaluating future episodes of unrest at Redoubt Volcano the fol-lowing observations should be considered:

(1) The earliest precursors to both the 1989–1990 and 2009 erup-tions were subtle and included increased gas emission, heatflux, and snow melt in the summit crater and ground deforma-tion in 2008 (Miller, 1994; Gardner et al., 1994; Bull andBuurman, 2013; Grapenthin et al., 2013).

(2) The 2009 eruption was preceded by a marked increase in DLPevents at 28 to 35 km depth that began roughly 2.5 monthsbefore the onset of magmatic explosions. Future monitoringprograms should be designed to capture seismicity in thisdepth range.

(3) The 2009 eruption was preceded by shallow volcanic tremorthat waxed and waned over a 58 day period. Similar activitywas not observed prior to the 1989–1990 eruption.

(4) Most energetic shallow swarms of repetitive LP events in the1989–1990 and 2009 eruption sequences culminated in an ex-plosive event. The exceptions to this were swarms that oc-curred on March 26–27 and May 5–8, 2009.

(5) A period of repetitive dome failures that produced large ashplumes took place during the end of the 1989–1990 eruption.This type of repetitive dome failure did not occur during the2009 eruption.

(6) Both the 1989–1990 and 2009 eruptions were followedwithin 6to 7 months by veryweak swarms of repetitive events seen prin-cipally on RSO. These swarms occurred in November 1990(Stephens et al., 1994), December 2009, and April 2010. Theseswarms did not result in noticeable changes within the summitcrater.

5. Conclusions

Based on the character and timing of seismicity observed at Re-doubt Volcano from 1989 through 2010, we feel the Redoubt mag-matic system consists of a diffuse magmatic source zone at depthsof 28 to 32 km depth, a mid-crustal magma storage area at depthsof roughly 3 to 9 km and a conduit and system of interconnectedcracks that extends from the mid-crustal storage zone to the Redoubt

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Crater floor. The deeper diffuse magmatic source area is defined bythe hypocenters of DLP events and VT earthquakes that are observedat these depths before and after the 2009 eruption. The mid-crustalstorage area is defined by hypocenters of VT earthquakes that increasedin rate following the initiation of eruptive activity in December 1989and April 2009 and are a persistent feature of the seismic record be-tween 1989 and 2010. The shallow system of cracks is defined seismi-cally by repetitive swarms of LP events, small VT earthquakes, andhybrid events that occur most prevalently during the eruptions in1989–1990 and 2009.

A notable aspect of Redoubt seismicity is the difference in precur-sory activity observed before the 1989–1990 and 2009 eruptions. In1989 clear seismic precursors to the magmatic eruption did notbegin until the onset of the 23-hour long swarm of repetitive LPevents on December 13, 1989. In contrast the 2009 eruption waspreceded by roughly 58 days of shallow seismicity characterized byextended periods of tremor, LP and hybrid events and phreatic explo-sions. The 2009 eruption was also preceded by a marked increase inDLP events, although the lack of a similar sequence of DLP events in1989 may have been a result of the insensitive event detection acqui-sition system in use in 1989 and short period of operation of theRedoubt seismic network prior to the 1989–1990 eruption.

Acknowledgements

We are grateful to John Paskievitch, Guy Tytgat, Cyrus Read, JohnRogers, Ed Clark, Tim Plucinski, Tom Murray, Tom Parker, John Lahr,and Jim Dixon, who worked to keep the Redoubt seismic networkand data acquisition systems operational between 1989 and 2010.During the preparation of this study we had useful conversationswith Michelle Coombs, Kate Bull, Helena Buurman, Jeff Freymueller,Peter Cervelli, Ronni Grapenthin, Stephanie Prejean, Alicia Hotovec,and Seth Moran. Cliff Thurber and Diana Roman provided formal re-views of the text and figures that greatly improved the manuscript.

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