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Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Atmospheric resonant oscillations by the 2014 eruption of the

Kelud volcano, Indonesia, observed with the ionospheric total electron contents and seismic signals

Yuki Nakashima

^a^,^∗

, Kosuke Heki

^a

, Akiko Takeo

^a^,^b

, Mokhamad N. Cahyadi

^c

, Arif Aditiya

^d

, Kazunori Yoshizawa

^a

aDepartmentofNaturalHistorySciences,HokkaidoUniversity,N10,W8,Kita-ku,Sapporo 060 0810,Japan bEarthquakeResearchInstitute,TheUniversityofTokyo,1-1-1Yayoi,Bunkyo-ku,Tokyo,Japan

cGeomaticsEngineeringInstitutTeknologiSepuluhNopember(ITS),Surabaya,Indonesia dBadanInformasiGeospasial,Jl. RayaJakarta,BogorKM. 46,Cibinong,Jawa Barat,Indonesia

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received6July2015

Receivedinrevisedform12November2015 Accepted19November2015

Availableonline4December2015 Editor:P.Shearer

Keywords:

GNSS TEC ionosphere

atmosphericfreeoscillation acousticwave

broadbandseismometer

Acoustic waves fromvolcaniceruptions are oftenobserved as infrasound innearfields. Part ofthem propagateupwardand disturbthe ionosphere,andcan beobservedwithTotalElectronContent(TEC) datafromGlobalNavigationSatelliteSystem(GNSS)receivers.HerewereportTECvariationsafterthe13 February2014PlinianeruptionoftheKeludvolcano,EastJava,Indonesia,observedwithregionalGNSS networks. Significantdisturbances inTECweredetectedwithsixGNSSsatellites, andwavelet analysis showed that harmonic oscillations started at ∼^{16:25 UT} ând ^continued ^for ∼^{2.5 h.} ^The âmplitude spectrum ofthe TECtime seriesshowed peaksat3.7 mHz, 4.8 mHzand 6.8 mHz.Long-wavelength standingwaves withawiderangeofwavelengthtrappedintheloweratmosphereare excitedbythe Plinian eruption. Amplitude spectra of the ground motion recorded by seismometers, however, had frequencycomponentsatdiscretewave-periods.Theconditionfortheresonantoscillationsbetweenthe atmosphereandthesolidEarthissatisfiedonlyatthesediscretewave-periodandhorizontalwavelength pairs,thereforeefficientenergytransferfromtheatmosphericstandingwavestothesolidEarthRayleigh wavesoccurredatdiscreteperiodsandresultedintheharmonicgroundmotion.

1. Introduction

Large volcanic eruptions often disturb the upper atmosphere including the ionosphere of the earth. Such disturbances can be observed by GNSS receivers aschanges in TEC. For example, an N-shapedchange inTEC, lastingfor ∼^{100 s,} ^was ^found ^after^the 2004SeptemberVulcanianexplosionoftheAsamavolcano,central Japan(Heki,2006).Suchdisturbancesarecausedbycompressional pulseintheneutralatmospherepropagatingupward asanacous- ticwave.A longer-lastingvolcaniceruptionoftenshow adifferent typeofatmosphericdisturbances.Forexample,Dautermannetal.

(2009b)detectedionosphericdisturbancesexcitedbytheeruption oftheSoufriereHillsvolcano,theLesserAntilles,in 2003.Theyin- cluded 1.4 mHz internal gravity wave and ∼^{4 mHz} atmospheric eigenfrequency components. Dautermann et al. (2009a) investi-

*

Correspondingauthor.

E-mailaddress:[email protected](Y. Nakashima).

gated thestrain gaugerecordsafterthiseruption, andalsofound the∼^{4 mHz}oscillationinthesolidearth.

Such resonant oscillations of the lower atmosphere are also observed by GNSS-TEC after large earthquakes. Choosakul et al.

(2009) reported thatthe 2004Sumatra–Andaman earthquakeex- cited ∼^{4 mHz} oscillation in the ionospheric lasting for hours.

Rolland et al. (2011) analyzed various types of ionospheric per- turbations by the 2011 Tohoku-oki earthquake. It includes an N-shaped wave excited by the Rayleigh wave, and the 3.7 mHz and 4.4 mHz acoustic-trap-modes excited in the lower atmosphere and leaked into the ionosphere. Then, the internal gravity wave appeared ∼45 min later and propagated concentrically outward by ∼^{225 m/s}^from^the ^epicenter. ^Saito^et^{al. (2011)} ^de- tected3.7 mHz,4.5 mHzand5.3 mHzfrequencypeaksintheTEC changes.Nishiokaetal. (2013)reportedinternalgravitywavesand lower atmospherictrappedwavesexcitedby the2013mooreEF5 tornado.

Atmosphericresonantoscillationsalsoexcitesecondaryoscilla- tionsinthesolidEarth,andtheirfrequenciesareexplainedbythe http://dx.doi.org/10.1016/j.epsl.2015.11.029

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normalmodetheory (Lognonnéetal., 1998).The Rayleighwaves in very long period seismograms, after the 1991Pinatubo eruption, hadtwo peaks,which are interpreted as coupling between theatmosphericoscillations excited by thevolcanic eruption and theRayleighwave (KanamoriandMori, 1992).Widmer andZürn (1992)alsoreportedverticaloscillationsofthegroundobservedby gravimeters around theworld. Later, Kanamoriet al. (1994) pro- posed physical mechanisms of such oscillations, but the ground motionperiodisunderestimatedabout10%.

Watada and Kanamori (2010) provided an excitation mecha- nism; the fundamental and overtone of the atmospheric long- wavelength acoustic waves trapped in the low-sound velocity channelintheatmospherebelowthethermosphere,andthefun- damentalmode whichpropagated asRayleigh wavesinthe solid Earthsharethesamehorizontalwavelengthalongthegroundsur- faceandthesamewave periods.Thisacousticresonancebetween theatmosphereandthesolid Earthresultedintheobservedhar- monic ground motion composed of Rayleigh waves at two res- onance periods. They employed the normal mode method for a combinedEarthmodelwiththesolidEarth,theoceanandtheat- mosphereupto200 kmaltitude,andsucceededinre-producingas a normalmode summation the harmonicground motionexcited by a point source that models a volcanic eruption in the atmosphere.

Inthispaper,we reportpreliminaryobservationresultsofhar- monic oscillations in ionospheric TEC and seismic records after the 2014 February Plinian eruption of the Kelud volcano, east- ernJava,Indonesia.TheKeludvolcanoisaveryexplosivevolcano, andhaserupted8timesforthelastonehundredyears.The2014 Februaryeruptionfracturedthelavadomemadebythe2007erup- tion and created a new crater (Sulaksana et al., 2014). Corentin et al. (2015) interpreted the eruption sequence from infrasound and seismic observations. The oscillation in the ionosphere and lithosphere would have been caused by the lower atmospheric trapped waves excited by this eruption. We compare the results fromthetwo kinds ofsensors,anddiscuss howthe 13February 2014 Kelud volcano eruption excited these atmospheric trapped waves,from the oscillations observed in the ionosphere and the solidEarth.

2. Dataanalysisandresults 2.1.GNSS-TECdata

Ionospheric delays of microwave signals from GNSS satellites are frequency dependent, andphase differences between the L1 and L2 carriers provide information on the ionosphere between the satellite and the ground station.TEC is the number of elec- tronsintegratedalongtheline-of-sight,andcanbecalculatedfrom the L1and L2 phase differences. We extracted the TEC information before and after the 2014 February eruption of the Kelud volcano from the raw data of 37 GNSS stations in and around Indonesia.The observation dataare fromthree networks, (1) the GNSSnetworkinJavarunbytheBadanInformasiGeospasial(BIG), (2) Sumatra GPS Array (SuGAr) operated in Sumatra by Indone- sianInstituteofScienceandCaliforniaInstituteofTechnology,and (3) International GNSS Service (IGS) stations (Fig. 1). The SuGAr stationsobservedonlyGlobalPositioningSystem(GPS),theAmer- icanGNSS, every 15 s. Other stations received signalsfrom both GPSandGLONASS(GLObal’nayaNAvigatsionnayaSputnikovayaSis- tema),theRussianGNSS,every 30 s,exceptforCOCOwhereonly GPSdataarerecorded.

Weextractedthecomponentswithperiodsaround270 s from slantTECtime seriesusing theMexican-hat wavelet. In orderto makeitpossibletocompareamplitudesbetweendifferentstation–

satellitepairs, we convertedtheamplitudesintothose invertical

Fig. 1.MapoftheJavaIslandandnearbyislands,andtheGNSSstationsandbroad- bandseismometersusedinthisstudy.TheKeludvolcanoismarkedwiththered triangleinEasternJavaIsland.BlacktrianglesandstarsindicateIGSandSuGARsta- tions,respectively.We usealso26stationswhicharerunbyBIG.Whitediamonds showGEOFONstationswhosewaveformsareshowninFigs. 3 and 4.(Forinterpre- tationofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtothe webversionofthisarticle.)

TECby multiplying withthecosineofthe incidenceangleofthe line-of-sightwithathinlayerat250 kmheight.Thisheightisalso usedtocalculatethecoordinatesofsub-ionosphericpoints(SIPs).

After extracting the 270 s components, we estimated their propagation velocity by plotting the time and distance from the volcanooftheTECdisturbancesfollowingRollandetal. (2011).The estimatedpropagationvelocitywas∼^{0.8 km/s,}^close^to^the^acous- ticwavespeed intheFregionoftheionosphere,andtheoscilla- tioninthisperiodcontinuedfrom16:25 UTto 19:00 UT(Fig. 2).

The SIPsofthe disturbanceinthisfrequency showclear concen- tric wavefronts propagating outward from the volcano (Fig. 2(c), Animation S1).

We obtained the frequency spectrum of the TEC oscillations.

First,we extractedcomponentswithfrequencies2.0–8.0 mHzwith a band-pass filterfor thestations within 1000 km fromthe volcano. At first,forindividual satellites, we adjusted thetime axes of thetime series ofdifferent stations assuming 0.8 km/sas the propagationvelocity. TheTECdata obtainedwiththe samesatel- literesults havesimilar geometry, andgeometryplays an impor- tantroleinTECobservations.We confirmedthatthesewaveforms have similar structure, and then we further stacked them again (Fig. 3(a)),andthenconvertedthedoubly-stackedtimeseriesinto the frequency domain.The spectrum showedtwo clearpeaks at 3.7 mHzand4.8 mHz(Fig. 3(a)).

2.2. Seismicdata

The 2014eruption of the Keludvolcanowas a very explosive oneanditcausedmanysolidEarthandatmosphericeventswhich destroyedobservationinstrumentsnearthevolcano.Corentinetal.

(2015)investigatedtheeruptionsequencewithremote(>200 km) low-frequency-seismic(<0.2Hz)andinfrasounddata.Theyfound three typesofseismicsignal (S_LL:long lastingwave, S_P1:only visiblenearbysitesandS_P2:short-durationenergeticsignal)and twotypesofinfrasoundsignal(I_1:firsteventandI_LL:longlast- ing second event) and constructed the eruption sequence from difference of these signals. Here we also look into the eruption time-line from the seismometer records of periods 17–33 s observed atthree stationswiththeSTS-2broadbandsensorsofGE- OFON ∼^{200 km} ^from ^the ^volcano, ând ânother ^GEOFON ^station

∼^{500 km} ^from ^the ^volcano ^{(Fig. 4).} ^The ^signal ^first âppears ât

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Fig. 2.(a)TrajectoriesofSIPoftheGNSSstationsviewedfromLNNGandBAKOGNSSstations.TheredtriangleindicatestheKeludvolcano.LargeandsmallcirclesontheSIP tracksindicateSIPsat17:53 UTandat16:00,17:00and18:00,respectively.(b) Thehorizontalandtheverticalaxesshowtimein UTanddistancefromtheKeludvolcano, respectively.Colorsindicateamplitudesofthecomponentwiththeperiodof270 sextractedbywavelettransform.(c) Spatialdistributionsofthe270 scomponentTEC oscillationsatthreetimeepochs,(i) 16:25 UT(ii) 16:42 UT(iii) 17:54 UT.(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtotheweb versionofthisarticle.)

around16 UTatthestations∼^{200 km}^from^the^volcano.^The^seismic recordsincludebothRayleigh wavesandgroundmotiondue to acoustic waves (e.g., Corentin et al., 2015). For distinguishing betweenthesewaves,we usedthedifferenceini) thepropagation velocity, and ii) the phase shifts betweenhorizontal andvertical components(Fig. S1).Thefirstsignalcouldbetheninterpretedas thegroundmotionduetoanacousticwave(i.e.notawaveinthe solid earth) that was excited at ∼^{15:46 UT} ât ^the ^volcano, ând propagated outward in the lower atmospherewith the speed of

∼^{0.3 km/s.} ^Tremor ^starting ât ^{16:12 UT}^wouldâlso ^be ôfâcous- tic origin, andwould have startedat ∼^{16:01 UT} ât ^the ^volcano.

It continuedformorethanonehour.In addition,arelativelylarge amplitudewavepacketpropagatingwiththeRayleighwavespeed,

∼^{4 km/s,}^seems^to^have^beenêxcitedât^the^volcanoât∼^{16:14 UT.}

From these seismic records, we interpreted that the initial explosion occurred at 15:46, which was followed by the start of thePlinian eruption ∼15 min later.Then, an unknown eventoc- curredat∼^{16:14 UT}^that^generated^the^Rayleigh^waveⁱⁿ^the^solid earth.

In Fig. 3(b),we decomposed seismic records atUGM, located

∼^{200 km}^west ^from^the ^volcano,înto^three ^bandsôf^periods, î.e.

17–33 s, 100–200 s and200–333 s. We interpret that the long- lasting ground oscillation in the long-period band includes the resonantgroundmotioncoupledtotheacousticresonanceinthe lower atmosphere. Components with periods longer than 100 s started at 16:25, which is different from the initiation at 16:01 at shorterperiods. It might be reﬂecting an increase of volcanic activity dueto an eventin the solid earth at 16:14such asthe domecollapseortheventexpansion.Componentswithperiodsof 17–33 s or 100–200 sterminated at ∼^{18:00 UT,} ^which ^possibly

reﬂecttheendofthePlinianeruption.Ontheotherhand,theos- cillations with periods of 200–333 scontinued until ∼^{19:00 UT.}

Because thisperiodband includes thoseofthe ionosphericoscil- lations, these oscillations wouldhave been excited by the atmospheric freeoscillationthatcontinued∼^{1 h after}^thetermination ofthePlinianeruption.

Seismic signals withperiods of 100–1000 swere observed at the78broadbandseismometers(STS-1)oftheGlobalSeismicNet- work (GSN) around the world (Fig. S2). The signal is shown to come from the Kelud volcano (Fig. S3) with the speed of the fundamental-modeRayleighwave(Fig. S4)forthePreliminaryRef- erence EarthModel(PREM) by DziewonskiandAnderson(1981).

Assuming the Rayleigh wave phase velocity, we estimated the source time function at the Kelud volcano by stacking velocity waveforms after the corrections of sensor responses (Fig. 3(b)).

Thespectrumshowsbroadsourcewithseveralclearpeaksinclud- ing thoseat3.7 mHz,4.8 mHz,5.7 mHzand6.8 mHz(Fig. 3(b)).

If we assume atmospherictop free surface boundary is ∼^{90 km} aboveground,theeigenfrequenciesofthestandingacousticmodes in the atmospheric layer are 3.7, 4.7, 5.8 and 7.1 mHz for fundamental, ﬁrst,second, thirdandfourthovertones inthe vertical direction, respectively(WatadaandKanamori,2010).The altitude of thethermopause (∼^{100 km)} ^calculated^from ^theInternational ReferenceIonosphere(IRI)(Bilitzaetal., 2011) is closetothisas- sumption. The continuous spectrum would reﬂect the turbulent atmospheric waves excited directly by the Plinian eruption. The discreteamplitudepeaksareexplainedbytheresonantoscillation of the lower atmosphere and its leakage into the solid Earthas demonstratedbyWatadaandKanamori (2010).

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Fig. 3.(a)TheleftpanelshowstheTECchangesobservedbysevenGNSSsatellites,ﬁveGPS(1,7,13,20,23)andtwoGLONASSsatellites(15,16).Verticallinesshowevents indicatedinFig. 4.Timeseriesofmultiplesitesarereducedtothoseatthesourceofthedisturbance(i.e.theKeludvolcano)byadjustingthetimeaxesassumingthe propagationvelocityof0.8 km/sbackwardtheSIPs.Theseseventimeseriesarestackedtoproduceonetimeseriesatthebottom,whosefrequencyspectrumisgiveninthe rightpanel.Grayverticallinesshow3.7 mHz,4.8 mHz,5.7 mHzand6.8 mHz.(b) BluecurvesintheleftpanelshowverticalvelocitywaveformsattheUGMstation(200 km fromtheKelud)withperiodsof200–333 s(top),100–200 s(middle)and17–33 s(bottom).ThesewavesaredetectedbyaSTS-2seismometerasgroundmotionexcited bytheloweratmosphere.Thegreencurveindicatesthesourcetimefunctioncalculatedfromverticalvelocitywaveformsof72STS-1seismometersoftheGlobalSeismic Network.It showsRayleighwavespropagatinginthesolidearth.VerticallinesshoweventsindicatedinFig. 4.Thefrequencyspectrumofthesourcetimefunctionisshown intherightpanel.Grayverticallinesarethe sameaspanels(a).(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversion ofthisarticle.)

Fig. 4.Waveformsobservedat4GEOFONseismicstationson13February2014.Verticalcomponentsoftherawrecordsataband-passﬁlterof17–33 sareplotted.Dashed linesshow gradientscorresponding topropagationspeedsof0.3 km/sand4.0 km/s.Thesignalappearingaround16:00wouldbethe acousticwavesignalexcitedat T1=15:46:00andpropagatedfromtheKeludvolcano.Thecontinuoustremor,alsoofacousticwaveorigin,wouldhavestartedatT2=16:01:30,andlastedforhours.We can alsoseethewavepacketatT3=16:14:30propagatingwiththeRayleighwavespeedof∼^{4.0 km/s.}

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3. Discussionandsummary

First, we discuss durations of the various observed phases of the eruption. For the initial explosion at ∼^15:46, ^{we could} ^not identify the N-shaped pulsation in ionospheric TEC which may appear ∼10 min later. As shown in the short period (17–33 s) seismicsignals(Fig. 3(b))possiblycausedbythegroundvibration inthecontinuouseruption,thePlinianeruption mayhavestarted at ∼^{16:01 UT} ând ^lasted^for ∼^{2 h.} Âfter^the termination ofthe Plinianeruption at∼^{18 UT,}^long ^period^signals(200–333 s)were observeduntil ∼^{19 UT}^both ⁱⁿ^the îonosphere (Fig. 3(a)) andon theground(Fig. 3(b),thetopcurve).Thiswouldindicatetheatmo- spheric free oscillationthat continuedeven afterthe termination ofthePlinianeruption,anddecayedslowlyinonehour.

Theseismic signalsfrom16:25 UTto 18:00 UT showedpeaks at 3.7 mHz, 4.8 mHz, 5.7 mHz and 6.8 mHz, suggesting that thelithosphere–atmospherecouplingalsooccurredalongwiththe ionosphere–atmosphere coupling. The oscillations of TEC started at ∼^{16:25 UT,} ∼20 min after the start of the Plinian eruption at ∼^16:01. ^This ^time ^lag ^would ^reflect ^the ^time ^for ^the ^growth of the lower atmospheric oscillation to reach the ionospheric F layer(∼^{300 km).}^The^Plinianêruptionênded^by∼^{18 UT,}^however theionosphericandloweratmosphericoscillationscontinueduntil

∼^{19:00 UT} ^due^to ^relatively^large ^quality^factor ^(Q).Atmospheric quality factorof 3.7and 4.6 mHz are about150and 20,respectively(Rollandetal.,2011),andtheseQsarehighenoughforthe atmosphericfreeoscillationtocontinueby1 h withoutfurtherex- citations.

Here westudied ionosphericsignaturesmade bythe eruption oftheKeludvolcanoon13February2014,withregionalnetworks of continuous GNSS receivers. This study gives the ﬁrst detailed reportofspatialdistributionandtimeevolutionofionosphericdis- turbances bya Plinian eruption.First, thePlinian eruption would havecausedatmosphericturbulencesinitsplume.Theturbulence itselfhadbroadfrequencyspectrum,andstandingwavesoracous- ticfundamental andovertonemodestrapped inthe lower atmo- spherewereexcited.Then,somepartoftheenergyofthesestand- ingwaveswouldhaveleakedintotheionosphereandlithosphere, resulting inspectral peaks inTEC andseismograms. This Plinian eruptionthatdrovethemulti-sphereoscillationsendedat∼^{18 UT,} andonlylowfrequencyoscillationcontinuedforonemorehourin theionosphereandloweratmosphere.

Thisstudycanbesummarizedasfollows.

(1) With the GNSS-TEC technique, we observed ionospheric oscillations forced by lower atmospheric natural vibration after the start of the 2014 February Plinian eruption of the Kelud volcano, Indonesia. The Plinian eruption lasted from ∼^{16:01 UT} to ∼^{18:00 UT} ^while ^the ^TEC oscillation lasted from ∼^16:25 to ∼^{19:00 UT.} ^The ^TEC oscillations showed frequency peaks at 3.7 mHzand4.8 mHz.

(2) Spectra of the seismic records showed peaks of a series of atmospheric acoustic modes, 3.7 mHz and4.8 mHz, and two higher modes. These frequency peaks suggest that the harmonic ground motions are excited by the coupling between the solid Earthandtheloweratmosphere.

Acknowledgements

Wewouldliketo thanktoGeospatialInformation Agency(In- donesia)forprovidingGNSSdatawithinlimitedterm.IGS dataare

available fromthe web(ftp://garner.ucsd.edu/). GEOFONandGSN dataareavailablefromGFZandIRISdatamanagementcenters,respectively.

Appendix A. Supplementarymaterial

Supplementarymaterialrelatedtothisarticlecanbefoundon- lineathttp://dx.doi.org/10.1016/j.epsl.2015.11.029.

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